Asymmetric fault current limiting method based on constructed network type energy storage current converter

Abstract

The invention discloses an asymmetric fault current limiting method based on a network-forming type energy storage current converter. The asymmetric fault current limiting method comprises three core links of power parameter setting, internal potential primary compensation and internal potential secondary compensation. In the power parameter setting step, reference amplitudes of an active current and a reactive current are determined based on a fault current limiting range and active support characteristics of a reactive voltage loop of the grid-forming type energy storage converter; setting of power parameters is completed; the internal potential primary compensation is used for compensating the internal potential compensation amount of the negative-sequence current required to be suppressed to the original internal potential; and the internal potential secondary compensation is used for limiting transient overcurrent under an asymmetric fault on the basis of the internal potential primary compensation and power parameter setting. According to the method, through primary compensation of the internal potential, the aim of balancing the current is achieved by suppressing the negative sequence current and limiting the overall current under network-forming type control, and meanwhile, secondary compensation of the internal potential is further introduced, so that transient over-current limitation under an asymmetric fault is achieved.

Description

Technical Field

[0001] This invention belongs to the field of smart grid control, and specifically relates to an asymmetric fault current limiting method based on a grid-type energy storage converter. Background Technology

[0002] With the continuous increase in the penetration rate of new energy power generation, grid-forming energy storage converters based on grid-forming control technology are widely used. They can simulate the external characteristics of synchronous generators and provide active support for new power systems.

[0003] When asymmetrical faults occur in the power grid (such as single-phase short circuits), grid-connected energy storage converters will generate large-scale fault currents. If this current exceeds the tolerance level of the power electronic devices, it will cause abnormal heating of the energy storage converter, potentially leading to equipment damage, loss of active support function, and jeopardizing system stability. Existing current limiting technologies for asymmetrical faults are broadly divided into direct and indirect methods: direct methods limit current by restricting the current reference value, but are not suitable for grid-connected control architectures; indirect methods limit current by adjusting the voltage reference or introducing virtual impedance, making it difficult to balance current limiting effectiveness with support capability, and few designs are optimized for the characteristics of asymmetrical fault currents. Current current limiting technologies cannot fully adapt to the operating characteristics and control requirements of grid-connected energy storage converters, making it difficult to achieve an efficient balance between current limiting effectiveness and active support capability, especially in the case of asymmetrical faults. Therefore, there is an urgent need to propose a novel asymmetric fault current limiting method for grid-type energy storage converters. This method can effectively suppress asymmetric fault currents in grid-type energy storage converters, protect power semiconductor devices, and ensure their active support function for the power grid, thereby ensuring the safe and stable operation of the new power system under fault conditions. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide an asymmetric fault current limiting method based on a grid-type energy storage converter with active support capability, so as to not only effectively limit fault overcurrent under asymmetric faults, but also retain the active support capability of the grid-type energy storage converter.

[0005] This invention provides an asymmetric fault current limiting method based on a grid-type energy storage converter, which includes three core steps: power parameter setting, primary internal potential compensation, and secondary internal potential compensation.

[0006] The power parameter tuning takes into account both the fault current limiting range and the active support characteristics of the reactive voltage loop of the grid-type energy storage converter. First, the reference amplitudes of the active and reactive currents are determined; then, based on these reference values ​​of active and reactive currents, the power parameters of the grid-type energy storage converter are tuned.

[0007] The internal potential compensation adopts a phase-by-phase compensation method, which compensates the internal potential compensation amount required to suppress negative sequence current to the original internal potential, keeping the original grid-type control framework unchanged.

[0008] The internal potential secondary compensation adopts an overall compensation method. Based on the internal potential primary compensation, the internal potential is compensated secondary according to the amplitude requirements of the transient peak current, so as to limit the transient overcurrent under asymmetrical fault.

[0009] The power parameter tuning is specifically as follows:

[0010] The output phase currents of a grid-type energy storage converter are expressed using the following formulas: ;in, Let be the average phase angle of the initial phase angle of the positive-sequence voltage and the initial phase angle of the negative-sequence voltage, and ; Let A be the current amplitude of phase k; A is the first intermediate parameter used to calculate the current amplitude of each phase, which is synthesized from the d-axis and q-axis components of the positive and negative sequence currents and the angle δ; B is the second intermediate parameter used to calculate the current amplitude of each phase. This is the initial phase angle of the positive sequence voltage; The initial phase angle of the negative sequence voltage; The d-axis component of the positive sequence current in the dq coordinate system; For the negative sequence current, the d-axis component is shown in the dq coordinate system. The q-axis component of the positive sequence current in the dq coordinate system; For the negative sequence current dq coordinate system, it represents the q-axis component. Let be the phase offset angle of the current in each phase, and for phases a, b, and c, take values ​​of respectively. 、( )、( (), used to distinguish the phase differences of three-phase currents;

[0011] While meeting the current imbalance requirements, the negative sequence component of the current is set to 0, and the converter output current meets the requirement of not exceeding the preset safety threshold. It can be expressed using the following formula: ;

[0012] Based on the reactive voltage loop support characteristics of the grid-type energy storage converter, when an asymmetrical fault occurs, the active support characteristics are activated, while limiting the fault current to within the allowable range and the active current... Express it using the following formula: ; Active current; This is reactive current;

[0013] The setting of the active-reactive current reference value is expressed using the following formula: ; The magnitude of the positive sequence component of the given q-axis current; The magnitude of the positive sequence component of the given q-axis current;

[0014] Simultaneously, based on the fault current limiting range, the power setting value of the grid-type energy storage converter is obtained, expressed by the following formula: ; Given the magnitude of the q-axis component of the current; Given the magnitude of the q-axis component of the current; This represents the positive sequence component of the converter output voltage along the d-axis. This represents the positive q-axis component of the converter output voltage. For reactive power commands of grid-type converters; This is the active power command for grid-type converters.

[0015] The primary compensation of the internal potential includes the following steps:

[0016] S1. Perform dq transformation on the current output by the converter to obtain the DC component and AC component; the positive sequence current component is represented by the DC component, and the negative sequence current component is represented by the AC component of the second harmonic; based on this characteristic, the three-phase negative sequence current can be finally extracted using a filter.

[0017] S2. The extracted three-phase negative sequence current is compared with zero value. The deviation signal after comparison is adjusted by the PR controller, and the PR controller outputs a dynamic voltage compensation term. This is superimposed on the main control circuit to suppress the negative sequence current component, improve the imbalance of the output current, achieve the goal of balanced current control, and complete the first compensation of internal potential.

[0018] Step S1 is as follows:

[0019] When an asymmetrical fault occurs, the three-phase current contains a negative sequence component. Based on the frequency mapping characteristics of Park variation under asymmetrical operating conditions, the three-phase current is transformed into abc / dq coordinates, so that the positive sequence component is converted into a DC component in the dq coordinate system and the negative sequence component is converted into a second harmonic AC component.

[0020] The coordinate transformation matrix of the abc / dq coordinate transformation for: ;in, To synchronize the angular frequency of the rotating coordinate system;

[0021] For the positive sequence current component After coordinate transformation matrix The DC component obtained after rotary synchronous conversion is expressed by the following formula: ; This is the initial phase angle of the positive sequence voltage; This refers to the a-phase component of the positive sequence current. This represents the b-phase component of the positive sequence current. This refers to the c-phase component of the positive sequence current. This represents the d-axis component of the positive sequence current. This represents the q-axis component of the positive sequence current.

[0022] For negative sequence current components After coordinate transformation matrix The second harmonic AC component is obtained after the inverse rotational conversion, and is expressed by the following formula: ; The initial phase angle of the negative sequence voltage; This is the a-phase component of the negative sequence current; This is the b-phase component of the negative sequence current; This is the c-phase component of the negative sequence current; This represents the d-axis component of the negative sequence current. This represents the q-axis component of the negative sequence current.

[0023] Finally, through the pass function The second harmonic AC component is separated from the total current, and the three-phase negative sequence current is obtained by inverse transformation of dq / abc.

[0024] The transfer function Express it using the following formula: ; where s is the Laplace operator, used to represent complex frequency variables.

[0025] In step S2, the PR controller has resonant characteristics and its gain is 0 at the DC component, so it will not affect the DC component.

[0026] The secondary internal potential compensation specifically involves: based on the primary internal potential compensation, performing peak detection on the three-phase output current of phases a, b, and c to obtain the current amplitude; and comparing the current amplitude with the maximum allowable current value. The current deviation signal is obtained by comparison; the current deviation signal is then processed by a current deviation gain coefficient. After weighting, the secondary compensation term of the internal potential is obtained. Use the following formula to represent: ;in, The output current of phase a after the internal potential is compensated once; The output current of phase b after the internal potential is compensated once; The output current of phase c after the internal potential is compensated once.

[0027] Because the transient peak current has a large amplitude and a short duration, secondary compensation of the internal potential is not required after the fault enters the steady state. Therefore, the aforementioned current deviation gain coefficient is used in the secondary compensation of the internal potential. To attenuate the compensation value and thus exit secondary compensation, the following formula is used: ;in, The time when the fault occurred; This is the moment when the fault enters a steady state; The decay time constant is the current deviation gain coefficient.

[0028] This invention discloses an asymmetric fault current limiting method based on a grid-type energy storage converter. By compensating the internal potential once, it suppresses the negative sequence current and limits the overall current without introducing an additional negative sequence control circuit, thereby achieving the balanced current target under grid-type control. At the same time, it further introduces power command setting and internal potential secondary compensation to achieve transient overcurrent suppression under asymmetric faults. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the primary compensation of internal potential described in this invention;

[0030] Figure 2 This is a schematic diagram showing the flow direction of the secondary compensation current for the internal potential described in this invention;

[0031] Figure 3 This is a single-unit grid-connected model of a grid-type converter used in the embodiments of the present invention;

[0032] Figure 4 This is the output waveform of the external characteristic simulation experiment without introducing a current limiting circuit in the embodiment of the present invention;

[0033] Figure 5 This is the output waveform of the external characteristic simulation experiment in this embodiment of the invention, which introduces the internal potential primary compensation and power parameter tuning stage.

[0034] Figure 6 The output waveform diagram is shown in the simulation experiment of external characteristics using the method of the present invention in the embodiment of the present invention. Detailed Implementation

[0035] This invention provides an asymmetric fault current limiting method based on a grid-type energy storage converter, which includes three core steps: power parameter setting, primary internal potential compensation, and secondary internal potential compensation.

[0036] The power parameter tuning takes into account both the fault current limiting range and the active support characteristics of the reactive voltage loop of the grid-type energy storage converter. First, the reference amplitudes of the active and reactive currents are determined; then, based on these active and reactive current reference values, the power parameters of the grid-type energy storage converter are tuned.

[0037] The internal potential compensation adopts a phase-by-phase compensation method, which compensates the internal potential compensation amount required to suppress negative sequence current to the original internal potential, thus maintaining the original control structure.

[0038] The internal potential secondary compensation is based on the internal potential primary compensation, and performs secondary compensation on the internal potential according to the amplitude requirements of the transient peak current, so as to limit the transient overcurrent under asymmetrical faults.

[0039] The power parameter tuning is specifically as follows:

[0040] The output phase currents of a grid-type energy storage converter are expressed using the following formulas: ;in, Let be the average phase angle of the initial phase angle of the positive-sequence voltage and the initial phase angle of the negative-sequence voltage, and ; Let A be the current amplitude of phase k; A is the first intermediate parameter used to calculate the current amplitude of each phase, which is synthesized from the d-axis and q-axis components of the positive and negative sequence currents and the angle δ; B is the second intermediate parameter used to calculate the current amplitude of each phase. This is the initial phase angle of the positive sequence voltage; The initial phase angle of the negative sequence voltage; The d-axis component of the positive sequence current in the dq coordinate system; For the negative sequence current, the d-axis component is shown in the dq coordinate system. The q-axis component of the positive sequence current in the dq coordinate system; For the negative sequence current dq coordinate system, it represents the q-axis component. Let be the phase offset angle of the current in each phase, and let be the values ​​for phases a, b, and c respectively. 、( )、( (), used to distinguish the phase differences of three-phase currents;

[0041] While meeting the current imbalance requirements, the negative sequence component of the current is set to 0, and the converter output current meets the requirement of not exceeding the preset safety threshold. It can be expressed using the following formula: ;

[0042] Based on the reactive voltage loop support characteristics of the grid-type energy storage converter, when an asymmetrical fault occurs, the active support characteristics are activated, while limiting the fault current to within the allowable range and the active current... Express it using the following formula: ; Active current; This is reactive current;

[0043] The setting of the active-reactive current reference value is expressed using the following formula: ; The magnitude of the positive sequence component of the given q-axis current; The magnitude of the positive sequence component of the given q-axis current;

[0044] Simultaneously, based on the fault current limiting range, the power setting value of the grid-type energy storage converter is obtained, expressed by the following formula: ; Given the magnitude of the q-axis component of the current; Given the magnitude of the q-axis component of the current; This represents the positive sequence component of the converter output voltage along the d-axis. This represents the positive q-axis component of the converter output voltage. For reactive power commands of grid-type converters; This is the active power command for grid-type converters.

[0045] The internal potential is compensated once, such as Figure 1 As shown, it includes the following steps:

[0046] S1. Perform dq transformation on the current output by the converter to obtain the DC component and AC component; the positive sequence current component is represented by the DC component, and the negative sequence current component is represented by the AC component of the second harmonic; based on this characteristic, the three-phase negative sequence current can be finally extracted using a filter.

[0047] S2. The extracted three-phase negative sequence current is compared with zero value. The deviation signal after comparison is adjusted by the PR controller, and the PR controller outputs a dynamic voltage compensation term. This is superimposed on the main control circuit to suppress the negative sequence current component, improve the imbalance of the output current, achieve the goal of balanced current control, and complete the first compensation of internal potential.

[0048] Step S1 is as follows:

[0049] When an asymmetrical fault occurs, the three-phase current contains a negative sequence component. Based on the frequency mapping characteristics of Park variation under asymmetrical operating conditions, the three-phase current is transformed into abc / dq coordinates, so that the positive sequence component is converted into a DC component in the dq coordinate system and the negative sequence component is converted into a second harmonic AC component.

[0050] The coordinate transformation matrix of the abc / dq coordinate transformation for: ;in, To synchronize the angular frequency of the rotating coordinate system;

[0051] For the positive sequence current component After coordinate transformation matrix The DC component obtained after rotary synchronous conversion is expressed by the following formula: ; This is the initial phase angle of the positive sequence voltage; This refers to the a-phase component of the positive sequence current. This represents the b-phase component of the positive sequence current. This refers to the c-phase component of the positive sequence current. This represents the d-axis component of the positive sequence current. This represents the q-axis component of the positive sequence current.

[0052] For negative sequence current components After coordinate transformation matrix The second harmonic AC component is obtained after the inverse rotational conversion, and is expressed by the following formula: ; The initial phase angle of the negative sequence voltage; This is the a-phase component of the negative sequence current; This is the b-phase component of the negative sequence current; This is the c-phase component of the negative sequence current; This represents the d-axis component of the negative sequence current. This represents the q-axis component of the negative sequence current.

[0053] Finally, through the pass function The second harmonic AC component is separated from the total current, and the three-phase negative sequence current is obtained by inverse transformation of dq / abc.

[0054] The transfer function Express it using the following formula: ; where s is the Laplace operator, used to represent complex frequency variables.

[0055] In step S2, the PR controller has resonant characteristics and a gain of 1 at the DC component, so it will not affect the DC component.

[0056] The internal potential secondary compensation, such as Figure 2 As shown, specifically: based on the initial compensation of the internal potential, peak detection is performed on the three-phase output current of phases abc to obtain the current amplitude; the current amplitude is then compared with the maximum allowable current value. The current deviation signal is obtained by comparison; the current deviation signal is then processed by a current deviation gain coefficient. After weighting, the secondary compensation term of the internal potential is obtained. Use the following formula to represent: ;in, The output current of phase a after the internal potential is compensated once; The output current of phase b after the internal potential is compensated once; The output current of phase c after the internal potential is compensated once.

[0057] Because the transient peak current has a large amplitude and a short duration, secondary compensation of the internal potential is not required after the fault enters the steady state. Therefore, the aforementioned current deviation gain coefficient is used in the secondary compensation of the internal potential. To attenuate the compensation value and thus exit secondary compensation, the following formula is used: ;in, The time when the fault occurred; This is the moment when the fault enters a steady state; The decay time constant is the current deviation gain coefficient.

[0058] The method of the present invention will be further described below with reference to an embodiment:

[0059] A grid-connected system model of a grid-connected energy storage converter is constructed based on simulation, such as... Figure 3 As shown.

[0060] In this simulation, the specific operating conditions are as follows: the grid-connected converter operates at rated conditions with a set output active power of 20kW and reactive power of 0var. At 0.5s, the grid voltage drops by 50%; at 1.5s, the fault is cleared. Based on the current safety threshold of 1.2 to 1.5 times the rated current, the steady-state overcurrent withstand coefficient n of the grid-connected energy storage converter is set. c =1.2, transient overcurrent resistance coefficient n t =1.5, the parameters are shown in Table 1 below.

[0061] Table 1 Experimental parameters Three simulation experiments under asymmetrical faults were conducted on the simulation platform, including a simulation experiment group without introducing current limiting circuitry, a simulation experiment group with internal potential compensation and power parameter tuning circuitry, and an experiment group using the method of this invention.

[0062] Simulation results without introducing current limiting are as follows Figure 4 As shown. By Figure 4 It is known that when a grid asymmetry fault occurs, the negative sequence component of the current causes the grid-type converter to output three-phase unbalanced current, with the maximum phase peak current exceeding 260A. At the same time, the second harmonic component will appear, causing fluctuations in active and reactive power. The fluctuation range of active power is about 80kW, and the fluctuation range of reactive power is about 70kvar, which seriously threatens the safe and stable operation of the grid-type converter and the grid.

[0063] The simulation results of the experimental group that incorporates primary internal potential compensation and power parameter tuning are as follows: Figure 5 As shown. By Figure 5It can be seen that, on the one hand, the internal potential compensation greatly reduces the imbalance of the three-phase current. On the other hand, due to the combination of the maximum current limit requirement and the readjustment of the given power setting, the steady-state overcurrent is limited to 1.2 times the rated current value, i.e., 51.2046A. The grid-type converter also generates additional reactive power to support the grid during the fault period. However, the transient overcurrent still cannot meet the requirements, which is about 132A, far exceeding 1.5 times the rated current value. Therefore, it is still necessary to consider limiting the transient overcurrent, which seriously threatens the safe and stable operation of the grid-type converter and the grid.

[0064] The experimental results using the method of this invention are as follows: Figure 6 As shown. By Figure 6 It can be seen that, after adopting the method proposed in this invention, the external characteristic output waveform during the fault steady-state period is maintained in accordance with the fault steady-state stage. During the fault transient period, it can be seen that with the temporary addition of the internal potential secondary compensation, there is a smooth current regulation during the fault transient period, limiting the transient overcurrent to 1.5 times the rated current value, which meets the requirements for overcurrent suppression during the fault period and verifies the effectiveness of the method. During the fault period, the current amplitude is limited within the safety threshold, that is, the three-phase current is limited to within 1.2 Pu, the maximum phase peak current is also effectively limited, and the three-phase current is balanced. In addition, during the fault period, the grid-type energy storage converter actively supports the grid voltage and generates additional reactive power.

[0065] In summary, by using primary internal potential compensation, the balanced current target under grid-type control can be achieved without introducing additional negative-sequence control circuitry. However, relying solely on primary internal potential compensation, the current amplitude still cannot meet the current limiting requirements. Therefore, power parameter tuning and secondary internal potential compensation are further introduced. Simulation results show that during fault periods, the output current of the grid-type energy storage converter can be effectively limited within the current safety threshold, actively supporting the power grid.

Claims

1. An asymmetric fault current limiting method based on a grid-type energy storage converter, characterized in that, It includes three core components: power parameter tuning, primary internal potential compensation, and secondary internal potential compensation.

2. The asymmetric fault current limiting method based on a grid-type energy storage converter according to claim 1, characterized in that, The power parameter tuning takes into account both the fault current limiting range and the active support characteristics of the reactive voltage loop of the grid-type energy storage converter. First, the reference amplitudes of the active and reactive currents are determined; then, based on these reference values ​​of active and reactive currents, the power parameters of the grid-type energy storage converter are tuned.

3. The asymmetric fault current limiting method based on a grid-type energy storage converter according to claim 2, characterized in that, The power parameter tuning is specifically as follows: The output phase currents of a grid-type energy storage converter are expressed using the following formulas: ;in, Let be the average phase angle of the initial phase angle of the positive-sequence voltage and the initial phase angle of the negative-sequence voltage, and ; Let A be the current amplitude of phase k; A is the first intermediate parameter used to calculate the current amplitude of each phase, which is synthesized from the d-axis and q-axis components of the positive and negative sequence currents and the angle δ; B is the second intermediate parameter used to calculate the current amplitude of each phase. This is the initial phase angle of the positive sequence voltage; The initial phase angle of the negative sequence voltage; The d-axis component of the positive sequence current in the dq coordinate system; For the negative sequence current, the d-axis component is shown in the dq coordinate system. The q-axis component of the positive sequence current in the dq coordinate system; For the negative sequence current dq coordinate system, it represents the q-axis component. Let be the phase offset angle of the current in each phase, and let be the values ​​for phases a, b, and c respectively. 、( )、( (), used to distinguish the phase differences of three-phase currents; While meeting the current imbalance requirements, the negative sequence component of the current is set to 0, and the converter output current meets the requirement of not exceeding the preset safety threshold. It can be expressed using the following formula: ; Based on the reactive voltage loop support characteristics of the grid-type energy storage converter, when an asymmetrical fault occurs, the active support characteristics are activated, while limiting the fault current to within the allowable range and the active current... Express it using the following formula: ; Active current; This is reactive current; The setting of the active-reactive current reference value is expressed using the following formula: ; The magnitude of the positive sequence component of the given q-axis current; The magnitude of the positive sequence component of the given q-axis current; Simultaneously, based on the fault current limiting range, the power setting value of the grid-type energy storage converter is obtained, expressed by the following formula: ; Given the magnitude of the q-axis component of the current; Given the magnitude of the q-axis component of the current; This represents the positive sequence component of the converter output voltage along the d-axis. This represents the positive q-axis component of the converter output voltage. For reactive power commands of grid-type converters; This is the active power command for grid-type converters.

4. The asymmetric fault current limiting method based on a grid-type energy storage converter according to claim 1, characterized in that, The internal potential compensation adopts a phase-by-phase compensation method, which compensates the internal potential required to suppress the negative sequence current to the original internal potential, thus maintaining the original control structure.

5. The asymmetric fault current limiting method based on a grid-type energy storage converter according to claim 4, characterized in that, The primary compensation of the internal potential includes the following steps: S1. Perform dq transformation on the current output by the converter to obtain the DC component and AC component; the positive sequence current component is represented by the DC component, and the negative sequence current component is represented by the second harmonic AC component; based on this characteristic, the three-phase negative sequence current is finally extracted using a filter. S2. The extracted three-phase negative sequence current is compared with zero value. The deviation signal after comparison is adjusted by the PR controller, and the PR controller outputs a dynamic voltage compensation term. This is superimposed on the main control circuit to suppress the negative sequence current component, improve the imbalance of the output current, achieve the goal of balanced current control, and complete the first compensation of internal potential.

6. The asymmetric fault current limiting method based on a grid-type energy storage converter according to claim 5, characterized in that, Step S1 is as follows: When an asymmetrical fault occurs, the three-phase current contains a negative sequence component. Based on the frequency mapping characteristics of Park variation under asymmetrical operating conditions, the three-phase current is transformed into abc / dq coordinates, so that the positive sequence component is converted into a DC component in the dq coordinate system and the negative sequence component is converted into a second harmonic AC component. The coordinate transformation matrix of the abc / dq coordinate transformation for: ;in, The angular frequency of the synchronous rotating coordinate system; For the positive sequence current component After coordinate transformation matrix The DC component obtained after rotary synchronous conversion is expressed by the following formula: ; This is the initial phase angle of the positive sequence voltage; This refers to the a-phase component of the positive sequence current. This represents the b-phase component of the positive sequence current. This refers to the c-phase component of the positive sequence current. The positive sequence current d-axis component; This represents the q-axis component of the positive sequence current. For negative sequence current components After coordinate transformation matrix The second harmonic AC component is obtained after the inverse rotational conversion, and is expressed by the following formula: ; The initial phase angle of the negative sequence voltage; This is the negative sequence current a-phase component; This is the b-phase component of the negative sequence current; This is the c-phase component of the negative sequence current; This represents the d-axis component of the negative sequence current. This represents the q-axis component of the negative sequence current. Finally, through the pass function The second harmonic AC component is separated from the total current, and the three-phase negative sequence current is obtained by inverse transformation of dq / abc.

7. The asymmetric fault current limiting method based on a grid-type energy storage converter according to claim 6, characterized in that, The transfer function Express it using the following formula: ; where s is the Laplace operator, used to represent complex frequency variables.

8. The asymmetric fault current limiting method based on a grid-type energy storage converter according to claim 1, characterized in that, The secondary internal potential compensation is based on the primary internal potential compensation, and performs secondary compensation on the internal potential according to the amplitude limit requirements of the transient peak current, so as to limit the transient overcurrent under asymmetrical faults.

9. The asymmetric fault current limiting method based on a grid-type energy storage converter according to claim 8, characterized in that, The secondary internal potential compensation specifically involves: based on the primary internal potential compensation, performing peak detection on the three-phase output current of phases a, b, and c to obtain the current amplitude; and comparing the current amplitude with the maximum allowable current value. The current deviation signal is obtained by comparison; the current deviation signal is then processed by a current deviation gain coefficient. After weighting, the secondary compensation term of the internal potential is obtained. ; Express it using the following formula: ;in, The output current of phase a after the internal potential is compensated once; The output current of phase b after the internal potential is compensated once; The output current of phase c after the internal potential is compensated once.

10. The asymmetric fault current limiting method based on a grid-type energy storage converter according to claim 9, characterized in that, The current deviation gain coefficient is used in the secondary compensation of internal potential. To attenuate the compensation value and thus exit secondary compensation, the following formula is used: ;in, The time when the fault occurred; This is the moment when the fault enters a steady state; is the decay time constant of the current deviation gain coefficient.

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