A method for evaluating fault current in electric vehicles considering the influence of battery state of charge

Abstract

The present invention relates to a method for evaluating electric vehicle fault current that takes into account the influence of battery state of charge. The method calculates the critical state of charge for switching between constant-current and constant-voltage charging of an electric vehicle, and determines whether the electric vehicle is being charged at constant current or constant voltage based on the state of charge of the electric vehicle at the time of the fault. Under the constant-current or constant-voltage charging state, the critical state of charge for overdamping / underdamping of the fault current under constant voltage or constant-current charging is calculated, respectively. Based on the state of charge of the electric vehicle at the time of the fault, the fault current is determined to be overdamped or underdamped, and the corresponding expressions are used to calculate the fault current of the electric vehicle. The present invention can consider the influence of the state of charge on the fault current of an electric vehicle, accurately estimate and calculate the fault current of the electric vehicle, and provide more accurate reference data for fault current analysis of new energy distribution networks connected to electric vehicles, thereby better ensuring that the charging management system can accurately provide charging fault warnings or power distribution and power supply management for the new energy distribution network.

Description

Technical Field

[0001] The present invention relates to the technical field of power system protection and fault analysis, and in particular to a method for evaluating fault current of an electric vehicle taking into account the influence of the battery state of charge. Background Art

[0002] Electric vehicles, characterized by cleanliness, pollution-free operation, and high energy efficiency, have seen their share of the market continue to rise in recent years. Electric vehicles are charged using a dual-active bridge converter and a voltage source converter, with DC capacitors acting as buffers. During the charging process, the power transmitted by the two-stage converter is deeply coupled with the energy of the DC capacitors. In new energy distribution networks, a charging management system is deployed at distribution stations or in electric vehicle equipment. This system analyzes voltage and current information during the charging process of electric vehicles and monitors the distribution and power supply status. In the event of a fault in the new energy distribution network, the charging management system further analyzes the fault current analysis and calculations of the electric vehicle charging system, and then implements appropriate charging fault warnings or distribution and power supply management. However, if the charging management system does not accurately analyze and calculate the fault current of electric vehicle charging, it will affect the accuracy of the power grid fault analysis, which may lead to false alarms of fault conditions, and it will be impossible to provide accurate early warnings or timely intervention to perform distribution and power supply management tasks, which will further cause the power grid fault to deteriorate, leading to serious accidents. Especially after a power grid fault, the DC voltage drops due to the active power drop absorbed by the voltage source converter, and at the same time affects the power transmitted to the battery by the dual-active active bridge converter, so that the fault current of the electric vehicle operating in the charging mode is affected by the charging power at the moment of the fault and the two-stage converter control link of the voltage source converter-dual-active active bridge converter, resulting in challenges for the charging management system to analyze and calculate the fault current of the electric vehicle.

[0003] There has been some research on calculating the fault current of electric vehicles (EVs) under grid faults. Researchers have pointed out that according to the national standard GB / T 18287.1-2015, the fault current of AC charging EVs is 16 to 32A. Researchers used a probability function to represent the concentrated discharge power of EVs at a given moment, thereby calculating the range of steady-state fault current values ​​at that moment and deriving a method for evaluating EV fault currents under different grid fault severity levels. Researchers independently analyzed two-stage converters, a voltage source converter (VSC) and a dual-active active bridge converter (DABC), and derived a fault current model for EV charging and discharging stations. However, this research primarily focused on calculating the steady-state magnitude of EV fault currents and failed to consider the impact of the EV's state of charge and charging control on the transient phase of the fault current calculation. Consequently, the transient process calculation of EV fault currents remains relatively underdeveloped.

[0004] Electric vehicle charging circuits share a similar structure to inverter-type distributed power sources (DCGs). Therefore, some researchers have used methods based on DCG fault currents to calculate EV fault currents. These researchers have derived fault current models for the constant power and current limiting phases of DCGs under fault conditions, but have neglected the converter control process. Some researchers have analyzed fault current models for DC voltage outer loop control of voltage-source converters (VSCCs) after grid faults and derived analytical expressions for wind turbine fault currents under control strategies. Researchers have derived fault current calculation methods for voltage-controlled DCGs with exponentially damped DC components and for photovoltaic array power affected by DC voltage. Some researchers have derived fault current models for DCGs that consider the dynamic response of phase-locked loops (PLLs). However, these methods primarily focus on the impact of single-stage VSC control on fault currents. The charging power of electric vehicles is closely related to the state of charge. In addition, electric vehicles use different constant current or constant voltage charging strategies for dual-active bridge converters under different states of charge. The two-level control of voltage source converters and dual-active bridge converters affects the fault current by changing the DC voltage, making the fault current calculation method of distributed power supplies difficult to apply to electric vehicles.

[0005] Therefore, how to accurately calculate the fault current of electric vehicles under different states of charge, and thus provide more accurate reference data for fault current analysis of new energy distribution networks with a large number of electric vehicles connected, has become an urgent problem that technicians in this field need to solve. Summary of the Invention

[0006] In response to the above-mentioned deficiencies in the prior art, the present invention provides an electric vehicle fault current assessment method that takes into account the influence of the battery state of charge, which is used to more accurately calculate the fault current of electric vehicles under different states of charge, and provide more accurate reference data for the fault current analysis of the new energy distribution network connected to the electric vehicle, thereby better ensuring that the charging management system can accurately provide charging fault warnings or distribution and power supply management for the new energy distribution network.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] A method for evaluating electric vehicle fault current, taking into account the influence of battery state of charge, is implemented by a battery management system in a new energy distribution network. The method analyzes and determines the fault current of an electric vehicle when it is connected to a charging station during a grid fault, and uses the fault current of the electric vehicle to perform charging fault warning or power distribution and supply management. The method includes the following steps:

[0009] S101, collecting battery charging parameters of the electric vehicle, determining the battery state of charge of the electric vehicle at the time of the grid failure, and then determining the charging strategy of the electric vehicle; if the electric vehicle is in a constant current charging state, executing S102; if the electric vehicle is in a constant voltage charging state, executing S103;

[0010] S102, judging the state of the electric vehicle's fault current under constant current charging according to the battery charge state of the electric vehicle at the time of the grid fault, calculating and determining the electric vehicle's fault current according to the fault current state under constant current charging, and executing step S104;

[0011] S103, judging the state of the electric vehicle fault current under constant voltage charging according to the battery charge state of the electric vehicle at the time of the grid fault, calculating and determining the electric vehicle fault current according to the fault current state under constant voltage charging, and executing step S104;

[0012] S104: Outputting the electric vehicle fault current result to execute charging fault warning or power distribution and supply management according to the electric vehicle fault current.

[0013] As a preferred solution, in step S101, the specific method of determining the charging strategy of the electric vehicle is: the battery state of charge (SOC) of the electric vehicle at the time of the fault is f Critical state of charge (SOC) for switching between constant current and constant voltage charging of electric vehicles sw Compare; if soc f ≤soc sw , then the electric vehicle is judged to be in constant current charging state; if soc f >soc sw , it is determined that the electric vehicle is in a constant voltage charging state.

[0014] As a preferred solution, the critical state of charge (SOC) of the electric vehicle constant current-constant voltage charging switch is sw Determine by solving the following equation:

[0015]

[0016] Where, It is the reference value of battery charging current under constant current charging; is the battery charging voltage reference value under constant voltage charging; R ba is the battery ohmic internal resistance; soc sw is the critical state of charge for constant current-constant voltage charging switching; E0 is the constant voltage of the battery; K is the battery polarization voltage constant; e is the natural constant; A ba 、B ba are the amplitude of the voltage drop in the exponential region of the typical discharge curve of the battery and the inverse of the time constant in the exponential region of the discharge curve; Q c is the battery capacity of the electric vehicle.

[0017] As a preferred solution, in S102, the specific method of determining the state of the fault current of the electric vehicle under constant current charging is: the battery state of charge (SOC) of the electric vehicle at the time of the fault is calculated. f The critical state of charge (SOC) of overdamping / underdamping under constant current charging cc Compare; if soc f >soc cc , then the fault current is judged to be in an over-damping state; if soc f ≤soc cc , then the fault current is determined to be in an underdamped state.

[0018] As a preferred solution, the critical state of charge soc of overdamping / underdamping under constant current charging is cc Determine by solving the following equation:

[0019]

[0020] Where, soc cc k is the critical state of charge of overdamping / underdamping under constant current charging; p 、k i k is the PI parameter of the DC voltage outer loop control of the voltage source converter; vp is the PI control parameter of the dual active bridge converter; C is the DC capacitor; u f is the AC voltage after the grid fault; a1 and a2 are parameters determined by the dual-active bridge converter parameters, the DC voltage reference value, and the charging voltage reference value, and are expressed as:

[0021]

[0022]

[0023] Where n is the turns ratio of the high-voltage side and low-voltage side coils of the dual-active bridge converter; L r is the energy storage inductor of the dual active bridge converter; f s is the switching frequency of the dual active bridge converter; is the DC voltage reference value.

[0024] As a preferred solution, in S102, when the fault current is in an over-damping state during constant current charging, the fault current is calculated as follows:

[0025]

[0026] Where i(t) is the fault current of the electric vehicle; t is the time variable; P sis the charging power of the electric vehicle before the fault; c1, c2, λ1, and λ2 are the over-damping fault current parameters under constant current charging, which are expressed as:

[0027]

[0028]

[0029]

[0030]

[0031] Where, σ is the voltage drop degree at the grid connection point; SOC is the battery state of charge of the electric vehicle at the time of failure f Determine the battery open circuit voltage;

[0032] When the fault current is underdamped during constant current charging, the fault current is calculated as follows:

[0033]

[0034] Where α1, β1, θ1, and A1 are the underdamped fault current parameters under constant current charging, which are expressed as:

[0035]

[0036]

[0037]

[0038]

[0039] As a preferred solution, in S103, the specific method of judging the state of the fault current of the electric vehicle under constant voltage charging is: the battery state of charge (SOC) of the electric vehicle at the time of the fault is calculated. f The critical state of charge (SOC) of overdamping / underdamping under constant voltage charging cv Compare; if soc f ≤soc cv , then the fault current is judged to be in an over-damping state; if soc f >soc cv , then the fault current is determined to be in an underdamped state.

[0040] As a preferred solution, the critical state of charge soc of overdamping / underdamping under constant voltage charging is cv Determine by solving the following equation:

[0041]

[0042] Where, is the critical state of charge (SOC) of overdamping / underdamping under constant voltage charging cv Determines the battery open circuit voltage.

[0043] As a preferred solution, in S103, when the fault current is in an over-damping state during constant voltage charging, the fault current is calculated as follows:

[0044]

[0045] Where c4, c5, λ3, and λ4 are the over-damping fault current parameters under constant voltage charging, which are expressed as:

[0046]

[0047]

[0048]

[0049] Where a3 and a4 are parameters determined by the dual-active bridge converter parameters, the DC voltage reference value, and the charging voltage reference value, and are expressed as:

[0050]

[0051]

[0052] Where, SOC is the battery state of charge of the electric vehicle at the time of failure f Determine the battery open circuit voltage;

[0053] When the fault current is underdamped under constant voltage charging, the fault current is calculated as follows:

[0054]

[0055] Where α2, β2, θ2, and A2 are underdamped fault current parameters under constant voltage charging, and are expressed as:

[0056]

[0057]

[0058]

[0059]

[0060] Compared with the prior art, the present invention has the following beneficial effects:

[0061] 1. The existing technology for calculating the steady-state current of electric vehicles cannot reflect the transient changes of fault current. The method of the present invention takes into account the influence of the state of charge and the control of the two-stage converter, so that the calculated value of the proposed calculation method can accurately fit the transient process of the fault current of electric vehicles.

[0062] 2. The existing technology uses the voltage source current conversion control equation of the electric vehicle charging circuit to calculate the fault current, ignoring the impact of the different constant current or constant voltage charging control of the dual-active active bridge converter, which may cause significant errors in the fault current calculation; the method of the present invention analyzes the control process of the dual-active active bridge converter under constant current or constant voltage charging to achieve accurate calculation of the electric vehicle fault current under constant current or constant voltage charging.

[0063] 3. The existing technology ignores the impact of changes in electric vehicle battery power during a fault on the fault current; the method of the present invention calculates the critical charge state of overdamping / underdamping under constant current / constant voltage charging by taking into account the changes in battery power during the fault process under the influence of the charge state, and then accurately determines whether the electric vehicle fault current is overdamped or underdamped based on the charge state at the time of the fault. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] In order to make the purpose, technical solutions and advantages of the invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings, in which:

[0065] Figure 1 This is a flow chart of the electric vehicle fault current assessment method taking into account the influence of battery state of charge of the present invention;

[0066] Figure 2 This is a topological grid-connected structure of the main circuit for charging an electric vehicle as exemplified in the embodiment;

[0067] Figure 3 This is a comparison chart of the fault currents obtained by different methods under constant current charging in the embodiment and the simulation results;

[0068] Figure 4 This is a comparison chart of the fault current obtained by different methods under constant voltage charging in the embodiment and the simulation results. DETAILED DESCRIPTION

[0069] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the invention claimed for protection, but only represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0070] 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. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0071] Existing methods primarily focus on the impact of single-stage voltage source converter control on fault current during faults, but are unable to calculate the fault current of electric vehicles under two-stage control, taking into account the impact of state of charge. This invention discloses a method for estimating electric vehicle fault current that accounts for the influence of battery state of charge, taking into account the impact of state of charge on electric vehicle fault current during grid faults. This method, executed by a battery management system in a new energy distribution network, analyzes and determines the fault current of an electric vehicle connected to charging during a grid fault, and uses this to implement charging fault warnings or power distribution and supply management based on the electric vehicle fault current.

[0072] like Figure 1 As shown, the electric vehicle fault current assessment method taking into account the influence of battery state of charge disclosed in the present invention includes the following steps:

[0073] S101, collecting battery charging parameters of the electric vehicle, determining the battery state of charge of the electric vehicle at the time of the grid failure, and then determining the charging strategy of the electric vehicle; if the electric vehicle is in a constant current charging state, executing S102; if the electric vehicle is in a constant voltage charging state, executing S103;

[0074] S102, judging the state of the electric vehicle's fault current under constant current charging according to the battery charge state of the electric vehicle at the time of the grid fault, calculating and determining the electric vehicle's fault current according to the fault current state under constant current charging, and executing step S104;

[0075] S103, judging the state of the electric vehicle fault current under constant voltage charging according to the battery charge state of the electric vehicle at the time of the grid fault, calculating and determining the electric vehicle fault current according to the fault current state under constant voltage charging, and executing step S104;

[0076] S104: Outputting the electric vehicle fault current result to execute charging fault warning or power distribution and supply management according to the electric vehicle fault current.

[0077] In the specific implementation, in step S101, the specific method of determining the charging strategy of the electric vehicle is: the battery state of charge (SOC) of the electric vehicle at the time of the fault is f Critical state of charge (SOC) for switching between constant current and constant voltage charging of electric vehicles sw Compare; if soc f ≤soc sw , then the electric vehicle is judged to be in constant current charging state; if soc f >soc sw , it is determined that the electric vehicle is in a constant voltage charging state.

[0078] Critical state of charge (SOC) for switching between constant current and constant voltage charging of electric vehicles sw Determine by solving the following equation:

[0079]

[0080] Where, It is the reference value of battery charging current under constant current charging; is the battery charging voltage reference value under constant voltage charging; R ba is the battery ohmic internal resistance; soc sw Critical state of charge during constant current-constant voltage charging switch; E(soc sw ) is the battery open circuit voltage determined by the critical state of charge during constant current-constant voltage charging.

[0081] The battery open circuit voltage E(soc) is determined by the critical state of charge of the constant current-constant voltage charging switch. sw ) is calculated as follows:

[0082]

[0083] Therefore, combining equations (1) and (2), the critical state of charge (SOC) of the electric vehicle constant current-constant voltage charging switch is sw Determine by solving the following equation:

[0084]

[0085] Where, E0 is the constant voltage of the battery; K is the polarization voltage constant of the battery; R ba is the battery ohmic internal resistance; Aba 、B ba are the amplitude of the voltage drop in the exponential region of the typical discharge curve of the battery and the inverse of the time constant in the exponential region of the discharge curve; Q c is the battery capacity of the electric vehicle.

[0086] In specific implementation, in step S102, the specific method of determining the state of the fault current of the electric vehicle under constant current charging is: the battery state of charge (SOC) of the electric vehicle at the time of the fault is calculated. f The critical state of charge (SOC) of overdamping / underdamping under constant current charging cc Compare; if soc f >soc cc , then the fault current is judged to be in an over-damping state; if soc f ≤soc cc , then the fault current is determined to be in an underdamped state.

[0087] The battery open circuit voltage E(soc) is determined by the critical state of charge of overdamping / underdamping under constant current charging. cc ) is calculated as follows:

[0088]

[0089] Therefore, combining equations (4) and (2), the critical state of charge soc of overdamping / underdamping under constant current charging is cc Determine by solving the following equation:

[0090]

[0091] Where, soc cc k is the critical state of charge of overdamping / underdamping under constant current charging; p 、k i k is the PI parameter of the DC voltage outer loop control of the voltage source converter; vp is the PI control parameter of the dual active bridge converter; C is the DC capacitor; u f is the AC voltage after the grid fault; a1 and a2 are parameters determined by the dual-active bridge converter parameters, the DC voltage reference value, and the charging voltage reference value, and are expressed as:

[0092]

[0093]

[0094] Where n is the turns ratio of the high-voltage side and low-voltage side coils of the dual-active bridge converter; L r is the energy storage inductor of the dual active bridge converter; f s is the switching frequency of the dual active bridge converter; is the DC voltage reference value.

[0095] In specific implementation, in step S102, when the fault current is in an over-damping state during constant current charging, the fault current is calculated as follows:

[0096]

[0097] Where i(t) is the fault current of the electric vehicle; t is the time variable; P s is the charging power of the electric vehicle before the fault; c1, c2, λ1, and λ2 are the over-damping fault current parameters under constant current charging, which are expressed as:

[0098]

[0099]

[0100]

[0101]

[0102] Where, σ is the voltage drop degree at the grid connection point; SOC is the battery state of charge of the electric vehicle at the time of failure f Determines the battery open circuit voltage.

[0103] When the fault current is underdamped during constant current charging, the fault current is calculated as follows:

[0104]

[0105] Where α1, β1, θ1, and A1 are the underdamped fault current parameters under constant current charging, which are expressed as:

[0106]

[0107]

[0108]

[0109]

[0110] In specific implementation, in S103, the specific method of judging the state of the fault current of the electric vehicle under constant voltage charging is: the battery state of charge (SOC) of the electric vehicle at the time of the fault is calculated. f The critical state of charge (SOC) of overdamping / underdamping under constant voltage charging cv Compare; if soc f ≤soc cv , then the fault current is judged to be in an over-damping state; if soc f >soccv , then the fault current is determined to be in an underdamped state.

[0111] Among them, the critical state of charge soc of overdamping / underdamping under constant voltage charging cv Determine by solving the following equation:

[0112]

[0113] Where, is the critical state of charge (SOC) of overdamping / underdamping under constant voltage charging cv Determines the battery open circuit voltage.

[0114] In specific implementation, in S103, when the fault current is in an over-damping state under constant voltage charging, the fault current is calculated as follows:

[0115]

[0116] Where c4, c5, λ3, and λ4 are the over-damping fault current parameters under constant voltage charging, which are expressed as:

[0117]

[0118]

[0119]

[0120]

[0121] Where a3 and a4 are parameters determined by the dual-active bridge converter parameters, the DC voltage reference value, and the charging voltage reference value, and are expressed as:

[0122]

[0123]

[0124] Where, SOC is the battery state of charge of the electric vehicle at the time of failure f Determines the battery open circuit voltage.

[0125] When the fault current is underdamped under constant voltage charging, the fault current is calculated as follows:

[0126]

[0127] Where α2, β2, θ2, and A2 are underdamped fault current parameters under constant voltage charging, and are expressed as:

[0128]

[0129]

[0130]

[0131]

[0132] Example:

[0133] To verify the effectiveness of the method of the present invention, this example is as follows Figure 2 The main circuit topology of electric vehicle charging is connected to the new energy distribution network for charging. The AC voltage rating is 380V, the DC voltage rating is 800V, and the k of the VSC outer loop control is 1. p =1, k i =50, DAB controlled k vp =0.0005, k vi =20. The parameters of the electric vehicle battery pack are E0=390.3692V, K=0.013485V, R ba =0.009Ω、A ba =30.2313V, B ba =0.30531(Ah) -1 The battery pack's voltage reference value for constant voltage charging is 415V, and its constant current charging is 380A, with a capacity of 200Ah. A three-phase short circuit occurred in the grid at 0.3s, causing the voltage at the electric vehicle's grid connection point to drop to 0.8pu.

[0134] According to the electric vehicle fault current evaluation method disclosed in the present invention, the critical state of charge for constant current-constant voltage charging switching is 91%, the constant current critical state of charge is 62.59%, and the constant voltage critical state of charge is 94.92%. The electric vehicle charge state at the moment of fault is set to 10%, 30%, 50%, 70%, 90% constant current charging and 92%, 94%, 96%, 98% constant voltage charging respectively. By comparing the fault currents under the three different methods, the correctness of the proposed method is verified. Among them, method 1 is an electric vehicle fault current evaluation method disclosed in the present invention that takes into account the influence of the battery charge state, method 2 is a fault current calculation method that only takes into account the outer loop control of the voltage source converter, and method 3 is the simulation result. The fault currents of electric vehicles under different charge states are respectively as follows: Figure 3 and Figure 4 As shown. Among them, Figure 3 (a), (b), (c), (d), and (e) are comparison diagrams of the fault currents obtained by different methods and the simulation results under the constant current charging state of the electric vehicle with a charge state of 10%, 30%, 50%, 70%, and 90% at the moment of fault respectively; Figure 4(a), (b), (c), and (d) are comparison diagrams of the fault current obtained by different methods and the simulation results under the constant voltage charging state of the electric vehicle with a charge state of 92%, 94%, 96%, and 98% at the moment of fault, respectively.

[0135] pass Figure 3 A comparison shows that as the state of charge increases from 10% to 90%, the fault current in Method 2 remains underdamped, while the simulated current gradually changes from underdamped to overdamped, causing the calculated fault current error in Method 2 to increase from 5% to 10%, and the DC voltage error to reach 30%. The disclosed electric vehicle fault current assessment method that accounts for the influence of battery state of charge takes into account the DAB power changes during the fault process. Under constant current charging, the calculated fault current can track the simulated fault current and DC voltage. The overdamped and underdamped states are consistent with the simulation results, and the calculated and simulated errors are less than 2%.

[0136] pass Figure 4 A comparison shows that as the state of charge decreases from 98% to 92%, the fault current in Method 2 remains underdamped, and the error between the calculated fault current and the simulation results gradually increases from 5% to 10%. The fault current estimation method for electric vehicle fault current that accounts for the influence of battery state of charge (SOC) disclosed in this invention consistently remains below 2%. Under constant voltage charging, the fault current estimated by the method for electric vehicle fault current estimation that accounts for the influence of battery state of charge (SOC) is substantially consistent with the simulation results. The calculation error of Method 2 is greatest when the SOC is between 90% SOC under constant current charging and 92% SOC under constant voltage charging, i.e., near the critical SOC of 91% for switching between constant current and constant voltage charging. The calculation method disclosed in this invention, however, accounts for the DAB control link and transmission power variations under the influence of DC voltage, and can better reflect the fault current transient process at the critical SOC of switching between constant current and constant voltage charging, where charging power is maximized.

[0137] As can be seen from the above, the present electric vehicle fault current assessment method, which accounts for the influence of battery state of charge (SOC), accurately reflects the transient process of the electric vehicle fault current by taking into account the influence of SOC and two-stage converter control. In comparison, the existing method calculates fault current using the voltage source current conversion control equation of the electric vehicle charging circuit, ignoring the influence of the different constant current or constant voltage charging control of the dual-active active bridge converter, which may result in significant errors in the fault current calculation. The present method accurately calculates the fault current of the electric vehicle under constant current or constant voltage charging by analyzing the control process of the dual-active active bridge converter under constant current or constant voltage charging. Furthermore, the existing method ignores the impact of changes in electric vehicle battery power during the fault period on the fault current. The present method, however, calculates the critical SOC of overdamping / underdamping under constant current / constant voltage charging by taking into account the change in battery power during the fault period under SOC. This allows accurate judgment of whether the electric vehicle fault current is overdamped or underdamped based on the SOC at the time of the fault. Therefore, the method of the present invention can more accurately calculate the fault current of electric vehicles under different charge states, provide more accurate reference data for the fault current analysis of the new energy distribution network under the connection of electric vehicles, and thus better ensure that the charging management system can accurately provide charging fault warning or distribution and power supply management for the new energy distribution network.

[0138] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the technical solutions. Those skilled in the art should understand that modifications or equivalent replacements of the technical solutions of the present invention that do not depart from the purpose and scope of the technical solutions of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A method for evaluating fault current of an electric vehicle taking into account the influence of battery state of charge, characterized in that: Executed by the battery management system in the new energy distribution network, it analyzes and determines the fault current when the electric vehicle is connected to the charging station when the grid fails, and is used to perform charging fault warning or power distribution management based on the fault current of the electric vehicle; The method comprises the following steps: S101, collect the battery charging working parameters of the electric vehicle, determine the battery state of charge of the electric vehicle at the time of the power grid failure, and then judge the charging strategy of the electric vehicle; if the electric vehicle is in the constant current charging state, execute S102; if the electric vehicle is in the constant voltage charging state, execute S103; in step S101, the specific method of judging the charging strategy of the electric vehicle is: the battery state of charge of the electric vehicle at the time of the failure is determined. f Critical state of charge (SOC) for switching between constant current and constant voltage charging of electric vehicles sw Compare; if soc f ≤soc sw , then the electric vehicle is judged to be in constant current charging state; if soc f >soc sw , it is determined that the electric vehicle is in a constant voltage charging state; the critical state of charge soc of the electric vehicle constant current-constant voltage charging switch sw Determine by solving the following equation: Where, It is the reference value of battery charging current under constant current charging; is the battery charging voltage reference value under constant voltage charging; R ba is the battery ohmic internal resistance; soc sw is the critical state of charge for constant current-constant voltage charging switching; E0 is the constant voltage of the battery; K is the battery polarization voltage constant; e is the natural constant; A ba 、B ba are the amplitude of the voltage drop in the exponential region of the typical discharge curve of the battery and the inverse of the time constant in the exponential region of the discharge curve; Q c is the battery capacity of the electric vehicle; S102, judging the state of the electric vehicle's fault current under constant current charging according to the battery charge state of the electric vehicle at the time of the grid fault, calculating and determining the electric vehicle's fault current according to the fault current state under constant current charging, and executing step S104; S103, judging the state of the electric vehicle fault current under constant voltage charging according to the battery charge state of the electric vehicle at the time of the grid fault, calculating and determining the electric vehicle fault current according to the fault current state under constant voltage charging, and executing step S104; S104: Outputting the electric vehicle fault current result to execute charging fault warning or power distribution and supply management according to the electric vehicle fault current.

2. The electric vehicle fault current assessment method taking into account the influence of battery state of charge according to claim 1, characterized in that: In S102, the specific method of determining the state of the fault current of the electric vehicle under constant current charging is: the battery state of charge (SOC) of the electric vehicle at the time of the fault is calculated. f The critical state of charge (SOC) of overdamping / underdamping under constant current charging cc Compare; if soc f >soc cc , then the fault current is judged to be in an over-damping state; if soc f ≤soc cc , then the fault current is determined to be in an underdamped state.

3. The electric vehicle fault current assessment method taking into account the influence of battery state of charge according to claim 2, characterized in that: The critical state of charge SOC of overdamping / underdamping under constant current charging cc Determine by solving the following equation: Where, It is the reference value of battery charging current under constant current charging; is the battery charging voltage reference value under constant voltage charging; R ba is the battery ohmic internal resistance; soc cc is the critical state of charge of overdamping / underdamping under constant current charging; E0 is the constant voltage of the battery; K is the battery polarization voltage constant; e is the natural constant; A ba 、B ba are the amplitude of the voltage drop in the exponential region of the typical discharge curve of the battery and the inverse of the time constant in the exponential region of the discharge curve; Q c is the battery capacity of the electric vehicle; k p 、k i k is the PI parameter of the DC voltage outer loop control of the voltage source converter; vp is the PI control parameter of the dual active bridge converter; C is the DC capacitor; u f is the AC voltage after the grid fault; a1 and a2 are parameters determined by the dual-active bridge converter parameters, the DC voltage reference value, and the charging voltage reference value, and are expressed as: Where n is the turns ratio of the high-voltage side and low-voltage side coils of the dual-active bridge converter; L r is the energy storage inductor of the dual active bridge converter; f s is the switching frequency of the dual active bridge converter; is the DC voltage reference value.

4. The electric vehicle fault current assessment method taking into account the influence of battery state of charge according to claim 3, characterized in that: In S102, when the fault current is in an over-damping state during constant current charging, the fault current is calculated as follows: Where i(t) is the fault current of the electric vehicle; t is the time variable; P s is the charging power of the electric vehicle before the fault; c1, c2, λ1, and λ2 are the over-damping fault current parameters under constant current charging, which are expressed as: Where, σ is the voltage drop degree at the grid connection point; SOC is the battery state of charge of the electric vehicle at the time of failure f Determine the battery open circuit voltage; When the fault current is underdamped during constant current charging, the fault current is calculated as follows: Where α1, β1, θ1, and A1 are the underdamped fault current parameters under constant current charging, which are expressed as:

5. The electric vehicle fault current assessment method taking into account the influence of battery state of charge according to claim 1, characterized in that: In S103, the specific method of determining the state of the fault current of the electric vehicle under constant voltage charging is: the battery state of charge (SOC) of the electric vehicle at the time of the fault is calculated. f The critical state of charge (SOC) of overdamping / underdamping under constant voltage charging cv Compare; if soc f ≤soc cv , then the fault current is judged to be in an over-damping state; if soc f >soc cv , then the fault current is determined to be in an underdamped state.

6. The electric vehicle fault current assessment method taking into account the influence of battery state of charge according to claim 5, characterized in that: The critical state of charge (SOC) of overdamping / underdamping under constant voltage charging is cv Determine by solving the following equation: In the formula, in the formula, It is the reference value of battery charging current under constant current charging; is the battery charging voltage reference value under constant voltage charging; R ba is the battery's ohmic internal resistance; is the critical state of charge (SOC) of overdamping / underdamping under constant voltage charging cv The open circuit voltage of the battery is determined by E0; E0 is the constant voltage of the battery; K is the polarization voltage constant of the battery; e is the natural constant; A ba 、B ba are the amplitude of the voltage drop in the exponential region of the typical discharge curve of the battery and the inverse of the time constant in the exponential region of the discharge curve; Q c is the battery capacity of the electric vehicle; k p 、k i k is the PI parameter of the DC voltage outer loop control of the voltage source converter; vp is the PI control parameter of the dual active bridge converter; C is the DC capacitor; u f is the AC voltage after the grid fault; n is the turns ratio of the high-voltage side and low-voltage side coils of the dual-active bridge converter; L r is the energy storage inductor of the dual active bridge converter; f s is the switching frequency of the dual active bridge converter; is the DC voltage reference value.

7. The electric vehicle fault current assessment method taking into account the influence of battery state of charge according to claim 6, characterized in that: In S103, when the fault current is in an over-damping state under constant voltage charging, the fault current is calculated as follows: Where i(t) is the fault current of the electric vehicle; t is the time variable; P s is the charging power of the electric vehicle before the fault; c4, c5, λ3, and λ4 are the over-damped fault current parameters under constant voltage charging, which are expressed as: Where a3 and a4 are parameters determined by the dual-active bridge converter parameters, the DC voltage reference value, and the charging voltage reference value, and are expressed as: Where, SOC is the battery state of charge of the electric vehicle at the time of failure f Determine the battery open circuit voltage; When the fault current is underdamped under constant voltage charging, the fault current is calculated as follows: Where α2, β2, θ2, and A2 are underdamped fault current parameters under constant voltage charging, and are expressed as:

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