Network-forming grid voltage active support control method based on virtual electromotive force impedance

Abstract

The application discloses a network-constructing type power grid voltage active support control method based on virtual electromotive force impedance, relates to the technical field of power system power electronic controlled converter control, and utilizes virtual electromotive force phase independent generation, does not depend on grid connection point phase feedback and active droop quantity, and avoids oscillation risk caused by a phase-locked loop; slow dynamic bus voltage regulation only compensates loss, has very low bandwidth, and does not respond to transient and harmonic disturbance; voltage amplitude reference is jointly driven by a deviation signal and a voltage change rate, so that reactive current responds quickly in the initial stage of voltage overrun; dynamic virtual impedance is jointly determined by voltage deviation and reactive current, steady-state low impedance support, and fault current limiting protection; finally, virtual electromotive force, amplitude, grid voltage and dynamic impedance are uniformly included into an electromotive force impedance network to generate a current reference, internal integration restraint of driving and current limiting is realized, and external loop saturation and distortion are avoided.

Description

Technical Field

[0001] This invention relates to the field of power electronic controlled converter technology in power systems, and specifically to a method for active voltage support control of grid-type power grids based on virtual electromotive force impedance. Background Technology

[0002] In distribution networks with new energy power plant busbars, long-distance transmission receiving ends, and high proportions of power electronics access, Static Var Generators (SVG) are widely used to quickly suppress grid connection point voltage exceedances. Their main circuit consists of a voltage source converter, AC filter inductor, DC capacitor, and control unit, with reactive power compensation and voltage regulation as their core objectives. Traditional SVGs often employ grid-following control, relying on a phase-locked loop (PLL) to obtain the grid connection point voltage phase for reactive current control. However, in weak grid scenarios with low short-circuit ratios, high line impedance, or high harmonic content, the grid connection point phase is no longer a stable reference. The PLL introduces voltage disturbances and harmonics into the phase path, causing a phase deviation between the reactive current reference and the actual voltage. This deviation alters the SVG's equivalent output impedance, creating a resonant path with the grid impedance, which can easily trigger current oscillations, overcurrent protection failures, or even grid disconnection. Grid-based control (such as virtual synchronous machines and droop control) enables the converter to exhibit voltage source characteristics, providing active support capabilities. However, this type of control typically relies on an active power-frequency coupling path, requiring the converter to have short-term active power throughput capability. The DC-side capacitor capacity of the SVG is limited by cost and size, making it unable to sustain active power exchange for extended periods. When the grid experiences voltage dips or phase angle jumps, traditional grid-connected control causes internal phase angle and capacitor energy interaction, leading to a rapid exceedance of the DC bus voltage due to a lack of continuous active power source. Furthermore, the operating losses of switching devices and filter inductors need to be replenished from the grid side over a long period. Without an independent bus voltage regulation channel, the DC voltage will gradually deviate from its steady-state value, affecting the long-term reliability of the system. During deep voltage dips at the grid connection point, the SVG's internal virtual electromotive force and the grid-side voltage difference increase sharply. If the equivalent internal impedance is insufficient, the output current will rise dramatically. Traditional countermeasures such as current limiting, switching to grid-connected mode, or shutdown severely damage the voltage support characteristics of the grid-connected voltage source, resulting in the loss of necessary reactive power support capability during fault periods. Summary of the Invention

[0003] The technical problem this invention aims to solve is that existing control schemes struggle to balance stable synchronization in weak power grids, voltage support without energy storage physical constraints, and current limiting and continuous compensation during faults. The goal is to provide a grid-based active voltage support control method for grid-connected power grids based on virtual electromotive force (EMF) impedance. This method utilizes independently generated virtual EMF phases, independent of grid connection point phase feedback and active power droop, avoiding the oscillation risks caused by phase-locked loops (PLLs). Slow dynamic bus voltage regulation only compensates for losses, has extremely low bandwidth, and does not respond to transient and harmonic disturbances. The voltage amplitude reference is driven by both the deviation signal and the voltage change rate, enabling rapid reactive current response in the initial stage of voltage overshoot. The dynamic virtual impedance is jointly determined by voltage deviation and reactive current, providing low impedance support in steady state and current limiting protection during faults. Finally, the virtual EMF, amplitude, grid voltage, and dynamic impedance are unified into the EMF impedance network to generate a current reference, achieving an integrated constraint between driving and current limiting, avoiding outer loop saturation and distortion.

[0004] This invention is achieved through the following technical solution:

[0005] The first aspect of this invention provides a method for active voltage support control of a grid-type power grid based on virtual electromotive force impedance, comprising the following specific steps:

[0006] Collect the three-phase signal at the grid connection point of the static var generator, perform coordinate transformation on the three-phase signal at the grid connection point of the static var generator to obtain the grid connection point voltage vector signal and the output reactive current signal;

[0007] The DC voltage deviation signal between the DC bus voltage signal and the DC voltage reference value is collected to generate a slow dynamic angular frequency correction. The rated angular frequency and the slow dynamic angular frequency correction are superimposed and integrated to generate a virtual electromotive force phase signal.

[0008] A virtual electromotive force amplitude reference signal is generated based on the voltage deviation signal between the grid connection point voltage amplitude signal and the voltage reference value, as well as the rate of change of the grid connection point voltage amplitude signal.

[0009] Impedance parameter mapping is calculated based on the voltage deviation signal and the output reactive current signal to generate a dynamic virtual impedance signal containing virtual resistance parameters and virtual inductance parameters.

[0010] The grid connection point voltage vector signal, virtual electromotive force phase signal, virtual electromotive force amplitude reference signal and dynamic virtual impedance signal are input into the electromotive force impedance equivalent network to generate a reference current signal.

[0011] Based on the reference current signal and the three-phase current signal output by the static var generator, current tracking control with resonance suppression is performed to obtain the modulation control quantity. The modulation control quantity is then converted into a pulse width modulation signal to control the static var generator to inject or absorb reactive current to the grid connection point.

[0012] Furthermore, the acquisition of the three-phase signal at the grid connection point of the static var generator, and the coordinate transformation of the three-phase signal at the grid connection point to obtain the grid connection point voltage vector signal and the output reactive current signal, includes:

[0013] Collect the three-phase voltage signal at the grid connection point of the static var generator and the three-phase current signal output by the static var generator;

[0014] Clarke transforms are performed on the three-phase voltage and three-phase current signals at the grid connection point of the static var generator to obtain the voltage and current components in the two-phase stationary coordinate system, respectively.

[0015] By performing Parker transformation on the voltage and current components, the grid connection point voltage vector signal and the output current vector signal in the synchronous rotating coordinate system are obtained.

[0016] The output reactive current signal is determined based on the component in the output current vector signal that corresponds to the reactive direction, and the positive direction of the output reactive current signal is obtained by the capacitive reactive current injected by the static var generator into the grid connection point.

[0017] Furthermore, the generation of the virtual electromotive force phase signal includes:

[0018] The generation of the slow dynamic angular frequency correction includes:

[0019] Acquire the DC bus voltage signal, and determine the DC voltage deviation signal based on the DC voltage reference value and the DC bus voltage signal;

[0020] Based on the DC voltage proportional adjustment coefficient, the DC voltage deviation signal, and the DC voltage integral adjustment coefficient, the slow dynamic angular frequency correction amount is obtained;

[0021] Read the first zero-crossing information of the three-phase voltage signal at the grid connection point of the static var generator to obtain the initial value of the virtual electromotive force phase signal;

[0022] Obtain the rated angular frequency. Based on the initial value of the virtual electromotive force phase signal, integrate the rated angular frequency and the slow dynamic angular frequency correction after superimposing them to obtain the virtual electromotive force phase signal.

[0023] When the virtual electromotive force phase signal exceeds the phase range corresponding to one power frequency cycle, the digital controller performs periodic processing on the virtual electromotive force phase signal, so that the virtual electromotive force phase signal cycles within the phase range corresponding to one power frequency cycle.

[0024] Furthermore, the acquisition of the voltage deviation signal between the grid connection point voltage amplitude signal and the voltage reference value includes:

[0025] Collect the voltage amplitude signal at the grid connection point and perform amplitude limiting filtering on the voltage amplitude signal at the grid connection point: if the change in the voltage amplitude signal at the grid connection point in adjacent sampling periods exceeds the upper limit of the physical change allowed by the equipment, it is truncated according to the upper limit of the physical change; if there are high-frequency spikes near the switching frequency in the voltage amplitude signal at the grid connection point, a first-order low-pass filter is used to obtain the voltage amplitude signal at the grid connection point after amplitude limiting filtering.

[0026] The voltage deviation signal is obtained by subtracting the voltage amplitude signal at the grid connection point after amplitude limiting and filtering from the voltage reference value.

[0027] When the grid connection point voltage is lower than the voltage reference value, the voltage deviation signal is positive. The digital controller increases the virtual electromotive force amplitude reference signal and drives the static var generator to inject capacitive reactive current.

[0028] When the grid connection point voltage is higher than the voltage reference value, the voltage deviation signal is negative. The digital controller reduces the virtual electromotive force amplitude reference signal or drives the static var generator to absorb inductive reactive current.

[0029] Furthermore, the step of generating a virtual electromotive force amplitude reference signal based on the voltage deviation signal between the grid connection point voltage amplitude signal and the voltage reference value, and the rate of change of the grid connection point voltage amplitude signal, specifically includes:

[0030] The rate of change of the grid connection point voltage amplitude signal is extracted based on the filtered grid connection point voltage amplitude signal of adjacent sampling periods, and an upper limit constraint is set on the rate of change of the grid connection point voltage amplitude signal to obtain the rate of change of the grid connection point voltage amplitude signal after the upper limit constraint.

[0031] The rate of change of the voltage deviation signal, the grid connection point voltage amplitude signal after upper limit constraint, and the voltage reference value are input into the virtual electromotive force amplitude generation process to obtain the virtual electromotive force amplitude reference signal. The process is as follows:

[0032]

[0033] In the formula, This is a reference signal for the amplitude of the virtual electromotive force. This is the voltage reference value. Based on the proportional gain, The coefficient for enhancing the rate of change. The rate of change of the grid connection point voltage amplitude signal after the upper limit constraint. This is a voltage deviation signal. Let be the voltage deviation signal in the m-th sampling period. For integral gain, This refers to the sampling period number in the integral calculation. The sampling period number is... The sampling period is This is the voltage feedforward coefficient.

[0034] Furthermore, after obtaining the virtual electromotive force amplitude reference signal, it also includes:

[0035] Set integral freeze and integral reverse release conditions: When the virtual electromotive force amplitude reference signal reaches the limit and the voltage deviation signal still drives the virtual electromotive force amplitude reference signal to change in the same direction, the digital controller freezes the integral adjustment amount; when the polarity of the voltage deviation signal is opposite to the direction of action of the integral adjustment amount, the digital controller releases the integral adjustment amount in reverse.

[0036] Furthermore, the step of performing impedance parameter mapping calculation based on the voltage deviation signal and the output reactive current signal to generate a dynamic virtual impedance signal containing virtual resistance parameters and virtual inductance parameters includes:

[0037] The voltage deviation signal and the output reactive current signal are acquired. The voltage deviation signal is used to reflect the voltage support requirements and fault depth at the grid connection point. The output reactive current signal is used to reflect the withstand capacity of the static var generator body.

[0038] Calculate the virtual inductance parameters based on the voltage deviation signal: ;

[0039] In the formula, For virtual inductance parameters, Based on the virtual inductance parameters, This is the virtual inductance amplification factor. It is a natural constant. For voltage deviation sensitivity coefficient, This is the voltage deviation signal expressed in per-unit values. When calculating using per-unit values, It is a dimensionless parameter; when calculating using a nominal voltage value, Its dimension is the reciprocal of voltage;

[0040] Calculate the virtual resistance parameters based on the output reactive current signal: ;

[0041] In the formula, For virtual resistance parameters, Based on the basic virtual resistance parameters, This is the current damping amplification factor. The output reactive current signal is expressed in per-unit value. This is the reactive current warning value expressed in per-unit terms. This is a function for finding the maximum value. When using per-unit values ​​for calculation, The dimensions of the current are consistent with those of the virtual resistance parameter; when using the nominal current for calculation, The dimension of resistance is the square of the current.

[0042] Furthermore, the step of inputting the grid connection point voltage vector signal, virtual electromotive force phase signal, virtual electromotive force amplitude reference signal, and dynamic virtual impedance signal into the electromotive force impedance equivalent network to generate a reference current signal includes:

[0043] The virtual electromotive force amplitude reference signal and the virtual electromotive force phase signal are combined into an internal electromotive force vector;

[0044] Use the grid connection point voltage vector signal as the boundary voltage vector;

[0045] Obtain the physical resistance parameters and physical inductance parameters of the AC side of the static var generator, and combine the dynamic virtual impedance signal and the physical impedance of the AC side of the static var generator into the total equivalent impedance.

[0046] A reference current signal is generated based on the difference between the internal electromotive force vector and the boundary voltage vector, as well as the total equivalent impedance.

[0047] Furthermore, the current tracking control with resonance suppression based on the reference current signal and the three-phase current signal output from the static var generator to obtain the modulation control quantity includes:

[0048] The reference current signal is converted into a three-phase reference current instantaneous quantity through the Parker inverse transform and the Clark inverse transform;

[0049] Calculate the difference between the three-phase current signal output by the static var generator in the current sampling period and the three-phase current signal output by the static var generator in the previous sampling period, and then subtract the difference from the output of the proportional resonant regulator after multiplying it by the damping coefficient to obtain the current deviation signal.

[0050] The current deviation signal is input to the proportional resonant regulator, and the output of the proportional resonant regulator and the damping feedback together form the modulation control quantity; the proportional resonant regulator includes a proportional branch and a fundamental resonant branch, the proportional branch amplifies the current deviation signal instantaneously, and the fundamental resonant branch performs cumulative compensation of the current deviation signal near the rated angular frequency.

[0051] Furthermore, the process of controlling the static var generator to inject or absorb reactive current into the grid connection point also includes real-time monitoring of the voltage amplitude signal at the grid connection point:

[0052] When the voltage amplitude signal at the grid connection point is lower than the low voltage threshold and the absolute value of the voltage deviation signal exceeds the voltage safety threshold, the digital controller determines that it has entered a deep voltage drop state.

[0053] After entering a deep voltage drop state, the digital controller does not switch to grid-following mode, does not enable the phase-locked loop, and does not stop the virtual electromotive force phase signal recursion.

[0054] The digital controller increases the current limiting increment in the dynamic virtual impedance signal and limits the integral of the virtual electromotive force amplitude reference signal from continuing to accumulate;

[0055] When the grid connection point voltage recovers to the voltage recovery threshold range, and the output reactive current signal is lower than the reactive current recovery threshold for several consecutive sampling cycles, the digital controller determines that it has entered the recovery state.

[0056] During the recovery phase, the digital controller reduces the current limiting increment in the virtual inductance and virtual resistance parameters according to the preset recovery slope, avoiding a sudden drop in the dynamic virtual impedance signal that could cause a secondary impact on the output current.

[0057] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0058] The virtual electromotive force phase signal in operation of this invention does not receive the actual phase feedback from the grid connection point, nor does it receive the frequency droop caused by active power deviation. The above phase generation method ensures that the phase reference of the static var generator is not affected by the phase disturbance of the weak grid, reducing the risk of weak grid oscillation caused by the phase-locked loop. The slow dynamic DC bus voltage regulation generates a slow dynamic angular frequency correction only based on the DC bus voltage deviation. The bandwidth of the slow dynamic DC bus voltage regulation is much lower than that of the voltage support loop and the current tracking loop, which can compensate for the conduction loss, switching loss and AC side filter inductor winding loss of power switching devices, while not responding to transient phase disturbances and harmonic disturbances. The above processing enables the static var generator to have long-term DC bus voltage stability capability and retains the weak grid anti-disturbance characteristics without phase-locked loop.

[0059] This invention utilizes the rate of change of voltage deviation signal and grid connection point voltage amplitude signal to jointly generate virtual electromotive force amplitude reference signal; when the grid connection point voltage deviates slowly, the virtual electromotive force amplitude reference signal is driven by the steady-state voltage deviation signal; when the grid connection point voltage drops or rises rapidly, the rate of change of the grid connection point voltage amplitude signal causes the virtual electromotive force amplitude reference signal to be enhanced in advance, so that the reactive current is pushed in the desired direction in the early stage of voltage over-limit.

[0060] This invention utilizes voltage deviation signal and output reactive current signal to generate dynamic virtual impedance signal; voltage deviation signal reflects the depth of grid-side fault, and output reactive current signal reflects the withstand capability of static var generator body; the two types of signals jointly determine virtual resistance parameters and virtual inductance parameters, so that the dynamic virtual impedance signal can maintain low impedance in steady state to improve support capability, and enter current-limiting impedance state when there is a deep voltage drop.

[0061] Simultaneously, the virtual electromotive force (EMF) phase signal, virtual EMF amplitude reference signal, grid connection point voltage vector signal, and dynamic virtual impedance signal are uniformly placed into the EMF impedance equivalent network to generate a reference current signal. The virtual EMF amplitude reference signal corresponds to the supporting driving force, and the dynamic virtual impedance signal corresponds to the current limiting boundary. The two constrain each other in the same current reference generation stage, avoiding the outer loop saturation and current distortion caused by first amplifying the current reference and then hard limiting it. This invention is applicable to static var generators without large-capacity active energy storage, and can also be used for parallel operation of multiple static var generators. Through reactive power ratio correction, the reactive circulating current caused by inconsistent line impedance in parallel equipment can be reduced. Attached Figure Description

[0062] To more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be considered as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort. In the drawings:

[0063] Figure 1 This is an overall flowchart of an embodiment of the present invention.

[0064] Figure 2 This is a flowchart of the signal acquisition and coordinate transformation sub-process in an embodiment of the present invention;

[0065] Figure 3 This is a flowchart of the virtual electromotive force phase signal generation sub-process in an embodiment of the present invention;

[0066] Figure 4 This is a flowchart of the virtual electromotive force amplitude reference signal generation sub-process in an embodiment of the present invention;

[0067] Figure 5 This is a flowchart of the dynamic virtual impedance signal generation sub-process in an embodiment of the present invention;

[0068] Figure 6 This is a flowchart of the electromotive force impedance equivalent network and reference current signal generation sub-process in an embodiment of the present invention;

[0069] Figure 7 This is a flowchart of the current tracking control with resonance suppression and pulse width modulation signal generation sub-process in an embodiment of the present invention;

[0070] Figure 8 This is a flowchart illustrating the reactive power ratio correction process when multiple static var generators are connected in parallel in an embodiment of the present invention. Detailed Implementation

[0071] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0072] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of the embodiments of the invention. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0073] The following is in conjunction with the appendix Figures 1-8 The embodiments of the present invention will be described in detail below. The numerical values ​​in the embodiments are used to illustrate the feasibility of the control flow and the parameter selection method, and are not intended to limit the scope of protection of the present invention. In actual engineering, the voltage level, converter capacity, filter inductor, sampling frequency and control parameters can be determined based on the equipment capacity, grid connection specifications, short-circuit ratio and safe operating area of ​​power semiconductor devices.

[0074] Example 1, as one possible implementation, such as Figure 1 As shown, this embodiment provides a method for active voltage support control of a grid-type power grid based on virtual electromotive force impedance, including the following specific steps:

[0075] Collect the three-phase signal at the grid connection point of the static var generator, perform coordinate transformation on the three-phase signal at the grid connection point of the static var generator to obtain the grid connection point voltage vector signal and the output reactive current signal;

[0076] The DC voltage deviation signal between the DC bus voltage signal and the DC voltage reference value is collected to generate a slow dynamic angular frequency correction. The rated angular frequency and the slow dynamic angular frequency correction are superimposed and integrated to generate a virtual electromotive force phase signal.

[0077] A virtual electromotive force amplitude reference signal is generated based on the voltage deviation signal between the grid connection point voltage amplitude signal and the voltage reference value, as well as the rate of change of the grid connection point voltage amplitude signal.

[0078] Impedance parameter mapping is calculated based on the voltage deviation signal and the output reactive current signal to generate a dynamic virtual impedance signal containing virtual resistance parameters and virtual inductance parameters.

[0079] The grid connection point voltage vector signal, virtual electromotive force phase signal, virtual electromotive force amplitude reference signal and dynamic virtual impedance signal are input into the electromotive force impedance equivalent network to generate a reference current signal.

[0080] Based on the reference current signal and the three-phase current signal output by the static var generator, current tracking control with resonance suppression is performed to obtain the modulation control quantity. The modulation control quantity is then converted into a pulse width modulation signal to control the static var generator to inject or absorb reactive current to the grid connection point.

[0081] In practical implementation, a static var generator (SVA) is connected to the busbar of the renewable energy power station. The SVA includes a three-phase voltage source converter, an AC-side filter inductor, a DC-side capacitor, voltage transformers, current transformers, a DC bus voltage sensor, a digital controller, and a pulse width modulation (PWM) drive unit. The DC side of the SVA does not have battery, flywheel, or supercapacitor energy storage units; the DC-side capacitor is only used to maintain the DC bus voltage balance of the converter. The AC-side filter inductor corresponds to the physical inductance parameters of the AC side of the SVA, and the equivalent resistance of the AC-side filter inductor and the line connection branch corresponds to the physical resistance parameters of the AC side of the SVA.

[0082] In this embodiment, the sampling period of the digital controller is The digital controller executes the control flow of this embodiment once per sampling cycle. The three-phase voltage signal at the grid connection point of the static var generator is acquired by a voltage transformer, the three-phase current signal output by the static var generator is acquired by a current transformer, and the DC bus voltage signal is acquired by a DC bus voltage sensor. The analog signals acquired by the voltage transformer, current transformer, and DC bus voltage sensor are first subjected to anti-aliasing filtering, and then converted into digital quantities by the analog-to-digital converter. The cutoff frequency of the anti-aliasing filter is higher than the fundamental frequency and lower than half of the switching frequency to prevent high-frequency switching ripple from entering the fundamental frequency calculation channel of the digital controller.

[0083] In this embodiment, the voltage signal, current signal, and impedance parameters are used in the control calculations within the digital controller using per-unit values. The per-unit references include the rated line voltage, rated current, rated impedance, and rated DC bus voltage of the static var generator (SVM). When output to the drive circuit or equipment protection circuit is required, the digital controller converts the control quantity into a nominal value according to the aforementioned per-unit references.

[0084] The specific implementation process is as follows:

[0085] S101, signal acquisition and Clarke transform. For example... Figure 2As shown, the digital controller reads the three-phase voltage signal at the grid connection point of the static var generator (SVM), the three-phase current signal output by the SVM, and the DC bus voltage signal. The digital controller performs a Clarke transform on the three-phase voltage signal at the grid connection point of the SVM to obtain the voltage component in a two-phase stationary coordinate system; the digital controller also performs a Clarke transform on the three-phase current signal output by the SVM to obtain the current component in a two-phase stationary coordinate system. The voltage and current components in the two-phase stationary coordinate systems are temporarily stored in the digital controller's data register, awaiting the generation of the virtual electromotive force phase signal for the current sampling period before undergoing a Parker transform.

[0086] S102, virtual electromotive force phase signal generation; such as Figure 3 As shown, before the static var generator (SVA) is unlocked from the grid, the digital controller reads the first zero-crossing information of the three-phase voltage signal at the grid connection point to obtain the initial value of the virtual electromotive force (EMF) phase signal. The soft alignment process refers to the process by which the digital controller calibrates the initial value of the virtual EMF phase signal based on the grid connection point voltage phase once before the SVA is unlocked from the grid. The soft alignment process is only performed once in the zero-output state; after grid connection is unlocked, the digital controller no longer uses the actual phase at the grid connection point to correct the virtual EMF phase signal. The soft alignment process differs from a phase-locked loop (PLL) that continuously tracks the grid connection point voltage phase.

[0087] After grid connection is unlocked, the digital controller no longer uses the actual phase at the grid connection point to correct the virtual electromotive force phase signal, nor does it use the active power deviation to form the frequency droop. The digital controller acquires the DC bus voltage signal and generates a slow dynamic angular frequency correction based on the DC voltage deviation signal between the DC bus voltage signal and the DC voltage reference value. The generation method of the slow dynamic angular frequency correction is as follows:

[0088]

[0089] In the formula, This is a DC voltage deviation signal. This is the DC voltage reference value. This is the DC bus voltage signal.

[0090]

[0091] In the formula, This is the slow dynamic angular frequency correction for the k-th sampling period. This is the DC voltage proportional adjustment coefficient. This represents the DC voltage deviation signal during the k-th sampling period. This is the DC voltage integral adjustment coefficient. This refers to the sampling period number in the DC voltage integral calculation. The sampling period number is... This is the DC voltage deviation signal for the nth sampling period. The sampling period.

[0092] The virtual electromotive force phase signal is updated according to the following formula:

[0093]

[0094] In the formula, The virtual electromotive force phase signal for the kth sampling period is... This represents the virtual electromotive force phase signal during the (k-1)th sampling period. The rated angular frequency, This is the slow dynamic angular frequency correction for the k-th sampling period. The sampling period is This is the sampling period number.

[0095] The control bandwidth of slow dynamic DC bus voltage regulation is to The control bandwidth is lower than that of the virtual electromotive force amplitude reference signal generation process and the current tracking control process. Slow-dynamic DC bus voltage regulation is only used to compensate for the conduction losses, switching losses, AC side filter inductor winding losses, and auxiliary losses of power switching devices, and does not participate in transient voltage support. When voltage dips, phase angle jumps, or harmonic disturbances occur at the grid connection point, slow-dynamic DC bus voltage regulation cannot change the virtual electromotive force phase signal within milliseconds; therefore, slow-dynamic DC bus voltage regulation will not form a phase disturbance feedback path similar to a phase-locked loop.

[0096] The slow dynamic angular frequency correction setting has upper and lower limits. These limits are determined based on the grid's permissible steady-state frequency offset range, the allowable deviation of the DC bus voltage, and the power loss of the static var generator. Through the slow dynamic angular frequency correction, the average frequency of the virtual electromotive force can adapt to the grid's steady-state frequency offset, avoiding continuous phase angle accumulation caused by completely fixing the rated angular frequency.

[0097] When the virtual electromotive force (EMF) phase signal exceeds the phase range corresponding to one power frequency cycle, the digital controller performs periodic processing on the virtual EMF phase signal, causing it to cycle within the phase range corresponding to one power frequency cycle. This periodic processing only changes the numerical range of the virtual EMF phase signal; it does not alter the source from which the virtual EMF phase signal is recursively derived from the rated angular frequency and slow dynamic angular frequency correction.

[0098] S103, Parker transformation and voltage and current signal extraction: The digital controller uses the virtual electromotive force phase signal of the current sampling period generated by S102 as the rotation angle to perform Parker transformation on the voltage component and current component in the two-phase stationary coordinate system obtained by S101, so as to obtain the grid connection point voltage vector signal and output current vector signal in the synchronous rotating coordinate system.

[0099] The grid connection point voltage vector signal includes the d-axis component and the q-axis component. The grid connection point voltage amplitude signal is obtained by the following formula:

[0100]

[0101] In the formula, This is the voltage amplitude signal at the grid connection point. The d-axis component of the grid connection point voltage vector signal. This represents the q-axis component of the grid connection point voltage vector signal.

[0102] The output reactive current signal is obtained from the component of the output current vector signal corresponding to the reactive direction. In this embodiment, the positive direction of the output reactive current signal is defined as the capacitive reactive current injected by the static var generator into the grid connection point. When the equipment manufacturer uses the opposite current direction definition, the sign convention can be uniformly changed in the control program, but the causal relationship between the voltage deviation signal, the output reactive current signal, and the dynamic virtual impedance signal remains unchanged.

[0103] The outputs of this step include the grid connection point voltage vector signal, the grid connection point voltage amplitude signal, and the output reactive current signal. The grid connection point voltage amplitude signal is used in the subsequent virtual electromotive force amplitude generation process, the grid connection point voltage vector signal is used in the subsequent electromotive force impedance equivalent network, and the output reactive current signal is used in the subsequent dynamic virtual impedance generation process.

[0104] S104, extraction of the rate of change of voltage deviation signal and grid connection point voltage amplitude signal. For example... Figure 4 As shown, the digital controller subtracts the voltage amplitude signal at the grid connection point from the voltage reference value to obtain the voltage deviation signal. The voltage deviation signal is obtained by the following formula:

[0105]

[0106] In the formula, This is a voltage deviation signal. This is the voltage reference value. This is the voltage amplitude signal at the grid connection point.

[0107] When the grid connection point voltage is lower than the voltage reference value, the voltage deviation signal is positive. The digital controller increases the virtual electromotive force amplitude reference signal and drives the static var generator to inject capacitive reactive current. When the grid connection point voltage is higher than the voltage reference value, the voltage deviation signal is negative. The digital controller decreases the virtual electromotive force amplitude reference signal or drives the static var generator to absorb inductive reactive current.

[0108] In practical implementation, to avoid sampling noise being misinterpreted as rapid voltage changes, the digital controller first performs amplitude limiting filtering on the grid-connected point voltage amplitude signal. Amplitude limiting filtering includes two processes: if the change in the grid-connected point voltage amplitude signal in adjacent sampling periods exceeds the upper limit of the physical change allowed by the equipment, it is truncated according to the upper limit; if there are high-frequency glitches near the switching frequency in the grid-connected point voltage amplitude signal, a first-order low-pass filter is used to obtain the filtered grid-connected point voltage amplitude signal. Subsequently, the digital controller extracts the rate of change of the grid-connected point voltage amplitude signal based on the filtered grid-connected point voltage amplitude signals in adjacent sampling periods and sets an upper limit constraint on the rate of change of the grid-connected point voltage amplitude signal, obtaining the rate of change of the grid-connected point voltage amplitude signal after the upper limit constraint. The rate of change of the grid-connected point voltage amplitude signal after the upper limit constraint is used as the input quantity for proportional gain adjustment in S105. The upper limit constraint is determined based on the maximum allowable voltage change rate of the grid-connected point, the analog-to-digital conversion resolution, and the current control bandwidth. The purpose of setting the upper limit constraint is to prevent single-point sampling noise from causing a sudden increase in the virtual electromotive force amplitude reference signal.

[0109] S105, Virtual EMF Amplitude Reference Signal Generation. The digital controller inputs the voltage deviation signal, the rate of change of the grid-connected point voltage amplitude signal after upper limit constraint, and the voltage reference value into the virtual EMF amplitude generation process. The virtual EMF amplitude reference signal is obtained by superimposing the proportional regulation, integral regulation, and voltage feedforward regulation.

[0110] In practice, one feasible generation method is as follows:

[0111]

[0112] In the formula, This is a reference signal for the amplitude of the virtual electromotive force. This is the voltage reference value. Based on the proportional gain, The coefficient for enhancing the rate of change. The rate of change of the grid connection point voltage amplitude signal after the upper limit constraint. This is a voltage deviation signal. Let be the voltage deviation signal in the m-th sampling period. For integral gain, This refers to the sampling period number in the integral calculation. The sampling period number is... The sampling period is This is the voltage feedforward coefficient.

[0113] The integral calculation begins at the moment the static var generator (SVM) is unlocked from the grid. When the SVM is shut down or disconnected from the grid, the integral regulation is reset to zero. The integral regulation has upper and lower limits, determined based on the allowable range of the virtual electromotive force amplitude reference signal, the upper current limit, and the voltage support target.

[0114] When the grid connection point voltage amplitude signal changes slowly, the rate of change of the grid connection point voltage amplitude signal after the upper limit constraint is close to zero, and the virtual electromotive force amplitude reference signal is mainly determined by the basic proportional gain, integral gain, and voltage feedforward coefficient. When the grid connection point voltage experiences a rapid drop or rapid rise, the rate of change of the grid connection point voltage amplitude signal after the upper limit constraint increases, and the response of the virtual electromotive force amplitude reference signal to the voltage deviation signal is enhanced.

[0115] To prevent integral saturation during deep faults, the digital controller is configured with integral freeze and integral reverse release conditions. When the virtual electromotive force (EMF) amplitude reference signal reaches its limit and the voltage deviation signal continues to drive the virtual EMF amplitude reference signal in the same direction, the digital controller freezes the integral regulation. When the polarity of the voltage deviation signal is opposite to the direction of the integral regulation, the digital controller releases the integral regulation in reverse. This process ensures that after the grid connection voltage recovers, the virtual EMF amplitude reference signal will not continue to push up the output current due to the accumulated integral.

[0116] The virtual electromotive force (EMF) amplitude reference signal output in this step is directly input into the subsequent EMF-impedance equivalent network. The virtual EMF amplitude reference signal is not the final current command, but rather the amplitude of the internal virtual EMF. The subsequent dynamic virtual impedance signal will simultaneously constrain the current increase caused by the virtual EMF amplitude reference signal when the reference current signal is generated. Therefore, the voltage support driving force and the fault current limiting boundary work together within the same network.

[0117] S106, Dynamic Virtual Impedance Signal Generation. For example... Figure 5 As shown, the digital controller receives a voltage deviation signal and outputs a reactive current signal. The voltage deviation signal reflects the grid connection point voltage support requirements and fault depth, while the output reactive current signal reflects the load-bearing capacity of the static var generator (SVR). Based on the voltage deviation signal and the output reactive current signal, the digital controller performs impedance parameter mapping calculations to generate a dynamic virtual impedance signal. The dynamic virtual impedance signal includes virtual resistance parameters and virtual inductance parameters.

[0118] In this embodiment, the virtual inductance parameter monotonically increases with the absolute value of the voltage deviation signal. The voltage deviation signal uses per-unit values ​​in the exponential mapping, making the independent variable of the exponential function a dimensionless quantity. The following exponential mapping can be used:

[0119]

[0120] In the formula, For virtual inductance parameters, Based on the virtual inductance parameters, This is the virtual inductance amplification factor. It is a natural constant. For voltage deviation sensitivity coefficient, This is the voltage deviation signal expressed in per-unit values. When calculating using per-unit values, It is a dimensionless parameter; when calculating using a nominal voltage value, Its dimension is the reciprocal of voltage.

[0121] When the grid connection point voltage deviates slightly, the absolute value of the voltage deviation signal is small, the virtual inductance parameter is close to the basic virtual inductance parameter, and the static var generator exhibits low internal impedance voltage source characteristics. When the grid connection point voltage drops sharply, the absolute value of the voltage deviation signal increases, the virtual inductance parameter increases rapidly, the total equivalent impedance in the electromotive force impedance equivalent network increases, and the reference current signal is restricted.

[0122] To cover different digital controller computing power and engineering tuning methods, virtual inductor parameters can also be obtained through piecewise linear mapping, piecewise quadratic mapping, or lookup table mapping.

[0123] For example, when the absolute value of the voltage deviation signal is below the first voltage threshold, the virtual inductance parameter maintains the basic virtual inductance parameter; when the absolute value of the voltage deviation signal is between the first and second voltage thresholds, the virtual inductance parameter increases with a linear slope; when the absolute value of the voltage deviation signal is above the second voltage threshold, the virtual inductance parameter increases with a higher slope and is constrained by the highest virtual inductance limit. Regardless of whether exponential mapping, piecewise linear mapping, piecewise quadratic mapping, or lookup table mapping is used, the mapped output satisfies the condition that it does not decrease as the absolute value of the voltage deviation signal increases.

[0124] The tuning principle for the virtual inductance parameters is as follows:

[0125] The basic virtual inductance parameters are selected based on the steady-state voltage support accuracy and steady-state reactive power response speed; the maximum virtual inductance limit is calculated based on the maximum allowable current limit, AC side physical inductance parameters, and the maximum allowable internal electromotive force difference under the deepest voltage drop condition; the virtual inductance amplification factor and voltage deviation sensitivity factor are determined based on the voltage deviation range from the basic virtual inductance parameters to the maximum virtual inductance limit.

[0126] The virtual resistance parameters are generated based on the output reactive current signal. The output reactive current signal uses per-unit values ​​in the virtual resistance calculation. It can be expressed in the following form:

[0127]

[0128] In the formula, For virtual resistance parameters, Based on the basic virtual resistance parameters, This is the current damping amplification factor. The output reactive current signal is expressed in per-unit value. This is the reactive current warning value expressed in per-unit terms. This is a function for finding the maximum value. When using per-unit values ​​for calculation, The dimensions of the current are consistent with those of the virtual resistance parameter; when using the nominal current for calculation, The dimension of resistance is the square of the current.

[0129] When the output reactive current signal is below the reactive current warning value, the virtual resistance parameter maintains basic damping. When the output reactive current signal exceeds the reactive current warning value, the virtual resistance parameter increases with the increase of the output reactive current signal. When the output reactive current signal reaches the reactive current limit, the digital controller further increases the virtual resistance parameter to limit the reference current signal from increasing further. This process increases damping in the static var generator as it approaches the current safety boundary, suppressing current oscillations and overshoot.

[0130] For scenarios requiring overvoltage mitigation, the digital controller can constrain the direction of reactive current based on the polarity of the voltage deviation signal. When the voltage deviation signal is positive, the static var generator (SVM) primarily injects capacitive reactive current to raise the grid connection voltage; when the voltage deviation signal is negative, the SVM primarily absorbs inductive reactive current to lower the grid connection voltage. This directional constraint does not alter the non-negative damping property of the virtual resistance parameters; it only changes the selection of the reactive current direction in the generation of the reference current signal, preventing incorrect damping direction during overvoltage mitigation.

[0131] In practice, the virtual resistance and inductance parameters generated in this step are not isolated algorithm results. They are directly fed into the electromotive force impedance equivalent network of S107, where they, along with the virtual electromotive force amplitude reference signal, generate the reference current signal. If a large voltage deviation causes the virtual electromotive force amplitude reference signal to rise, and simultaneously the dynamic virtual impedance signal increases, the electromotive force impedance equivalent network will make a trade-off between voltage support requirements and current safety boundaries. This trade-off occurs before the reference current signal is generated, rather than after it is generated through hard clipping, thereby reducing current waveform distortion and outer loop saturation.

[0132] S107, the electromotive force impedance equivalent network generates a reference current signal. For example... Figure 6 As shown, the digital controller synthesizes the virtual electromotive force (EMF) amplitude reference signal and the virtual EMF phase signal into an internal EMF vector, uses the grid connection point voltage vector signal as the boundary voltage vector, and synthesizes the dynamic virtual impedance signal and the physical impedance of the static var generator (SVM) AC side into a total equivalent impedance. The physical impedance of the SVM AC side includes the physical resistance parameters and the physical inductance parameters of the SVM AC side. The reference current signal is generated based on the difference between the internal EMF vector and the boundary voltage vector, as well as the total equivalent impedance.

[0133] In a synchronously rotating coordinate system established using a virtual electromotive force (EMF) phase signal, the d-axis component of the internal EMF vector serves as the virtual EMF amplitude reference signal, while the q-axis component of the internal EMF vector is zero. The reference current signal can be obtained through the following matrix relationship:

[0134]

[0135] In the formula, The d-axis component of the reference current signal. The q-axis component of the reference current signal. The total equivalent resistance is... For total equivalent reactance, This is a reference signal for the amplitude of the virtual electromotive force. The d-axis component of the grid connection point voltage vector signal. This represents the q-axis component of the grid connection point voltage vector signal.

[0136] The total equivalent resistance and total equivalent reactance are obtained by the following formulas:

[0137]

[0138] In the formula, The total equivalent resistance is... For virtual resistance parameters, These are the physical resistance parameters of the AC side of the static var generator. For total equivalent reactance, The rated angular frequency, For virtual inductance parameters, These are the physical inductance parameters of the AC side of the static var generator.

[0139] The virtual electromotive force (EMF) amplitude reference signal comes from S105, the virtual EMF phase signal comes from S102, the grid connection point voltage vector signal comes from S103, the virtual resistance and virtual inductance parameters come from S106, and the static var generator (SVM) AC side physical resistance and inductance parameters come from the equipment parameter calibration. The basic virtual resistance, basic virtual inductance, SVM AC side physical resistance, and SVM AC side physical inductance parameters are all set as non-negative parameters to ensure the denominator in the matrix formula is not zero. The reference current signal does not contain any variables without a source.

[0140] Under steady-state conditions with slight voltage deviation, the virtual resistance and virtual inductance parameters are close to their base values. The reference current signal is mainly determined by the difference between the internal electromotive force vector and the boundary voltage vector, and the static var generator has a strong voltage support capability.

[0141] Under deep voltage dip conditions, the virtual electromotive force (EMF) amplitude reference signal increases due to the increased voltage deviation signal. However, the virtual inductance parameter also increases due to the increased absolute value of the voltage deviation signal, and the virtual resistance parameter may also increase due to the increased output reactive current signal. With the increase in total equivalent impedance, the reference current signal is limited to the allowable range of the static var generator. This mechanism differs from direct current limiting. Direct current limiting cuts off the current after the reference current signal has been calculated to be too large, while this embodiment modifies the equivalent circuit parameters within the EMF impedance equivalent network, allowing the reference current signal to be generated naturally according to the limited voltage source circuit.

[0142] S108, current tracking control with resonance suppression and pulse width modulation signal generation. For example... Figure 7 As shown, the digital controller converts the reference current signal into a three-phase instantaneous reference current quantity through inverse Parker transform and inverse Clarke transform. The digital controller then calculates the phase-by-phase difference between the three-phase instantaneous reference current quantity and the three-phase current signal output from the static var generator (SVM) to obtain the current deviation signal. This current deviation signal is input to a proportional resonant regulator. The proportional resonant regulator includes a proportional branch and a fundamental resonant branch. The proportional branch instantaneously amplifies the current deviation signal, while the fundamental resonant branch performs cumulative compensation of the current deviation signal near the rated angular frequency. The proportional resonant regulator provides high gain near the fundamental frequency, enabling the three-phase current signal output from the SVM to track the three-phase reference current instantaneous quantity.

[0143] In practical implementation, to suppress resonance caused by the filter inductor, filter capacitor, and line impedance under weak power grid conditions, damping feedback is superimposed on the digital controller. The damping feedback can be selected as the input of the output current change or the filter capacitor current.

[0144] This embodiment uses the change in output current as damping feedback. The digital controller calculates the difference between the three-phase current signal output by the static var generator in the current sampling period and the three-phase current signal output by the static var generator in the previous sampling period, and subtracts this difference from the output of the proportional resonant regulator after multiplying it by the damping coefficient. This process is equivalent to adding virtual damping in a specific frequency band, suppressing the modulation control quantity when the output current changes too rapidly.

[0145] The proportional resonant regulator output and damping feedback together form the modulation control quantity. This modulation control quantity, after being limited, is input to the pulse width modulation (PWM) drive unit. The PWM drive unit compares the modulation control quantity with the carrier signal, and, combined with the DC bus voltage and dead time, generates a PWM signal. This PWM signal is used to drive the converter power switching devices to turn on and off. When the converter output voltage changes, the three-phase current signal output by the static var generator (SVA) changes accordingly. The new SVA three-phase current signal is acquired in the next sampling cycle, forming a closed loop.

[0146] S109, Fault Entry and Recovery Handling. The low voltage threshold is used to determine if the grid connection point voltage has entered a fault range. The deep fault threshold is used to determine if the grid connection point has entered a severe voltage drop state. The deep fault threshold can be set according to grid connection guidelines or equipment protection settings, and is typically taken as a percentage of the rated voltage. Doubled When the voltage amplitude signal at the grid connection point is lower than the low voltage threshold and the absolute value of the voltage deviation signal exceeds the voltage safety threshold, the digital controller determines that it has entered a deep voltage sag state. Upon entering the deep voltage sag state, the digital controller does not switch to grid-following mode, does not activate the phase-locked loop, and does not stop the recursion of the virtual electromotive force phase signal. The digital controller increases the current-limiting increment in the dynamic virtual impedance signal through S106 and limits the continued accumulation of the integral of the virtual electromotive force amplitude reference signal through S105.

[0147] When the grid connection point voltage recovers to the voltage recovery threshold range, and the output reactive current signal remains below the reactive current recovery threshold for several sampling cycles, the digital controller determines that it has entered the recovery state. In the recovery state, the digital controller reduces the current-limiting increments in the virtual inductance and virtual resistance parameters according to a preset recovery slope, preventing a sudden drop in the dynamic virtual impedance signal from causing a secondary impact on the output current. The virtual electromotive force phase signal is recursively generated by the rated angular frequency and the slow dynamic angular frequency correction during fault entry, fault holding, and recovery.

[0148] For example, the rated capacity of the static var generator is The rated line voltage is The sampling frequency is The equivalent inductance on the AC side is Per-unit value, maximum current is Per-unit value. The short-circuit ratio for weak grids is set to... The rated capacity and rated line voltage of the static var generator are used as the per-unit reference. The equivalent inductance on the AC side is calculated based on the nameplate value of the filter inductor. The upper limit of the current is set based on the continuous overload capacity of the power semiconductor devices. The control bandwidth of the slow dynamic DC bus voltage regulation is set as follows: The equivalent bandwidth of the voltage support ring is set to The equivalent bandwidth of the current tracking loop is set to The aforementioned bandwidth configuration ensures that DC bus voltage regulation only compensates for steady-state losses and does not participate in millisecond-level transient voltage support.

[0149] exist The voltage at the grid connection point should drop to a certain level at all times. Per-unit value, continued decline When using the method in this embodiment, the grid connection point voltage after the fault... Internally The per-unit value has been raised to The per-unit value limits the peak output reactive current to a certain value. Within the per-unit value, the DC bus voltage deviation remains within the rated value. Within the specified range. The above data represents the results of the control strategy, illustrating that the control flow of this embodiment can simultaneously provide voltage support, current limiting, and DC energy constraint.

[0150] Example 2, as a possible implementation, further illustrates the reactive power ratio correction when multiple static var generators are operating in parallel, based on Example 1. For example... Figure 8 As shown in this embodiment, multiple static var generators (SVAs) or multiple chained SVA branches are typically arranged in the new energy collection station. Due to differences in line impedance, filter inductance, and control delay between each SVA and the collection bus, if each SVA independently generates a virtual electromotive force amplitude reference signal with the same voltage reference value, it is possible that one SVA will handle excessive reactive current while another outputs less reactive current. Uneven reactive power output can lead to uneven equipment heat load, localized overcurrent, and reactive power circulation.

[0151] In this embodiment, each static var generator (SVR) receives a reactive current allocation reference value. This reference value can be allocated by the station-level digital controller based on equipment capacity, or it can be calculated locally by the SVR based on a proportion of its rated capacity. Before generating the virtual electromotive force amplitude reference signal, the digital controller first calculates the reactive current share deviation signal.

[0152]

[0153] In the formula, This is a reactive power component deviation signal. To output reactive current signal, To allocate a reference value for reactive current.

[0154] The digital controller corrects the grid connection point voltage amplitude signal based on the reactive power component deviation signal:

[0155]

[0156] In the formula, This is the corrected grid connection point voltage amplitude signal. This is the voltage amplitude signal at the grid connection point. This is the reactive power component correction factor. This is the reactive power component deviation signal.

[0157] When the reactive current signal output by this static var generator is higher than the allocated reactive current reference value, the reactive power share deviation signal is positive, and the corrected grid connection point voltage amplitude signal is greater than the grid connection point voltage amplitude signal. The digital controller uses the corrected grid connection point voltage amplitude signal in the voltage deviation signal calculation, resulting in a smaller voltage support requirement, thereby reducing the virtual electromotive force amplitude reference signal of this static var generator. When the reactive current signal output by this static var generator is lower than the allocated reactive current reference value, the corrected grid connection point voltage amplitude signal is lower than or close to the grid connection point voltage amplitude signal, the digital controller receives a larger voltage support requirement, thereby increasing the virtual electromotive force amplitude reference signal of this static var generator. Multiple static var generators achieve reactive power share balance in this way.

[0158] To prevent the parallel system from weakening its overall voltage support capability due to share correction during transient faults, this embodiment sets a share correction exit condition. When the voltage amplitude signal at the grid connection point is lower than the deep fault threshold, the digital controller reduces the reactive power share correction coefficient or freezes the share correction, allowing each static var generator to participate in fault support first. When the grid connection point voltage recovers, the digital controller restores the reactive power share correction coefficient according to the slope, allowing the reactive power share balancing function to be reactivated. The above processing enables this embodiment to achieve operating condition adaptation between voltage support and multi-machine current sharing, without affecting the virtual electromotive force phase generation and electromotive force impedance equivalent network of Embodiment 1.

[0159] In some preferred embodiments, amplitude correction is performed after the virtual electromotive force (EMF) amplitude reference signal is generated. The digital controller lowers or raises the virtual EMF amplitude reference signal based on the reactive power share deviation signal, causing the output reactive current signal to tend towards the allocated reactive current reference value. This implementation does not change the voltage deviation signal itself, but adjusts the reactive power output of each static var generator at the virtual EMF amplitude level. The corrected virtual EMF amplitude reference signal continues to be input into the EMF impedance equivalent network, and together with the virtual EMF phase signal, the grid connection point voltage vector signal, and the dynamic virtual impedance signal, generates the reference current signal.

[0160] In this embodiment, two static var generators of the same capacity are connected in parallel to the same... The line equivalent reactance of a static var generator at the busbar is: The per-unit value, the line equivalent reactance of the other static var generator is Per-unit value. When reactive power compensation is not enabled, the voltage drops to [a certain value]. After per-unit values, the peak reactive current outputs of the two static var generators are respectively per unit value and The per-unit value shows a large deviation in reactive power output. After enabling reactive power ratio correction, the peak reactive current outputs of the two static var generators are respectively... per unit value and Per-unit value, reactive power output deviation is reduced. The above indicates that reactive power share correction can improve reactive power sharing in multi-machine parallel systems while maintaining the virtual electromotive force phase generation mechanism.

[0161] Example 3, as one possible implementation, provides a grid-type active voltage support control system based on virtual electromotive force impedance. The control system can be deployed in the digital controller of a static var generator (SVM). The digital controller may include a digital signal processor (DSP), a field-programmable gate array (FPGA), a microcontroller, or an industrial control computing platform. The control system includes a signal processing and virtual phase generation module, a virtual electromotive force amplitude generation module, a dynamic virtual impedance generation module, and a current reference and modulation control module.

[0162] The signal processing and virtual phase generation module connects to the voltage sampling channel, current sampling channel, and DC bus voltage sampling channel. It receives the three-phase voltage signal from the grid connection point of the static var generator (SVM), the three-phase current signal from the SVM output, and the DC bus voltage signal. It performs sampling synchronization, zero-drift correction, anti-aliasing filtering, Clarke transform, slow-dynamic DC bus voltage regulation, virtual electromotive force (EMF) phase signal recursion, and Parker transform, outputting the grid connection point voltage vector signal, the grid connection point voltage amplitude signal, the reactive current signal, and the virtual EMF phase signal. The Parker transform uses the virtual EMF phase signal output by the phase recursion unit as the rotation angle. The phase recursion unit receives initial phase information once before grid connection unlocking and receives the rated angular frequency, slow-dynamic angular frequency correction, and sampling period after grid connection unlocking, outputting the virtual EMF phase signal. The input of the phase recursion unit is not connected to the output of the phase-locked loop (PLL) or the active power frequency droop calculation terminal.

[0163] The virtual electromotive force (EMF) amplitude generation module connects to the voltage amplitude output of the signal processing and virtual phase generation modules. The virtual EMF amplitude generation module includes a voltage deviation calculation unit, a rate of change extraction unit, a dynamic proportional gain unit, an integral adjustment unit, and a feedforward adjustment unit. The voltage deviation calculation unit outputs the voltage deviation signal. The rate of change extraction unit outputs the rate of change of the grid-connected point voltage amplitude signal after upper limit constraint. The dynamic proportional gain unit adjusts the proportional gain according to the rate of change of the grid-connected point voltage amplitude signal after upper limit constraint. The integral adjustment unit has freeze and reverse release functions. The feedforward adjustment unit generates a voltage feedforward adjustment amount based on the voltage deviation signal. The virtual EMF amplitude generation module superimposes and limits the above adjustment amounts, outputting a virtual EMF amplitude reference signal.

[0164] The dynamic virtual impedance generation module connects to the reactive current output of both the voltage deviation calculation unit and the signal processing and virtual phase generation module. The module includes a voltage depth assessment unit, a reactive current state assessment unit, a virtual inductance mapping unit, and a virtual resistance mapping unit. The voltage depth assessment unit determines voltage support requirements and fault depth based on the absolute value of the voltage deviation signal. The reactive current state assessment unit determines the static var generator's current handling capacity based on the output reactive current signal. The virtual inductance mapping unit outputs virtual inductance parameters based on the absolute value of the voltage deviation signal. The virtual resistance mapping unit outputs virtual resistance parameters based on the output reactive current signal. The dynamic virtual impedance generation module outputs the virtual resistance and virtual inductance parameters to the current reference and modulation control module.

[0165] The current reference and modulation control module connects to the signal processing and virtual phase generation module, the virtual electromotive force (EMF) amplitude generation module, and the dynamic virtual impedance generation module. The current reference and modulation control module includes an EMF-impedance equivalent network unit, a current coordinate inverse transformation unit, a proportional resonant tracking unit, a damping feedback unit, and a pulse width modulation (PWM) drive unit. The EMF-impedance equivalent network unit receives the virtual EMF phase signal, the virtual EMF amplitude reference signal, the grid connection point voltage vector signal, and the dynamic virtual impedance signal, and calculates the reference current signal. The current coordinate inverse transformation unit converts the reference current signal into a three-phase reference current instantaneous quantity. The proportional resonant tracking unit calculates the current deviation signal between the three-phase reference current instantaneous quantity and the three-phase current signal output by the static var generator (SVM) and forms the fundamental wave tracking control quantity. The damping feedback unit forms a damping control quantity based on the output current change or the filter capacitor current. The fundamental wave tracking control quantity and the damping control quantity are superimposed to form the modulation control quantity, and the PWM drive unit compares the modulation control quantity with the carrier signal to generate a PWM signal.

[0166] In this embodiment, the modules in the control system are not independent functional stacks. The grid-connected point voltage amplitude signal output by the signal processing and virtual phase generation module is used by the virtual electromotive force amplitude generation module. The grid-connected point voltage vector signal output by the signal processing and virtual phase generation module is used by the current reference and modulation control module. The output reactive current signal output by the signal processing and virtual phase generation module is used by the dynamic virtual impedance generation module. The virtual electromotive force phase signal output by the signal processing and virtual phase generation module is used by the current reference and modulation control module. The virtual electromotive force amplitude reference signal output by the virtual electromotive force amplitude generation module is used by the current reference and modulation control module. The dynamic virtual impedance signal output by the dynamic virtual impedance generation module is used by the current reference and modulation control module. The pulse width modulation signal output by the current reference and modulation control module changes the three-phase current signal output by the static var generator. The new three-phase current signal output by the static var generator is returned to the signal processing and virtual phase generation module in the next sampling cycle, forming a closed loop.

[0167] Comparative Example 1: Comparative Example 1 uses a grid-connected static var generator (SVR). The digital controller tracks the voltage phase at the grid connection point via a phase-locked loop (PLL) and generates a reactive current reference based on the outer voltage loop and inner current loop. At a short-circuit ratio of... In the weak grid model, The voltage drop at the grid connection point is constantly applied. A per-unit fault. In the initial stage of the fault, the voltage phase at the grid connection point experiences rapid disturbances, causing oscillations between the phase-locked loop (PLL) output phase and the actual fundamental phase. Since the inner current loop uses the PLL output phase as its coordinate axis, second-harmonic and low-frequency oscillation components appear in the output reactive current. Results show that the peak output reactive current of the comparative example reaches... The per-unit value exceeds the set current limit, and the grid connection point voltage recovers slowly. Comparative Example 1 illustrates that in weak power grids, grid-following control that relies on the actual phase feedback of the grid connection point easily introduces voltage disturbances into the current control axis.

[0168] Comparative Example 2: Comparative Example 2 employs a traditional virtual synchronous generator (VSG) with grid control. The digital controller retains the active power and frequency control paths as well as the reactive power and voltage control paths. The internal phase is generated by the active power deviation and the virtual inertia equation. The static var generator (SVA) hardware is identical to that of Example 1, with only ordinary DC capacitors on the DC side. After a voltage drop in a weak grid, the internal phase angle of the traditional virtual synchronous generator control dynamically oscillates, resulting in transient active power exchange between the converter and the grid. Due to the limited energy of the DC-side capacitors, the DC bus voltage drops significantly shortly after the fault. Comparative Example 2, after the fault... If the internal DC bus voltage drops to near the undervoltage protection threshold, the protection may be triggered if the grid control output is not reduced. Comparative Example 2 illustrates that without a large-capacity active power storage static var generator, it is not suitable to directly use active power and frequency coupled grid control.

[0169] Comparative Example 3: Comparative Example 3 employs fixed virtual electromotive force (EMF) amplitude and fixed virtual impedance control. The digital controller does not use a phase-locked loop (PLL), but the virtual EMF amplitude is generated only based on a fixed voltage reference value, and the dynamic virtual impedance signal is replaced by a fixed impedance value. The voltage of the weak grid drops to... At per-unit values, the voltage difference between the internal virtual electromotive force and the grid connection point voltage increases, while the fixed impedance value cannot increase with the fault depth, and the peak output reactive current reaches [a certain value]. Per-unit value. If the output current is forcibly limited by current limiting, the current waveform will clip, the proportional resonant regulator output will saturate, and a secondary current surge will occur during fault recovery. This indicates that simply fixing the virtual phase is insufficient to solve the fault current limiting problem; the voltage deviation signal and the output reactive current signal must be introduced into the dynamic virtual impedance signal generation process.

[0170] Comparative Example 4: Comparative Example 4 employs virtual electromotive force impedance control without slow-dynamic DC bus voltage regulation. The digital controller does not use a phase-locked loop, nor does it provide a path for the DC bus voltage deviation to the slow-dynamic angular frequency correction. Under the same weak power grid and the same loss parameters, during long-term operation, the conduction losses, switching losses of the power switching devices, and the losses of the AC-side filter inductor winding continuously consume the DC-side capacitor energy. The results show that Comparative Example 4... After steady-state operation, the DC bus voltage is lower than the rated value. This triggers the DC undervoltage protection. This demonstrates that slow dynamic DC bus voltage regulation is a necessary compensation mechanism for maintaining the long-term operation of a static var generator without a large-capacity active power storage system.

[0171] The results of the examples and comparative examples are explained below:

[0172] Under the same weak power grid and fault conditions, the peak output reactive current of Example 1 is Per-unit value, comparative example 1: peak output reactive current is Per-unit value; Comparative Example 2 is limited by DC undervoltage and cannot be continuously supported; Comparative Example 3 output reactive current peak value is... Per-unit value. Example 1 in... The voltage at the grid connection point will be changed from The per-unit value has been raised to The per-unit value, in Comparative Example 1, is unstable due to phase oscillation; in Comparative Example 3, it faces overcurrent risk due to insufficient current limiting. Example 1... After steady-state operation, the DC bus voltage remains at the rated value. Within the range, Comparative Example 4 shows a DC bus voltage drop under the same steady-state loss conditions.

[0173] This invention is not limited to the above embodiments. For static var generators of different voltage levels, the rated angular frequency can be selected as... or Corresponding angular frequency. For chain-type static var generators, modular multilevel static var generators, or two-level voltage source static var generators, the processes for signal acquisition, virtual electromotive force (EMF) phase signal generation, virtual EMF amplitude reference signal generation, dynamic virtual impedance signal generation, and reference current signal generation can all remain consistent. For asymmetrical fault scenarios, positive and negative sequence separation can be added to the signal processing and virtual phase generation modules, allowing the positive sequence voltage amplitude to participate in the generation of the virtual EMF amplitude reference signal, and allowing the negative sequence current to participate in the generation of additional damping or current limiting increments. The above extensions still focus on the coordination of phase-locked loop-free virtual EMF phase signal, slow dynamic DC bus voltage loss compensation, and dynamic virtual impedance signal, without departing from the concept of this embodiment.

[0174] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0175] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for active voltage support control of a grid-type power grid based on virtual electromotive force impedance, characterized in that, include: Collect the three-phase signal at the grid connection point of the static var generator, perform coordinate transformation on the three-phase signal at the grid connection point of the static var generator to obtain the grid connection point voltage vector signal and the output reactive current signal; The DC voltage deviation signal between the DC bus voltage signal and the DC voltage reference value is collected to generate a slow dynamic angular frequency correction. The rated angular frequency and the slow dynamic angular frequency correction are superimposed and integrated to generate a virtual electromotive force phase signal. A virtual electromotive force amplitude reference signal is generated based on the voltage deviation signal between the grid connection point voltage amplitude signal and the voltage reference value, as well as the rate of change of the grid connection point voltage amplitude signal. Impedance parameter mapping is calculated based on the voltage deviation signal and the output reactive current signal to generate a dynamic virtual impedance signal containing virtual resistance parameters and virtual inductance parameters. The grid connection point voltage vector signal, virtual electromotive force phase signal, virtual electromotive force amplitude reference signal and dynamic virtual impedance signal are input into the electromotive force impedance equivalent network to generate a reference current signal. Based on the reference current signal and the three-phase current signal output by the static var generator, current tracking control with resonance suppression is performed to obtain the modulation control quantity. The modulation control quantity is then converted into a pulse width modulation signal to control the static var generator to inject or absorb reactive current to the grid connection point.

2. The active voltage support control method for grid-type power grids based on virtual electromotive force impedance according to claim 1, characterized in that, The process of acquiring the three-phase signal at the grid connection point of the static var generator (SVM), performing coordinate transformation on the three-phase signal at the grid connection point to obtain the grid connection point voltage vector signal and the output reactive current signal includes: Collect the three-phase voltage signal at the grid connection point of the static var generator and the three-phase current signal output by the static var generator; Clarke transforms are performed on the three-phase voltage and three-phase current signals at the grid connection point of the static var generator to obtain the voltage and current components in the two-phase stationary coordinate system, respectively. By performing Parker transformation on the voltage and current components, the grid connection point voltage vector signal and the output current vector signal in the synchronous rotating coordinate system are obtained. The output reactive current signal is determined based on the component in the output current vector signal that corresponds to the reactive direction, and the positive direction of the output reactive current signal is obtained by the capacitive reactive current injected by the static var generator into the grid connection point.

3. The active voltage support control method for grid-type power grids based on virtual electromotive force impedance according to claim 1, characterized in that, The generation of the virtual electromotive force phase signal includes: The generation of the slow dynamic angular frequency correction includes: Acquire the DC bus voltage signal, and determine the DC voltage deviation signal based on the DC voltage reference value and the DC bus voltage signal; Based on the DC voltage proportional adjustment coefficient, the DC voltage deviation signal, and the DC voltage integral adjustment coefficient, the slow dynamic angular frequency correction amount is obtained; Read the first zero-crossing information of the three-phase voltage signal at the grid connection point of the static var generator to obtain the initial value of the virtual electromotive force phase signal; Obtain the rated angular frequency. Based on the initial value of the virtual electromotive force phase signal, integrate the rated angular frequency and the slow dynamic angular frequency correction after superimposing them to obtain the virtual electromotive force phase signal. When the virtual electromotive force phase signal exceeds the phase range corresponding to one power frequency cycle, the digital controller performs periodic processing on the virtual electromotive force phase signal, so that the virtual electromotive force phase signal cycles within the phase range corresponding to one power frequency cycle.

4. The active voltage support control method for grid-type power grids based on virtual electromotive force impedance according to claim 1, characterized in that, The acquisition of the voltage deviation signal between the grid connection point voltage amplitude signal and the voltage reference value includes: Collect the voltage amplitude signal at the grid connection point and perform amplitude limiting filtering on the voltage amplitude signal at the grid connection point: if the change in the voltage amplitude signal at the grid connection point in adjacent sampling periods exceeds the upper limit of the physical change allowed by the equipment, it is truncated according to the upper limit of the physical change; if there are high-frequency spikes near the switching frequency in the voltage amplitude signal at the grid connection point, a first-order low-pass filter is used to obtain the voltage amplitude signal at the grid connection point after amplitude limiting filtering. The voltage deviation signal is obtained by subtracting the voltage amplitude signal at the grid connection point after amplitude limiting and filtering from the voltage reference value. When the grid connection point voltage is lower than the voltage reference value, the voltage deviation signal is positive. The digital controller increases the virtual electromotive force amplitude reference signal and drives the static var generator to inject capacitive reactive current. When the grid connection point voltage is higher than the voltage reference value, the voltage deviation signal is negative. The digital controller reduces the virtual electromotive force amplitude reference signal or drives the static var generator to absorb inductive reactive current.

5. The active voltage support control method for grid-type power grids based on virtual electromotive force impedance according to claim 1, characterized in that, The step of generating a virtual electromotive force amplitude reference signal based on the voltage deviation signal between the grid connection point voltage amplitude signal and the voltage reference value, and the rate of change of the grid connection point voltage amplitude signal, specifically includes: The rate of change of the grid connection point voltage amplitude signal is extracted based on the filtered grid connection point voltage amplitude signal of adjacent sampling periods, and an upper limit constraint is set on the rate of change of the grid connection point voltage amplitude signal to obtain the rate of change of the grid connection point voltage amplitude signal after the upper limit constraint. The rate of change of the voltage deviation signal, the grid connection point voltage amplitude signal after upper limit constraint, and the voltage reference value are input into the virtual electromotive force amplitude generation process to obtain the virtual electromotive force amplitude reference signal. The process is as follows: ； In the formula, This is a reference signal for the amplitude of the virtual electromotive force. This is the voltage reference value. Based on the proportional gain, The coefficient for enhancing the rate of change. The rate of change of the grid connection point voltage amplitude signal after the upper limit constraint. This is a voltage deviation signal. Let be the voltage deviation signal in the m-th sampling period. For integral gain, This refers to the sampling period number in the integral calculation. The sampling period number is... The sampling period is This is the voltage feedforward coefficient.

6. The active voltage support control method for grid-type power grids based on virtual electromotive force impedance according to claim 5, characterized in that, After obtaining the virtual electromotive force amplitude reference signal, the following is also included: Set integral freeze and integral reverse release conditions: When the virtual electromotive force amplitude reference signal reaches the limit and the voltage deviation signal still drives the virtual electromotive force amplitude reference signal to change in the same direction, the digital controller freezes the integral adjustment amount; when the polarity of the voltage deviation signal is opposite to the direction of action of the integral adjustment amount, the digital controller releases the integral adjustment amount in reverse.

7. The active voltage support control method for grid-type power grids based on virtual electromotive force impedance according to claim 1, characterized in that, The step of performing impedance parameter mapping calculation based on the voltage deviation signal and the output reactive current signal to generate a dynamic virtual impedance signal containing virtual resistance and virtual inductance parameters includes: The voltage deviation signal and the output reactive current signal are acquired. The voltage deviation signal is used to reflect the voltage support requirements and fault depth at the grid connection point. The output reactive current signal is used to reflect the withstand capacity of the static var generator body. Calculate the virtual inductance parameters based on the voltage deviation signal: ; In the formula, For virtual inductance parameters, Based on the virtual inductance parameters, This is the virtual inductance amplification factor. It is a natural constant. For voltage deviation sensitivity coefficient, This is the voltage deviation signal expressed in per-unit values. When calculating using per-unit values, It is a dimensionless parameter; when calculating using a nominal voltage value, Its dimension is the reciprocal of voltage; Calculate the virtual resistance parameters based on the output reactive current signal: ; In the formula, For virtual resistance parameters, Based on the basic virtual resistance parameters, This is the current damping amplification factor. The output reactive current signal is expressed in per-unit value. This is the reactive current warning value expressed in per-unit terms. This is a function for finding the maximum value. When using per-unit values ​​for calculation, The dimensions of the current are consistent with those of the virtual resistance parameter; when using the nominal current for calculation, The dimension of resistance is the square of the current.

8. The active voltage support control method for grid-type power grids based on virtual electromotive force impedance according to claim 1, characterized in that, The step of inputting the grid connection point voltage vector signal, virtual electromotive force phase signal, virtual electromotive force amplitude reference signal, and dynamic virtual impedance signal into the electromotive force impedance equivalent network to generate a reference current signal includes: The virtual electromotive force amplitude reference signal and the virtual electromotive force phase signal are combined into an internal electromotive force vector; Use the grid connection point voltage vector signal as the boundary voltage vector; Obtain the physical resistance parameters and physical inductance parameters of the AC side of the static var generator, and combine the dynamic virtual impedance signal and the physical impedance of the AC side of the static var generator into the total equivalent impedance. A reference current signal is generated based on the difference between the internal electromotive force vector and the boundary voltage vector, as well as the total equivalent impedance.

9. The active voltage support control method for grid-type power grids based on virtual electromotive force impedance according to claim 1, characterized in that, The current tracking control with resonance suppression based on the reference current signal and the three-phase current signal output from the static var generator yields the modulation control quantity, including: The reference current signal is converted into a three-phase reference current instantaneous quantity through the Parker inverse transform and the Clark inverse transform; Calculate the difference between the three-phase current signal output by the static var generator in the current sampling period and the three-phase current signal output by the static var generator in the previous sampling period, and then subtract the difference from the output of the proportional resonant regulator after multiplying it by the damping coefficient to obtain the current deviation signal. The current deviation signal is input to the proportional resonant regulator, and the output of the proportional resonant regulator and the damping feedback together form the modulation control quantity; the proportional resonant regulator includes a proportional branch and a fundamental resonant branch, the proportional branch amplifies the current deviation signal instantaneously, and the fundamental resonant branch performs cumulative compensation of the current deviation signal near the rated angular frequency.

10. The active voltage support control method for grid-type power grids based on virtual electromotive force impedance according to claim 1, characterized in that, The process of controlling the static var generator to inject or absorb reactive current into the grid connection point also includes real-time monitoring of the voltage amplitude signal at the grid connection point: When the voltage amplitude signal at the grid connection point is lower than the low voltage threshold and the absolute value of the voltage deviation signal exceeds the voltage safety threshold, the digital controller determines that it has entered a deep voltage drop state. After entering a deep voltage drop state, the digital controller does not switch to grid-following mode, does not enable the phase-locked loop, and does not stop the virtual electromotive force phase signal recursion. The digital controller increases the current limiting increment in the dynamic virtual impedance signal and limits the integral of the virtual electromotive force amplitude reference signal from continuing to accumulate; When the grid connection point voltage recovers to the voltage recovery threshold range, and the output reactive current signal is lower than the reactive current recovery threshold for several consecutive sampling cycles, the digital controller determines that it has entered the recovery state. During the recovery phase, the digital controller reduces the current limiting increment in the virtual inductance and virtual resistance parameters according to the preset recovery slope, avoiding a sudden drop in the dynamic virtual impedance signal that could cause a secondary impact on the output current.

Images