CN113824129A - Power compensation control for improving bidirectional power stability of grid-connected converter system - Google Patents
Power compensation control for improving bidirectional power stability of grid-connected converter system Download PDFInfo
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- H—ELECTRICITY
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- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
- H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
- H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/79—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/797—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
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- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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Abstract
The invention discloses a power compensation control strategy for improving the stability of a DC/AC grid-connected converter system under the condition of power bidirectional flow. The disturbance component in the actual alternating-current side voltage is extracted by using the second-order low-pass filter with better filtering performance, and the disturbance component is corrected to an active power set value after passing through the newly-added compensation controller, so that the transmission power under the transient state can be correspondingly adjusted according to the change of the actual alternating-current voltage, the change trends of the voltage and the current are the same, namely, the positive impedance characteristic is shown, and the damping and disturbance resistance of the system are enhanced. The method can effectively improve the stability difference of the grid-connected system under the traditional power control caused by the bidirectional power transmission, and finally improves the stability of the system.
Description
Technical Field
The invention relates to the technical field of power electronics, in particular to a power compensation control strategy for improving stability of a grid-connected system with a power control converter under bidirectional power flow.
Background
In recent years, with the gradual scarcity of fossil energy and the increasingly severe environmental problems, the rapid development of new energy power generation technology is promoted, and more energy storage devices and new energy power generation devices mainly comprising photovoltaic and wind power are incorporated into a power grid. In addition, the energy supply and demand among different regions are unbalanced, and interconnection among multi-region power grids is promoted. These all make the power generation mode change from traditional single centralized to centralized and distributed coexistence, and the electric energy transmission also changes from traditional one-way transmission to two-way transmission.
The grid-connected converter is used as a grid-connected interface unit of the direct current system and the alternating current power grid, and can effectively control energy exchange between the direct current system and the alternating current power grid. The method is widely applied to occasions such as alternating current and direct current micro-grids, alternating current low-voltage micro-grid interconnection, direct current power transmission and transformation, motor driving, alternating current motor variable frequency speed regulation and the like. Under the condition of power bidirectional transmission, the stability of a system comprising a grid-connected converter is different along with the change of a power transmission direction, namely the bidirectional power stability difference exists. In recent years, scholars at home and abroad research the problem of the difference of the bidirectional power stability in various systems comprising grid-connected converters, analyze the fundamental mechanism of the bidirectional power stability and the influence on the system stability, and provide various solutions.
Research has revealed that the problem of bidirectional power stability difference exists in a double-end flexible direct-current transmission system through impedance analysis and a Nyquist diagram, and the specific expression is that the system can stably operate when power is transmitted from a power control converter to a voltage control converter, and the system can be unstable when power is transmitted reversely. The method also takes a cascaded converter system formed by a bidirectional full-Bridge converter (DAB) and a grid-connected converter as a research object, and analyzes the fundamental mechanism of the problem that the impedance characteristics of the port impedance of the power control converter are different under bidirectional power: when the grid-connected converter is used as a power load, the direct-current side input impedance of the grid-connected converter has a negative damping characteristic, and when the grid-connected converter is used as an equivalent power source, the direct-current side output impedance of the grid-connected converter has a positive damping characteristic. This problem can cause fluctuations in the dc bus voltage that can destabilize the system at high power levels. Two different coordination control strategies are provided for the problem, and the stability of the system under the bidirectional power can be effectively enhanced. In addition, a cascade system formed by a bidirectional DC/DC converter for controlling bus voltage and a bidirectional DC/AC converter for controlling power in the AC/DC micro-grid is used as a research object, and the fact that the equivalent impedance and the stability of the cascade system are influenced by the load power level and the power transmission direction is revealed. And a cooperative control strategy is provided to inhibit oscillation and improve the system stability.
In the research, the problem of stability difference of a grid-connected converter direct-current side system under power bidirectional flow is mainly optimized when the traditional power control is adopted. Similarly to the dc side, when power is transmitted from the ac grid to the grid-connected converter, the grid-connected converter acts as a constant power load with respect to the ac grid, and the ac-side input impedance has a negative impedance characteristic, and therefore, this problem also occurs in the ac-side system.
Disclosure of Invention
The invention aims to provide a power compensation control strategy for improving the stability of a DC/AC grid-connected converter system under the condition of power bidirectional flow, so that reverse input impedance is converted from negative impedance characteristic to positive impedance characteristic, the stability difference of the bidirectional power flow on a constant-power control grid-connected system is effectively improved, the damping of the system is finally enhanced, and the power transmission capability and the stability of the system are improved.
In order to achieve the purpose, the invention provides the following scheme:
a power compensation control strategy for improving stability of a DC/AC grid-connected converter system under power bidirectional flow is applied to a grid-connected system containing a power control converter under power bidirectional flow, the system comprises a three-phase grid-connected converter and an alternating current power grid, and a traditional direct power control strategy is adopted, and the method comprises the following steps:
in DC/AC grid-connected converter systems, there is an interaction between the grid-connected inverter and the AC grid. Under bidirectional power flow, when power is transmitted from a grid-connected converter to an alternating current power grid, the expression of the output impedance of the alternating current side of the converter is as follows:
wherein, in the formula: zinv-outThe output impedance of the alternating current side of the converter is used for transmitting power from the grid-connected converter to the alternating current power grid; Δ ugThe variation of the grid voltage is obtained; Δ igIs the variation of the grid-connected current.
Similarly, when power is transmitted from the ac grid to the converter, the converter ac side input impedance expression is as follows:
under the traditional power control, the grid voltage and grid-connected current have opposite variation trends, so when power is transmitted from a grid-connected converter to an alternating current grid, the grid-connected converter as a power source shows a positive impedance characteristic. When power is transmitted from the ac grid to the converter, the grid-connected converter exhibits a negative impedance characteristic as a power load. That is, under bidirectional flow of power, there is a significant difference in impedance characteristics. And because positive damping stabilizes the system, negative damping can reduce system stability. Therefore, the stability of the grid-connected system is obviously different along with the change of the power flowing direction, and the stability difference easily causes system oscillation.
Extracting disturbance component in actual alternating-current side voltage by using a second-order low-pass filter with better filtering performance, and extracting d-axis component u of actual alternating-current voltagegdThe voltage disturbance quantity u is obtained by subtracting the low-frequency steady-state quantity obtained after the low-frequency steady-state quantity passes through a low-pass filtergd ～This process can be equivalent to extracting the amount of disturbance in the actual voltage by a high pass filter. u. ofgd ～The expression of (a) is:
ugd ～＝ugd(1-GLPF)＝ugdGfc，
wherein, in the formula: gLPFIs a transfer function of a second order low-pass filter, GfcFor the transfer function of an equivalent high-pass filter:
wherein, in the formula: omeganIs the undamped natural frequency of the system; ζ is a damping ratio of the system, and is generally 0.707.
The voltage disturbance component is almost zero in the steady state, i.e. the regulator input signal is zero. In order to not change the transmission power under the steady state, only the power set value under the transient state is corrected, namely the output signal of the compensation regulator under the steady state is zero, and the compensation regulator acts under the transient state. If the compensation controller is designed as a proportional-integral controller, the output quantity of an integral link of the compensation controller in a steady state may not be zero, namely the requirement is not met. The compensation regulator is therefore designed as a proportional controller. Because the input signal of the regulator is a voltage signal and the output signal is a power signal, the proportional coefficient of the regulator is related to a current signal, and the expression of the compensation regulator is as follows:
wherein, in the formula: i isgdThe steady state average value of the grid-connected current d-axis component is obtained; k is a radical ofmIs a margin factor.
The compensated active power branch circuit satisfies the following formula:
the extracted disturbance component is corrected to an active power set value after passing through a newly added compensation controller, so that the transmission power under the transient state can be correspondingly adjusted according to the change of the actual alternating voltage, the change trends of the voltage and the current are the same, namely, the positive impedance characteristic is shown, and the damping and disturbance resistance of the system are enhanced.
According to the specific embodiment provided by the invention, the invention discloses the following technical effects: according to the power compensation control strategy for improving the stability of the DC/AC grid-connected converter system under the power bidirectional flow, on the basis of the traditional direct power control strategy, the disturbance component in the actual alternating-current side voltage is extracted through the second-order low-pass filter with better filtering performance, the disturbance component is corrected to the active power set value after passing through the newly-added compensation controller, the transmission power under the transient state can be correspondingly adjusted according to the change of the actual alternating-current voltage, the change trends of the voltage and the current are the same, namely, the positive impedance characteristic is shown, the damping and the disturbance resistance of the system are enhanced, the problem of stability difference caused by the bidirectional power flow to the grid-connected system containing the power control converter is effectively solved, and the stability of the system is finally improved.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without inventive exercise.
FIG. 1 is a block diagram of the DC/AC grid-connected converter system of the present invention;
FIG. 2 is a block diagram of a conventional power control employed by the DC/AC grid-connected inverter of the present invention;
FIG. 3 is a small signal model of an inverter with a phase locked loop constant power control closed loop according to the present invention;
FIG. 4 is a block diagram of a power compensation control strategy of the grid-connected converter of the present invention;
FIG. 5 is a small signal model after the power compensation control strategy is adopted in the present invention;
FIG. 6 is a simulated waveform under operating conditions before optimization according to the present invention;
FIG. 7 is a simulation waveform under the operating conditions after the optimization of the present invention;
FIG. 8 is a simulated waveform under condition two before optimization according to the present invention;
FIG. 9 is a simulation waveform under the second operating condition after the optimization of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The invention aims to provide a power compensation control strategy for improving the stability of a DC/AC grid-connected converter system under the condition of power bidirectional flow, so that reverse input impedance is converted from negative impedance characteristic to positive impedance characteristic, the stability difference of the bidirectional power flow on a constant-power control grid-connected system is effectively improved, the damping of the system is finally enhanced, and the power transmission capability and the stability of the system are improved.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
Fig. 1 is a structural block diagram of a DC/AC grid-connected converter system according to the present invention, as shown in fig. 1, the DC/AC grid-connected converter system includes a DC/AC grid-connected converter and an AC grid, a main circuit of the grid-connected converter includes a three-phase bridge circuit of a DC-side energy storage capacitor C, IGBT and an AC-side filter inductor L, and the AC grid considers line impedance. Power may flow in both directions in the system. In the figure: u shapedcIs a direct current side voltage; e.g. of the typej(j ═ a, b, c) is the converter ac side terminal voltage; u. ofgjIs an alternating current grid voltage; i.e. igjIs three-phase grid-connected current.
Fig. 2 is a block diagram of a conventional power control adopted by the DC/AC grid-connected converter of the present invention, and as shown in fig. 2, an inner loop of the conventional power control is current control under a d-q coordinate system. In the figure: hiA current PI regulator; omegagIs the fundamental frequency of the power grid; dj(j ═ d, q) is the duty cycle dq axis component. Upper label "*"denotes a given value of the corresponding variable, and capital letters denote steady-state values of the corresponding variable.
FIG. 3 is a small signal model of a grid-connected inverter adopting conventional power control when power flows in the forward direction, with the alternating current side under the d-q axis, according to the present invention:
inverter AC side output impedance Z when power flows in forward directioninv-outInput impedance Z of inverter at AC side when sum power flows reverselyinv-inAre respectively:
wherein, in the formula: zinv-outFor power from grid-connected converter toThe output impedance of the AC side of the converter during AC power grid transmission; Δ ugThe variation of the grid voltage is obtained; Δ igIs the variation of the grid-connected current.
Similarly, when power is transmitted from the ac grid to the converter, the converter ac side input impedance expression is as follows:
under the traditional power control, the grid voltage and grid-connected current have opposite variation trends, so when power is transmitted from a grid-connected converter to an alternating current grid, the grid-connected converter as a power source shows a positive impedance characteristic. When power is transmitted from the ac grid to the converter, the grid-connected converter exhibits a negative impedance characteristic as a power load. That is, under bidirectional flow of power, there is a significant difference in impedance characteristics. And because positive damping stabilizes the system, negative damping can reduce system stability. Therefore, the stability of the grid-connected system is obviously different along with the change of the power flowing direction, and the stability difference easily causes system oscillation.
Selecting the transmission direction of power from the grid-connected converter to the alternating current power grid as a positive direction; the reverse direction is the direction of power transfer from the ac grid to the converter. Firstly, establishing an impedance small signal model during power forward transmission to obtain an impedance expression; the power value and the current value are negative values to indicate reverse power transmission.
Assuming constant DC side voltage, i.e. Δ u dc0. Let Δ d be based on the superposition theorem in circuit theoryd＝ΔdqThe inverter ac-side impedance Z can be obtained as 0inv_olExpression (c):
in the formula: zdd(Zqq) The voltage variation of the direct (alternating) axis caused by the unit current disturbance quantity of the direct (alternating) axis; zdq(Zqd) For inducing unit current disturbance of direct (alternating) axisA change in voltage of the AC (DC) axis, Zdq(Zqd) The mutual coupling effect between the direct axis and the quadrature axis is embodied; l is a filter inductance value; r is inductance internal resistance; omegagIs the grid angular frequency.
Let Δ ud＝Δu q0, an expression between the duty ratio and the grid-connected current under small signal modeling can be obtained:
in the formula: u shapedcIs a dc input voltage.
In the figure: upper label "s"represents a variable of the system d-q coordinate system; upper label "c"represents a variable of the controller d-q coordinate system; Δ represents a small perturbation of the variable. ZinvAn output impedance matrix under the open loop of a phase-locked loop is not considered for the alternating current side of the grid-connected converter; midA small signal transfer function matrix from duty ratio to grid-connected current under open loop; t isdA transfer function matrix that is a controller delay; f is a transfer function matrix of the digital filter; pud、PuiAnd PuuRespectively obtaining small signal disturbance path transfer function matrixes from the alternating current power grid voltage under the system d-q coordinate system to the duty ratio, the grid-connected current and the alternating current power grid voltage under the controller d-q coordinate system; miAnd McpRespectively, a transfer function matrix and a coupling term matrix of the current PI regulator.
The disturbance matrix of the grid-connected current given value is derived by a formula as follows:
Δig *c＝C1MpqΔug c，
in the formula: constant expression C1＝-2/(3Ugd 2)，UgdTaking 311 as the average value of the grid voltage under the steady state; mpqMatrix for power setpoint:
in the formula: p*The given value of active power; q*The given value of reactive power.
Fig. 4 is a block diagram of a power compensation control strategy of the grid-connected converter.
The d-axis component u of the actual AC voltagegdThe voltage disturbance quantity u is obtained by subtracting the low-frequency steady-state quantity obtained after the low-frequency steady-state quantity passes through a low-pass filtergd ～This process can be equivalent to extracting the amount of disturbance in the actual voltage by a high pass filter. u. ofgd ～The expression of (a) is:
ugd ～＝ugd(1-GLPF)＝ugdGfc，
wherein, in the formula: gLPFIs a transfer function of a second order low-pass filter, GfcFor the transfer function of an equivalent high-pass filter:
wherein, in the formula: omeganIs the undamped natural frequency of the system; ζ is a damping ratio of the system, and is generally 0.707.
Compensation regulator Hc(s) the input signal is a voltage disturbance ugd ～And the output signal is the active power correction quantity. The voltage disturbance component is almost zero in the steady state, i.e. the regulator input signal is zero. In order to not change the transmission power under the steady state, only the power set value under the transient state is corrected, namely the output signal of the compensation regulator under the steady state is zero, and the compensation regulator acts under the transient state. If the compensation controller is designed as a proportional-integral controller, the output quantity of an integral link of the compensation controller in a steady state may not be zero, namely the requirement is not met. The compensation regulator is therefore designed as a proportional controller. Because the input signal of the regulator is a voltage signal and the output signal is a power signal, the proportional coefficient of the regulator is related to a current signal, and the expression of the compensation regulator is as follows:
wherein, in the formula: i isgdThe steady state average value of the grid-connected current d-axis component is obtained; k is a radical ofmThe margin coefficient, i.e. a larger proportionality coefficient than in the steady state is required to maintain the positive impedance characteristic of the converter input impedance in the transient state in view of the sudden increase of the power level. Since the regulator mainly corrects the power value when the power flows reversely, the power value and the voltage are changed in the same trend when the power flows reversely by setting the negative sign in the expression.
The compensated active power branch circuit satisfies the following formula:
the disturbance matrix of the grid-connected current given value after compensation is as follows:
the above equation can be simplified as:
Δig *c＝C1MpqΔug c+C2McΔug c，
wherein, in the formula: c2Is a constant expression: c2＝2/(3Ugd)；McTo compensate the matrix:
wherein, in the formula: kp_cTo compensate for the scaling factor of the regulator.
Fig. 5 is a small signal model after the power compensation control strategy is adopted in the invention.
Building a simulation model of the DC/AC grid-connected converter system shown in FIG. 1, wherein the voltage of a direct current bus side is 800V; the given reactive power value is 0 kW; the fundamental frequency is 50 Hz; the switching frequency is 10 kHz; the effective value of the voltage of the alternating current power grid line is 380V; the line inductance is 1.5 mH.
The working condition I is as follows: and +/-10 kW of power flows in two directions.
FIG. 6 is a simulation waveform under the working condition before the optimization of the present invention. It can be seen from the simulation waveform that the system fluctuation amplitude is large when the power flow direction is switched from the forward direction to the reverse direction, and the system fluctuation amplitude is small when the power flow direction is switched from the reverse direction to the forward direction, and a significant stability difference exists.
FIG. 7 is a simulation waveform under the operating condition after the optimization of the present invention. As can be seen from the simulation waveform, after the proposed power compensation control is adopted, the power and voltage fluctuation amplitude is greatly reduced, the step response adjustment time is about 12ms, and the kinetic energy performance is better; the three-phase current is close to a sine wave, and large fluctuation and impact can not occur in the transient state of switching of the power flow direction. Compared with the AC side waveforms under the two control methods, the system stability is better when the proposed compensation control is adopted under the power bidirectional flow, and the stability difference between different power flow directions is smaller; and the response speed is higher, which shows that the filter has less influence on the dynamic performance of the system.
Working conditions are as follows: the different power flow down power levels gradually increased from 10kW to 30 kW.
FIG. 8 is a simulation waveform under the second operating condition before the optimization of the present invention. According to the simulation waveform, when the power reversely flows and the power level is improved to 30kW, the power on the alternating current side fluctuates violently, and the grid-connected system loses instability; when the power level is increased to 30kW when the power flows in the forward direction, the fluctuation range of the power at the alternating current side is small, and the grid-connected system can be kept stable. The obvious stability difference of the system under high power level when the power control is adopted is shown.
FIG. 9 is a simulation waveform under the second operating condition after the optimization of the present invention. From the simulation waveform, after the power level is improved to 30kW after compensation, the power fluctuation of the alternating current side under the forward and reverse flows of power is greatly reduced, the stability is greatly improved, and the stability margin is greatly improved.
According to the power compensation control strategy for improving the stability of the DC/AC grid-connected converter system under the condition of power bidirectional flow, the disturbance component in the actual alternating-current side voltage is extracted by using the second-order low-pass filter with better filtering performance, the disturbance component is corrected to the active power set value after passing through the newly-added compensation controller, the transmission power under the transient state can be correspondingly adjusted according to the change of the actual alternating-current voltage, the change trends of the voltage and the current are the same, namely, the positive impedance characteristic is shown, and the damping and the disturbance resistance of the system are enhanced.
The principles and embodiments of the present invention have been described herein using specific examples, which are provided only to help understand the method and the core concept of the present invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, the specific embodiments and the application range may be changed. In view of the above, the present disclosure should not be construed as limiting the invention.
Claims (2)
1. A power compensation control strategy for improving stability of a DC/AC grid-connected converter system under power bidirectional flow is applied to a grid-connected system containing a power control converter under power bidirectional flow, the system comprises a three-phase grid-connected converter and an alternating current power grid, and is characterized by comprising the following steps:
in DC/AC grid-connected converter systems, there is an interaction between the grid-connected inverter and the AC grid. Under bidirectional power flow, when power is transmitted from a grid-connected converter to an alternating current power grid, the expression of the output impedance of the alternating current side of the converter is as follows:
wherein, in the formula: zinv-outThe output impedance of the alternating current side of the converter is used for transmitting power from the grid-connected converter to the alternating current power grid; au coatinggThe variation of the grid voltage is obtained; Δ igThe variable quantity of the grid-connected current is obtained;
similarly, when power is transmitted from the ac grid to the converter, the converter ac side input impedance expression is as follows:
under the traditional power control, the grid voltage and grid-connected current have opposite variation trends, so when power is transmitted from a grid-connected converter to an alternating current grid, the grid-connected converter as a power source shows a positive impedance characteristic. When power is transmitted from the ac grid to the converter, the grid-connected converter exhibits a negative impedance characteristic as a power load. That is, under bidirectional flow of power, there is a significant difference in impedance characteristics. And because positive damping stabilizes the system, negative damping can reduce system stability. Therefore, the stability of the grid-connected system is obviously different along with the change of the power flowing direction, and the stability difference easily causes system oscillation.
2. The power compensation control strategy for improving the stability of the DC/AC grid-connected converter system under the condition of bidirectional power flow according to claim 1 is characterized by comprising the following steps:
extracting disturbance component in actual alternating-current side voltage by using a second-order low-pass filter with better filtering performance, and extracting d-axis component u of actual alternating-current voltagegdThe voltage disturbance quantity u is obtained by subtracting the low-frequency steady-state quantity obtained after the low-frequency steady-state quantity passes through a low-pass filtergd ～This process can be equivalent to extracting the amount of disturbance in the actual voltage by a high pass filter. u. ofgd ～The expression of (a) is:
ugd ～＝ugd(1-GLPF)＝ugdGfc，
wherein, in the formula: gLPFIs a transfer function of a second order low-pass filter, GfcFor the transfer function of an equivalent high-pass filter:
whereinIn the formula: omeganIs the undamped natural frequency of the system; zeta is the damping ratio of the system, and generally zeta is 0.707;
the voltage disturbance component is almost zero in the steady state, i.e. the regulator input signal is zero. In order to not change the transmission power under the steady state, only the power set value under the transient state is corrected, namely the output signal of the compensation regulator under the steady state is zero, and the compensation regulator acts under the transient state. If the compensation controller is designed as a proportional-integral controller, the output quantity of an integral link of the compensation controller in a steady state may not be zero, namely the requirement is not met. The compensation regulator is therefore designed as a proportional controller. Because the input signal of the regulator is a voltage signal and the output signal is a power signal, the proportional coefficient of the regulator is related to a current signal, and the expression of the compensation regulator is as follows:
wherein, in the formula: i isgdThe steady state average value of the grid-connected current d-axis component is obtained; k is a radical ofmIs a margin coefficient;
the compensated active power branch circuit satisfies the following formula:
the extracted disturbance component is corrected to an active power set value after passing through a newly added compensation controller, so that the transmission power under the transient state can be correspondingly adjusted according to the change of the actual alternating voltage, the change trends of the voltage and the current are the same, namely, the positive impedance characteristic is shown, and the damping and disturbance resistance of the system are enhanced.
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