Voltage sag amplitude calculation method considering transient response characteristics of optical storage resources

Abstract

The invention provides a voltage sag amplitude calculation method considering transient response characteristics of optical storage resources, and relates to the technical field of power electronics. The method comprises the following steps: calculating available voltage regulation capacity of energy storage based on system load and generated power of a power system; based on the voltage recording data of the photovoltaic grid-connected point, a parameter identification model is constructed and solved, photovoltaic transient current equivalent control parameters are obtained, and photovoltaic transient output current is further obtained through calculation; calculating photovoltaic off-network time according to the photovoltaic low-voltage ride-through curve, and constructing a voltage sag evolution time sequence; and calculating the voltage sag amplitude by combining the voltage sag evolution time sequence, the available voltage regulation capacity and the photovoltaic transient output current. According to the method, the light storage transient response characteristics during the voltage sag period can be depicted, and the light storage resource voltage support margin during the voltage sag period can be quantitatively calculated; the method can be well adapted to a novel power system, and is beneficial to improving the calculation accuracy and precision of the voltage sag amplitude characteristics.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a method for calculating a voltage sag amplitude taking into account transient response characteristics of photovoltaic storage resources. Background Art

[0002] Voltage sags cause AC contactors, adjustable speed drives, computers and other equipment to trip, causing huge economic losses to industrial users. With the development of new power systems, the scale of photovoltaic storage resources in the power grid has increased year by year. Electrochemical energy storage has a fast transient response speed and can respond to power disturbances in milliseconds, which can buffer the impact of supply voltage and frequency fluctuations on sensitive loads. During voltage sags, energy storage resources in the system can support node voltages to alleviate voltage sags. The impact of photovoltaic access on voltage sags is reflected in the following two aspects. On the one hand, the response of photovoltaic low voltage ride-through control strategy under voltage sag can support the amplitude of voltage sag. On the other hand, photovoltaic disconnection under voltage sag events may cause further voltage deterioration. The traditional voltage sag amplitude calculation method lacks consideration of the transient response characteristics of photovoltaic storage resources, which may lead to large errors in the calculation of voltage sag amplitude.

[0003] The existing technology mainly has the following problems:

[0004] 1) The existing technology lacks a method to characterize the transient response characteristics of photovoltaic and energy storage resources, and the voltage support capacity of photovoltaic and energy storage under temporary voltage sag is unclear.

[0005] 2) The existing voltage sag amplitude calculation method lacks consideration of the transient response process of photovoltaic storage. The traditional calculation method has poor applicability to new power systems and lacks calculation accuracy and precision. Summary of the invention

[0006] In order to solve the above technical problems existing in the prior art, the present invention provides a method for calculating the voltage sag amplitude taking into account the transient response characteristics of photovoltaic storage resources.

[0007] Specifically, the technical solution includes:

[0008] Calculate the available voltage regulation capacity of energy storage based on the system load and power generation of the power system;

[0009] Based on the voltage recording data of the photovoltaic grid-connected point, a parameter identification model is constructed and solved to obtain the photovoltaic transient current equivalent control parameters; based on the photovoltaic transient current equivalent control parameters, the photovoltaic transient output current is calculated;

[0010] Calculate the photovoltaic grid-off time based on the photovoltaic low voltage ride-through curve and construct the voltage sag evolution time series;

[0011] The voltage sag amplitude characteristics are calculated by combining the voltage sag evolution time sequence, available voltage regulation capacity and PV transient output current.

[0012] Preferably, based on the system load and power generation of the power system, the available voltage regulation capacity of the energy storage is calculated, specifically including:

[0013] Calculate the energy storage active power P ESSi ,

[0014]

[0015] Where P V is the load valley power value, P net,i is the net load power at the i-th moment, P F is the load peak shaving line power value, t1 is the start time of energy storage charging, t2 is the end time of energy storage charging, t3 is the start time of energy storage discharging, and t4 is the end time of energy storage discharging;

[0016] Calculate the remaining energy storage capacity S at the i-th moment i ,

[0017]

[0018] Where η c is the charging efficiency of energy storage, η d is the discharge efficiency of energy storage, S n is the rated capacity of the energy storage;

[0019] Calculate the available voltage regulation capacity of the energy storage at the i-th moment during the voltage sag, including the adjustable active power P′ ESS，i and adjustable reactive power Q′ ESS,i ,

[0020]

[0021] Where, t p is the duration of voltage sag, t ESS is the energy storage response delay, t p -t ESS Indicates the energy storage voltage support time, P ESS,max is the maximum discharge power of energy storage, P n is the active power shortage of the node during voltage sag, Q n It is the reactive power shortage of the node during voltage sag.

[0022] Preferably, based on the voltage recording data of the photovoltaic grid-connected point, a parameter identification model is constructed and solved to obtain the photovoltaic transient current equivalent control parameter; based on the photovoltaic transient current equivalent control parameter, the photovoltaic transient output current is calculated, specifically including:

[0023] Construct a photovoltaic transient control parameter identification model,

[0024]

[0025] In the formula, f is the optimization objective function, U PV,dm and U PV,qm are the d-axis voltage component and q-axis voltage component after the three-phase monitoring voltage of the photovoltaic grid-connected node is transformed, U PV,de and U PV,qe are the estimated values ​​of the d-axis voltage component and the q-axis voltage component at the photovoltaic station grid connection point, respectively, and U PV is the voltage amplitude of the photovoltaic grid-connected node, U min and U max are the lower and upper limits of the photovoltaic grid-connected node voltage, I d and I q are the d-axis current and q-axis current of the photovoltaic grid-connected node, I max is the output current limit of the photovoltaic grid-connected node, P PV and Q PV are the active power and reactive power output by the photovoltaic grid-connected node, S PV is the photovoltaic station capacity, and are the minimum and maximum values ​​of reactive power output by the photovoltaic grid-connected node respectively;

[0026] in,

[0027]

[0028] Where θ is the rotation angle of the dq coordinate system relative to the three-phase coordinate system, U PV,a , U PV,b and U PV,c They are the photovoltaic grid-connected point voltage U PV The a-phase voltage component, b-phase voltage component and c-phase voltage component;

[0029]

[0030] Where U PV,pred and U PV,preq are the d-axis voltage component monitoring value and q-axis voltage component monitoring value before the transient response of the photovoltaic grid-connected node, R g is the equivalent resistance of the photovoltaic station, L g is the equivalent inductance of the photovoltaic station, w is the angular frequency of the power system;

[0031] Input the voltage recording data into the parameter identification model and solve it to obtain the photovoltaic transient current equivalent control parameters K1 and K2;

[0032] Calculate the PV transient output current, including the d-axis current I of the PV grid-connected node d and q-axis current I q ,

[0033]

[0034] In the formula, I dr and I qr are the d-axis reference current and q-axis reference current of the photovoltaic grid-connected node, I n is the rated current of the photovoltaic grid-connected node.

[0035] Preferably, the photovoltaic grid-off time is calculated according to the photovoltaic low voltage ride-through curve, and the voltage sag evolution time sequence is constructed, which specifically includes:

[0036] Calculate the photovoltaic off-grid time t trip ,

[0037]

[0038] Where U PV is the voltage amplitude of the photovoltaic grid-connected node, U X and U M are the maximum and minimum values ​​of the ordinate of the low voltage ride-through curve, T max and T min They are the maximum and minimum values ​​of the horizontal axis of the low voltage ride through inclined section curve respectively;

[0039] Construct a voltage sag evolution time sequence T including multiple stages,

[0040] T=rank(t PV ,t ESS ,t trip ,t p );

[0041] In the formula, rank(·) represents the ascending order operator, t PV is the duration of photovoltaic low voltage ride-through, t ESS is the energy storage response time, t p is the duration of voltage sag.

[0042] Preferably, the voltage sag amplitude characteristics are calculated in combination with the voltage sag evolution time sequence, available voltage regulation capacity and photovoltaic transient output current, specifically including:

[0043] Before the PV-storage response, the voltage divider model is used to calculate the voltage sag amplitude matrix U of the power system. sag ;

[0044] When the energy storage responds, calculate the transient current I′ injected by the photovoltaic at the i-th moment ESS，i ,

[0045]

[0046] In the formula, QESS,i is the energy storage reactive power at the i-th moment, Z ESS is the transient equivalent impedance of energy storage;

[0047] When the photovoltaic low voltage rides through, calculate the transient current fault component I′ at the i-th moment PV，i ,

[0048]

[0049] In the formula, I d.i and I q.i are the d-axis current and q-axis current of the photovoltaic grid-connected node at the i-th moment, j is the sign of the imaginary part, P PV,i and Q PV,i are the active power and reactive power output by the photovoltaic grid-connected node at the i-th moment, and is the transient equivalent impedance of the photovoltaic grid-connected node;

[0050] When the photovoltaic power plant is off-grid, calculate the photovoltaic injection current deviation value I at the i-th moment PV,i ,

[0051]

[0052] Calculate the voltage sag amplitude U at multiple voltage sag evolution time sequence stages sag,k ,

[0053] U sag,k =U sag,k-1 +I k ·Z S,k ;

[0054] Where U sag,k and Usag,k-1 are the voltage sag amplitudes in the kth stage and the k-1th stage respectively, I k is the transient injection current of the kth stage, Z S,k is the system impedance matrix of the kth stage;

[0055] in,

[0056]

[0057] Where t is time.

[0058] In summary, the technical solution provided by the present invention achieves the characterization of the transient response characteristics of photovoltaic storage during voltage sags, as well as the quantitative calculation of the voltage support margin of photovoltaic storage resources during voltage sags, through the calculation of available capacity and the identification of photovoltaic transient control parameters. By constructing a voltage sag amplitude evolution time series and a multi-stage voltage sag amplitude calculation model, the multi-stage voltage sag amplitude characteristics considering the transient response of photovoltaic storage can be calculated, which can be better adapted to new power systems and help improve the accuracy and precision of the voltage sag amplitude characteristic calculation. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 The figure is a flow chart of the method for calculating the voltage sag amplitude in the present invention.

[0060] Figure 2 It is the photovoltaic low voltage ride-through curve in the present invention. DETAILED DESCRIPTION

[0061] Hereinafter, the technical solution provided by the present invention will be further elaborated in detail with reference to the accompanying drawings.

[0062] Voltage sag amplitude calculation is the basis for voltage sag frequency assessment and risk characterization. In order to improve the applicability and calculation accuracy of the voltage sag amplitude calculation method for the new power system, the present invention proposes a voltage sag amplitude calculation method taking into account the transient response characteristics of photovoltaic and storage resources.

[0063] like Figure 1 As shown, the basic flow chart is divided into four steps:

[0064] Step S1: Taking system load and power generation data as input, a transient voltage regulation margin calculation model for energy storage is constructed to calculate the available voltage regulation capacity of energy storage.

[0065] Assume that the system load power data set and power generation data set are P L and P G , then the net load data set P net for:

[0066] P net =P L -P G ;

[0067] Under the variable power control strategy, the energy storage power can be calculated using the following formula:

[0068]

[0069] Among them, P ESS,i is the energy storage power at the i-th moment, P V is the load valley power value, P net,i is the net load power at the i-th moment, P F is the load peak shaving line power value, t1 is the start time of energy storage charging, t2 is the end time of energy storage charging, t3 is the start time of energy storage discharging, t4 is the end time of energy storage discharging, when i∈[t1,t2], the energy storage charging absorbs power, when i∈[t1,t2], the energy storage discharging releases power.

[0070] The remaining energy storage capacity S at the i-th moment i Use the following formula to calculate:

[0071]

[0072] Among them, η c is the charging efficiency of energy storage, η d is the discharge efficiency of energy storage, S n is the rated capacity of the energy storage.

[0073] During the voltage sag, the adjustable capacity of the energy storage at the i-th moment is calculated using the following formula:

[0074]

[0075] Among them, t p is the duration of voltage sag, t ESS is the energy storage response delay, t p -t ESS Indicates the energy storage voltage support time, P ESS,max is the maximum discharge power of energy storage, P n is the active power shortage of the node during voltage sag, Q n It is the reactive power shortage of the node during voltage sag.

[0076] Step S2: Taking the voltage recording data of the photovoltaic access point monitoring device as input, a parameter identification model is constructed and solved to obtain the photovoltaic transient current equivalent control parameters.

[0077] The photovoltaic transient control parameter identification model is as follows:

[0078]

[0079] Where f is the optimization objective function, U PV,dm and U PV,qm are the d-axis voltage component and q-axis voltage component after the three-phase monitoring voltage of the photovoltaic grid-connected node is transformed, U PV,de and U PV,qe are the estimated values ​​of the d-axis voltage component and the q-axis voltage component of the photovoltaic grid-connected point, respectively, and U PV is the voltage amplitude of the photovoltaic grid-connected node, U min and U max are the lower and upper limits of the photovoltaic grid-connected node voltage, I d and I q are the d-axis current and q-axis current of the photovoltaic grid-connected node, I max is the output current limit of the photovoltaic grid-connected node, P PV and Q PV are the active power and reactive power output by the photovoltaic grid-connected node, S PV is the photovoltaic station capacity, and They are the minimum and maximum values ​​of reactive power output by the photovoltaic grid-connected node, respectively.

[0080] U is obtained by transforming the three-phase voltage monitoring value PV,dm and U PV,qm The formula is as follows:

[0081]

[0082] Where θ is the rotation angle of the dq coordinate system relative to the three-phase coordinate system, U PV,a , U PV,b and U PV,c They are the photovoltaic grid-connected point voltage U PV The a-phase voltage component, b-phase voltage component and c-phase voltage component are the fault recording monitoring values.

[0083] Estimated value of d-axis and q-axis voltage components at the photovoltaic grid-connected point U PV,de and U PV,qe The calculation process is as follows:

[0084]

[0085] Where U PV,pred and U PV,preq are the d-axis voltage component monitoring value and q-axis voltage component monitoring value before the transient response of the photovoltaic grid-connected node, R g is the equivalent resistance of the photovoltaic station, L g is the equivalent inductance of the photovoltaic station, and w is the angular frequency of the power system.

[0086] According to the photovoltaic low voltage ride-through control strategy, the d-axis component of the photovoltaic transient output current I d , q-axis component I q As shown below:

[0087]

[0088] In the formula, I dr and I qr are the d-axis reference current and q-axis reference current of the photovoltaic grid-connected node, U PV is the photovoltaic grid-connected point voltage, I n is the rated current of the photovoltaic station. Photovoltaic transient current equivalent control parameters K1 and K2 are parameters to be identified.

[0089] The parameter identification model takes the monitoring data as input and takes the minimum voltage estimation value and monitoring value as the optimization goal. The equivalent control parameters K1 and K2 of the photovoltaic transient current are obtained by solving the parameter identification model, and the photovoltaic transient output current can be further calculated.

[0090] Step S3: Figure 2As shown, the photovoltaic grid-off time is calculated according to the photovoltaic low voltage ride-through curve (obtained according to the national standard low voltage ride-through requirements), and the voltage sag evolution time series is constructed.

[0091] Photovoltaic off-grid time t trip The calculation formula is as follows:

[0092]

[0093] Where U PV is the voltage amplitude of the photovoltaic grid-connected node, U X and U M They are the maximum and minimum values ​​of the vertical coordinates of the low voltage ride-through curve required by the national standard, T max and T min They are respectively the maximum and minimum values ​​of the horizontal axis of the low voltage ride through inclined section curve.

[0094] The voltage sag evolution time sequence T is as follows:

[0095] T=rank(t PV ,t ESS ,t trip ,t p );

[0096] Among them, rank(·) is an ascending order operator, which arranges the time in the brackets in ascending order to form a time series. PV is the duration of photovoltaic low voltage ride-through, t ESS is the energy storage response time, t p is the duration of voltage sag.

[0097] Step S4: After obtaining the multi-stage evolution time series of the voltage sag, the multi-stage voltage sag amplitude characteristics are calculated by combining the energy storage available capacity and the photovoltaic transient output current obtained by S1 and S2.

[0098] Before the PV-storage response, the traditional voltage divider model is used to calculate the voltage sag amplitude matrix U of the power system. sag , that is, the voltage sag amplitude matrix of the first stage.

[0099] Energy storage in t ESS When the transient current I' is injected ESS，i Calculated by the following formula:

[0100]

[0101] Among them, Q ESSi is the energy storage reactive power at the i-th moment, Z ESS is the transient equivalent impedance of energy storage.

[0102] When the photovoltaic low voltage ride-through occurs, the photovoltaic low voltage ride-through control strategy responds, and the transient current fault component is I′ PV,i :

[0103]

[0104] In the formula, I d.i and I q.i are the d-axis current and q-axis current of the photovoltaic grid-connected node at the i-th moment, j is the sign of the imaginary part, P PV,i and Q PV,i are the active power and reactive power output by the photovoltaic grid-connected node at the i-th moment, and is the transient equivalent impedance of the photovoltaic grid-connected node.

[0105] When the photovoltaic power plant is disconnected from the grid, no current is injected into the grid connection point. At this time, the photovoltaic injection current deviation value is I” PV,i It is expressed as:

[0106]

[0107] It can be seen that t trip ≥t PV When the photovoltaic low voltage ride-through moment is earlier than the grid disconnection moment, the photovoltaic grid disconnection loss transient current fault component, t trip <t PV When the PV is disconnected from the grid, the grid-tripping moment occurs before the PV low voltage ride-through control strategy responds, and the PV grid-tripping loses the injected current under normal operating conditions.

[0108] Corresponding to multiple voltage sag evolution time series, the voltage sag amplitude calculation formula is expressed as:

[0109] U sag,k =U sag,k-1 +I k ·Z S,k ;

[0110] Among them, U sag,k and U sag,k-1 are the voltage sag amplitudes of the kth stage and the k-1th stage, I k is the transient injection current of the kth stage, Z S,k is the system impedance matrix of the kth stage. k The value selection rules are as follows:

[0111]

[0112] Where t is a time variable. When t is any element in the voltage sag evolution time series T, the voltage sag amplitude evolves and the corresponding I needs to be updated. k .

[0113] It should be noted that step S1, step S2 and step S3 are independent of each other, and executing any one of the steps alone will not be affected by the other two steps. Therefore, step S1, step S2 and step S3 can be executed in any order.

[0114] In summary, the technical solution provided by the present invention achieves the characterization of the transient response characteristics of photovoltaic storage during voltage sags, as well as the quantitative calculation of the voltage support margin of photovoltaic storage resources during voltage sags, through the calculation of available capacity and the identification of photovoltaic transient control parameters. By constructing a voltage sag amplitude evolution time series and a multi-stage voltage sag amplitude calculation model, the multi-stage voltage sag amplitude characteristics considering the transient response of photovoltaic storage can be calculated, which can be better adapted to new power systems and help improve the accuracy and precision of the voltage sag amplitude characteristic calculation.

Claims

1. A method for calculating voltage sag amplitude taking into account the transient response characteristics of photovoltaic and energy storage resources, characterized in that: include: Calculate the available voltage regulation capacity of energy storage based on the system load and power generation of the power system; Based on the voltage recording data of the photovoltaic grid-connected point, a parameter identification model is constructed and solved to obtain the photovoltaic transient current equivalent control parameters; based on the photovoltaic transient current equivalent control parameters, the photovoltaic transient output current is calculated; Calculate the photovoltaic grid-off time based on the photovoltaic low voltage ride-through curve and construct the voltage sag evolution time series; The voltage sag amplitude characteristics are calculated by combining the voltage sag evolution time sequence, available voltage regulation capacity and PV transient output current.

2. A method for calculating voltage sag amplitude taking into account transient response characteristics of photovoltaic energy storage resources as claimed in claim 1, characterized in that: The calculation of the available voltage regulation capacity of the energy storage based on the system load and the power generation power of the power system specifically includes: Calculate the energy storage active power P ESSi , Where P V is the load valley power value, P net,i is the net load power at the i-th moment, P F is the load peak shaving line power value, t1 is the start time of energy storage charging, t2 is the end time of energy storage charging, t3 is the start time of energy storage discharging, and t4 is the end time of energy storage discharging; Calculate the remaining energy storage capacity S at the i-th moment i , Where η c is the charging efficiency of energy storage, η d is the discharge efficiency of energy storage, S n is the rated capacity of the energy storage; Calculate the available voltage regulation capacity of the energy storage at the i-th moment during the voltage sag, including the adjustable active power P′ ESS,i and adjustable reactive power Q′ ESS,i , Where, t p is the duration of voltage sag, t ESS is the energy storage response delay, t p -t ESS Indicates the energy storage voltage support time, P ESS,max is the maximum discharge power of energy storage, P n is the active power shortage of the node during voltage sag, Q n It is the reactive power shortage of the node during voltage sag.

3. A method for calculating voltage sag amplitude taking into account transient response characteristics of photovoltaic energy storage resources as claimed in claim 1, characterized in that: The voltage recording data of the photovoltaic grid-connected point is used to construct and solve the parameter identification model to obtain the photovoltaic transient current equivalent control parameter; based on the photovoltaic transient current equivalent control parameter, the photovoltaic transient output current is calculated, which specifically includes: Construct a photovoltaic transient control parameter identification model, Where f is the optimization objective function, U PV,dm and U PV,qm are the d-axis voltage component and q-axis voltage component after the three-phase monitoring voltage of the photovoltaic grid-connected node is transformed, U PV,de and U PV,qe are the estimated values ​​of the d-axis voltage component and the q-axis voltage component at the photovoltaic station grid connection point, respectively, and U PV is the voltage amplitude of the photovoltaic grid-connected node, U min and U max are the lower and upper limits of the photovoltaic grid-connected node voltage, I d and I q are the d-axis current and q-axis current of the photovoltaic grid-connected node, I max is the output current limit of the photovoltaic grid-connected node, P PV and Q PV are the active power and reactive power output by the photovoltaic grid-connected node, S PV is the photovoltaic station capacity, and are the minimum and maximum values ​​of reactive power output by the photovoltaic grid-connected node respectively; in, Where θ is the rotation angle of the dq coordinate system relative to the three-phase coordinate system, U PV,a , U PV,b and U PV,c They are the photovoltaic grid-connected point voltage U PV The a-phase voltage component, b-phase voltage component and c-phase voltage component; Where U PV,pred and U PV,preq are the d-axis voltage component monitoring value and q-axis voltage component monitoring value before the transient response of the photovoltaic grid-connected node, R g is the equivalent resistance of the photovoltaic station, L g is the equivalent inductance of the photovoltaic station, w is the angular frequency of the power system; Input the voltage recording data into the parameter identification model and solve it to obtain the photovoltaic transient current equivalent control parameters K1 and K2; Calculate the PV transient output current, including the d-axis current I of the PV grid-connected node d and q-axis current I q , In the formula, I dr and I qr are the d-axis reference current and q-axis reference current of the photovoltaic grid-connected node, I n is the rated current of the photovoltaic grid-connected node.

4. A method for calculating voltage sag amplitude taking into account transient response characteristics of photovoltaic energy storage resources as claimed in claim 1, characterized in that: The method of calculating the photovoltaic grid-off time according to the photovoltaic low voltage ride-through curve and constructing the voltage sag evolution time sequence specifically includes: Calculate the photovoltaic off-grid time t trip , Where U PV is the voltage amplitude of the photovoltaic grid-connected node, U X and U M are the maximum and minimum values ​​of the ordinate of the low voltage ride-through curve, T max and T min They are the maximum and minimum values ​​of the horizontal axis of the low voltage ride through inclined section curve respectively; Construct a voltage sag evolution time sequence T including multiple stages, T=rank(t PV ,t ESS ,t trip ,t p ); In the formula, rank(·) represents the ascending order operator, t PV is the duration of photovoltaic low voltage ride-through, t ESS is the energy storage response time, t p is the duration of voltage sag.

5. The method for calculating the voltage sag amplitude taking into account the transient response characteristics of photovoltaic energy storage resources according to claim 1, characterized in that: The voltage sag amplitude characteristics are calculated by combining the voltage sag evolution time sequence, available voltage regulation capacity and photovoltaic transient output current, specifically including: Before the PV-storage response, the voltage divider model is used to calculate the voltage sag amplitude matrix U of the power system. sag ; When the energy storage responds, calculate the transient current I′ injected by the photovoltaic at the i-th moment ESS,i , In the formula, P′ ESS,i is the adjustable active power, Q′ ESS,i is the adjustable reactive power, Q ESS,i is the energy storage reactive power at the i-th moment, j is the sign of the imaginary part, Z ESS is the transient equivalent impedance of energy storage; When the photovoltaic low voltage rides through, calculate the transient current fault component I′ at the i-th moment PV,i , In the formula, I d.i and I q.i are the d-axis current and q-axis current of the photovoltaic grid-connected node at the i-th moment, respectively, PV,i and Q PV,i are the active power and reactive power output by the photovoltaic grid-connected node at the i-th moment, Z PV is the transient equivalent impedance of the photovoltaic grid-connected node; When the photovoltaic power plant is disconnected from the grid, calculate the injection current deviation value I″ at the i-th moment PV,i , In the formula, t trip is the photovoltaic off-grid time, P PV,i and Q PV,i are the active power and reactive power output by the photovoltaic grid-connected node at the i-th moment, Z PV is the transient equivalent impedance of the photovoltaic grid-connected node, t PV is the duration of PV low voltage ride-through; Calculate the voltage sag amplitude U at multiple voltage sag evolution time sequence stages sag,k , U sag,k =U sag,k-1 +I k ·Z S,k ; Where U sag,k and U sag,k-1 are the voltage sag amplitudes of the kth stage and the k-1th stage, I k is the transient injection current of the kth stage, Z S,k is the system impedance matrix of the kth stage; in, Where t is time.

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