WO2022108420A1

WO2022108420A1 - Apparatus and method for controlling grid-forming power conversion

Info

Publication number: WO2022108420A1
Application number: PCT/KR2021/017281
Authority: WO
Inventors: 강지성; 허견
Original assignee: ㈜한국그리드포밍
Priority date: 2020-11-23
Filing date: 2021-11-23
Publication date: 2022-05-27

Abstract

The present invention relates to an apparatus for controlling grid-forming power conversion. The apparatus may comprise: a grid-forming power conversion unit which converts power supplied from a renewable energy generator into a voltage for supplying to a system power source, and thereby supplies power to the system power source; an optimum voltage control unit which is connected to the grid-forming power conversion unit and which, by calculating a fault current when a fault occurs and calculating a reverse current that is generated when voltage is lowered when the fault occurs and a recovery reverse current that is generated when the fault is removed, performs optimum voltage control that involves finding an operating point at which overcurrent due to fault current supply characteristics required by the system is minimized, and finding an operating point which satisfies the requirements of the power system as much as possible within a range that does not exceed the limit of the current that a power conversion apparatus can supply; and an impedance constraint evaluation control unit which collects parameters for impedance evaluation using information from the power system, and sets a reference MVA connection impedance input and a grid-forming source (GFM) voltage so as to obtain a minimum impedance that does not exceed the limit of the current that the power conversion apparatus can supply.

Description

Grid forming power conversion control device and method

The present invention relates to a grid-forming power conversion control apparatus, and more particularly, to a grid-forming power conversion control apparatus and method capable of stable operation based on system information connected with information of a power conversion apparatus to be connected.

The future of the global energy system depends on the development of new energy technologies due to the limitations of fossil fuels and how to solve greenhouse gas (GHG) emissions from fossil fuels.

And the flow of future energy is expected to shift from the oil era to the natural gas era to the hydrogen-based new & renewable energy era.

New and renewable energy are defined slightly differently by each country. In Korea, it is defined as energy other than oil, coal, nuclear power, or natural gas, and it is defined as energy in eight fields (solar energy, biomass, wind power, small hydropower, geothermal energy). , marine energy, waste energy) and three fields (fuel cell, coal liquefaction and hydrogen energy) as new energy belong to this category.

Renewable energy can be classified into several types such as solar heat, photovoltaic power generation, wind power generation, small hydro power generation, waste incineration heat and power generation, biomass energy (biogas, gasification power generation, biofuel), geothermal energy, and marine energy, depending on the technology and type of final energy. can be divided into Since such renewable energy is to obtain clean energy using the natural energy sources of the sun (light, heat), wind, water, and the sea, which are the primary energy sources, the amount of resources is almost infinite.

Globally, investment in the expansion and distribution of new and renewable energy such as wind power and solar power is concentrated, but power generation by new and renewable energy sources such as wind and solar power, which have intermittent power generation characteristics, is difficult to predict and has severe output fluctuation characteristics. This will greatly affect the stable operation of the connected system.

Therefore, in order to dramatically expand the supply of new and renewable energy sources such as wind power and solar power, stable supply of power generation output with severe output fluctuations and improvement of power quality are urgently required.

To this end, a power converter is installed between the renewable energy generator and the grid power source, converts the power supplied from the renewable energy generator into a voltage suitable for supplying the grid power, and supplies power to the grid power.

In addition, the power converter charges the power supplied from the renewable energy generator or grid power, and when the power supply from the grid power is stopped due to an abnormality such as a power outage in the grid power, the power supply is pre-charged according to the command of the upper controller. It performs the function of supplying power to the load.

On the other hand, the grid-forming power converter is a device that independently generates power in the power system and performs a role similar to that of a generator.

As the number of renewable power sources increases, the spread of grid-forming power converters that can replace the reduced role of synchronous generators is required.

In particular, the grid-forming power converter maintains independent voltage generation even in the event of an external system failure and is different from other power converters in that it is required to supply a fault current.

In the prior art, a simple method is adopted to respond to the failure of the grid-forming power converter, and the voltage is lowered until the failure current does not exceed the limit, and there is no consideration of unbalanced failure and reverse current.

That is, in the case of an unbalanced failure, a healthy reverse current is generated as the voltage of the power converter decreases, and a recovery reverse current is generated according to the potential difference when the failure is removed regardless of the failure type. There is a problem that power electronic equipment is damaged by current and recovery reverse current.

In particular, there was an analysis of the phenomenon according to the failure of the power system when the grid-forming power converter is connected to the power system, but it is limited to the analysis of the balanced failure, and there was no analysis of the phenomenon in the case of an unbalanced failure, which is an important part. There was no way to estimate the impedance constraints required to operate.

Therefore, it is required to evaluate the impedance adequacy based on the information of the power converter to be connected and the system information to be connected, and to develop a technology for correctly designing, manufacturing, and using the power converter based on this evaluation tool.

Background art of the present invention is disclosed in Korean Patent Registration No. 10-1337437.

The present invention provides a grid-forming power conversion control device and method capable of stable operation by evaluating impedance adequacy based on system information connected with information of a power conversion device to be connected.

The present invention provides a grid-forming power conversion control apparatus and method capable of optimal operation voltage control in which the power conversion apparatus minimizes the intensity of overcurrent in consideration of both the failure current and the reverse current in the event of a failure.

The present invention calculates the impedance constraint that enables stable grid forming operation even in all failure situations and, when the impedance criterion is not met, through power conversion device design change, controller addition, transformer wiring change, external system grounding factor adjustment, etc. A grid-forming power conversion control apparatus and method capable of alleviating restrictions are provided.

The present invention provides a grid-forming power conversion control device and method that can efficiently meet the demand for a grid-forming power conversion device according to the expansion of new and renewable power sources by determining whether stable operation is performed in the step of connecting the grid-forming power conversion device to provide.

The present invention provides a grid-forming power conversion control apparatus and method capable of controlling the frequency of the voltage output by the grid-forming power conversion device without controlling the direct current, and capable of operating at a high speed.

Other objects of the present invention are not limited to the above-mentioned objects, and other objects not mentioned will be clearly understood by those skilled in the art from the following description.

According to one aspect of the present invention, there is provided a grid-forming power conversion control device.

The grid-forming power conversion control device according to an embodiment of the present invention converts the power supplied from the renewable energy generator into a voltage for supplying the grid power to the grid-forming power conversion unit, the grid-forming power conversion unit for supplying power It is connected to calculate the fault current when a fault occurs and calculates the reverse current that occurs when the voltage is lowered in case of a fault and the recovery reverse current that occurs when the fault is removed according to the fault current supply characteristics required by the system. Information on the optimum voltage control unit and power system to find the operating point where overcurrent is minimized and find the operating point that satisfies the requirements of the power system as much as possible within the range that does not exceed the limit of the current that the power converter can supply Impedance constraint evaluation control unit that collects parameters for impedance evaluation using may include

According to another aspect of the present invention, there is provided a grid-forming power conversion control method.

In the grid forming power conversion control method according to an embodiment of the present invention, when a failure occurs in the system power supply, the fault current supply characteristic is controlled by calculating the fault current and the reverse current, and the parameter collection is performed using the information of the power system. Step, setting the reference MVA connection impedance input and GFM (Grid Forming Source) voltage in the input setting section, 3-phase short-circuit fault current, line-to-line short-circuit fault current, line-to-line short-circuit fault current and 1-line ground fault at fault current calculation section Calculating the fault current, storing the current GFM voltage, calculating the recovery reverse current, and judging whether the calculated recovery reverse current exceeds the set limit value and operating with the input impedance It may include the step of determining whether it is possible.

The grid-forming power conversion control apparatus and method according to the present invention as described above has the following effects.

First, stable operation is possible by evaluating the impedance adequacy based on the information of the connected power converter and the connected system information.

Second, it is possible to control the optimal operating voltage to minimize the overcurrent intensity by considering both the fault current and the reverse current of the power converter in case of a fault.

Third, if the impedance criterion is not met by calculating the impedance constraint that enables stable grid forming operation even in all failure conditions, it is restricted through power conversion device design change, controller addition, transformer wiring change, external system grounding factor adjustment, etc. can alleviate

Fourth, stable operation can be determined in the step of connecting the grid-forming power converter, so that the demand for the grid-forming power converter according to the expansion of new and renewable power sources can be efficiently met.

Fifth, it is possible to control the frequency of the voltage output by the grid-forming power converter without controlling the direct current, and to operate at a high speed.

1 is a model configuration diagram for the optimal voltage tracking control and impedance constraint evaluation of the grid-forming power conversion control device according to the present invention;

2 is a block diagram of a device for optimal voltage tracking control and impedance constraint evaluation of a grid-forming power conversion control device according to the present invention;

3 is a flowchart illustrating a method for optimal voltage tracking control and impedance constraint evaluation of a grid-forming power conversion control device according to the present invention;

4 is a block diagram of a failure model of a three-phase short circuit failure;

5 is a graph showing the current change of the converter according to different connection impedances in a 3-phase fault;

6 is a configuration diagram of a failure model of a one-line ground fault failure;

7 is a configuration diagram of the calculation model of HV Side Fault of 1-line ground fault.

8 is a Fault Current graph in the case of a ground fault in the HV side PCC point 1-line of the GFM Source linked to the AC system.

9 is a block diagram of a failure model of a short circuit between lines

10 is a configuration diagram of a calculation model of HV Side Fault of line-to-line short circuit failure

11 and 12 are Fault Current graphs in case of short circuit failure between HV side PCC points of GFM Source linked to AC system.

13 is a flowchart illustrating an overview of an output frequency control method of a grid-forming power conversion control apparatus according to the present invention

14 is a block diagram schematically showing an output frequency control unit of a grid-forming power conversion control device according to the present invention;

15 is a droop plot illustrating that the power generation system and the grid-forming power converter divide and provide power to the load.

Figure 16 is a view showing the outline of the output frequency control method of the grid-forming power converter according to the prior art together with the droop diagram illustrated in Figure 15

17 is an exemplary control diagram for controlling the matching control of the grid forming power conversion unit to control the output frequency according to the voltage

18 is a view showing the outline of the output frequency control method of the grid-forming power conversion control apparatus according to the present invention together with the droop diagram illustrated in FIG.

19 is a view showing an output frequency for a DC voltage (VDC) provided to a grid-forming power converter according to the present invention

20 is a view for explaining a third embodiment of the output frequency control method of the grid-forming power conversion control apparatus according to the present invention

21 is a view for explaining a fourth embodiment of the output frequency control method of the grid-forming power conversion control apparatus according to the present invention

22 is a view for explaining a fifth embodiment of the output frequency control method of the grid-forming power conversion control apparatus according to the present invention

23 is a view for explaining a sixth embodiment of the output frequency control method of the grid-forming power conversion control apparatus according to the present invention

24 is a diagram illustrating a DC voltage and an output frequency provided to the grid-forming power converter when the grid-forming power converter is controlled in the first embodiment, the second embodiment, and the third embodiment, respectively;

Figure 25 (a) is a diagram showing the output frequency of the DC voltage provided to the grid-forming power converter according to the prior art and the first embodiment and the second embodiment, Figure 25 (b) is Figure 25 (a) A drawing showing an enlarged range of 225kV to 245KV of

26 (a) and 26 (b) are diagrams showing the experimental results of the grid forming power conversion unit (solid line) and the power system (dotted line) controlled by the prior art

27 (a) and 27 (b) are diagrams showing the experimental results of the grid forming power conversion unit (solid line) and the power system (dashed line) controlled in the first embodiment

28 (a) and 28 (b) are diagrams showing experimental results of the grid forming power conversion unit (solid line) and the power system (dashed line) controlled in the second embodiment

29 (a) and 29 (b) are diagrams showing the experimental results of the grid forming power conversion unit (solid line) and the power system (dashed line) controlled in the third embodiment

30 (a) and 30 (b) are diagrams showing the experimental results of the grid forming power conversion unit (solid line) and the power system (dashed line) controlled in the fourth embodiment

Figure 31 (a) shows the experimental results of the grid forming power conversion unit (solid line) and the power system (dotted line) controlled in the fifth embodiment, Figure 31 (b) is from 1.2 seconds to 3.2 seconds in Figure 31 (a) A drawing showing an enlarged second part

32(a) and 32(b) are diagrams illustrating a case in which control is performed by droop control according to the prior art under the experimental conditions illustrated in FIG. 19;

33 and 34 are diagrams showing experimental results of the grid forming power conversion unit (solid line) and the power system (dotted line) controlled in the sixth embodiment

Hereinafter, a preferred embodiment of the grid-forming power conversion control apparatus and method according to the present invention will be described in detail as follows.

Features and advantages of the grid-forming power conversion control apparatus and method according to the present invention will become apparent through the detailed description of each embodiment below.

1 is a model configuration diagram for the optimal voltage tracking control and impedance constraint evaluation of the grid-forming power conversion control device according to the present invention.

Grid-forming power conversion control apparatus and method according to the present invention based on the information of the connected power conversion device and the system information connected to the power conversion device in the event of a failure in consideration of both the fault current and the reverse current optimal operation to minimize the overcurrent intensity Stable operation is possible by controlling voltage (optimal voltage) and evaluating impedance adequacy.

To this end, the present invention calculates the impedance constraint that enables stable grid forming operation even in all failure situations, and when the impedance criterion is not met, the power conversion device design change, the controller addition, the transformer wiring method change, the grounding factor adjustment of the external system It may include a configuration that can relieve the constraint through the

The present invention is a failure calculation method, and may include the steps of calculating the Thevenin equivalent impedance and calculating the abc phase → symmetrical coordinates → converting the abc phase back to the abc phase to calculate the unbalanced fault.

However, instead of calculating by equalizing one voltage source and one Thevenin impedance, the magnitude of the voltage acts as a variable in the grid forming power source, and it is impossible to equalize it by adding it to the AC voltage source. We use the superposition method, which calculates each and then adds them to one another.

When the power converter is operated by grid forming, it is required to supply the fault current to the allowable limit. Considering this, the method of lowering the voltage by considering only the fault current in the case of a 3-phase short-circuit failure without considering the reverse current is the power It is judged that it can have a fatal effect on the converter.

In the present invention, the control in which the power converter finds the optimal voltage when an external power system failure occurs is the characteristic of the fault current and the reverse current, the characteristic that there is a limit of the current that the power converter can flow, and the power system is grid Consider all the requirements for the forming power converter.

The present invention calculates all failure types in consideration of the characteristics of the fault current and the reverse current, finds the operating point where the overcurrent is minimized even in the most severe fault among them, and does not exceed the limit of the current that the power conversion device can flow. It is to track the optimum voltage that satisfies the requirements of the power system as much as possible in the range.

And impedance evaluation is to find the minimum impedance that does not exceed the limit range of the current that the power converter can flow based on the fault current and reverse current.

The present invention is particularly useful in the engineering phase of the power converter because it can determine the impedance of the power converter or determine whether the connection point of the system to be connected is appropriate by understanding the impedance constraint.

And when removing and recovering from a fault, regardless of the fault type, it is considered that the recovery reverse current from the external system exists while the power converter that has reduced the voltage once recovers the original voltage. In comparison with the lock fault current, consider that the limit voltage that prevents the reverse current from becoming the largest even in a situation where the external system impedance is close to 0 is 0.5pu.

And the present invention considers the following characteristics of the power conversion device.

A generator and a grid-forming power converter both have a voltage source in common. A generator has no current limit, whereas a power converter has a low current limit value as a power electronic device.

Therefore, the grid-forming power converter operates as a voltage source, but a method of controlling the current not to be exceeded by reducing the voltage in a situation such as a failure is required.

Here, in the present invention, when the current is limited, it is considered that not only the fault current supplied by the power conversion device but also the reverse current and the recovery reverse current entering the power conversion device are limited.

This is because, if limited by focusing only on the fault current, the reverse current becomes an overcurrent and may cause damage to the power converter.

Symbols used in the following description may be defined as follows.

Symbols for physical quantities V and E are voltage, I is current, and Z is impedance , F is the current flowing to the fault point.

And the symbol for the name of the facility is GFM for converter, SM for external system power source, and ext for external system excluding GFM.

The symbol for equivalence Z _th is the Thevenin equivalent impedance, and I _th is the current flowing through the Thevenin equivalent impedance, that is, the current generated in the GFM or the current generated in the SM.

And when a branch point occurs in the fault circuit in the middle of the calculation, when a name is assigned to each route, out means going out and in means returning to the inside.

And GFM, SMdirection is the abbreviation of the current generated from GFM and directed to the external system, while SM and GFMdirection are the current generated from SM and directed to the GFM.

If an expression becomes too long, abbreviation is made using s, b, e, t, m, and triangles.

And when it is necessary to distinguish the symmetric component, the image (0), normal (1), and reverse image (2) are distinguished.

2 is a block diagram of a grid-forming power conversion control apparatus according to the present invention.

The grid-forming power conversion control device according to the present invention is installed between the renewable energy generator and the grid power source, as shown in FIG. 3, and converts the power supplied from the renewable energy generator into a voltage for supplying the grid power, The grid-forming power conversion unit 100 that supplies power to the power source, and the grid-forming power conversion unit 100 are connected to the calculation of the fault current when a fault occurs and the reverse current generated when the voltage is lowered in case of a fault. Calculate the recovery reverse current generated during removal to find the operating point where the overcurrent is minimized according to the fault current supply characteristics required by the system, and maximize it within the range that does not exceed the limit of current that the power converter can supply The optimum voltage control unit 200 performs optimal voltage control to find an operating point that satisfies the requirements of the power system, and the parameters for impedance evaluation are collected using information on the power system, and the connection impedance and GFM voltage setting values are changed. and an impedance constraint evaluation control unit 300 to obtain a minimum impedance that does not exceed the limit of the current that the power conversion device can supply.

Here, the optimum voltage control unit 200 includes an optimum voltage calculation unit 10 that calculates an optimum voltage as the FRT voltage command value of the optimum voltage control unit, and a limit voltage calculation unit ( 11), the minimum voltage calculation unit 12 that calculates the minimum voltage as the lower limit of safety measures in case of incomplete voltage control of the optimum voltage control unit, and the operating point where the overcurrent is minimized in consideration of the fault current supply characteristics required by the system and a fault current supply characteristic control unit 13 that performs optimal voltage control to find an operating point that satisfies the requirements of the power system as much as possible in a range that does not exceed the limit of the current that the converter can supply.

And optimal voltage control lowers the voltage when the current exceeds, and solves the overcurrent by raising the voltage in a specific section where the reverse current is high. Therefore, it is not lowered below the limit voltage.

And the impedance constraint evaluation control unit 300 includes a parameter collection unit 20 that collects parameters for impedance evaluation using information of the power system, and an input setting unit 21 that sets the reference MVA connection impedance input and GFM voltage and , a fault current calculation unit 22 that calculates a fault current to obtain the minimum impedance that does not exceed the limit of the current that the power converter can supply, and the current GFM by determining whether the power converter current exceeds the set limit value The recovery reverse current calculation and determination unit 23 that stores the voltage, calculates the recovery reverse current, and determines whether the recovery reverse current exceeds the set limit value, and operates with the input impedance if the recovery reverse current does not exceed the set limit value It includes an impedance evaluation unit 24 that determines that it is possible.

The fault current calculation unit 22 calculates three-phase short-circuit fault current, line-to-line short-circuit fault current, line-to-line short-circuit fault current, and 1-line ground fault fault current.

3 is a flowchart illustrating a method for optimal voltage tracking control and impedance constraint evaluation of a grid-forming power conversion control device according to the present invention.

First, when a fault occurs in the system power supply, fault current calculation and reverse current calculation are performed to control fault current supply characteristics, and parameters are collected using information from the power system (S301).

The input setting unit 21 sets the reference MVA connection impedance input and the GFM (Grid Forming Source) voltage. (S302)

Next, in the fault current calculation unit 22, three-phase short-circuit fault current, line-to-line short-circuit fault current, line-to-line short-circuit fault current, and 1-wire ground fault current are calculated, and the recovery reverse current is calculated. (S304) )

And it is determined whether the calculated power converter current exceeds the set limit value (S305), and if the current does not exceed the set limit value, it is determined that the operation is possible with the input impedance (S310), otherwise the GFM voltage is compared with the minimum voltage. (S306)

If the GFM voltage is greater than or equal to the minimum voltage, the GFM voltage is reduced by 1 step and the fault current is calculated again (S307), otherwise it is judged that operation is impossible with the input impedance (S308), and the reference MVA connection impedance is increased by 1 step and the fault current is reduced Calculate again. (S309)

If the current does not exceed the set limit value, it is determined that operation with the input impedance is possible (S310), and it is determined that the minimum impedance that does not exceed the limit of the current that the power converter can supply is obtained, and this value is set as the required minimum impedance value. interface. (S311)

Here, in the step of setting the first connection impedance input and GFM (Grid Forming Source) voltage in the input setting unit 21, 100MVA is input as the reference impedance, and the GFM voltage is set to 1.0 [pu] (S303).

And the process of reducing the GFM voltage by 1 step and calculating the fault current again and the process of inputting the reference MVA connection impedance again after reducing the connection impedance by 1 step to obtain the minimum impedance that does not exceed the limit of the current that the power converter can supply Repeat until

With reference to FIG. 1, the model configuration for the optimal voltage tracking control and impedance constraint evaluation of the grid-forming power conversion control device according to the present invention will be described as follows.

The model and calculation method for the optimal voltage tracking control and impedance constraint evaluation to be described below have been described as an example applied to the present invention and are not limited thereto.

Failures in GFM environment show a completely different distribution from failures in existing AC systems due to Virtual Impedance and consequent terminal voltage drop.

Up to now, based on the principle that only the AC power source is the only fault current source, the Single Source Thevenin Equivalent Circuit viewed from the point of failure was able to simplify and express all circuits with one impedance.

However, in the GFM environment, since the source voltages of GFM and AC Machine are different, they cannot be combined into one source, and the phenomenon must be interpreted with the superposition of each source.

Calculating the distribution of fault current and voltage for each fault type in the GFM and AC machine environments of 1 unit is as follows.

For fault points, consider 3-phase short circuit on the high voltage side of the converter transformer, short circuit between lines, and ground fault on one line. The reason for supposing the HV side failure is that this point is a general PCC point.

The formulas for the power converter voltage/current and PCC Ry voltage/current for each case are derived from the six failure models as follows.

The parameters used in the calculation are as follows.

GFM(Grid Forming Source), SM(Synchronous AC Machine Source), F(Total Fault Current), G(Grounding Voltage), 0,1,2(Zero Sequence, Positive Sequence, Negative Sequence), th(Thevenin equivalent impedance) , E (Source Voltage), the virtual ideal power inside GFM is

indicated as

voltage magnitude

and the phase angle

is -30° when connected with Yd1-connected 3-phase transformer, and may vary depending on the impedance angle of VI.

the important point is

is a virtual power source and not an actual voltage. The power that the GFM converter actually generates is a voltage less than or equal to this.

The equalized external AC system is

indicated as The normal and reverse phase impedances are the same, but the image impedance is set as a variable depending on the level of grounding.

put it as Since LV and HV of the GFM source are different points of failure, the transformer impedance is not expressed as equivalent.

connect through

Transformer is connected wye on HV side and delta connection on LV side. When voltage and current vary depending on the measurement point, they are displayed separately as 'HV' and 'LV'.

Impedance of GFM consists of transformer and interfacing impedance. In order to keep the control characteristics of the converter constant regardless of the capacity, the impedance compared to the self-capacitance (DC MVA) is taken constant.

[Equation 1]

here,

is the transformer impedance,

is the converter series impedance (including filter impedance).

Rotating equipment has different impedance for each sequence.

Therefore, the AC source impedance is also classified. On the other hand, the converter

, of the transformer

is a passive element, R, L, and C, so it can be expressed as the same value without distinction between 0, 1 and 2.

The failure model of 3-phase short circuit failure is as follows.

4 is a block diagram of a three-phase short circuit failure model.

In the Grid Forming environment, a 3-phase short circuit fault circuit as shown in Fig. 4 is written as a parallel circuit for each GFM and AC source, and the problem is analyzed by superposition.

HV Side Fault is calculated as follows.

[Equation 2]

here,

is Thevenin equivalent impedance, normal (1), zero (0), negative (2) division, converter (GFM) side and external system (SM) side,

is the voltage, converter (GFM) side and external system (SM) side,

is the fault current supplied by the converter, and classifies a, b, and c phases.

GFM terminal voltage is controlled as follows.

[Equation 3]

5 is a graph showing the current change of the converter according to different connection impedances in a 3-phase fault.

The conventional failure model of 1-wire ground fault is as follows.

6 is a configuration diagram of a failure model of a one-line ground fault failure.

In the Grid Forming environment, one-line ground fault circuit as shown in the figure above is written as a parallel circuit for each GFM and AC (SM) source, and the problem is analyzed by superposition.

Each Seq Circuit is a series structure, and the + side of the Pos Circuit is connected to the - side of the Neg Circuit. Neg Circuit + side is connected to Zero Circuit - side. And the + side of the Zero Circuit is connected to the - side of the Pos Circuit. If there is an impedance between the fault point and the ground, connect it to the + side of each Seq circuit.

HV Side Fault is calculated as follows.

7 is a configuration diagram of a calculation model of HV Side Fault in 1-line ground fault failure.

[Equation 4]

here,

is the external system impedance, and is divided into normal (1) and zero (0).

and

To calculate , define toward zero seq route current notation 'out' and toward opponent route current notation 'in' as sub-routes. Since the two routes are distinguished only within the pos sequence, the seq notation '1' is omitted.

The current generated from the Grid Forming Converter is calculated as follows.

[Equation 5]

here,

is divided into sub-root impedance, inward (in), and outward (out) directions,

is the fault impedance.

In case of HV 1-line ground fault, the zero-phase current does not flow on the LV side because the zero-phase circuit is opened.

That is, the zero-phase current generated in the converter is defined as in Equation (6).

[Equation 6]

Among reverse-phase and zero-phase currents, the current in the direction of the external AC system is calculated as follows.

[Equation 7]

here,

is divided into zero (0) and reversed (2) current, which is generated in the converter and flows to the external system side.

The current generated from the AC side SM Source is calculated as follows.

[Equation 8]

here,

represents the current generated in the external system, and is divided into internal (in) and external (out) directions, normal component (1), and reverse phase component (2).

is the root impedance applied to find outward and inward currents.

That is, it is defined as in Equation 9.

[Equation 9]

[Equation 10]

here,

is the inward (in) current generated from the external system, and is divided into zero (0) and reverse phase (2).

As described above, from the current calculated by the superposition for each source, the Ry current on the HV side in case of a ground fault in HV 1 line is as follows.

[Equation 11]

Here, HV is the current that finally flows to the high voltage side.

In case of HV 1-line ground fault, the external system current on the HV side is as follows.

[Equation 12]

here,

is the current that finally flows into the external system.

In case of HV 1-line ground fault, the HV side voltage is as follows.

[Equation 13]

here,

is the voltage finally measured on the high voltage side.

In case of HV 1-line ground fault, the converter current on the LV side is as follows.

[Equation 14]

here,

is the final low voltage side, that is, the current flowing through the converter stage.

In case of HV 1-line ground fault, the LV side voltage is as follows.

[Equation 15]

here,

is the final low voltage side, that is, the voltage formed at the converter stage.

Abnormal HV side 1-wire ground fault In each equation, abbreviated impedance

Is as follows.

[Equation 16]

The important point is that, unlike traditional failure analysis, the healthy current cannot be zero in the case of a one-wire ground fault derived by superposition.

this is

class

This is because each sequence current component has a different value as the size changes. Accordingly, not only the a-phase current but also the b and c-phase currents are the voltage of the GFM.

It is determined as a function according to

As , the b and c phase currents increase.

Finally

while

In this case, there is no problem with current limiting successful, that is, FRT in case of failure.

The reason for taking the absolute value is that each phase current can be either in the forward direction or in the reverse direction. If this formula is not satisfied, limiting fails, a distorted current waveform is output, and the FRT fails (unstable) because resynchronization is not performed even after the fault is removed. In this case, it is necessary to protect the IGBT by blocking the converter. In this case, the equipment can be protected, but the grid forming property is lost, so the stability in terms of system operation is deteriorated, and the correct operation of the protective relay cannot be induced because the fault current cannot be supplied. none.

In order to confirm the above analysis, the Fault Current Graph in the case of a ground fault in the HV side PCC point of the GFM source connected to the AC system is as follows. )

8 is a Fault Current graph in case of a ground fault in the HV side PCC point 1-line of the GFM Source linked to the AC system.

Unlike Balanced Fault, it can be seen that there is a DC MVA area where current limiting is not possible even if the GFM Voltage is lowered. This means that if DC MVA increases over a certain level, GFM operation cannot be maintained in a single line to ground fault.

The failure model of line-to-line short circuit failure is as follows.

9 is a configuration diagram of a failure model of a line-to-line short circuit failure.

In this case, it is possible to create different types of failures depending on the location of the failure impedance. Only the case where there is no impedance between the fault phases and there is a ground fault impedance between the fault point and earth is considered.

In the Grid Forming environment, the line-to-line short circuit fault circuit as shown in FIG. 9 is written as a parallel circuit for each GFM and AC source, and the problem is analyzed by superposition.

Each Seq Circuit is a parallel structure, connecting the + sides and connecting the - sides. The fault impedance connects 3Zf to the + side of Zero Seq.

HV Side Fault is calculated as follows.

10 is a configuration diagram of a calculation model of HV Side Fault of line-to-line short circuit failure.

[Equation 17]

here,

A Neg.Seq direction

with Zero Seq direction

Since it is divided into

is defined as

[Equation 18]

In the event of a short circuit between HV lines, the LV side has an open video circuit and no zero current flows.

That is, it is defined as in Equation 19.

[Equation 19]

[Equation 20]

The current generated from the AC side SM Source is calculated as follows.

[Equation 21]

That is, it is defined as in Equation 22.

[Equation 22]

[Equation 23]

As described above, from the current calculated by the superposition for each source, the Ry current on the HV side is defined as follows in case of a short circuit between HV lines.

[Equation 24]

In case of a short circuit between HV lines, the external system current on the HV side is as follows.

[Equation 25]

In case of short circuit between HV lines, the voltage on the HV side is as follows.

[Equation 26]

In case of short circuit between HV lines, the converter current on the LV side is as follows.

[Equation 27]

In case the HV failure mode is a short circuit between lines without including a ground fault, the image component is removed and it is simplified as follows.

[Equation 28]

In case of a short circuit between HV lines, the voltage on the LV side is as follows.

[Equation 29]

Abnormal HV side short circuit failure, abbreviated impedance in each formula

is as follows

[Equation 30]

Similarly, the healthy current does not become 0 in case of a short circuit between lines derived from Superposition.

Finally

while

In case of failure, current limiting is successful, that is, FRT is successful.

The reason for taking the absolute value is that each phase current can be either in the forward direction or in the reverse direction.

As in the case of the previous 1-wire ground fault, if this formula is not satisfied, limiting fails and the current waveform is distorted.

In order to confirm the above analysis, the Fault Current Graph in case of a short circuit failure between the HV side PCC point of the GFM Source connected to the AC system is as follows. In the case of a line-to-line short circuit that does not include a ground fault,

apply

11 and 12 are Fault Current graphs in case of a short circuit failure between the HV side PCC points of the GFM Source connected to the AC system.

Unlike Balanced Fault, it can be seen that there is a connection impedance region where current limiting is not possible even if the GFM Voltage is lowered. This shows that GFM operation cannot be maintained in a single line to ground fault if the connection impedance is not secured over a certain level.

In addition, the limit value of DC MVA, which fails to limit in line-to-line short-circuit failure, is lower than that of line-to-line short circuit.

That is, as a result of the analysis so far, in GFM, the short-circuit failure between lines is the most severe.

In addition, the optimum voltage, the limit voltage, and the minimum voltage to be used as the setting values of the voltage control unit based on the above-mentioned 1-line ground fault, line-to-line short circuit, and line short-circuit fault calculation are as follows.

The optimum voltage is the voltage at which the power converter is expected to produce the best FRT performance. At the optimum voltage, the power converter supplies the maximum fault current within the range that does not exceed the current limit.

The optimum voltage is obtained from the condition that the phase current of the line-to-line short circuit becomes equal to the limit value. The formula is:

[Equation 31]

If abbreviated as , it is the same as in Equations 32 and 33.

[Equation 32]

Here, s is an abbreviated display of converter impedance, g is an abbreviated display of Thevenin equivalent impedance viewed from the converter, e is an abbreviated display of the Thevenin equivalent impedance viewed from an external system, b is an abbreviated display of external system impedance, and L is an abbreviated display of current limit.

[Equation 33]

However, if the impedance constraint evaluation is not satisfied, the current limit does not succeed, so the optimal voltage does not exist fundamentally. Therefore, impedance constraint satisfaction takes precedence over control unit operation.

The success of current limit means that it is a sinusoidal three-phase AC waveform of the rated frequency, and the result is supplied so that the maximum wave height value among three phases does not exceed the limit.

The limit voltage is the voltage at the point where the magnitude of the fault current and the reverse current are equal. At the limit voltage, the power converter can no longer obtain the gain of lowering the voltage further. If the voltage is lowered than the limit voltage, the reverse current (RC) exceeds the fault current, so there is no real benefit of lowering the maximum current. Therefore, the voltage control unit performs control not to lower the voltage than the limit voltage.

The limit voltage is obtained from the condition that the two phase currents of the line-to-line short circuit fault are equal. The formula is:

[Equation 34]

If abbreviated as , it is the same as in Equations 35 and 36.

t is an abbreviation of the parallel composite impedance of the Thevenin equivalent impedance and the external system impedance viewed from the converter.

[Equation 35]

[Equation 36]

Since the limit voltage must be a scalar value, θk is set so that the phase angle of the right side becomes 0.

The limit voltage is always present regardless of whether the impedance constraint evaluation is satisfied. Therefore, if the limit current at the limit voltage does not exceed the current limit value, the impedance constraint is considered to be satisfied.

The minimum voltage is the lowest voltage the power converter can operate without leaving the range of a successful FRT. The minimum voltage means both the voltage at the point where the reverse current exceeds the current limit when the voltage is lowered in a 1-line ground fault, and the voltage at the point where the recovery reverse current exceeds the limit regardless of the fault type. If the impedance limit is exceeded during operation due to an unexpected change in the system impedance, there is a risk that the voltage control unit will command the minimum voltage or less. As a safety measure, if the voltage command value is lower than the minimum voltage, anti-wind-up clamping is performed. Anti-wind-up clamping is a control that resets the voltage control unit integrator. That is, it is changed from PI control to P control, and the effect that errors do not accumulate can be obtained. Accordingly, distortion of the current waveform cannot be avoided, but resynchronization can be achieved after the fault is removed. Therefore, anti-wind-up is implemented as a safety measure in case of incomplete FRT.

Two conditions are considered to calculate the minimum voltage. First, it is obtained as follows from the RRC condition.

[Equation 37]

* Since the minimum voltage must be a scalar value, θk is set so that the phase angle of the right side becomes 0.

Second, it is obtained as follows from the reverse current (RC) condition of 1-line ground fault.

[Equation 38]

If abbreviated as , it is the same as in Equations 39 and 40.

t0 and t2 are the zero-phase and inverse parts of the abbreviated impedance t, m is the abbreviated display of the sum of the zero-phase and inverse parts of the Thevenin equivalent impedance seen from the converter, and the triangle is the abbreviated display of the loop impedance.

[Equation 39]

[Equation 40]

Therefore, the sizes of EGFM and MINIMUM are obtained in the same way in both conditions.

Like the optimal voltage, if the impedance constraint evaluation is not satisfied, the optimal voltage does not exist fundamentally. Therefore, impedance constraint satisfaction takes precedence over control unit operation.

Hereinafter, embodiments of an output frequency control method of a grid-forming power conversion control apparatus will be described with reference to the accompanying drawings.

13 is a flowchart illustrating an outline of an output frequency control method of a grid-forming power conversion control apparatus according to the present invention. Referring to FIG. 13 , the output frequency control method of the grid-forming power conversion control apparatus according to the present embodiment includes: calculating a frequency corresponding to the DC voltage provided to the grid-forming power conversion unit 100 ( S1310 ) and the grid The forming power conversion unit 100 includes a step (S1320) of outputting a frequency corresponding to the provided DC voltage, but outputting the calculated frequency does not control the DC current provided to the grid forming power conversion unit 100 is performed without

14 is a block diagram schematically showing an output frequency control unit of the grid-forming power conversion control apparatus according to the present invention. The output frequency control unit 400 of the grid-forming power conversion control apparatus according to the present embodiment includes an input unit 410 , an output unit 420 , a processor 450 , a memory 440 , and a database 430 . The output frequency control unit 400 of FIG. 14 is according to an embodiment, and not all blocks shown in FIG. 14 are essential components, and in another embodiment, some blocks included in the output frequency control unit 400 are added or changed. Or it can be deleted. On the other hand, the output frequency control unit 400 may be implemented as a computing device for controlling the grid forming power conversion unit 100, each component included in the output frequency control unit 400 is implemented as a separate software device, It may be implemented as a separate hardware device combined with software.

The output frequency control unit 400 calculates a frequency corresponding to the DC voltage provided to the grid forming power conversion unit 100, and controls the grid forming power conversion unit 100 to output a frequency corresponding to the provided DC voltage, The step of outputting the calculated frequency is performed without controlling the DC current provided to the grid-forming power conversion unit 100 .

The input unit 410 means a means for inputting or obtaining a signal or data for controlling the grid-forming power conversion unit 100 . The input unit 410 may input various types of signals or data in association with the processor 450 , or may directly acquire data in association with an external device and transmit the data to the processor 450 . The input unit 410 may be a device or a server for inputting or receiving an output voltage, an output frequency, a droop rate, set point information, etc. of the grid forming power conversion unit 100, but is not necessarily limited thereto.

The output unit 420 may display an output voltage, an output frequency, a droop rate, setpoint information, and the like of the grid forming power conversion unit 100 in conjunction with the processor 450 . The output unit 420 preferably displays various information through a display (not shown), a speaker, etc. provided in the output frequency control unit 400 in order to output predetermined information, but is not necessarily limited thereto.

The processor 450 performs a function of executing at least one instruction or program included in the memory 440 .

The processor 450 according to this embodiment calculates a frequency corresponding to the DC voltage provided to the grid forming power converter 100 based on the data obtained from the input unit 410 or the database 430, and the grid forming power An operation of controlling the converter 100 to output a frequency corresponding to the provided DC voltage is performed.

The memory 440 includes at least one instruction or program executable by the processor 450 . The memory 440 may include instructions or programs for performing processing. The memory 440 may store a program for calculating a frequency and the calculated frequency values.

The database 430 refers to a general data structure implemented in the storage space (hard disk or memory) of a computer system using a database management program (DBMS), and performs data search (extraction), deletion, editing, addition, etc. Relational database management system (RDBMS) such as Oracle, Infomix, Sybase, DB2, Gemston, Orion ), an object-oriented database management system (OODBMS) such as O2, and an XML Native Database such as Excelon, Tamino, Sekaiju, etc. It can be implemented according to the requirements, and has appropriate fields or elements to achieve its function.

The database 430 according to the present embodiment may store an algorithm for calculating the frequency of the grid forming power converter 100, and the like, and may provide the stored data. Meanwhile, although the database 140 is described as being implemented in the output frequency control unit 400, it is not necessarily limited thereto, and may be implemented as a separate data storage device.

15 is a droop plot illustrating that the power generation system and the grid-forming power converter according to an embodiment of the present invention share and provide power to a load. 15 illustrates a case in which the power generation system and the grid-forming power conversion unit supply power to the same load. In FIG. 15, AC droop' is a plot showing the output frequency versus the power provided by the power generation system to the load, and the GFM droop shows the output frequency versus the power provided by the grid forming power converter 100 to the load. It is a plot.

In the embodiment illustrated in Fig. 15, both the AC droop' diagram and the GFM droop diagram have a characteristic that the output frequency decreases as the power provided to the load increases. Here, if the AC droop' diagram is inverted left and right, an AC droop diagram can be formed, and a set point (Psp, set point), which is an intersection point with the GFM droop diagram, is formed.

The power generation system and the grid-forming power converter 100 have different ratios of sharing the power provided to the load based on the set point Psp. In the illustrated embodiment, it is assumed that the PGFM is 50 MW, the PAC is 10 MW, and the grid forming power conversion unit 100 and the power generation system provide a total of 60 MW to the load. That is, the 50 MW grid-forming power conversion unit 100 provides power to the load, and the remaining 10 MW provides power to the load by the external AC power generation system.

The frequency output by the grid forming power converter 100 in the no-load state is the no-load frequency (fNLGFM), and as the power provided to the load increases, it decreases to the maximum load frequency (fFLGFM). At the set point (Psp), the grid forming power converter 100 and the power generation system output a common frequency, and the frequency at that time is referred to as the set point frequency (fsp).

When the grid forming power conversion unit 100 is operated together with the conventional power generation system to provide power to the load, the grid forming power conversion unit 100 controls the power provided to the load by the grid forming power conversion unit 100 ) can be performed by controlling the GFM Droop.

As an example, the droop diagram (GFM Droop) of the grid-forming power converter 100 is a droop rate corresponding to the slope of the droop diagram (GFM droop) of the grid-forming power converter 100, a target set point , It can be made by adjusting the no-load frequency (fNLGFM) output by the grid-forming power converter 100 in a no-load state and the maximum load frequency (fFLGFM) output by the grid-forming power converter 100 in a maximum load state. Here, droop refers to the control principle for proportionally sharing the power load jointly supplied by generators running in parallel. This is called the droop rate. That is, the droop rate is defined as in Equation 41 below.

[Equation 41]

FIG. 16 is a diagram illustrating an overview of an output frequency control method of a grid forming power converter according to the prior art together with a droop diagram illustrated in FIG. 15 . In FIG. 16, the droop diagram illustrated in FIG. 15 is shown in the first quadrant, and in the second quadrant, a function for the DC voltage (VDC) provided with the output frequency of the grid forming power converter 100 is shown.

Referring to Figure 16, when the droop control of the grid forming power conversion unit 100, the grid forming power conversion unit 100 in the droop diagram (GFM Droop) changes the output frequency from the maximum load frequency fFLGFM to the no-load frequency fNLGFM It can be outputted and interlocked with the power generation system to provide power to the load.

When the grid-forming power conversion unit 100 is matched and controlled, the output frequency is controlled according to the provided DC voltage. That is, the output frequency of the grid-forming power converter 100 has the form of a linear function passing through the origin. Therefore, in order for the output frequency of the grid forming power conversion unit 100 to change from the maximum load frequency fFLGFM to the no-load output frequency fNLGFM, the voltage provided to the grid forming power conversion unit 100 must change within the range of ΔVDC. do.

According to the prior art, the matching control of the grid forming power converter 100 is provided as illustrated in FIG. 17 in order to control the output frequency to correspond to the voltage (VDC), the rated DC voltage (VDC*) and the rating The direct current (idc) formed from the power (P*) and the power loss (Ploss) is controlled and must be provided to the grid forming power converter 100 . In this way, the DC current must be controlled so that the voltage provided to the grid forming power converter 100 exists within the range of ΔVDC, and accordingly, the output frequency changes from fFLGFM, which is the output frequency at maximum load, to fNLGFM, which is the output frequency at no load.

The kdc included in the control loop illustrated in FIG. 17 may be expressed as in Equation 42 below.

[Equation 42]

(η: the slope of the output frequency with respect to the DC voltage of the conventional grid-forming power conversion unit 100, mp: the droop rate, vdc*: the rated voltage)

As described in the above equation, the denominator of kdc includes a droop rate (mp). That is, for the matching control, DC current control is required to implement the droop characteristic separately from the control unit that generates the AC frequency, and the droop rate, which is the power sharing ratio of the grid forming power conversion unit 100, is reflected during the DC current control. There is a difficulty in that it is necessary to implement a complex and sophisticated controller.

18 is a diagram illustrating an outline of an output frequency control method of a grid-forming power conversion control apparatus according to the present invention together with the droop diagram illustrated in FIG. 15 . In FIG. 18, the first quadrant is a droop diagram illustrated in FIG. 15, and the diagram shown in the second quadrant shows the output frequency of the grid-forming power conversion control device according to this embodiment as a function of the DC voltage (VDC) provided. .

According to this embodiment, the output frequency shown by the thick solid line of the grid-forming power converter 100 may change according to a linear function with respect to the input DC voltage (VDC). As an example, the first-order function may be a first-order function having a slope η′ passing through the output frequency fFLGFM at maximum load. The linear function can be expressed as Equation 43 below.

[Equation 43]

As another example, the linear function may be a linear function having a slope η′ over the no-load output frequency fNLGFM. The first-order function may be a first-order function passing through the no-load output frequency (fNLGFM) and the maximum-load output frequency (fFLGFM).

The slope (η′) of the linear function can be obtained from Equation 44 below.

[Equation 44]

(f*: rated frequency, VDC*: rated DC voltage, droop%: droop rate)

The rated frequency is set to 60Hz in Korea and 50Hz or 60Hz in the case of overseas, and the rated DC voltage is a value determined according to the manufacturing specifications of the equipment. The frequency control method of the grid-forming power conversion control apparatus according to this embodiment is a grid-forming power conversion unit when the DC voltage (VDC) provided to the grid-forming power conversion unit 100 is 0, unlike the prior art illustrated in FIG. 17 . (100) outputs the output frequency (fFLGFM) at the maximum load, and as the DC voltage (VDC) provided to the grid forming power converter 100 increases, the output frequency (f) increases along the slope η'. In addition, when the maximum voltage of the DC voltage VDC provided to the grid forming power conversion unit 100 is provided, the grid forming power conversion control device outputs an output frequency fNLGFM at no load.

In addition, the method for controlling the output frequency of the grid-forming power conversion control apparatus according to the prior art controls the DC current provided to the grid-forming power conversion unit 100 to obtain an output frequency within a desired range, thereby controlling the grid-forming power conversion unit 100 ) was maintained within the desired range. According to the prior art, a controller for controlling a direct current has been required, but such a controller is complicated and uneconomical in terms of cost.

However, according to the present embodiment, the output frequency can be formed from the DC voltage provided to the grid forming frequency without controlling the DC current. Accordingly, a controller for controlling a complicated current is unnecessary, and thus an advantage of economical efficiency is provided.

Hereinafter, a second embodiment of the output frequency control method of the grid-forming power conversion control device will be described.

19 is a diagram illustrating an output frequency for a DC voltage (VDC) provided to a grid-forming power converter according to a second embodiment of the present invention. Referring to FIG. 19 , in the output frequency control method according to the present embodiment, the grid-forming power converter 100 changes exponentially when the input voltage VDC changes.

When the grid-forming power conversion unit 100 is prepared as a generator, the rotational speed error (g) of the generator rotor is expressed in terms of the DC voltage (VDC) provided to the grid-forming generator, and Equation ① and Assuming that it can be expressed in the form of an exponential function, the acceleration (g') obtained by differentiating it can be expressed by Equation (2).

[Equation 45]

...①

...②

(a, b, c, d, k: design parameters that are undetermined constants)

From the result expressed by Equation 45, the output frequency (f) of the grid forming frequency is expressed as the rotation speed (g) and the acceleration (g') as shown in Equation 46 below.

[Equation 46]

The coefficients a, b, c and d are design parameters. As exemplified by Equation 47 below, a coefficient a is related to a droop characteristic, and b is related to an inertia characteristic. c corresponds to -Vd0 in which a minus sign is added to the DC voltage provided to the grid-forming power converter 100 when the grid-forming power converter 100 outputs the maximum load frequency.

As boundary conditions, when VDC = Vd0, the output frequency f = fNLGFM, and when VDC = 0, the output frequency f = fFLGFM is used to obtain the indeterminate constants d and k. a, b, c, d, and k are the same as in the following equation.

[Equation 47]

(droop%: droop rate, H: inertia, Vd0: DC voltage at full load frequency output, fNLGFM: no-load frequency)

From this, the output frequency f of the grid forming frequency is expressed as Equation 48 below.

[Equation 48]

The DC voltage provided to the grid forming power converter 100 obtained by Equation 48 - the relationship between the output frequency is in the form of an exponential function convex downward as shown in FIG. 19 . Even in the second embodiment, it is possible to form an output frequency with the voltage provided to the grid-forming power converter 100 without controlling the current formed in the grid-forming power converter 100 . Accordingly, there is provided an advantage that there is no need to introduce a separate and complicated current controller.

Hereinafter, a third embodiment of the output frequency control method of the grid-forming power conversion control apparatus will be described with reference to FIG. 20 . For concise and clear description, the description of elements that are the same as or similar to those of the above-described embodiments may be omitted. Referring to FIG. 20 , in the output frequency control method according to the present embodiment, the grid-forming power converter 100 changes exponentially when the input voltage VDC changes.

As in the embodiment described above, when the second grid-forming power conversion unit 100 is replaced with a generator, the rotational speed error (g) of the generator rotor is expressed in terms of the DC voltage (VDC) provided to the grid-forming generator. Assuming that it can be expressed in the form of an exponential function as in Equation (1) of Equation 49 below, an acceleration (g') obtained by differentiating it can be expressed as Equation (2). In Equation 49, a minus sign is added to the coefficients a and b in Equation 6 described above.

[Equation 49]

...①

...②

From the result expressed by Equation 48, the output frequency (f) of the grid forming frequency is expressed as the rotation speed (g) and the acceleration (g') as shown in Equation 50 below.

[Equation 50]

The coefficients a, b, c and d are design parameters. As exemplified by Equation 51 below, a coefficient a is related to a droop characteristic, and b is related to an inertia characteristic. c corresponds to -Vd0 by adding a minus sign to the DC voltage provided to the grid-forming power conversion unit 100 when the grid-forming power conversion unit 100 outputs the maximum load frequency.

[Equation 51]

From this, the output frequency f of the grid forming frequency is expressed as in Equation 52 below.

[Equation 52]

The DC voltage provided to the grid forming power converter 100 obtained by Equation 52 - the relationship between the output frequency is in the form of an exponential function convex upward as shown in FIG. 20 . Even in the third embodiment, it is possible to form an output frequency with the voltage provided to the grid-forming power converter 100 without controlling the current formed in the grid-forming power converter 100 . Accordingly, there is provided an advantage that there is no need to introduce a separate and complicated current controller.

Hereinafter, a fourth embodiment of the output frequency control method of the grid-forming power conversion control apparatus according to the present invention will be described with reference to FIG. 21 . For concise and clear description, the description of elements that are the same as or similar to those of the above-described embodiments may be omitted.

From Equations 49 and 50, unlike the previously set boundary condition, when VDC = 0, the output frequency f = 0, and when VDC = Vd0, the output frequency f = fNLGFM is set. Accordingly, the DC voltage versus frequency curve of this embodiment passes through the origin and the output frequency f = fNLGFM when VDC = Vd0. Using this boundary condition to obtain undetermined constants d and k, it is expressed in Equation 53 below.

[Equation 53]

From this, the output frequency f of the grid forming frequency is expressed as in Equation 54 below.

[Equation 54]

The DC voltage provided to the grid-forming power converter 100 obtained by Equation 54-output frequency relationship is in the form of an exponential function convex upwards passing through the origin as shown in FIG. 21 . Even in the fourth embodiment, it is possible to form an output frequency with the voltage provided to the grid-forming power converter 100 without controlling the current formed in the grid-forming power converter 100 . Accordingly, there is provided an advantage that there is no need to introduce a separate and complicated current controller.

Hereinafter, a fifth embodiment of the output frequency control method of the grid-forming power conversion control apparatus according to the present invention will be described with reference to FIG. 22 . For concise and clear description, the description of elements that are the same as or similar to those of the above-described embodiments may be omitted. 22 (a) is a diagram illustrating a DC voltage versus frequency relationship according to the prior art and a voltage versus frequency according to the present embodiment, and FIG. 22 (b) is an enlarged view of the operating point in FIG. 22 (a). It is a drawing.

In Fig. 22(a), reference numeral 221 denotes a frequency change according to the prior art, 222 denotes a case where the steady-state frequency fss is the maximum load frequency, and 223 denotes the steady-state frequency fss. ) represents the case of no-load frequency, and (224) represents the case where the steady-state frequency (fss) is the rated frequency. For example, the rated frequency in Korea is 60 Hz, and the rated frequency in foreign countries is 50 Hz or 60 Hz. Also, in Fig. 22(b), reference numeral 225 denotes a full load state output frequency, 226 denotes a steady state output frequency in a full load state, 227 denotes an output frequency in a no-load state, and 228 denotes a no-load state. represents the steady-state output frequency. Reference numeral 229 denotes the output frequency according to the prior art, and 230 denotes the output frequency at the set point.

In the second, third, and fourth embodiments, the frequency conversion equation was derived in a bottom-up manner by applying the boundary condition after reflecting the intended droop ratio and inertia property to each parameter of the equation in advance. In this embodiment, a top-down method of deriving an appropriate value of each parameter after first applying a boundary condition is adopted. As in the embodiment described above, when the second grid-forming power conversion unit 100 is replaced with a generator, the rotational speed of the generator rotor is displayed in terms of DC voltage (VDC) provided from the grid-forming power conversion unit 100 Then, it is defined as being able to be expressed in the form of an exponential function gVDC as in Equation 55.

[Equation 55]

(a, d: positive numbers, undetermined coefficients that change with time, b, c: positive numbers and coefficients that do not change with time)

Each parameter a, b, c, and d in Equation 55 is determined as described below. c is set to c=Vd0 to determine the condition when the DC voltage is rated. (Vd0: rated DC voltage)

For any DC voltage VDC, the point is considered to have reached the steady state. If this state is compared to a generator, the same force acts from the rotor speed in the direction opposite to the rotational direction from the speed of the rotor according to the laws of physics (Generator Swing Equation, see Equation 64), so that the acceleration of the rotor becomes 0 and equilibrium is achieved. Only the frequencies f of the steady state appearing later are considered.

Accordingly, without considering the differentiation of g(VDC) in the previous embodiment, it is understood that the balanced rotational speed appears as a DC voltage, and is converted into a frequency through an exponential function. Therefore, if the DC voltage is the same as the rated voltage, it is regarded as a no-load state and the no-load frequency fFLGFM is output.

Similarly, from any voltage VDC between no load and full load, a droop control frequency fss corresponding to the load is output. fss is set to fs=60 [Hz], which is the rated frequency for independent operation, and applies the frequency according to Equation 56 below in case of parallel operation with other generators or converters.

[Equation 56]

When the frequency converted for each voltage level is expressed as an equation, the relationship between parameters a and d is arranged as shown in Equation 57. In Equation 57 ①, ②, ③ and ④ respectively, the DC voltage is rated, the voltage is dropped to an arbitrary value by the load supply, the voltage is the lowest by the full load supply, and the voltage is 0 corresponding to the state of being From this, it can be seen that the parameters of the fifth embodiment are not determined as constants, unlike the previous embodiment, but are determined as a function of VDC that changes according to the voltage level in order to match the measured voltage level and the target droop fss.

For the undetermined coefficient b of the exponential function, condition 1 of g(VDC = Vdo) =FNLGFM, condition 2 of g(VDC = Vop) =Fss, condition 3 of g(VDC = VFL) =FFLGFM, g(VDC = 0) ) is calculated using condition 4 of =0. Results obtained from condition 1, condition 2, condition 3, and condition 4 are the same as

Equations

①, ②, ③ and ④ of Equation 57 below, respectively.

[Equation 57]

...①

...②

...③

...④

For a given droop, it is selected to pass through all three points, g(0)=0, g(Vdo)=fNLGFM, and g(VFL)=fFLGFM based on the no-load operating point. Under this condition,

expressions

①, ③, and ④ in Equation 57 all have the same value of a. The value of b, which is a positive number satisfying this, may be expressed by Equation 58.

[Equation 58]

On the other hand, for b, the DC voltage is valid only at VFL, and in order to derive the desired steady-state frequency fss, the value of b must be determined at all voltage points VDC as in Equation 59.

[Equation 59]

For b, even if the voltage changes, the value calculated based on VFL is applied as it is, and the change is applied to parameter a instead. a varies depending on the voltage level, and the smaller the load, the closer to the curve passing through fNL and the origin, and the larger the load, the closer to the curve passing through fNL and fss. Therefore, according to the load factor i=(Pmeasured/Pmax), the weighted average a is actually applied by multiplying the right side of Equation 59 by the load factor i and the left side by (1-i). Accordingly, the finally determined a and d follow Equation (60).

[Equation 60]

...(One)

...(2)

The final frequency through the above setting is calculated as in Equation 61.

[Equation 61]

In Equation 60, a may be expressed by Equation (1) of Equation 60. b may be defined as in Equation 59. However, as described above, a value selected based on the full load (refer to Equation 58) may be used.

Referring to Equation 61, (wc/(s+wc)) denotes a low-frequency bandpass filter. That is, the exponential term operates instantaneously and bar a and bar d operate with a delay, so that the inertia effect is expressed from the DC voltage in the transient section, and finally converges to the desired steady-state frequency fss.

The relationship between the input DC voltage (VDC) and the output frequency according to this embodiment may be similar to the embodiment illustrated in FIG. 61, but in this embodiment, the final value of the output frequency is determined according to the frequency fss intended by the droop. It is different in that it draws a variety of different curves for the VDC.

Even in the fifth embodiment, it is possible to form an output frequency with the voltage provided to the grid-forming power converter 100 without controlling the current formed in the grid-forming power converter 100 . Accordingly, there is provided an advantage that there is no need to introduce a separate and complicated current controller. In addition, it is possible to control faster than droop because DC voltage is used for control as it is.

Hereinafter, a sixth embodiment of the output frequency control method of the grid-forming power conversion control apparatus according to the present invention will be described with reference to FIG. 23 . For concise and clear description, the description of elements that are the same as or similar to those of the above-described embodiments may be omitted. Referring to FIG. 23 , in the output frequency control method according to the present embodiment, the output frequency of the grid forming power converter 100 is linearly changed when the input voltage VDC is changed.

The operation of the sixth embodiment is as follows.

In order to achieve a desired load sharing from parallel operation of a plurality of generators, control according to the droop principle as shown in Equation 62 is common. All physical quantities are described according to the per unit unit method.

[Equation 62]

fout: output frequency, m: droop, P: output measured value, Psp: output setpoint, f0: rated frequency

Controlling the GFM converter with droop converges to the steady-state frequency fss depending on interaction with other parallel operators, and if you want more load sharing, you can do this by lowering the droop m or increasing the setpoint PSP. If you want to reduce the load sharing, you can do it by increasing m or decreasing PSP. This is the output when the PSP system reaches f0, so the actual output may be higher or lower than the PSP depending on the fss.

An object of the present embodiment is to achieve control that achieves load sharing performance equivalent to droop by using a measurement value other than P. According to the principle of the existing matching control, it is claimed that the frequency fout can be controlled by having a proportional relationship with the DC voltage, so that the droop control can be replaced by controlling it according to Equation 63.

[Equation 63]

(Unit: per unit)

However, in order for this control to be established, the rate at which fout fluctuates according to the droop m and the rate at which the measured voltage VDC fluctuates for any load change must be the same. Therefore, matching control cannot be properly implemented without a separate controller that guarantees this.

On the other hand, according to Equation 64, which is the fluctuation equation of the generator, it can be seen that P can also be indirectly grasped through VDC, paying attention to the matching principle that DC voltage means the rotation speed of the synchronous generator rotor.

[Equation 64]

(Unit: per unit)

H: coefficient of inertia K: damping coefficient w, w’: rotor rotation angular velocity, angular acceleration

That is, in the equilibrium state in which the rotor does not decelerate or accelerate any more, the output can be known from the rotational speed of the rotor. Also, since the rotation speed can be known from the DC voltage, the output can be known from the DC voltage as a result. Therefore, if ① the relationship between DC voltage and output, ② relationship between DC voltage and frequency, and ③ relationship between output and frequency, is accurately defined, then droop control can be replaced by DC voltage measurement.

When the converter is no-load, the DC voltage is maintained close to the rated voltage, and at full load, the DC voltage changes to the minimum and reaches VFL. Accordingly, the relationship between the DC voltage and the output is shown in Equation 65.

[Equation 65]

...(One)

...(2)

(unit: per unit) VFL: full-load voltage, VNL: no-load voltage

2 in Equation 65 is p.u. The existing matching principle of equalizing unit reference DC voltage and rotation speed is applied.

If the converter voltage is rated, it is in the no-load state, so the no-load frequency is output and the DC voltage is

If it falls to , the full load frequency is output. Accordingly, the relationship between the DC voltage and the frequency is shown in Equation 66.

[Equation 66]

...(One)

...(2)

fNL : No-load frequency, fFL : Full-load frequency, (unit: per unit)

The relationship between output and frequency means droop, and if Equation 62 is rewritten according to the relationship with DC voltage, Equation 67 is obtained.

[Equation 67]

(Unit: per unit)

Also, in FIG. 23 , the frequency when the input voltage is 0 corresponds to f(v=0)=fNL-m/(vNL-vFL)=(1+mP)-m/(vNL-vFL).

Equation 67 can be expressed as Equation 68.

[Equation 68]

k: control parameter, wc: low-frequency filter

Therefore, if the frequency is controlled as in Equation 68, the same droop frequency as Equation 62 can be achieved in a steady state, and frequency control can be performed only by measuring the DC voltage without a separate control unit for controlling the DC current. The purpose of the control parameter k is to reduce noise by increasing k when the high frequency component of the DC voltage is severe and to enable smooth tuning.

모의실험 결과Simulation result

24 is a diagram illustrating a DC voltage and an output frequency provided to the grid-forming power converter when the grid-forming power converter is controlled in the first embodiment, the second embodiment, and the third embodiment, respectively. Referring to FIG. 24 , as the DC voltage provided to the grid-forming power converter 100 increases, the grid-forming power converter 100 controlled by the first embodiment outputs a frequency that increases in a first-order function. And, the grid forming power conversion unit 100 controlled by the second embodiment outputs a frequency increasing exponentially convex downwards, and the grid forming power conversion unit 100 controlled by the third embodiment It can be seen that outputs an exponentially increasing frequency that is convex upwards.

25 (a) is a diagram showing the output frequency versus the DC voltage provided to the grid-forming power converter according to the prior art and the first embodiment and the second embodiment. FIG. 25(b) is an enlarged view of the range of 225kV to 245KV of FIG. 25(a). Referring to FIG. 25 ( a ), when a DC voltage of 0 to 250 kV is provided to the grid forming power conversion unit 100 , the grid forming power conversion unit 100 controlled by the prior art is linear in the magnitude of the provided DC voltage. Outputs a voltage with a frequency proportional to . However, this is performed by controlling the current provided to the grid forming power conversion unit 100 using a separate DC current controller.

However, it can be seen that the change width of the frequency is relatively small even when the first embodiment changes linearly with respect to the DC voltage and the second embodiment changes exponentially. The range of change in frequency according to the embodiment and the second embodiment can be seen.

According to the present embodiments, it can be confirmed that the advantage that a frequency corresponding to the provided DC voltage can be formed and output is provided without employing a DC current controller.

26 to 30 show when the power system (dotted line) and the grid-forming power conversion unit 100 (solid line) supply power to the load, and when disturbance occurs at 2 seconds, 3 seconds, and 4 seconds, power according to time It is a figure which shows the fluctuation|variation and frequency fluctuation|variation with time. When the outputs of the two power sources converge to a new operating point after each disturbance, it is judged to be stable.

26(a) and 26(b) show experimental results of a grid-forming power conversion unit (solid line) and a power system (dotted line) controlled by the prior art. Referring to FIG. 26( a ), the grid-forming power conversion unit 100 and the power system supply power to the load by 30Mw each from approximately 1.3 seconds. When a disturbance occurs in 2.0 seconds, it is confirmed that both the power system and the grid-forming power conversion unit 100 vibrate, and the vibration is stabilized after approximately 2.5 seconds. Also, vibration occurs in the power supplied by the disturbance generated in 3 seconds, and the vibration is stabilized after 3.5 seconds. After 4 seconds, the grid-forming power conversion unit 100 provides 60MW to the load. Referring to FIG. 26(b) , it can be seen that the output frequency fluctuates significantly whenever a disturbance occurs.

27 (a) and 27 (b) show the experimental results of the grid forming power conversion unit (solid line) and the power system (dotted line) controlled in the first embodiment, FIGS. 28 (a), 28 (b) ) shows the experimental results of the grid forming power conversion unit (solid line) and the power system (dotted line) controlled in the second embodiment, and FIGS. 29 (a) and 29 (b) are the grids controlled by the third embodiment Showing the experimental results of the forming power conversion unit (solid line) and the power system (dotted line), FIGS. 30 (a) and 30 (b) are grid forming power conversion unit 100 (solid line) controlled in the fourth embodiment and the experimental results of the power system (dotted line) are shown.

Referring to FIGS. 27(a) to 30(b), the grid-forming power conversion unit 100 and the power system supply power to the load by 30Mw, respectively, from about 1.3 seconds later. When a disturbance occurs at 2.0 seconds, it is confirmed that both the power system and the grid-forming power conversion unit 100 vibrate, and the vibration is stabilized after approximately 2.2 seconds. Also, in the case of a disturbance occurring in 3 seconds, the vibration is stabilized after 15.2 seconds. After 4 seconds, the grid-forming power conversion unit 100 provides 60MW to the load.

However, according to the second and third embodiments shown in FIGS. 28( a ) and 29 ( a ), it can be seen that overshoot occurs largely due to disturbance occurring at 2.0 seconds, and FIG. In the fourth embodiment shown as 30(a), a relatively small size of overshoot occurs.

As shown in FIGS. 27 to 30, the grid-forming power conversion unit 100 controlled by the first to fourth embodiments can confirm that the vibration is stably stabilized faster than in the prior art even when a disturbance occurs, It can be seen that even when disturbance occurs, the range of variation of the frequency output by the grid-forming power conversion unit 100 is not large compared to the prior art.

Figure 31 (a) shows the experimental results of the grid forming power conversion unit (solid line) and the power system (dotted line) controlled in the fifth embodiment, Figure 31 (b) is from 1.2 seconds to 3.2 seconds in Figure 31 (a) It is an enlarged view of the second part. Referring to FIGS. 31 ( a ) and 31 ( b ), the grid-forming power conversion unit 100 and the AC power generation system are connected in 1.5 seconds. At 3.0 seconds, a disturbance occurs that increases the load, and the set-point of the grid forming converter decreases in 4.5 seconds, and the set-point of the AC power generation system is maintained. At 6.0 seconds, there was a disturbance in which the AC power generation system generator fell off. Referring to Figure 31 (c), the output frequency is the same as Figure 31 (c). It can be confirmed that smooth load sharing with the AC system is achieved by having different steady-state frequencies according to the load level.

The third guide line G3, the first guide line G1, and the second guide line G2 each show an output distribution according to an intended droop at the corresponding time point. Since the output of the grid forming power converter 100 shown by the red solid line converges on each guide line, it can be confirmed that the desired load sharing is performed.

In addition, as confirmed at the time of 4.5 seconds, it can be confirmed that the load can be shared as intended without DC current control, and since it is controlled quickly through DC DC voltage measurement, it is more stable than the conventional droop control. reach quickly

32(a) and 32(b) show a case in which control is performed by droop control according to the prior art in the experimental condition illustrated in FIG. 19 . It can be seen that the vibration is attenuated in about 0.35 seconds in the present embodiment illustrated in FIG. 31(b), but according to the prior art illustrated in FIG. 32(a), 0.6 seconds, which is approximately twice the time of this embodiment, is It can be seen that the vibration is attenuated over time. Furthermore, when the droop control according to the prior art is to be controlled at high speed as in the present embodiment, it can be confirmed that a large vibration occurs as shown in FIG. 32(b), and the droop control according to the prior art cannot be controlled at high speed. can confirm.

33 and 34 are diagrams showing experimental results of grid forming power conversion (solid line) and power system (dashed line) controlled in the sixth embodiment. Referring to FIGS. 33 and 34 , the grid-forming power conversion unit 100 and the power system supply power to the load by 30Mw each from about 1 second. When a disturbance occurs at 1.3 seconds, it is confirmed that both the power system and the grid-forming power conversion unit 100 vibrate, and the vibration is stabilized after approximately 2.0 seconds. Also, in the case of disturbances occurring at 3 seconds and 4.2 seconds, the vibration is stabilized after approximately 0.3 seconds after the disturbance occurs. After 6 seconds, the grid-forming power conversion unit 100 provides power to the load alone.

As shown in FIGS. 33 and 34 , the grid forming power conversion unit 100 controlled according to the sixth embodiment can confirm that the vibration is stably stabilized faster than in the prior art even when a disturbance occurs, and the disturbance is Even if it occurs, it can be seen that the range of variation of the frequency output by the grid-forming power conversion unit 100 is not large compared to the prior art.

As described above, it will be understood that the present invention is implemented in a modified form without departing from the essential characteristics of the present invention.

Therefore, the specified embodiments are to be considered in an illustrative rather than a restrictive point of view, the scope of the present invention is indicated in the claims rather than the foregoing description, and all differences within the scope equivalent thereto are included in the present invention. will have to be interpreted.

Modes for carrying out the invention have been described together in the best mode for carrying out the invention above.

The present invention relates to a grid-forming power conversion control device, and since it can provide a stable operating environment based on system information connected with information of a power conversion device to be connected, there is potential for industrial application.

Claims

a grid-forming power conversion unit for supplying power by converting the power supplied from the renewable energy generator into a voltage for supplying power to the grid;

It is connected to the grid forming power converter and calculates the fault current when a fault occurs and calculates the reverse current that occurs when the voltage is lowered in case of a fault and the recovery reverse current that occurs when the fault is removed. Optimal voltage control that finds an operating point at which overcurrent is minimized according to fault current supply characteristics and finds an operating point that satisfies the requirements of the power system as much as possible within the range that does not exceed the limit of current that the power converter can supply control unit; and

It collects parameters for impedance evaluation using power system information, sets the reference MVA connection impedance input and GFM (Grid Forming Source) voltage to obtain the minimum impedance that does not exceed the limit of the current that the power converter can supply. A grid-forming power conversion control device comprising an impedance constraint evaluation control unit.
According to claim 1,

The optimum voltage control unit is

an optimum voltage calculation unit that calculates the optimum voltage as the FRT voltage command value of the optimum voltage control unit;

a limit voltage calculator that calculates a limit voltage as the FRT voltage control lower limit of the optimal voltage controller;

A minimum voltage calculator that calculates the minimum voltage as a lower limit of safety measures in case of incomplete voltage control of the optimal voltage controller;

The optimum voltage to find the operating point where overcurrent is minimized considering the fault current supply characteristics required by the system and to find the operating point that satisfies the requirements of the power system as much as possible within the range that does not exceed the limit of the current that the power converter can supply A grid-forming power conversion control device comprising a fault current supply characteristic control unit for controlling.
According to claim 1,

The optimum voltage control of the optimum voltage control unit is

When the current exceeds the voltage, the voltage is lowered, and the overcurrent is resolved by raising the voltage in a specific section where the reverse current exceeds the current limit. A grid-forming power conversion control device that bypasses the integral controller by clamping and promotes resynchronization success after removing a fault.
4. The method of claim 3,

A grid-forming power conversion control device that does not lower the voltage below the limit voltage because the voltage is raised when the current is low and the limit is free, and when the voltage is excessively low, the reverse current and recovery reverse current increase.
According to claim 1,

Impedance constraint evaluation control unit,

a parameter collecting unit that collects parameters for impedance evaluation by using information on the power system;

Input setting unit for setting the reference MVA connection impedance input and GFM voltage;

a fault current calculation unit for calculating a fault current to obtain a minimum impedance that does not exceed the limit of the current that the power converter can supply;

a recovery reverse current calculation and determination unit that determines whether the power converter current exceeds a set limit value, stores the current GFM voltage, calculates a recovery reverse current, and determines whether the recovery reverse current exceeds a set limit value; and

A grid-forming power conversion control device including an impedance evaluation unit that determines that it is possible to operate with the input impedance when the recovery reverse current does not exceed the set limit value.
6. The method of claim 5,

The fault current calculator,

A grid-forming power conversion control device that calculates 3-phase short-circuit fault current, line-to-line short-circuit fault current, line-to-line short-circuit fault current, and 1-line ground fault fault current.
When a failure occurs in the system power supply, calculating the fault current and the reverse current to control the fault current supply characteristics, and collecting parameters using the information of the power system;

setting the reference MVA connection impedance input and GFM (Grid Forming Source) voltage in the input setting unit;

calculating three-phase short-circuit fault current, line-to-line short-circuit fault current, line-to-line short-circuit fault current, and 1-line ground fault current in the fault current calculation unit;

storing the current GFM voltage and calculating a recovery reverse current; and

Grid forming power conversion control method comprising the step of determining whether the calculated recovery reverse current (Recovery reverse current) exceeds a set limit value to determine whether operation with the input impedance.
8. The method of claim 7,

A grid-forming power conversion control method that determines whether the calculated power converter current exceeds the set limit value, and if the current does not exceed the set limit value, saves the current GFM voltage and calculates the recovery reverse current.
8. The method of claim 7,

When the calculated power converter current exceeds the set limit value, the GFM voltage is reduced by 1 step within the range above the minimum voltage and the fault current is recalculated.
8. The method of claim 7,

If the calculated recovery reverse current exceeds the set limit value, it is judged that operation with the input impedance is impossible, and the reference MVA connection impedance is increased by 1 step and the fault current is calculated again.
8. The method of claim 7,

A grid-forming power conversion control method in which 100MVA is input as the reference impedance in the step of setting the connection impedance in the input setting unit and the GFM voltage is set to 1.0 [pu].
11. The method of claim 9 or 10,

The process of reducing the GFM voltage by 1 step, calculating the fault current again, and applying the connection impedance increased by 1 step again is repeated until the minimum impedance that does not exceed the limit of the current that the power converter can supply is found. Forming power conversion control method.
An output frequency control method of a grid-forming power conversion control device, the control method comprising:

calculating a frequency corresponding to the DC voltage provided to the grid forming power converter; and

Comprising the step of outputting a frequency corresponding to the DC voltage provided by the grid-forming power converter,

The outputting of the calculated frequency is an output frequency control method of a grid-forming power conversion control device that is performed without controlling the DC current provided to the grid-forming power conversion unit.
14. The method of claim 13,

Calculating a frequency corresponding to the DC voltage provided to the grid-forming power converter includes:

Grid-forming power conversion performed by calculating the frequency for the DC voltage provided to the grid-forming power converter from a function of passing the no-load output frequency when the grid-forming power converter is no load and outputting the full-load output frequency when it is full load How to control the output frequency of the control device.
15. The method of claim 14,

The function is a first-order function,

The slope of the linear function is,

formula

A method of controlling the output frequency of a grid-forming power conversion control device determined by

(η‘: slope, f*: rated frequency, vdc*: rated voltage, droop%: droop slope)
15. The method of claim 14,

The function is

A method of controlling the output frequency of a grid-forming power conversion control device expressed as .

(dra: droop rate, Vd: DC voltage, f: output frequency for DC voltage (Vd), fNL: no-load output frequency, H: inertia, Vd0: rated DC voltage)
15. The method of claim 14,

The function is

A method of controlling the output frequency of a grid-forming power conversion control device expressed as .

(dra: droop rate, Vd: DC voltage, f: output frequency for DC voltage (Vd), fNL: no-load output frequency, H: inertia, Vd0: rated DC voltage)
15. The method of claim 14,

The function is

A method of controlling the output frequency of a grid-forming power conversion control device expressed as .

(dra: droop rate, Vd: DC voltage, f: output frequency for DC voltage (Vd), fNL: no-load output frequency, H: inertia, Vd0: rated DC voltage)
15. The method of claim 14,

The function is

A method of controlling the output frequency of a grid-forming power conversion control device expressed as .

(dra: droop rate, Vd: DC voltage, f: output frequency for DC voltage (Vd), fNL: no-load output frequency, H: inertia, Vd0: rated DC voltage)
15. The method of claim 14,

The function is

formula

is displayed as

Parameter a is the formula

satisfied with

Parameter b is the formula

An output frequency control method of a grid-forming power conversion control device that satisfies

(Vd: DC voltage, f: output frequency for DC voltage (Vd), fNL: output frequency at no-load, fFL: output frequency at full load, Vd0: rated DC voltage, i: load factor, fss: steady-state frequency according to droop )
15. The method of claim 14,

The function is

A method of controlling the output frequency of a grid-forming power conversion control device expressed as .

(fout: output frequency, ωc: rotor rotation angular velocity, VDC: input DC voltage, VFL: full-load voltage, m: droop, k: control parameter, PSP: output setpoint)
The output frequency control unit connected to the grid forming power conversion unit to control the frequency includes:

at least one processor; and

a memory storing one or more programs executed by the processor, wherein the programs, when executed by the one or more processors, in the one or more processors;

calculating a frequency corresponding to the DC voltage provided to the grid-forming power converter; and

Comprising the step of controlling the grid-forming power converter to output a frequency corresponding to the provided DC voltage,

The controlling to output the calculated frequency is a grid-forming power conversion control device that is performed without controlling the DC current provided to the grid-forming power conversion unit.
23. The method of claim 22,

Calculating a frequency corresponding to the DC voltage provided to the grid-forming power converter includes:

Grid-forming power conversion control device for performing by calculating the frequency of the DC voltage provided to the grid-forming power conversion unit from a linear function passing the output frequency when the grid-forming power conversion unit is no load.
24. The method of claim 23,

The slope of the linear function is,

formula

A grid-forming power conversion control device determined by

(η‘: slope, f*: rated frequency, vdc*: rated voltage, droop%: droop slope)
23. The method of claim 22,

Calculating a frequency corresponding to the DC voltage provided to the grid-forming power converter includes:

Grid-forming power conversion control device for performing by calculating the frequency for the DC voltage provided to the grid-forming power conversion unit from a function passing the output frequency when the grid-forming power conversion unit is no load.
26. The method of claim 25,

The function is

A grid-forming power conversion control device represented by .

(dra: droop rate, Vd: DC voltage, f: output frequency for DC voltage (Vd), fNL: no-load output frequency, H: inertia, Vd0: rated DC voltage)
26. The method of claim 25,

The function is

A grid-forming power conversion control device represented by .

(Vd: DC voltage, f: Output frequency for DC voltage (Vd), fNL: No-load output frequency, H: Inertia, Vd0: Rated DC voltage)
26. The method of claim 25,

The function is

A grid-forming power conversion control device represented by .

(Vd: DC voltage, f: Output frequency for DC voltage (Vd), fNL: No-load output frequency, H: Inertia, Vd0: Rated DC voltage)
26. The method of claim 25,

The function is

A grid-forming power conversion control device represented by .

(dra: droop rate, Vd: dc voltage, f: output frequency for dc voltage (Vd), fNL: no-load output frequency, H: inertia, Vd0: rated DC voltage)
26. The method of claim 25,

The function is

is expressed as

a and b are each

A grid-forming power conversion control device that satisfies

(Vd: DC voltage, f: output frequency for DC voltage (Vd), fNL: output frequency at no-load, fFL: output frequency at full load, Vd0: rated DC voltage, i: load factor, fss: steady-state frequency according to droop )
26. The method of claim 25,

The function is

A grid-forming power conversion control device represented by .

(fout: output frequency, ωc: rotor rotation angular velocity, VDC: input DC voltage, VFL: full-load voltage, m: droop, k: control parameter, PSP: output setpoint)