A fault characteristic analysis method and system for electric vehicle charging station

Abstract

The present invention discloses a method and system for analyzing the fault characteristics of an electric vehicle charging station. The method comprises: converting the voltage equations of the three-phase circuit of the electric vehicle charging station during charging and discharging to obtain the charging control strategy and discharging control strategy of the electric vehicle charging station, respectively; simulating a short-circuit fault at different locations on the DC side of the electric vehicle charging station and recording the change in fault current on the DC side before and after the fault; simulating a short-circuit fault on the AC side of the electric vehicle charging station and recording the change in fault current on the AC side before and after the fault; and obtaining the fault characteristics of the electric vehicle charging station based on the change in fault current on the DC side and the change in fault current on the AC side before and after the fault. The present invention constructs a fault current output model for the electric vehicle charging station while considering low voltage ride-through, providing data support for optimizing relay protection of the distribution network containing the electric vehicle charging station and improving the operational stability of the electric vehicle charging station.

Description

Technical Field

[0001] The present invention belongs to the technical field of fault analysis, and in particular relates to a fault characteristic analysis method and system for an electric vehicle charging station. Background Art

[0002] The charging and discharging behavior of large-scale electric vehicle charging stations connected to the distribution network will have a certain impact on the level and direction of the distribution network's power flow. When a charging station fails, the fault characteristics of the distribution network will also change, causing a certain impact on the power grid. Therefore, it is necessary to study the fault characteristics of electric vehicle charging stations to provide a basis for the development of relay protection methods suitable for electric vehicle charging stations connected to the distribution network.

[0003] Current research focuses on the fault characteristics of electric vehicle charging stations during charging. This includes analyzing the fault characteristics of non-isolated DC charging stations based on converters. Under various fault conditions, the DC bus voltage decreases to varying degrees, accompanied by varying degrees of oscillation. Other studies have analyzed the characteristics of AC-side three-phase short-circuit faults and DC-side inter-pole short-circuit faults during the charging and discharging process of DC charging and discharging stations from the power electronics perspective. However, these studies have not considered the fault characteristics of electric vehicle charging stations providing reactive power support to the distribution network during low-voltage ride-through (LVRT). Therefore, a comprehensive analysis of the fault characteristics of electric vehicle charging stations, which also provide charging and discharging functions, is necessary to provide support for the relay protection of the distribution network containing these stations. Summary of the Invention

[0004] The present invention proposes a fault characteristic analysis method and system for electric vehicle charging stations. Taking low voltage ride-through into consideration, a fault current output model of the electric vehicle charging station is constructed, and the charging and discharging fault characteristics of the electric vehicle charging station are comprehensively analyzed. This provides data support for the relay protection optimization of the distribution network containing the electric vehicle charging station, thereby improving the operational stability of the electric vehicle charging station.

[0005] A first aspect of the present invention provides a method for analyzing fault characteristics of an electric vehicle charging station, the method comprising:

[0006] The voltage equation of the three-phase circuit of the electric vehicle charging station during charging and the voltage equation of the three-phase circuit during discharging are converted to obtain the charging control strategy and the discharging control strategy of the electric vehicle station respectively;

[0007] According to the charging control strategy and the discharging control strategy, a short circuit fault is simulated at different locations on the DC side of the electric vehicle charging station, and the change of the fault current on the DC side before and after the fault is recorded;

[0008] According to the charging control strategy and the discharging control strategy, a short circuit fault on the AC side of the electric vehicle charging station and a low voltage ride-through at the grid connection point are simulated, and the change in the fault current on the AC side before and after the fault is recorded;

[0009] The fault current changes on the DC side and the AC side before and after the fault are compared and analyzed to obtain the fault characteristics of the electric vehicle charging station.

[0010] The above scheme first simulates short-circuit faults occurring at different locations on the DC side during charging to obtain the changes in the fault current on the DC side. It then simulates the changes in the fault current when a short-circuit fault occurs on the AC side, resulting in low voltage ride-through at the grid connection point. Finally, based on the analysis and comparison of the fault current changes on the DC side and the fault current changes on the AC side, the fault characteristics of the electric vehicle charging station under low voltage ride-through are obtained, providing data support for the optimization of relay protection of the distribution network containing electric vehicle charging stations, and improving the operational stability of electric vehicle charging stations.

[0011] In a possible implementation method of the first aspect, the voltage equation of the three-phase circuit during charging and the voltage equation of the three-phase circuit during discharging of the electric vehicle charging station are converted to obtain the charging control strategy and the discharging control strategy of the electric vehicle station, respectively. Specifically,

[0012] Performing coordinate transformation and feedforward decoupling control on the voltage equation of the three-phase circuit of the electric vehicle charging station during charging to obtain a first voltage control equation;

[0013] Calculating according to the first voltage control equation and the current information on the DC side to obtain a charging control strategy for the electric vehicle station;

[0014] The voltage equation of the three-phase circuit of the electric vehicle charging station during discharge is subjected to coordinate transformation and feedforward decoupling control processing to obtain a second voltage control equation;

[0015] According to the second voltage control equation and the output power of the electric vehicle charging station during discharge, the discharge control strategy of the electric vehicle station is obtained.

[0016] The above scheme performs coordinate transformation on the voltage equations of the three-phase circuit of the electric vehicle charging station during charging and discharging, and then combines the output power and current information of the electric vehicle charging station to obtain the discharge control strategy and charging control strategy of the electric vehicle station, respectively. This provides data support for the subsequent regulation of the active current and reactive current on the DC and AC sides of the electric vehicle charging station, ensuring that the changes in the fault current on the DC and AC sides are monitored more accurately.

[0017] In a possible implementation method of the first aspect, according to the charging control strategy and the discharging control strategy, a short circuit fault is simulated at different locations on the DC side of the electric vehicle charging station, and changes in the fault current on the DC side before and after the fault are recorded, specifically:

[0018] According to the charging control strategy and the discharging control strategy, a short circuit fault is simulated on the DC side of the electric vehicle charging station, and the change of the fault current on the DC side before and after the short circuit fault is recorded;

[0019] According to the charging control strategy and the discharging control strategy, a polar short circuit fault is simulated on the DC side of the electric vehicle charging station through a preset grounding method, and the fault current change on the DC side before and after the polar short circuit fault is recorded.

[0020] The above scheme simulates inter-pole short-circuit faults and pole-to-ground short-circuit faults on the DC side of the electric vehicle charging station, monitors the direction and value changes of the fault current, determines the changes between the fault current and the fault point on the DC side, and provides data support for the subsequent fault analysis when a short-circuit fault occurs on the DC side of the electric vehicle charging station.

[0021] In a possible implementation method of the first aspect, according to the charging control strategy and the discharging control strategy, a short circuit fault on the AC side of the electric vehicle charging station and a low voltage ride-through at the grid connection point are simulated, and changes in the fault current on the AC side before and after the fault are recorded, specifically:

[0022] According to a preset short-circuit fault form, a short-circuit fault is simulated on the AC side of the electric vehicle charging station, causing low voltage ride-through at the grid connection point, and adjusting the reactive current and active current of the electric vehicle charging station under the charging control strategy, discharging control strategy and preset inverter limit conditions;

[0023] According to the reactive current and the active current, the current change on the AC side before and after the fault is obtained.

[0024] The above scheme simulates a short-circuit fault on the AC side of the electric vehicle charging station through a preset short-circuit fault form, causing a low-voltage ride-through at the grid connection point. Then, the reactive current and active current of the electric vehicle charging station are adjusted to control the response speed of the charging station, thereby accurately obtaining the current changes on the AC side before and after the fault, providing data support for the following fault analysis when a short-circuit fault occurs on the AC side of the electric vehicle charging station.

[0025] In a possible implementation method of the first aspect, comparing and analyzing the fault current changes on the DC side and the fault current changes on the AC side before and after the fault to obtain the fault characteristics of the electric vehicle charging station includes:

[0026] Comparing and analyzing the changes in the fault current on the DC side before and after the fault, obtaining first change data of the fault current on the DC side when an inter-pole short circuit fault and a pole-to-ground short circuit fault occur on the DC side; wherein the first change data includes a change in direction of the fault current on the DC side and a value of the fault current on the DC side when the fault current reaches a steady state;

[0027] Comparing and analyzing the changes in the fault current on the AC side before and after the fault, obtaining second change data of the fault current on the AC side when a preset short-circuit fault occurs on the AC side, resulting in a low voltage ride-through at the grid connection point; wherein the numerical change in the fault current includes a change in the ratio of the fault current to the rated output current of the electric vehicle charging station;

[0028] The fault characteristics of the electric vehicle charging station are obtained according to the first change data and the second change data.

[0029] By comparing and analyzing the changes in fault current on the DC side and the AC side before and after a fault, the above solution derives the fault characteristics of the DC and AC sides of an EV charging station under different types of short-circuit faults. This fault characteristic is based on a comprehensive fault environment, making the fault characteristic analysis more accurate and complete. Furthermore, based on this complete and accurate fault characteristic, the relay protection of the distribution network containing the EV charging station can be optimized, improving the operational stability of the EV charging station.

[0030] A second aspect of the present invention provides a magnetic field strength measurement system based on segmented fixed frequency, the system comprising: a charging / discharging control strategy generation module, a DC side fault current monitoring module, an AC side fault current monitoring module and a charging station fault analysis module;

[0031] The charging / discharging control strategy generation module is used to convert the voltage equation of the three-phase circuit of the electric vehicle charging station during charging and the voltage equation of the three-phase circuit during discharging to obtain the charging control strategy and the discharging control strategy of the electric vehicle station respectively;

[0032] The DC side fault current monitoring module is used to simulate short circuit faults at different locations on the DC side of the electric vehicle charging station according to the charging control strategy and the discharging control strategy, and record the changes in the fault current on the DC side before and after the fault;

[0033] The AC side fault current monitoring module is used to simulate a short circuit fault on the AC side of the electric vehicle charging station and a low voltage ride-through at the grid connection point according to the charging control strategy and the discharging control strategy, and record the changes in the fault current on the AC side before and after the fault;

[0034] The charging station fault analysis module is used to compare and analyze the fault current changes on the DC side and the fault current changes on the AC side before and after the fault to obtain the fault characteristics of the electric vehicle charging station.

[0035] In a possible implementation of the second aspect, the charging / discharging control strategy generating module includes: a charging / discharging control strategy acquiring unit;

[0036] The charging / discharging control strategy acquisition unit is used to perform coordinate transformation and feedforward decoupling control processing on the voltage equation of the three-phase circuit of the electric vehicle charging station during charging to obtain a first voltage control equation; perform calculation based on the first voltage control equation and the current information on the DC side to obtain the charging control strategy of the electric vehicle station; perform coordinate transformation and feedforward decoupling control processing on the voltage equation of the three-phase circuit of the electric vehicle charging station during discharging to obtain a second voltage control equation; and obtain the discharge control strategy of the electric vehicle station based on the second voltage control equation and the output power of the electric vehicle charging station during discharge.

[0037] In a possible implementation of the second aspect, the DC side fault current monitoring module includes: a DC side fault current change recording unit;

[0038] The DC side fault current change recording unit is used to simulate an inter-pole short circuit fault on the DC side of the electric vehicle charging station according to the charging control strategy and the discharging control strategy, and record the fault current change on the DC side before and after the inter-pole short circuit fault; according to the charging control strategy and the discharging control strategy, through a preset grounding method, simulate the occurrence of a pole-to-ground short circuit fault on the DC side of the electric vehicle charging station, and record the fault current change on the DC side before and after the pole-to-ground short circuit fault.

[0039] In a possible implementation of the second aspect, the AC side fault current monitoring module includes: an AC side fault current change recording unit;

[0040] The AC side fault current change recording unit is used to simulate a short circuit fault on the AC side of the electric vehicle charging station according to a preset short circuit fault form, causing low voltage ride-through at the grid connection point, and adjust the reactive current and active current of the electric vehicle charging station under the charging control strategy, discharging control strategy and preset inverter limit conditions; based on the reactive current and active current, the current change on the AC side before and after the fault is obtained.

[0041] In a possible implementation of the second aspect, the charging station fault analysis module includes: a charging station fault characteristic analysis unit;

[0042] The charging station fault characteristic analysis unit is used to compare and analyze the changes in the fault current on the DC side before and after the fault, and obtain first change data of the fault current on the DC side when an inter-pole short circuit fault and a pole-to-ground short circuit fault occur on the DC side; wherein, the first change data includes the change in direction of the fault current on the DC side, and the value of the fault current on the DC side when it reaches a steady state; compare and analyze the changes in the fault current on the AC side before and after the fault, and obtain second change data of the fault current on the AC side when a preset short circuit fault occurs on the AC side, resulting in a low voltage ride-through at the grid connection point; wherein, the change in the value of the fault current includes a change in the ratio of the fault current to the rated output current of the electric vehicle charging station; based on the first change data and the second change data, the fault characteristics of the electric vehicle charging station are obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the technical solution of the present invention, the following is a brief introduction to the drawings required for use in the implementation. Obviously, the drawings described below are only some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0044] Figure 1 A schematic diagram of a specific flow chart of a method for analyzing fault characteristics of an electric vehicle charging station is provided in one embodiment of the present invention;

[0045] Figure 2 An equivalent circuit diagram of an electric vehicle charging station connected to a power distribution network, in which a method for analyzing fault characteristics of an electric vehicle charging station is provided in a certain embodiment of the present invention;

[0046] Figure 3 A current loop decoupling control diagram of a fault characteristic analysis method for an electric vehicle charging station is provided in a certain embodiment of the present invention;

[0047] Figure 4 A voltage loop control diagram of a method for analyzing fault characteristics of an electric vehicle charging station is provided in accordance with a certain embodiment of the present invention;

[0048] Figure 5 A current inner loop and voltage outer loop control diagram of a fault characteristic analysis method for an electric vehicle charging station is provided in a certain embodiment of the present invention;

[0049] Figure 6 An embodiment of the present invention provides a method for analyzing fault characteristics of an electric vehicle charging station, which is an equivalent circuit diagram of a DC side inter-electrode short circuit fault;

[0050] Figure 7A diagram of a second-order RLC discharge circuit on the DC side of a method for analyzing fault characteristics of an electric vehicle charging station is provided in accordance with one embodiment of the present invention;

[0051] Figure 8 An embodiment of the present invention provides an AC side equivalent circuit diagram of a method for analyzing fault characteristics of an electric vehicle charging station;

[0052] Figure 9 A DC side equivalent circuit diagram of a method for analyzing fault characteristics of an electric vehicle charging station is provided in accordance with a certain embodiment of the present invention;

[0053] Figure 10 A polar short circuit fault diagram on the DC side of a method for analyzing fault characteristics of an electric vehicle charging station is provided in a certain embodiment of the present invention;

[0054] Figure 11 A low voltage ride-through control flow chart of a fault characteristic analysis method for an electric vehicle charging station is provided in a certain embodiment of the present invention;

[0055] Figure 12 A schematic diagram of low voltage ride-through requirements for a fault characteristic analysis method for an electric vehicle charging station is provided in accordance with one embodiment of the present invention;

[0056] Figure 13 A structural diagram of a fault characteristic analysis system for an electric vehicle charging station is provided in accordance with one embodiment of the present invention. DETAILED DESCRIPTION

[0057] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0058] It should be understood that the step numbers used herein are only for convenience of description and are not intended to limit the order in which the steps are to be executed.

[0059] like Figure 1 As shown, Figure 1 A specific flow chart of a method for analyzing the fault characteristics of an electric vehicle charging station is provided for one embodiment of the present invention. The method for analyzing the fault characteristics of an electric vehicle charging station of this embodiment includes steps S1 to S4, which are described in detail as follows:

[0060] Step S1, converting the voltage equation of the three-phase circuit during charging and the voltage equation of the three-phase circuit during discharging of the electric vehicle charging station to obtain the charging control strategy and the discharging control strategy of the electric vehicle station respectively;

[0061] In this step, we first construct the equivalent circuit of the electric vehicle charging station connected to the distribution network. Figure 2 As shown, Figure 2 This is the equivalent circuit diagram of an electric vehicle charging station connected to the distribution network. It simulates the three-phase AC current flowing through the AC / DC converter, outputting DC power, and charging the electric vehicle power battery through the DC / DC converter. The AC / DC and DC / DC achieve bidirectional energy flow through the coordination of control strategies.

[0062] On the equivalent circuit of the distribution network, according to the voltage equations of the three-phase circuit of the electric vehicle charging station during charging and discharging, combined with the operation information of the equivalent circuit, the charging control strategy and the discharging control strategy of the electric vehicle station are respectively obtained.

[0063] In some embodiments, the step S1 is specifically:

[0064] The charging state is simulated in the equivalent circuit connected to the distribution network. The voltage equation of the three-phase circuit during charging is obtained according to the KVL law. The specific formula is:

[0065]

[0066] Among them, u sa 、u sb 、u sc They are the three-phase currents on the grid side, u a 、u b 、u c are the three-phase voltages at the converter side ports, i a 、i b 、i c are the three-phase currents on the AC side, L is the filter inductance on the AC side, and R is the equivalent resistance of the filter inductance;

[0067] Then, the voltage equation of the three-phase circuit during charging is transformed by PARK to obtain the mathematical equation of the AC / DC converter in the two-phase synchronous rotating dq coordinate system. The specific formula is:

[0068]

[0069] Among them, u sd 、u sq is the d-axis and q-axis component of the distribution network voltage, u d 、u q is the d-axis and q-axis components of the port voltage on the AC side of the AD / DC converter, i d 、i q are the d-axis and q-axis components of the current on the AC side of the AD / DC converter,  is the synchronous angular velocity, and s is the frequency domain variable after PARK transformation, which is called the differential operator;

[0070] Then, feedforward decoupling control is adopted and the current regulator adopts PI control to respectively control the d and q axis components i of the current. d 、i q Adjust to get u d 、u q The control equation, that is, the first voltage control equation, is as follows:

[0071]

[0072] Among them, k p 、k i They are the current inner loop proportional adjustment coefficient and integral adjustment coefficient respectively. 、 are the command values ​​of the d-axis and q-axis components of the AC side current, u d 、u q is the inverter command voltage on the AC side.

[0073] According to the first voltage control equation, the final current loop decoupling control method can be obtained. In order to better describe the final current loop decoupling control method, Figure 3 A current loop decoupling control diagram of this embodiment is provided, which describes information such as the flow direction of each current in detail.

[0074] Combining the DC side current information with the DC side and AC side electrical quantities, the switching function S of the three-phase circuit can be obtained. k , the specific formula is:

[0075]

[0076] Where k is one of the three-phase circuits a, b, and c;

[0077] Combining the switching function of the three-phase circuit with the KCL law, the formula for the three-phase circuit on the AC side is obtained:

[0078]

[0079] Among them, u dc is the output voltage on the DC side, i dc is the current on the DC side, and C is the large capacitor on the DC side;

[0080] Then perform Park transformation on the formula of the three-phase circuit on the AC side to obtain:

[0081]

[0082] Assume i q = 0, the above formula can be simplified as:

[0083]

[0084] According to this formula, the switching function of the three-phase circuit can be approximated as:

[0085]

[0086] Where, θ is the initial phase angle of the three-phase current before the fault, and m is the duty cycle;

[0087] In this embodiment, let S d Take the preset maximum impact value; where S d For S a 、S b 、S c After the rotation transformation, since the DC component is 0 after the rotation transformation, the fundamental component is 0.5, so S d It can be approximately taken as 0.5, so the formula for the three-phase circuit on the AC side is:

[0088]

[0089] The above formula is transformed by Laplace transform to obtain the final formula of the AC side three-phase circuit:

[0090] ;

[0091] Combined with the final AC side three-phase circuit formula Figure 4 The voltage loop control diagram provided can obtain the output voltage u on the DC side dc and i d According to the relationship, the voltage loop and the current loop are combined to obtain the control mode of the current inner loop and the voltage outer loop. For detailed description, see Figure 5 Provided current inner loop and voltage outer loop control diagram.

[0092] according to Figure 5 The AC / DC converter mathematical model is used to obtain the AC / DC converter control strategy for the electric vehicle charging station. The q-axis reference current is set to 0 in order to control the electric vehicle charging station to maintain a power factor of 1, that is, the reactive power is 0.

[0093] According to the AC / DC converter control strategy of the electric vehicle charging station, the charging control strategy of the electric vehicle station can be obtained; wherein, the control objective of the charging control strategy is to maintain the stability of the DC bus voltage and the power battery port voltage. The specific method is to make a difference between the actual value of the DC bus voltage and the battery port voltage and the reference value, perform PI operation on the error signal, and combine the battery charge state and voltage limit to generate the driving signal of the switch tube. During this process, the DC bus voltage and the battery port voltage remain unchanged.

[0094] The discharge state is simulated in the equivalent circuit connected to the distribution network. The voltage equation of the three-phase circuit during discharge is obtained according to the KVL law. The specific formula is:

[0095]

[0096] The voltage equation of the three-phase circuit during discharge is transformed by PARK to obtain the mathematical equation of the AC / DC converter in the two-phase synchronous rotating dq coordinate system:

[0097]

[0098] Then adopt feedforward decoupling control and PI control for the current regulator to obtain u d 、u q The control equation of the second voltage control equation is as follows:

[0099]

[0100] When the electric vehicle charging station is discharging, the grid voltage oriented vector control technology is used to orient the d axis to the grid voltage vector. sd 、u sq Related: , =0; adopt PQ control strategy to control the output power through active current and reactive current, so that the electric vehicle charging station is always in normal working state; the output power under normal working state is:

[0101]

[0102] Among them, P is active power and Q is reactive power;

[0103] By controlling d 、i q It can realize the control of active power and reactive power of the converter. Generally, Under normal circumstances, the electric vehicle charging station operates at unity power factor, which can be obtained before a fault occurs. Therefore, under normal circumstances, the formula for the three-phase current when the distribution network discharges is:

[0104]

[0105] Among them, I sa0 , I sb0 , I sc0 are the currents of phase a, phase b, and phase c respectively;

[0106] According to the formula of three-phase current when the distribution network is discharged, the discharge control strategy of the electric vehicle station is determined.

[0107] Step S2, simulating a short circuit fault at different locations on the DC side of the electric vehicle charging station according to the charging control strategy and the discharging control strategy, and recording the change in the fault current of the DC side before and after the fault;

[0108] In this step, the main purpose is to simulate the occurrence of an inter-pole short circuit fault and a pole-to-ground short circuit fault on the DC side of the electric vehicle charging station. According to the charging control strategy and the discharging control strategy, the inter-pole short circuit fault is simulated on the DC side of the electric vehicle charging station, and the change in the fault current on the DC side before and after the inter-pole short circuit fault is recorded;

[0109] According to the charging control strategy and the discharging control strategy, a polar short circuit fault is simulated on the DC side of the electric vehicle charging station through a preset grounding method, and the fault current change on the DC side before and after the polar short circuit fault is recorded.

[0110] In some embodiments, step S2 is specifically as follows:

[0111] First, simulate the inter-pole short circuit fault on the DC side of the electric vehicle charging station. The equivalent circuit of the DC side inter-pole short circuit fault is as follows: Figure 6 As shown in the figure, the VSC uses a three-phase, two-level structure, and the DC link adopts a Type II equivalent model. Due to the large capacitor C on the DC side, the DC link capacitance to ground is ignored. At the moment of an inter-pole short-circuit fault, the current through the AC / DC converter increases dramatically. The IGBT will lock out of operation under its own protection, while the antiparallel diode remains connected in the circuit, forming a fault loop. The fault process can be divided into three stages: capacitor discharge, initial uncontrolled rectification, and steady-state uncontrolled rectification.

[0112] At the initial moment of an inter-pole short-circuit fault on the DC side, the DC side capacitor voltage is higher than the AC side three-phase line voltage, the diode is cut off, and the AC side provides a freewheeling current of the current-limiting reactor, which is much smaller than the capacitor discharge current. Therefore, the DC side fault current is mainly composed of rapid capacitor discharge, and the AC side freewheeling current is ignored. The DC side capacitor, inductor and resistor form an RLC second-order discharge circuit. Figure 7 The RLC second-order discharge circuit diagram on the DC side is provided.

[0113] Assuming that the instantaneous capacitor voltage of the inter-pole short circuit fault is U0 and the DC side current is I0, the current equation of the three-phase circuit can be obtained as follows:

[0114]

[0115] According to the current equation of the three-phase circuit, it can be deduced:

[0116]

[0117] When the short-circuit impedance R is large, that is, When the discharge process of the electric vehicle charging station is a second-order over-damped attenuation process, combined with the current equation of the three-phase circuit, the output voltage u on the DC side can be obtained. dc The expression:

[0118]

[0119] Among them, the discharge current i c The specific formula is:

[0120]

[0121] in, 、 、

[0122] p1, p2 are characteristic roots, A1, A2 are constant terms in the discharge current expression;

[0123] In addition, when the short-circuit impedance R is small, that is, When , the discharge process of the electric vehicle charging station is a second-order underdamped oscillation process. The solution method is the same as when the short-circuit impedance R is large.

[0124] Since the capacitor discharge process is extremely fast and takes a very short time, when the DC side capacitor voltage is lower than the AC side three-phase line voltage, the diode begins to conduct and enters the initial stage of uncontrolled rectification. The AC side provides fault current to the fault point through the diode.

[0125] When the short-circuit impedance is large, the capacitor discharge current rises slowly, the AC side current boosting effect is more obvious, and the entire current gradually enters a steady state under the action of the AC current, and the fault transition process is relatively smooth.

[0126] When the short-circuit impedance is small, the capacitor discharge current rises quickly, and the AC side current does not play a dominant role in a short period of time. The capacitor will continue to discharge until the voltage is 0. At this time, the diode is turned on at the same time, and the DC side forms an RL first-order discharge circuit. The capacitor current is 0, and the AC side is equivalent to a three-phase short circuit. The AC side and the DC side can be divided into two independent currents. Figure 8 The AC side equivalent circuit diagram and Figure 9 Provided DC side equivalent circuit diagram.

[0127] Depend on Figure 8 It can be seen that, taking phase A as an example, the phase A voltage U sa and phase A current i sa The expression is:

[0128]

[0129]

[0130]

[0131]

[0132] Among them, R s is the AC side resistance, L s is the AC side filter inductor, U m is the maximum voltage on the AC side,  is the short-circuit impedance angle on the AC side, I sm is the amplitude of the periodic component of the fault current, I a0 is the instantaneous value of phase A current when the capacitor voltage drops to 0;

[0133] Depend on Figure 9 It can be seen that the DC side equivalent circuit is equivalent to the RL first-order discharge circuit, the fault current continues to decay, and each diode flows through 1 / 3 of the discharge current. The fault current i dc for:

[0134]

[0135] Among them, I dc1 is the fault current value at the instant when the capacitor voltage drops to 0;

[0136] When the circuit reaches the uncontrolled rectification steady state stage, the upper and lower bridge arm diodes of each phase are turned on for half a cycle respectively, and the output voltage is approximately a square wave. Assume that the DC side voltage at this time is U d Taking phase A as an example, phase B and phase C lag and lead phase A by 1 / 3 cycle respectively. When the upper bridge arm of phase A is opened, S a = 1, when the lower bridge arm of phase A is open, S a = 0, the output voltage U of phase A can be obtained ca for:

[0137]

[0138] Ignoring the reactance on the AC side, the voltage U on the AC side can be obtained s and current I s The relationship is:

[0139]

[0140] It can be seen that the effective value of the phase current on the AC side is I s for:

[0141] .

[0142] Then, a polar short circuit fault on the DC side of the electric vehicle charging station is simulated. Among them, there are several types of grounding methods for the neutral point of the low-voltage distribution system. The characteristics of the polar short circuit fault on the DC side under different grounding methods are different. This embodiment mainly adopts the neutral point direct grounding method and the neutral point ungrounded method.

[0143] In the neutral point ungrounded mode, the DC bus positive pole is short-circuited to ground. Figure 10 When a positive or negative pole short-circuit fault occurs, a discharge loop to the ground is formed between the fault point and the neutral point. After the fault loop is formed, the load is short-circuited and the current flowing through the load is very small, which cannot meet the output requirements and cannot work normally.

[0144] In the neutral point direct grounding mode, the short-circuit point and the system neutral point are grounded at the same time to form a loop. The fault current will return to the neutral point through the ground loop, and the voltage and current will change significantly. This transient process is similar to the DC side inter-pole short-circuit fault, and is also divided into capacitor discharge, uncontrolled rectification initial stage and uncontrolled rectification steady-state stage; therefore, the fault current i in the uncontrolled rectification steady-state stage can be derived. dc for:

[0145]

[0146]

[0147]

[0148] Among them, V s is the voltage on the AC side, R f is the fault resistance, C is the distributed capacitance that is short-circuited;

[0149] It can be seen from this that the polar short circuit fault is an asymmetric fault, which will cause three-phase imbalance of the AC side current at the moment of the fault.

[0150] In an ungrounded system, when a positive-to-ground short-circuit fault occurs, the potential of the faulty (positive) electrode approaches zero. However, due to the presence of capacitance C between the positive and negative electrodes, the voltage of the non-faulty (negative) electrode relative to ground also changes, and the voltage between the positive and negative electrodes does not change immediately. Due to the effect of the capacitance, the voltage between the positive and negative electrodes does not drop to zero instantly, but instead gradually decreases. The discharge time of a capacitor depends on its capacitance and the resistance in the circuit. If the capacitance is large and the resistance in the circuit is small, the discharge time will be longer, and the voltage at the negative electrode will gradually decrease. Because the fault current is small, the voltage change between the positive and negative electrodes is not significant, allowing the system to continue energized operation for a period of time, ensuring system power supply reliability. However, this also reduces system sensitivity and speed, making it difficult to detect faults in a timely manner.

[0151] Step S3, simulating a short circuit fault on the AC side of the electric vehicle charging station and a low voltage ride-through at the grid connection point according to the charging control strategy and the discharging control strategy, and recording the change in the fault current on the AC side before and after the fault;

[0152] In this step, a short circuit fault is simulated on the AC side of the electric vehicle charging station according to a preset short circuit fault form, causing a low voltage ride-through at the grid connection point, and adjusting the reactive current and active current of the electric vehicle charging station under the charging control strategy, discharging control strategy and preset inverter limit conditions;

[0153] According to the reactive current and the active current, the current change on the AC side before and after the fault is obtained.

[0154] In some embodiments, step S3 is specifically as follows:

[0155] When an electric vehicle charging station maintains a charging state during normal operation, and the grid voltage drops below 0.85pu, the electric vehicle charging station will be required to provide reactive power compensation to support the grid voltage. This phenomenon is called low voltage ride-through. To better describe the process of low voltage ride-through, Figure 11 Provides a low voltage ride-through control flow chart.

[0156] Comparing electric vehicle charging stations to distributed energy storage, according to GB / T 34120-2017 "Technical Specifications for Energy Storage Converters of Electrochemical Energy Storage Systems": When a power system fault occurs, it is required to remain connected to the grid for a period of time and provide certain support to the grid. If the voltage at the grid connection point is all within the voltage contour line required by the low voltage ride-through and above, the following conditions must be met: Figure 12 The low voltage ride-through requirements shown can ensure that the electric vehicle charging station continues to operate without being disconnected from the grid; otherwise, the electric vehicle charging station is allowed to be disconnected. Figure 12 It can be seen that when the grid connection point voltage drops to 0, it can ensure continuous operation for 0.15 s without disconnecting from the grid; when the grid connection point voltage drops below curve 1, it can be disconnected from the grid; for charging stations that are not disconnected during the power system fault, their active power should be able to recover quickly after the fault is cleared, and recover to the pre-fault value at a power change rate of at least 30% of the rated power / second from the moment the fault is cleared.

[0157] For three-phase short circuit faults and two-phase short circuit faults, the test voltage of the grid connection point is the grid connection point line voltage; for single-phase ground short circuit faults, the test voltage is the grid connection point phase voltage.

[0158] When a short circuit fault occurs in the simulated power system and causes a voltage drop, when the voltage drop exceeds 15%, the electric vehicle charging station must provide at least 1.6% of the reactive current for every 1% voltage drop. From the moment the grid voltage drops, the response time of the dynamic reactive current is within 30 ms. From the moment the dynamic reactive current responds until the voltage recovers to 0.85 (pu), the dynamic reactive current i injected into the power system is q The voltage change at the grid connection point should be tracked in real time and should satisfy the following formula:

[0159]

[0160] Among them, I N is the rated current output by the converter, U s is the per-unit value of the grid-connected point voltage.

[0161] When a fault occurs in the simulated distribution network, the reactive reference current of the electric vehicle charging station is determined by the grid connection point voltage, and the corresponding reactive current is provided according to the different degrees of the above voltage drop. Since the grid connection point voltage drops during a fault, the reactive reference current and the active reference current will increase. Under the charging control strategy and the discharging control strategy, the reactive current i q and active current i d The power supply will also change accordingly. In order to improve the response speed of the charging station during the low voltage ride-through process, the outer power loop is locked and the reference values ​​of the active current and reactive current are directly given to control the inner current loop. Due to the current limiting effect of the inverter, the maximum allowable output current is usually 1.2 to 1.5 times the rated current. According to regulations, during a grid fault, the reactive current is controlled first, and the output active current is limited by the inverter limiter. In order to output the maximum allowable active power in the case of inverter overcurrent, the active current is taken as and Taking into account the influence of the charging control strategy and the discharging control strategy, the output fault current is limited by limiting the reference current, which can be expressed as:

[0162]

[0163] in, It is the reactive current command value that should be provided after the grid voltage drops. It is the active current command value provided before the grid voltage drops, I max The maximum short-circuit current allowed for output, I f and  are the amplitude and phase angle of the output fault current.

[0164] When a fault occurs, the grid voltage , prioritize reactive current control according to the requirements of charging control strategy and discharging control strategy ,when When the value is 1, Imax is 1.2 times of the rated current, according to , should satisfy According to the boundary conditions of the output short-circuit current, we can get When the active power reference Provides the required active current, and when hour, The required active current can no longer be provided according to the active current reference, otherwise the fault current output by the inverter will be greater than the maximum current it can withstand. At this time, the command value of the active current should meet According to the output fault current formula, we can get When ;when When , the inverter output current is equal to the maximum current it can withstand 1.2.

[0165] When the grid voltage drops, the fault current output by the electric vehicle charging station is as follows:

[0166]

[0167] From this, the three-phase fault current output by the electric vehicle charging station after the fault can be derived:

[0168]

[0169]

[0170] Among them, i d1 、i q1 They are respectively the active current and reactive current output after a fault;

[0171] By comparing the three-phase fault currents before and after the fault, it can be found that the current output by the electric vehicle charging station has changed in both amplitude and phase. This change is related to the degree of voltage drop at the grid connection point.

[0172] When an asymmetric fault occurs, a negative-sequence component will appear in the voltage. If conventional control strategies are used, the charging station will output a current containing this component. Therefore, to improve the current output characteristics during a fault, a phase-locked loop is used to quickly and accurately obtain the positive-sequence voltage amplitude and phase. Positive-sequence component control is adopted, using the positive-sequence component of the grid connection point voltage as a reference while filtering out the influence of the negative-sequence component. In this case, the charging station can be equivalent to a current source controlled by the PCC positive-sequence voltage. In this case, the following is true: .

[0173] During the low voltage ride-through process, the voltage drop degree and voltage phase of the grid connection point will affect the fault current output by the electric vehicle charging station, but its maximum output current will not exceed 1.2 times the rated current. The line impedance and fault type will affect the positive sequence component of the voltage after the fault, causing the magnitude and phase of the fault current output by the charging station to vary with different fault conditions.

[0174] Step S4, comparing and analyzing the fault current changes on the DC side and the fault current changes on the AC side before and after the fault to obtain the fault characteristics of the electric vehicle charging station;

[0175] In this step, firstly, the fault current changes on the DC side before and after the inter-pole fault occurs on the DC side are compared and analyzed to obtain first change data of the fault current on the DC side when the inter-pole short circuit fault occurs on the DC side;

[0176] Then, comparing and analyzing the changes in the fault current on the DC side before and after the polar short circuit fault occurs on the DC side, to obtain first change data of the fault current on the DC side when the polar short circuit fault occurs on the DC side;

[0177] Finally, the fault current changes on the AC side before and after the fault are compared and analyzed to obtain second change data of the fault current on the AC side when a preset short circuit fault occurs on the AC side causing low voltage ride-through at the grid connection point.

[0178] According to the first change data and the second change data, the fault characteristics of the electric vehicle charging station are obtained. In some embodiments, the step S4 is specifically as follows:

[0179] When an inter-pole short-circuit fault occurs on the DC side, the AC / DC converter locks up, causing the DC bus voltage to drop rapidly due to capacitor discharge. When the capacitor voltage drops to zero, all the AC / DC converter diodes conduct, entering the uncontrolled rectification steady-state phase, stabilizing the DC bus voltage. The AC side voltage does not change significantly, but the AC current initially exhibits a significant three-phase imbalance. After a short period of time, the three phases regain balance, increasing by several dozen times. The DC side voltage drops to zero at the instant of the fault, then stabilizes at around 100V after the uncontrolled steady-state phase, rendering it inoperable. The DC current then changes direction, flowing toward the fault point.

[0180] When a polar short circuit fault occurs on the DC side, under the direct grounding mode of the neutral point, the AC side current can only discharge to the fault point through the anti-parallel diode of the upper bridge arm, so its current direction can only be positive, and tends to be stable after a substantial increase, while the AC side voltage does not change. Due to the surge in the AC side current, the DC side current also increases significantly and then tends to be stable, but the current direction on the DC side changes from charging the battery to flowing to the fault point. Since the current on the AC side can only pass through the anti-parallel diode of the upper bridge arm, the DC bus voltage decreases, and is about half of the normal state in steady state; in the ungrounded mode, the fault point forms a loop through the distributed capacitance of the earth and the negative pole. At this time, the voltage becomes 0 after the positive pole is grounded and short-circuited, but due to the existence of the supporting capacitor, the DC bus voltage remains unchanged, the DC side current does not change, and the AC side voltage and current do not change significantly.

[0181] When a single-phase ground fault or a two-phase short circuit fault occurs on the AC side, the voltage drop at the grid connection point is in the second range, i.e. At this time, the fault current output by the charging station has not exceeded the limit of the converter, so the active current output is considered to be 1p.u. When a two-phase ground short circuit fault occurs, the voltage drop at the grid connection point is in the third range, that is, At this time, the fault current output by the charging station has exceeded the limit of the converter. Therefore, in addition to the requirements for the output reactive current, the active current should also be limited. Therefore, the active current provided by the charging station is no longer 1 p.u., but becomes 0.93 p.u. When a three-phase short circuit fault occurs, the grid voltage drops to zero and is in the fourth range, that is, At this time, the reactive current also reaches the limit of 1.04pu, and the active current also reaches the limit of 0.6pu.

[0182] Therefore, regardless of the type of fault, the fault current output by the charging station will not exceed 1.2 times the rated current, which is the maximum allowable output current of the converter. Regardless of whether a symmetrical or asymmetrical fault occurs, the fault current output by the charging station contains only a positive-sequence component. Therefore, when analyzing distribution network fault characteristics, the charging station can be treated as a positive-sequence current source controlled by the PCC voltage at the grid connection point, providing a foundation for subsequent protection of the distribution network containing electric vehicle charging stations.

[0183] Furthermore, in order to execute the fault characteristic analysis system of the electric vehicle charging station corresponding to the above method embodiment to achieve the corresponding functions and technical effects, Figure 13 A structural diagram of a fault characteristic analysis system for an electric vehicle charging station is provided. For ease of illustration, only the parts relevant to this embodiment are shown. The fault characteristic analysis system for an electric vehicle charging station provided by the embodiment of the present invention includes:

[0184] The charging / discharging control strategy generation module 201 is used to convert the voltage equation of the three-phase circuit of the electric vehicle charging station during charging and the voltage equation of the three-phase circuit during discharging to obtain the charging control strategy and the discharging control strategy of the electric vehicle charging station respectively;

[0185] A DC side fault current monitoring module 202 is configured to simulate short circuit faults occurring at different locations on the DC side of the electric vehicle charging station according to the charging control strategy and the discharging control strategy, and record changes in the DC side fault current before and after the fault;

[0186] The AC side fault current monitoring module 203 is used to simulate a short circuit fault on the AC side of the electric vehicle charging station and a low voltage ride-through at the grid connection point according to the charging control strategy and the discharging control strategy, and record the changes in the AC side fault current before and after the fault;

[0187] The charging station fault analysis module 204 is used to compare and analyze the fault current changes on the DC side and the fault current changes on the AC side before and after the fault to obtain the fault characteristics of the electric vehicle charging station.

[0188] In some embodiments, the charge / discharge control strategy generation module 201 further includes:

[0189] The charging / discharging control strategy acquisition unit is used to perform coordinate transformation and feedforward decoupling control processing on the voltage equation of the three-phase circuit of the electric vehicle charging station during charging to obtain a first voltage control equation; perform calculation based on the first voltage control equation and the current information on the DC side to obtain the charging control strategy of the electric vehicle station; perform coordinate transformation and feedforward decoupling control processing on the voltage equation of the three-phase circuit of the electric vehicle charging station during discharging to obtain a second voltage control equation; and obtain the discharge control strategy of the electric vehicle station based on the second voltage control equation and the output power of the electric vehicle charging station during discharge.

[0190] In some embodiments, the DC side fault current monitoring module 202 further includes:

[0191] The DC side fault current change recording unit is used to simulate an inter-pole short circuit fault on the DC side of the electric vehicle charging station according to the charging control strategy and the discharging control strategy, and record the fault current change on the DC side before and after the inter-pole short circuit fault; according to the charging control strategy and the discharging control strategy, through a preset grounding method, simulate an inter-pole short circuit fault on the DC side of the electric vehicle charging station, and record the fault current change on the DC side before and after the inter-pole short circuit fault.

[0192] In some embodiments, the AC side fault current monitoring module 203 further includes:

[0193] The AC side fault current change recording unit is used to simulate a short circuit fault on the AC side of the electric vehicle charging station according to a preset short circuit fault form, causing low voltage ride-through at the grid connection point, and adjust the reactive current and active current of the electric vehicle charging station under the charging control strategy, discharging control strategy and preset inverter limit conditions; based on the reactive current and active current, obtain the current change on the AC side before and after the fault.

[0194] In some embodiments, the charging station fault analysis module 204 further includes:

[0195] The charging station fault characteristic analysis unit is used to compare and analyze the changes in the fault current on the DC side before and after the fault, and obtain first change data of the fault current on the DC side when an inter-pole short circuit fault and a pole-to-ground short circuit fault occur on the DC side; wherein the first change data includes the change in direction of the fault current on the DC side, and the value of the fault current on the DC side when it reaches a steady state; compare and analyze the changes in the fault current on the AC side before and after the fault, and obtain second change data of the fault current on the AC side when a preset short circuit fault occurs on the AC side, resulting in a low voltage ride-through at the grid connection point; wherein the change in the value of the fault current includes a change in the ratio of the fault current to the rated output current of the electric vehicle charging station; based on the first change data and the second change data, the fault characteristics of the electric vehicle charging station are obtained.

[0196] This embodiment proposes a method and system for analyzing the fault characteristics of an electric vehicle charging station. The method converts the voltage equations of the three-phase circuit during charging and discharging to obtain the charging and discharging control strategies of the electric vehicle charging station, respectively. Based on the charging and discharging control strategies, a short-circuit fault is simulated at different locations on the DC side of the electric vehicle charging station, and the changes in the fault current on the DC side before and after the fault are recorded. Based on the charging and discharging control strategies, a short-circuit fault and low voltage ride-through (LVRT) on the AC side of the electric vehicle charging station are simulated, and the changes in the fault current on the AC side before and after the fault are recorded. The changes in the DC and AC side fault currents before and after the fault are compared and analyzed to obtain the fault characteristics of the electric vehicle charging station. The method has the beneficial effect of constructing a fault current output model for the electric vehicle charging station while considering LVRT, comprehensively analyzing the charging and discharging fault characteristics of the electric vehicle charging station, providing data support for optimizing relay protection in the distribution network containing the electric vehicle charging station, and improving the operational stability of the electric vehicle charging station.

[0197] The specific embodiments described above further illustrate the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the above description is merely a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention by those skilled in the art should be included in the scope of protection of the present invention.

Claims

1. A method for analyzing fault characteristics of an electric vehicle charging station, characterized in that: include: The voltage equation of the three-phase circuit of the electric vehicle charging station during charging and the voltage equation of the three-phase circuit during discharging are converted to obtain the charging control strategy and the discharging control strategy of the electric vehicle station respectively; According to the charging control strategy and the discharging control strategy, a short circuit fault is simulated at different locations on the DC side of the electric vehicle charging station, and the change of the fault current on the DC side before and after the fault is recorded; According to the charging control strategy and the discharging control strategy, a short circuit fault on the AC side of the electric vehicle charging station and a low voltage ride-through at the grid connection point are simulated, and the change in the fault current on the AC side before and after the fault is recorded. Specifically, according to a preset short circuit fault form, a short circuit fault is simulated on the AC side of the electric vehicle charging station, causing a low voltage ride-through at the grid connection point, and the reactive current and active current of the electric vehicle charging station are adjusted under the charging control strategy, the discharging control strategy, and preset inverter limit conditions; based on the reactive current and active current, the current change on the AC side before and after the fault is obtained; Comparing and analyzing the changes in the fault current on the DC side and the fault current on the AC side before and after the fault, and obtaining the fault characteristics of the electric vehicle charging station, including: comparing and analyzing the changes in the fault current on the DC side before and after the fault, and obtaining first change data of the fault current on the DC side when an inter-pole short circuit fault and a pole-to-ground short circuit fault occur on the DC side; wherein, the first change data includes the change in direction of the fault current on the DC side, and the value of the fault current on the DC side when it reaches a steady state; comparing and analyzing the changes in the fault current on the AC side before and after the fault, and obtaining second change data of the fault current on the AC side when a preset short circuit fault occurs on the AC side, resulting in a low voltage ride-through at the grid connection point; wherein, the change in the value of the fault current includes a change in the ratio of the fault current to the rated output current of the electric vehicle charging station; and obtaining the fault characteristics of the electric vehicle charging station based on the first change data and the second change data.

2. The fault characteristic analysis method of an electric vehicle charging station according to claim 1, characterized in that: The voltage equation of the three-phase circuit during charging and the voltage equation of the three-phase circuit during discharging of the electric vehicle charging station are converted to obtain the charging control strategy and the discharging control strategy of the electric vehicle station, respectively. Specifically, Performing coordinate transformation and feedforward decoupling control on the voltage equation of the three-phase circuit of the electric vehicle charging station during charging to obtain a first voltage control equation; Calculating according to the first voltage control equation and the current information on the DC side to obtain a charging control strategy for the electric vehicle station; The voltage equation of the three-phase circuit of the electric vehicle charging station during discharge is subjected to coordinate transformation and feedforward decoupling control processing to obtain a second voltage control equation; According to the second voltage control equation and the output power of the electric vehicle charging station during discharge, the discharge control strategy of the electric vehicle station is obtained.

3. The fault characteristic analysis method of an electric vehicle charging station according to claim 1, characterized in that: According to the charging control strategy and the discharging control strategy, a short circuit fault is simulated at different locations on the DC side of the electric vehicle charging station, and the fault current changes on the DC side before and after the fault are recorded, specifically: According to the charging control strategy and the discharging control strategy, a short circuit fault is simulated on the DC side of the electric vehicle charging station, and the change of the fault current on the DC side before and after the short circuit fault is recorded; According to the charging control strategy and the discharging control strategy, a polar short circuit fault is simulated on the DC side of the electric vehicle charging station through a preset grounding method, and the fault current change on the DC side before and after the polar short circuit fault is recorded.

4. A fault characteristic analysis system for an electric vehicle charging station, characterized in that: include: Charging / discharging control strategy generation module, DC side fault current monitoring module, AC side fault current monitoring module and charging station fault analysis module; The charging / discharging control strategy generation module is used to convert the voltage equation of the three-phase circuit of the electric vehicle charging station during charging and the voltage equation of the three-phase circuit during discharging to obtain the charging control strategy and the discharging control strategy of the electric vehicle station respectively; The DC side fault current monitoring module is used to simulate short circuit faults at different locations on the DC side of the electric vehicle charging station according to the charging control strategy and the discharging control strategy, and record the changes in the fault current on the DC side before and after the fault; The AC side fault current monitoring module is used to simulate a short circuit fault on the AC side of the electric vehicle charging station and a low voltage ride-through at the grid connection point according to the charging control strategy and the discharging control strategy, and record the change in the fault current on the AC side before and after the fault; wherein, the AC side fault current monitoring module includes: an AC side fault current change recording unit, which is used to simulate a short circuit fault on the AC side of the electric vehicle charging station according to a preset short circuit fault form, causing a low voltage ride-through at the grid connection point, and adjusting the reactive current and active current of the electric vehicle charging station under the charging control strategy, the discharging control strategy and the preset inverter limit conditions; and obtain the current change on the AC side before and after the fault based on the reactive current and the active current; The charging station fault analysis module is used to compare and analyze the changes in the fault current on the DC side and the fault current on the AC side before and after the fault to obtain the fault characteristics of the electric vehicle charging station; wherein, the charging station fault analysis module includes: a charging station fault characteristic analysis unit, used to compare and analyze the changes in the fault current on the DC side before and after the fault to obtain first change data of the fault current on the DC side when an inter-pole short circuit fault and a pole-to-ground short circuit fault occur on the DC side; wherein, the first change data includes the change in direction of the fault current on the DC side and the value of the fault current on the DC side when it reaches a steady state; compare and analyze the changes in the fault current on the AC side before and after the fault to obtain second change data of the fault current on the AC side when a preset short circuit fault form occurs on the AC side, resulting in a low voltage ride-through at the grid connection point; wherein, the change in the value of the fault current includes a change in the ratio of the fault current to the rated output current of the electric vehicle charging station; based on the first change data and the second change data, the fault characteristics of the electric vehicle charging station are obtained.

5. The fault characteristic analysis system for an electric vehicle charging station according to claim 4, characterized in that: The charging / discharging control strategy generation module includes: a charging / discharging control strategy acquisition unit; The charging / discharging control strategy acquisition unit is used to perform coordinate transformation and feedforward decoupling control processing on the voltage equation of the three-phase circuit of the electric vehicle charging station during charging to obtain a first voltage control equation; perform calculation based on the first voltage control equation and the current information on the DC side to obtain the charging control strategy of the electric vehicle station; perform coordinate transformation and feedforward decoupling control processing on the voltage equation of the three-phase circuit of the electric vehicle charging station during discharging to obtain a second voltage control equation; and obtain the discharge control strategy of the electric vehicle station based on the second voltage control equation and the output power of the electric vehicle charging station during discharge.

6. The fault characteristic analysis system for an electric vehicle charging station according to claim 4, characterized in that: The DC side fault current monitoring module includes: a DC side fault current change recording unit; The DC side fault current change recording unit is used to simulate an inter-pole short circuit fault on the DC side of the electric vehicle charging station according to the charging control strategy and the discharging control strategy, and record the fault current change on the DC side before and after the inter-pole short circuit fault; according to the charging control strategy and the discharging control strategy, through a preset grounding method, simulate the occurrence of a pole-to-ground short circuit fault on the DC side of the electric vehicle charging station, and record the fault current change on the DC side before and after the pole-to-ground short circuit fault.

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