WO2022198764A1 - Control method and system for offshore wind power grid-connected system - Google Patents

Abstract

Disclosed are a control method and system for the inertia of an offshore wind power grid-connected system. The control method comprises: when a fault occurs in an AC power grid at a receiving end, on the basis of the frequency of the AC power grid at the receiving end, determining a DC side voltage reference value of a grid side converter of a VSC-HVDC system in an offshore wind power grid-connected system; using double closed-loop control technology of the grid side converter to control the DC side voltage of the grid side converter to the DC side voltage reference value of the grid side converter; on the basis of the DC side voltage of a wind power farm side converter of the VSC-HVDC system, determining an AC side frequency reference value of the wind power farm side converter, wherein the DC side voltage of the wind power farm side converter is equal to the DC side voltage of the grid side converter; using double closed-loop control technology of the wind power farm side converter to control the AC side frequency of the wind power farm side converter to the AC side frequency reference value of the wind power farm side converter; and using the controlled AC side frequency of the wind power farm side converter to control the active power output of a wind power farm in the offshore wind power grid-connected system.

Description

Control method and system for offshore wind power grid-connected system

This application claims the priority of the Chinese patent application with application number 202110312502.8 filed with the China Patent Office on March 24, 2021, the entire contents of which are incorporated herein by reference.

technical field

The present application relates to the technical field of offshore wind power generation, for example, to a control method and system for an offshore wind power grid-connected system.

Background technique

With the rapid development of new energy power generation, the installed capacity of new energy, including wind power and photovoltaics, has occupied a considerable proportion in the power system.

In large-scale offshore wind power grid-connected systems, there are a large number of new energy grid-connected converters. These power electronic devices are static equipment and lack the moment of inertia similar to traditional synchronous generators; in addition, flexible DC transmission (voltage source commutation) VSC-HVDC (Voltage Source Converter-High Voltage Direct Current, VSC-HVDC) is a typical grid connection method for large-scale offshore wind power. VSC-HVDC decouples the offshore wind farm and the receiving AC power grid, making the offshore wind farm The rotational kinetic energy of the DFIG wind turbine is hidden, and the offshore wind farm and the AC power grid at the receiving end cannot provide mutual support. The above factors lead to insufficient mechanical moment of inertia in the offshore wind power grid-connected system, that is, the offshore wind farm cannot obtain the frequency change information of the receiving AC power grid in real time, and thus cannot provide inertia support for the receiving AC power grid.

With the continuous increase of the proportion of offshore wind power grid-connected, insufficient mechanical rotational inertia will make it difficult for the offshore wind power grid-connected system to cope with the frequency offset problem caused by load fluctuations of the AC power grid at the receiving end and system faults, which will seriously affect the offshore wind power grid-connected system and the receiving end. Safe and stable operation of AC power grid.

In the traditional control strategy, the frequency deviation information of the AC power grid at the receiving end is often transmitted to the offshore wind farm by means of remote communication, and the inertial support is realized by using the rotational kinetic energy of the wind turbine in the offshore wind farm. This method relies on long-distance communication, which not only increases equipment investment, but also has problems such as delay and poor reliability.

Therefore, it is necessary to study the control method of the offshore wind power grid-connected system, so that the offshore wind power grid-connected system has the function of inertia support and primary frequency modulation similar to the traditional synchronous motor.

SUMMARY OF THE INVENTION

The present application provides a control method and system for an offshore wind power grid-connected system.

Provided is a control method for an offshore wind power grid-connected system, wherein the offshore wind power grid-connected system adopts a flexible direct current transmission technology for grid connection, and the method includes:

In the case of failure of the AC power grid at the receiving end, based on the frequency of the AC power grid at the receiving end, determining the reference value of the DC side voltage of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system;

By adopting the double closed-loop control technology of the grid-side converter, the DC-side voltage of the grid-side converter is controlled to be the reference value of the DC-side voltage of the grid-side converter;

Based on the DC side voltage of the wind farm side converter of the VSC-HVDC system, the AC side frequency reference value of the wind farm side converter is determined; wherein, the DC side voltage of the wind farm side converter is equal to the DC side voltage of the grid-side converter;

Using the double closed-loop control technology of the wind farm side converter, the AC side frequency of the wind farm side converter is controlled to be the AC side frequency reference value of the wind farm side converter;

The active power output of the wind farm in the offshore wind power grid-connected system is controlled by using the controlled AC side frequency of the wind farm side converter.

Also provided is a control system for an offshore wind power grid-connected system, wherein the offshore wind power grid-connected system adopts flexible direct current transmission technology for grid connection, and the system includes:

a first determining module, configured to determine the DC side of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system based on the frequency of the receiving-end AC power grid in the event of a failure of the receiving-end AC power grid voltage reference;

a first control module, configured to adopt the double closed-loop control technology of the grid-side converter, to control the DC-side voltage of the grid-side converter to be a reference value of the DC-side voltage of the grid-side converter;

The second determination module is configured to determine the AC side frequency reference value of the wind farm side converter based on the DC side voltage of the wind farm side converter of the VSC-HVDC system; wherein, the wind farm side converter The DC side voltage of the converter is equal to the DC side voltage of the grid side converter;

The second control module is configured to adopt the double closed-loop control technology of the wind farm side converter, and control the AC side frequency of the wind farm side converter as the reference frequency of the AC side of the wind farm side converter value;

The third control module is configured to use the controlled AC side frequency of the wind farm side converter to control the active power output of the wind farm in the offshore wind power grid-connected system.

Description of drawings

1 is a flowchart of a control method for enhancing the inertia response capability of an offshore wind power grid-connected system provided by an embodiment of the present application;

2 is a structural diagram of a control system for enhancing the inertia response capability of an offshore wind power grid-connected system provided by an embodiment of the present application;

3 is a block diagram of a control strategy of an offshore wind power grid-connected system provided by an embodiment of the present application;

4 is a schematic diagram of a double closed-loop control of a grid-side converter of a VSC-HVDC in an offshore wind power grid-connected system provided by an embodiment of the present application;

FIG. 5 is a schematic diagram of a double closed-loop control of a wind farm side converter of a VSC-HVDC in an offshore wind power grid-connected system provided by an embodiment of the present application.

Detailed ways

The technical solutions in the embodiments of the present application will be described below with reference to the accompanying drawings in the embodiments of the present application.

Example 1:

The present application provides a control method for enhancing the inertia response capability of an offshore wind power grid-connected system, where the offshore wind power grid-connected system adopts flexible DC power transmission technology for grid connection, as shown in FIG. 1 , including:

Step 101 , when the receiving end AC power grid fails, determine the DC side voltage reference value of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system based on the receiving end AC power grid frequency.

Step 102 , using the double closed-loop control technology of the grid-side converter, to control the DC-side voltage of the grid-side converter to be the reference value of the DC-side voltage of the grid-side converter.

Step 103 , based on the DC side voltage of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system, determine the AC side frequency reference value of the wind farm side converter.

Step 104 , using the double closed-loop control technology of the wind farm side converter to control the AC side frequency of the wind farm side converter to be the AC side frequency reference value of the wind farm side converter.

Step 105, using the AC side frequency of the wind farm side converter to control the active power output of the wind farm in the offshore wind power grid-connected system.

In this embodiment, the DC side voltage of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid connected system is equal to the DC side voltage of the grid side converter of the VSC-HVDC system in the offshore wind power grid connected system.

In the step 101, the calculation formula of the DC-side voltage reference value of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system is as follows:

In the formula,

is the DC side voltage reference value of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system, T _GSVSC is the inertia time constant of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system, S _GSVSC is the rated capacity of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system, C _GSVSC is the DC-side equivalent capacitance of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system, and f _N is The rated frequency of the receiving side AC grid, f is the receiving side AC grid frequency, U _{GSVSC, dcN} is the DC side voltage rating of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system.

In the step 102, the calculation formula of the AC side frequency reference value of the wind farm side converter is as follows:

In the formula,

is the AC side frequency reference value of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system, f _WF0 is the AC side initial frequency of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system , C _WFVSC is the DC side equivalent capacitance of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system, T _WFVSC is the inertia of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system Time constant, S _WFVSC is the rated capacity of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system, f _N is the rated frequency of the AC power grid at the receiving end, U _{WFVSC, dcN} is the VSC in the offshore wind power grid-connected system - The DC side voltage rating of the wind farm side converter of the HVDC system, ΔU _WFVSC,dc is the DC side voltage deviation of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system; the ΔU _WFVSC, The calculation formula of _dc is as follows:

ΔU _WFVSC,dc =U _dc -U _WFVSC,dcN

In the formula, U _dc is the DC side voltage of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system.

The step 103 includes:

Step 103-1, based on the AC side frequency of the wind farm side converter, determine the active power adjustment amount with frequency change information corresponding to the wind farm in the offshore wind power grid-connected system.

Step 103-2, control the active power output of the wind farm in the offshore wind power grid-connected system to be the active power adjustment amount with frequency change information corresponding to the wind farm and the original maximum power point tracking (Maximum Power Point Tracking, MPPT) of the wind farm Summation of active reference values under control.

In step 103-1, the calculation formula of the active power adjustment amount with frequency change information corresponding to the wind farm in the offshore wind power grid-connected system is as follows:

In the formula, _Pad is the active power adjustment with frequency change information corresponding to the wind farm in the offshore wind power grid-connected system, K _I is the proportional coefficient in the PID controller, Δf _WF is the VSC-HVDC system in the offshore wind power grid-connected system is the AC side frequency deviation of the wind farm side converter, K _D is the integral coefficient in the PID controller, f _WF is the AC side frequency of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system; where , the calculation formula of Δf _WF is as follows:

Δf _WF =f _WF -f _WF0

Among them, f _WF0 is the initial frequency of the AC side of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system.

In the solution provided by the embodiments of the present application, in the event of a fault in the AC power grid at the receiving end, the grid-side converter adopts inertia control to couple the frequency change information of the AC power grid at the receiving end with the DC voltage, and at the same time the capacitor absorbs/releases its own energy to stabilize When the system frequency changes, the inverter on the wind farm side adopts frequency control, and the DC voltage change information is proportionally reflected as the frequency change on the wind farm side. Active power output, in response to system frequency changes, improves the inertia support ability of the offshore wind power grid-connected system to the AC power grid at the receiving end.

Example 2:

The present application provides a control system for enhancing the inertia response capability of an offshore wind power grid-connected system. The offshore wind power grid-connected system adopts flexible DC power transmission technology for grid connection. As shown in FIG. 2 , the system includes:

The first determining module is configured to determine the DC side voltage reference value of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system based on the frequency of the receiving side AC grid when the receiving end AC power grid fails; the first control The module is set to adopt the double closed-loop control technology of the grid-side converter to control the DC-side voltage of the grid-side converter to be the reference value of the DC-side voltage of the grid-side converter; the second determination module is set to Based on the DC side voltage of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system, the AC side frequency reference value of the wind farm side converter is determined; the second control module is set to adopt the wind farm side frequency reference value. The double closed-loop control technology of the converter controls the AC side frequency of the wind farm side converter to be the AC side frequency reference value of the wind farm side converter; the third control module is set to use the wind farm side inverter. The AC side frequency of the side converter controls the active power output of the wind farm in the offshore wind power grid-connected system; among them, the DC side voltage of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system is equal to the offshore wind power grid-connected. The DC side voltage of the grid-side converter of the VSC-HVDC system in the system.

The calculation formula of the DC side voltage reference value of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system is as follows:

In the formula,

is the DC side voltage reference value of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system, T _GSVSC is the inertia time constant of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system, S _GSVSC is the rated capacity of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system, C _GSVSC is the DC-side equivalent capacitance of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system, and f _N is The rated frequency of the receiving side AC grid, f is the receiving side AC grid frequency, U _{GSVSC, dcN} is the DC side voltage rating of the grid-side converter of the VSC-HVDC system in the offshore wind power grid-connected system.

The calculation formula of the AC side frequency reference value of the wind farm side converter is as follows:

In the formula,

is the AC side frequency reference value of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system, f _WF0 is the AC side initial frequency of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system , C _WFVSC is the DC side equivalent capacitance of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system, T _WFVSC is the inertia of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system Time constant, S _WFVSC is the rated capacity of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system, f _N is the rated frequency of the AC power grid at the receiving end, U _{WFVSC, dcN} is the VSC in the offshore wind power grid-connected system - The DC side voltage rating of the wind farm side converter of the HVDC system, ΔU _WFVSC,dc is the DC side voltage deviation of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system; the ΔU _WFVSC, The calculation formula of _dc is as follows:

ΔU _WFVSC,dc =U _dc -U _WFVSC,dcN

In the formula, U _dc is the DC side voltage of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system.

The third control module includes:

The first determining unit is configured to determine, based on the AC side frequency of the wind farm side converter, the active power adjustment amount with frequency change information corresponding to the wind farm in the offshore wind power grid-connected system; the control unit is configured to control the offshore wind power The active power output of the wind farm in the grid-connected system is the sum of the active power adjustment value with frequency change information corresponding to the wind farm and the active power reference value of the wind farm under the original MPPT control.

The calculation formula of the active power adjustment amount with frequency change information corresponding to the wind farm in the offshore wind power grid-connected system is as follows:

In the formula, _Pad is the active power adjustment with frequency change information corresponding to the wind farm in the offshore wind power grid-connected system, K _I is the proportional coefficient in the PID controller, Δf _WF is the VSC-HVDC system in the offshore wind power grid-connected system is the AC side frequency deviation of the wind farm side converter, K _D is the integral coefficient in the PID controller, f _WF is the AC side frequency of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system; where , the calculation formula of Δf _WF is as follows:

Δf _WF =f _WF -f _WF0

Among them, f _WF0 is the initial frequency of the AC side of the wind farm side converter of the VSC-HVDC system in the offshore wind power grid-connected system.

Example 3:

An offshore wind power grid-connected system VSC-HVDC wind farm side voltage source converter (Wind Farm Side Voltage Source Converter, WFVSC) (referred to as wind farm side converter) DC side voltage and the offshore wind power grid-connected system The DC side voltage of the grid side voltage source converter (Grid Side Voltage Source Converter, GSVSC) (referred to as the grid side converter) of the VSC-HVDC in the middle is equal, and the DC side voltage rating of WFVSC and the DC side voltage rating of GSVSC are rated If the values are equal, the same specifications are used for WFVSC and GSVSC, that is, the inertia time constants corresponding to WFVSC and GSVSC are the same, the rated capacity for WFVSC and GSVSC is the same, and the equivalent capacitance for the DC side is the same for WFVSC and GSVSC. In order to solve the problem of insufficient inertia response capability of the offshore wind power grid-connected system, and it is difficult to deal with the frequency offset problem caused by the load fluctuation of the AC power grid at the receiving end and system failure, the present application proposes a control method for enhancing the inertia response capability of the offshore wind power grid-connected system, This method focuses on coordinating and controlling the power of grid-side converters, wind farm-side converters and wind turbines to enhance the inertia response capability of the offshore wind power grid-connected system. The characteristics of the voltage, the frequency change information of the AC power grid at the receiving end is converted into the change of the DC voltage through the GSVSC. At the same time, the wind farm side converter (WFVSC) adopts frequency conversion control, which transmits the frequency change information on the AC grid side of the receiving end to the offshore wind farm side, and realizes the phase-locked loop tracking of the grid frequency. Finally, the kinetic energy release of the wind turbine is controlled to realize the inertia support for the AC power grid at the receiving end. In this control mode, the inertia of the offshore wind farm and the VSC-HVDC are mobilized at the same time, and the inertia support capability of the AC power grid at the receiving end is enhanced.

Because the whole control method includes the coordinated control of the grid-side converter (GSVSC), the wind farm-side converter (WFVSC) and the wind turbine, the energy of the DC capacitor and the rotor kinetic energy of the doubly-fed wind turbine partially compensate for the inconsistency of the grid frequency fluctuation. Therefore, the provided control method mainly includes three parts: (1) grid side converter (GSVSC) DC capacitive inertial support control; (2) wind farm side converter ( WFVSC) frequency conversion control; (3) wind turbine power control.

(1) Grid-side converter (GSVSC) DC capacitive inertial support control.

In the case of ignoring damping, the rotational kinetic energy E _r of the rotor of the synchronous generator can be expressed as:

Where: J is the moment of inertia of the synchronous generator, ω _m is the electrical angular velocity of the synchronous generator.

It can be seen from (1) that the traditional synchronous generator converts the rotor kinetic energy into active power output by adjusting the rotor speed, which plays the role of adjusting the active power balance and smoothing the frequency fluctuation. The speed adjustment range of the synchronous generator determines its dynamic response capability to grid frequency changes.

The inertia time constant T _J of the synchronous generator can be expressed as:

In the formula: Ω _N is the rated mechanical angular speed of the synchronous generator, and S _N is the rated power of the synchronous generator.

The inertia response characteristics of the synchronous generator can be expressed as:

where P _m is the mechanical power of the synchronous generator, _Pe is the electromagnetic power of the synchronous generator, and Ω is the mechanical angular velocity of the synchronous generator.

Suppose the number of pole pairs of the rotor of the synchronous generator is p. According to the theory of electromechanics, the mechanical angular velocity Ω of the synchronous generator is related to the electrical angular velocity ω _m , and the rated mechanical angular velocity Ω _N of the synchronous motor is related to the rated electrical angular velocity ω _mN as follows:

Formula (3) can be expressed as:

In the formula: f is the grid frequency, f _N grid frequency rating.

The DC capacitance of the converter is a key factor affecting the stability of the DC side voltage. The time constant τ _C of the DC capacitance of the converter is:

In the formula: C is the equivalent capacitance of the DC side of the converter, U _dc is the DC side voltage of the converter, and S _VSC is the rated capacity of the converter. The converters described here include grid-side converters (GSVSC) and wind farm-side converters (WFVSC). side voltage fluctuations. The dynamic characteristics of DC voltage can be expressed by equation (7):

In the formula: P _WF is the active power output by WFVSC; P _GS is the active power connected to the grid by GSVSC.

From the analogy between equation (2) and equation (6), it can be seen that the inertia time constant of the synchronous generator and the equivalent time constant of the DC side capacitance of the converter have similar properties. Among them, the DC voltage U _dc and the mechanical angular velocity of the synchronous motor rotor Ω _N corresponds. Comparing Equation (5) with Equation (7), it can be seen that the inertia response characteristics of the synchronous generator are similar to the dynamic characteristics of the DC voltage, both of which play a buffering role in the impact of unbalanced power.

Therefore, the change of the DC voltage can be analogized to the change of the speed of the synchronous generator, and the energy storage of the DC voltage can be equivalent to the mechanical energy of the synchronous generator, that is, the power released by the change of the DC voltage can be equivalent to the change of the kinetic energy of the rotor of the synchronous generator. Therefore, the charging and discharging power can be quantified by changing the voltage of the DC capacitor, so that the DC side can provide a virtual VSC inertia.

Let the DC capacitor power change equal to the synchronous motor power change, the combination of formulas (5) and (7) has:

Where: T _VSC is the inertia time constant of the converter.

Integrate both sides of Equation (8) to get:

Where: U _dcN is the DC voltage rating.

From equation (10), the GSVSC analog inertia control algorithm can be obtained as:

In Equation 11, U _dc is the DC voltage controlled by the grid-side converter (GSVSC) DC capacitive inertial support. In order to better distinguish between the DC voltage and the current DC voltage, for subsequent use

represents the DC voltage controlled by the grid-side converter (GSVSC) DC capacitive inertial support, and U _dc represents the current DC voltage, that is, Equation 11 is rewritten as:

Generally, the system frequency deviation should not exceed 1% of the rated value, that is, it should be within ±0.5Hz, and the DC voltage fluctuation should not exceed 5% of the rated voltage. The DC voltage only fluctuates in a small range, so the first-order Taylor expansion of Equation (11) at U _dcN can be used to obtain:

Equation (12) constructs the relationship between power grid frequency fluctuation and DC voltage change, which can be obtained:

Wherein, Δf=ff _N , ΔU _dc =U _dc -U _dcN .

It can be seen from the above analysis that when the grid frequency deviates from the rated value, the DC voltage will fluctuate due to the change of the GSVSC transmission power. From equation (12), it can be seen that the increase of the grid frequency will lead to the increase of the DC voltage. Therefore, the grid-side frequency change information is proportional to It is reflected as DC voltage change information. At the same time, the larger the DC equivalent capacitance is, the higher the T _VSC is, the smaller the fluctuation of the DC voltage under the same frequency change is, and the stronger the anti-interference ability of the system frequency to the active power mutation is.

Using the coupling relationship between DC voltage and grid frequency, the grid side frequency information is converted into DC voltage changes, and at the same time, the electromagnetic energy in the DC side is used to simulate it as the mechanical kinetic energy of the synchronous machine rotor, which realizes the inertia support effect on the grid. , the control strategy is shown in Figure 3. Among them, the adjustment of the AC power grid voltage at the receiving end can be realized by changing the reactive power setting value.

(2) Frequency conversion control of wind farm side converter (WFVSC).

In the traditional method, the frequency of the power grid is generally transmitted to the converter station on the wind farm side by means of Supervisory Control and Data Acquisition (SCADA) communication, which increases the cost and has poor reliability, and is not suitable for offshore wind farms. , so it is necessary to transmit the frequency information from the grid side to the wind farm side through the WFVSC.

In the flexible DC transmission system of offshore wind power grid-connected, the WFVSC controls the AC frequency of the wind farm. This method utilizes the feature that the WFVSC can work in the frequency conversion mode, and manually couples the AC frequency at both ends by tracking the DC voltage change, so that the wind farm can perceive the affected frequency. The frequency of the terminal AC power grid changes, and its own output is adjusted to balance the active power of the system.

As mentioned above, when the frequency of the AC power grid at the receiving end changes, in order to quickly provide inertia support, the GSVSC first adjusts the DC voltage value, and uses the energy stored in the DC capacitor to suppress frequency fluctuations, while the WFVSC adjusts the wind farm side AC according to the change of the DC voltage. frequency, so that the wind farm senses the frequency change of the AC power grid at the receiving end to adjust its own output.

Applying the inverse process of frequency coupling control of GSVSC to WFVSC to make it track the change of DC voltage, it can be obtained from equation (12):

WFVSC frequency control can be expressed as:

where:

is the reference frequency of the wind farm side, and f _WF0 is the initial frequency of the wind farm side. WFVSC controls the frequency of the wind farm side, and should be controlled based on the initial frequency of the wind farm side (the frequency before the start of the variable frequency control), and the control strategy is shown in Figure 3.

(3) Wind turbine power control.

In the normal operation control mode, the doubly-fed wind turbine generally operates in the MPPT mode. The double-fed wind turbine can change speed within the range of ±30% of its rated speed, and can provide a large inertia support in a short time, even exceeding its inherent rotational inertia. Therefore, the rotor kinetic energy can be changed by adjusting the speed to respond to system frequency changes. . If the rotational speed of the doubly-fed fan changes from ω _r to ω _r +Δω _r , the rotor kinetic energy variable ΔE _K can be expressed as:

In the formula: Δω _r is the variation of the rotor speed of the doubly-fed wind turbine; J _DFIG is the inherent moment of inertia of the doubly-fed wind turbine.

When the speed of the doubly-fed fan changes, the change of the effective kinetic energy of the wind turbine includes two parts: the change of the actual kinetic energy of the fan rotor and the change of the wind power capture.

ΔE _K =ΔE _P +ΔE _D (17)

In the formula: _ΔEP is the change in the kinetic energy of the fan rotor; ΔE _D is the change in the wind power capture of the fan.

However, due to the decoupling of the converter, the wind turbine cannot directly obtain the frequency change information of the AC power grid at the receiving end, and thus cannot control the rotor to absorb or release the stored kinetic energy to smooth the frequency deviation. Therefore, in order to realize the inertia support for the AC power grid at the receiving end, it is necessary to couple the output of the wind farm with the frequency change of the AC power grid at the receiving end. On the active power reference value P _ref under the original MPPT control of the wind turbine, a power variable P _ad with frequency change information is added to become a new active power reference value

The action time of the converter is much smaller than the inertia response time of the mechanical system, so it can be considered that

That is, the actual output power P _WF of the wind turbine.

Using Proportion Integral Differential (PID) control to design the virtual inertia controller of wind turbine, K _I Δf _WF proportional controller realizes inertia control of DFIG,

The differential controller realizes the electromagnetic power control function, and _Pad can be expressed as:

In the formula: f _WF is the AC side frequency of the wind farm side converter, Δf _WF = f _WF -f _WF0 is the AC side frequency deviation of the wind farm side converter, f _WF0 is the AC side frequency of the wind farm side converter Initial frequency, K _I is the proportional coefficient of the PID controller; K _D is the differential coefficient of the PID controller.

new active reference value

It can be seen from the above analysis that the wind power flexible DC grid-connected system can be equivalently regarded as a traditional synchronous machine with the total inertia support capacity of the wind turbine and the DC capacitor for the receiving system. Under the coordinated control strategy provided by the present application, the inertia response capability of the flexible DC system and the wind farm is mobilized, which is stronger than the capability of using a single system for inertia support. In addition, it can be seen that the inertia support capacity of the wind farm is determined by the speed regulation range of the fan speed. The greater the speed deviation of the fan speed, the stronger the support capacity. The integrated GSVSC inertia control, WFVSC frequency control and wind farm active power adjustment constitute a coordinated control method with frequency modulation and inertia support for the AC power grid at the receiving end, as shown in Figure 3, U _WF in Figure 3 is the wind farm side AC The actual value of the voltage, U _WFN is the rated value of the AC voltage on the wind farm side, and the Phase Locked Loop (PLL) is used to control the kinetic energy change of the wind turbine by adjusting the rotor speed of the wind turbine, thereby adjusting the output power change of the wind turbine. . If the frequency of the AC power grid at the receiving end increases, the reference value of the DC side voltage will be increased, and the converter station on the wind farm side will transmit the increased DC voltage to the wind farm to reduce the output and reduce the amplitude and rate of grid frequency fluctuations.

Aiming at the lack of inertia caused by large-scale offshore wind power connection to the power system, this application proposes a control method for coupling the frequency of the grid with the frequency of the wind farm by using the DC voltage, and coordinating the DC side capacitive energy storage of the flexible DC transmission system and the rotor kinetic energy of the doubly-fed wind turbine. , the whole system is equivalent to the AC power grid at the receiving end as a synchronous generator with both the DC system and the inertia of the wind turbine, which improves the inertia support capability of the AC power grid at the receiving end.

In the grid-side converter (GSVSC) DC capacitive inertial support control stage, the control methods are as follows:

First: Substitute f and f ₀ into equation (11) to get

Then: use the double closed-loop control technology of the grid-side converter to control the DC-side voltage of the grid-side converter to be

Among them, the double closed-loop control schematic diagram of the grid-side converter is shown in Figure 4. In Figure 4:

Compare the measured value U _dc of the DC side voltage of the grid-side converter (GSVSC) and the reference value of the DC side voltage of the grid-side converter (GSVSC)

Substitute into the PI controller to obtain the reference value i _dref of the d-axis current on the AC side of the grid-side converter (GSVSC) _. ) The AC side d-axis current component id is substituted into the PI controller to obtain the grid-side converter (GSVSC) AC-side _d -axis voltage reference value U _dref , based on U _dref , the grid-side converter (GSVSC) AC side q-axis current The component i _q , the d-axis voltage component U _td of the AC side of the grid-side converter (GSVSC), the inductance value of the grid-side converter (GSVSC) and the frequency of the AC grid at the receiving end determine the AC side d of the grid-side converter (GSVSC) Shaft modulation voltage U _d ; substitute the measured reactive power value Q of the grid-side converter (GSVSC) and the reactive power reference value Qref output by the grid-side converter ( _GSVSC ) into the PI controller to obtain grid-side commutation Substitute the reference value i _qref of the q-axis current on the AC side of the grid-side converter ( _GSVSC ) and the q-axis current component i _q on the AC side of the grid-side converter (GSVSC) into The PI controller obtains the reference value U _qref of the AC side q-axis voltage of the grid-side converter (GSVSC), based on U _qref , the grid-side converter (GSVSC) AC side _d -axis current component id , the grid-side converter ( GSVSC) AC side q-axis voltage component U _tq and the inductance value of the grid-side converter (GSVSC) determine the q-axis modulation voltage U _q on the AC side of the grid-side converter ( _GSVSC ); U _q is input to the pulse width modulation (Pulse width modulation, PWM) controller to get the pulse, which controls the insulated gate bipolar transistor (Insulated Gate Bipolar Transistor, IGBT) in the GSVSC to operate, so as to control the DC side voltage of the GSVSC to be

In the variable frequency control stage of the wind farm side converter (WFVSC), the control methods are as follows:

First, calculate the deviation ΔU _dc between the DC side voltages U _dc and U _dcN of the wind farm side converter (WFVSC); secondly, substitute ΔU _dc into equation (15) to get

Finally, the double closed-loop control technology of the wind farm side converter is used to control the AC side frequency of the WFVSC as

The schematic diagram of WFVSC double closed-loop control is shown in Figure 5. In Figure 5, U _WFd is the d-axis component of the measured DC side voltage U _dc of the wind farm side converter (WFVSC), and U _WFq is the wind farm side converter. (WFVSC) is the q-axis component of the measured DC side voltage U _dc , U _WFNd is the d-axis component of the DC side voltage rating U _WFN of the wind farm side converter (WFVSC), U _WFNq is the wind farm side converter (WFVSC) DC side voltage rating U q-axis component of _WFN , i _dref is the d-axis current reference value of the AC side of the wind farm side converter (WFVSC), i _qref is the AC side of the wind farm side converter (WFVSC) The reference value of the q-axis current, i _d is the d-axis current on the AC side of the wind farm side converter (WFVSC), i _q is the q-axis current on the AC side of the wind farm side converter (WFVSC), and L is the wind farm side converter (WFVSC) inductance value.

Embodiments of the present application may be provided as methods, systems, or computer program products. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present application may employ one or more computer-usable storage media (including but not limited to magnetic disk memory, Compact Disc Read-Only Memory, CD-ROM) having computer-usable program code embodied therein , optical storage, etc.) in the form of a computer program product.

The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the present application. It will be understood that each flow and/or block in the flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to the processor of a general purpose computer, special purpose computer, embedded processor or other programmable data processing device to produce a machine such that the instructions executed by the processor of the computer or other programmable data processing device produce Means for implementing the functions specified in a flow or flow of a flowchart and/or a block or blocks of a block diagram.

These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory result in an article of manufacture comprising instruction means, the instructions The apparatus implements the functions specified in the flow or flow of the flowcharts and/or the block or blocks of the block diagrams.

These computer program instructions can also be loaded on a computer or other programmable data processing device to cause a series of operational steps to be performed on the computer or other programmable device to produce a computer-implemented process such that The instructions provide steps for implementing the functions specified in the flow or blocks of the flowcharts and/or the block or blocks of the block diagrams.

Claims (10)

1. A control method for an offshore wind power grid-connected system, wherein the offshore wind power grid-connected system adopts a flexible direct current transmission technology for grid connection, and the method includes:

   In the case of failure of the receiving-end AC power grid, based on the frequency of the receiving-end AC power grid, determine the grid-side converter of the voltage source converter-high voltage direct current transmission VSC-HVDC system in the offshore wind power grid-connected system. DC side voltage reference value;

   By adopting the double closed-loop control technology of the grid-side converter, the DC-side voltage of the grid-side converter is controlled to be the reference value of the DC-side voltage of the grid-side converter;

   Based on the DC side voltage of the wind farm side converter of the VSC-HVDC system, the AC side frequency reference value of the wind farm side converter is determined; wherein, the DC side voltage of the wind farm side converter is equal to the DC side voltage of the grid-side converter;

   Using the double closed-loop control technology of the wind farm side converter, the AC side frequency of the wind farm side converter is controlled to be the AC side frequency reference value of the wind farm side converter;

   The active power output of the wind farm in the offshore wind power grid-connected system is controlled by using the controlled AC side frequency of the wind farm side converter.
2. The method according to claim 1, wherein the calculation formula of the DC side voltage reference value of the grid-side converter is as follows:

   

   in,

   

   is the DC side voltage reference value of the grid-side converter, T _GSVSC is the inertia time constant of the grid-side converter, S _GSVSC is the rated capacity of the grid-side converter, and C _GSVSC is the grid-side converter DC side equivalent capacitance of the side converter, f _N is the rated frequency of the AC power grid at the receiving end, f is the frequency of the AC power grid at the receiving end, U _{GSVSC, dcN} is the DC side of the grid-side converter voltage rating.
3. The method according to claim 1, wherein the calculation formula of the AC side frequency reference value of the wind farm side converter is as follows:

   

   in,

   

   is the AC side frequency reference value of the wind farm side converter, f _WF0 is the AC side initial frequency of the wind farm side converter, C _WFVSC is the DC side equivalent capacitance of the wind farm side converter , T _WFVSC is the inertia time constant of the wind farm side converter, S _WFVSC is the rated capacity of the wind farm side converter, f _N is the rated frequency of the AC power grid at the receiving end, U _WFVSC,dcN is the the DC side voltage rating of the wind farm side converter, ΔU _WFVSC,dc is the DC side voltage deviation of the wind farm side converter;

   The calculation formula of the ΔU _WFVSC,dc is as follows:

   ΔU _WFVSC,dc =U _dc -U _WFVSC,dcN ;

   Wherein, U _dc is the DC side voltage of the wind farm side converter.
4. The method according to claim 1, wherein the controlling the active power output of the wind farm in the offshore wind power grid-connected system using the controlled AC side frequency of the wind farm side converter comprises:

   Based on the controlled AC side frequency of the wind farm side converter, determining an active power adjustment amount corresponding to the wind farm with frequency change information;

   Controlling the active power output of the wind farm is the sum of the active power adjustment amount and the active power reference value of the wind farm under the original maximum power point tracking MPPT control.
5. The method of claim 4, wherein the calculation formula of the active power adjustment amount is as follows:

   

   Among them, P _ad is the active power adjustment amount, K _I is the proportional coefficient in the proportional-integral-derivative PID controller, Δf _WF is the AC side frequency deviation of the wind farm side converter, and K _D is the PID controller The integral coefficient in , f _WF is the AC side frequency of the wind farm side converter;

   Wherein, the calculation formula of the Δf _WF is as follows:

   Δf _WF =f _WF −f _WF0 ;

   Wherein, f _WF0 is the initial frequency of the AC side of the wind farm side converter.
6. A control system for an offshore wind power grid-connected system, the offshore wind power grid-connected system adopts flexible direct current transmission technology for grid connection, and the system includes:

   a first determining module, configured to determine a voltage source converter-high voltage direct current transmission VSC-HVDC system in the offshore wind power grid-connected system based on the frequency of the receiving-end AC power grid in the event of a failure of the receiving-end AC power grid The reference value of the DC side voltage of the grid-side converter;

   a first control module, configured to adopt the double closed-loop control technology of the grid-side converter, to control the DC-side voltage of the grid-side converter to be a reference value of the DC-side voltage of the grid-side converter;

   The second determination module is configured to determine the AC side frequency reference value of the wind farm side converter based on the DC side voltage of the wind farm side converter of the VSC-HVDC system; wherein, the wind farm side converter The DC side voltage of the converter is equal to the DC side voltage of the grid side converter;

   The second control module is configured to adopt the double closed-loop control technology of the wind farm side converter, and control the AC side frequency of the wind farm side converter as the reference frequency of the AC side of the wind farm side converter value;

   The third control module is configured to use the controlled AC side frequency of the wind farm side converter to control the active power output of the wind farm in the offshore wind power grid-connected system.
7. The system according to claim 6, wherein the calculation formula of the DC side voltage reference value of the grid-side converter is as follows:

   

   in,

   

   is the DC side voltage reference value of the grid-side converter, T _GSVSC is the inertia time constant of the grid-side converter, S _GSVSC is the rated capacity of the grid-side converter, and C _GSVSC is the grid-side converter DC side equivalent capacitance of the side converter, f _N is the rated frequency of the AC power grid at the receiving end, f is the frequency of the AC power grid at the receiving end, U _{GSVSC, dcN} is the DC side of the grid-side converter voltage rating.
8. The system according to claim 6, wherein the calculation formula of the AC side frequency reference value of the wind farm side converter is as follows:

   

   in,

   

   is the AC side frequency reference value of the wind farm side converter, f _WF0 is the AC side initial frequency of the wind farm side converter, C _WFVSC is the DC side equivalent capacitance of the wind farm side converter , T _WFVSC is the inertia time constant of the wind farm side converter, S _WFVSC is the rated capacity of the wind farm side converter, f _N is the rated frequency of the AC power grid at the receiving end, U _WFVSC,dcN is The DC side voltage rating of the wind farm side converter, ΔU _WFVSC,dc is the DC side voltage deviation of the wind farm side converter;

   The calculation formula of the ΔU _WFVSC,dc is as follows:

   ΔU _WFVSC,dc =U _dc -U _WFVSC,dcN ;

   Wherein, U _dc is the DC side voltage of the wind farm side converter.
9. The system of claim 6, wherein the third control module comprises:

   a first determining unit, configured to determine an active power adjustment amount with frequency change information corresponding to the wind farm based on the controlled AC side frequency of the wind farm side converter;

   A control unit, configured to control the active power output of the wind farm to be the sum of the active power adjustment amount and the active power reference value of the wind farm under the original MPPT control.
10. The system of claim 9, wherein the calculation formula of the active power adjustment amount is as follows:

    

    Among them, P _ad is the active power adjustment amount, K _I is the proportional coefficient in the proportional-integral-derivative PID controller, Δf _WF is the AC side frequency deviation of the wind farm side converter, and K _D is the PID controller The integral coefficient in , f _WF is the AC side frequency of the wind farm side converter;

    Wherein, the calculation formula of the Δf _WF is as follows:

    Δf _WF =f _WF −f _WF0 ;

    Wherein, f _WF0 is the initial frequency of the AC side of the wind farm side converter.

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