A control method of a chain-type angularly connected network STATCOM based on virtual impedance

Abstract

The application relates to the field of power electronics, in particular to a control method of a chain type STATCOM based on virtual impedance, which comprises positive sequence voltage and negative sequence voltage phasor calculation, positive sequence voltage and negative sequence current unit phasor calculation, voltage feedforward unit phasor calculation, reactive voltage control, bridge arm positive sequence reactive current instruction calculation, bridge arm negative sequence reactive current instruction calculation, bridge arm current reference instruction limiting logic, converter valve DC voltage control and modulation wave instruction calculation; the application realizes that the negative sequence current output by the chain type STATCOM is inductive in an asymmetric transient fault; the virtual impedance calculation link is designed as a feedback system mode, so that the situation that the bridge arm current overshoot is too large is avoided when the STATCOM is in a transient fault.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a control method for a chain-type STATCOM based on virtual impedance. Background Technology

[0002] Traditional Static Var Generators (STATCOMs) lack proactive support and natural response capabilities for system frequency and voltage, making them prone to oscillations and overvoltages in weak grid conditions. Grid-based STATCOMs, however, can simulate voltage source characteristics, possessing inertial response and autonomous voltage support capabilities, resulting in strong adaptability to weak grids. Grid-based STATCOMs can actively provide voltage support, with a topology identical to conventional STATCOMs, but without energy storage components. They feature high reliability and low cost, making them suitable for weak grid applications where the goal is to stabilize system voltage, thus addressing the pain points of traditional STATCOMs, such as oscillation and slow response in weak grids. During transient faults, grid-based STATCOMs exhibit rapid current response, leading to faster grid voltage recovery and less overvoltage. Grid-based STATCOMs utilize virtual, inertial-free internal potentials, primarily for voltage support.

[0003] When an asymmetrical fault occurs in the power grid, the grid-connected STATCOM generates a large amount of negative sequence current under this condition, causing voltage divergence between phases. Therefore, the grid-connected STATCOM requires an additional balancing control strategy. Furthermore, when compensating for negative sequence voltage in the power grid, the polarity of the output negative sequence current of the grid-connected STATCOM should always be inductive to avoid overvoltage in the non-faulty phases of the power grid, which could trigger protection mechanisms on other connected power equipment and lead to overvoltage disconnection. To address these issues, this invention proposes a control method for a chain-connected grid-connected STATCOM based on virtual impedance. Summary of the Invention

[0004] This invention provides a control method for a chain-connected grid-type STATCOM based on virtual impedance, which ensures that the negative sequence current output by the chain-connected grid-type STATCOM remains inductive during asymmetric transient faults. The virtual impedance calculation stage is designed as a feedback system to avoid excessive overshoot of the bridge arm current during transient faults in the grid-type STATCOM.

[0005] To achieve the above objectives, the present invention employs the following technical solution:

[0006] A control method for a chain-type delta-connected STATCOM based on virtual impedance includes:

[0007] (1) Calculation of positive sequence voltage and negative sequence voltage phasors: The positive and negative sequence component method based on the second-order generalized integrator is used to decompose the grid voltage to obtain the positive sequence voltage and negative sequence voltage phasors;

[0008] (2) Calculation of positive sequence voltage and negative sequence current unit phasors: The positive and negative sequence component method based on the second-order generalized integrator is adopted to obtain the positive sequence voltage and negative sequence current unit phasors;

[0009] (3) Calculation of unit phasor of voltage feedforward: A second-order generalized integrator is used for single-phase grid voltage to generate two-phase quadrature signals, thereby calculating the unit phasor of the three-phase grid feedforward voltage and filtering out high-order harmonics.

[0010] (4) Reactive voltage control: The deviation of the virtual internal electromotive force is obtained through the reactive voltage droop control loop, which increases the limit on the integral and output of the PI regulator respectively. At the same time, the integral term is cleared during the short circuit fault.

[0011] (5) Calculation of positive sequence reactive current command of bridge arm: The virtual internal electromotive force adopts the positive sequence phase of the grid voltage, and the positive sequence current command is obtained by subtracting the positive sequence voltage of the grid.

[0012] (6) Calculation of negative sequence reactive current command of bridge arm: Set the target value of negative sequence voltage to 0, subtract it from the negative sequence voltage of the grid to obtain the negative sequence current command, and add the negative sequence current command adjustment coefficient;

[0013] (7) Bridge arm current reference command limiting logic: ensure that the maximum current reference command is always less than the maximum withstand current. The input signal of the current reference command limiting logic is the total current reference command, and only the reactive current is dynamically limited. The active current is used for DC voltage equalization control and is not limited.

[0014] (8) DC voltage control of converter valve: For single-phase grounding faults in the power grid, zero-sequence current is injected to achieve energy balance between bridge arms; for phase-to-phase grounding faults in the power grid, in addition to injecting zero-sequence current, inductive negative-sequence current is injected into the power grid at the same time to ensure that the DC voltage of the bridge arm is not over-voltage.

[0015] (9) Modulation wave command calculation: The difference between the reference command of each phase arm current and the feedback of the arm current is passed through a three-phase independent quasi-proportional resonant regulator to obtain the control voltage of the fundamental current of each phase. After being superimposed with the grid voltage feedforward, the final modulation signal is generated.

[0016] Furthermore, the voltage feedforward unit phasor calculation specifically includes: Phase A grid voltage. Divide by the peak value of the phase voltage rating Then, the per-unit value of the A-phase grid voltage is obtained. ; through a second-order generalized integrator and Obtain in-phase signal and orthogonal signals Taking the square root of the sum of squares yields the peak values ​​of the two orthogonal signals. ;Will and Divide by peak value respectively The voltage of phase A of the power grid can then be obtained. feedforward phasor Phasors with a phase lag of 90° The calculation method for the unit phasor of the voltage feedforward in phases B and C is consistent with that in phase A; where k is the damping coefficient. Let be the center frequency, and s be the complex frequency variable.

[0017] Furthermore, the reactive power voltage control specifically includes: voltage reference value. The settings are configured via the HMI (Host Management Interface) and are related to the effective value of the mains voltage. Subtracting them gives the voltage difference between the two, which is then divided by the slope. The corresponding reactive power reference value is obtained. And thus related to reactive power feedback value Subtract, and pass through a PI regulator The deviation of the virtual electromotive force is obtained. In this context, the amplitude limit of the integral term in the PI controller is set independently, and the final output amplitude limit of the PI controller is also set independently. The proportional coefficient of the regulator. The integral coefficient of the regulator. For complex frequency variables; the deviation of the virtual electromotive force. With virtual electromotive force reference value Adding them together, we obtain the virtual electromotive force reference value. ; Through the delay phase Generate a delayed virtual electromotive force reference value. Among them, the delay phase The time constant in The virtual electromotive force reference value is 40ms during the transient fault period. equal This refers to the value before the short-circuit fault occurred; at the same time, the integral value in the PI controller is cleared to prevent the error from accumulating and saturating.

[0018] Furthermore, the calculation of the positive sequence reactive current command for the bridge arm specifically includes: a virtual internal electromotive force reference value. With the positive sequence voltage unit phasor respectively After multiplication, the phasor of the virtual internal electromotive force is obtained. Virtual internal electromotive force phasor With positive sequence voltage components After subtraction, the voltage deviation is obtained. Then multiply by the reciprocal of the virtual inductance. Obtain the positive sequence current reference command value , where the time constant The value is the virtual inductance. Inductance value; positive sequence current reference command value Through a second-order bandpass filter The signal component with a frequency of 50Hz was obtained. ,in, The damping coefficient is... For complex frequency variables, Center frequency 50Hz; positive sequence current reference command value minus This creates a band-stop filter effect, resulting in a signal that has passed through the band-stop filter. When the system is in steady state, the signal passing through the band-stop filter The signal frequency is close to 0, and the 50Hz component cannot pass through; when a transient fault occurs in the system, in addition to the 50Hz signal being blocked, other frequency signals and virtual resistors can pass through. Multiplying them together yields the voltage component across the virtual resistor. ; and thus the voltage deviation Subtract the voltage component across the virtual resistor. This creates negative feedback, i.e., in the voltage deviation amount In this process, the DC component present during transient faults is dissipated through a virtual resistor, thereby allowing the current flowing through the virtual inductor to reference the command. The 50Hz component is used; the calculation method for the positive sequence reactive current command of the bridge arms of phases B and C is consistent with that of phase A.

[0019] Furthermore, the calculation of the negative sequence reactive current command for the bridge arm specifically includes: negative sequence voltage component. , , The difference from the target value of 0 is calculated by multiplying the negative sequence voltage deviation by the reciprocal of the virtual inductance. Obtain the negative sequence current reference command value , where the time constant The value is the virtual inductance. Inductance value; negative sequence current reference command value Through a second-order bandpass filter The signal component with a frequency of 50Hz was obtained. ,in, The damping coefficient is... For complex frequency variables, The center frequency is 50Hz; the negative sequence current reference command value is... minus This creates a band-stop filter effect, resulting in a signal that has passed through the band-stop filter. When the system is in steady state, the signal passing through the band-stop filter The signal frequency is close to 0, and the 50Hz component cannot pass through; when a transient fault occurs in the system, in addition to the 50Hz signal being blocked, other frequency signals and virtual resistors can pass through. Multiplying them together yields the voltage component across the virtual resistor. ; and thus the voltage deviation Subtract the voltage component across the virtual resistor. This creates negative feedback, i.e., in the voltage deviation amount In this process, the DC component present during transient faults is dissipated through a virtual resistor, thereby allowing the current flowing through the virtual inductor to reference the command. The 50Hz component; the calculation method for the negative sequence reactive current command of the bridge arms of phases B and C is consistent with that of phase A.

[0020] Virtual internal electromotive force reference value Phase voltage rating After multiplying, we obtain the named value of the virtual internal electromotive force. ; the positive sequence voltage component and Taking the square root of the sum of squares yields the magnitude of the positive-sequence voltage. Peak value of phase voltage rating After multiplication, the nominal value of the positive sequence voltage is obtained. ; and thus the named value of the virtual internal electromotive force reference value. After subtraction, the deviation value of the positive sequence voltage is obtained. ; the negative sequence voltage component and Taking the square root of the sum of squares yields the magnitude of the positive-sequence voltage. Peak value of phase voltage rating After multiplication, the nominal value of the negative sequence voltage is obtained. That is, the deviation value of the negative sequence voltage. ; Divide by Since the virtual impedances are the same, they are all... Therefore, it is actually a positive sequence current. With negative sequence current The proportion of amplitude, i.e. Then, using a limiter, the result after limiting is recorded as a coefficient. ;Right now ; in coefficient Then, again with the coefficient Multiplication, denoted as Thus, the negative sequence current that can be adjusted by the distribution ratio coefficient is obtained, i.e. , and the negative sequence current reference command value After multiplication, we obtain a positive-sequence current with a magnitude of 1. Multiple of negative sequence current reference command The calculation method for the reference command of the bridge arm current of phase B and phase C is consistent with that of phase A.

[0021] Furthermore, the arm current reference command limiting logic specifically includes: A-phase arm positive sequence reactive current reference command. Reference command for negative sequence reactive current of phase A bridge arm After summing, the total reactive current reference command for phase A bridge arm is obtained. Furthermore, through a second-order low-pass filter Then, the DC component was obtained. In order to isolate the DC component in the current reference command value, Subtracting the DC component yields the unlimited reactive current reference command for the bridge arm. ;in, This refers to the filter's passband gain. The transfer function cutoff frequency; The damping coefficient is the filter damping coefficient; the calculation method for the unlimited reactive current reference command of the bridge arms of phases B and C is consistent with that of phase A.

[0022] Unlimited reactive current reference command for bridge arm , , Divide by the peak value of the bridge arm current rating Then, its per-unit value is obtained, and a Clark transformation is performed on the current command after the per-unit value to transform the three-phase stationary coordinate system into a two-phase stationary coordinate system, thus obtaining an orthogonal AC signal. and Then, the two-phase stationary coordinate system is transformed into a two-phase rotating orthogonal coordinate system, namely the dq coordinate system. The angle used in the coordinate transformation process is the positive sequence voltage angle of phase A, resulting in orthogonal DC signals. and The amplitude of the signal can be obtained by taking the square root of the sum of squares. ;

[0023] Three-phase bridge arm active current reference command , , Each is obtained through a second-order generalized integrator. and Subsequently, two orthogonal AC signals were obtained. Then, the two-phase stationary coordinate system was transformed into a two-phase rotating orthogonal coordinate system, i.e., the dq coordinate system. During the coordinate transformation, the angles used were the unit phasors of the three-phase voltage feedforward at A, B, and C, respectively, to obtain orthogonal DC signals. , , , and , Taking the square root of the sum of squares yields the peak value of the three-phase active current reference command. , , ;

[0024] By solving the quadratic equation for the reactive current limiting coefficient in real time, the reference command value is ensured to be within the range of the maximum design value of short-time overcurrent. Priority is given to ensuring that the active current command is not limited for DC voltage balance control, while the reactive current is limited, resulting in the final unified limiting coefficient for the three-phase bridge arm current. Pick , , The minimum of the three.

[0025] Furthermore, the DC voltage control of the converter valve specifically includes: the average value of the three-phase DC voltage of the converter valve. , , Passing sequentially through the delayed sampling stage And points process The sliding filter, with a mean time constant of 10ms, is used to filter out the second harmonic component in the DC voltage. The filtered signals are as follows: , , The average of the three is denoted as Average value of three-phase DC voltage of converter valve This reflects the overall level of DC voltage, and the result is subtracted from the target value of 1.0 and then passed through a PI regulator. The output of the regulator is the amplitude of the active quantity deviation, and the positive sequence voltage unit phasor. , , After multiplication, the reference command for the active positive sequence current of the bridge arm is obtained. , , ;

[0026] Average value of three-phase DC voltage of converter valve The average value of the three-phase DC voltage of the converter valve, respectively , , Subtracting them gives the DC voltage difference. , , Then, through three-phase independent PI regulators Then, the output result is denoted as , , Calculated with voltage feedforward unit phasor respectively , , After multiplying and superimposing, the injected zero-sequence current component is obtained. It is used to balance the functional quantities among the three-phase DC voltages of the converter valve;

[0027] The average value of the DC voltage after filtering in phase A of the converter valve The average value of the DC voltage after filtering in phase B Average value of C-phase DC voltage after filtering Select the maximum value from the middle Divide by 1.1, i.e., standardize the overvoltage threshold of 1.1 pu per unit, subtract the reference value of 1.0, and then pass it through the PI controller. The output of the regulator is the active variable adjustment quantity. ,when When the value is less than 1.1 pu, the output of the PI controller is 0, which is the lower limit of the amplitude limit; the active variable adjustment value... Respectively with the negative sequence current unit phasor , , After multiplication, the injected negative sequence current reference command is obtained. , , This ensures that the three-phase DC voltage of the converter valve does not experience overvoltage under severely unbalanced operating conditions.

[0028] Furthermore, the modulation wave command calculation specifically includes: bridge arm unlimited reactive current reference command. , , With dynamic limiting coefficient After multiplication, the final bridge arm reactive current reference command is obtained. , , ;

[0029] Bridge arm active positive sequence current reference command , , , and the injected zero-sequence current component Add them together, and then add them to the injected negative sequence current reference command. , , Adding them together, we obtain the total active current reference command for the three-phase bridge arms. , , Final bridge arm reactive current reference command , , Reference command for total active current of bridge arm , , After summing, the total bridge arm current reference command is obtained. , , ; and then with the three-phase bridge arm current feedback , , The difference is calculated, and the resulting current deviation is the sum of the three independent proportional resonant regulators. The input signal is the closed-loop output of each of the three-phase current loops. , , That is, the fundamental current controls the voltage; the feedforward unit phasor of the three-phase grid voltage. , , Phasor and proportionality coefficient Multiplying them yields the per-unit grid voltage feedforward phasor; where, The value is 0.75; after superimposing the feedforward phasor of the fundamental current control voltage with the per-unit three-phase grid voltage, the final modulated wave signal is obtained. , , .

[0030] Compared with the prior art, the beneficial effects of the present invention are:

[0031] 1) This invention designs a DC voltage control method for converter valves, which ensures that the negative sequence current output by the chain-connected grid-type STATCOM is always inductive during asymmetrical transient faults, effectively guaranteeing that the grid voltage of the non-faulty phases is reduced. This is especially important in weak grid conditions, thus avoiding the increase in grid voltage of the non-faulty phases caused by capacitive negative sequence current, which could lead to overvoltage disconnection of other equipment and improve the stability of the power system.

[0032] 2) This invention designs a method for calculating the reference command of the positive and negative sequence reactive current of the bridge arm. The virtual impedance calculation link is designed as a feedback system to avoid the excessive overshoot of the bridge arm current in the network-type STATCOM during transient faults, thereby improving the control effect of the network-type STATCOM. Attached Figure Description

[0033] Figure 1 This is a connection topology diagram of the chain-type corner-connected STATCOM system described in this invention;

[0034] Figure 2 This is a schematic diagram of the calculation structure for the positive and negative sequence components of the power grid voltage described in this invention;

[0035] Figure 3 This is a schematic diagram of the unit phasor calculation structure for positive sequence voltage and negative sequence current described in this invention;

[0036] Figure 4 This is a schematic diagram of the voltage feedforward unit phasor calculation structure described in this invention;

[0037] Figure 5 This is a schematic diagram of the reactive voltage control and bridge arm positive and negative sequence reactive current reference command calculation structure described in this invention;

[0038] Figure 6 This is a schematic diagram of the bridge arm current reference command limiting logic structure described in this invention;

[0039] Figure 7 This is a schematic diagram of the DC voltage control method for the converter valve described in this invention;

[0040] Figure 8 This is a schematic diagram of the modulation wave command calculation stage of the present invention. Detailed Implementation

[0041] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings:

[0042] See Figure 1 This is a connection topology diagram of the chain-connected STATCOM system described in this invention. This invention provides a control method for a chain-connected STATCOM system based on virtual impedance, including positive-sequence and negative-sequence voltage phasor calculation, positive-sequence and negative-sequence current unit phasor calculation, voltage feedforward unit phasor calculation, reactive voltage control, bridge arm positive-sequence reactive current command calculation, bridge arm negative-sequence reactive current command calculation, bridge arm current reference command limiting logic, converter valve DC voltage control, and modulation wave command calculation.

[0043] (1) Calculation of positive and negative sequence phasors of grid voltage, unit phasor of positive sequence voltage, and unit phasor of negative sequence current: see Figure 2-3 The positive and negative sequence component method based on the second-order generalized integrator is used to decompose the grid voltage, thereby obtaining the positive and negative sequence phasors of the grid voltage, as well as the positive sequence voltage unit phasor and the negative sequence current unit vector.

[0044] (2) Voltage feedforward unit phasor calculation: A second-order generalized integrator is used for single-phase grid voltage to generate two-phase quadrature signals, thereby calculating the unit phasor of three-phase grid voltage and filtering out high-order harmonics;

[0045] See Figure 4 Phase A grid voltage Divide by the peak value of the phase voltage rating Then, the per-unit value of the A-phase grid voltage is obtained. ; through a second-order generalized integrator and Obtain in-phase signal and orthogonal signals Taking the square root of the sum of squares yields the peak values ​​of the two orthogonal signals. ;Will and Divide by peak value respectively The voltage of phase A of the power grid can then be obtained. feedforward phasor Phasors with a phase lag of 90° The calculation method for the voltage feedforward phasors of phases B and C is consistent with that of phase A; where k is the damping coefficient. The center frequency is s, and the complex frequency variable is s.

[0046] The meanings of some variables in this control method are as follows:

[0047] : The transfer function of the second-order generalized integrator SOGI;

[0048] : The transfer function of the second-order generalized integrator SOGI.

[0049] (3) Reactive voltage control: The deviation of the virtual internal electromotive force is obtained through the reactive voltage droop control loop, which increases the limit on the integral and output of the PI regulator respectively. At the same time, the integral term is cleared during the short circuit fault.

[0050] See Figure 5 Voltage reference value The settings are configured via the HMI (Host Management Interface) and are related to the effective value of the mains voltage. Subtracting them gives the voltage difference between the two, which is then divided by the slope. The corresponding reactive power reference value is obtained. And thus related to reactive power feedback value Subtract, and pass through a PI regulator The deviation of the virtual internal electromotive force is obtained. In this context, the amplitude limit of the integral term in the PI controller is set independently, and the final output amplitude limit of the PI controller is also set independently. The proportional coefficient of the regulator. The integral coefficient of the regulator. For complex frequency variables; the deviation of the virtual internal electromotive force. Compared with the virtual internal electromotive force reference value Adding them together, we obtain the virtual internal electromotive force reference value. ; Through the delay phase Generate a delayed virtual internal electromotive force reference value. Among them, the delay phase The time constant in The virtual internal electromotive force reference value is 40ms during the transient fault period. equal This refers to the value before the short-circuit fault occurred; at the same time, the integral value in the PI controller is cleared to prevent the error from accumulating and saturating.

[0051] (4) Calculation of positive sequence reactive current command of bridge arm: The virtual internal electromotive force adopts the positive sequence phase of the grid voltage, and the positive sequence voltage of the grid is subtracted to obtain the positive sequence reactive current command of the bridge arm.

[0052] See Figure 5 Virtual internal electromotive force reference value With the positive sequence voltage unit phasor respectively After multiplication, the phasor of the virtual internal electromotive force is obtained. Virtual internal electromotive force phasor With positive sequence voltage components After subtraction, the voltage deviation is obtained. Then multiply by the reciprocal of the virtual inductance. Obtain the positive sequence current reference command value , where the time constant The value is the virtual inductance. Inductance value; positive sequence current reference command value Through a second-order bandpass filter The signal component with a frequency of 50Hz was obtained. ,in, The damping coefficient is... For complex frequency variables, Center frequency 50Hz; positive sequence current reference command value minus This creates a band-stop filter effect, resulting in a signal that has passed through the band-stop filter. When the system is in steady state, the signal passing through the band-stop filter The signal frequency is close to 0, and the 50Hz component cannot pass through; when a transient fault occurs in the system, in addition to the 50Hz signal being blocked, other frequency signals and virtual resistors can pass through. Multiplying them together yields the voltage component across the virtual resistor. This will further reduce the voltage deviation. Subtract the voltage component across the virtual resistor. This creates negative feedback, i.e., in the voltage deviation amount In this process, the DC component present during transient faults is dissipated through a virtual resistor, thereby allowing the current flowing through the virtual inductor to reference the command. The 50Hz component effectively solves the overshoot phenomenon caused by the large DC component in the current reference command at the time of transient fault occurrence. The calculation method for the positive sequence reactive current command of phase B and phase C bridge arms is consistent with that of phase A.

[0053] (5) Calculation of negative sequence reactive current command for bridge arm: Set the target value of negative sequence voltage to 0, subtract it from the negative sequence voltage of the grid to obtain the negative sequence reactive current command for bridge arm, and add the negative sequence current command adjustment coefficient.

[0054] See Figure 5 Negative sequence voltage component , , The difference from the target value of 0 is calculated by multiplying the negative sequence voltage deviation by the reciprocal of the virtual inductance. Obtain the negative sequence current reference command value , where the time constant The value is the virtual inductance. Inductance value; negative sequence current reference command value Through a second-order bandpass filter The signal component with a frequency of 50Hz was obtained. ,in, The damping coefficient is... For complex frequency variables, The center frequency is 50Hz; the negative sequence current reference command value is... minus This creates a band-stop filter effect, resulting in a signal that has passed through the band-stop filter. When the system is in steady state, the signal passing through the band-stop filter The signal frequency is close to 0, and the 50Hz component cannot pass through; when a transient fault occurs in the system, in addition to the 50Hz signal being blocked, other frequency signals and virtual resistors can pass through. Multiplying them together yields the voltage component across the virtual resistor. ; and thus the voltage deviation Subtract the voltage component across the virtual resistor. This creates negative feedback, i.e., in the voltage deviation amount In this process, the DC component present during transient faults is dissipated through a virtual resistor, thereby allowing the current flowing through the virtual inductor to reference the command. The 50Hz component effectively solves the overshoot phenomenon caused by the large DC component in the current reference command at the moment of transient fault occurrence; the calculation method of negative sequence reactive current command for phase B and phase C bridge arms is consistent with that of phase A.

[0055] Virtual internal electromotive force reference value Phase voltage rating After multiplying, we obtain the named value of the virtual internal electromotive force. ; the positive sequence voltage component and Taking the square root of the sum of squares yields the magnitude of the positive-sequence voltage. Peak value of phase voltage rating After multiplication, the nominal value of the positive sequence voltage is obtained. ; and thus the named value of the virtual internal electromotive force reference value. After subtraction, the deviation value of the positive sequence voltage is obtained. ; the negative sequence voltage component and Taking the square root of the sum of squares yields the magnitude of the positive-sequence voltage. Peak value of phase voltage rating After multiplication, the nominal value of the negative sequence voltage is obtained. That is, the deviation value of the negative sequence voltage. ; Divide by Since the virtual impedances are the same, they are all... Therefore, it is actually a positive sequence current. With negative sequence current The proportion of amplitude, i.e. Then, using a limiter, the result after limiting is recorded as a coefficient. ;Right now ; in coefficient Then, again with the coefficient Multiplication, denoted as Thus, the negative sequence current that can be adjusted by the distribution ratio coefficient is obtained, i.e. , and the negative sequence current reference command value After multiplication, we obtain a positive-sequence current with a magnitude of 1. Multiple of negative sequence current reference command The calculation method for the reference command of the bridge arm current of phase B and phase C is consistent with that of phase A.

[0056] (6) Bridge arm current reference command limiting logic: ensure that the maximum current reference command is always less than the maximum withstand current. The input signal of the current reference command limiting logic is the total current reference command, and only the reactive current is dynamically limited. The active current is used for DC voltage balancing control and is not limited.

[0057] A-phase bridge arm positive sequence reactive current reference command Reference command for negative sequence reactive current of phase A bridge arm After summing, the total reactive current reference command for phase A bridge arm is obtained. Furthermore, through a second-order low-pass filter Then, the DC component was obtained. In order to isolate the DC component in the current reference command value, Subtracting the DC component yields the unlimited reactive current reference command for the bridge arm. ;in, This refers to the filter's passband gain. The transfer function cutoff frequency; The damping coefficient is the filter damping coefficient; the calculation method for the unlimited reactive current reference command of the bridge arms of phases B and C is consistent with that of phase A.

[0058] The meanings of some variables in the reactive voltage control method are as follows:

[0059] : Transfer function of reactive voltage control PI regulator;

[0060] : Regulator proportional coefficient;

[0061] : Integral coefficient of the regulator;

[0062] Complex frequency variables;

[0063] The meanings of some variables in the virtual impedance calculation are as follows:

[0064] Virtual resistance;

[0065] Virtual inductance;

[0066] The transfer function of the second-order low-pass filter (LPF) used is as follows:

[0067]

[0068] The meanings of the variables in this section are as follows:

[0069] Complex frequency variables;

[0070] Filter passband gain;

[0071] : Transfer function cutoff frequency;

[0072] : Filter damping coefficient;

[0073] See Figure 6 Unlimited reactive current reference command for bridge arm , , Divide by the peak value of the bridge arm current rating Then, its per-unit value is obtained; a Clark transformation is performed on the per-unit current command to transform the three-phase stationary coordinate system to a two-phase stationary coordinate system, resulting in an orthogonal AC signal. and This process transforms the two-phase stationary coordinate system into a two-phase rotating orthogonal coordinate system, namely the dq coordinate system. The angle used in the coordinate transformation is the positive sequence voltage angle of phase A, resulting in orthogonal DC signals. and The amplitude of the signal can be obtained by taking the square root of the sum of squares. ;

[0074] Three-phase active current reference command , , Each is obtained through a second-order generalized integrator. and Subsequently, two orthogonal AC signals were obtained. Then, the two-phase stationary coordinate system was transformed into a two-phase rotating orthogonal coordinate system, i.e., the dq coordinate system. During the coordinate transformation, the angles used were the unit phasors of the three-phase voltage feedforward at A, B, and C, respectively, to obtain orthogonal DC signals. , , , and , Taking the square root of the sum of squares yields the peak value of the three-phase active current reference command. , , ;

[0075] The current command limiting logic ensures that the reference command value is within the maximum design value range of short-time overcurrent by solving the quadratic equation of reactive current limiting coefficient in real time. It also prioritizes ensuring that the active current command is not limited, which is used to control DC voltage balance, while the reactive current is reasonably limited. This method is more reasonable and avoids directly limiting only the total current command, which would affect the active power control link.

[0076] Taking phase A current limiting as an example, the calculation process of the formula is explained in detail:

[0077] variable Let be the reactive current dynamic limiting coefficient to be solved, and let be the total transient short-time current overload factor. By projecting the positive and negative sequence current reference commands and the active current onto the dq coordinate system, the following equation can be obtained:

[0078] ;

[0079] After the formula is expanded,

[0080] ;

[0081] It can be seen that the position variable in the formula is only a variable. It is about variables. The quadratic equation of .

[0082] because , Therefore, the above formula can be simplified to:

[0083] ;

[0084] The standard form of a quadratic equation in one variable is:

[0085] , ;

[0086] According to the quadratic formula: , thus obtaining variables The root is:

[0087] ;

[0088] B and C phase current dynamic limiting coefficients , The calculation process is consistent with that of phase A current, and will not be repeated here; the final unified limiting coefficient for the three-phase bridge arm current is obtained. Pick , , The minimum value among the three;

[0089] The meanings of the variables in this section are as follows:

[0090] Transient short-time current overload factor;

[0091] The formula for the Park transform used in this section is as follows:

[0092]

[0093] The meanings of the variables in this section are as follows:

[0094] Before transformation Axial components;

[0095] Before transformation Axial components;

[0096] : The transformed D-axis component;

[0097] : Transformed Q-axis component;

[0098] : Phase angle of the coordinate transformation signal.

[0099] (7) DC voltage control of converter valve: For single-phase grounding faults in the power grid, zero-sequence current is injected to achieve energy balance between bridge arms; for phase-to-phase grounding faults in the power grid, in addition to injecting zero-sequence current, an inductive negative-sequence current is injected into the power grid to ensure that the DC voltage of the bridge arm is not over-voltage.

[0100] See Figure 7 Average value of three-phase DC voltage of converter valve , , Passing sequentially through the delayed sampling stage And points process The sliding filter, with a mean time constant of 10ms, is used to filter out the second harmonic component in the DC voltage. The filtered signals are as follows: , , The average of the three is denoted as Average value of three-phase DC voltage of converter valve This reflects the overall level of DC voltage, and the result is subtracted from the target value of 1.0 and then passed through a PI regulator. The output of the regulator is the amplitude of the active quantity deviation, and the positive sequence voltage unit phasor. , , After multiplication, the reference command for the active positive sequence current of the bridge arm is obtained. , , ;

[0101] Average value of three-phase DC voltage of converter valve The average value of the three-phase DC voltage of the converter valve, respectively , , Subtracting them gives the DC voltage difference. , , Then, through three-phase independent PI regulators Then, the output result is denoted as , , Calculated with voltage feedforward unit phasor respectively , , After multiplying and superimposing, the injected zero-sequence current component is obtained. It is used to balance the functional quantities among the three-phase DC voltages of the converter valve;

[0102] The average value of the DC voltage after filtering in phase A of the converter valve The average value of the DC voltage after filtering in phase B Average value of C-phase DC voltage after filtering Select the maximum value from the middle Divide by 1.1, i.e., standardize the overvoltage threshold of 1.1 pu per unit, subtract the reference value of 1.0, and then pass it through the PI controller. The output of the regulator is the active variable adjustment quantity. ,when When the value is less than 1.1 pu, the output of the PI controller is 0, which is the lower limit of the amplitude limit; the active variable adjustment value... Respectively with the negative sequence current unit phasor , , After multiplication, the injected negative sequence current reference command is obtained. , , This ensures that the three-phase DC voltage of the converter valve does not experience overvoltage under severely unbalanced operating conditions.

[0103] (8) Modulation wave command calculation: The difference between the reference command of each phase arm current and the feedback of the arm current is passed through a three-phase independent quasi-proportional resonant regulator to obtain the control voltage of the fundamental current of each phase. After being superimposed with the grid voltage feedforward, the final modulation signal is generated.

[0104] See Figure 8 Unlimited reactive current reference command for bridge arm , , With dynamic limiting coefficient After multiplication, the final bridge arm reactive current reference command is obtained. , , Bridge arm active positive sequence current reference command , , , and the injected zero-sequence current component Add them together, and then add them to the injected negative sequence current reference command. , , Adding them together, we obtain the total active current reference command for the three-phase bridge arms. , , Final bridge arm reactive current reference command , , Reference command for total active current of bridge arm , , After summing, the total bridge arm current reference command is obtained. , , ; and then with the three-phase bridge arm current feedback , , The difference is calculated, and the resulting current deviation is the sum of the three independent proportional resonant regulators. The input signal is the closed-loop output of each of the three-phase current loops. , , That is, the fundamental current controls the voltage; the feedforward unit phasor of the three-phase grid voltage. , , Phasor and proportionality coefficient Multiplying them yields the per-unit grid voltage feedforward phasor; where, The value is 0.75; after superimposing the feedforward phasor of the fundamental current control voltage with the per-unit three-phase grid voltage, the final modulated wave signal is obtained. , , ;

[0105] The meanings of some variables in the modulation wave command calculation method are as follows:

[0106] : The transfer function of the PI regulator for the average value of the three-phase DC voltage;

[0107] : Quasi-PR regulator transfer function.

[0108] The above embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the above embodiments. Unless otherwise specified, the methods used in the above embodiments are conventional methods.

Claims

1. A control method for a chain-type delta-connected STATCOM based on virtual impedance, characterized in that, include: (1) Calculation of positive sequence voltage and negative sequence voltage phasors: The positive and negative sequence component method based on the second-order generalized integrator is used to decompose the grid voltage to obtain the positive sequence voltage and negative sequence voltage phasors; (2) Calculation of positive sequence voltage and negative sequence current unit phasors: The positive and negative sequence component method based on the second-order generalized integrator is adopted to obtain the positive sequence voltage and negative sequence current unit phasors; (3) Calculation of unit phasor of voltage feedforward: A second-order generalized integrator is used for single-phase grid voltage to generate two-phase quadrature signals, thereby calculating the unit phasor of the three-phase grid feedforward voltage and filtering out high-order harmonics. (4) Reactive voltage control: The deviation of the virtual internal electromotive force is obtained through the reactive voltage droop control loop. The value of the reactive reference value and the reactive feedback value in the reactive voltage droop control loop are increased by the integral and output of the PI regulator respectively. At the same time, the integral term is cleared during the short circuit fault. (5) Calculation of positive sequence reactive current reference command of bridge arm: The virtual internal electromotive force reference value is obtained by using the deviation of the virtual internal electromotive force. The virtual internal electromotive force reference value is multiplied by the positive sequence voltage unit phasor, and the positive sequence current command is obtained by subtracting it from the positive sequence voltage of the grid. (6) Calculation of negative sequence reactive current reference command for bridge arm: Set the target value of negative sequence voltage to 0, subtract it from the negative sequence voltage of the grid to obtain the negative sequence current command, and add the negative sequence current command adjustment coefficient; (7) Bridge arm current reference command limiting logic: ensure that the maximum current reference command is always less than the maximum withstand current. The input signals of the current reference command limiting logic are the bridge arm positive sequence reactive current reference command and the bridge arm negative sequence reactive current reference command, and only the reactive current is dynamically limited. The active current is used for DC voltage equalization control and is not limited. (8) DC voltage control of converter valve: For single-phase grounding faults in the power grid, zero-sequence current is injected to achieve energy balance between bridge arms; for phase-to-phase grounding faults in the power grid, in addition to injecting zero-sequence current, an inductive negative-sequence current unit phasor is injected into the power grid to ensure that the DC voltage of the bridge arm is not over-voltage. (9) Modulation wave command calculation: The difference between the reference command of each phase arm current and the feedback of the arm current is passed through a three-phase independent quasi-proportional resonant regulator to obtain the control voltage of the fundamental current of each phase. After being superimposed with the grid voltage feedforward, the final modulation signal is generated.

2. The control method for a chain-type corner-connected STATCOM based on virtual impedance according to claim 1, characterized in that, The voltage feedforward unit phasor calculation specifically includes: Phase A grid voltage. Divide by the peak value of the phase voltage rating Then, the per-unit value of the A-phase grid voltage is obtained. ; through a second-order generalized integrator and Obtain in-phase signal and orthogonal signals Taking the square root of the sum of squares yields the peak values ​​of the two orthogonal signals. ;Will and Divide by peak value respectively The voltage of phase A of the power grid can then be obtained. feedforward phasor Phasors with a phase lag of 90° The calculation method for the unit phasor of the voltage feedforward in phases B and C is consistent with that in phase A; where k is the damping coefficient. Let be the center frequency, and s be the complex frequency variable.

3. The control method for a chain-type corner-connected STATCOM based on virtual impedance according to claim 2, characterized in that, The reactive power control specifically includes: voltage reference value. The settings are configured via the HMI (Host Management Interface) and are related to the effective value of the mains voltage. Subtracting them gives the voltage difference between the two, which is then divided by the slope. The corresponding reactive power reference value is obtained. And thus related to reactive power feedback value Subtract, and pass through a PI regulator The deviation of the virtual internal electromotive force is obtained. In this context, the amplitude limit of the integral term in the PI controller is set independently, and the final output amplitude limit of the PI controller is also set independently. The proportional coefficient of the regulator. The integral coefficient of the regulator. For complex frequency variables; the deviation of the virtual internal electromotive force. Compared with the virtual internal electromotive force reference value Adding them together, we obtain the virtual internal electromotive force reference value. ; Through the delay phase Generate a delayed virtual internal electromotive force reference value. Among them, the delay phase The time constant in The virtual internal electromotive force reference value is 40ms during the transient fault period. equal This refers to the value before the short-circuit fault occurred; at the same time, the integral value in the PI controller is cleared to prevent the error from accumulating and saturating.

4. The control method for a chain-type corner-connected STATCOM based on virtual impedance according to claim 3, characterized in that, The calculation of the bridge arm positive sequence reactive current reference command specifically includes: virtual internal electromotive force reference value. With the positive sequence voltage unit phasor respectively After multiplication, the phasor of the virtual internal electromotive force is obtained. Virtual internal electromotive force phasor Positive sequence voltage components in a three-phase stationary coordinate system After subtraction, the voltage deviation is obtained. Then multiply by the reciprocal of the virtual inductance. Obtain the positive sequence current reference command value , where the time constant The value is the virtual inductance. Inductance value; positive sequence current reference command value Through a second-order bandpass filter The signal component with a frequency of 50Hz was obtained. ,in, The damping coefficient is... For complex frequency variables, Center frequency 50Hz; positive sequence current reference command value minus This creates a band-stop filter effect, resulting in a signal that has passed through the band-stop filter. When the system is in steady state, the signal passing through the band-stop filter The signal frequency is close to 0, and the 50Hz component cannot pass through; when a transient fault occurs in the system, in addition to the 50Hz signal being blocked, other frequency signals and virtual resistors can pass through. Multiplying them together yields the voltage component across the virtual resistor. ; and thus the voltage deviation Subtract the voltage component across the virtual resistor. This creates negative feedback, i.e., in the voltage deviation amount In this process, the DC component present during transient faults is dissipated through a virtual resistor, thereby allowing the current flowing through the virtual inductor to reference the command. The 50Hz component is used; the calculation method for the positive sequence reactive current reference command of phase B and phase C bridge arms is consistent with that of phase A.

5. The control method for a chain-type corner-connected STATCOM based on virtual impedance according to claim 4, characterized in that, The calculation of the bridge arm negative sequence reactive current reference command specifically includes: negative sequence voltage components in the three-phase stationary coordinate system. , , The difference from the target value of 0 is calculated by multiplying the negative sequence voltage deviation by the reciprocal of the virtual inductance. Obtain the negative sequence current reference command value , where the time constant The value is the virtual inductance. Inductance value; negative sequence current reference command value Through a second-order bandpass filter The signal component with a frequency of 50Hz was obtained. ,in, The damping coefficient is... For complex frequency variables, The center frequency is 50Hz; the negative sequence current reference command value is... minus This creates a band-stop filter effect, resulting in a signal that has passed through the band-stop filter. When the system is in steady state, the signal passing through the band-stop filter The signal frequency is close to 0, and the 50Hz component cannot pass through; when a transient fault occurs in the system, in addition to the 50Hz signal being blocked, other frequency signals and virtual resistors can pass through. Multiplying them together yields the voltage component across the virtual resistor. ; and thus the voltage deviation Subtract the voltage component across the virtual resistor. This creates negative feedback, i.e., in the voltage deviation amount In this process, the DC component present during transient faults is dissipated through a virtual resistor, thereby allowing the current flowing through the virtual inductor to reference the command. The 50Hz component; the calculation method for the negative sequence reactive current reference command of phase B and phase C bridge arms is consistent with that of phase A. Virtual internal electromotive force reference value Phase voltage rating After multiplication, we obtain the named value of the virtual internal electromotive force. ; the positive sequence voltage components in the two-phase stationary coordinate system and Taking the square root of the sum of squares yields the magnitude of the positive-sequence voltage. Peak value of phase voltage rating After multiplication, the nominal value of the positive sequence voltage is obtained. ; and thus the named value of the virtual internal electromotive force reference value. After subtraction, the deviation value of the positive sequence voltage is obtained. The negative sequence voltage component in the two-phase stationary coordinate system and Taking the square root of the sum of squares yields the magnitude of the positive-sequence voltage. Peak value of phase voltage rating After multiplication, the nominal value of the negative sequence voltage is obtained. That is, the deviation value of the negative sequence voltage. ; Divide by Since the virtual impedances are the same, they are all... Therefore, it is actually a positive sequence current. With negative sequence current The proportion of amplitude, i.e. Then, using a limiter, the result after limiting is recorded as a coefficient. ;Right now ; in coefficient Then, again with the coefficient Multiplication, denoted as Thus, the negative sequence current that can be adjusted by the distribution ratio coefficient is obtained, i.e. , and the negative sequence current reference command value After multiplication, we obtain the positive sequence current with the magnitude of the product. Multiple of negative sequence current reference command The calculation method for the reference command of the bridge arm current of phase B and phase C is consistent with that of phase A.

6. The control method for a chain-type corner-connected STATCOM based on virtual impedance according to claim 5, characterized in that, The bridge arm current reference command limiting logic specifically includes: A-phase bridge arm positive sequence reactive current reference command. Reference command for negative sequence reactive current of phase A bridge arm After summing, the total reactive current reference command for phase A bridge arm is obtained. Furthermore, through a second-order low-pass filter Then, the DC component was obtained. In order to isolate the DC component in the current reference command value, Subtracting the DC component yields the unlimited reactive current reference command for the bridge arm. ;in, This refers to the filter's passband gain. The transfer function cutoff frequency; The damping coefficient is the filter damping coefficient; the calculation method for the unlimited reactive current reference command of the bridge arms of phases B and C is consistent with that of phase A. Unlimited reactive current reference command for bridge arm , , Divide by the peak value of the bridge arm current rating Then, its per-unit value is obtained, and a Clark transformation is performed on the current command after the per-unit value to transform the three-phase stationary coordinate system into a two-phase stationary coordinate system, thus obtaining an orthogonal AC signal. and Then, the two-phase stationary coordinate system is transformed into a two-phase rotating orthogonal coordinate system, namely the dq coordinate system. The angle used in the coordinate transformation process is the positive sequence voltage angle of phase A, resulting in orthogonal DC signals. and The amplitude of the signal can be obtained by taking the square root of the sum of squares. ; Three-phase bridge arm active current reference command , , Each is obtained through a second-order generalized integrator. and Subsequently, two orthogonal AC signals were obtained. Then, the two-phase stationary coordinate system was transformed into a two-phase rotating orthogonal coordinate system, i.e., the dq coordinate system. During the coordinate transformation, the angles used were the unit phasors of the three-phase voltage feedforward at A, B, and C, respectively, to obtain orthogonal DC signals. , , , and , Taking the square root of the sum of squares yields the peak value of the three-phase active current reference command. , , ; By solving the quadratic equation for the reactive current limiting coefficient in real time, the reference command value is ensured to be within the range of the maximum design value of short-time overcurrent. Priority is given to ensuring that the active current command is not limited for DC voltage balance control, while the reactive current is limited, resulting in the final unified limiting coefficient for the three-phase bridge arm current. Pick , , The minimum of the three.

7. The control method for a chain-type corner-connected STATCOM based on virtual impedance according to claim 6, characterized in that, The DC voltage control of the converter valve specifically includes: the average value of the three-phase DC voltage of the converter valve. , , Passing sequentially through the delayed sampling stage And points process The sliding filter, with a mean time constant of 10ms, is used to filter out the second harmonic component in the DC voltage. The filtered signals are as follows: , , The average of the three is denoted as Average value of three-phase DC voltage of converter valve This reflects the overall level of DC voltage, and the result is subtracted from the target value of 1.0 and then passed through a PI regulator. The output of the regulator is the amplitude of the active quantity deviation, and the positive sequence voltage unit phasor. , , After multiplication, the reference command for the active positive sequence current of the bridge arm is obtained. , , ; Average value of three-phase DC voltage of converter valve The average value of the three-phase DC voltage of the converter valve, respectively , , Subtracting them gives the DC voltage difference. , , Then, through three-phase independent PI regulators Then, the output result is denoted as , , Calculated with voltage feedforward unit phasor respectively , , After multiplying and superimposing, the injected zero-sequence current component is obtained. It is used to balance the functional quantities among the three-phase DC voltages of the converter valve; The average value of the DC voltage after filtering in phase A of the converter valve The average value of the DC voltage after filtering in phase B Average value of C-phase DC voltage after filtering Select the maximum value from the middle Divide by 1.1, i.e., standardize the overvoltage threshold of 1.1 pu per unit, subtract the reference value of 1.0, and then pass it through the PI controller. The output of the regulator is the active variable adjustment quantity. ,when When the value is less than 1.1 pu, the output of the PI controller is 0, which is the lower limit of the amplitude limit; the active variable adjustment value... Respectively with the negative sequence current unit phasor , , After multiplication, the injected negative sequence current reference command is obtained. , , This ensures that the three-phase DC voltage of the converter valve does not experience overvoltage under severely unbalanced operating conditions.

8. The control method for a chain-type corner-connected STATCOM based on virtual impedance according to claim 6, characterized in that, The modulation wave command calculation specifically includes: bridge arm unlimited reactive current reference command. , , With dynamic limiting coefficient After multiplication, the final bridge arm reactive current reference command is obtained. , , ; Bridge arm active positive sequence current reference command , , , and the injected zero-sequence current component Add them together, and then add them to the injected negative sequence current reference command. , , Adding them together, we obtain the total active current reference command for the three-phase bridge arms. , , Final bridge arm reactive current reference command , , Reference command for total active current of bridge arms , , After summing, the total bridge arm current reference command is obtained. , , ; and then with the three-phase bridge arm current feedback , , The difference is calculated, and the resulting current deviation is the sum of the three independent proportional resonant regulators. The input signal is the closed-loop output of each of the three-phase current loops. , , That is, the fundamental current controls the voltage; the feedforward unit phasor of the three-phase grid voltage. , , Phasor and proportionality coefficient Multiplying them yields the per-unit grid voltage feedforward phasor; where, The value is 0.75; after superimposing the feedforward phasor of the fundamental current control voltage with the per-unit three-phase grid voltage, the final modulated wave signal is obtained. , , .

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