A fault current limiting method for grid-type energy storage converters based on dynamic virtual impedance

Abstract

This invention discloses a fault current limiting method for grid-connected energy storage converters based on dynamic virtual impedance. By introducing positive and negative sequence dynamic virtual impedance, the virtual impedance is adjusted to its optimal value in real time according to the degree of grid voltage drop and the magnitude of grid-connected current. This method effectively suppresses short-circuit current under all types of short-circuit faults, including symmetrical and asymmetrical faults, protecting the operational safety of the converter equipment. Furthermore, this method adjusts the power command according to the fault condition, achieving rapid reactive power support while maintaining transient power angle stability. This method facilitates grid fault recovery, enhances the operational stability of new power systems, and effectively solves the problem that existing control methods struggle to simultaneously address fault current limiting and transient instability under severe grid short-circuit faults.

Description

Technical Field

[0001] This invention relates to the field of battery energy storage conversion technology, and in particular to a fault current limiting method for a grid-type energy storage converter based on dynamic virtual impedance. Background Technology

[0002] Through grid-based control technology, energy storage converters can simulate the physical characteristics of traditional synchronous generators, functioning externally as voltage sources and providing inertia compensation and voltage / frequency support to the power grid. However, when faced with sudden grid short-circuit faults, grid-based energy storage converters will experience significant overcurrents to maintain their internal potential. Compared to the 7-9 times short-time overcurrent capacity of traditional synchronous generators, grid-based energy storage converters, composed of semiconductor devices, can only withstand 1.2-1.5 times their rated overcurrent for short periods. This means that severe short-circuit currents can damage equipment and threaten the safe and stable operation of the new power system.

[0003] Currently, methods for suppressing fault overcurrent mainly fall into two categories: current saturation limiting and adding virtual impedance. Current saturation limiting involves switching the grid-connected energy storage converter to current source control mode when a grid fault occurs, thus limiting the grid-connected current. However, this method causes the converter to lose its grid-connected capability and suffers from transient stability issues after grid recovery. Adding virtual impedance involves introducing additional virtual impedance to limit the current when the grid-connected energy storage converter detects a current value exceeding a set threshold. Traditional virtual impedance methods are relatively ideal but difficult to implement, and their current-limiting effect is poor under fault conditions, especially asymmetrical short-circuit faults, making them difficult to apply in engineering practice. Therefore, there is an urgent need for an effective fault current suppression method that can suppress short-circuit current under all types of short-circuit faults to improve the fault ride-through capability and transient power angle stability of grid-connected energy storage converters, ensuring the efficient and reliable operation of the power system. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a fault current limiting method for grid-type energy storage converters based on dynamic virtual impedance, which effectively suppresses short-circuit current surges under symmetrical and asymmetrical grid faults, and enhances the fault ride-through capability and dynamic characteristics of grid-type energy storage converters in actively supporting the grid.

[0005] To solve the above-mentioned technical problems, the technical solution adopted in this invention is: a fault current limiting method for grid-type energy storage converters based on dynamic virtual impedance, applicable to short-circuit current suppression of grid-type energy storage converters under grid fault conditions. The grid-type energy storage converter operates using a direct voltage modulation method without a current loop, maintaining grid-type characteristics at any time, and is connected to the external grid through power synchronization control. By subtracting the virtual impedance voltage drop from the output side of the power synchronization control module, a dynamic virtual impedance is rapidly generated when a grid fault occurs, and the power command is adjusted in real time to cooperate, limiting the short-circuit current while maintaining transient power angle stability, thus achieving rapid reactive power output. The fault current limiting method for grid-type energy storage converters includes the following steps:

[0006] 1) At the beginning of each sampling period, the grid-connected current i x (k) The fault current limiting module sent to the control unit tracks the maximum amplitude I of the three-phase grid-connected current in real time. max (k), the calculation formula is:

[0007]

[0008] In the formula, max{·} is the maximum value operator; notch 100Hz (·) is the notch filter operator, with a notch frequency of 100Hz; k represents the sequence number of the kth sampling period;

[0009] 2) The maximum amplitude I of the three-phase current obtained in step 1) max (k) and the virtual impedance magnitude Z of the previous sampling period v Using (k-1) as input, current state evaluation is performed to obtain the current state quantity State(k) of the grid-type energy storage converter. The criterion is as follows:

[0010]

[0011] In the formula, I N is the rated current of the grid-type energy storage converter, ∩ is the intersection operator, and ∪ is the union operator;

[0012] 3) Convert the grid voltage u gx (k) is used as the input quantity to extract the positive sequence component of the grid voltage, thus obtaining the magnitude of the positive sequence component of the grid voltage. And perform power grid fault detection, generating a fault characteristic pulse flag bit Flag(k), the expression of which is:

[0013]

[0014] It is worth noting that under normal operating conditions, the flag bit Flag(k) is 00; when a fault occurs, the flag bit Flag(k) increments to 01, and after several sampling cycles, it returns to 00, generating a trigger narrow pulse; when the fault is recovered, the flag bit Flag(k) increments to 10, and after several sampling cycles, it returns to 00, generating a trigger narrow pulse; in this invention, the pulse duration is 100 sampling cycles.

[0015] 4) Send the current state State(k) obtained in step 2) to the state bit of the 4-to-1 data selector to select the virtual impedance amplitude Z under different current states. v The relationship of (k) is specifically modified by using the fault characteristic pulse flag bit Flag(k) in step 3) to correct the initial value of the integrator at the fault occurrence and recovery times, so as to accelerate the convergence rate of the fault current suppression method. The calculation formula is as follows:

[0016]

[0017] In the formula, K i The integral coefficients of the virtual impedance integrator are used to optimize the virtual impedance under each current state; ΔT s Z is the sampling period length; v0 Z is the initial value provided to the integrator at the moment the fault occurs; it is generally taken as a large value. v1 The initial value provided to the integrator at the fault recovery time is generally a small value;

[0018] 5) The virtual impedance amplitude Z obtained through step 4) v (k) and the set virtual impedance phase θ Zv (k), the virtual resistance R is calculated. v (k) and virtual reactance X v (k), the calculation formula is:

[0019]

[0020] 6) The grid-connected current i x (k) Construct the differential terms of the positive and negative sequence components and connect them with the virtual resistance R. v (k) and virtual reactance X v (k) Perform Ohm's law calculations to obtain the virtual impedance voltage drop ΔU. Zvx (x = a, b, c), the calculation formula is:

[0021]

[0022] In the formula, This is the differential term of the positive-sequence grid-connected current. For the differential term of the negative-sequence grid-connected current, ωN This is the rated angular frequency.

[0023] Simply increasing the virtual impedance carries the risk of transient instability; therefore, power command correction is needed to mitigate this problem. Furthermore, the power command adjustment method of the fault current limiting method of this invention includes the following steps:

[0024] 1) The magnitude of the positive sequence component of the grid voltage calculated by the fault current limiting module and the maximum amplitude of grid-connected current I max (k) is used to calculate the maximum capacity S of the grid-type energy storage converter under a grid short-circuit fault. F (k), the calculation formula is:

[0025]

[0026] 2) Based on the amplitude of the positive sequence component of the grid voltage With rated internal potential E N The ratio of the initial active power command P ref0 (k) is corrected to obtain the final active power command P. ref (k), the calculation formula is:

[0027]

[0028] 3) Based on S obtained in steps 1) and 2), F (k) and P ref (k), for the initial reactive power command Q ref0 (k) is corrected to obtain the final reactive power command Q. ref (k), the calculation formula is:

[0029]

[0030] In the formula, I QGB (k) represents the reactive current that the grid-type energy storage converter is required to respond to under fault conditions according to national standards.

[0031] Compared with existing technologies, the beneficial effects of this invention are as follows: By introducing positive and negative sequence dynamic virtual impedance, this invention dynamically adjusts the virtual impedance in real time according to the grid voltage and grid-connected current until the optimal value is reached. This effectively suppresses fault current under all types of short-circuit faults, protecting the operational safety of converter equipment. Furthermore, this invention corrects the power command according to the fault condition, maintaining transient power angle stability while rapidly responding to reactive power, which helps grid fault recovery and enhances the operational stability of the new power system. The method proposed in this invention is widely applicable to various power electronic equipment related to grid construction technology, significantly improving the fault ride-through and continuous operation capabilities of the equipment, and has good prospects for promotion and application value in engineering. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of a grid-type energy storage converter according to an embodiment of the present invention;

[0033] Figure 2 This is a block diagram of a control system for a grid-type energy storage converter according to an embodiment of the present invention;

[0034] Figure 3 This is a control block diagram of a fault current limiting method for a grid-type energy storage converter according to an embodiment of the present invention;

[0035] Figure 4 This is a schematic diagram of the active power command adjustment of a grid-type energy storage converter according to an embodiment of the present invention;

[0036] Figure 5 In one embodiment of the present invention, when the grid-connected energy storage converter experiences a three-phase symmetrical voltage drop to 0.2 pu, the grid voltage u gx Grid-connected current i x Instantaneous active power P out Instantaneous reactive power Q out The simulated waveform; where A11 is the grid voltage waveform u gx A12 is the grid-connected current i x Waveform, A13 represents the converter output active power P out and output reactive power Q out Waveform;

[0037] Figure 6(a) shows the grid voltage u after the grid-type energy storage converter adopts the fault current suppression method in an embodiment of the present invention, when the grid voltage drops symmetrically to 0.2pu. gx Grid-connected current i x Instantaneous active power P out Instantaneous reactive power Q out The simulated waveform; where A21 is the grid voltage u gx Waveform, A22 is the grid-connected current i x Waveform, A23 represents the converter output active power P out and output reactive power Q out Waveform; Figure 6(b) shows the virtual impedance amplitude Z of the grid-type energy storage converter under the same conditions as Figure 6(a). v Virtual impedance phase θ Zv The simulated waveform; where A24 is the virtual impedance amplitude Z. v Waveform, A25 represents the virtual impedance phase θ Zv Waveform;

[0038] Figure 7 In one embodiment of the present invention, when the grid-connected energy storage converter experiences a single-phase symmetrical voltage drop to 0.5 pu, the grid voltage ugx Grid-connected current i x Instantaneous active power P out Instantaneous reactive power Q out The simulated waveform; where A31 is the grid voltage waveform u gx A32 represents the grid-connected current i x Waveform, A33 represents the converter output active power P out and output reactive power Q out Waveform;

[0039] Figure 8(a) shows the grid voltage u after the grid-type energy storage converter adopts the fault current suppression method in an embodiment of the present invention, when the grid voltage drops symmetrically to 0.5pu. gx Grid-connected current i x Instantaneous active power P out Instantaneous reactive power Q out The simulated waveform; where A41 is the grid voltage u gx Waveform, A42 is the grid-connected current i x Waveform, A43 represents the converter output active power P out and output reactive power Q out Waveform; Figure 8(b) shows the virtual impedance amplitude Z of the grid-type energy storage converter under the same conditions as Figure 8(a). v Virtual impedance phase θ Zv The simulated waveform; where A44 is the virtual impedance amplitude Z. v Waveform, A45 represents the virtual impedance phase θ Zv Waveform. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] Figure 1This is a schematic diagram of a grid-type energy storage converter according to an embodiment of the present invention. It includes an energy storage battery, a DC capacitor, an ANPC-type three-level inverter, an AC filter inductor, and a control system. The control system includes a sampling unit, a control unit, and a drive protection unit. The energy storage battery is connected in parallel to both sides of the DC capacitor. The DC capacitor is connected to the ANPC-type three-level inverter. The ANPC-type three-level inverter is connected to the AC filter inductor. The sampling unit is connected to the control unit. The control unit is connected to the drive protection unit. The drive protection unit is connected to the ANPC-type three-level inverter. The control unit mainly consists of a power synchronization control module and a fault current limiting module.

[0042] Figure 2 This is a block diagram of the control system of a grid-connected energy storage converter according to an embodiment of the present invention. The power synchronization control module is responsible for the grid-connected control technology of the converter, simulating the physical characteristics of a synchronous generator to provide voltage and frequency support to the power grid. The fault current limiting module is responsible for implementing virtual impedance when a short-circuit fault occurs in the power grid, limiting the short-circuit current, and improving the transient power angle stability of the converter, providing rapid reactive power response support to the power grid. The specific control method includes the following steps:

[0043] 1) At the beginning of each sampling period, the sampling unit samples the AC mains voltage u. gx Grid-connected current i of grid-connected energy storage converter x Sampling and low-pass filtering are performed (x = a, b, c);

[0044] 2) The AC grid voltage u obtained in step 1) gx and grid-connected current i x Perform instantaneous power calculation to obtain the instantaneous active power P output by the current energy storage converter. out and instantaneous reactive power Q out The calculation formula is:

[0045]

[0046] 3) Convert the AC grid voltage u gx Grid-connected current i x Active power initial command P ref0 Reactive power initial command Q ref0 The fault current limiting module sent to the control unit adjusts the power command to obtain the corrected active power command P. ref and reactive power command Q ref ;

[0047] 4) The instantaneous active power P calculated in step 2) out Instantaneous reactive power Q out And the active power command P obtained in step 3)ref and reactive power command Q ref The power synchronization control module sent to the control unit receives the internal potential amplitude command E for the current sampling period. ref and phase command θ ref The calculation formula is:

[0048]

[0049] In the formula, ω N E is the rated angular frequency. N Where J is the rated internal potential, and D is the moment of inertia. p D is the damping coefficient. q is the reactive power-voltage coefficient, and s is the Laplace operator;

[0050] 5) Set the internal potential amplitude command E from step 4) to... ref and phase command θ ref Voltage vector synthesis is performed to obtain the positive sequence modulation voltage E. x (x = a, b, c), the calculation formula is:

[0051]

[0052] 6) Convert the AC grid voltage u gx and grid-connected current i x The fault current limiting module fed into the control unit uses the fault current limiting method of the grid-type energy storage converter to obtain the virtual impedance voltage drop ΔU. Zvx (x = a, b, c);

[0053] 7) The virtual impedance voltage drop ΔU obtained in step 6) Zvx With positive sequence modulation voltage E x Superimposed and normalized, the modulation wave reference voltage E is obtained. invx (x = a, b, c), the calculation formula is:

[0054]

[0055] In the formula, U dc This represents the DC-side voltage of the grid-type energy storage converter; after the reference voltage is limited and compared with a triangular carrier wave, the drive signal of the ANPC type three-level inverter is obtained.

[0056] 8) The drive signal of the ANPC type three-level inverter obtained in step 7) is transmitted to the drive protection unit to drive the grid-type energy storage converter.

[0057] Figure 3This is a control block diagram of a fault current limiting method for a grid-type energy storage converter according to an embodiment of the present invention. It includes two parts: virtual impedance generation and power command adjustment. Virtual impedance generation involves real-time evaluation of the current state and applying corresponding virtual impedance relationships under different levels of overcurrent to flexibly and dynamically adjust the virtual impedance, thereby limiting the short-circuit current. Specifically, under asymmetrical faults, the control algorithm can generate virtual impedance under negative-sequence networks, effectively suppressing both positive-sequence and negative-sequence currents, ensuring that the overcurrent of the grid-type energy storage converter does not exceed 1.1 times its rated value under all types of short-circuit faults. Power command adjustment is made to adjust the power command according to the actual fault conditions to assist the virtual impedance algorithm in achieving optimal current limiting performance.

[0058] Under the premise of unchanged active power command, increasing virtual impedance will cause the power angle characteristic curve to decrease, resulting in a reduction in the maximum deceleration area. Grid-type energy storage converters will also be at risk of transient instability. Therefore, power command correction is needed to avoid these drawbacks. Dynamic addition of virtual impedance and adaptive adjustment of power command together constitute the fault current limiting method proposed in this embodiment of the invention. The two are interlinked and form an organic whole. Figure 4 The diagram shows the adjustment of the active power command. The active power command is reduced by the same magnitude based on the voltage drop, the acceleration area is reduced from S1 to S1', and the maximum deceleration area is increased from S2 to S2'. At the same time, the power angle δ before and after the fault remains basically unchanged, effectively improving the transient stability of the converter. The reactive power command is set according to the reactive current response requirements and low voltage ride-through curve of the national standard to ensure that it provides the grid with rapid emergency reactive power support to the greatest extent within the capacity limit.

[0059] To enhance the versatility and practicality of this method, the following presentation uses a discrete form. The specific fault current limiting control method includes the following steps:

[0060] 1) At the beginning of each sampling period, the grid-connected current i x (k) The fault current limiting module sent to the control unit tracks the maximum amplitude I of the three-phase grid-connected current in real time. max (k), the calculation formula is:

[0061]

[0062] In the formula, max{·} is the maximum value operator; notch 100Hz (·) is the notch filter operator, with a notch frequency of 100Hz; k represents the sequence number of the kth sampling period;

[0063] 2) The maximum amplitude I of the three-phase current obtained in step 1) max (k) and the virtual impedance magnitude Z of the previous sampling period vUsing (k-1) as input, current state evaluation is performed to obtain the current state quantity State(k) of the grid-type energy storage converter. The criterion is as follows:

[0064]

[0065] In the formula, I N is the rated current of the grid-type energy storage converter, ∩ is the intersection operator, and ∪ is the union operator;

[0066] When the grid-connected energy storage converter is in normal operating condition, State(k) is marked as 00, indicating a "normal" state; when the maximum magnitude of the grid-connected current of the grid-connected energy storage converter exceeds 1.1 times I... N When State(k) is 01, it indicates a "serious fault" state, requiring an increase in virtual impedance to suppress current; when the maximum amplitude of the grid-connected current of the grid-connected energy storage converter is 1.1 times I... N and 1.0 times I N When the range is within 10, State(k) is marked as 10, indicating a "fault-resistant" state. At this time, the virtual impedance is in the steady-state operating range under grid fault conditions and requires no adjustment. When the maximum amplitude of the grid-connected current of the grid-connected energy storage converter is less than I... N When the State(k) is 11, it indicates the "recovery preparation" state, at which time the grid-type energy storage converter waits for the external power grid to recover from the fault;

[0067] 3) Convert the grid voltage u gx (k) is used as the input quantity to extract the positive sequence component of the grid voltage, thus obtaining the magnitude of the positive sequence component of the grid voltage. And perform power grid fault detection, generating a fault characteristic pulse flag bit Flag(k), the expression of which is:

[0068]

[0069] It is worth noting that under normal operating conditions, the flag bit Flag(k) is 00; when a fault occurs, the flag bit Flag(k) increments to 01, and after several sampling cycles, it returns to 00, generating a trigger narrow pulse; when the fault is recovered, the flag bit Flag(k) increments to 10, and after several sampling cycles, it returns to 00, generating a trigger narrow pulse; in this invention, the pulse duration is 100 sampling cycles.

[0070] 4) Send the current state State(k) obtained in step 2) to the state bit of the 4-to-1 data selector to select the virtual impedance amplitude Z under different current states. vThe relationship of (k) is specifically modified by using the fault characteristic pulse flag bit Flag(k) in step 3) to correct the initial value of the integrator at the fault occurrence and recovery times, so as to accelerate the convergence rate of the fault current suppression method. The calculation formula is as follows:

[0071]

[0072] In the formula, K i The integral coefficients of the virtual impedance integrator are used to optimize the virtual impedance under each current state; ΔT s Z is the sampling period length; v0 Z is the initial value provided to the integrator at the moment the fault occurs; it is generally taken as a large value. v1 The initial value provided to the integrator at the fault recovery time is generally a small value;

[0073] In this method, an integrator is used to solve for the optimal value of the virtual impedance under different current states, with 1.1 times I. N and 1.0 times I N As the upper and lower limits of the action threshold, the fault current is eventually smoothly converged to below the maximum current limit allowed by the grid-type energy storage converter; when State(k) is 00 and Flag(k) is 00, the virtual impedance is cleared; when State(k) is 01 and Flag(k) is 00, I... max (k) and 1.1I N The difference is used as the input to the integrator to increase the virtual impedance, thereby suppressing the increase of the short-circuit current; when State(k) is 10 and Flag(k) is 00, 0 is used as the input to the integrator, which is equivalent to keeping the virtual impedance amplitude constant to prevent the short-circuit current from changing slightly near the critical value, causing frequent switching of State(k); when State(k) is 11 and Flag(k) is 00, I is used as the input to the integrator. max (k) and I N The difference is used as the input to the integrator to reduce the excessive virtual impedance; when Flag(k) is 0 or 1, it indicates that a short-circuit fault has occurred in the power grid, at which time a larger Z is used. v0 As the initial value for the integrator, it effectively suppresses transient overcurrent; when Flag(k) is 10, it indicates that the grid fault has been recovered, at which point a smaller Z... v1 As the initial value for the integrator, it enables the converter to quickly and smoothly transition from the current-limited state to the normal operating state.

[0074] 5) The virtual impedance amplitude Z obtained through step 4) v (k) and the set virtual impedance phase θ Zv (k), the virtual resistance R is calculated. v (k) and virtual reactance Xv (k), the calculation formula is:

[0075]

[0076] 6) For the grid-connected current i x (k) Construct the differential terms for the positive and negative order components, and calculate them using the following formula:

[0077]

[0078]

[0079] In the above formula, θ is used respectively ref (k) and -θ ref (k) is used as the reference phase. Park coordinate transformation is performed on the grid-connected current, and the second harmonic component is filtered out using a 100Hz notch filter to obtain the positive and negative sequence components of the grid-connected current. Then, inverse Park coordinate transformation is performed in the dq coordinate system with a 90° lead, thus completing the grid-connected current i x (k) Construct the differential terms for the positive and negative order components. It is worth noting that the differential terms constructed using spatial coordinate transformation are similar in magnitude to i. x (k) is equal, but leads by 90° in phase, therefore this value is consistent with the actual di. x (k) / dt numerically exists 1 / ω N The gain difference, this effect will subsequently affect its relationship with virtual reactance X. v (k) is eliminated during the calculation process;

[0080] 7) Compare the grid-connected current and its differential term obtained in step 6) with the virtual resistance R. v (k) and virtual reactance X v (k) Perform Ohm's law calculations to obtain the virtual impedance voltage drop ΔU. Zvx (x = a, b, c), the calculation formula is:

[0081]

[0082] In the formula, This is the differential term of the positive-sequence grid-connected current. This is the differential term of the negative sequence grid-connected current.

[0083] Furthermore, to mitigate the transient power angle stability degradation caused by the introduction of additional virtual impedance and to enhance the rapid reactive power support capability of the grid-connected energy storage converter under fault conditions, this embodiment of the invention adaptively modifies the power command to maximize the performance of the fault current limiting method. The power command adjustment method for the grid-connected energy storage converter includes the following steps:

[0084] 1) The magnitude of the positive sequence component of the grid voltage calculated by the fault current limiting module and the maximum amplitude of grid-connected current I max (k) is used to calculate the maximum capacity S of the grid-type energy storage converter under grid fault conditions. F (k), the calculation formula is:

[0085]

[0086] 2) Based on the amplitude of the positive sequence component of the grid voltage With rated internal potential E N The ratio of the initial active power command P ref0 (k) is corrected to obtain the final active power command P. ref (k), the calculation formula is:

[0087]

[0088] 3) Based on S obtained in steps 1) and 2), F (k) and P ref (k), for the initial reactive power command Q ref0 (k) is corrected to obtain the final reactive power command Q. ref (k), the calculation formula is:

[0089]

[0090] In the formula, I QGB (k) represents the reactive current that the grid-type energy storage converter must respond to under fault conditions according to national standards, and its value is...

[0091]

[0092] The effectiveness and advancement of the control method proposed in this invention embodiment were verified using MATLAB / Simulink software.

[0093] The grid-type energy storage converter has a rated capacity of 93kVA, a voltage level of DC 800V / AC 380V, a rated current amplitude of 200A at 1.0 times the rated current amplitude, a rated current amplitude of 220A at 1.1 times the rated current amplitude, an AC filter inductance of 500μH, a DC side capacitor of 2115μF, and before the fault occurred, the converter output active power of 50kW and reactive power of 0kVar.

[0094] See Figure 5 For the grid-type energy storage converter, when the three-phase grid voltage symmetrically drops to 0.2 pu, the grid voltage u gx Grid-connected current i x Instantaneous active power P out Instantaneous reactive power Q outThe simulated waveform shows a power grid fault occurring at 1 second and recovery occurring at 2 seconds. Without any intervention, the current rapidly spikes from 115A to 1500A at 1 second, and this short-circuit overcurrent will directly cause equipment shutdown or damage.

[0095] Referring to Figures 6(a) and 6(b), after the grid-type energy storage converter adopts the fault current suppression method, when the grid voltage drops symmetrically to 0.2pu, the grid voltage u gx Grid-connected current i x Instantaneous active power P out Instantaneous reactive power Q out and virtual impedance Z v ∠θ Zv The simulation waveforms are shown in Figure 6(a). A grid fault occurs at 1s and recovers at 2s. After current limiting measures are adopted, the fault transient current is reduced to 260A, which is 1.3 times the rated value, and is the overcurrent level that the converter can withstand for a short time. The fault current suppression algorithm under fault steady state maintains the short-circuit current at 200A. Under the performance requirement of the energy storage converter meeting 1.1 times the rated value for long-term operation, the algorithm can effectively ensure the operation safety and fault endurance of the equipment, and provide sufficient short-circuit current guidance for the grid's relay protection device. Due to the adjustment of the power command, the converter responds quickly to reactive power, providing 28kVar of reactive power support to the grid within 10ms. In addition, the active power delivered to the grid is limited during the fault, which improves the dynamic response capability and transient power angle stability. In Figure 6(b), the virtual impedance amplitude rapidly increases to 1.90Ω at the moment of fault, verifying that the converter has not adjusted its internal potential at the beginning of the fault. At this time, a larger impedance is needed to suppress the overcurrent caused by the large voltage difference across the inductor. Subsequently, the integrator gradually approaches the optimal value, and the impedance amplitude eventually stabilizes at 1.12Ω, stabilizing the fault current at 1.0 times the rated value. The virtual impedance phase is set to a fixed value of 57.5°, with a larger inductive component to weaken the power coupling effect, while retaining a part of the resistive component to provide damping for the attenuation of transient inrush current.

[0096] See Figure 7 For the grid-type energy storage converter, when the grid voltage drops symmetrically to 0.5 pu, the grid voltage u gx Grid-connected current i x Instantaneous active power P out Instantaneous reactive power Q out The simulated waveforms are shown. A power grid fault occurs at 1 second and recovers at 2 seconds. Without any intervention, the transient current spike reaches 1100A, and the steady-state fault amplitude is 500A, which also exceeds the current withstand limit of the equipment.

[0097] Referring to Figures 8(a) and 8(b), after the grid-type energy storage converter adopts the fault current suppression method, when the grid voltage drops symmetrically to 0.5pu, the grid voltage u gx Grid-connected current i x Instantaneous active power P out Instantaneous reactive power Q out and virtual impedance Z v ∠θ Zv The simulation waveforms are shown. A grid fault occurs at 1 second and recovers at 2 seconds. In Figure 8(a), after the fault occurs, due to the current limiting measures, the maximum amplitude of the short-circuit current is quickly limited to the rated value of 200A, verifying the effectiveness and feasibility of the method under asymmetrical short-circuit faults. At the instant of the asymmetrical short-circuit fault, the converter can also respond quickly to reactive power, providing 42kVar of power support to the grid within 10ms, demonstrating good dynamic response characteristics. In Figure 8(b), at 1 second, the algorithm provides a large initial integral value (i.e., 1.86Ω) for the virtual impedance, effectively avoiding the occurrence of fault current spikes. When the voltage recovers, the amplitude of the virtual impedance is corrected to 0, improving the recovery rate from the current-limiting state to the normal operating state.

[0098] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0099] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A fault current limiting method for a grid-type energy storage converter based on dynamic virtual impedance, characterized in that, Includes the following steps: S1, at the starting point of each sampling period, the grid-connected current i x (k) is utilized max (k) is utilized to track the maximum amplitude I max (k) of the three-phase grid-connected current in real time, wherein the calculation formula of I max (k) is: max{·} is the maximum operator, notch 100Hz (·) is the notch operator, k denotes the sequence number of the kth sampling period; x = a, b, c, S2, using I max (k) and the virtual impedance magnitude Z of the previous sampling period v (k-1) Perform current state assessment to obtain the current state quantity State(k) of the grid-type energy storage converter. The criterion is as follows: Among them, I N is the rated current of the grid-type energy storage converter, ∩ is the intersection operator, and ∪ is the union operator; S3. Send the current state State(k) to the state bit of the 4-to-1 data selector to select the virtual impedance amplitude Z under different current states. v The relational expression for (k); S4. Utilizing the virtual impedance amplitude Z v (k) and the set virtual impedance phase θ Zv (k), the virtual resistance R is calculated. v (k) and virtual reactance X v (k), the calculation formula is: S5, the grid-connected current i x (k) Construct the differential terms of the positive and negative sequence components and connect them with the virtual resistance R. v (k) and virtual reactance X v (k) Perform Ohm's law calculations to obtain the virtual impedance voltage drop ΔU. Zvx : in, This is the differential term of the positive-sequence grid-connected current. For the differential term of the negative-sequence grid-connected current, ω N This is the rated angular frequency.

2. The fault current limiting method for grid-type energy storage converters based on dynamic virtual impedance according to claim 1, characterized in that, Between steps S3, the method also includes: adjusting the grid voltage u gx (k) Perform sequence component extraction to obtain the positive sequence component amplitude of the grid voltage. And perform grid fault detection, generating a fault characteristic pulse flag bit Flag(k); at this time, step S3 also includes: The initial value of the integrator is corrected at the time of fault occurrence and recovery by using the fault characteristic pulse flag bit Flag(k).

3. The fault current limiting method for grid-type energy storage converters based on dynamic virtual impedance according to claim 2, characterized in that, Among them, K i The integral coefficients of the virtual impedance integrator, ΔT s Z is the sampling period length. v0 Z is the initial value provided to the integrator at the moment the fault occurs. v1 The initial values ​​provided to the integrator at the fault recovery time.

4. The fault current limiting method for grid-type energy storage converters based on dynamic virtual impedance according to claim 1, characterized in that, The method also includes: 1) Utilizing the amplitude of the positive sequence component of the grid voltage and the maximum amplitude of grid-connected current I max (k) Calculate the maximum capacity S of the grid-type energy storage converter under grid short-circuit fault. F (k): 2) Based on the amplitude of the positive sequence component of the grid voltage With rated internal potential E N The ratio of the initial active power command P ref0 (k) is corrected to obtain the final active power command P. ref (k): 3) Based on S obtained in steps 1) and 2), F (k) and P ref (k), for the initial reactive power command Q ref0 (k) is corrected to obtain the final reactive power command Q. ref (k): Among them, I QGB (k) represents the reactive current that the grid-type energy storage converter needs to respond to under fault conditions.

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