A method for evaluating transient stability of phase-locked loop of inverter-type power supply in series compensation power grid based on voltage analytic decomposition

Abstract

A transient stability evaluation method of phase-locked loop of inverter-type power supply in series compensation power grid based on voltage analytical decomposition, comprising the following steps: step 1, calculating the equivalent comprehensive internal impedance Z of the new energy field station containing the inverter eq ; step 2, constructing the frequency domain linearization equivalent impedance of the series compensation device; step 3, calculating the IBR internal current source response time domain voltage, calculating the grid Thevenin voltage source response time domain voltage, superimposing to generate the time domain voltage component of the grid connection point; step 4: calculating and extracting the analytical term of the q-axis voltage of the grid connection point: Park coordinate transformation is carried out on the time domain voltage component, and the q-axis voltage component v sq used for driving the phase-locked loop control loop is extracted, and the analytical decomposition is three voltage terms; step 5: transient stability boundary evaluation: constructing an absolute value inequality for algebraic determination; the application can directly and quickly determine the transient stability boundary of the phase-locked loop of the inverter-type power supply in the series compensation power grid during the fault, and meets the demand of real-time online evaluation and control of the smart grid.

Description

Technical Field

[0001] This invention belongs to the field of power system stability analysis and evaluation technology, specifically relating to a transient stability evaluation method for inverter power source phase-locked loop in a series-compensated power grid based on analytical decomposition. Technical Background

[0002] In modern power systems, the penetration rate of inverter-driven power sources (IBRs) such as wind and solar power is rapidly increasing. The vast majority of industrial-grade grid-connected inverters use phase-locked loops (PLLs) to track the grid voltage phase and maintain synchronization. Meanwhile, to improve the power transmission capacity of existing long-distance transmission lines, series capacitor compensation technology (series compensation) is often used on the grid side. Because series capacitors are prone to overvoltage during grid short-circuit faults, series compensation devices are typically configured with nonlinear protection components such as metal oxide varistors (MOVs) in parallel. However, existing methods fail to accurately quantify the harmful coupling mechanism between the nonlinear characteristics of series capacitors and MOVs and the dynamics of PLLs during faults, making it difficult for dispatch systems to predict PLL instability risks and easily leading to large-scale inverter disconnection accidents. Summary of the Invention

[0003] This invention provides a method for evaluating the transient stability of inverter power supplies under series compensation. Based on the evaluation method of voltage analytical decomposition and algebraic inequality, the method directly and quickly determines the transient stability boundary of the inverter power supply phase-locked loop during a fault by decomposing the grid connection point voltage into three independent components with clear physical meaning, thus meeting the needs of real-time online evaluation and control of smart grids.

[0004] The technical solution of this invention is: a method for evaluating the transient stability of a phase-locked loop (PLL) for an inverter-type power supply in a series-compensated power grid based on voltage analytical decomposition, comprising the following steps:

[0005] Step 1: Extraction of equivalent parameters and initialization of current source for new energy power plants with inverters

[0006] Collect physical parameters of new energy power plants containing inverters, and calculate the equivalent integrated internal impedance Z of the new energy power plants containing inverters. eq ;

[0007] Based on the active / reactive current command settings in the inverter's low-voltage ride-through control strategy, the initial phase angle offset of the current relative to the d-axis of the phase-locked loop is determined. ;

[0008] Step 2: Construct the frequency domain linearized equivalent impedance of the series compensation device

[0009] Extracting the instantaneous estimated angular frequency of a phase-locked loop during a transient process. ;

[0010] The instantaneous estimated angular frequency is calculated using the Goldsworthy linearized model. The series complement of complex linearized impedance ;

[0011] Step 3: Construct a dual-frequency excitation-response circuit based on the superposition principle

[0012] Calculate the time-domain voltage response of the internal current source of the IBR, calculate the time-domain voltage response of the Thevenin voltage source of the power grid, and superimpose them to generate the time-domain voltage component at the grid connection point;

[0013] Step 4: Calculate and extract the analytical term of the q-axis voltage at the grid connection point.

[0014] Park coordinate transformation is performed on the time-domain voltage components after grid connection point superposition to extract the q-axis voltage component v used to drive the phase-locked loop control loop. sq The analysis decomposes the voltage into three terms: synchronization voltage term, desynchronization voltage term, and series compensation voltage term.

[0015] Step 5: Transient stability boundary assessment

[0016] Extract the three voltage terms from step 4, construct an absolute value inequality, and perform algebraic judgment to determine whether the amplitude of the synchronization voltage term is not less than the sum of the amplitudes of the desynchronization voltage term and the series compensation voltage term.

[0017] If the conditions are met, it is determined that the amplitude of the synchronous voltage provided by the equivalent grid is sufficient to offset the synchronization effect, the phase-locked loop can recapture the phase, and the inverter control system remains stable.

[0018] If this condition is not met, regardless of the bandwidth and parameters of the inverter control system, the q-axis voltage component v used to drive the phase-locked loop control loop will be... sq If the value cannot be returned to zero across the entire real number field, the phase-locked loop will lose synchronization.

[0019] In step 1,

[0020] Collect physical parameters of new energy power plants containing inverters, and determine the number of inverter units n. u Collecting circuit resistor R clc and reactance X clc and internal transformer leakage reactance X su Leakage reactance X of main transformer cp The equivalent integrated internal impedance Z of a new energy power station including an inverter eq , where R eq and X eq These are equivalent resistance and equivalent reactance, respectively.

[0021] ;

[0022] Based on the active / reactive current command settings in the inverter's low-voltage ride-through control strategy, the d-axis current reference value in the synchronous rotating coordinate system is... and q-axis current reference value Determine the initial phase angle offset of the current relative to the d-axis of the phase-locked loop. ,

[0023] ;

[0024] In step 2, since the metal oxide varistor exhibits nonlinear conduction during the fault period, the Goldsworthy linearization model is used to linearize the series capacitor and its parallel nonlinear metal oxide varistor, and the instantaneous estimated angular frequency is calculated. The series complement of complex linearized impedance :

[0025] ;

[0026] in, and These are the extracted equivalent resistance and equivalent reactance of the metal oxide varistor, respectively. This operation transforms the nonlinear element into a linear impedance to which the superposition theorem applies; where... To instantaneously estimate the voltage of the series capacitor near the new energy source side at the angular frequency, To instantaneously estimate the voltage of the series capacitor near the equivalent grid side at the angular frequency, This is the phasor of the inverter unit current at the instantaneous estimated angular frequency.

[0027] In step 3,

[0028] First, construct the equivalent impedance of the dual-frequency loop.

[0029] Collect physical parameters of the power grid, including the total impedance Z of the lines. l The Thevenin voltage of the equivalent grid V Th and equivalent impedance Z Th It also collects system fault parameters, including fault distance m and fault resistance R. f Calculate the inverter-side impedance respectively. and grid-side impedance ;

[0030] Among them, inverter-side impedance The equivalent integrated internal impedance Z of a new energy power station containing an inverter eq It consists of the line impedance from the fault point to the new energy power station containing the inverter connected in series;

[0031] ;

[0032] Where m is the ratio of fault distance to total line length, R l X is the line resistance. l For line reactance;

[0033] Grid-side impedance It consists of the line impedance from the fault point to the grid side and the Thevenin equivalent impedance of the equivalent grid, connected in series.

[0034] ;

[0035] Among them, R Th X is the resistive part of the Thevenin impedance. Th The reactance component of the Thevenin impedance; the current injected through the inverter flows through the grid-side impedance and is shunt after reaching the fault point, therefore the total impedance at the inverter-side output is... , is the fault resistor R f and grid-side impedance The parallel connection is obtained by connecting it in series with the inverter-side impedance.

[0036] ;

[0037] Among them, R r for The resistance part, X r (ω est )for The reactance part, R e for The resistance part, X e for The reactance part, for amplitude, for The phase angle;

[0038] Then, based on the fault resistance, the fundamental frequency is calculated. Fault voltage division factor of the grid voltage referred to the grid connection point ,

[0039] ;

[0040] Where R r for The resistor part, for Reactance at the fundamental frequency for amplitude, for The phase angle;

[0041] Finally, the faulty line is equivalent to a superimposed circuit excited by two independent frequency sources. The frequency domain response is solved separately and then superimposed in the time domain.

[0042] First, the response of the internal current source: given a frequency of... Total inverter current Individual excitation. Calculate the total impedance flowing through the inverter-side outlet. and series-complement complex linearized impedance The phasor and time-domain voltage component of the resulting voltage drop component;

[0043] ;

[0044] ;

[0045] in, for phase angle, for phase angle, for The phase;

[0046] Second, the response of the external power grid source: determined by the actual power grid fundamental frequency. Thevenin equivalent voltage V Th (ω est Individual excitation, calculate the fault voltage divider coefficient of the grid voltage. The phasor and time-domain voltage components of the voltage components after conversion to the grid connection point.

[0047] ;

[0048] ;

[0049] in, Thevenin equivalent voltage phase angle, This is the fault voltage divider coefficient. for phase angle, V Th (ω est () represents the Thevenin equivalent voltage;

[0050] According to the superposition principle, the final time-domain voltage component v after superposition at the grid connection point s (t) is: .

[0051] In step 4

[0052] Perform Park transform on the time-domain voltage components after superposition at the grid connection point in step 3 to extract the q-axis voltage component v used to drive the phase-locked loop control loop. sq It can be decomposed into the sum of three voltage terms.

[0053] ;

[0054] Among them, the synchronization voltage term V u1 : Represents the voltage support capability provided by the external power grid, and its amplitude is ,

[0055] ;

[0056] in, for phase angle, For instantaneous estimation of voltage phase angle by a phase-locked loop, The thevenin equivalent voltage phase angle.

[0057] Remove the synchronization voltage term V u2 : Represents the negative offset voltage generated by the IBR current flowing through the system's conventional impedance, and its analytical expression is,

[0058] ;

[0059] in, for phase angle, The inverter current offset angle relative to the initial phase angle of the phase-locked loop d-axis;

[0060] Series compensation voltage term V u3 This specifically represents the coupling voltage generated by the series compensation capacitor and its MOV protection device. Its analytical expression is:

[0061] ;

[0062] in, impedance phase angle, for The amplitude.

[0063] In step 5

[0064] First, an amplitude judgment is performed, since a necessary condition for the phase-locked loop to maintain stability is the existence of a certain amplitude. Enabling Therefore, the three voltage terms in step 4 are extracted and their absolute values ​​are compared. The following inequality is calculated and determined to be true. (The validity of this inequality can directly evaluate the transient stability boundary of the phase-locked loop.)

[0065] ;

[0066] If the inequality holds, it is determined that the amplitude of the synchronous voltage provided by the current equivalent grid is sufficient to offset the synchronization effect, the phase-locked loop can recapture the phase, and the inverter control system remains stable.

[0067] If the inequality does not hold, regardless of how the bandwidth and parameters of the inverter control system are designed, v sq If the value cannot be returned to zero across the entire real number field, the phase-locked loop will lose synchronization.

[0068] A transient stability assessment system for inverter-type power supply phase-locked loops in a series-compensated power grid based on voltage analytical decomposition includes the following modules:

[0069] Module for Extracting Equivalent Parameters and Initializing Current Sources for New Energy Power Stations with Inverters

[0070] Collect physical parameters of new energy power plants containing inverters, and calculate the equivalent integrated internal impedance Z of the new energy power plants containing inverters. eq ;

[0071] Based on the active / reactive current command settings in the inverter's low-voltage ride-through control strategy, the initial phase angle offset of the current relative to the d-axis of the phase-locked loop is determined. ;

[0072] Frequency Domain Linearization Equivalent Impedance Construction Module for Series Compensation Devices

[0073] Extracting the instantaneous estimated angular frequency of a phase-locked loop during a transient process. ;

[0074] The instantaneous estimated angular frequency is calculated using the Goldsworthy linearized model. The series complement of complex linearized impedance ;

[0075] A dual-frequency excitation-response circuit module is constructed based on the superposition principle.

[0076] Calculate the time-domain voltage response of the internal current source of the IBR, calculate the time-domain voltage response of the Thevenin voltage source of the power grid, and superimpose them to generate the time-domain voltage component at the grid connection point;

[0077] Module for calculating and extracting the analytical term of the q-axis voltage at the grid connection point

[0078] Park coordinate transformation is performed on the time-domain voltage components after grid connection point superposition to extract the q-axis voltage component v used to drive the phase-locked loop control loop. sq The analysis decomposes the voltage into three terms: synchronization voltage term, desynchronization voltage term, and series compensation voltage term.

[0079] Transient stability boundary assessment module

[0080] Extract the three voltage terms from step 4, construct an absolute value inequality, and perform algebraic judgment to determine whether the amplitude of the synchronization voltage term is not less than the sum of the amplitudes of the desynchronization voltage term and the series compensation voltage term.

[0081] If the conditions are met, it is determined that the amplitude of the synchronous voltage provided by the equivalent grid is sufficient to offset the synchronization effect, the phase-locked loop can recapture the phase, and the inverter control system remains stable.

[0082] If this condition is not met, regardless of the bandwidth and parameters of the inverter control system, the q-axis voltage component v used to drive the phase-locked loop control loop will be... sq If the value cannot be returned to zero across the entire real number field, the phase-locked loop will lose synchronization.

[0083] The overall process of this invention is as follows: After obtaining the system's physical and fault parameters, the new energy power station containing the inverter is equivalent to a constant current source, and the nonlinear effect of the MOV is handled by a linearized impedance model; the superposition theorem is used to calculate the voltage components generated by the grid support voltage and the new energy output current in the fault loop; finally, the q-axis voltage of the grid connection point is strictly decomposed into "synchronization voltage term", "desynchronization voltage term" and "series compensation voltage term" in the dq coordinate system, and the physical feasibility of the PLL to maintain synchronization is directly evaluated by algebraic inequalities.

[0084] The beneficial effects of this invention are:

[0085] First, the grid connection point q-axis voltage is strictly and physically decomposed into three parts: "synchronization voltage term", "desynchronization voltage term" and "series compensation voltage term". This accurately quantifies the harmful coupling mechanism between series capacitor, MOV nonlinear characteristics and PLL dynamics during low voltage ride-through, effectively avoiding large-scale inverter grid disconnection accidents caused by misjudging stability in the dispatching system.

[0086] Secondly, it accurately quantifies the multi-frequency coupling effect between the MOV nonlinear characteristics and the PLL dynamic frequency, enabling instantaneous estimation of the angular frequency in the phase-locked loop. The Goldsworthy model is applied to linearize the MOV and series compensation capacitor, and a dual-frequency excitation response circuit is constructed using the superposition theorem. This accurately restores the nonlinear coupling characteristics of the series compensation capacitor and the new energy control link during the transient process, and solves the problem of evaluation distortion in existing methods.

[0087] Third, the invention replaces electromagnetic transient simulation with algebraic criteria, greatly reducing computational complexity. This invention transforms the complex process of finding dynamic instability boundaries into evaluating whether the algebraic equations at transient operating points have real solutions, reducing computation time from the traditional minutes / hours to milliseconds, thus meeting the requirements for online evaluation. Attached Figure Description

[0088] Figure 1 It is the system configuration structure of the transient stability assessment device;

[0089] Figure 2This is a flowchart of a transient stability assessment method for inverter-type power supply phase-locked loops in a series-compensated power grid. Detailed Implementation

[0090] like Figure 1 The transient stability assessment device system configuration includes a new energy power station with an inverter, a transmission line side with a series compensation capacitor, and an equivalent power grid. The inverter output is sequentially stepped up by an internal transformer, combined by a collection circuit, and stepped up again by the main transformer before being transmitted to the grid connection point. The phase-locked loop (PLL) provides control commands to the inverter to maintain system synchronization by collecting the voltage phase at the grid connection point.

[0091] like Figure 2 The specific solution of the present invention is as follows:

[0092] Step 1: Extraction of equivalent parameters and initialization of current source for new energy power plants with inverters

[0093] Collect physical parameters of new energy power plants containing inverters, including the number of inverter units n. u Collecting circuit resistor R clc and reactance X clc and internal transformer leakage reactance X su Leakage reactance of main transformer X cp The equivalent integrated internal impedance Z of a new energy power station including an inverter eq , where R eq and X eq These are equivalent resistance and equivalent reactance, respectively.

[0094] ;

[0095] Active / reactive current command settings based on inverter low voltage ride-through (LVRT) control strategy (d-axis current reference value in synchronous rotating coordinate system) and q-axis current reference value Determine the initial phase angle offset of the current relative to the d-axis of the PLL. ,

[0096]

[0097] Step 2: Construct the frequency domain linearized equivalent impedance of the series compensation device

[0098] Extracting the instantaneous estimated angular frequency of a phase-locked loop (PLL) during a transient process. ;

[0099] Since metal oxide varistors (MOVs) exhibit nonlinear conduction during faults, the Goldsworthy linearization model is used to linearize the series capacitor and its parallel nonlinear MOV, and the instantaneous estimated angular frequency is calculated. The series complement of complex linearized impedance :

[0100] ;

[0101] in, and These are the extracted equivalent resistance and equivalent reactance of the metal oxide varistor, respectively. This operation transforms the nonlinear element into a linear impedance to which the superposition theorem applies. To instantaneously estimate the voltage of the series capacitor near the new energy source side at the angular frequency, To instantaneously estimate the voltage of the series capacitor near the equivalent grid side at the angular frequency, This is the phasor of the inverter unit current at the instantaneous estimated angular frequency.

[0102] Step 3: Construct a dual-frequency excitation-response circuit based on the superposition principle

[0103] First, construct the equivalent impedance of the dual-frequency loop. Collect the physical parameters of the power grid, including the total line impedance Z. l The Thevenin voltage of the equivalent grid V Th and equivalent impedance Z Th It also collects system fault parameters, including fault distance m and fault resistance R. f Calculate the inverter-side impedance. and grid-side impedance .

[0104] Among them, inverter-side impedance The equivalent integrated internal impedance Z of a new energy power station containing an inverter eq It consists of the line impedance from the fault point to the new energy power station containing the inverter connected in series.

[0105]

[0106] Where m is the ratio of fault distance to total line length, R l X is the line resistance. l This refers to the line reactance.

[0107] Grid-side impedance It consists of the line impedance from the fault point to the grid side and the Thevenin equivalent impedance of the equivalent grid, connected in series.

[0108] ;

[0109] Among them, R Th X is the resistive part of the Thevenin impedance. Th This represents the reactance portion of the Thevenin impedance. The current injected through the inverter flows through the internal impedance and is shunt at the fault point; therefore, the total impedance at the inverter-side outlet is... , is the fault resistor R f and grid-side impedance The parallel connection is obtained by connecting it in series with the inverter-side impedance.

[0110] ;

[0111] Among them, R r for The resistance part, X r (ω est )for The reactance part, R e for The resistance part, X e for The reactance part, for amplitude, for The phase angle.

[0112] Then, based on the fault resistance, the fundamental frequency is calculated. Fault voltage division factor of the grid voltage referred to the grid connection point

[0113]

[0114] Where R r for The resistor part, for Reactance at the fundamental frequency for amplitude, for The phase angle.

[0115] Finally, the complex power grid under fault conditions is equivalent to a superimposed circuit excited by two independent frequency sources. The frequency domain response is solved separately and then superimposed in the time domain.

[0116] First, the response of the internal current source: given a frequency of... Total inverter current Individual excitation. Calculate the total impedance flowing through the inverter-side outlet. and series-complementary complex linearized impedance The resulting voltage drop component is a phasor of the time-domain voltage component.

[0117]

[0118]

[0119] in, for phase angle, for phase angle, for The phase.

[0120] Second, the response of the external power grid source: determined by the actual power grid fundamental frequency. Thevenin equivalent voltage V Th (ω est Individual excitation. Calculate the fault voltage divider coefficient of the grid voltage. The phasor and time-domain voltage components of the voltage components after being converted to the grid connection point.

[0121]

[0122]

[0123] in, Thevenin equivalent voltage phase angle, This is the fault voltage divider coefficient. for phase angle, V Th (ω est ) represents the Thevenin equivalent voltage.

[0124] According to the superposition principle, the final time-domain voltage component v after superposition at the grid connection point s (t) is the sum of the time-domain voltage components of the above two items.

[0125]

[0126] Step 4: Calculate and extract the analytical term of the q-axis voltage at the grid connection point.

[0127] The Park transform is applied to the time-domain expression of the superimposed three-phase voltage at the grid connection point to extract the q-axis voltage component v used to drive the phase-locked loop control loop. sq It can be precisely decomposed into the sum of three independent and physically meaningful voltage terms.

[0128]

[0129] Among them, the synchronization voltage term (V u1 ): Represents the voltage support capability provided by the external power grid. Its amplitude is ,

[0130] ,

[0131] in, for phase angle, For instantaneous estimation of voltage phase angle by a phase-locked loop, The thevenin equivalent voltage phase angle.

[0132] Remove synchronization voltage term (V) u2 ): This represents the negative offset voltage generated by the IBR current flowing through the system's conventional impedance. The analytical expression is,

[0133]

[0134] in, for phase angle, The inverter current offset angle relative to the initial phase angle of the phase-locked loop d-axis.

[0135] Series compensation voltage term (V u3 ): This specifically represents the coupling voltage generated by the series compensation capacitor and its MOV protection device. The analytical expression is,

[0136]

[0137] in, impedance phase angle, for The amplitude.

[0138] Step 5: Transient stability boundary assessment

[0139] First, an amplitude judgment is performed, since a necessary condition for the phase-locked loop to maintain stability is the existence of a certain amplitude. Enabling Therefore, the three voltage terms from step 4 are extracted and their absolute value inequalities are compared. The following inequalities are calculated and determined to be valid, serving as the boundary conditions for directly evaluating the transient stability of the phase-locked loop.

[0140]

[0141] If the inequality holds, it is determined that the amplitude of the synchronous voltage provided by the equivalent grid is sufficient to offset the synchronization effect, the phase-locked loop can recapture the phase, and the inverter control system remains stable.

[0142] If the inequality does not hold, regardless of how the bandwidth and parameters of the inverter control system are designed, v sq If the value cannot be returned to zero across the entire real number field, the phase-locked loop will lose synchronization.

Claims

1. A method for evaluating the transient stability of a phase-locked loop (PLL) for an inverter-type power source in a series-compensated power grid based on voltage analytical decomposition, characterized in that: Includes the following steps: Step 1: Extraction of equivalent parameters and initialization of current source for new energy power plants with inverters Collect physical parameters of new energy power plants containing inverters, and calculate the equivalent integrated internal impedance Z of the new energy power plants containing inverters. eq ; Based on the active / reactive current command settings in the inverter's low-voltage ride-through control strategy, the initial phase angle offset of the current relative to the d-axis of the phase-locked loop is determined. ; Step 2: Construct the frequency domain linearized equivalent impedance of the series compensation device Extracting the instantaneous estimated angular frequency of a phase-locked loop during a transient process. ; The instantaneous estimated angular frequency is calculated using the Goldsworthy linearized model. The series complement of complex linearized impedance ; Step 3: Construct a dual-frequency excitation-response circuit based on the superposition principle Calculate the time-domain voltage response of the internal current source of the IBR, calculate the time-domain voltage response of the Thevenin voltage source of the power grid, and superimpose them to generate the time-domain voltage component at the grid connection point; Step 4: Calculate and extract the analytical term of the q-axis voltage at the grid connection point. Park coordinate transformation is performed on the time-domain voltage components after grid connection point superposition to extract the q-axis voltage component v used to drive the phase-locked loop control loop. sq The analysis decomposes the voltage into three terms: synchronization voltage term, desynchronization voltage term, and series compensation voltage term. Step 5: Transient stability boundary assessment Extract the three voltage terms from step 4, construct an absolute value inequality, and perform algebraic judgment to determine whether the amplitude of the synchronization voltage term is not less than the sum of the amplitudes of the desynchronization voltage term and the series compensation voltage term. If the conditions are met, it is determined that the amplitude of the synchronous voltage provided by the equivalent grid is sufficient to offset the synchronization effect, the phase-locked loop can recapture the phase, and the inverter control system remains stable. If this condition is not met, regardless of how the bandwidth and parameters of the inverter control system are designed, the q-axis voltage component v used to drive the phase-locked loop control loop will be... sq If the value cannot be returned to zero across the entire real number field, the phase-locked loop will lose synchronization.

2. The transient stability assessment method for inverter-type power supply phase-locked loop in a series-compensated power grid based on voltage analytical decomposition according to claim 1, characterized in that: In step 1, Collect physical parameters of new energy power plants containing inverters, and determine the number of inverter units n. u Collecting circuit resistor R clc and reactance X clc and internal transformer leakage reactance X su Leakage reactance X of main transformer cp The equivalent integrated internal impedance Z of a new energy power station including an inverter eq , where R eq and X eq These are equivalent resistance and equivalent reactance, respectively. ； Based on the active / reactive current command settings in the inverter's low-voltage ride-through control strategy, the d-axis current reference value in the synchronous rotating coordinate system is... and q-axis current reference value Determine the initial phase angle offset of the current relative to the d-axis of the phase-locked loop. , ； In step 2, since the metal oxide varistor exhibits nonlinear conduction during the fault period, the Goldsworthy linearization model is used to linearize the series capacitor and its parallel nonlinear metal oxide varistor, and the instantaneous estimated angular frequency is calculated. The series complement of complex linearized impedance : ； in, and These are the equivalent resistance and equivalent reactance of the extracted metal oxide varistor, respectively. This operation transforms the nonlinear element into a linear impedance to which the superposition theorem can be applied. in To instantaneously estimate the voltage of the series capacitor near the new energy source side at the angular frequency, To instantaneously estimate the voltage of the series capacitor near the equivalent grid side at the angular frequency, This is the phasor of the inverter unit current at the instantaneous estimated angular frequency.

3. The transient stability assessment method for inverter-type power supply phase-locked loop in a series-compensated power grid based on voltage analytical decomposition according to claim 2, characterized in that: In step 3, First, construct the equivalent impedance of the dual-frequency loop. Collect physical parameters of the power grid, including the total impedance Z of the lines. l The Thevenin voltage of the equivalent grid V Th and equivalent impedance Z Th It also collects system fault parameters, including fault distance m and fault resistance R. f Calculate the inverter-side impedance respectively. and grid-side impedance ; Among them, inverter-side impedance The equivalent integrated internal impedance Z of a new energy power station containing an inverter eq It consists of the line impedance from the fault point to the new energy power station containing the inverter connected in series; ； Where m is the ratio of fault distance to total line length, R l X is the line resistance. l For line reactance; Grid-side impedance It consists of the line impedance from the fault point to the grid side and the Thevenin equivalent impedance of the equivalent grid, connected in series. ； Among them, R Th X is the resistive part of the Thevenin impedance. Th The reactance component of the Thevenin impedance; the current injected through the inverter flows through the grid-side impedance and is shunt after reaching the fault point, therefore the total impedance at the inverter-side output is... , is the fault resistor R f and grid-side impedance The parallel connection is obtained by connecting it in series with the inverter-side impedance. ； Among them, R r for The resistance part, X r (ω est )for The reactance part, R e for The resistance part, X e for The reactance part, for amplitude, for The phase angle; Then, based on the fault resistance, the fundamental frequency is calculated. Fault voltage division factor of the grid voltage referred to the grid connection point , ； Where R r for The resistor part, for Reactance at the fundamental frequency for amplitude, for The phase angle; Finally, the faulty line is equivalent to a superimposed circuit excited by two independent frequency sources. The frequency domain response is solved separately and then superimposed in the time domain. First, the response of the internal current source: given a frequency of... Total inverter current Individual excitation. Calculate the total impedance flowing through the inverter-side outlet. and series-complement complex linearized impedance The phasor and time-domain voltage component of the resulting voltage drop component; ； ； in, for phase angle, for phase angle, for The phase; Second, the response of the external power grid source: determined by the actual power grid fundamental frequency. Thevenin equivalent voltage V Th (ω est Individual excitation, calculate the fault voltage divider coefficient of the grid voltage. The phasor and time-domain voltage components of the voltage components after conversion to the grid connection point. ； ； in, Thevenin equivalent voltage phase angle, This is the fault voltage divider coefficient. for phase angle, V Th (ω est () represents the Thevenin equivalent voltage; According to the superposition principle, the final time-domain voltage component v after superposition at the grid connection point s (t) is: .

4. The transient stability assessment method for inverter-type power supply phase-locked loop in a series-compensated power grid based on voltage analytical decomposition according to claim 3, characterized in that: In step 4 Perform Park transform on the time-domain voltage components after superposition at the grid connection point in step 3 to extract the q-axis voltage component v used to drive the phase-locked loop control loop. sq It can be decomposed into the sum of three voltage terms. ； Among them, the synchronization voltage term V u1 : Represents the voltage support capability provided by the external power grid, and its amplitude is , ； in, for phase angle, For instantaneous estimation of voltage phase angle by a phase-locked loop, The Thevenin equivalent voltage phase angle; Remove the synchronization voltage term V u2 : Represents the negative offset voltage generated by the IBR current flowing through the system's conventional impedance, and its analytical expression is, ； in, for phase angle, The inverter current offset angle relative to the initial phase angle of the phase-locked loop d-axis; Series compensation voltage term V u3 This specifically represents the coupling voltage generated by the series compensation capacitor and its MOV protection device. Its analytical expression is: ； in, impedance phase angle, for The amplitude.

5. The transient stability assessment method for inverter-type power supply phase-locked loop in a series-compensated power grid based on voltage analytical decomposition according to claim 4, in step 5... First, an amplitude judgment is performed, since a necessary condition for the phase-locked loop to maintain stability is the existence of a certain amplitude. Enabling Therefore, the three voltage terms in step 4 are extracted and their absolute values ​​are compared. The following inequality is calculated and determined to be true. (The validity of this inequality can directly evaluate the transient stability boundary of the phase-locked loop.) ； If the inequality holds, it is determined that the amplitude of the synchronous voltage provided by the current equivalent grid is sufficient to offset the synchronization effect, the phase-locked loop can recapture the phase, and the inverter control system remains stable. If the inequality does not hold, regardless of how the bandwidth and parameters of the inverter control system are designed, v sq If the value cannot be returned to zero across the entire real number field, the phase-locked loop will lose synchronization.

6. A transient stability assessment system for inverter-type power supply phase-locked loops in a series-compensated power grid based on voltage analytical decomposition, characterized in that: Includes the following modules: Module for Extracting Equivalent Parameters and Initializing Current Sources for New Energy Power Stations with Inverters Collect physical parameters of new energy power plants containing inverters, and calculate the equivalent integrated internal impedance Z of the new energy power plants containing inverters. eq ; Based on the active / reactive current command settings in the inverter's low-voltage ride-through control strategy, the initial phase angle offset of the current relative to the d-axis of the phase-locked loop is determined. ; Frequency Domain Linearization Equivalent Impedance Construction Module for Series Compensation Devices Extracting the instantaneous estimated angular frequency of a phase-locked loop during a transient process. ; The instantaneous estimated angular frequency is calculated using the Goldsworthy linearized model. The series complement of complex linearized impedance ; A dual-frequency excitation-response circuit module is constructed based on the superposition principle. Calculate the time-domain voltage response of the internal current source of the IBR, calculate the time-domain voltage response of the Thevenin voltage source of the power grid, and superimpose them to generate the time-domain voltage component at the grid connection point; Module for calculating and extracting the analytical term of the q-axis voltage at the grid connection point Park coordinate transformation is performed on the time-domain voltage components after grid connection point superposition to extract the q-axis voltage component v used to drive the phase-locked loop control loop. sq The analysis decomposes the voltage into three terms: synchronization voltage term, desynchronization voltage term, and series compensation voltage term. Transient stability boundary assessment module Extract the three voltage terms from step 4, construct an absolute value inequality, and perform algebraic judgment to determine whether the amplitude of the synchronization voltage term is not less than the sum of the amplitudes of the desynchronization voltage term and the series compensation voltage term. If the conditions are met, it is determined that the amplitude of the synchronous voltage provided by the equivalent grid is sufficient to offset the synchronization effect, the phase-locked loop can recapture the phase, and the inverter control system remains stable. If this condition is not met, regardless of how the bandwidth and parameters of the inverter control system are designed, the q-axis voltage component v used to drive the phase-locked loop control loop will be... sq If the value cannot be returned to zero across the entire real number field, the phase-locked loop will lose synchronization.

Images