A method for starting a wind power flexible direct current system and connecting a wind farm to the grid based on grid-connected energy storage

Abstract

The present invention discloses a method for starting a wind power flexible direct current system and connecting a wind farm to the grid based on grid-connected energy storage. The flexible direct current transmission system adopts the form of charging the receiving-end grid to charge the submodule capacitors of the receiving-end converter station and the sending-end converter station, establishes the AC side voltage of the sending-end converter station, and uses the grid-connected energy storage station system to realize the self-starting of the wind turbine. After receiving the grid-connected signal, the AC side voltage of the wind turbine is adjusted to meet the grid-connected requirements through pre-synchronization control, and the grid-connected switch valve is closed to complete the starting process of the entire system. The present invention adopts energy storage as an auxiliary equipment of the wind farm to provide stable voltage and frequency support for the wind turbine set. The energy storage adopts pre-synchronization control to achieve seamless switching from off-grid to grid-connected process, avoiding the influence of impact current. The grid-connected energy storage station device and the wind farm can form a microgrid system, which is connected to the distribution network in the form of a microgrid, thereby improving the frequency and voltage stability of the new power system.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and in particular relates to a method for starting a wind power flexible direct current system and connecting a wind farm to the grid based on grid-connected energy storage. Background Art

[0002] Wind power flexible direct current system is the flexible direct current transmission system of wind farm. With the continuous advancement of the construction of new power systems, flexible direct current transmission technology has become an important means to promote the integrated direct current transmission of hydropower, wind power and solar power bundling due to its advantages of fast control regulation performance and large-capacity long-distance power transmission. However, unlike the current startup strategy of flexible direct current transmission system based on active power at both ends, the startup strategy of MMC-HVDC system with wind farm as the sending end can only charge from one end. MMC is modular multilevel converter, HVDC is high voltage direct current transmission, and MMC-HVDC system is high voltage direct current transmission based on modular multilevel converter (Modular Multilevel Converter Based High Voltage Direct Current, MMC-HVDC) system; most of the existing MMC-HVDC startup processes only consider the form where both the sending end and the receiving end are active power sources, and do not consider the startup process when the wind farm, a passive device, is connected. There are few startup methods for wind farm-flexible direct current systems that use MMC as submodules. Before the charging of the flexible direct current system is completed, the wind farm needs to achieve self-starting, but there is currently a process in which DFIG (DFIG, Doubly-fed Induction Generator) wind turbines are connected to a multi-terminal MMC-HVDC system. A simple direct-drive wind turbine set (PMSG) cannot achieve self-starting. The existing technology lacks technology for the startup and grid-connected process of direct-drive wind turbines (PMSG) connected to the MMC-HVDC system; therefore, it is necessary to design a wind power flexible direct current system startup and wind farm grid-connected method based on grid-type energy storage to solve the above problems. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a method for starting a wind power flexible direct current system and connecting a wind farm to the grid based on grid-connected energy storage. In this method, the flexible direct current transmission system adopts the form of charging the receiving-end power grid to charge the sub-module capacitors of the receiving-end converter station and the sending-end converter station, establish the AC side voltage of the sending-end converter station, and use the grid-connected energy storage station to realize the self-starting of the wind turbine. After receiving the grid-connected signal, the AC side voltage of the wind turbine is adjusted to meet the grid-connected requirements through pre-synchronization control, and the grid-connected switch valve is closed to complete the startup process of the entire system.

[0004] In order to achieve the above technical effects, the technical solution adopted by the present invention is:

[0005] A method for starting a wind power flexible direct current system and connecting a wind farm to the grid based on grid-connected energy storage comprises the following steps:

[0006] Construct a wind farm flexible DC transmission system based on a grid-type energy storage station, including a wind farm, a flexible DC transmission system and a grid-type energy storage station;

[0007] Set up a startup charging strategy for the flexible DC transmission system;

[0008] Use grid-type energy storage stations as auxiliary equipment to achieve self-starting of wind farms;

[0009] The grid-connected energy storage station was pre-synchronized and the wind farm was connected to the grid, and the wind farm-flexible DC transmission system was started.

[0010] Preferably, the wind farm flexible DC transmission system based on the grid-type energy storage station includes a receiving-end power grid, the receiving-end power grid is connected to the receiving-end converter station through a cable; the receiving-end converter station is connected to the sending-end converter station through a DC cable; the wind farm and the grid-type energy storage station are connected through a switch K c After parallel connection, it is connected to the sending-end converter station through a cable. A current limiting resistor R2 is set on the line between the receiving-end converter station and the receiving-end power grid, and a switch K is put into operation at the receiving-end converter station. G and bypass switch K connected in parallel with current limiting resistor R2 R The PCC points between the sending-end converter station, wind farm and grid-type energy storage station are equipped with an on-off switch K W .

[0011] Furthermore, an inductor L, a capacitor C and a protective resistor R1 are provided between the wind farm and the grid-type energy storage station.

[0012] Preferably, setting a startup charging strategy for the flexible DC transmission system includes:

[0013] Set the uncontrollable charging strategy for the receiving and sending converter stations:

[0014] In the initial state, the switch K G and K R The converter station submodule is in a locked state; the converter station submodule is connected to the receiving end converter station. N The submodules and 2N submodules of the sending-end converter station;

[0015] t 0 time, closed K G , the current limiting resistor R2 is put into operation, and the receiving-end power grid charges the converter station submodule through the diode uncontrolled rectification; at this time, the receiving-end power grid is applied to the receiving-end converter station N The voltage of each submodule is the peak value of the valve-side line voltage of the AC system , when charging is finished, the receiving end converter stationN Submodule capacitor voltage and the first DC link voltage as follows:

[0016] ;

[0017] ;

[0018] At this time, the sending end converter station passes the DC line to the upper and lower bridge arms, a total of 2 N The sub-modules are charged simultaneously, and the sub-module voltage of the sending-end converter station is the capacitor voltage of the submodule of the receiving converter station Half:

[0019] ;

[0020] Where N is the number of submodules in the receiving-end converter station; the capacitor voltage of the submodule in the receiving-end converter station is is the capacitor voltage of the submodule of the receiving end converter station during uncontrollable charging; The voltage of the submodule of the converter station at the sending end during uncontrollable charging; the voltage of the first DC side It is the DC side voltage during uncontrolled charging.

[0021] Furthermore, during the charging process of the converter, the submodule in the locked state needs to be charged first, and the submodule can be unlocked only when the submodule capacitor voltage reaches the operating voltage of the control system; the charging process at this time is called uncontrollable charging.

[0022] Preferably, the method for starting charging of the flexible direct current transmission system comprises:

[0023] Set the controllable charging strategy of the sending-end converter station:

[0024] The charging process after valve control unlocking introduces a control strategy to charge the capacitor voltage and raise the DC voltage to the rated value. This process is called controlled charging. When the submodule capacitor voltage tends to be stable, the uncontrolled charging stage is completed. t At time 1, the sending-end converter station submodule is unlocked, and passive AC voltage control is started. The AC system charges the capacitor of the sending-end converter station submodule only through the DC line; t 1 moment occurs at t After 0 time;

[0025] After unlocking the submodules of the sending-end converter station, the number of submodules put into operation is halved. When unlocking the sending-end converter station, the second DC side voltage It instantly dropped to 1 / 2 of its original height, and then gradually rose;

[0026] After charging is completed, the capacitor voltage of the submodule at the receiving end converter station remains unchanged, and the capacitor voltage of the submodule at the sending end converter station continues to rise until it is equal to the capacitor voltage of the submodule at the receiving end converter station. The formula for the capacitor voltage of the submodule after charging is:

[0027] ;

[0028] Where, the second DC side voltage is the DC side voltage in the controllable charging stage; The capacitor voltage of the submodule of the receiving-end converter station during the controllable charging stage; The voltage of the submodule of the converter station at the sending end during the controllable charging stage; is the peak value of the valve-side line voltage of the AC system, and N is the number of submodules in the receiving-end converter station.

[0029] Preferably, the method for starting charging of the flexible direct current transmission system comprises:

[0030] Set the controllable charging strategy of the receiving converter station:

[0031] In order to make the DC side of the MMC-HVDC system reach the rated voltage and the submodule capacitor voltage reach the rated value, it is necessary to start the controllable charging at the receiving converter station side after the passive controllable charging;

[0032] t 2. Start active controllable charging at all times and close the bypass switch K R ; t 2 Moments occur at t 1 moment later;

[0033] Unlock the DC voltage control of the receiving converter station with a slope to control the power injected into the converter. The specific method is:

[0034] Set the DC voltage control of the receiving converter station with a slope, and increase or decrease the slope of the control command value within the range of 0.734pu~1pu, so that the capacitance of the MMC submodules at both ends rises to / N After charging is completed, the DC voltage reaches the rated value; the MMC submodule is a modular multi-level converter built into the receiving end converter station.

[0035] Preferably, using a grid-type energy storage station as an auxiliary device to realize the self-starting of a wind farm includes:

[0036] Setting the zero-start boost start control method for grid-type energy storage station:

[0037] Before providing voltage and frequency support for the start-up of direct-drive wind turbines, the grid-type energy storage station must first achieve its own start-up;

[0038] By improving the reactive power loop, the grid-type energy storage station can be started, so that the grid-type energy storage station has the function of zero-starting voltage boost and establishes the voltage on the AC side of the wind turbine. The calculation formula is:

[0039] ;

[0040] ;

[0041] In the formula, is the no-load voltage amplitude of the grid-type energy storage station; E Q is the output of reactive power regulation, k p is the reactive power regulation coefficient, is the reactive power reference value, Q is the actual value of reactive power; yes The slope of the rise, ; is the rated voltage of the grid-type energy storage station; yes From 0 to time; when Rise to After that, the output voltage of the grid-type energy storage station remains unchanged.

[0042] Preferably, using a grid-type energy storage station as an auxiliary device to realize the self-starting of a wind farm includes:

[0043] After setting up the zero-start boost start control method for the grid-type energy storage station, the grid-type energy storage station is used to control the active loop output to have a constant frequency, so as to establish the AC side frequency of the wind turbine; the control strategy is as follows:

[0044] By introducing inertia and damping into the grid-type energy storage station to simulate the characteristics of the synchronous machine:

[0045] ;

[0046] in:

[0047] ;

[0048] ;

[0049] In the formula, T M is the mechanical torque, T E is the electromagnetic torque, D p is the damping coefficient, θ is the power angle, ωis the derivative of the power angle with respect to time, is the rated frequency; is the active power reference value; It is electromagnetic power; the grid-forming energy storage station simulates the active power and frequency and reactive power and voltage droop characteristics of the synchronous generator to achieve primary frequency regulation and primary voltage regulation, and provides stable voltage and frequency to the wind farm. At this point, the zero-voltage starting process of the grid-forming energy storage station is completed.

[0050] Preferably, using a grid-type energy storage station as an auxiliary device to realize the self-starting of a wind farm includes:

[0051] The grid-type energy storage station is used to control the active loop output to have a constant frequency. After the frequency of the wind turbine AC side is established, the energy storage device of the grid-type energy storage station is set to charge the direct-drive wind turbine:

[0052] When the grid-type energy storage station is started, close the switch between the wind farm and the grid-type energy storage station. K , connecting wind farms;

[0053] When the voltage and frequency of the wind farm are stable and the external wind speed meets the starting conditions, the wind turbines are started to realize the conversion of wind energy into electrical energy; the starting strategy is:

[0054] When the external wind speed meets the starting wind speed requirement of the wind turbine, the pitch angle of the wind rotor blades begins to adjust from 90° to 0°, the wind direction and the blade power angle increase, and the wind turbine starts to rotate;

[0055] After the fan is started, if the speed does not reach the cut-in speed, the motor is in a non-working state; when the speed meets the cut-in speed, the motor is connected to the grid and enters the optimal power output stage;

[0056] After the start-up phase, the wind turbine enters the optimal power output phase. The wind turbine is connected to the grid. At this time, the wind speed does not reach the rated wind speed, the output power is lower than the rated power, and the blade pitch angle remains near 0°. At this time, the main task of the wind turbine control system is to adjust the wind turbine speed so that the wind turbine can absorb wind energy to the maximum extent. However, due to the limitations of the wind turbine mechanical properties and other factors, the optimal power output phase is divided into a variable speed operation area and a constant speed operation area. The optimal power output value is in the variable speed operation area. The optimal power output strategy is:

[0057] The wind turbine adopts the maximum power point tracking control method to make the wind energy utilization coefficient maximum, Tip speed ratio λ Related to the pitch angle β, fix the fan blade pitch angle β to obtain the optimal tip speed ratio Maximize wind energy utilization coefficient ; At this time, the fan conversion efficiency is the highest, and the fan can capture wind energy to the maximum extent. At this stage, the fan will change speed when the wind speed changes;

[0058] At the same time, considering that the wind farm is in an isolated grid state and the load is low, if the wind turbine outputs at rated power, the energy storage device of the grid-type energy storage station may not be able to absorb all the electricity it generates, so the wind turbine is controlled to reduce power output. When the output power of the wind turbine reaches the set low operating power, the wind turbine enters the constant power output stage. At this time, the control system adjusts the wind energy utilization coefficient by changing the pitch angle B to achieve control of the output power.

[0059] Preferably, pre-synchronizing the grid-connected energy storage station and connecting the wind farm to the grid includes:

[0060] After the wind farm is self-started, the sending-end converter station sends a grid-connected signal:

[0061] When the wind farm enters the constant power output stage and the capacitor voltage of the flexible DC transmission system submodule reaches the rated value, the sending-end converter station sends a grid-connected signal to the grid-connected energy storage station.

[0062] Preferably, pre-synchronizing the grid-connected energy storage station and connecting the wind farm to the grid includes:

[0063] Set the pre-synchronization strategy of the grid-connected energy storage station device:

[0064] Set the pre-synchronization strategy of the grid-forming energy storage station device, including the grid-forming virtual synchronous generator VSG control strategy:

[0065] Close the built-in synchronization control switch of the grid-type energy storage station to start the pre-synchronization control; generate a new E and ω Instruction; wherein, the phase synchronization signal is the voltage phase difference between the AC side of the sending-end converter station and the AC side of the grid-forming energy storage station, and the amplitude synchronization signal is the voltage amplitude difference between the AC side of the sending-end converter station and the AC side of the grid-forming energy storage station;

[0066] Pre-synchronization control is used for the grid-type virtual synchronous generator VSG output voltage to track the grid voltage, including amplitude synchronization and phase synchronization, respectively, by adjusting E and ω accomplish:

[0067] ;

[0068] in To superimpose ω N The phase synchronization signal, To superimpose E Amplitude synchronization signal on 0, is the grid voltage phase, 、 are the synchronous integrator coefficients for phase and amplitude; is the grid voltage; s is the complex frequency variable in Laplace transform;

[0069] Closing the synchronous control switch starts the pre-synchronous control, opening the synchronous control switch exits the pre-synchronous control, and resetting the synchronous integral regulator of the phase and amplitude;

[0070] Connecting wind farms to the grid:

[0071] When the voltage amplitude difference between the AC side of the sending-end converter station and the grid-type energy storage station is less than 10% of the rated value, and the phase angle deviation is less than 5°, pre-synchronization is completed and the synchronous grid connection conditions are met; after a delay of T time, the pre-synchronization switch is disconnected, and the PPC point between the wind farm and the grid-type energy storage station, i.e., the common coupling point, is closed at the switch K at the sending-end converter station. W , the wind farm and the grid-type energy storage station can transmit power to the wind farm and complete the startup of the system.

[0072] Furthermore, when the microgrid is operating off-grid, there may be a certain deviation between the amplitude and phase of the output voltage of the grid-connected energy storage station and the grid voltage; if the output voltage, especially the phase, is not controlled, excessive current shock may be generated after the wind farm is connected to the grid, resulting in pre-synchronization failure. In order to smoothly connect the wind farm to the grid, it is necessary to pre-synchronize the energy storage device of the grid-connected energy storage station.

[0073] Furthermore, when the conditions for synchronous grid connection are met, in order to minimize the current impact as much as possible or to make it zero, it is necessary to disconnect the pre-synchronous switch after a delay, and close the switch K at the sending-end converter station, which is the common coupling point between the wind farm and the grid-connected energy storage station PPC point. W .

[0074] The beneficial effects of the present invention are as follows:

[0075] 1. Through the present invention, the MMC-HVDC system can realize the charging process of the submodules of the receiving-end converter station and the sending-end converter station by relying on the receiving-end AC side power grid, and complete the startup process of the MMC-HVDC system.

[0076] 2. The present invention uses a wind farm plus an energy storage system to complete the self-starting process of the wind farm. In the absence of external power support, energy storage can be used as an auxiliary equipment of the wind farm to provide stable voltage and frequency support for wind turbines, complete the grid connection requirements before the MMC-HVDC system is started, and wait for the grid connection signal of the MMC-HVDC system. After receiving the grid connection signal, the energy storage adopts pre-synchronization control to achieve seamless switching from off-grid to grid connection, avoiding the impact of impact current.

[0077] 3. Compared with ordinary energy storage devices, the present invention adopts a grid-type control energy storage device. The grid-type converter adopts a power synchronization strategy similar to that of a synchronous generator, can independently construct the AC side output voltage, and actively support the frequency and voltage regulation of the converter; the present invention adopts a grid-type virtual synchronous generator (VSG) control strategy in the grid-type energy storage station to enhance the system inertia; and the grid-type energy storage station and the wind farm can form a microgrid system, which is connected to the distribution network in the form of a microgrid, thereby improving the frequency and voltage stability of the new power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 is a flowchart of the method of the present invention;

[0079] Figure 2 It is a structural schematic diagram of a wind farm flexible direct current system based on a grid-type energy storage station established in the present invention;

[0080] Figure 3 is a curve diagram showing the relationship between the tip speed ratio and the wind energy utilization coefficient in the maximum power point tracking control method in an embodiment of the present invention;

[0081] Figure 4 It is a strategy block diagram of pre-synchronization control of a grid-type energy storage station in an embodiment of the present invention. DETAILED DESCRIPTION

[0082] Embodiment 1:

[0083] like Figure 1 As shown, a method for starting a wind power flexible direct current system and connecting a wind farm to the grid based on grid-connected energy storage comprises the following steps:

[0084] Construct a wind farm flexible DC transmission system based on a grid-type energy storage station, including a wind farm, a flexible DC transmission system and a grid-type energy storage station;

[0085] Set up a startup charging strategy for the flexible DC transmission system;

[0086] Use grid-type energy storage stations as auxiliary equipment to achieve self-starting of wind farms;

[0087] The grid-connected energy storage station was pre-synchronized and the wind farm was connected to the grid, and the wind farm-flexible DC transmission system was started.

[0088] Embodiment 2:

[0089] like Figure 2As shown in FIG. 1 , the wind farm flexible DC transmission system based on the grid-type energy storage station includes a receiving-end power grid, which is connected to the receiving-end converter station through a cable; the receiving-end converter station is connected to the sending-end converter station through a DC cable; the wind farm and the grid-type energy storage station are connected through a switch K c After parallel connection, it is connected to the sending-end converter station through a cable. A current limiting resistor R2 is set on the line between the receiving-end converter station and the receiving-end power grid, and a switch K is put into operation at the receiving-end converter station. G and bypass switch K connected in parallel with current limiting resistor R2 R The PCC points between the sending-end converter station, wind farm and grid-type energy storage station are equipped with an on-off switch K W .

[0090] Furthermore, an inductor L, a capacitor C and a protective resistor R1 are provided between the wind farm and the grid-type energy storage station.

[0091] Preferably, setting a startup charging strategy for the flexible DC transmission system includes:

[0092] Set the uncontrollable charging strategy for the receiving and sending converter stations:

[0093] In the initial state, the switch K G and K R The converter station submodule is in a locked state; the converter station submodule is connected to the receiving end converter station. N The submodules and 2N submodules of the sending-end converter station;

[0094] t 0 time, closed K G , the current limiting resistor R2 is put into operation, and the receiving-end power grid charges the converter station submodule through the diode uncontrolled rectification; at this time, the receiving-end power grid is applied to the receiving-end converter station N The voltage of each submodule is the peak value of the valve-side line voltage of the AC system , when charging is finished, the receiving end converter station N Submodule capacitor voltage and the first DC link voltage as follows:

[0095] ;

[0096] ;

[0097] At this time, the sending end converter station passes the DC line to the upper and lower bridge arms, a total of 2 N The sub-modules are charged simultaneously, and the sub-module voltage of the sending-end converter station is the capacitor voltage of the submodule of the receiving converter station Half:

[0098] ;

[0099] Where N is the number of submodules in the receiving-end converter station; the capacitor voltage of the submodule in the receiving-end converter station is is the capacitor voltage of the submodule of the receiving end converter station during uncontrollable charging; The voltage of the submodule of the converter station at the sending end during uncontrollable charging; the voltage of the first DC side It is the DC side voltage during uncontrolled charging.

[0100] Furthermore, during the charging process of the converter, the submodule in the locked state needs to be charged first, and the submodule can be unlocked only when the submodule capacitor voltage reaches the operating voltage of the control system; the charging process at this time is called uncontrollable charging.

[0101] Preferably, the method for starting charging of the flexible direct current transmission system comprises:

[0102] Set the controllable charging strategy of the sending-end converter station:

[0103] The charging process after valve control unlocking introduces a control strategy to charge the capacitor voltage and raise the DC voltage to the rated value. This process is called controlled charging. When the submodule capacitor voltage tends to be stable, the uncontrolled charging stage is completed. t At time 1, the sending-end converter station submodule is unlocked, and passive AC voltage control is started. The AC system charges the capacitor of the sending-end converter station submodule only through the DC line; t 1 moment occurs at t After 0 time;

[0104] After unlocking the submodules of the sending-end converter station, the number of submodules put into operation is halved. When unlocking the sending-end converter station, the second DC side voltage It instantly dropped to 1 / 2 of its original height, and then gradually rose;

[0105] After charging is completed, the capacitor voltage of the submodule at the receiving end converter station remains unchanged, and the capacitor voltage of the submodule at the sending end converter station continues to rise until it is equal to the capacitor voltage of the submodule at the receiving end converter station. The formula for the capacitor voltage of the submodule after charging is:

[0106] ;

[0107] Where, the second DC side voltage is the DC side voltage in the controllable charging stage; The capacitor voltage of the submodule of the receiving-end converter station during the controllable charging stage; The voltage of the submodule of the converter station at the sending end during the controllable charging stage; is the peak value of the valve-side line voltage of the AC system, and N is the number of submodules in the receiving-end converter station.

[0108] Preferably, the method for starting charging of the flexible direct current transmission system comprises:

[0109] Set the controllable charging strategy of the receiving converter station:

[0110] In order to make the DC side of the MMC-HVDC system reach the rated voltage and the submodule capacitor voltage reach the rated value, it is necessary to start the controllable charging at the receiving converter station side after the passive controllable charging;

[0111] t 2. Start active controllable charging at all times and close the bypass switch K R ; t 2 Moments occur at t 1 moment later;

[0112] Unlock the DC voltage control of the receiving converter station with a slope to control the power injected into the converter. The specific method is:

[0113] Set the DC voltage control of the receiving converter station with a slope, and increase or decrease the slope of the control command value within the range of 0.734pu~1pu, so that the capacitance of the MMC submodules at both ends rises to / N After charging is completed, the DC voltage reaches the rated value; the MMC submodule is a modular multi-level converter built into the receiving end converter station.

[0114] Preferably, using a grid-type energy storage station as an auxiliary device to realize the self-starting of a wind farm includes:

[0115] Setting the zero-start boost start control method for grid-type energy storage station:

[0116] Before providing voltage and frequency support for the start-up of direct-drive wind turbines, the grid-type energy storage station must first achieve its own start-up;

[0117] By improving the reactive power loop, the grid-type energy storage station can be started, so that the grid-type energy storage station has the function of zero-starting voltage boost and establishes the voltage on the AC side of the wind turbine. The calculation formula is:

[0118] ;

[0119] ;

[0120] In the formula, is the no-load voltage amplitude of the grid-type energy storage station; E Q is the output of reactive power regulation, k p is the reactive power regulation coefficient, is the reactive power reference value, Q is the actual value of reactive power; yes The slope of the rise, ; is the rated voltage of the grid-type energy storage station; yes From 0 to time; when Rise to After that, the output voltage of the grid-type energy storage station remains unchanged.

[0121] Preferably, using a grid-type energy storage station as an auxiliary device to realize the self-starting of a wind farm includes:

[0122] After setting up the zero-start boost start control method for the grid-type energy storage station, the grid-type energy storage station is used to control the active loop output to have a constant frequency, so as to establish the AC side frequency of the wind turbine; the control strategy is as follows:

[0123] By introducing inertia and damping into the grid-type energy storage station to simulate the characteristics of the synchronous machine:

[0124] ;

[0125] in:

[0126] ;

[0127] ;

[0128] In the formula, T M is the mechanical torque, T E is the electromagnetic torque, D p is the damping coefficient, θ is the power angle, ω is the derivative of the power angle with respect to time, is the rated frequency; is the active power reference value; It is electromagnetic power; the grid-forming energy storage station simulates the active power and frequency and reactive power and voltage droop characteristics of the synchronous generator to achieve primary frequency regulation and primary voltage regulation, and provides stable voltage and frequency to the wind farm. At this point, the zero-voltage starting process of the grid-forming energy storage station is completed.

[0129] Preferably, using a grid-type energy storage station as an auxiliary device to realize the self-starting of a wind farm includes:

[0130] The grid-type energy storage station is used to control the active loop output to have a constant frequency. After the frequency of the wind turbine AC side is established, the energy storage device of the grid-type energy storage station is set to charge the direct-drive wind turbine:

[0131] When the grid-type energy storage station is started, close the switch between the wind farm and the grid-type energy storage station. K , connecting wind farms;

[0132] When the voltage and frequency of the wind farm are stable and the external wind speed meets the starting conditions, the wind turbines are started to realize the conversion of wind energy into electrical energy; the starting strategy is:

[0133] When the external wind speed meets the starting wind speed requirement of the wind turbine, the pitch angle of the wind rotor blades begins to adjust from 90° to 0°, the wind direction and the blade power angle increase, and the wind turbine starts to rotate;

[0134] After the fan is started, if the speed does not reach the cut-in speed, the motor is in a non-working state; when the speed meets the cut-in speed, the motor is connected to the grid and enters the optimal power output stage;

[0135] like Figure 3 As shown in the figure, after the start-up phase, the wind turbine enters the optimal power output phase, and the wind turbine is connected to the grid. At this time, the wind speed does not reach the rated wind speed, the output power is lower than the rated power, and the blade pitch angle remains near 0°; at this time, the main task of the wind turbine control system is to adjust the wind turbine speed so that the wind turbine can absorb wind energy to the maximum extent. However, due to the limitations of the wind turbine mechanical properties and other factors, the optimal power output phase is divided into a variable speed operation area and a constant speed operation area, and the optimal power output value is in the variable speed operation area; the optimal power output strategy is:

[0136] The wind turbine adopts the maximum power point tracking control method to make the wind energy utilization coefficient maximum, Tip speed ratio λ Related to the pitch angle β, fix the fan blade pitch angle β to obtain the optimal tip speed ratio Maximize wind energy utilization coefficient ; At this time, the fan conversion efficiency is the highest, and the fan can capture wind energy to the maximum extent. At this stage, the fan will change speed when the wind speed changes;

[0137] At the same time, considering that the wind farm is in an isolated grid state and the load is low, if the wind turbine outputs at rated power, the energy storage device of the grid-type energy storage station may not be able to absorb all the electricity it generates, so the wind turbine is controlled to reduce power output. When the output power of the wind turbine reaches the set low operating power, the wind turbine enters the constant power output stage. At this time, the control system adjusts the wind energy utilization coefficient by changing the pitch angle B to achieve control of the output power.

[0138] Preferably, pre-synchronizing the grid-connected energy storage station and connecting the wind farm to the grid includes:

[0139] After the wind farm is self-started, the sending-end converter station sends a grid-connected signal:

[0140] When the wind farm enters the constant power output stage and the capacitor voltage of the flexible DC transmission system submodule reaches the rated value, the sending-end converter station sends a grid-connected signal to the grid-connected energy storage station.

[0141] Preferably, pre-synchronizing the grid-connected energy storage station and connecting the wind farm to the grid includes:

[0142] Set the pre-synchronization strategy of the grid-connected energy storage station device:

[0143] Set the pre-synchronization strategy of the grid-forming energy storage station device, including the grid-forming virtual synchronous generator VSG control strategy:

[0144] Close the built-in synchronization control switch of the grid-type energy storage station to start the pre-synchronization control; generate a new E and ω Instruction; wherein, the phase synchronization signal is the voltage phase difference between the AC side of the sending-end converter station and the AC side of the grid-forming energy storage station, and the amplitude synchronization signal is the voltage amplitude difference between the AC side of the sending-end converter station and the AC side of the grid-forming energy storage station;

[0145] like Figure 4 As shown in the figure, the pre-synchronization control is used to build a virtual synchronous generator (VSG) whose output voltage tracks the grid voltage, including amplitude synchronization and phase synchronization, respectively, by adjusting E and ω accomplish:

[0146] ;

[0147] in To superimpose ω N The phase synchronization signal, To superimpose E Amplitude synchronization signal on 0, is the grid voltage phase, 、 are the synchronous integrator coefficients for phase and amplitude; is the grid voltage; s is the complex frequency variable in Laplace transform;

[0148] Closing the synchronous control switch starts the pre-synchronous control, opening the synchronous control switch exits the pre-synchronous control, and resetting the synchronous integral regulator of the phase and amplitude;

[0149] Connecting wind farms to the grid:

[0150] When the voltage amplitude difference between the AC side of the sending-end converter station and the grid-type energy storage station is less than 10% of the rated value, and the phase angle deviation is less than 5°, pre-synchronization is completed and the synchronous grid connection conditions are met; after a delay of T time, the pre-synchronization switch is disconnected, and the PPC point between the wind farm and the grid-type energy storage station, i.e., the common coupling point, is closed at the switch K at the sending-end converter station. W , the wind farm and the grid-type energy storage station can transmit power to the wind farm and complete the startup of the system.

[0151] Furthermore, when the microgrid is operating off-grid, there may be a certain deviation between the amplitude and phase of the output voltage of the grid-connected energy storage station and the grid voltage; if the output voltage, especially the phase, is not controlled, excessive current shock may be generated after the wind farm is connected to the grid, resulting in pre-synchronization failure. In order to smoothly connect the wind farm to the grid, it is necessary to pre-synchronize the energy storage device of the grid-connected energy storage station.

[0152] Furthermore, when the conditions for synchronous grid connection are met, in order to minimize the current impact as much as possible or to make it zero, it is necessary to disconnect the pre-synchronous switch after a delay, and close the switch K at the sending-end converter station, which is the common coupling point between the wind farm and the grid-connected energy storage station PPC point. W .

Claims

1. A method for starting a wind power flexible direct current system and connecting a wind farm to the grid based on grid-connected energy storage, characterized in that: The following steps are involved: Construct a wind farm flexible DC transmission system based on a grid-type energy storage station, including a wind farm, a flexible DC transmission system and a grid-type energy storage station; Set up a startup charging strategy for the flexible DC transmission system; Use grid-type energy storage stations as auxiliary equipment to achieve self-starting of wind farms; The grid-connected energy storage station was pre-synchronized and the wind farm was connected to the grid. The wind farm-flexible DC transmission system was started. The wind farm flexible DC transmission system based on the grid-type energy storage station includes a receiving-end power grid, which is connected to the receiving-end converter station through a cable; the receiving-end converter station is connected to the sending-end converter station through a DC cable; the wind farm and the grid-type energy storage station are connected through a switch K c After parallel connection, it is connected to the sending-end converter station through a cable. A current limiting resistor R2 is set on the line between the receiving-end converter station and the receiving-end power grid, and a switch K is put into operation at the receiving-end converter station. G and bypass switch K connected in parallel with current limiting resistor R2 R The PCC points between the sending-end converter station, wind farm and grid-type energy storage station are equipped with an on-off switch K W ; Using a grid-connected energy storage station as an auxiliary device to achieve self-starting of a wind farm includes: Setting the zero-start boost start control method for grid-type energy storage station: By improving the reactive power loop, the grid-type energy storage station can be started, so that the grid-type energy storage station has the function of zero-starting voltage boost and establishes the voltage on the AC side of the wind turbine. The calculation formula is: ； ； In the formula, is the no-load voltage amplitude of the grid-type energy storage station; E Q is the output of reactive power regulation, k p is the reactive power regulation coefficient, is the reactive power reference value, Q is the actual value of reactive power; yes The slope of the rise, ; is the rated voltage of the grid-type energy storage station; yes From 0 to time; when Rise to After that, the output voltage of the grid-type energy storage station remains unchanged; After setting the zero-start boost start control method of the grid-type energy storage station, the grid-type energy storage station is used to control the active loop output to a constant frequency to establish the AC side frequency of the wind turbine. The control strategy is as follows: By introducing inertia and damping into the grid-type energy storage station to simulate the characteristics of the synchronous machine: ； in: ； ； In the formula, T M is the mechanical torque, is the electromagnetic torque, D p is the damping coefficient, θ is the power angle, ω is the derivative of the power angle with respect to time; is the rated frequency; is the active power reference value; is the electromagnetic power; The grid-type energy storage station simulates the active power, frequency, reactive power and voltage droop characteristics of the synchronous generator to achieve primary frequency regulation and primary voltage regulation. The grid-type energy storage station is used to control the active loop output to have a constant frequency. After the frequency of the wind turbine AC side is established, the energy storage device of the grid-type energy storage station is set to charge the direct-drive wind turbine: When the grid-type energy storage station is started, close the switch between the wind farm and the grid-type energy storage station. K , connecting wind farms; When the voltage and frequency of the wind farm are stable and the external wind speed meets the starting conditions, the wind turbines are started. The starting strategy is: When the external wind speed meets the starting wind speed requirement of the wind turbine, the pitch angle of the wind rotor blades begins to adjust from 90° to 0°, the wind direction and the blade power angle increase, and the wind turbine starts to rotate; After the fan is started, if the speed does not reach the cut-in speed, the motor is in a non-working state; when the speed meets the cut-in speed, the motor is connected to the grid and enters the optimal power output stage; After the start-up phase, the wind turbine enters the optimal power output phase, and the wind turbine is connected to the grid. At this time, the wind speed does not reach the rated wind speed, and the output power is lower than the rated power. The optimal power output phase is divided into a variable speed operation area and a constant speed operation area, and the optimal power output value is in the variable speed operation area. The optimal power output strategy is: The wind turbine adopts the maximum power point tracking control method to make the wind energy utilization coefficient maximum, Tip speed ratio λ Related to the pitch angle β, fix the fan blade pitch angle β to obtain the optimal tip speed ratio Maximize wind energy utilization coefficient ; The wind turbine is controlled to reduce power output. When the output power of the wind turbine reaches the set low operating power, the wind turbine enters the constant power output stage. At this time, the control system adjusts the wind energy utilization coefficient by changing the pitch angle B to achieve control of the output power.

2. A method for starting a wind power flexible direct current system and connecting a wind farm to the grid based on grid-connected energy storage according to claim 1, characterized in that: Setting the startup charging strategy of the flexible DC transmission system includes: Set the uncontrollable charging strategy for the receiving and sending converter stations: In the initial state, the switch K G and K R The converter station submodule is in a locked state; the converter station submodule is connected to the receiving end converter station. N The submodules and 2N submodules of the sending-end converter station; t 0 time, closed K G , the current limiting resistor R2 is put into operation, and the receiving-end power grid charges the converter station submodule through the diode uncontrolled rectification; at this time, the receiving-end power grid is applied to the receiving-end converter station N The voltage of each submodule is the peak value of the valve-side line voltage of the AC system , when charging is finished, the receiving end converter station N Submodule capacitor voltage and the first DC link voltage as follows: ； ； At this time, the sending end converter station passes the DC line to the upper and lower bridge arms, a total of 2 N The sub-modules are charged simultaneously, and the sub-module voltage of the sending-end converter station is the capacitor voltage of the submodule of the receiving converter station Half: ； Where N is the number of submodules in the receiving-end converter station; the capacitor voltage of the submodule in the receiving-end converter station is is the capacitor voltage of the submodule of the receiving end converter station during uncontrollable charging; The voltage of the submodule of the converter station at the sending end during uncontrollable charging; the voltage of the first DC side It is the DC side voltage during uncontrolled charging.

3. A method for starting a wind power flexible direct current system and connecting a wind farm to the grid based on grid-connected energy storage according to claim 2, characterized in that: The startup charging method for setting up a flexible DC transmission system includes: Set the controllable charging strategy of the sending-end converter station: t At time 1, the sending-end converter station submodule is unlocked, and passive AC voltage control is started. The AC system charges the capacitor of the sending-end converter station submodule only through the DC line; t 1 moment occurs at t After 0 time; After unlocking the submodules of the sending-end converter station, the number of submodules put into operation is halved. When unlocking the sending-end converter station, the second DC side voltage It instantly dropped to 1 / 2 of its original height, and then gradually rose; After charging is completed, the capacitor voltage of the submodule at the receiving end converter station remains unchanged, and the capacitor voltage of the submodule at the sending end converter station continues to rise until it is equal to the capacitor voltage of the submodule at the receiving end converter station. The formula for the capacitor voltage of the submodule after charging is: ； Where, the second DC side voltage is the DC side voltage in the controllable charging stage; The capacitor voltage of the submodule of the receiving-end converter station during the controllable charging stage; The voltage of the submodule of the converter station at the sending end during the controllable charging stage; is the peak value of the valve-side line voltage of the AC system, and N is the number of submodules in the receiving-end converter station.

4. A method for starting a wind power flexible direct current system and connecting a wind farm to the grid based on grid-connected energy storage according to claim 3, characterized in that: The startup charging method for setting up a flexible DC transmission system includes: Set the controllable charging strategy of the receiving converter station: t 2. Start active controllable charging at all times and close the bypass switch K R ; t 2 Moments occur at t 1 moment later; Unlock the DC voltage control of the receiving converter station with a slope to control the power injected into the converter. The specific method is: Set the DC voltage control of the receiving converter station with a slope, and increase or decrease the slope of the control command value within the range of 0.734pu~1pu, so that the capacitance of the MMC submodules at both ends rises to / N After charging is completed, the DC voltage reaches the rated value; the MMC submodule is a modular multi-level converter built into the receiving end converter station.

5. A method for starting a wind power flexible direct current system and connecting a wind farm to the grid based on grid-connected energy storage according to claim 1, characterized in that: Pre-synchronization of grid-connected energy storage stations and wind farm grid connection include: After the wind farm is self-started, the sending-end converter station sends a grid-connected signal: When the wind farm enters the constant power output stage and the capacitor voltage of the flexible DC transmission system submodule reaches the rated value, the sending-end converter station sends a grid-connected signal to the grid-connected energy storage station.

6. A method for starting a wind power flexible direct current system and connecting a wind farm to the grid based on grid-connected energy storage according to claim 5, characterized in that: Pre-synchronization of grid-connected energy storage stations and wind farm grid connection include: Set the pre-synchronization strategy of the grid-forming energy storage station, including the grid-forming virtual synchronous generator VSG control strategy: Close the built-in synchronization control switch of the grid-type energy storage station to start the pre-synchronization control; generate a new E and ω Instruction; wherein, the phase synchronization signal is the voltage phase difference between the AC side of the sending-end converter station and the AC side of the grid-forming energy storage station, and the amplitude synchronization signal is the voltage amplitude difference between the AC side of the sending-end converter station and the AC side of the grid-forming energy storage station; Pre-synchronization control is used for the grid-type virtual synchronous generator VSG output voltage to track the grid voltage, including amplitude synchronization and phase synchronization, respectively, by adjusting E and ω accomplish: ； in To superimpose ω N The phase synchronization signal, To superimpose E Amplitude synchronization signal on 0, is the grid voltage phase, 、 are the synchronous integrator coefficients for phase and amplitude; is the grid voltage; s is the complex frequency variable in Laplace transform; Closing the synchronous control switch starts the pre-synchronous control, opening the synchronous control switch exits the pre-synchronous control, and resetting the synchronous integral regulator of the phase and amplitude; Connecting wind farms to the grid: When the voltage amplitude difference between the AC side of the sending-end converter station and the grid-type energy storage station is less than 10% of the rated value, and the phase angle deviation is less than 5°, pre-synchronization is completed and the synchronous grid connection conditions are met; after a delay of T time, the pre-synchronization switch is disconnected, and the switch K between the wind farm and the grid-type energy storage station PPC point located at the sending-end converter station is closed. W , the wind farm and the grid-type energy storage station can transmit power to the wind farm and complete the startup of the system.

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