Average value model establishing method suitable for small disturbance stability research of string type photovoltaic grid-connected system

Abstract

The invention provides an average value model establishment method suitable for small disturbance stability research of a string type photovoltaic grid-connected system, and the method comprises the steps: firstly, carrying out the mathematical modeling of a string type photovoltaic converter SPWM modulation process; then, respectively establishing an alternating current side equivalent model and a direct current side equivalent model of the average value model of the converter; and finally, establishing a string photovoltaic independent grid-connected system through a two-stage boosting transformer in PSCAD / EMTDC electromagnetic transient simulation software, and comparing time domain electromagnetic transient simulation results of a detailed model and an average value model which are adopted by converter modulation in the same small disturbance instability scene, so as to verify the accuracy of the average value model. According to the method, the average value model of the string type photovoltaic converter is established, the problems of low simulation efficiency and overlarge calculation energy consumption caused by detailed modeling in the converter modulation process can be effectively avoided, and the method is suitable for small disturbance stability research under string type photovoltaic sudden increase of a complex alternating current system.

Description

Technical Field

[0001] The present invention relates to the field of electromagnetic transient simulation modeling of photovoltaic power stations, and specifically to a method for establishing an average value model suitable for the study of small-signal stability of a series-connected photovoltaic grid-connected system. Background Art

[0002] Under the background of the construction of a new energy system, the new energy has experienced a rapid growth, and the degree of power electronics of equipment on the source, grid, and load sides has been gradually improved, resulting in the evolution of the traditional power system into a double-high form, and the dynamic characteristics of the power system have changed significantly. Multiple types of power electronic devices interact and couple complexly on multiple time scales, and the interaction between power electronic devices and the power grid exacerbates the oscillation risks in the sub-synchronous frequency band, super-synchronous frequency band, and high-frequency band of the system. The new electromagnetic oscillation problems caused by new energy will seriously restrict the efficient consumption of new energy and affect the safe and stable operation of the system. Therefore, it is urgent to study the problem of new energy grid-connected stability. However, when using the time-domain simulation analysis method to study the small-signal stability of a photovoltaic grid-connected system at present, the modulation process of the photovoltaic power station converter usually adopts a detailed electromagnetic transient simulation model. The large-scale modeling of a large number of photovoltaic power stations is likely to cause the differential-algebraic equations reflecting the dynamic characteristics of the system to be difficult to solve, reducing the electromagnetic transient simulation speed and being unfavorable for analyzing various operating conditions of complex systems.

[0003] Therefore, it is urgent to study a method for establishing an average value model suitable for the study of small-signal stability of a series-connected photovoltaic grid-connected system to adapt to the grid-connected stability study under the rapid growth of series-connected photovoltaics in a complex AC system. The present invention establishes an average value model of a series-connected photovoltaic inverter, which can effectively avoid the problems of low simulation efficiency and excessive computational energy consumption caused by detailed modeling of the converter modulation process, and is applicable to the grid-connected stability study under the rapid growth of series-connected photovoltaics in a complex AC system. Summary of the Invention

[0004] The purpose of the present invention is to solve the above-mentioned deficiencies of the prior art, and thus provide a method for establishing an average value model suitable for the study of small-signal stability of a series-connected photovoltaic grid-connected system.

[0005] Based on the SPWM modulation working principle and its mathematical equations of a series-connected photovoltaic converter, the present invention establishes an equivalent model of the AC side and an equivalent model of the DC side of the converter average value model, and builds a series-connected photovoltaic system directly connected to the grid through a two-stage step-up transformer in the PSCAD / EMTDC electromagnetic transient simulation software. Under the same small-signal instability scenario, the time-domain electromagnetic transient simulation results of the converter modulation using a detailed model and an average value model are compared, verifying the accuracy of the average value model, which can be applied to the study of small-signal stability of a large-scale series-connected photovoltaic grid-connected system.

[0006] A method for establishing an average value model applicable to the small disturbance stability study of a string-type photovoltaic grid-connected system, comprising the following steps:

[0007] (1) Mathematically model the SPWM modulation process of the converter of the string-type photovoltaic;

[0008] (2) According to the mathematical model of the converter modulation process established in step (1), establish an equivalent model of the AC side and an equivalent model of the DC side of the converter average value model;

[0009] (3) Build a string-type photovoltaic single-grid-connected system through a two-stage step-up transformer in the PSCAD / EMTDC electromagnetic transient simulation software, and compare the time-domain electromagnetic transient simulation results of the two methods of using a detailed model and an average value model for converter modulation under the same small disturbance instability scenario to verify the accuracy of the average value model.

[0010] Step (1) specifically includes:

[0011] When the string-type photovoltaic converter uses SPWM modulation, the actual output voltage u i of the converter after modulation is as follows:

[0012]

[0013] In the formula, K pwm is the voltage equivalent gain when the converter uses the SPWM modulation method; u iref is the reference value of the inverter output voltage; T d is the switching delay time constant of the converter control delay link; U dc is the DC side bus voltage; U c is the carrier amplitude of the modulation process. When U c takes the value of U dc , the voltage equivalent gain K pwm is 0.5;

[0014] The first-order inertia link is used to linearly process the converter delay link. After the linear processing of formula (1), it is as follows:

[0015]

[0016] Before and after the linear processing, the amplitude-frequency and phase-frequency characteristics of the two in the sub / super-synchronous frequency band are the same, and the small disturbance characteristics of the model in the sub / super-synchronous frequency band can be correctly reflected;

[0017] Perform d, q decomposition on formula (3) to obtain the mathematical model of the converter modulation process of the voltage d, q components, as follows:

[0018]

[0019] Where, u id and u iq are respectively the d-axis component and the q-axis component of the converter modulation output voltage; u idref and u iqref are respectively the reference value of the d-axis component and the reference value of the q-axis component of the converter modulation output voltage.

[0020] Step (2) includes the following steps:

[0021] (21) Establish an equivalent model of the AC side of the converter average value model according to the mathematical model of the converter modulation process established in step (1);

[0022] (22) Establish a DC side equivalent model according to the equivalent model of the AC side of the converter average value model.

[0023] According to the mathematical model of the converter modulation process established in step (1), establish an equivalent model of the AC side of the converter average value model, specifically:

[0024] The AC side equivalent model consists of a three-phase controlled voltage source and a diode three-phase full bridge. Among them, the three-phase controlled voltage sources u sa , u sb , u sc are used to simulate the voltage output result of the converter, and the diode bridge is used to ensure power balance and simulation accuracy when the DC voltage drops below the peak value of the AC voltage;

[0025] To keep the output voltage of the converter consistent before and after equivalence, the input voltages u sa , u sb , u sc of the three-phase controlled voltage source of the AC side equivalent system are obtained from the converter modulation output voltages u id , u iq through Park transformation, as shown in the following formula:

[0026]

[0027] Where, θ is the inverter phase angle output by the phase-locked loop link; U b is the voltage reference value.

[0028] Establish a DC side equivalent model according to the equivalent model of the AC side of the converter average value model, specifically including:

[0029] The DC side equivalent system consists of a controlled DC current source. According to the output power of the controlled three-phase voltage source and the power absorbed by the controlled current source being consistent, the input I dc of the DC controlled current source is as follows:

[0030]

[0031] where, i sa , i sb , i sc are the measured three-phase current values of the inverter output.

[0032] Step (3) specifically includes:

[0033] 1) Build a string-type PV system connected to the grid separately through a two-stage step-up transformer in the PSCAD / EMTDC electromagnetic transient simulation software. Consider two methods, namely the detailed model and the average value model, for the converter modulation process. At the same time, ensure that the PV array, Boost circuit and its control system, inverter control system, PLL and its control system models of the string-type PV are consistent;

[0034] 2) Take the system with the detailed model for the string-type PV converter modulation process as the research object, and construct a sub-synchronous oscillation scenario for the string-type PV power station: According to the key influencing factors for the sub-synchronous oscillation of the string-type PV power station, which are usually the proportional gain coefficient of the current loop control and the short-circuit ratio of the station port of the string-type PV power station, set the proportional gain coefficient of the current loop control as a fixed value, and gradually reduce the short-circuit ratio of the string-type PV power station until the system has a sub-synchronous oscillation, and record the critical instability operating conditions of the system before and after the oscillation;

[0035] 3) Draw the time-domain simulation waveform diagram of the PV output power of the system with the detailed model for the string-type PV converter modulation process under the critical instability scenario;

[0036] 4) Take the system with the average value model for the string-type PV converter modulation process as the research object, set the operating conditions of the system to be the same as the critical instability operating conditions before and after the oscillation of the detailed model system, and draw the time-domain simulation waveform diagram of the PV output power;

[0037] 5) Prove the accuracy of the average value model: Perform discrete Fourier transforms on the time-domain simulation waveform diagrams of the PV output power obtained from the sub-synchronous oscillation scenarios of the string-type PV detailed model and the average value model systems respectively, and read their oscillation frequencies and oscillation amplitudes. If the difference in oscillation frequencies of the output power waveforms of the two models is within 1 Hz and the difference in oscillation amplitudes is within 5%, then the average value model is accurate.

[0038] The critical instability operating conditions include the proportional gain coefficient of the current loop control and the short-circuit ratio of the station port of the string-type PV power station.

[0039] The beneficial effects of the present invention are as follows: The present invention explores a method for establishing an average value model suitable for the small disturbance stability research of a string-type photovoltaic grid-connected system, realizes the simplified modeling of the detailed modulation process of a traditional converter, can correctly reflect the oscillation stability of the system after small disturbances, can effectively avoid the problems of low simulation efficiency and excessive computational energy consumption caused by the detailed modeling of the converter modulation process, and is applicable to the time-domain simulation analysis of the small disturbance stability under the rapid growth of string-type photovoltaics in a complex AC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 A string-type photovoltaic circuit and its control system in which the detailed model is adopted for the converter modulation process of the present invention;

[0041] Figure 2 A string-type photovoltaic circuit in which the average value model is adopted for the converter modulation process of the present invention;

[0042] Figure 3 A schematic diagram of the string-type photovoltaic grid-connected system of the present invention;

[0043] Figure 4 The small disturbance time-domain simulation result of the system in which the detailed model is adopted for the converter modulation process of the string-type photovoltaics of the present invention;

[0044] Figure 5 The small disturbance time-domain simulation result of the system in which the average value model is adopted for the converter modulation process of the string-type photovoltaics of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0045] The principles and features of the present invention will be described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0046] The embodiment of the present invention provides a method for establishing an average value model suitable for the small disturbance stability research of a string-type photovoltaic grid-connected system. First, a mathematical model is established for the SPWM modulation process of the converter of the string-type photovoltaics, and the delay link of the converter is linearized; then, based on the working principle and mathematical equation of the SPWM modulation, an equivalent model of the AC side and an equivalent model of the DC side of the average value model of the converter are established; finally, a string-type photovoltaic system connected to the grid alone through a two-stage step-up transformer is built in the PSCAD / EMTDC electromagnetic transient simulation software, and the time-domain electromagnetic transient simulation results of two methods of adopting the detailed model and the average value model for the converter modulation are compared under the same small disturbance instability scenario to verify the accuracy of the average value model.

[0047] A method for establishing an average value model suitable for the small disturbance stability research of a string-type photovoltaic grid-connected system includes the following steps:

[0048] (1) Establish a mathematical model for the SPWM modulation process of the converter of the string-type photovoltaics;

[0049] The series-connected photovoltaic circuit and its control system using a detailed model in the converter modulation process are as follows Figure 1 shown. The definitions of the parameter variables in the figure are shown in Table 1. Its typical control and basic principles are relatively clear in existing research, and will not be elaborated in this invention. The establishment of the average value model is actually a simplification of the converter modulation process, that is, simplifying the three-level SPWM modulation process in the detailed model and directly using an equivalent potential source and an equivalent current source for equivalence. The series-connected photovoltaic circuit using the average value model in the converter modulation process is as follows Figure 2 shown. The definitions of the parameter variables in the figure are shown in Table 2.

[0050] Table 1 Main parameters of the series-connected photovoltaic circuit and its control system using a detailed model in the converter modulation process

[0051]

[0052] Table 2 Main parameters of the series-connected photovoltaic circuit using the average value model in the converter modulation process

[0053]

[0054] When the series-connected photovoltaic converter uses SPWM modulation, the actual output voltage u of the converter after modulation is i , as follows

[0055]

[0056] In the formula, K pwm is the voltage equivalent gain when the converter uses the SPWM modulation method; u iref is the reference value of the inverter output voltage; T d is the switching delay time constant of the converter control delay link; U dc is the DC side bus voltage; U c is the carrier amplitude of the modulation process. When U c takes the value of U dc , the voltage equivalent gain K pwm is 0.5;

[0057] The first-order inertia link is used to linearize the converter delay link. After the linearization process of Equation (1), it is as follows

[0058]

[0059] Before and after the linearization process, the amplitude-frequency and phase-frequency characteristics of the two are the same in the sub / super-synchronous frequency band, and can correctly reflect the small disturbance characteristics of the model in the sub / super-synchronous frequency band;

[0060] Considering that the control link of the string-type photovoltaic inverter adopts a vector-based orientation control strategy, and the output voltage reference values are the d and q components. Further decompose Equation (3) into d and q components to obtain the converter modulation mathematical model of the voltage d and q components, as shown in the following equation:

[0061]

[0062] In the formula, u id and u iq are the d-axis component and q-axis component of the converter modulation output voltage respectively; u idref and u iqref are the d-axis component reference value and q-axis component reference value of the converter modulation output voltage respectively.

[0063] (2) According to the converter modulation process mathematical model established in step (1), establish the AC-side equivalent model and DC-side equivalent model of the converter average value model;

[0064] (21) According to the converter modulation process mathematical model established in step (1), establish the AC-side equivalent model of the converter average value model:

[0065] The AC-side equivalent model consists of a three-phase controlled voltage source and a diode three-phase full bridge. Among them, the three-phase controlled voltage sources u sa , u sb , u sc are used to simulate the voltage output result of the converter, and the diode bridge is used to ensure power balance and simulation accuracy when the DC voltage drops below the AC voltage peak;

[0066] To keep the output voltage of the converter consistent before and after equivalence, the input voltages u sa , u sb , u sc of the three-phase controlled voltage source of the AC-side equivalent system are obtained through Park transformation according to the converter modulation output voltages u id , u iq , as shown in the following formula:

[0067]

[0068] In the formula, θ is the inverter phase angle output by the phase-locked loop link; U b is the voltage reference value.

[0069] (22) Establish the DC-side equivalent model according to the AC-side equivalent model of the converter average value model:

[0070] The DC-side equivalent system consists of a controlled DC current source. According to the output power of the controlled three-phase voltage source and the power absorbed by the controlled current source being consistent, the input I dc of the DC controlled current source is as follows:

[0071]

[0072] where \(i\) sa 、 \(i\) sb 、 \(i\) sc are the measured three-phase current values of the inverter output.

[0073] (3) Build a series-connected PV system connected to the grid independently via a two-stage step-up transformer in the PSCAD / EMTDC electromagnetic transient simulation software. Compare the time-domain electromagnetic transient simulation results of the converter modulation using the detailed model and the average value model under the same small-disturbance instability scenario to verify the accuracy of the average value model. The verification steps are as follows:

[0074] 1) Build a series-connected PV system connected to the grid independently via a two-stage step-up transformer in the PSCAD / EMTDC electromagnetic transient simulation software. Consider the converter modulation process using the detailed model and the average value model respectively, and at the same time ensure that the PV array, Boost circuit and its control system, inverter control system, PLL and its control system models of the series-connected PV system are consistent;

[0075] 2) Take the system with the converter modulation process using the detailed model of the series-connected PV as the research object, and construct a sub-synchronous oscillation scenario for the series-connected PV power station: According to the key influencing factors for the sub-synchronous oscillation of the series-connected PV power station, which are usually the proportional gain coefficient of the current loop control and the short-circuit ratio of the station port of the series-connected PV power station, set the proportional gain coefficient of the current loop control as a fixed value, and gradually reduce the short-circuit ratio of the series-connected PV power station until the system has a sub-synchronous oscillation, and record the critical instability operating conditions of the system before and after the oscillation;

[0076] 3) Draw the time-domain simulation waveform diagram of the PV output power of the system with the converter modulation process using the detailed model of the series-connected PV under the critical instability scenario;

[0077] 4) Take the system with the converter modulation process using the average value model of the series-connected PV as the research object, set the operating conditions of the system to be consistent with the critical instability operating conditions before and after the oscillation of the detailed model system, and draw the time-domain simulation waveform diagram of the PV output power;

[0078] 5) Prove the accuracy of the average value model: Perform discrete Fourier transforms on the time-domain simulation waveform diagrams of the PV output power obtained from the sub-synchronous oscillation scenarios of the series-connected PV detailed model and the average value model systems respectively, read their oscillation frequencies and oscillation amplitudes. If the difference in the oscillation frequencies of the output power waveforms of the two models is within 1 Hz and the difference in the oscillation amplitudes is within 5%, then the average value model is accurate.

[0079] The critical instability operating conditions include the proportional gain coefficient of the current loop control and the short-circuit ratio of the substation port of the string-type photovoltaic power station.

[0080] Time-domain simulation verification analysis:

[0081] The effectiveness of applying the average value model of the converter to the small-signal stability study of the string-type photovoltaic grid-connected system is verified and analyzed in the PSCAD / EMTDC electromagnetic transient simulation software. As Figure 3 shown, the example system is a string-type photovoltaic system connected to the grid separately through two-stage step-up transformers. The string-type photovoltaic power station adopts a 100 MW aggregated equivalent model. The circuit parameters and control parameters of the photovoltaic power station are shown in Table 3. When the light intensity G is 800 W / m 2 , the output of the substation is about 0.7 p.u.

[0082] Table 3 Control parameters of the string-type photovoltaic grid-connected system

[0083]

[0084] In the PSCAD / EMTDC electromagnetic transient simulation software, keeping other parameters unchanged, at 2 s, the short-circuit ratio of the substation port of the string-type photovoltaic power station is changed from 3 to 2. The small-signal time-domain simulation results when the detailed model is adopted in the modulation process of the string-type photovoltaic converter are as Figure 4 shown, Figure 4 in which (a) is the active power waveform diagram of the photovoltaic power station output, Figure 4 in which (b) is the FFT analysis diagram of the active power output of the photovoltaic power station, and a power oscillation of 31.4 Hz appears in the system.

[0085] Keeping the operating conditions and control parameters the same, the small-signal time-domain simulation results when the average value model is adopted for the string-type photovoltaic converter are as Figure 5 shown, Figure 5 in which (a) is the active power waveform diagram of the photovoltaic power station output, Figure 5 in which (b) is the FFT analysis diagram of the active power output of the photovoltaic power station, a power oscillation of 30.76 Hz appears in the system, the error of the oscillation frequency from that of the detailed model is within 1 Hz, and the difference of the oscillation amplitude is within 5%, verifying the accuracy of the average value model.

Claims

1. A method for establishing an average value model applicable to the study of small-signal stability of a string-type photovoltaic grid-connected system, characterized in that, It includes the following steps: (1) Mathematically model the SPWM modulation process of the converter in the string-type photovoltaic system; (2) Based on the mathematical model of the converter modulation process established in step (1), establish the AC-side equivalent model and DC-side equivalent model of the converter average value model; (3) Build a string-type photovoltaic system connected to the grid separately through a two-stage step-up transformer in the PSCAD / EMTDC electromagnetic transient simulation software. Compare the time-domain electromagnetic transient simulation results of the converter modulation using the detailed model and the average value model under the same small disturbance instability scenario to verify the accuracy of the average value model.

2. A method for establishing an average value model applicable to the study of small-signal stability of a string-type photovoltaic grid-connected system, characterized in that, (1) Step (1) specifically includes: When the string type photovoltaic inverter adopts SPWM modulation, the actual output voltage u of the inverter after modulation is i , as shown in the following formula: Where K pwm is the voltage equivalent gain when the converter adopts the SPWM modulation method; u iref is the reference value of the inverter output voltage; T d is the switching delay time constant of the converter control delay link; U dc is the DC side bus voltage; U c is the carrier amplitude of the modulation process. When U c takes the value of U dc , the voltage equivalent gain K pwm is 0.5; Use a first-order inertia link to linearize the delay link of the converter. After the linearization process in formula (1), it is as follows: Before and after the linearization process, the amplitude-frequency and phase-frequency characteristics of both in the sub / super-synchronous frequency band are the same, which can correctly reflect the small disturbance characteristics of the model in the sub / super-synchronous frequency band; Perform d, q decomposition on formula (3) to obtain the mathematical model of the converter modulation process of the voltage d, q components, as follows: where, u id and u iq are the d-axis component and q-axis component of the converter modulation output voltage respectively; u idref and u iqref are the reference values of the d-axis component and q-axis component of the converter modulation output voltage respectively.

3. A method for establishing an average value model for small disturbance stability research of a string-type photovoltaic grid-connected system according to claim 1, characterized in that, (2) Step (2) includes the following steps: (21) Based on the mathematical model of the converter modulation process established in step (1), establish the AC-side equivalent model of the converter average value model; (22) Establish the DC-side equivalent model based on the AC-side equivalent model of the converter average value model.

4. A method for establishing an average value model applicable to the small-signal stability study of a string-type photovoltaic grid-connected system, as claimed in claim 3, wherein Based on the mathematical model of the converter modulation process established in step (1), establish the AC-side equivalent model of the converter average value model, specifically: The AC-side equivalent model consists of a three-phase controlled voltage source and a three-phase full-bridge diode rectifier. Among them, the three-phase controlled voltage sources u sa 、u sb 、u sc are used to simulate the voltage output results of the converter, and the diode bridge is used to ensure power balance and simulation accuracy when the DC voltage drops below the peak value of the AC voltage; To keep the output voltage of the converter consistent before and after equivalence, the input voltages u sa 、u sb 、u sc of the three-phase controlled voltage sources of the AC-side equivalent system are obtained from the converter modulation output voltage u id 、u iq through Park transformation, as shown in the following formula: where θ is the inverter phase angle output by the phase-locked loop link; U b is the voltage reference value.

5. The method for establishing an average value model applicable to the study of small disturbance stability of a string-type photovoltaic grid-connected system according to claim 3, characterized in that Establish the DC-side equivalent model based on the AC-side equivalent model of the converter average value model, specifically including: The DC-side equivalent system consists of a controlled DC current source. According to the output power of the controlled three-phase voltage source and the power absorbed by the controlled current source being kept consistent, the input I of the DC controlled current source dc , is as follows: where, i sa , i sb , i sc are the measured three-phase current values of the inverter output.

6. The method for establishing an average value model for small disturbance stability research of a string-type photovoltaic grid-connected system according to claim 1, wherein (3) Step (3) specifically includes: 1) Build a string-type photovoltaic system connected to the grid separately through a two-stage step-up transformer in the PSCAD / EMTDC electromagnetic transient simulation software. Consider the converter modulation process using the detailed model and the average value model respectively, and at the same time ensure that the models of the photovoltaic array, Boost circuit and its control system, inverter control system, phase-locked loop and its control system of the string-type photovoltaic system are consistent; 2) Take the system with the detailed model of the converter modulation process in the string-type photovoltaic system as the research object, and construct a sub-synchronous oscillation scenario for the string-type photovoltaic power station: According to the key influencing factors for the sub-synchronous oscillation of the string-type photovoltaic power station, which are usually the proportional gain coefficient of the current loop control and the short-circuit ratio of the station port of the string-type photovoltaic power station, set the proportional gain coefficient of the current loop control as a fixed value, and gradually reduce the short-circuit ratio of the string-type photovoltaic power station until the system undergoes sub-synchronous oscillation, and record the critical instability operating conditions of the system before and after the oscillation; 3) Draw the time-domain simulation waveform diagram of the photovoltaic output power of the system with the detailed model of the converter modulation process in the string-type photovoltaic power station under the critical instability scenario; 4) Take the system with the average value model of the converter modulation process in the string-type photovoltaic system as the research object, set the operating conditions of the system to be the same as the critical instability operating conditions before and after the oscillation of the detailed model system, and draw the time-domain simulation waveform diagram of the photovoltaic output power. 5) Prove the accuracy of the average value model: Perform discrete Fourier transforms on the time-domain simulation waveforms of the PV output power obtained from the detailed model of series-connected PV and the average value model for the sub-synchronous oscillation scenario of the system, and read their oscillation frequencies and amplitudes. If the difference in oscillation frequencies of the output power waveforms of the two models is within 1 Hz and the difference in oscillation amplitudes is within 5%, then the average value model is accurate.

7. A method for establishing an average value model for small-signal stability study of a string-type photovoltaic grid-connected system according to claim 6, characterized in that: The critical instability operating conditions include the proportional gain coefficient of the current loop control and the short-circuit ratio at the substation port of the series-connected PV power station.

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