WO2021040197A1

WO2021040197A1 - Independent microgrid system and inverter device

Info

Publication number: WO2021040197A1
Application number: PCT/KR2020/007121
Authority: WO
Inventors: 정상민; 손의권; 최기영
Original assignee: 효성중공업 주식회사
Priority date: 2019-08-29
Filing date: 2020-06-02
Publication date: 2021-03-04
Also published as: KR102274048B1; KR20210026165A

Abstract

An independent microgrid system of the present invention is always or selectively separated from an upper grid to operate independently in a local grid and may comprise: a generator for supplying alternating current power to the local grid and supplying generated power to the local grid via a transformer for insulation without an inverter; an energy storage means for providing power to the local grid when there is insufficient power available and storing remaining power; a main inverter block for supplying, to the local grid through a first transformer means in a normal state, the power stored in the energy storage means as alternating current power; and a sub inverter block for, when the main inverter block stops, supplying, to the local grid through a second transformer means, the power stored in the energy storage means as alternating current power.

Description

Independent microgrid system and inverter device

The present invention relates to an independent microgrid system and an inverter device that can be applied to remote areas such as island areas.

Recently, in order to reduce fossil energy, the use of renewable energy such as solar power, wind power, and hydrogen energy is rapidly increasing.

Renewable energy accounted for 10% of the world's primary energy consumption in 2017 due to the improvement of technology and economics. Along with the Paris Agreement, each country's policies to support renewable energy are expected to continue, and the proportion is expected to increase to 17% by 2040. In particular, the region achieving grid parity is expanding due to continuous price reductions for wind and solar power, and the sustainability of power generation is further improved through integrated operation with an energy storage system (ESS). The development of new and renewable energy technologies has promoted a distributed generation method through a number of new and renewable energies from the existing central power generation method, and expanded application and active research to microgrids for small-scale local systems have become possible.

Islands, where it is difficult to connect to the existing system due to technical or economic reasons, have long been supplied with power through diesel power plants. Diesel power generation has problems such as high fuel cost, fuel transport, storage and management, and CO2 emission. In order to compensate for the above-described problems, an island microgrid system incorporating an energy storage device (ESS) and a renewable energy generation device such as wind and solar power has been proposed. The island microgrid system is a semi-independent local system that operates after being cut off from the upper system for a considerable period of time.

However, the conventional PCS inverter control method and structure of a general energy storage device (ESS) is for a system with a low burden on the entire system, so it is not suitable for a microgrid for islands where the burden of the energy storage device (ESS) is high. Did.

An object of the present invention is to provide a system and device structure for controlling an inverter of a PCS of an energy storage device (ESS) suitable for a microgrid for islands where the power burden of the energy storage device (ESS) is high.

An object of the present invention is to provide an independent microgrid system or inverter device capable of maintaining power supply to a local system of a microgrid for islands even when an abnormal situation in which the inverter may be stopped occurs.

In the independent microgrid system according to an aspect of the present invention, in an independent microgrid system that is always or selectively separated from an upper system and independently operated in a local system, AC power is supplied to the local system, and the generated power is converted into an inverter. A generator for supplying to the local system via a transformer for insulation without using; Energy storage means for providing insufficient power to the local system and storing remaining power; A main inverter block for supplying the power stored in the energy storage means to the local grid as AC power through a first transformer means in a normal state; And a sub-inverter block for supplying the power stored in the energy storage means to the local grid as AC power through a second transformer means when the main inverter block is stopped.

Here, the main inverter block includes: a main inverter converting DC power output from the energy storage means into AC power; A first reactor connected between the output terminal of the main inverter and the input terminal of the first transformer means; And a first capacitor connected to one end of the first reactor and a neutral point or a ground point to form an LC filter configuration with the first reactor,

The sub-inverter block may include a sub-inverter for converting DC power output from the energy storage means into AC power; A second reactor connected between the sub-inverter output terminal and the input terminal of the first transformer means; And a second capacitor connected to one end of the second reactor and a neutral point or a ground point to form an LC filter configuration with the second reactor.

Here, the main inverter block may further include a third reactor connected in parallel with the first capacitor, and the sub-inverter block may further include a fourth reactor connected in parallel with the second capacitor.

Here, the generator may further include a renewable energy generation means for supplying unchanged AC power to the local system, generating power from renewable energy such as solar or wind power, and supplying it to the local system.

An inverter device according to another aspect of the present invention is an inverter device that converts DC power stored in a battery into AC power and supplies the converted AC power to a grid in a normal state. Inverter block; A sub-inverter block for supplying AC power converted through a second transformer means to a grid when the main inverter block is stopped; And a control unit controlling a power conversion operation of the main inverter block and the sub inverter block.

Here, the main inverter block includes: a main inverter converting DC power output from the battery into AC power; A first reactor connected between the output terminal of the main inverter and the input terminal of the first transformer means; And a first capacitor connected to one end of the first reactor and a neutral point or a ground point to form an LC filter configuration with the first reactor,

The sub-inverter block may include a sub-inverter for converting DC power output from the battery into AC power; A second reactor connected between the sub-inverter output terminal and the input terminal of the first transformer means; And a second capacitor connected to one end of the second reactor and a neutral point or a ground point to form an LC filter configuration with the second reactor.

Here, when it is determined that an abnormality has occurred in the main inverter block, the control unit stops the switching operation of the main inverter and starts switching of the sub inverter, but the magnetic flux defined according to the difference between the reference voltage and the actual changed voltage Depending on the offset, the sub-inverter may be controlled to immediately follow the power of the grid, or to perform a switching operation after being lowered to less than a predetermined reference voltage supplied from the battery to the grid.

Here, the control unit controls the sub-inverter to immediately follow the power of the system when Be is larger among the values of Be and Bk obtained according to the following equation, and when Bk is larger, a predetermined value supplied from the battery to the system It can be controlled to perform a switching operation after lowering below the reference voltage.

B _e =B _s -B _r

(Where, Nt: number of transformer turns, A: transformer cross-section, T _delay : time delay,

ω _r ,: nominal frequency, ω _d : natural resonance frequency, v _r : nominal voltage,

α: time constant, B _s : saturation magnetic flux density of transformer, B _r : Transformer magnetic flux density at nominal voltage)

If the independent microgrid system or inverter device of the present invention according to the above configuration is implemented, there is an advantage of operating a PCS inverter of an energy storage device (ESS) suitable for a microgrid for islands where the burden of the energy storage device (ESS) is high.

The independent microgrid system or inverter device of the present invention has the advantage of being able to maintain the power supply of the island microgrid to the local system even when an abnormal situation in which the inverter may be stopped occurs.

The independent microgrid system or inverter device of the present invention has the advantage that the sub-inverter can immediately follow the rated voltage without the inrush current of the transformer under a specific load capacity when the inverter in use is stopped.

The independent microgrid system or inverter device of the present invention has the advantage of being able to recover the system within one cycle without a power outage, compared to the conventional technology in which a power outage of several to several tens of cycles occurs to bypass the inrush current.

The independent microgrid system or inverter device of the present invention has the advantage of implementing the independent microgrid requiring high stability at low cost.

1 is a conceptual diagram showing a microgrid system for books to which an inverter device according to the spirit of the present invention can be applied.

2 is a circuit diagram of an inverter device and a microgrid configuration according to an embodiment of the present invention.

3 is an experimental waveform diagram showing a grid voltage according to an inverter stop when implemented with the LC filter type inverter device of FIG. 2;

4 is an experimental waveform diagram showing a load voltage and an inrush current of a transformer when the inverter starts up when implemented with the LC filter type inverter device of FIG.

5 is a conceptual waveform diagram showing a voltage fluctuation of a microgrid area system when the main inverter is stopped in an ESS single operation mode when implemented as an LC filter type inverter device.

6 is a circuit diagram of an inverter device and a microgrid configuration according to another embodiment of the present invention.

7A and 7B are conceptual waveform diagrams showing the grid voltage according to the stop of the inverter when implemented as an LCPL filter type inverter device.

8 is a flowchart illustrating a method of starting a sub-inverter according to power conversion control according to the spirit of the present invention.

9 is a block diagram showing a control algorithm block of a sub-inverter for performing the sub-inverter starting method of FIG. 8;

10 is a conceptual diagram showing a test configuration for verifying the LCPL filter design method and start control method proposed by the present invention.

11A to 11C are experimental waveform diagrams showing experimental results performed under

experimental conditions

1, 2, and 3;

12 is a waveform diagram displayed together to compare respective Vab waveforms, which are experimental results performed under

test conditions

1, 2, and 3;

13 is an experimental waveform diagram showing that the voltage decreases less and recovers to the rated voltage more quickly under test condition 4;

14 is an experimental waveform diagram showing that under test condition 5, it is started in a soft start method and recovered to a rated voltage.

Hereinafter, a specific embodiment for carrying out the present invention will be described with reference to the accompanying drawings.

In describing the present invention, terms such as first and second may be used to describe various elements, but the elements may not be limited by terms. The terms are only for the purpose of distinguishing one component from another component. For example, without departing from the scope of the present invention, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element.

When a component is connected to or is referred to as being connected to another component, it can be understood that it is directly connected to or may be connected to the other component, but other components may exist in the middle. .

The terms used in the present specification are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions may include plural expressions unless the context clearly indicates otherwise.

In the present specification, terms such as include or include are intended to designate the presence of features, numbers, steps, actions, components, parts, or a combination thereof described in the specification, and one or more other features or numbers, It may be understood that the possibility of the presence or addition of steps, actions, components, parts, or combinations thereof, is not preliminarily excluded.

In addition, shapes and sizes of elements in the drawings may be exaggerated for clearer explanation.

1 shows a microgrid system for a book to which an inverter device according to the spirit of the present invention can be applied.

For example, three 150kVA diesel generators were being supplied with power, and wind, solar, and ESS may be additionally installed to constitute the illustrated microgrid system. The solar power generation unit 114kW is mainly installed in the home (load), and the wind power generator is installed with two 250kVA units, acting as the main power source together with the diesel generator.

Installed wind power generators can be applied with squirrel-cage induction generators that can be directly connected to the AC system in the installation area. This does not go through a separate inverter, but directly connects to the local system through only an isolation transformer, which has an economical advantage that control is simple and the hardware of the system can be minimized.

However, since diesel power generation is directly connected to the grid without an inverter, large voltage fluctuations may occur due to inrush current and transient current during startup. As shown in the figure so that the influence of the squirrel cage wind generator with high instantaneous variability is eliminated and can be operated independently regardless of the local grid voltage of the microgrid, the wind turbine generator and ESS each equipped with an inverter are configured back to back. do.

The energy storage device (ESS) may be operated alone or integrated with a diesel generator to control the voltage of the microgrid and supply power.

In the illustrated island microgrid system, the power generation amount of the renewable energy generation device, the state of charge of the ESS battery (SoC), and operation combinations of the diesel generator and the energy storage device (ESS) according to the load are shown in Table 1 below.

[Table 1]

Diesel generator when the SoC of the ESS battery is larger than the Set SoC, which is the SoC set by the microgrid designer to enable the single operation of the energy storage device, and when the amount of renewable energy generation is larger than the minimum SoC and the amount of renewable energy generation is larger than the load. Is stopped, and power is supplied only by the energy stored in the ESS battery and the renewable energy generation device.

In this case, the local grid voltage of the microgrid is controlled by the ESS's PCS (more specifically, the inverter), and if an overcurrent occurs or a breakdown occurs in the ESS inverter due to an external accident at this time, and the corresponding inverter stops, the local grid is powered off. This can happen.

As a countermeasure against this, the present invention proposes a standalone microgrid system having a structure in which sub-inverters are installed in parallel as a spare means for the ESS inverter (main inverter) that can be stopped in case of an accident described above.

The proposed independent microgrid system may be a fully independent microgrid, which is always separated from the upper system, or a microgrid that is selectively separated from the upper system and operates independently in the local system.

The proposed independent microgrid system includes: a generator (20) for supplying unchanged AC power to a local system and supplying the generated power to the local system via a transformer for insulation without an inverter; Energy storage means (60) for providing insufficient power to the local system and storing remaining power; A main inverter block for supplying the power stored in the energy storage means to the local grid as AC power through the first transformer means 110 in a normal state; And a sub-inverter block for supplying the power stored in the energy storage means 60 to the local grid as AC power through a second transformer means 210 when the main inverter block is stopped.

However, in a situation in which a failure occurs in the main inverter 120, it is not possible to guarantee the normal operation of the transformer part directly connected thereto. As a result, the sub-inverter block is parallel to the main inverter block including the transformer device directly connected to the inverter as well as the inverter. It is desirable to install and prepare for power outages.

The sub-inverter block according to the idea of the present invention is a preliminary configuration that does not operate normally and operates when an abnormality occurs in the main inverter block, but depending on the implementation, the sub-inverter block is a spare inverter in preparation for future expansion of the battery storage capacity of the ESS. Can be

Alternatively, depending on the implementation, the sub-inverter block may be an inverter for power input/output of a battery that is preliminarily provided in case the load in the connected grid or microgrid increases. In this case, it is used as a spare inverter according to the idea of the present invention at a low load without using a spare battery.

The illustrated island microgrid may be supplied with auxiliary power from the upper system in case of emergency, but it is independent from the outside through a renewable energy generation means that mainly produces power from renewable energy such as solar or wind power and supplies it to the local system. It aims to supply power. To this end, as a renewable energy generation means as shown, the wind power generation means 40 and the solar power generation means 80 supply main power to the local system. However, in the renewable energy generation means, power fluctuations exist according to the weather, and in order to compensate for this, an energy storage device (ESS) 60 and a generator supplying stable AC power without fluctuations to the local system (diesel in the drawing Generator) (20) is provided.

In accordance with the above-described idea, the main inverter connection structure and the sub-inverter connection structure shown in FIG. 1 are shown in FIG. 2. 2 is an inverter device according to an embodiment of the present invention, and shows a circuit diagram of a simplified microgrid configuration under the condition that the independent microgrid for books of FIG. 1 is supplied with power only from renewable energy sources and ESS.

On the other hand, the wind power generator not shown in FIG. 2 is composed of ESS and back to back, and does not directly affect the microgrid voltage, so it is substantially the same as the ESS single operation and is excluded from the configuration of FIG. 2.

The main inverter block 100 shown in FIG. 2 includes a main inverter 120 for converting DC power output from the energy storage means 60 into AC power; A first reactor 142 connected between the output terminal of the main inverter 120 and the input terminal of the first transformer means 110; And a first capacitor 144 connected to one end of the first reactor 142 and a neutral point or a ground point to form an LC filter configuration with the first reactor 142,

The sub-inverter block 200 may include a sub-inverter 220 for converting DC power output from the energy storage means 60 into AC power; A second reactor (242) connected between the output terminal of the sub-inverter 220 and the input terminal of the second transformer means (210); And a second capacitor 244 connected to one end of the second reactor and a neutral point or a ground point to form an LC filter configuration with the second reactor 242.

Meanwhile, a control unit (not shown) for controlling the power conversion operation of the main inverter block 100 and the sub inverter block 200 may be further included.

Here, the main inverter 120 and the sub-inverter 220 are generally structured to generate AC power from DC power by switching transistors. In this case, the controller is connected to each gate (or base) terminal of the switching transistors. The power conversion operation can be controlled by applying a PWM pulse.

In the inverter device of the illustrated configuration, when the main inverter 120 is stopped while the main inverter 120 supplies power and the sub inverter is waiting, the local grid voltage of the microgrid is rapidly reduced by the sub inverter 220. Can be controlled (followed) accordingly.

For this operation, it is effective to maintain the grid voltage by transmitting a stop signal of the main inverter 120 to the sub-inverter 220 to respond so that the sub-inverter 220 is operated. Communication or wired (eg hard wiring) can be used as a method for transferring information. By the way, the time delay of signal transmission according to the transmission method (T _delay (Referred to as) occurs.

An abnormality occurs in the main inverter block 100 having the LC filter structure as shown, and the PWM (pulse width modulation) applied to the switching transistor of the main inverter 120 is stopped. Accordingly, the microgrid is used as the main inverter. It can be simplified with a parallel circuit of 120 LC filter capacitors and load resistors. The voltage of the RC parallel circuit without power can be expressed as a negative exponential function as shown in Equation 1 below. Here, C is the filter capacitor of the inverter, and R is the load resistance.

[Equation 1]

v(t) = v _o e ^-t/ ^RC

As shown in the above equation, the voltage of the system operates as attenuation of the RC filter circuit, and the detailed operation is illustrated in FIG. That is, FIG. 3 is an experimental waveform of a grid voltage according to an inverter stop when implemented as an LC filter type inverter device.

As shown, when the main inverter 120 stops at a point where one of the three phase voltages is zero, the zero voltage is maintained as it is, and the other two phases are in negative exponential form at voltages having the same magnitude and opposite signs. Decreases to and converges to zero. Where Vab, Vbc, and Vca are the line voltages of the load.

_{A time delay (T delay} ) may inevitably occur in the process of the sub-inverter 220 recognizes and operates the stop of the main inverter 120. If there is a large difference between the actual grid voltage and the sub-inverter output voltage due to such a time delay, an offset may occur in the magnetic flux of the transformer, resulting in an inrush current due to the flux saturation. Since this inrush current can be generated several times more than the rated current of each inverter, it may cause overcurrent protection operation when the sub-inverter is turned on. Due to the overcurrent protection operation, the power failure state of the local system of the microgrid may be deteriorated.

4 shows the load voltage and the inrush current of the transformer (transformer) when starting the inverter. Specifically, it shows the experimental waveform of the inrush current according to the load voltage that occurs when the inverter starts when the soft-start function that gradually increases the grid voltage is not used for quick recovery of the grid under a load of 10kW. . Here, Vab, Vbc, and Vca are the line voltages of the load, and Ia_tr, Ib_tr, and Ic_tr are transformer currents. The current protection level of each inverter is 200A, and when the transformer current exceeds 200A, it can be confirmed that the inverter stops.

5 illustrates voltage fluctuations in the microgrid area system when the main inverter is stopped in the ESS single operation mode when implemented as an LC filter type inverter device.

The phenomenon of the illustrated voltage waveform is the microgrid voltage during a _{time delay (T delay} ) when the sub-inverter recognizes the stop of the main inverter 120 after a _{time delay (T delay) and controls the voltage as a reference voltage.} It is caused by F, which is the difference between the and the original reference voltage. A magnetic flux offset corresponding to F occurs in the transformer, and the magnetic flux is saturated and may generate an inrush current.

Due to the inrush current due to the magnetic flux saturation, the controller may operate the sub-inverter block 200 in a soft start method when power is converted between the main inverter block 100 and the sub-inverter block 200.

6 is an inverter device according to another embodiment of the present invention, and proposes an inverter structure including an LCPL filter (filter) for suppressing sudden changes in the system when the operation is switched between the main inverter 120 and the sub inverter 220. .

The illustrated inverter device converts DC power stored in the battery 60 into AC power and supplies it to the grid.

The illustrated inverter device includes: a main inverter block 100-1 for supplying the converted AC power to the grid through the first transformer unit 110 in a normal state; When the main inverter block (100-1) is stopped, the sub-inverter block (200-1) for supplying the converted AC power to the grid through the second transformer means (210); A control unit (not shown) for controlling the power conversion operation of the main inverter block 100-1 and the sub inverter block 200-1 may be included.

The illustrated main inverter block 100-1 includes: a main inverter 120 for converting DC power output from the battery 60 into AC power; A first reactor 142 connected between the output terminal of the main inverter 120 and the input terminal of the first transformer means 110; And a first capacitor 144 connected to one end of the first reactor 142 and a neutral point or a ground point,

The sub-inverter block 200-1 may include a sub-inverter 220 for converting DC power output from the battery 60 into AC power; A second reactor (242) connected between the output terminal of the sub-inverter 220 and the input terminal of the second transformer means (210); And a second capacitor 244 connected to one end of the second reactor 242 and a neutral point or a ground point.

In addition, the illustrated main inverter block 100-1 further includes a third reactor 146 connected in parallel with the first capacitor 144, and the sub inverter block 200-1 is the second capacitor It further includes a fourth reactor 246 connected in parallel with the 244.

Accordingly, the first capacitor 144, the first reactor 142, and the third reactor 146 constitute an LCPL filter for the main inverter 120 and the first transformer unit 110, and the second The capacitor 244, the second reactor 242 and the fourth reactor 246 constitute an LCPL filter for the sub-inverter 220 and the second transformer means 210.

The microgrid system in which the inverter device of this embodiment with the LCPL filter is installed can quickly recover (i.e., immediately follow) the grid voltage without the inrush current of the transformer when the sub-inverter 220 is input due to the stop of the main inverter 120. I can.

In order to quickly control the microgrid voltage to the original voltage without inrush current, the difference between the microgrid voltage and the original normal voltage must be reduced during a _{time delay (T delay).} It is suggested to use C||L filter). In the LCPL filter, the inductor connected in parallel with the capacitor not only prevents the collapse of the local grid voltage when the inverter stops starting, but also provides a function to compensate for reactive power flowing through the capacitor of the inverter in a combined operation with a diesel generator. .

7A and 7B illustrate voltage fluctuations in the microgrid area system when the main inverter is stopped in the ESS single operation mode when implemented as an LCPL filter (L+C||L filter) type inverter device.

When the main inverter 120 with the LCPL filter is stopped, the microgrid can be simplified to an RLC parallel circuit. The RLC circuit voltage having the under-attenuation characteristic is as shown in Equation 2 below, and has an attenuated sinusoidal response characteristic in the form as shown in FIGS. 7A and 7B.

[Equation 2]

As shown, as shown in _{the voltage response waveform of the RLC parallel circuit during the time delay (T delay} ), the area of F is reduced compared to the voltage response of the RC parallel circuit (shown in Fig. 5), and the larger the load resistance (R) is The area of F is further reduced. Accordingly, the LCPL filter structure serves to suppress a decrease in voltage during a time delay due to the start of the sub-inverter 220.

Next, a design method of an LCPL filter (L+C||L filter) optimized to quickly recover the grid voltage without the inrush current of the transformer when the sub-inverter 220 is input due to the stop of the main inverter 120 is presented. Considering the transformer saturation magnetic flux density, determine the load capacity that can be operated without the inrush current of the transformer according to the design of the LCPL filter (L+C||L filter). The case where the load is balanced and can be approximated by a resistive load was considered.

The filter of the voltage source inverter for reducing the switching frequency voltage generally uses an LC filter (in this case of FIG. 2). The resonant frequency of the LC filter is generally determined to be about 1/5 to 1/10 of the switching frequency, and the combination of the LC filter satisfying this is infinite. Accordingly, the combination of the LC filter may be determined differently depending on the installation site environmental condition or application. The inductor of the LC filter can be determined by limiting the current ripple and the magnitude of the fundamental wave voltage drop, and the capacitor can be determined by suppressing voltage fluctuations due to load fluctuations and limiting capacitive reactive power.

In terms of reducing the switching frequency voltage, the LCPL filter (L+C||L filter) has the impedance of the filter capacitor (the first and second capacitors described above) and the inductor connected in parallel (the third and fourth inductors described above) compared to the capacitor. In the frequency band of several kHz, it becomes negligibly large, so it can be equivalent to an LC filter and can be designed similarly to the LC filter design. In addition, the voltage must be designed to have the attenuation characteristics of a non-powered RLC parallel circuit in a specific load range during a _{time delay (T delay} ) after the main inverter stops. In addition, when the sub-inverter is operated after the voltage fluctuation accordingly to recover to the normal voltage, the load range in which the magnetic flux of the transformer is not saturated must be calculated. Also, the reactive power of the LC parallel circuit must be considered.

The design procedure reflecting the above considerations is as follows.

1. Determine the cut-off frequency of the LC filter.

The specific equation of the RLC parallel circuit without power is shown in Equation 3 below.

[Equation 3]

Determine the cut-off frequency ω ₀ of the L ₁ C _{1 filter, where}

to be.

In order to have an under-attenuation response, the _{condition α <ω 0} must be satisfied. In case of having an under-attenuation response, the voltage is the same as in Equation 2 above, and the natural resonance frequency ω _d is

to be.

2. Determine the transformer's saturation magnetic flux density B _s , the nominal voltage v _r , the nominal frequency ω _r , and the transformer magnetic flux density B _{r at the nominal voltage.}

3. Determine the time delay (T _delay ).

4. The magnetic flux margin B _e of the _{transformer is B s} -B _r .

5. Determine the natural resonance frequency ω _d as the nominal frequency.

6. Determine α and B _k from Equation 4 below. Equation 4 below defines _{a magnetic flux offset B k} generated due to a difference between an original reference voltage (reference voltage) and a microgrid voltage during a time delay (T _delay).

[Equation 4]

Here, N _T is the number of turns of the transformer and A is the cross-sectional area of the transformer.

7. Determine the power factor limit at the nominal frequency in the situation where the diesel generator and inverter are operated in combination. Reactive power Q is obtained from the determined power factor PF using Equation 5 below. Here, P _d is the rated power of one phase of the diesel generator.

[Equation 5]

8. Find the total capacitance of the microgrid required by _{w d} , Q, α, v _r and ω _r. Equation 6 below is _{a reactive power equation of C T} ||L _T and can be summarized as Equation 7 below. Here, C _T is the total capacitance and L _T is the total inductance.

[Equation 6]

[Equation 7]

Under the under-attenuation response condition, the relationship of Equation 8 below holds.

[Equation 8]

Equation 9 below can be derived from Equation 7 and Equation 8.

[Equation 9]

When w _d and ω _r are the same, C _T is the same as in Equation 10 below, and if the expression of α is substituted, it is as Equation 11 below.

[Equation 10]

[Equation 11]

9. Divide C _T by the number of inverters (N) to get the C1 value of each inverter.

10. Calculate R from _{C T and α.}

11. ω ₀ can be obtained through Equation 8, L _T is 1/(ω ₀ ² C _T ), L ₂ is L _T × N.

12. L ₁ is 1/(ω _f ² C ₁ ).

Next, look at how the sub-inverter starts when the main inverter is stopped.

When the main inverter is stopped and the time delay (T _delay ), the voltage of the microgrid is reduced, which is sized according to the designed LCPL filter (L+C||L filter) and the load. The starting method of the sub-inverter may be determined based on the sensed microgrid voltage during a time delay (T _delay).

To this end, when it is determined that an abnormality has occurred in the main inverter block, the control unit of the inverter device stops the switching operation of the main inverter and starts switching of the sub inverter,

Depending on the degree of the residual magnetic flux of the first transformer means, the sub-inverter is controlled to immediately follow the power of the grid, or the main inverter is controlled according to a magnetic flux offset defined according to a difference between a reference voltage and an actual fluctuated voltage. Through this, it is possible to control to perform a switching operation after the power supplied to the local grid is sufficiently lowered (a value close to zero).

8 is a flowchart illustrating a method of starting a sub-inverter according to the power conversion control of the control unit described above.

When the difference between the change in magnetic flux density according to the reference (reference) voltage and the change in magnetic flux density according to the reduced microgrid voltage is less than the transformer magnetic flux margin, the sub-inverter immediately estimates the reference voltage to ensure the stability of the continuous side of local load supply. can do.

On the other hand, in other cases, the sub-inverter performs a soft start only after the microgrid voltage is reduced to less than the minimum voltage. This is to prevent the occurrence of inrush current due to magnetic flux offset. Of course, a method of changing the reference voltage itself so that the transformer inrush current does not occur can be considered, but it is not desirable in terms of increasing voltage fluctuations in the local load.

According to the illustrated flowchart, the control unit of the inverter device,

Equations 4 and B _e described above areIf Be is greater among the values of Be and Bk obtained according to the relationship B _s -B _r , the sub-inverter is controlled to immediately follow the power of the grid, and if Bk is greater, it is less than the predetermined reference voltage supplied from the battery to the grid. It is controlled to perform the switching operation after being lowered to. Here, Nt: number of transformer turns, A: transformer cross-sectional area, T _delay : time delay, ω _r ,: nominal frequency, ω _d : natural resonance frequency, v _r : nominal voltage, α: time constant, B _s : saturation magnetic flux density of the transformer , B _r : It is the magnetic flux density of the transformer at the nominal voltage.

The sub-inverter estimates the phase of the microgrid voltage in the standby state, and when receiving a stop signal from the main inverter, it has a fixed reference phase from the estimated phase before the _{time delay (T delay).}

9 is a block diagram of a control algorithm of a sub-inverter for performing the sub-inverter starting method of FIG. 8. In the drawing, it can be seen that the voltage controller feedback controls only the magnitude of the voltage, and the phase calculates only the start phase and generates the reference phase as it is.

10 shows a test configuration for verifying the proposed LCPL filter (L+C||L filter) design method and start control method.

The test was configured with a 480kW DC power supply, two 100kW three-phase inverters, and a 100kW load device. The stop signal transmission of the main inverter used hard wiring and a circuit was constructed with a relay. Time delay (T _delay ) occurs within about 5ms. Here, the inverter parameters are shown in Table 1 and Table 2 below, and the LCPL filter (L+C||L filter) parameters as test conditions are shown in Table 3 below.

[Table 2]

[Table 3]

The test conditions depend on the load power and the stop point of the main inverter. For each test condition, a test was conducted in which the PWM of the main inverter was stopped and the sub inverter controlled the voltage. The test results were divided into seamless transfer and soft start. Seamless transfer is a case where the voltage is immediately followed by a reference voltage, and soft start is a method of gradually recovering the voltage within several tens of ms after the microgrid voltage decreases below the reference voltage close to zero. Means. The experimental conditions according to the load, the stop position of the system and the voltage control method are shown in Table 4 below.

[Table 4]

11A to 11C show the results of experiments performed under

test conditions

1, 2, and 3 of Table 4 above. When the PWM of the main inverter is stopped, the line voltage of the three-phase load shows an under-attenuation characteristic. 11A is a case where the main inverter is stopped at a phase 0 degree of Vab, and it can be seen that the sub-inverter operates after about 5 ms and quickly recovers to the rated voltage without a power outage.

11B and 11C show similar characteristics as the case where the main inverter is stopped at 45 degrees and 90 degrees of Vab.

12 is displayed together to compare the respective Vab waveforms, which are the results of experiments performed under

test conditions

1, 2, and 3, and the rated voltage without power failure within 1 cycle from the time when the PWM of the main inverter is stopped. Show recovery.

13 corresponds to the experimental condition 4 in Table 4 above. It can be seen that the voltage decrease is less than that of the previous test at the 30kW load and recovers to the rated voltage more quickly.

14 corresponds to the experimental condition 5 in Table 4 above. When tested on an 80kW load, the LCPL filter (L+C||L filter) is designed to recover directly to the rated voltage without transformer inrush current only up to about 30kW load, so it is started with a soft start method. It can be seen that the sub-inverter reaches the rated voltage within tens of ms by starting operation after the load voltage is reduced to a certain voltage.

Through the test for five conditions, when the main inverter is stopped, the LCPL filter (L+C||L filter) designed by the sub inverter and the rated voltage are immediately followed without power failure under load conditions, and in other conditions, it is started with a soft start method. Confirmed.

It should be noted that the above-described embodiments are for the purpose of explanation and not limitation. In addition, those of ordinary skill in the technical field of the present invention will understand that various embodiments are possible within the scope of the technical idea of the present invention.

* Explanation of the sign

20: generator 60: energy storage means

100, 100-1: main inverter block 110: first transformer means

120: main inverter 142: first reactor

144: first capacitor 146: third reactor

200, 200-1: sub-inverter block 210: second transformer means

220: sub inverter 242: second reactor

244: second capacitor 246: fourth reactor

The present invention relates to an independent microgrid system and an inverter device that can be applied to remote areas such as island areas, and can be used in the field of microgrid systems.

Claims

In an independent microgrid system that is always or selectively separated from the upper system and operates independently from the local system,

A generator that supplies AC power to the local system and supplies the generated power to the local system via a transformer for insulation without an inverter;

Energy storage means for providing insufficient power to the local system and storing remaining power;

A main inverter block for supplying the power stored in the energy storage means to the local grid as AC power through a first transformer means in a normal state; And

When the main inverter block is stopped, a sub-inverter block for supplying the power stored in the energy storage means to the local grid as AC power through a second transformer means

Independent microgrid system comprising a.
The method of claim 1, wherein the main inverter block,

A main inverter converting DC power output from the energy storage means into AC power;

A first reactor connected between the output terminal of the main inverter and the input terminal of the first transformer means; And

And a first capacitor connected to one end of the first reactor and a neutral point or a ground point to form an LC filter configuration with the first reactor,

The sub inverter block,

A sub-inverter converting DC power output from the energy storage means into AC power;

A second reactor connected between the sub-inverter output terminal and the input terminal of the first transformer means; And

A second capacitor connected to one end of the second reactor and a neutral point or a ground point to form an LC filter configuration with the second reactor

Independent microgrid system comprising a.
The method of claim 2,

The main inverter block further includes a third reactor connected in parallel with the first capacitor,

The sub-inverter block further comprises a fourth reactor connected in parallel with the second capacitor.
The method of claim 1,

The generator supplies unchanged AC power to the local system,

Renewable energy generation means that produces electric power from renewable energy such as solar or wind power and supplies it to the local system

Independent microgrid system further comprising a.
In an inverter device that converts DC power stored in a battery into AC power and supplies it to a grid,

A main inverter block for supplying the converted AC power to the grid through the first transformer means in a normal state;

A sub-inverter block for supplying AC power converted through a second transformer means to a grid when the main inverter block is stopped; And

A control unit that controls the power conversion operation of the main inverter block and the sub inverter block

Inverter device comprising a.
The method of claim 5,

The main inverter block,

A main inverter converting DC power output from the battery into AC power;

A first reactor connected between the output terminal of the main inverter and the input terminal of the first transformer means; And

And a first capacitor connected to one end of the first reactor and a neutral point or a ground point to form an LC filter configuration with the first reactor,

The sub inverter block,

A sub-inverter converting DC power output from the battery into AC power;

A second reactor connected between the sub-inverter output terminal and the input terminal of the first transformer means; And

A second capacitor connected to one end of the second reactor and a neutral point or a ground point to form an LC filter configuration with the second reactor

Inverter device comprising a.
The method of claim 6,

The main inverter block further includes a third reactor connected in parallel with the first capacitor,

The sub-inverter block further comprises a fourth reactor connected in parallel with the second capacitor.
The method of claim 5,

The control unit,

When it is determined that an abnormality has occurred in the main inverter block, stopping the switching operation of the main inverter and starting the switching of the sub inverter,

According to the magnetic flux offset defined according to the difference between the reference voltage and the actually changed voltage, the sub-inverter is controlled to immediately follow the power of the grid, or the switching operation is lowered to less than a predetermined reference voltage supplied from the battery to the grid. Inverter device that controls to perform.
The method of claim 5,

The control unit,

If Be is larger among the Be value and Bk value obtained according to the following equation, the sub-inverter is controlled to immediately follow the power of the system,

When Bk is greater, the inverter device controls to perform a switching operation after being lowered to less than a predetermined reference voltage supplied from the battery to the grid.

B e = B s -B r

(Where, Nt: number of transformer turns, A: transformer cross-section, T delay : time delay,

ω r ,: nominal frequency, ω d : natural resonance frequency, v r : nominal voltage,

α: time constant, B s : saturation magnetic flux density of transformer, B r : Transformer magnetic flux density at nominal voltage)