Electric vehicle fault current evaluation method considering influence of battery charge state

Abstract

The invention relates to an electric vehicle fault current evaluation method considering the influence of the state of charge of a battery, and the method comprises the steps: calculating the critical state of charge of constant current-constant voltage charging switching of an electric vehicle, and judging whether the electric vehicle is charged at constant current or constant voltage according to the state of charge of the electric vehicle at a fault moment; and in a constant-current or constant-voltage charging state, respectively calculating a critical charge state of fault current over-damping / under-damping under constant-voltage or constant-current charging of the electric vehicle, judging whether the fault current is over-damping or under-damping according to the charge state of the electric vehicle at a fault moment, and respectively calculating the fault current of the electric vehicle by adopting a corresponding expression. The method can consider the influence of the state of charge on the fault current of the electric vehicle, accurately estimates and calculates the fault current of the electric vehicle, and provides more accurate reference data for the fault current analysis of the new energy power distribution network under the access of the electric vehicle. And therefore, accurate charging fault early warning or power distribution and power supply management of the charging management system on the new energy power distribution network can be better ensured.

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 battery charge state. Background Art

[0002] Electric vehicles are clean, pollution-free, and have high energy utilization rates, and their share has been increasing in recent years. Electric vehicles are charged through dual-active active bridge converters and voltage source converters, using DC capacitors as buffer links. During the charging process of electric vehicles, the two-stage converter transmission power is deeply coupled with the DC capacitor energy. In the new energy distribution network, the charging management system is arranged in the distribution station or the electric equipment in the tram, and the voltage / current and other information of the electric vehicle during the charging process are analyzed by the charging management system to monitor the distribution power supply status. When a fault occurs in the new energy distribution network, the charging management system further analyzes the fault current of the electric vehicle charging according to the analytical calculation results, and then performs the corresponding charging fault warning or distribution 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 accurately warn in advance or intervene in time to perform distribution and power supply management tasks, which will cause the power grid fault to further deteriorate, leading to serious accidents. Especially after a power grid failure, the DC voltage decreases due to the influence of the voltage source converter absorbing the active power drop, and at the same time affects the power transmitted to the battery by the dual 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 bridge converter, resulting in challenges for the charging management system to analyze and calculate the fault current of the electric vehicle.

[0003] At present, there have been some studies on the calculation of fault current of electric vehicles under power grid faults. Technicians pointed out that according to the provisions of the national standard GB / T 18287.1-2015, the fault current of electric vehicle AC charging is 16~32A. The researchers used a probability function to represent the concentrated discharge power of electric vehicles at a certain moment, and then calculated the range of the steady-state value of the fault current at that moment, and derived the fault current evaluation method of electric vehicles under different power grid fault levels. Technicians independently analyzed the two-stage converters of the voltage source converter and the dual active bridge converter, and derived the fault current model of the electric vehicle charging and discharging station. However, the above research mainly focuses on the calculation of the steady-state amplitude of the fault current of electric vehicles, and does not consider the influence of the state of charge of electric vehicles and the charging control link on the calculation of the transient stage of the fault current. The transient process calculation of the fault current of electric vehicles is still relatively lacking.

[0004] The electric vehicle charging circuit has a similar structure to the inverter-type distributed power supply, so some researchers have used the method of inverter-type distributed power supply fault current to calculate the fault current of electric vehicles. The researchers derived the fault current model of the constant power stage and the current limiting stage of the inverter-type distributed power supply under fault, but ignored the control link of the converter. Some technicians analyzed the fault current model under the DC voltage outer loop control of the voltage source converter after the grid fault and derived the analytical expression of the fault current of the wind turbine under the influence of the control strategy. The researchers derived the fault current calculation method of the voltage-controlled distributed power supply with exponential damping DC component under fault and derived the fault current calculation method of the photovoltaic array power affected by DC voltage. Some researchers derived the fault current model of the distributed power supply considering the dynamic response of the phase-locked loop. However, the above methods mainly focus on the influence of the control of the single-stage voltage source converter of the distributed power supply on the fault current under fault. The charging power of electric vehicles is closely related to the state of charge. In addition, the dual-active active bridge converter of electric vehicles adopts different constant current or constant voltage charging strategies under different states of charge. The two-level control of the voltage source converter and the dual-active active bridge converter affects the fault current by changing the DC voltage, making the fault current calculation method of distributed power supply difficult to apply to electric vehicles.

[0005] Therefore, how to accurately calculate the fault current of electric vehicles under different charge states, and then 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 view of the deficiencies of the above-mentioned prior art, the present invention provides an electric vehicle fault current assessment method taking into account the influence of the battery charge state, so as to more accurately calculate the fault current of the electric vehicle under different charge states, 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 warning 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 fault current of an electric vehicle taking into account the influence of the battery charge state is implemented by a battery management system in a new energy distribution network. When a power grid fails, the fault current of the electric vehicle when connected to charging is analyzed and determined, so as to perform charging fault warning or power distribution and power supply management according to the fault current of the electric vehicle. The method comprises the following steps:

[0009] S101, collecting the battery charging working parameters of the electric vehicle, determining the battery charge state of the electric vehicle at the time of the power grid failure, and then judging 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 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 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 For comparison; 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 sw Determined by solving the following equation:

[0015]

[0016] 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 ohmic internal resistance of the battery; soc sw is the critical state of charge of the constant current-constant voltage charging switch; 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 an 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: f The critical state of charge SOC of over-damping / under-damping under constant current charging cc For comparison; 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 judged to be in an under-damped state.

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

[0019]

[0020] In the formula, soc cc k is the critical charge state 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 jointly 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 ratio of the number of turns 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 under 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, λ2 are the over-damping fault current parameters under constant current charging, which are expressed as:

[0027]

[0028]

[0029]

[0030]

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

[0032] When the fault current is under-damped under constant current charging, the fault current is calculated as:

[0033]

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

[0035]

[0036]

[0037]

[0038]

[0039] As a preferred solution, in S103, the specific method of determining the state of the fault current of the electric vehicle under constant voltage charging is: f The critical state of charge (SOC) of overdamping / underdamping under constant voltage charging cv For comparison; 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 judged to be in an under-damped state.

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

[0041]

[0042] In the formula, 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 under 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] Wherein, a3 and a4 are parameters jointly 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] In the formula, is the battery state of charge (SOC) of the electric vehicle at the time of failure f Determine the battery open circuit voltage;

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

[0054]

[0055] Where α2, β2, θ2, and A2 are under-damped fault current parameters under constant voltage charging, respectively 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 is used to calculate the steady-state current of electric vehicles, which 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 the electric vehicle.

[0062] 2. The prior art calculates the fault current using the voltage source current conversion control equation of the electric vehicle charging circuit, ignoring the influence of different charging controls of the dual active bridge converter under constant current or constant voltage, which may cause significant errors in the fault current calculation; the method of the present invention analyzes the control process of the dual active bridge converter under constant current or constant voltage charging to achieve accurate calculation of the fault current of the electric vehicle under constant current or constant voltage charging.

[0063] 3. The prior art 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 over-damping / under-damping 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 over-damped or under-damped according to the charge state at the moment of the fault. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] In order to make the purpose, technical solution 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 A flow chart of a method for evaluating fault current of an electric vehicle taking into account the influence of battery charge state of the present invention;

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

[0067] Figure 3 A comparison diagram of the fault current obtained by different methods under constant current charging and the simulation results in the embodiment;

[0068] Figure 4 It is a comparison diagram 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 here 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 claimed invention, 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 belong to the scope of protection of the present invention.

[0070] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. 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 creative work are within the scope of protection of the present invention.

[0071] In view of the problem that the existing methods mainly focus on the influence of single-stage voltage source converter control on fault current under fault, and cannot calculate the fault current of electric vehicles under two-stage control taking into account the influence of charge state, the present invention discloses a method for evaluating the fault current of electric vehicles taking into account the influence of battery charge state, which takes into account the influence of charge state on the fault current of electric vehicles under power grid fault. The method is executed by the battery management system in the new energy distribution network, and analyzes and determines the fault current of the electric vehicle when it is connected to charging when the power grid fails, so as to perform charging fault warning or distribution and power supply management according to the fault current of the electric vehicle.

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

[0073] S101, collecting the battery charging working parameters of the electric vehicle, determining the battery charge state of the electric vehicle at the time of the power grid failure, and then judging 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 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 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 For comparison; 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) of constant current-constant voltage charging switch for electric vehicles sw Determined by solving the following equation:

[0079]

[0080] 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 ohmic internal resistance of the battery; soc sw Critical state of charge when switching between constant current and constant voltage charging; E(soc sw ) is the battery open circuit voltage determined by the critical state of charge of the constant current-constant voltage charging switch.

[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 Determined 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 an electric vehicle.

[0086] In the 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: f The critical state of charge SOC of over-damping / under-damping under constant current charging cc For comparison; 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 judged to be in an under-damped 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 equation (4) and equation (2), the critical state of charge soc of overdamping / underdamping under constant current charging is cc Determined by solving the following equation:

[0090]

[0091] In the formula, soc cc k is the critical charge state 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 jointly 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 ratio of the number of turns 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 under 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, λ2 are the over-damping fault current parameters under constant current charging, which are expressed as:

[0098]

[0099]

[0100]

[0101]

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

[0103] When the fault current is under-damped under constant current charging, the fault current is calculated as:

[0104]

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

[0106]

[0107]

[0108]

[0109]

[0110] In specific implementation, in S103, the specific method of determining the state of the fault current of the electric vehicle under constant voltage charging is: f The critical state of charge (SOC) of overdamping / underdamping under constant voltage charging cv For comparison; 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 judged to be in an under-damped state.

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

[0112]

[0113] In the formula, 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] Wherein, a3 and a4 are parameters jointly 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] In the formula, is the battery state of charge (SOC) of the electric vehicle at the time of failure f Determines the battery open circuit voltage.

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

[0126]

[0127] Where α2, β2, θ2, and A2 are under-damped fault current parameters under constant voltage charging, respectively 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 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 voltage reference value of the battery pack under constant voltage charging is 415V, and the constant current charging is 380A, with a capacity of 200Ah. A three-phase short circuit fault occurred in the power grid at 0.3s, and the voltage at the electric vehicle grid connection point dropped to 0.8pu.

[0134] According to the electric vehicle fault current assessment 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 assessment 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 fault currents obtained by different methods and simulation results under constant current charging conditions of 10%, 30%, 50%, 70%, and 90% of the electric vehicle charge state at the time of fault, respectively; Figure 4(a), (b), (c), and (d) are comparison diagrams of the fault current and simulation results obtained by different methods under constant voltage charging conditions with the electric vehicle charge state of 92%, 94%, 96%, and 98% at the moment of fault, respectively.

[0135] pass Figure 3 By comparison, it can be seen that as the state of charge increases from 10% to 90%, the fault current of method 2 is always in an underdamped state, while the simulated current gradually changes from underdamped to overdamped, causing the calculation error of the fault current of method 2 to increase from 5% to 10%, and the DC voltage error reaches 30%. The electric vehicle fault current evaluation method disclosed in the present invention taking into account the influence of the battery state of charge considers the DAB power change during the fault process. The calculated fault current under constant current charging can follow the simulated fault current and DC voltage, and the overdamped and underdamped states are also the same as the simulation results, and the calculation and simulation errors are less than 2%.

[0136] pass Figure 4 By comparison, it can be seen that as the state of charge decreases from 98% to 92%, the fault current of method 2 is always in an underdamped state, and the error between the calculated fault current and the simulation result gradually increases from 5% to 10%. The electric vehicle fault current evaluation method taking into account the influence of the battery state of charge disclosed in the present invention is always less than 2%, and the fault current of the electric vehicle fault current evaluation method taking into account the influence of the battery state of charge disclosed in the present invention under constant voltage charging is basically consistent with the simulation result. When the state of charge of the electric vehicle is 90% under constant current charging and 92% under constant voltage charging, that is, near the critical state of charge of 91% of constant current-constant voltage charging switching, the calculation error of method 2 is the largest. The calculation method disclosed in the present invention takes into account the DAB control link and transmission power changes under the influence of DC voltage, and can better reflect the transient process of the fault current of the critical state of charge of the constant current-constant voltage charging switching with the maximum charging power.

[0137] It can be seen that the electric vehicle fault current evaluation method of the present invention taking into account the influence of the battery charge state, by taking into account the influence of the charge state and the two-stage converter control, enables the calculated fault current value to accurately fit the transient process of the electric vehicle fault current. In comparison, the prior art uses the voltage source current conversion control equation of the electric vehicle charging circuit to calculate the fault current, ignoring the influence of different charging controls of the dual active active bridge converter under constant current or constant voltage, which may produce 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 fault current of the electric vehicle under constant current or constant voltage charging. On the other hand, the prior art ignores the influence of the change of the battery power of the electric vehicle during the fault period on the fault current; while the method of the present invention calculates the critical charge state of over-damping / under-damping under constant current / constant voltage charging by taking into account the change of the battery power under the influence of the charge state during the fault process, and then accurately judges whether the fault current of the electric vehicle is over-damped or under-damped by the charge state 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 fault current analysis of new energy distribution networks connected to 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 solution of the present invention rather than to limit the technical solution. Those skilled in the art should understand that those modifications or equivalent substitutions of the technical solution of the present invention that do not depart from the purpose and scope of the technical solution 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 charge state, characterized in that: It is executed by the battery management system in the new energy distribution network. When the grid fails, it analyzes and determines the fault current when the electric vehicle is connected to charge, and performs charging fault warning or power distribution management according to the fault current of the electric vehicle; The method comprises the following steps: S101, collecting the battery charging working parameters of the electric vehicle, determining the battery charge state of the electric vehicle at the time of the power grid failure, and then judging 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; S102, judging the state of the electric vehicle 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 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 charge state according to claim 1, characterized in that: 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 calculated. f Critical state of charge (SOC) for switching between constant current and constant voltage charging of electric vehicles sw For comparison; 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.

3. The electric vehicle fault current assessment method taking into account the influence of battery charge state according to claim 2 is characterized in that: The critical state of charge SOC of the electric vehicle constant current-constant voltage charging switch sw Determined by solving the following equation: 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 ohmic internal resistance of the battery; soc sw is the critical state of charge of the constant current-constant voltage charging switch; 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 an electric vehicle.

4. The electric vehicle fault current assessment method taking into account the influence of battery charge state 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 f The critical state of charge SOC of over-damping / under-damping under constant current charging cc For comparison; 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 judged to be in an under-damped state.

5. The electric vehicle fault current assessment method taking into account the influence of battery charge state according to claim 4, characterized in that: The critical state of charge SOC of over-damping / under-damping under constant current charging cc Determined by solving the following equation: 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 ohmic internal resistance of the battery; 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 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; a1 and a2 are parameters jointly 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 ratio of the number of turns 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.

6. The electric vehicle fault current assessment method taking into account the influence of battery charge state according to claim 5, characterized in that: In S102, when the fault current is in the over-damping state under 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, λ2 are the over-damping fault current parameters under constant current charging, which are expressed as: In the formula, σ is the voltage drop degree at the grid connection point; is the battery state of charge (SOC) of the electric vehicle at the time of failure f Determine the battery open circuit voltage; When the fault current is under-damped under constant current charging, the fault current is calculated as: Where α1, β1, θ1, and A1 are under-damped fault current parameters under constant current charging, and are expressed as:

7. The electric vehicle fault current assessment method taking into account the influence of battery charge state 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: f The critical state of charge (SOC) of overdamping / underdamping under constant voltage charging cv For comparison; 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 judged to be in an under-damped state.

8. The electric vehicle fault current assessment method taking into account the influence of battery charge state according to claim 7, characterized in that: The critical state of charge soc of overdamping / underdamping under constant voltage charging cv Determined 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 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 ratio of the number of turns of the high-voltage side and the 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.

9. The electric vehicle fault current assessment method taking into account the influence of battery charge state according to claim 8, 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, λ4 are the over-damped fault current parameters under constant voltage charging, which are expressed as: Wherein, a3 and a4 are parameters jointly determined by the dual active bridge converter parameters, the DC voltage reference value and the charging voltage reference value, and are expressed as: In the formula, is the battery state of charge (SOC) 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 under-damped fault current parameters under constant voltage charging, respectively expressed as:

Images