A control strategy for flywheel energy storage grid-side converter under asymmetric grid fault

Abstract

The present invention relates to a control strategy of a flywheel energy storage grid-side converter under a grid asymmetric fault. First, a mathematical model of the grid-side converter based on a two-phase rotating dq coordinate system is constructed; combined with the control target of the grid-side converter, the grid-side converter adopts a direct current control strategy based on grid voltage orientation, and the control system consists of a DC voltage outer loop and an active and reactive current inner loop; the positive and negative sequence current reference values ​​are obtained through the matrix relationship between power and the positive and negative sequence voltage and current, and a double closed-loop current control strategy under positive and negative sequence separation is adopted to realize the control of the positive and negative sequence current, thereby effectively suppressing the active power double frequency fluctuation; the maximum value of the three-phase current amplitude output by the grid-side converter under the abc coordinate system is calculated in real time, and compared with the current limit value. If the current exceeds the limit, the current is adjusted by adjusting the power reference value to make the current not exceed the limit. The present invention has the advantages of fast calculation and fast response.

Description

Technical Field

[0001] The present invention relates to an energy storage operation control method, in particular to a control strategy for a flywheel energy storage grid-side converter under an asymmetric fault of a power grid. Background Art

[0002] With the rapid development of energy storage technology, it has been widely used in the fields of smoothing wind power and photovoltaic output, providing reactive power support, and participating in grid frequency regulation. Flywheel energy storage is a physical energy storage. Due to its advantages such as high power density, long service life, unlimited charging and discharging times, and non-coupling between the machine and the grid, it is more suitable for participating in high-frequency grid frequency regulation and smoothing wind power output.

[0003] When the voltage at the flywheel energy storage grid connection point is asymmetrical, the voltage and current on the AC side of the grid-side converter will generate positive and negative sequence components. The unbalanced components will generate even harmonics on the DC side, causing the DC bus capacitor to charge and discharge repeatedly, shortening the capacitor's service life and reducing the reliability of the flywheel energy storage operation. The voltage fluctuation on the DC side causes odd non-characteristic harmonics on the AC side, deteriorating the performance of the converter and possibly even damaging the equipment.

[0004] Without considering the losses of the back-to-back converter itself, according to the principle of power conservation, the power on the generator side of the converter is equal to the power on the grid side. Since the back-to-back converter of the flywheel energy storage grid-connected system isolates the generator side from the grid side, when a grid drop fault occurs, the generator side cannot respond to the fault immediately, resulting in a rapid increase in the current on the grid side, posing a serious threat to the safety of the entire system.

[0005] Since the grid-side converter with flywheel energy storage is the first to be affected during a power grid fault, it is of great significance to study the control strategy of the grid-side converter during the fault. Summary of the invention

[0006] The purpose of the present invention is to provide a control strategy for a flywheel energy storage grid-side converter under an asymmetric fault in a power grid.

[0007] Based on the structure and operation principle of flywheel energy storage itself, the present invention first analyzes the topological structure of the flywheel energy storage grid-connected system; studies the steady-state control strategy of the flywheel energy storage grid-side converter; analyzes the transient process of the flywheel energy storage grid-side under asymmetric grid voltage fault; on the basis of the steady-state control strategy of the grid-side converter, develops a grid-side converter control strategy that can effectively suppress the double frequency fluctuation of the grid-side output active power and ensure that the converter current does not exceed the limit under an asymmetric grid fault; finally, builds a grid-connected model of the flywheel energy storage grid-side converter to verify the control strategy.

[0008] The present invention adopts the following technical solution:

[0009] A control strategy for a flywheel energy storage grid-side converter under an asymmetric fault in a power grid, characterized by comprising:

[0010] The transient process of the flywheel energy storage grid side under the asymmetric fault of the power grid is analyzed, and the positive and negative sequence current reference values ​​are obtained through the matrix relationship between power and positive and negative sequence voltage and current vectors. Through four current inner loop PI regulators, a double closed-loop current control strategy under positive and negative sequence separation is adopted: the output positive and negative sequence currents are controlled to track the positive and negative sequence current reference values ​​without error under the positive and negative sequence voltage networks respectively, and the double frequency fluctuation of active power is suppressed;

[0011] The maximum value of the three-phase current amplitude output by the grid-side converter in the abc coordinate system is calculated in real time and compared with the current limit value; if the current exceeds the limit, the current is adjusted by adjusting the power reference value so that the current does not exceed the limit.

[0012] The control strategy of the flywheel energy storage grid-side converter under the above-mentioned asymmetric grid fault is as follows:

[0013] Through Park transformation and positive and negative sequence separation, the grid voltage vector and current vector are decomposed into two-phase rotating dq coordinate system respectively when the grid is in asymmetric fault state, and the matrix relationship between grid-side power and positive and negative sequence voltage and current vectors is obtained through instantaneous power theory.

[0014] Select corresponding control variables and controlled variables; obtain positive and negative sequence current reference values ​​through matrix transformation;

[0015] Through four current inner loop PI regulators, a double closed-loop current control strategy is adopted under positive and negative sequence separation: the output positive and negative sequence currents are controlled to track the positive and negative sequence current reference values ​​without error under the positive and negative sequence voltage networks respectively, so as to suppress the double frequency fluctuation of active power.

[0016] The control strategy of the flywheel energy storage grid-side converter under the above-mentioned asymmetric grid fault is as follows:

[0017] The positive and negative sequence current reference values ​​are transformed into the abc coordinate system through the Park inverse transformation matrix, and the maximum value of the three-phase current amplitude in the abc coordinate system is calculated;

[0018] The maximum value of the three-phase current amplitude is compared with the current limit value. If the judgment result is that the current exceeds the limit, the amplitude of the output current is adjusted by adjusting the power control method.

[0019] The control strategy of the flywheel energy storage grid-side converter under the above-mentioned asymmetric grid fault is based on the mathematical model of the grid-side converter in the constructed two-phase rotating dq coordinate system:

[0020]

[0021] Among them, ugd 、u gq 、i gd 、i gq 、e d 、e q are the output voltage and current of the grid-side converter and the components of the grid voltage on the dq axis, ω is the speed of the rotating coordinate system, L g , R g They are the inductance and resistance on the grid side respectively.

[0022] In the control strategy of the flywheel energy storage grid-side converter under the above-mentioned asymmetric fault of the power grid, when suppressing the double frequency fluctuation of the active power, the specific control strategy includes:

[0023] S5.1, through Park transformation and positive and negative sequence separation, the grid voltage vector and current vector are decomposed into two-phase rotating dq coordinate system respectively when the grid is in asymmetric fault, and the matrix relationship between the grid side power and the positive and negative sequence voltage and current vectors is obtained through instantaneous power theory;

[0024] Through Park transformation and positive and negative sequence separation, the grid voltage vector and grid current vector are expressed in the two-phase rotating dq coordinate system as follows:

[0025]

[0026] in, is the positive sequence component of variable z in the positive sequence dq coordinate system; is the negative sequence component of variable z in the negative sequence dq coordinate system; θ + ,θ - is the instantaneous phase angle of the positive and negative sequence components of the grid voltage;

[0027] The active power p and reactive power q output by the grid-side converter are expressed as follows:

[0028]

[0029] p0 and q0 are the DC components of active power and reactive power; p1, q1, p2, and q2 are the amplitudes of the cosine and sine components of the double power frequency fluctuations of active power and reactive power respectively;

[0030] The voltage vector and current vector output by the grid-side converter are decomposed into positive and negative sequences to obtain the matrix relationship between power and positive and negative sequence voltage and current vectors:

[0031]

[0032] in are the positive sequence components of the grid voltage and current respectively; are the negative sequence components of the grid voltage and current respectively;

[0033] S5.2, with the control target of suppressing the double frequency fluctuation of active power, select the corresponding control variables and controlled variables; obtain the positive and negative sequence current reference values ​​through matrix transformation; select the four power values ​​of p0, p1, p2, and q0 as control variables; let That is, the double frequency component in the output active power can be eliminated; this paper controls the power factor to unity, so The current reference values ​​are as follows:

[0034]

[0035] in It is the positive and negative sequence current reference value of the current inner loop;

[0036] The average active power reference value in formula (9) It can be obtained from the voltage outer loop:

[0037]

[0038] in and u dc are the reference value and actual value of DC bus voltage, k vp and k vi are the proportional and integral parameters of the voltage outer loop PI regulator;

[0039] S5.3, through 4 current inner loop PI regulators, adopts a double closed-loop current control strategy under positive and negative sequence separation: the output positive and negative sequence currents are controlled to track the positive and negative sequence current reference values ​​without error under the positive and negative sequence voltage networks respectively, so as to suppress the double frequency fluctuation of active power;

[0040] The control equations for the positive and negative sequence networks are as follows:

[0041]

[0042] are the output voltages of the grid-side converter under positive and negative sequence, respectively, ip and k il It is the proportional parameter and integral parameter of the current inner loop PI regulator.

[0043] The control strategy of the flywheel energy storage grid-side converter under the above-mentioned asymmetric fault of the power grid is to adjust the current so that the current does not exceed the time limit, specifically including:

[0044] Step 5.1, transform the positive and negative sequence current reference values ​​into the abc coordinate system through the Park inverse transformation matrix, and calculate the maximum value of the three-phase current amplitude in the abc coordinate system;

[0045] The maximum value of the three-phase current can be calculated from the positive and negative sequence current command values, as shown in formula (12):

[0046]

[0047] where i amax 、i bmax 、i cmax They are the maximum values ​​of three-phase currents abc respectively;

[0048] Step 5.2, compare the maximum value of the three-phase current amplitude with the current limit value, if the judgment result is that the current exceeds the limit, adjust the output current amplitude by adjusting the power control method;

[0049] Since p0 and i are proportional, when the maximum current exceeds the limit value, p0 can be adjusted to make the current meet the limit requirement, as shown in formula (11);

[0050]

[0051] where p0 * is the given value of active power, i abcmax is the maximum amplitude of the three-phase current, i refmax is the current limit value of the grid-side converter, p * It is the set value of active power after adjustment.

[0052] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0053] (1) The double closed-loop current control strategy under positive and negative sequence separation designs the positive and negative sequence current reference values ​​based on the control target of eliminating the double frequency fluctuation of active power. The positive and negative sequence currents are controlled separately in the positive and negative sequence network, which can effectively suppress the double frequency fluctuation of active power.

[0054] (2) Because the dual closed-loop current control strategy under positive and negative sequence separation can achieve power factor regulation and DC voltage stability, it can also be used for grid-side converter control during steady-state operation. Therefore, there is no need to switch the control strategy when a fault occurs or when the fault is restored;

[0055] (3) Due to the strong coupling between the four current command values, if the current is limited separately in the current inner loop, it is difficult to ensure that the final three-phase current does not exceed the limit. The method of controlling the output current by adjusting the power reference value has the advantages of fast calculation and fast response, and avoids the interference of other feedback quantities, ensuring the accuracy of current limiting. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 It is the structural diagram of the flywheel energy storage grid-connected system.

[0057] Figure 2 It is the structural diagram of the grid-side converter.

[0058] Figure 3 It is the control block diagram of the flywheel energy storage grid-side converter.

[0059] Figure 4 It is the control block diagram of the flywheel energy storage grid-side converter under asymmetric grid fault.

[0060] Figure 5a It is the simulation result under the single-phase drop fault of the power grid voltage (grid-side voltage simulation result).

[0061] Figure 5b It is the simulation result (current simulation result) under the single-phase drop fault of the power grid voltage.

[0062] Figure 5c It is the simulation result (power reference value and actual value) under the single-phase drop fault of the power grid voltage.

[0063] Figure 5d It is the simulation result under the single-phase drop fault of the power grid voltage (comparison of the active power output of the grid-side converter under the control strategy proposed by the present invention and the traditional double closed-loop control strategy).

[0064] Figure 6a It is the simulation result under the two-phase drop fault of the power grid voltage (grid side voltage simulation result).

[0065] Figure 6b It is the simulation result (current simulation result) under the two-phase drop fault of the power grid voltage.

[0066] Figure 6c It is the simulation result (power reference value and actual value) under the two-phase drop fault of the power grid voltage.

[0067] Figure 6d It is the simulation result under the two-phase drop fault of the power grid voltage (the control strategy proposed in the present invention effectively suppresses the secondary fluctuation of the active power). DETAILED DESCRIPTION

[0068] The technical solution of the present invention will be further described in detail below in conjunction with specific implementation methods and drawings.

[0069] A control strategy for a flywheel energy storage grid-side converter under an asymmetric grid fault, the specific steps are as follows:

[0070] Step 1, analyzing the composition and topology of the flywheel energy storage grid-connected system;

[0071] This step is the basis and prerequisite for the flywheel energy storage grid-connected operation control, and this step is described in detail for easy understanding.

[0072] Flywheel energy storage is generally composed of a flywheel rotor, a permanent magnet synchronous motor, a back-to-back converter and a control system. The structure diagram of the flywheel energy storage grid-connected system is shown in the figure below. Figure 1 shown.

[0073] The flywheel rotor determines the energy stored in the flywheel.

[0074] Permanent magnet synchronous motor is used to realize the conversion of the rotational kinetic energy of the flywheel into electrical energy. When the flywheel is charging, it absorbs energy from the power grid, the flywheel's rotational kinetic energy increases, and the speed increases; while when the flywheel stores energy and discharges it, it transmits electrical energy to the power grid, the flywheel's rotational kinetic energy decreases, and the speed decreases.

[0075] The back-to-back converter consists of a machine-side converter, a grid-side converter and a DC bus capacitor. The machine-side converter controls the operation of the permanent magnet synchronous motor, and the grid-side converter controls the stability of the DC bus voltage and the power factor.

[0076] Step 2: Based on the topological structure and operation principle of the three-phase two-level voltage source inverter, a mathematical model of the grid-side converter based on a rotating dq coordinate system is constructed;

[0077] The voltage equation of the grid-side converter in the dq coordinate system is shown in equation (2).

[0078]

[0079] Among them, u gd 、u gq 、i gd 、i gq 、e d 、e q are the output voltage and current of the grid-side converter and the components of the grid voltage on the dq axis respectively, and ω is the rotation speed of the rotating coordinate system.

[0080] Step 3: Combined with the control target and instantaneous power theory of the grid-side converter, the grid-side converter adopts the direct current control strategy based on grid voltage orientation (VO-DCC). The VO-DCC control system consists of a DC voltage outer loop and an active and reactive current inner loop;

[0081] Step 3.1, determine that the control of the grid-side converter is mainly aimed at stabilizing the DC bus voltage and achieving grid-connected power control;

[0082] Step 3.2, the d-axis of the grid-side converter based on VO-DCC adopts the control mode of DC voltage outer loop and current inner loop, and the output of voltage is given by the current inner loop to maintain the DC side voltage stability;

[0083] Step 3.3, the q-axis of the grid-side converter based on VO-DCC adopts the control mode of reactive power outer loop and current inner loop, and the output of the outer loop is the given of the current inner loop to adjust the grid-connected power factor;

[0084] In step 3.4, the current inner loop adopts current control based on feedforward decoupling. Since the transfer functions of the d-axis and q-axis current inner loops of the grid-side converter are the same, a set of PI regulators with the same parameters are used to achieve zero steady-state error tracking of the current.

[0085] It can be obtained from formula (1) that the converter's control of current not only depends on the tracking control of the dynamic current, but is also affected by the d- and q-axis inductance voltage drops and the grid electromotive force disturbance. In addition, there is coupling between the d- and q-axis components. Therefore, a current control strategy based on feedforward decoupling is required.

[0086] The current controller is designed with a PI regulator. The current differential term in equation (1) is obtained by the PI regulator operation, and other coupling disturbance terms are obtained by feedforward compensation operation. gd 、u gq The control equation of the inner loop of the grid-side converter current is:

[0087]

[0088] Where: K iP and K il They are respectively the proportional and integral coefficients of the PI regulator in the inner current loop of the grid-side converter.

[0089] In VO-DCC control, the d-axis always coincides with the grid voltage vector E, so e d =|E|,e q =0. According to the instantaneous power theory, the active and reactive power output by the grid-side converter are:

[0090]

[0091] exist Figure 2 In the grid-side converter shown in the figure, the instantaneous value of the DC input active power is p = u dc i dc , if the power loss in the converter is not considered, then:

[0092]

[0093] From formula (4), we can get the DC voltage u dc with i d is proportional to, so we can control i d To control the DC bus voltage u dc , so the control equation of the voltage outer loop can be written as:

[0094]

[0095] In formula (5): KvP and K vl They are respectively the proportional and integral coefficients of the PI regulator in the voltage outer loop of the grid-side converter.

[0096] In summary, the steady-state control block diagram of the flywheel energy storage grid-side converter is as follows: Figure 3 shown.

[0097] Step 4, analyzing the transient process of the flywheel energy storage grid side under the asymmetric fault of the power grid, obtaining the positive and negative sequence current reference values ​​through the matrix relationship between power and the positive and negative sequence voltage and current, and adopting the double closed-loop current control strategy under the positive and negative sequence separation to realize the control of the positive and negative sequence currents, thereby effectively suppressing the double frequency fluctuation of the active power;

[0098] Step 4.1, through Park transformation and positive and negative sequence separation, the grid voltage vector and current vector are decomposed into two-phase rotating dq coordinate system respectively when the grid is in asymmetric fault state, and the matrix relationship between the grid side power and the positive and negative sequence voltage and current vectors is obtained through instantaneous power theory;

[0099] Through Park transformation and positive and negative sequence separation, the grid voltage vector and grid current vector can be expressed in the two-phase rotating dq coordinate system as follows:

[0100]

[0101] Therefore, the active power p and reactive power q output by the grid-side converter can be expressed as follows:

[0102]

[0103] In formula (7), p0 and q0 are the DC components of active power and reactive power; p1, q1, p2, and q2 are the amplitudes of the cosine and sine components of the double power frequency fluctuations of active power and reactive power, respectively.

[0104] Based on formula (7), the voltage vector and current vector output by the grid-side converter are decomposed into the positive sequence network and the negative sequence network respectively, and the matrix relationship between power and positive and negative sequence voltage and current vectors can be obtained:

[0105]

[0106] Step 4.2, with the control objective of suppressing the double frequency fluctuation of active power, select the corresponding control variables and controlled variables. Obtain the positive and negative sequence current reference values ​​through matrix transformation;

[0107] In order to suppress the double frequency fluctuation of active power, four power values ​​p0, p1, p2, and q0 are selected as control variables. The double frequency component in the output active power can be eliminated. This paper controls the power factor to unity, The current reference values ​​are as follows:

[0108]

[0109] The average active power reference value in formula (9) It can be obtained from the voltage outer loop:

[0110]

[0111] In step 4.3, a double closed-loop current control strategy with positive and negative sequence separation is adopted through four current inner-loop PI regulators: the output positive and negative sequence currents are controlled to track the positive and negative sequence current reference values ​​without error under the positive and negative sequence voltage networks respectively, so as to suppress the double frequency fluctuation of active power.

[0112] The control equations under positive and negative sequence networks are shown in (8):

[0113]

[0114] The double closed-loop current control block diagram of the flywheel energy storage grid-side converter under positive and negative sequence separation under asymmetric grid fault is as follows: Figure 4 shown.

[0115] Step 5, calculate the maximum value of the three-phase current amplitude output by the grid-side converter in the abc coordinate system in real time, and compare it with the current limit value. If the current exceeds the limit, adjust the current by adjusting the power reference value so that the current does not exceed the limit.

[0116] Step 5.1, transform the positive and negative sequence current reference values ​​into the abc coordinate system through the Park inverse transformation matrix, and calculate the maximum value of the three-phase current amplitude in the abc coordinate system;

[0117] The maximum value of the three-phase current can be calculated from the positive and negative sequence current command values, as shown in formula (12):

[0118]

[0119] Step 5.2, compare the maximum value of the three-phase current amplitude with the current limit value. If the judgment result is that the current exceeds the limit, the output current amplitude is adjusted by adjusting the power control method.

[0120] Since p0 and i are proportional, when the maximum current exceeds the limit value, p0 can be adjusted to make the current meet the limit requirement, as shown in formula (11).

[0121]

[0122] In formula (13): i refmax is the amplitude limit value of the abc three-phase current vector, i abcmaxis the maximum value of the three-phase current calculated in equation (12). * is the adjusted power reference value.

[0123] Example:

[0124] Verification of the effectiveness of the flywheel energy storage grid-side converter control strategy under asymmetric grid faults.

[0125] Parameter design: The DC bus voltage is set to 350V, the output rated power is set to 2kW, the rated phase voltage of the grid-connected point is effectively set to 110V, the rated frequency is set to 50Hz, and the limit value of the three-phase current on the grid side is 10A.

[0126] (i) At 0.1s, an asymmetric fault occurred in which the voltage of phase B dropped to 0.2pu, and the fault was restored at 0.4s. In order to verify the effectiveness of the current limit control, the current limit control was put into operation again at 0.2s. Figures 5(a) and (b) are the simulation results of the grid-side voltage and current, respectively.

[0127] In Figure 5(b), the current increases after the fault occurs and exceeds the current limit. After 0.2s of current limit control, the current amplitude can be effectively limited by adjusting p. Figure 5(c) shows the power reference value and actual value.

[0128] FIG5(d) compares the active power output of the grid-side converter based on the control strategy proposed in the present invention and based on the traditional dual closed-loop control strategy. It can be seen that by adopting the control strategy proposed in the present invention, the double frequency fluctuation of the active power is effectively reduced.

[0129] (ii) At 0.1s, an asymmetric fault occurred in which the voltage of the AB two-phase grid dropped to 0.5pu, and the fault recovered at 0.4s. Figures 6(a) and 6(b) are the simulation results of the grid-side voltage and current, respectively. As shown in Figure 6(a), the currents of phases A and B increased after the fault occurred and exceeded the current limit. After 0.2s, the current amplitude was effectively limited by adjusting the active power. In Figure 6(d), the control strategy proposed by the present invention effectively suppresses the secondary fluctuation of active power.

[0130] In addition, during the stable operation of the power grid, the negative sequence component in the power grid is 0. As can be seen from Figures 5 and 6, the control strategy proposed by the present invention can still be used for steady-state control. Therefore, when a fault occurs, it is not necessary to switch the control strategy of the grid-side converter of the flywheel energy storage grid-connected system.

[0131] The specific embodiments described herein are merely examples of the spirit of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in similar ways without departing from the spirit of the present invention or exceeding the scope defined by the appended claims.

Claims

1. A control strategy for flywheel energy storage grid-side converter under asymmetric grid fault. It is characterized in that include The transient process of the flywheel energy storage grid side under the asymmetric fault of the power grid is analyzed, and the positive and negative sequence current reference values ​​are obtained through the matrix relationship between power and positive and negative sequence voltage and current vectors. Through four current inner loop PI regulators, a double closed-loop current control strategy under positive and negative sequence separation is adopted: the output positive and negative sequence currents are controlled to track the positive and negative sequence current reference values ​​without error under the positive and negative sequence voltage networks respectively, and the double frequency fluctuation of active power is suppressed; Calculate the maximum value of the three-phase current amplitude output by the grid-side converter in the abc coordinate system in real time and compare it with the current limit value; If the current exceeds the limit, the current is adjusted by adjusting the power reference value so that the current does not exceed the limit; Based on the mathematical model of the grid-side converter in the constructed two-phase rotating dq coordinate system: Among them, u gd 、u gq 、i gd 、i gq 、e d 、e q are the output voltage and current of the grid-side converter and the components of the grid voltage on the dq axis, ω is the speed of the rotating coordinate system, L g , R g They are the inductance and resistance on the grid side; When suppressing the double frequency fluctuation of active power, it includes S5.1, through Park transformation and positive and negative sequence separation, the grid voltage vector and current vector are decomposed into two-phase rotating dq coordinate system respectively when the grid is in asymmetric fault, and the matrix relationship between the grid side power and the positive and negative sequence voltage and current vectors is obtained through instantaneous power theory; Through Park transformation and positive and negative sequence separation, the grid voltage vector and grid current vector are expressed in the two-phase rotating dq coordinate system as follows: in, is the positive sequence component of variable z in the positive sequence dq coordinate system; is the negative sequence component of variable z in the negative sequence dq coordinate system; θ + ,θ - is the instantaneous phase angle of the positive and negative sequence components of the grid voltage; The active power p and reactive power q output by the grid-side converter are expressed as follows: p 0 ,q 0 is the DC component of active power and reactive power; p 1 ,q 1 、p 2 ,q 2 are the amplitudes of the cosine and sine components of the double power frequency fluctuations of active power and reactive power respectively; The voltage vector and current vector output by the grid-side converter are decomposed into positive and negative sequences to obtain the matrix relationship between power and positive and negative sequence voltage and current vectors: in are the positive sequence components of the grid voltage and current respectively; are the negative sequence components of the grid voltage and current respectively; S5.2, with the control target of suppressing the double frequency fluctuation of active power, select the corresponding control variable and controlled variable; obtain the positive and negative sequence current reference values ​​through matrix transformation; select p 0 、p 1 、p 2 ,q 0 The four power values ​​are the control variables; That is, the double frequency component in the output active power can be eliminated; this paper controls the power factor to unity, so The current reference values ​​are as follows: in It is the positive and negative sequence current reference value of the current inner loop; The average active power reference value in formula (9) It can be obtained from the voltage outer loop: in and u dc are the reference value and actual value of DC bus voltage, k vp and k vi are the proportional and integral parameters of the voltage outer loop PI regulator; S5.3, through 4 current inner loop PI regulators, adopts a double closed-loop current control strategy under positive and negative sequence separation: the output positive and negative sequence currents are controlled to track the positive and negative sequence current reference values ​​without error under the positive and negative sequence voltage networks respectively, so as to suppress the double frequency fluctuation of active power; The control equations for the positive and negative sequence networks are as follows: are the output voltages of the grid-side converter under positive and negative sequence, respectively, ip and k il It is the proportional parameter and integral parameter of the current inner loop PI regulator.

2. A control strategy for a flywheel energy storage grid-side converter under an asymmetric grid fault according to claim 1, It is characterized in that Specifically: Through Park transformation and positive and negative sequence separation, the grid voltage vector and current vector are decomposed into two-phase rotating dq coordinate system respectively when the grid is in asymmetric fault state, and the matrix relationship between grid-side power and positive and negative sequence voltage and current vectors is obtained through instantaneous power theory. Select corresponding control variables and controlled variables; obtain positive and negative sequence current reference values ​​through matrix transformation; Through four current inner loop PI regulators, a double closed-loop current control strategy is adopted under positive and negative sequence separation: the output positive and negative sequence currents are controlled to track the positive and negative sequence current reference values ​​without error under the positive and negative sequence voltage networks respectively, so as to suppress the double frequency fluctuation of active power.

3. According to claim 1, a control strategy for a flywheel energy storage grid-side converter under an asymmetric grid fault, It is characterized in that Specifically: The positive and negative sequence current reference values ​​are transformed into the abc coordinate system through the Park inverse transformation matrix, and the maximum value of the three-phase current amplitude in the abc coordinate system is calculated; The maximum value of the three-phase current amplitude is compared with the current limit value. If the judgment result is that the current exceeds the limit, the amplitude of the output current is adjusted by adjusting the power control method.

4. According to claim 1, a control strategy for a flywheel energy storage grid-side converter under an asymmetric grid fault, It is characterized in that Adjust the current so that it does not exceed the limit, including Step 5.1, transform the positive and negative sequence current reference values ​​into the abc coordinate system through the Park inverse transformation matrix, and calculate the maximum value of the three-phase current amplitude in the abc coordinate system; The maximum value of the three-phase current can be calculated from the positive and negative sequence current command values, as shown in formula (12): where i amax 、i bmax 、i cmax They are the maximum values ​​of three-phase currents abc respectively; Step 5.2, compare the maximum value of the three-phase current amplitude with the current limit value, if the judgment result is that the current exceeds the limit, adjust the output current amplitude by adjusting the power control method; Because p 0 There is a proportional relationship between the current and i. When the maximum current exceeds the limit value, the p 0 Adjustment can make the current meet the limit requirement, as shown in formula (11); where p 0 * is the given value of active power, i abcmax is the maximum amplitude of the three-phase current, i refmax is the current limit value of the grid-side converter, p * It is the set value of active power after adjustment.

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