Method for evaluating the impact of cascaded charging station access on AC / DC system stability

Abstract

The present invention discloses a method for evaluating the influence of cascade charging stations accessing an AC / DC system on stability, and relates to the technical field of power systems. The present invention further obtains the transfer function of the cascade electric vehicle charging station through a linearized model of a single electric vehicle charging station, thereby obtaining a closed-loop system with an AC / DC system as a feedforward link, line parameters and the cascade electric vehicle charging station as a feedback link, and then obtains the damping provided by the cascade electric vehicle charging station to the AC / DC system through the closed-loop system, and finally realizes the judgment of the stability of the AC / DC system based on the damping size, which not only overcomes the defect that the time domain simulation method cannot quantify the influence of stability and the difficulty of solving the problem brought by the mode analysis method in high-order systems, but also obtains the damping contribution of the cascade electric vehicle charging station to the AC / DC system by processing the feedback link, avoids the influence of the DC line, and effectively improves the modeling accuracy and adaptability.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and more particularly to a method for evaluating the impact of cascade charging station access on the stability of an AC / DC system. Background Art

[0002] Environmental pollution is a topic of widespread concern around the world. The emission pollution caused by traditional motor vehicles has caused serious harm to the environment. As an effective solution, electric vehicles have been born. In recent years, their supporting technologies have been gradually improved and put into use in the market in large quantities, and have been rapidly developed. With the increase in the number of electric vehicles connected to the DC network, it is expected that with the deployment of the DC grid, the grid with a large number of electric vehicles will become an important part of the distribution network. Scholars have conducted many studies on electric vehicle charging stations, with the aim of obtaining a more stable power supply. The first is the optimization of electric vehicle charging station planning, such as optimizing economy, control and reliability, mainly to reduce costs by considering economic and traffic factors in planning.

[0003] However, in the traditional approach, each EV charging station contains only one DC / DC converter and uses a parallel connection to the AC / DC network. Single parallel-connected EV charging stations are bulky, expensive, and can only support the service of one EV at any given time. Therefore, a cascade connection that expands capacity and reduces cost is proposed, providing a new and more efficient connection method for DC / DC converters in EV charging stations. Here, the DC / DC converters are connected in a cascade manner, that is, one EV charging station contains multiple DC / DC converters and the services of multiple EVs, that is, a cascade EV charging station. Currently, there is little modeling of cascade EV charging stations, and in order to obtain its impact on the stability of the AC / DC system, it is necessary to establish its transfer function model. How to convert multiple EV charging stations as a whole into a single-input single-output transfer function form is a difficult problem. In addition, the reliability of the cascade connection still needs to be improved. The number of EVs, charging and discharging, etc. will affect the stability. It is necessary to explore the impact of the access of cascade EV charging stations on the stability of the system.

[0004] At present, there are still unresolved issues in the study of the impact of cascaded electric vehicle charging stations on the stability of AC and DC systems. For example, it is difficult to reveal the system instability mechanism based on traditional stability methods, time domain simulation, pattern analysis, etc., and the curse of dimensionality occurs in high-order systems. In addition, current research is more based on simple parallel access of electric vehicle charging stations, and the dynamic modeling of cascaded electric vehicle charging stations is insufficient.

[0005] In terms of stability analysis methods, time domain analysis, pattern analysis and frequency domain analysis are still universal, but time domain simulation method is difficult to provide quantitative analysis results. Pattern analysis method has large computational complexity and even dimensionality curse problems when facing high-order systems. It cannot accurately reveal the impact mechanism of cascaded electric vehicle charging stations on the stability of AC and DC systems. Both methods are difficult to quantify the impact of cascaded electric vehicle charging stations on the stability of AC and DC systems. Summary of the invention

[0006] In order to overcome the defects and deficiencies in the above-mentioned prior art, the present invention provides a method for evaluating the impact of cascade charging station access on the stability of the AC / DC system. The purpose of the present invention is to clarify the impact of the cascade electric vehicle charging station on the system stability. The present invention further obtains the transfer function of the cascade electric vehicle charging station through the linearized model of a single electric vehicle charging station, thereby obtaining a closed-loop system with the AC / DC system as the feedforward link, the line parameters and the cascade electric vehicle charging station as the feedback link, and then obtains the damping provided by the cascade electric vehicle charging station to the AC / DC system through the closed-loop system, and finally realizes the stability of the AC / DC system based on the damping size, which not only overcomes the defects of the time domain simulation method that cannot quantify the impact of stability and the difficulty of solving the problem brought by the mode analysis method in high-order systems, but also obtains the damping contribution of the cascade electric vehicle charging station to the AC / DC system by processing the feedback link, avoids the influence of the DC line, effectively improves the modeling accuracy and adaptability, and verifies the correctness of the proposed method, providing a theoretical reference for the planning of cascade electric vehicle charging stations from the system stability level.

[0007] In order to solve the above problems existing in the prior art, the present invention is implemented through the following technical solutions.

[0008] The present invention provides a method for evaluating the impact of cascade charging station access on the stability of an AC / DC system, the method comprising the following steps:

[0009] S1. Based on the cascaded energy storage network diagram and the constant voltage control of the energy storage DC / AC converter, a linearized model of a single electric vehicle charging station is established;

[0010] S2. Based on the established linearization model of a single electric vehicle charging station, a full-order linearization model and transfer function of a cascade electric vehicle charging station are obtained;

[0011] S3, obtain the linearized model of AC / DC system based on the cascaded energy storage network diagram;

[0012] S4, combining the transfer function of the cascade electric vehicle charging station with the AC / DC linearization model to obtain a closed-loop system with the AC / DC system as a feedforward link and the line parameters and the cascade electric vehicle charging station as a feedback link;

[0013] S5, based on the closed-loop system, the damping provided by the feedback link to the AC and DC systems;

[0014] S6, further obtaining the damping provided by the cascaded electric vehicle charging station to the AC and DC systems according to the damping provided to the system by the feedback link;

[0015] S7. According to the damping value provided by the cascaded electric vehicle charging station to the AC / DC system, determine the stability state of the AC / DC system and verify the correctness of the method.

[0016] Further preferably, in step S1, the linearization model of a single electric vehicle charging station is:

[0017]

[0018]

[0019]

[0020]

[0021]

[0022] Δd i =Δx il +K ip (Δx vl -K vi ΔV di -ΔI di )

[0023]

[0024] In the formula, Δ represents the linear form of the scalar, subscript 0 represents the steady-state value of the variable, and V dc represents the DC voltage between the cascaded electric vehicle charging station and the DC network, I dc represents the DC current of the cascaded electric vehicle charging station and the DC network, C fi is the capacitance of the filter on the input side of the i-th module; I dci is the capacitance C fi Current on the input side, I fi Represents capacitance C fi Output current, V fi Represents the capacitance C of the input filter of the i-th module fi Voltage, L fi is the inductance of the filter on the input side of the i-th module, R fi is the resistance of the filter on the input side of the i-th module, N represents the number of modules, C di is the capacitance of the output filter of the ith module, V diis the capacitor voltage at the output side of the i-th module, I Li is the output side capacitance C of the i-th module di Injected DC current, I di is the output side capacitance C of the i-th module di Output DC current, I bi is the internal current of the battery in the i-th module charging station, K vp is the proportional coefficient of DC voltage control, K vi is the integral coefficient of DC voltage control, K ip is the proportional coefficient of the DC current control loop, K ii is the integral coefficient of the DC current control loop, d i is the duty cycle of the ith module, x vl 、x il is the intermediate variable, R di is the resistance of the connection line between the i-th module and the electric vehicle, and the superscript ref is the reference value of the variable.

[0025] More preferably, in step S1, the connection variable between the cascaded electric vehicle charging station and the DC network is the DC voltage V dc and DC current I dc The cascaded electric vehicle charging station consists of N modules, each of which is an independent DC / DC converter for charging and discharging electric vehicles; the internal resistance model of the vehicle battery adopts: the DC voltage of the i-th battery is V ei , the internal resistance is R ei ; The current relationship of each electric vehicle is I dc1 =I dc2 =I dc3 =…=I dci =…=I dcN (2); where I dci is the capacitance C of the filter on the input side of the i-th module fi The current of the filter is:

[0026]

[0027]

[0028] Among them, V dc is the DC voltage between the cascaded electric vehicle charging station and the DC network; V fi is the capacitance C of the filter on the input side of the i-th module fi Voltage; L fi is the inductance of the filter on the input side of the i-th module; R fi is the resistance of the filter on the input side of the i-th module; variable C fi is the capacitance of the filter on the input side of the i-th module; Ifi is the capacitance C fi Output DC current;

[0029] The kinetic equation on the battery side is obtained as:

[0030] Among them, C di is the capacitance on the output side of the ith module, V di is the capacitor voltage at the output side of the i-th module, I Li and I bi are the DC currents injected into and output from the output capacitor of the i-th module respectively.

[0031] More preferably, the cascade electric vehicle charging station adopts a control method of constant DC bus voltage and constant battery current as a basic control strategy to achieve independent control of the modules.

[0032] Further preferably, other state variables in the control strategy of the cascade electric vehicle charging station are:

[0033]

[0034] The connection between the filter kinetic equation and the battery kinetic equation is realized, that is, I fi =d i I Li , d i V fi =R di I Li +V di (7);

[0035] Among them, K vp is the proportional coefficient of DC voltage control, K vi is the integral coefficient of DC voltage control, K ip is the proportional coefficient of the DC current control loop, K ii is the integral coefficient of the DC current control loop, d i is the duty cycle of the ith module, x vl 、x il is the intermediate variable, R di is the resistance of the connection line between the ith module and the electric vehicle, and the superscript ref indicates the parameter reference value.

[0036] Further preferably, in step S2, the full-order linearization model of the cascade electric vehicle charging station is:

[0037]

[0038] ΔI dc =c s ΔV dc (8)

[0039] Among them, X s is the state variable matrix, ΔX s =[ΔV f1 ,...,ΔV fN ,ΔI dc ,ΔV d1 ,...,ΔV dN ,Δx vl ,Δx il ], A s is the state matrix, b s is the input matrix, c s is the output matrix.

[0040] More preferably, the transfer function of the cascade electric vehicle charging station can be obtained from the full-order linearization model of the cascade electric vehicle charging station as ΔI dc =c s (sI-A s ) -1 b s ΔV dc =G e (s)ΔV dc (9), G e (s) is the impedance representation of the cascaded electric vehicle charging station.

[0041] Further preferably, in step S3, the cascaded electric vehicle charging station is connected to the AC / DC system, and the voltage direction at the common connection point is V s is the AC bus voltage, taking the d-axis direction of the AC / DC converter dq coordinate, V sq =0, V sd =V s , we get ΔP s =I sd0 ΔV s +V s0 ΔI sd =I l0 ΔV l +V l0 ΔI l (10), where P s is the line active power, I sd is the d-axis current of the AC system, I l is the current flowing into the DC / AC converter of the cascaded electric vehicle charging station, Vl is the DC / AC port voltage, and subscript 0 is the value of a variable in the steady state.

[0042] More preferably, in step S3, the line current equation of the AC / DC converter end linearization is:

[0043]

[0044]

[0045] Where X is the line impedance, I sq is the q-axis current of the AC system, ω 0 is the frequency reference value, V cd is the AC / DC output voltage, V sd 、V sq is the AC d and q axis terminal voltage.

[0046] More preferably, according to the AC / DC converter control strategy and the line current equation of the AC / DC converter terminal linearization, the following formula can be obtained:

[0047] Among them, K vp is the DC voltage outer loop parameter, K ip is the DC voltage inner loop parameter, K pl is the transfer function between the current reference value and the current, and the superscript ref is the reference value of the variable; the above formula (12) is the dynamics of the active control loop, including the outer loop and the inner loop, where ω 0 is the synchronous frequency, expressed in unit scale; the AC grid is simply represented as an infinite bus, on which V s is a constant;

[0048] Substituting formula (12) into formula (10), we get

[0049] Among them, K vp (s) = K p_vp +K i_vp / s,K ip (s) = K p_ip +K i_ip / s,K p_vp K is the DC voltage outer loop proportional coefficient, i_vp is the DC voltage integral coefficient, K p_ip K is the DC voltage inner loop proportional coefficient, i_ip is the DC voltage inner loop integral coefficient,

[0050]

[0051] G l (s) is the transfer function of the AC system. Further, from equation (13), the open-loop oscillation mode of the AC / DC system can be obtained as

[0052]

[0053] Where: B, C, K represent coefficients, M(s) = -V l0ω 0 K p_ip s 2 -V l0 ω 0 K i_ip s, B = ω 0 I l0 K p_ip +V s0 ω 0 K p_ ip K p_vp ,

[0054] C=V s0 (ω 0 I l0 K i_ip +K i_ip K p_vp +K p_ip K i_vp ),K=-V s0 K i_ip K i_vp , G l The denominator of (s) is a second-order loop of an AC / DC system.

[0055] Further preferably, in step S4, the AC / DC system is connected to the cascade electric vehicle charging station through an impedance matrix to obtain:

[0056]

[0057] Among them, y 11 ,y 22 is the self-admittance, y 12 ,y 21 is the mutual admittance, y 11 =y 22 =-y 12 =-y 21 , the specific value is determined by the line parameters, and it can be obtained from formula (15):

[0058]

[0059] In formula (15), That is, ΔI l =F sum (s)ΔV l , F(s) represents the line parameters and the feedback link of the cascaded electric vehicle charging station to the AC / DC system.

[0060] More preferably, firstly, the damping torque analysis of the AC / DC system is established according to formula (14), and its expression is:

[0061] (Bs 2 +Cs+K)ΔVl =M(s)ΔI l (17);

[0062] The damping provided to the AC / DC system by the feedback link formed by the DC line and the cascaded electric vehicle charging station is:

[0063] ΔI l =F sum (s)ΔV l (18)

[0064] Substituting equation (18) into equation (19) yields

[0065] (Bs 2 +Cs+KM(s)F sum (s))ΔV l =0 (19)

[0066] Formula (19) represents the characteristic equation of the closed-loop system. The solution of the AC / DC system voltage control loop formula (19) is: Will Substituting into formula (19) we get

[0067] Bs 2 +Cs+KM(s)F sum (λ d )=0 (20)

[0068] Among them, F(λ d ) affects the damping and frequency of the oscillation mode;

[0069] In the frequency domain, F(λ d ) can be decomposed into

[0070]

[0071] In formula (21), T ds Changing the damping of the oscillation mode is called the damping component provided by the feedback link to the AC / DC system, and we can get:

[0072]

[0073] In formula (21), T ks The imaginary part of the oscillation mode is changed, which is called the synchronization component provided by the feedback loop to the AC / DC system.

[0074]

[0075] Im-(·) and Re-(·)-represent the imaginary and real parts of the variable, respectively.

[0076] Further preferably, the step S6 includes the following sub-steps:

[0077] S601, calculating the damping and synchronization components provided by the feedback link to the second-order loop of the AC / DC system;

[0078] S602, calculating the damping and synchronization components provided by the feedback link containing only the DC line to the second-order loop of the AC / DC system;

[0079] S603. Calculate the damping and synchronization components provided by the cascaded electric vehicle charging station to the second-order loop of the AC / DC system.

[0080] More preferably, in step S601, the transfer function expression of the feedback link is ΔI l =F sum (s)ΔV l ,in, The damping provided by the feedback link to the AC / DC system can be obtained as T ds_sum =Im[F sum (λ d )] / ω d , the feedback link provides the synchronous component T of the AC / DC system ks_sum =Re[F sum (λ d )]-T ds ξ d .

[0081] More preferably, in step S602, the transfer function of the feedback link containing only the DC line is ΔI l =F l (s)ΔV l (24), where F l (s) = y 11 +y 12 (-y 22 ) -1 y 21 The damping provided to the AC / DC system by the feedback link containing only the DC line can be obtained as T ds_l =Im[F l (λ d )] / ω d , synchronization component

[0082] More preferably, in step S603, the transfer function expression ΔI based on the feedback link l =F sum (s)ΔV l The transfer function of the feedback link with only the DC link is ΔI l =F l (s)ΔV l , the transfer function of the cascaded electric vehicle charging station is ΔI l =(F sum(s)-F l (s))ΔV l (25) The damping provided by the cascaded electric vehicle charging station to the AC / DC system can be obtained as T ds_e =T ds_sum -T ds_l , T ks_e =T ks_sum -T ks_l .

[0083] More preferably, by ΔI l =(F sum (s)-F l (s))ΔV l It can be concluded that when T ds_e >0, the cascaded electric vehicle charging station provides positive damping to the AC / DC system, and the damping of the oscillation mode is improved; when T ds_e When <0, the cascaded electric vehicle charging station provides negative damping to the AC / DC system, and the damping of the oscillation mode is reduced; the stability criterion of the system is T s =T ds_e +T ds_l +C>0 (26) When the parameter and damping relationship satisfies equation (26), the AC / DC system is stable, otherwise it is unstable.

[0084] More preferably, in step S7, the correctness of the method is verified by the following scheme:

[0085] When the parameters are set as follows: B = 5, C = 0.198, K = 0.433, when there are five 0.5pu cascaded electric vehicle charging stations, the damping provided by the feedback link to the AC / DC system is T ds_sum =0.023, T ks_sum =0.0944; the damping provided by the DC line to the AC / DC system is T ds_l =0.026, T ks_l =0.1071; the damping provided by the cascaded electric vehicle charging station to the AC and DC systems is T ds_e =-0.003, T ks_e =-0.0127; it can be seen that T ds_e <-0.003, therefore, the control link of the cascade electric vehicle charging station provides negative damping for the system, and the damping index is used to determine T s ,T s =T ds_e +T ds_l +C=0.026-0.003+0.198>0; therefore the system is stable;

[0086] To verify the above analysis results, T ds_sum Substitute Bs2 +Cs+KM(s)F sum (λ d )=0, we get Bs 2 +(C+0.0230)s+K+0.0944=0 (27), the solution is That is (-5.005+j122.343rad / s); this is consistent with the actual oscillation mode of the system, thus verifying the correctness of the above decomposition ideas and conclusions.

[0087] Compared with the prior art, the beneficial technical effects brought by the present invention are as follows:

[0088] 1. This patent clearly proposes for the first time a method for evaluating the impact of cascaded electric vehicle charging station access on the stability of the AC / DC system. First, the transfer function of the cascaded electric vehicle charging station is further obtained through the linearization model of a single electric vehicle charging station, and the AC / DC system linearization model is obtained through the network diagram, thereby obtaining a closed-loop system with the AC / DC system as the feedforward link, and the line parameters and the cascaded electric vehicle charging station as the feedback link. Then, the damping provided by the cascaded electric vehicle charging station to the AC / DC system is obtained through the closed-loop system, and finally the stability of the AC / DC system is judged based on the damping size, and the accuracy of the proposed method is verified. The method of the present invention takes into account the impact of the cascaded electric vehicle charging station on the stability of the AC / DC system, and provides a reference for the planning of electric vehicles and the stable operation of the system.

[0089] 2. The present invention proposes an analysis method for evaluating the impact of cascade electric vehicle charging station access on the stability of AC / DC systems. The linearization model of cascade electric vehicle charging stations in existing studies needs to be clarified, and in order to obtain the impact of cascade electric vehicle charging stations on the stability of AC / DC systems, it is necessary to establish its transfer function model. The present invention further obtains the transfer function of the cascade electric vehicle charging station through the linearization model of a single electric vehicle charging station, thereby obtaining a closed-loop system with the AC / DC system as the feedforward link and the line parameters and the cascade electric vehicle charging station as the feedback link. Traditional stability analysis methods are difficult to reveal the impact mechanism and cannot quantify the impact of cascade electric vehicles on the AC / DC system. The present invention obtains the damping contribution of the cascade electric vehicle charging station to the AC / DC system through a closed-loop feedback link, and further judges the stability state of the AC / DC system. Then, the damping provided by the cascaded electric vehicle charging station to the AC / DC system is obtained through a closed-loop system, and finally the stability of the AC / DC system is judged based on the damping size, and the correctness of the proposed method is verified. This method overcomes the defect that the time domain simulation method cannot quantify the impact of stability and the difficulty of solving the problem brought by the pattern analysis method in high-order systems, effectively improves the modeling accuracy and adaptability, and provides a theoretical reference for the planning of cascaded electric vehicle charging stations from the system stability level. BRIEF DESCRIPTION OF THE DRAWINGS

[0090] Figure 1 is a basic flow chart of the present invention;

[0091] Figure 2 A configuration structure for a cascade electric vehicle charging station;

[0092] Figure 3 Control strategies for cascaded electric vehicle charging stations;

[0093] Figure 4 Schematic diagram of connecting a cascaded electric vehicle charging station to an AC / DC system;

[0094] Figure 5 AC / DC converter control strategy;

[0095] Figure 6 Closed-loop block diagram of the AC / DC system connected to a cascaded electric vehicle charging station. DETAILED DESCRIPTION

[0096] The following is an exemplary embodiment of the present invention that helps to fully understand the claims and their equivalents in conjunction with the accompanying drawings, wherein the specific details will be considered as exemplary only, and do not limit the scope of the present invention. Therefore, those of ordinary skill in the art can make various changes and modifications to the embodiments without departing from the scope and spirit of the present invention.

[0097] After the cascaded electric vehicle charging station is connected to the AC / DC system, its power electronic characteristics will have a certain impact on the system stability. In order to clarify the impact of the cascaded electric vehicle charging station on the system stability, the present invention proposes a method for evaluating the impact of the cascaded electric vehicle charging station access on the stability of the AC / DC system. The basic flow chart is as follows: Figure 1 As shown, it is divided into five steps S1-S7.

[0098] S1: Based on the cascaded energy storage network diagram and the constant voltage control of the energy storage DC / AC converter, a linearized model of a single electric vehicle charging station is established

[0099] The structure of a cascade electric vehicle charging station is as follows: Figure 1 As shown, the i-th cascade electric vehicle is taken as an example to represent the configuration of other cascade electric vehicle charging stations. Figure 1 In the example, the connection variable between the cascaded electric vehicle charging station and the DC network is the DC voltage V dc and DC current I dc The cascaded electric vehicle charging station consists of N modules, each of which is an independent DC / DC converter for charging and discharging electric vehicles. The internal resistance model of the vehicle battery adopts: the DC voltage of the i-th battery is V ei , the internal resistance is R eiConsidering that the modules of the cascade electric vehicle are cascaded, the current relationship of each electric vehicle is:

[0100] I dc1 =I dc1 =…=I dci =…=I dcN (1)

[0101] Among them, I dci is the capacitance C of the filter on the input side of the i-th module fi The dynamic equation of the filter is

[0102]

[0103]

[0104] Among them, V dc is the DC voltage between the cascaded electric vehicle charging station and the DC network; V fi is the capacitance C of the filter on the input side of the i-th module fi Voltage; L fi is the inductance of the filter on the input side of the i-th module; R fi The resistance of the filter on the input side of the i-th module; variable C fi is the capacitance of the filter on the input side of the i-th module; I dci and I fi They are respectively the capacitor C fi The DC current of input and output; the kinetic equation on the battery side is obtained as:

[0105]

[0106] Among them, C di is the capacitance at the output side of the ith electric vehicle module; V di is the capacitor voltage at the output side of the i-th electric vehicle module; I Li and I bi are the output side C of the i-th electric vehicle module di The DC current injected and output. The configuration and control strategy of cascaded electric vehicle charging station are as follows Figure 2 and Figure 3 As shown, the electric vehicle adopts a control method of constant DC bus voltage and constant battery current as the basic control strategy to achieve independent control of the modules. Figure 3 The other state variables shown are:

[0107]

[0108] Among them, K vp and K vi is the proportional coefficient and integral coefficient of DC voltage control, Kip and K ii are the proportional coefficient and integral coefficient of the DC current control loop, d i is the duty cycle of the ith module, x vl 、x il is an intermediate variable that realizes the connection between dynamic (3) and (4)

[0109] I fi =d i I Li , d i V fi =R di I Li +V di (6)

[0110] d i is the duty cycle of the i-th converter; R di is the resistance of the connection line between the i-th module and the electric vehicle.

[0111] S2: Based on the established linear model of a single electric vehicle charging station, the full-order linear model and transfer function of the cascade electric vehicle charging station are obtained

[0112] From (1)-(6), it can be seen that the linearized equation of the i-th module in the cascade electric vehicle charging station is as follows:

[0113]

[0114]

[0115]

[0116]

[0117]

[0118] Δd i =Δx il +K ip (Δx vl -K vi ΔV di -ΔI di )

[0119]

[0120] Where Δ represents the linearized form of the scalar, and the subscript 0 represents the steady-state value of the variable. From equation (8), the full-order state space model of the cascaded electric vehicle charging station can be obtained as follows:

[0121]

[0122] ΔI dc =c s ΔV dc (8)

[0123] Among them, X s is the state variable matrix, ΔX s =[ΔV f1 …ΔV fN ΔI dc ΔV d1 …ΔV dN Δx vl Δx il ], A s is the state matrix, b s is the input matrix, c s is the output matrix. According to (8), the transfer function of the cascaded electric vehicle charging station can be obtained as follows:

[0124] ΔI dc =c s (sI-A s ) -1 b s ΔV dc =G e (s)ΔV dc (9)

[0125] G e (s) is the impedance representation of the cascade electric vehicle.

[0126] S3: Get the linearized model of AC / DC system based on the cascaded energy storage network diagram

[0127] Cascade electric vehicles connected to AC and DC systems Figure 4 As shown, in Figure 4 The voltage direction at the common connection point, V s is the AC bus voltage, taking the d-axis direction of the AC / DC converter dq coordinate (therefore V sq =0, V sd =V s ),get

[0128] ΔP s =I sd0 ΔV s +V s0 ΔI sd =I l0 ΔV l +V l0 ΔI l (10)

[0129] Among them, P s is the line active power, I sdis the d-axis current of the AC system, I l V is the current flowing into the DC / AC converter of the electric vehicle l is the DC / AC port voltage, and the subscript 0 is the value of a variable in steady state. In addition, the linearized line current equation at the end of the AC / DC converter is also given:

[0130]

[0131]

[0132] Where X is the line impedance, I sq is the q-axis current of the AC system, ω 0 is the frequency reference value, V cd is the AC / DC output voltage, V sd , V sq is the AC d and q axis terminal voltage, the AC / DC converter control strategy is as follows Figure 5 As shown by Figure 5 And formula (11) can get the following formula

[0133]

[0134]

[0135] Among them, K vp is the DC voltage outer loop parameter, K ip is the DC voltage inner loop parameter, K pl is the transfer function between the current reference value and the current, the superscript ref is the reference value of the variable, and equation (12) is the dynamics of the active control loop, including the outer loop and the inner loop. 0 is the synchronous frequency, expressed in unit scale. Considering that the capacity of the AC grid is much larger than that of the cascaded electric vehicle, the AC grid can be simply represented as an infinite bus, on which V s is a constant. Substituting equation (12) into equation (10), we get

[0136]

[0137] Among them, K vp (s) = K p_vp +K i_vp / s,K ip (s) = K p_ip +K i_ip / s,K p_vp K is the DC voltage outer loop proportional coefficient, i_vp is the DC voltage integral coefficient, K p_ip K is the DC voltage inner loop proportional coefficient, i_ip is the DC voltage inner loop integral coefficient,

[0138]

[0139] G l (s) is the transfer function of the AC system. Further, from equation (13), the open-loop oscillation mode of the AC / DC system can be obtained as

[0140]

[0141] Where: B, C, K represent coefficients, M(s) = -V l0 ω 0 K p_ip s 2 -V l0 ω 0 K i_ip s, B = ω 0 I l0 K p_ip +V s0 ω 0 K p_ ip K p_vp ,

[0142] C=V s0 (ω 0 I l0 K i_ip +K i_ip K p_vp +K p_ip K i_vp ),K=-V s0 K i_ip K i_vp , G l The denominator of (s) is a second-order loop of an AC / DC system.

[0143] S4: Combine the transfer function of the cascaded electric vehicle charging station with the AC / DC linearization model to obtain a closed-loop system with the AC / DC system as the feedforward link and the line parameters and the cascaded electric vehicle charging station as the feedback link.

[0144] Connecting the AC / DC system to the cascaded electric vehicle charging station through the impedance matrix yields:

[0145]

[0146] Among them, y 11 ,y 22 is the self-admittance, y 12 ,y 21 is the mutual admittance, y 11 =y 22 =-y 12 =-y 21, the specific value is determined by the line parameters, and it can be obtained from formula (15):

[0147]

[0148] In formula (15), That is, ΔI l =F sum (s)ΔV l , F(s) represents the line parameters and the feedback link of the cascaded electric vehicle charging station to the AC / DC system. The closed-loop block diagram of the AC / DC system connected by the cascaded electric vehicle is shown in Figure 6 shown.

[0149] S5: Based on the closed-loop system, the damping provided by the feedback link to the AC and DC systems

[0150] In order to obtain the damping provided by the feedback link to the AC / DC system, the operation is as follows:

[0151] First, the damping torque analysis of the AC / DC system is established according to formula (14), and its expression is:

[0152] (Bs 2 +Cs+K)ΔV l =M(s)ΔI l (17)

[0153] according to Figure 6 It can be seen that the damping provided to the AC / DC system by the feedback link formed by the DC line and the cascaded electric vehicle charging station is:

[0154] ΔI l =F sum (s)ΔV l (18)

[0155] Substituting equation (18) into equation (19) yields

[0156] (Bs 2 +Cs+KM(s)F sum (s))ΔV l =0 (19)

[0157] Formula (19) represents the characteristic equation of the closed-loop system. The solution of the AC / DC system voltage control loop formula (19) is: The real part of the oscillation mode determines the damping and stability of the oscillations in the AC and DC systems. Substituting into formula (19) we get

[0158] Bs 2 +Cs+KM(s)F sum (λ d )=0 (20)

[0159] Where F(λ d ) affects the damping and frequency of the oscillation mode. In the frequency domain, F(λ d ) can be decomposed into

[0160]

[0161] In formula (21), T ds Changing the damping of the oscillation mode is called the damping component provided by the feedback link to the AC / DC system, and we can get:

[0162]

[0163] In formula (21), T ks The imaginary part of the oscillation mode is changed, which is called the synchronization component provided by the feedback loop to the AC / DC system.

[0164]

[0165] Im-(·) and Re-(·)-represent the imaginary and real parts of the variable, respectively.

[0166] S6: Based on the damping provided by the feedback link to the system, the damping provided by the cascaded electric vehicle charging station to the AC / DC system is further obtained.

[0167] Formula (22) represents the damping component provided by the feedback loop to the AC / DC system. The feedback link includes the influence of the DC line and the cascaded electric vehicle on the AC / DC system. Therefore, it is difficult to directly calculate the damping provided by the cascaded electric vehicle charging station to the AC / DC system only from the feedback link. In order to solve this problem, the following steps are used to obtain the damping provided by the cascaded electric vehicle charging station to the AC / DC system:

[0168] Step 1: Calculate the damping and synchronization components provided by the feedback link to the second-order loop of the AC / DC system

[0169] The transfer function expression of the feedback link is shown in formula (18), where: The damping provided by the feedback link to the AC / DC system can be obtained as T ds_sum =Im[F sum (λ d )] / ω d , the feedback link provides the synchronous component T of the AC / DC system ks_sum =Re[F sum (λ d )]-T ds ξ d

[0170] Step 2: Calculate the damping and synchronization components provided by the feedback link containing only the DC line to the second-order loop of the AC / DC system

[0171] The transfer function of the feedback link containing only the DC link is as follows:

[0172] ΔI l =F l (s)ΔV l (twenty four)

[0173] Among them: F l (s) = y 11 +y 12 (-y 22 ) -1 y 21 The damping provided to the AC / DC system by the feedback link containing only the DC line can be obtained as T ds_l =Im[F l (λ d )] / ω d , synchronization component

[0174] Step 3: Calculate the damping and synchronization components provided by the cascaded EV charging station to the second-order loop of the AC / DC system

[0175] Based on equations (18) and (24), the transfer function of the cascade electric vehicle is:

[0176] ΔI l =(F sum (s)-F l (s))ΔV l (25)

[0177] The damping provided by the cascaded electric vehicle charging station to the AC and DC systems can be obtained as T ds_e =T ds_sum -T ds_l , T ks_e =T ks_sum -T ks_l .

[0178] S7: Determine the stability state of the AC / DC system based on the damping value provided by the cascaded electric vehicle charging station to the AC / DC system and verify the correctness of the method

[0179] From formula (25), it can be concluded that when T ds_e When T > 0, the cascaded electric vehicle charging station provides positive damping to the AC / DC system, and the damping of the oscillation mode is improved. ds_e When <0, the cascaded electric vehicle charging station provides negative damping to the AC / DC system, and the damping of the oscillation mode is reduced. Further, the stability criterion of the system is obtained as follows:

[0180] T s =T ds_e +T ds_l+C>0 (26)

[0181] When the relationship between parameters and damping satisfies equation (26), the AC / DC system is stable, otherwise it is unstable.

[0182] When the parameters are set as follows: B = 5, C = 0.198, K = 0.433, when there are five 0.5pu cascaded electric vehicle charging stations, the damping provided by the feedback link to the AC / DC system is T ds_sum =0.023, T ks_sum =0.0944; the damping provided by the DC line to the AC / DC system is T ds_l =0.026, T ks_l =0.1071; the damping provided by the cascaded electric vehicle charging station to the AC and DC systems is T ds_e =-0.003, T ks_e =-0.0127. It can be seen that T ds_e <-0.003, therefore, the control link of the cascade electric vehicle charging station provides negative damping for the system, and the damping index is used to determine T s ,T s =T ds_e +T ds_l +C=0.026-0.003+0.198>0; therefore the system is stable. To verify the above analysis results, T ds_sum Substituting into formula (20), we get:

[0183] Bs 2 +(C+0.0230)s+K+0.0944=0 (27)

[0184] The solution of formula (27) is That is (-5.005+j122.343rad / s). This is consistent with the actual oscillation mode of the system, thus verifying the correctness of the above decomposition ideas and conclusions.

Claims

1. A method for evaluating the impact of cascaded charging station access on AC / DC system stability, characterized by: The following steps are included: S1. Based on the cascaded energy storage network diagram and the constant voltage control of the energy storage DC / AC converter, a linearized model of a single electric vehicle charging station is established; S2. Based on the established linearization model of a single electric vehicle charging station, a full-order linearization model and transfer function of a cascade electric vehicle charging station are obtained; S3, obtain the linearized model of AC / DC system based on the cascaded energy storage network diagram; S4, combining the transfer function of the cascade electric vehicle charging station with the AC / DC linearization model to obtain a closed-loop system with the AC / DC system as a feedforward link and the line parameters and the cascade electric vehicle charging station as a feedback link; S5, based on the closed-loop system, the damping provided by the feedback link to the AC and DC systems; S6, further obtaining the damping provided by the cascaded electric vehicle charging station to the AC and DC systems according to the damping provided to the system by the feedback link; S7. According to the damping value provided by the cascaded electric vehicle charging station to the AC / DC system, determine the stability state of the AC / DC system and verify the correctness of the method.

2. The method for evaluating the impact of cascade charging station access on AC / DC system stability as claimed in claim 1, characterized in that: In step S1, the linearized model of a single electric vehicle charging station is: Δd i =Δx il +K ip (Δx vl -K vi ΔV di -ΔI di ) In the formula, Δ represents the linear form of the scalar, subscript 0 represents the steady-state value of the variable, and V dc represents the DC voltage between the cascaded electric vehicle charging station and the DC network, I dc represents the DC current of the cascaded electric vehicle charging station and the DC network, C fi is the capacitance of the filter on the input side of the i-th module; I dci is the capacitance C fi Current on the input side, I fi Represents capacitance C fi Output current, V fi Represents the capacitance C of the input filter of the i-th module fi Voltage, L fi is the inductance of the input filter of the ith module, R fi is the resistance of the filter on the input side of the i-th module, N represents the number of modules, C di is the capacitance of the output filter of the ith module, V di is the capacitor voltage at the output side of the i-th module, I Li is the output side capacitance C of the i-th module di Injected DC current, I di is the output side capacitance C of the i-th module di Output DC current, I bi is the internal current of the battery in the i-th module charging station, K vi is the integral coefficient of DC voltage control, K ip is the proportional coefficient of the DC current control loop, K ii is the integral coefficient of the DC current control loop, d i is the duty cycle of the ith module, x vl 、x il is the intermediate variable, R di is the resistance of the connection line between the i-th module and the electric vehicle; In step S1, the connection variable between the cascaded electric vehicle charging station and the DC network is the DC voltage V dc and DC current I dc The cascaded electric vehicle charging station consists of N modules, each of which is an independent DC / DC converter for charging and discharging electric vehicles; the internal resistance model of the vehicle battery adopts: the DC voltage of the i-th battery is V ei , the internal resistance is R ei ; The current relationship of each electric vehicle is I dc1 =I dc2 =I dc3 =…=I dci =…=I dcN (2); Among them, I dci is the capacitance C of the filter on the input side of the i-th module fi The dynamic equation of the filter is Among them, V dc is the DC voltage between the cascaded electric vehicle charging station and the DC network; V fi is the capacitance C of the filter on the input side of the i-th module fi Voltage; L fi is the inductance of the filter on the input side of the i-th module; R fi is the resistance of the filter on the input side of the i-th module; variable C fi is the capacitance of the filter on the input side of the i-th module; I fi is the capacitance C fi Output DC current; The kinetic equation on the battery side is obtained as follows: Among them, C di is the capacitance on the output side of the ith module, V di is the capacitor voltage at the output side of the i-th module, I Li and I bi are the DC currents injected into and output from the output capacitor of the i-th module respectively.

3. The method for evaluating the impact of cascade charging station access on AC / DC system stability as claimed in claim 2, characterized in that: The cascaded electric vehicle charging station adopts the control method of constant DC bus voltage and constant battery current as the basic control strategy to achieve independent control of the modules; Other state variables in the control strategy of the cascaded electric vehicle charging station are The connection between the filter kinetic equation and the battery kinetic equation is realized, that is, I fi =d i I Li ,d i V fi =R di I Li +V di (7); Among them, K vp is the proportional coefficient of DC voltage control, K vi is the integral coefficient of DC voltage control, K ip is the proportional coefficient of the DC current control loop, K ii is the integral coefficient of the DC current control loop, d i is the duty cycle of the ith module, x vl 、x il is the intermediate variable, R di is the resistance of the connection line between the i-th module and the electric vehicle, and the superscript ref is the reference value of the variable.

4. The method for evaluating the impact of cascade charging station access on AC / DC system stability as claimed in claim 3, characterized in that: In step S2, the full-order linearization model of the cascaded electric vehicle charging station is: Where, X s is the state variable matrix, ΔX s =[ΔV f1 ,…,ΔV fN ,ΔI dc ,ΔV d1 ,...,ΔV dN ,Δx vl ,Δx il ], A s is the state matrix, b s is the input matrix, c s is the output matrix; From the full-order linearization model of the cascade electric vehicle charging station, the transfer function of the cascade electric vehicle charging station can be obtained as ΔI dc =c s (sI-A s ) -1 b s ΔV dc =G e (s)ΔV dc (9) G e (s) is the impedance representation of the cascaded electric vehicle charging station.

5. The method for evaluating the impact of cascade charging station access on AC / DC system stability as claimed in claim 4, characterized in that: In step S3, the cascaded electric vehicle charging station is connected to the AC / DC system. The voltage direction at the common connection point is Vs, which is the AC bus voltage. The d-axis direction of the dq coordinate of the AC / DC converter is taken, Vsq = 0, Vsd = Vs, and the result is ΔP s =I sd0 ΔV s +V s0 ΔI sd =I l0 ΔV l +V l0 ΔI l (10), Among them, P s is the line active power, I sd is the d-axis current of the AC system, I l is the current flowing into the DC / AC converter of the cascaded electric vehicle charging station, V l is the DC / AC port voltage, and the subscript 0 is the value of a variable in the steady state.

6. The method for evaluating the impact of cascade charging station access on AC / DC system stability as claimed in claim 5, characterized in that: In step S3, the linearized line current equation at the end of the AC / DC converter is: Where X is the line impedance, I sq is the q-axis current of the AC system, ω0 is the frequency reference value, V cd is the AC / DC output voltage, V sd 、V sq is the AC d and q axis terminal voltage.

7. The method for evaluating the impact of cascade charging station access on AC / DC system stability as claimed in claim 6, characterized in that: According to the AC / DC converter control strategy and the line current equation of the AC / DC converter end linearization, the following formula can be obtained: Among them, K vp is the DC voltage outer loop parameter, K ip is the DC voltage inner loop parameter, K pl is the transfer function between the current reference value and the current, and the superscript ref is the reference value of the variable; the above formula (12) is the dynamics of the active control loop, including the outer loop and the inner loop, where ω0 is the synchronous frequency, expressed in unit scale; the AC power grid is simply represented as an infinite bus, on which V s is a constant; Substituting formula (12) into formula (10), we get Among them, K vp (s) = K p_vp +K i_vp / s,K ip (s) = K p_ip +K i_ip / s,K p_vp K is the DC voltage outer loop proportional coefficient, i_vp is the DC voltage integral coefficient, K p_ip K is the DC voltage inner loop proportional coefficient, i_ip is the DC voltage inner loop integral coefficient, G l (s) is the transfer function of the AC system. Further, from equation (13), the open-loop oscillation mode of the AC / DC system can be obtained as Where: B, C, K represent coefficients, M(s) = -V l0 ω0K p_ip s 2 -V l0 ω0K i_ip s,B=ω0I l0 K p_ip +V s0 ω0K p_ip K p_vp ,C=V s0 (ω0I l0 K i_ip +K i_ip K p_vp +K p_ip K i_vp ),K=-V s0 K i_ip K i_vp , G l The denominator of (s) is a second-order loop of an AC / DC system.

8. The method for evaluating the impact of cascade charging station access on AC / DC system stability as claimed in claim 7, characterized in that: In step S4, the AC / DC system is connected to the cascaded electric vehicle charging station through the impedance matrix: Among them, y 11 ,y 22 is the self-admittance, y 12 ,y 21 is the mutual admittance, y 11 =y 22 =-y 12 =-y 21 , the specific value is determined by the line parameters, and it can be obtained from formula (15): In formula (15), That is, ΔI l =F sum (s)ΔV l , F(s) represents the feedback link of line parameters and cascaded electric vehicle charging station to AC / DC system; First, according to the formula The damping torque analysis of AC / DC system is established, and its expression is: (Bs 2 +Cs+K)ΔV l =M(s)ΔI l (17); The damping provided to the AC / DC system by the feedback link formed by the DC line and the cascaded electric vehicle charging station is: ΔI l =F sum (s)ΔV l (18), Substituting equation (18) into equation (19) yields (Bs 2 +Cs+K-M(s)F sum (s))ΔV l =0(19), Formula (19) represents the characteristic equation of the closed-loop system. The solution of the AC / DC system voltage control loop formula (19) is: Will Substituting into formula (19) we get Bs 2 +Cs+KM(s)F sum (λ d )=0(20), Where F(λ d ) affects the damping and frequency of the oscillation mode; In the frequency domain, F(λ d ) can be decomposed into In formula (21), T ds Changing the damping of the oscillation mode is called the damping component provided by the feedback link to the AC / DC system, and we can get: In formula (21), T ks The imaginary part of the oscillation mode is changed, which is called the synchronization component provided by the feedback loop to the AC / DC system. Im(·) and Re(·) represent the imaginary and real parts of the variable, respectively.

9. The method for evaluating the impact of cascade charging station access on AC / DC system stability as claimed in claim 8, characterized in that: The S6 step includes the following sub-steps: S601, calculating the damping and synchronization components provided by the feedback link to the second-order loop of the AC / DC system; S602, calculating the damping and synchronization components provided by the feedback link containing only the DC line to the second-order loop of the AC / DC system; S603. Calculate the damping and synchronization components provided by the cascaded electric vehicle charging station to the second-order loop of the AC / DC system.

10. The method for evaluating the impact of cascade charging station access on AC / DC system stability as claimed in claim 9, characterized in that: In step S601, the transfer function expression of the feedback link is ΔI l =F sum (s)ΔV l ,in, The damping provided by the feedback link to the AC / DC system can be obtained as T ds_sum =Im[F sum (λ d )] / ω d , the feedback link provides the synchronous component T of the AC / DC system ks_sum =Re[F sum (λ d )]-T ds ξ d ; In step S602, the transfer function of the feedback link containing only the DC line is ΔI l =F l (s)ΔV l (twenty four), Among them, F l (s) = y 11 +y 12 (-y 22 ) -1 y 21 The damping provided to the AC / DC system by the feedback link containing only the DC line can be obtained as T ds_l =Im[F l (λ d )] / ω d , synchronization component In step S603, based on the transfer function expression of the feedback link ΔI l =F sum (s)ΔV l The transfer function of the feedback link with only the DC link is ΔI l =F l (s)ΔV l , the transfer function of the cascaded electric vehicle charging station is ΔI l =(F sum (s)-F l (s))ΔV l (25), The damping provided by the cascaded electric vehicle charging station to the AC and DC systems can be obtained as T ds_e =T ds_sum -T ds_l , T ks_e =T ks_sum -T ks_l ; By ΔI l =(F sum (s)-F l (s))ΔV l It can be concluded that when T ds_e >0, the cascaded electric vehicle charging station provides positive damping to the AC / DC system, and the damping of the oscillation mode is improved; when T ds_e When <0, the cascaded electric vehicle charging station provides negative damping to the AC / DC system, and the damping of the oscillation mode is reduced; the stability criterion of the system is obtained as follows: T s =T ds_e +T ds_l +C>0(26), When the relationship between parameters and damping satisfies equation (26), the AC / DC system is stable, otherwise it is unstable.

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