CN115313483A - Grid-connected control method and system for voltage source type converter under weak power grid connection condition - Google Patents
Grid-connected control method and system for voltage source type converter under weak power grid connection condition Download PDFInfo
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Abstract
The invention discloses a grid-connected control method and a system of a voltage source type converter under the condition of weak grid connection, wherein the grid-connected control method comprises the following steps: establishing a mathematical model according to the typical topology of the voltage source type converter grid-connected system, and linearizing and simplifying the mathematical model to obtain a voltage source type converter grid-connected system output current small signal mathematical model; for the output current small signal mathematical model, extracting a control current reference value to compensate the loop gain; and adding the current reference value compensation loop gain into the current loop reference value, decoupling the voltage source type converter grid-connected system into a first-order loop only related to the current loop parameters, and realizing grid-connected control of the voltage source type converter. According to the invention, by adding the feedforward control into the current loop reference value of the converter grid-connected system, dynamic errors introduced by phase-locked loop dynamics and alternating current system impedance can be compensated, so that decoupling of d-axis control and q-axis control of the converter grid-connected system is realized, and the stability of the grid-connected system is effectively improved.
Description
Technical Field
The invention relates to the technical field of grid-connected system control of power electronic equipment, in particular to a grid-connected control method and system of a voltage source type converter under the condition of weak grid connection.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
The weak grid refers to a system with an excessive equivalent impedance of an alternating current system, and a short circuit ratio is generally adopted. The alternating current system with the short circuit ratio of 2-3 is generally considered as a weak alternating current system, also called as a weak power grid; an ac system with a short circuit ratio below 2 is a very weak ac system, also known as a very weak grid. When the voltage source type converter is connected to a weak alternating current system, strong interaction exists between a typical control link and the weak alternating current system, so that the grid-connected system is unstable under severe conditions, and the safe and stable operation of the grid-connected system is endangered.
The phase-locked loop is a key control link for causing the voltage source type converter to be connected into an alternating current system to be unstable, and the stability of the grid-connected system can be effectively improved by weakening control parameters and a control structure or compensating the output error of the phase-locked loop after the grid-connected system is disturbed. The existing research generally provides a control parameter and structure optimization method for enhancing the stability of a grid-connected system from three aspects of control parameter optimization, phase-locked loop structure optimization and additional feedforward compensation control.
The core idea of the control parameter optimization method is to remold the output characteristic of the converter through optimizing the parameters of a control link so as to further enhance the grid-connected stability of the voltage source type converter system, and the control method has the advantages that the classical double closed-loop control strategy and the phase-locked loop control structure controlled by the voltage source type converter do not need to be changed, but the relationship between the response speed and the stability of the control system needs to be balanced, and the relationship among the control links of the converter needs to be considered;
the core idea of the control structure optimization method is to enhance the grid-connected stability of the voltage source type converter system by remolding the impedance characteristic of a phase-locked loop control link or designing reasonable additional control. The method for reshaping the output characteristic of the phase-locked loop mainly improves the overall stability of a grid-connected system by enhancing the self-stability of the phase-locked loop, can be realized by adding active damping into the phase-locked loop, adopting a phase-locked loop with a symmetrical structure or adopting virtual synchronous control and the like, and has the advantages that only the control parameters of the phase-locked loop need to be re-adjusted, and the overall structure of double closed loop control does not need to be changed; the additional control method is characterized in that structures such as virtual impedance or feedforward compensation are added into a double closed-loop control structure, the phase-locked loop dynamics are compensated by improving the stability of the whole grid-connected system, and the control parameters of the grid-connected system need to be redesigned due to the fact that the control structure of the grid-connected system is changed in the additional control link.
The problems of the existing feedforward compensation control method are mainly as follows: the control design is complex and the parameter design is complex. The existing additional feedforward compensation control method generally changes the control structure of the system, so that the amplitude-frequency characteristic of the whole control loop is changed, the complexity of the control system is increased, and the control parameters of a grid-connected system need to be redesigned.
Disclosure of Invention
In order to solve the problems, the invention provides a grid-connected control method and a grid-connected control system for a voltage source type converter under the condition of weak power grid connection.
In some embodiments, the following technical scheme is adopted:
the grid-connected control method of the voltage source type converter under the weak grid connection condition comprises the following steps:
establishing a mathematical model according to the typical topology of the voltage source type converter grid-connected system, and linearizing and simplifying the mathematical model to obtain a mathematical model of a small signal of output current of the voltage source type converter grid-connected system;
for the output current small signal mathematical model, extracting a control current reference value to compensate the loop gain;
and adding the current reference value compensation loop gain into the current loop reference value, decoupling the voltage source type converter grid-connected system into a first-order loop only related to the current loop parameters, and realizing grid-connected control of the voltage source type converter.
As an optional implementation manner, the obtained mathematical model of the small signal of the output current of the voltage source type converter grid-connected system specifically includes:
wherein s represents a laplacian operator; the superscripts "gf" and "cf" represent electrical signals in the main circuit coordinate system and the control coordinate system, respectively; Δ i cfsdref 、Δi cfsqref 、Δi gfsd And Δ i gfsq Respectively representing the response of d-axis and q-axis current reference values and d-axis and q-axis currents in a reference coordinate system in a main circuit coordinate system; delta theta pll Outputting phase angle disturbance for the phase-locked loop; g CL Represents the current loop transfer function; i is d0 、I q0 Respectively representing the steady-state components of the d-axis current and the q-axis current; r is eq 、L eq Representing the equivalent resistance and the equivalent inductance of the alternating current system.
As an optional implementation manner, for the output current small signal mathematical model, extracting a control current reference value to compensate the loop gain, specifically:
wherein, K pgsc And K igsc Respectively representing a current loop proportional coefficient and an integral coefficient, and s represents a Laplace operator; r is eq Representing the equivalent resistance of the AC system, I d0 、I q0 Representing the d-axis current and the q-axis current steady-state components, respectively.
As an optional implementation, adding the current reference value compensation loop gain to the current loop reference value specifically includes:
wherein s represents a laplacian operator; the superscripts "gf" and "cf" represent electrical signals in the main circuit coordinate system and the control coordinate system, respectively; delta i cfsdref 、Δi cfsqref 、Δi gfsd And Δ i gfsq Respectively representing the response of d-axis and q-axis current reference values and d-axis and q-axis currents in a reference coordinate system in a main circuit coordinate system; delta theta pll Outputting phase angle disturbance for the phase-locked loop; g CL Representing a current loop transfer function; i is d0 、I q0 Respectively representing the steady-state components of the d-axis current and the q-axis current; r eq 、L eq Representing the equivalent resistance and the equivalent inductance of the AC system, G dcd 、G dcq The control current reference values for the d-axis and q-axis, respectively, compensate for the loop gain.
As an optional embodiment, before the mathematical model is established according to the typical topology of the voltage source converter grid-connected system, the following settings are performed:
the filter time constant of the voltage feedforward loop and the equivalent delay of the current converter control loop are very small, and the transfer function of the voltage feedforward filter loop and the equivalent delay of the current converter G are approximately considered f ≈1、G d ≈1；
Because the control bandwidth of the outer ring is generally far smaller than that of the inner ring, the control of the outer ring of the grid-connected converter is not responded within the time scale of the current ring bandwidth, and the influence of the outer ring dynamic on a grid-connected system is ignored.
In other embodiments, the following technical solutions are adopted:
a voltage source type converter grid-connected control system under the weak power grid connection condition comprises:
the mathematical modeling module is used for establishing a mathematical model according to the typical topology of the voltage source type converter grid-connected system, linearizing and simplifying the mathematical model and obtaining a mathematical model of the output current small signal of the voltage source type converter grid-connected system;
the compensation loop gain calculation module is used for extracting a control current reference value compensation loop gain for the output current small signal mathematical model;
and the grid-connected control module is used for adding the current reference value compensation loop gain into the current loop reference value, decoupling the voltage source type converter grid-connected system into a first-order loop only related to the current loop parameter, and realizing the grid-connected control of the voltage source type converter.
In other embodiments, the following technical solutions are adopted:
adding the current reference value compensation loop gain into the current loop reference value, specifically:
wherein s represents the laplace operator; the superscripts "gf" and "cf" represent the principal electrons, respectivelyElectrical signals under a road coordinate system and a control coordinate system; Δ i cfsdref 、Δi cfsqref 、Δi gfsd And Δ i gfsq Respectively representing the response of d-axis and q-axis current reference values and d-axis and q-axis currents in a reference coordinate system in a main circuit coordinate system; delta theta pll Outputting phase angle disturbance for the phase-locked loop; g CL Represents the current loop transfer function; i is d0 、I q0 Respectively representing the steady-state components of the d-axis current and the q-axis current; r eq 、L eq Representing the equivalent resistance and the equivalent inductance of the AC system, G dcd 、G dcq The control current reference values for the d-axis and q-axis, respectively, compensate the loop gain.
In other embodiments, the following technical solutions are adopted:
a terminal device comprising a processor and a memory, the processor being arranged to implement instructions; the storage is used for storing a plurality of instructions which are suitable for being loaded by the processor and executing the grid-connected control method of the voltage source type converter under the weak grid connection condition.
In other embodiments, the following technical solutions are adopted:
a computer readable storage medium, wherein a plurality of instructions are stored, the instructions are suitable for being loaded by a processor of a terminal device and executing the grid-connected control method of the voltage source type converter under the weak grid connection condition.
Compared with the prior art, the invention has the beneficial effects that:
(1) According to the invention, by adding the feedforward control into the current loop reference value of the converter grid-connected system, dynamic errors introduced by phase-locked loop dynamics and alternating current system impedance can be compensated, so that decoupling of d-axis control and q-axis control of the converter grid-connected system is realized, and the stability of the grid-connected system is effectively improved.
Meanwhile, the current reference value feedforward compensation control also has the advantage that the stability of a grid-connected system can be enhanced only by adding a compensation branch without parameter design, and has great significance for ensuring the safe and stable operation of the converter connected to a weak power grid through an optimized control structure.
Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
Drawings
Fig. 1 is a schematic diagram of a typical voltage source converter grid-connected system;
FIG. 2 is a block diagram of a current inner loop small signal;
fig. 3 is a small signal block diagram of a grid-connected system by a feedforward compensation decoupling method in the embodiment of the invention.
Detailed Description
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
Example one
In one or more embodiments, a voltage source converter grid-connected control method under the condition of weak grid connection is disclosed, aiming at a voltage source converter grid-connected system, an analytic transfer function model and an output current small signal model are established, feedback loop gain caused by impedance coupling of phase-locked loop dynamics and an alternating current system is extracted, and a feedforward compensation loop is added into a reference value to enhance stability of the grid-connected system.
The method of the embodiment specifically comprises the following processes:
(1) Establishing a mathematical model according to the typical topology of the voltage source type converter grid-connected system, linearizing and simplifying the mathematical model to obtain a voltage source type converter grid-connected system output current small signal mathematical model;
a typical voltage source converter grid-connected system is shown in fig. 1, and a typical structure of the voltage source converter grid-connected system is divided into a main circuit module, a phase-locked loop module, a current loop module and a modulation module. The phase-locked loop module is used for extracting the frequency of the alternating current system; the current loop is used for generating a modulation signal for realizing a control target of the grid-connected system; the modulation module generates a trigger pulse by the modulation signal and controls the voltage prototype converter to output alternating-current voltage.
The following assumptions are required before establishing a mathematical model according to a typical topology of a converter grid-connected system, and specifically:
the filter time constant of the voltage feedforward loop and the equivalent delay of the control loop of the converter are very small, and the transfer function of the voltage feedforward filter loop and the equivalent delay of the converter G are approximately considered f ≈1、G d ≈ 1 ；
Because the control bandwidth of the outer ring is generally far smaller than that of the inner ring, the control of the outer ring of the grid-connected converter is not responded within the time scale of the current ring bandwidth, and the influence of the outer ring dynamic on a grid-connected system is ignored.
According to the typical topological structure mathematical model of the converter grid-connected system, the output current small signal model of the converter grid-connected system is easy to obtain:
wherein s represents a laplacian operator; the superscripts "gf" and "cf" represent electrical signals in the main circuit coordinate system and the control coordinate system, respectively; Δ i cfsdref 、Δi cfsqref 、Δi gfsd And Δ i gfsq Respectively representing the response of d-axis and q-axis current reference values and d-axis and q-axis currents in a reference coordinate system in a main circuit coordinate system; delta theta pll Outputting phase angle disturbance for the phase-locked loop; g CL Representing a current loop transfer function; i is d0 、I q0 Respectively representing the steady-state components of the d-axis current and the q-axis current; r eq 、L eq Representing the equivalent resistance and the equivalent inductance of the alternating current system.
According to the output current small-signal model of the converter grid-connected system, a corresponding current closed-loop small-signal block diagram can be obtained, as shown in fig. 2.
As can be seen from FIG. 2, if there is a fluctuation Δ i in the current loop reference value cfsdref 、Δi cfsqref During the process, the disturbing signal is fed back and input to the signal link through the loop A and the loop B respectively under the combined action of the current loop transfer function link and the main circuit parameters in the alternating current system impedance and the phase-locked loop.
This is because the phase-locked loop will have phase angle tracking error after the system is disturbed, and there will be relative phase angle error between the control coordinate system established by the phase-locked loop output phase angle and the main circuit coordinate system established by the actual voltage phase angle of the point of connection. Under the action of equivalent impedance of an alternating current system, coordinate system deviation caused by phase angle output error of a phase-locked loop acts on corresponding current control loops through a dq axis feedback loop A and a feedback loop B respectively. The smaller the intensity of the alternating current power grid is, the larger the equivalent impedance of the corresponding alternating current power grid is. Therefore, the larger the influence of the current feedback branch on the output current under the same control parameter is, the reason that the instability of the system is caused by the interaction generated in the phase-locked loop control link under the condition of larger impedance of the alternating current system is also easy.
(2) For the output current small signal mathematical model, extracting a control current reference value to compensate the loop gain;
in the embodiment, according to the output current model of the grid-connected converter, the dq axis control current reference value compensation loop gain G of the converter grid-connected system is respectively calculated dcd And G dcq And obtaining a voltage source type converter feedforward compensation control model under the condition of weak power grid connection:
wherein, K pgsc And K igsc Respectively representing the current loop proportionality coefficient and the integral coefficient.
(4) Referring to fig. 3, a current reference value compensation loop gain is added to the input d-axis current reference value and q-axis current reference value respectivelyG dcd And G dcq Directly compensating the coupling effect caused by the interaction of the alternating current system and the phase-locked loop, and further decoupling the control link of the voltage source type converter grid-connected system into a control link only comprising a current loop transfer function G CL Equivalent resistance and inductance R with current converter eq 、L eq The first order decoupling loop of interest.
Specifically, adding the current reference value to the current loop reference value to compensate for the loop gain specifically includes:
since the last two terms in the above two equations are substantially the same, they can be cancelled out. After adding the current reference value to the current loop reference value to compensate the loop gain, the following steps are performed:
when the operation state of the grid-connected system is known, the feedforward compensation branches are respectively added into the dq axis control loop, so that the voltage source type converter grid-connected system can be decoupled into a first-order loop only related to current loop parameters. Into
And the stability of the grid-connected system is effectively improved while the control system parameters are not required to be redesigned.
Example two
In one or more embodiments, a grid-connected control system for a voltage source converter under a weak grid connection condition is disclosed, which includes:
the mathematical modeling module is used for establishing a mathematical model according to the typical topology of the voltage source type converter grid-connected system, linearizing and simplifying the mathematical model and obtaining a mathematical model of the output current small signal of the voltage source type converter grid-connected system;
the compensation loop gain calculation module is used for extracting a control current reference value compensation loop gain for the output current small signal mathematical model;
and the grid-connected control module is used for adding the current reference value compensation loop gain into the current loop reference value, decoupling the voltage source type converter grid-connected system into a first-order loop only related to the current loop parameter, and realizing the grid-connected control of the voltage source type converter.
In this embodiment, adding the current reference value to the current loop reference value to compensate the loop gain specifically includes:
wherein s represents the laplace operator; the superscripts gf and cf represent electrical signals under the main circuit coordinate system and the control coordinate system respectively; Δ i cfsdref 、Δi cfsqref 、Δi gfsd And Δ i gfsq Respectively representing the response of d-axis and q-axis current reference values and d-axis and q-axis currents in a reference coordinate system in a main circuit coordinate system; delta theta pll Outputting phase angle perturbation for the phase locked loop; g CL Representing a current loop transfer function; i is d0 、I q0 Respectively representing the steady-state components of the d-axis current and the q-axis current; r eq 、L eq Representing the equivalent resistance and the equivalent inductance of the AC system, G dcd 、G dcq The control current reference values for the d-axis and q-axis, respectively, compensate the loop gain.
EXAMPLE III
In one or more embodiments, a terminal device is disclosed, which includes a server, where the server includes a memory, a processor, and a computer program stored in the memory and executable on the processor, and the processor executes the computer program to implement the grid-connected control method for a voltage source converter under a weak grid connection condition in the first embodiment. For brevity, no further description is provided herein.
It should be understood that in this embodiment, the processor may be a central processing unit CPU, and the processor may also be other general purpose processors, digital signal processors DSP, application specific integrated circuits ASIC, off-the-shelf programmable gate arrays FPGA or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, and so on. A general purpose processor may be a microprocessor or the processor may be any conventional processor or the like.
The memory may include both read-only memory and random access memory, and may provide instructions and data to the processor, and a portion of the memory may also include non-volatile random access memory. For example, the memory may also store device type information.
In implementation, the steps of the above method may be performed by integrated logic circuits of hardware in a processor or instructions in the form of software.
Example four
In one or more embodiments, a computer-readable storage medium is disclosed, in which a plurality of instructions are stored, the instructions being adapted to be loaded by a processor of a terminal device and to execute the method for grid-connected control of a voltage source converter under weak grid connection conditions as described in the first embodiment.
Although the embodiments of the present invention have been described with reference to the accompanying drawings, it is not intended to limit the scope of the invention, and it should be understood by those skilled in the art that various modifications and variations can be made without inventive changes in the technical solutions of the present invention.
Claims (9)
1. The grid-connected control method of the voltage source type converter under the weak power grid connection condition is characterized by comprising the following steps of:
establishing a mathematical model according to the typical topology of the voltage source type converter grid-connected system, and linearizing and simplifying the mathematical model to obtain a voltage source type converter grid-connected system output current small signal mathematical model;
for the output current small signal mathematical model, extracting a control current reference value to compensate the loop gain;
and adding the current reference value compensation loop gain into the current loop reference value, decoupling the voltage source type converter grid-connected system into a first-order loop only related to the current loop parameters, and realizing the grid-connected control of the voltage source type converter.
2. The grid-connected control method of the voltage source converter under the weak grid connection condition according to claim 1, wherein a mathematical model of a small signal of an output current of a grid-connected system of the voltage source converter is obtained, and specifically:
wherein s represents the laplace operator; the superscripts "gf" and "cf" represent electrical signals in the main circuit coordinate system and the control coordinate system, respectively; Δ i cfsdref 、Δi cfsqref 、Δi gfsd And Δ i gfsq Respectively representing the d-axis current reference value and the q-axis current reference value under a reference coordinate system and the response of the d-axis current and the q-axis current under a main circuit coordinate system; delta theta pll Outputting phase angle disturbance for the phase-locked loop; g CL Represents the current loop transfer function; i is d0 、I q0 Respectively representing the steady-state components of the d-axis current and the q-axis current; r eq 、L eq Representing the equivalent resistance and the equivalent inductance of the alternating current system.
3. The grid-connected control method of the voltage source converter under the condition of weak grid connection according to claim 1, characterized in that for the output current small signal mathematical model, extracting a control current reference value to compensate the loop gain, specifically:
wherein, K pgsc And K igsc Respectively representing a current loop proportional coefficient and an integral coefficient, and s represents a Laplace operator; r eq Representing the equivalent resistance of the AC system, I d0 、I q0 Representing the d-axis current and the q-axis current steady-state components, respectively.
4. The grid-connected control method of the voltage source converter under the condition of weak grid connection according to claim 1, characterized in that the current reference value is added to the current loop reference value to compensate the loop gain, specifically:
wherein s represents the laplace operator; the superscripts "gf" and "cf" represent electrical signals in the main circuit coordinate system and the control coordinate system, respectively; Δ i cfsdref 、Δi cfsqref 、Δi gfsd And Δ i gfsq Respectively representing the response of d-axis and q-axis current reference values and d-axis and q-axis currents in a reference coordinate system in a main circuit coordinate system; delta theta pll Outputting phase angle disturbance for the phase-locked loop; g CL Represents the current loop transfer function; i is d0 、I q0 Respectively representing the steady-state components of the d-axis current and the q-axis current; r eq 、L eq Representing the equivalent resistance and the equivalent inductance of the AC system, G dcd 、G dcq The control current reference values for the d-axis and q-axis, respectively, compensate for the loop gain.
5. The grid-connected control method of the voltage source converter under the condition of weak grid connection according to claim 1, characterized in that before establishing the mathematical model according to the typical topology of the grid-connected system of the voltage source converter, the following settings are made:
the filter time constant of the voltage feedforward loop and the equivalent delay of the control loop of the converter are very small, and the transfer function of the voltage feedforward filter loop and the equivalent delay of the converter G are approximately considered f ≈1、G d ≈1；
Because the control bandwidth of the outer ring is generally far smaller than that of the inner ring, the control of the outer ring of the grid-connected converter is not responded within the time scale of the current ring bandwidth, and the influence of the outer ring dynamic on a grid-connected system is ignored.
6. Voltage source type transverter control system that is incorporated into power networks under weak grid connection condition, its characterized in that includes:
the mathematical modeling module is used for establishing a mathematical model according to the typical topology of the voltage source type converter grid-connected system, linearizing and simplifying the mathematical model and obtaining a mathematical model of the output current small signal of the voltage source type converter grid-connected system;
the compensation loop gain calculation module is used for extracting a control current reference value compensation loop gain for the output current small signal mathematical model;
and the grid-connected control module is used for adding the current reference value compensation loop gain into the current loop reference value, decoupling the voltage source type converter grid-connected system into a first-order loop only related to the current loop parameter, and realizing the grid-connected control of the voltage source type converter.
7. The grid-connected control system of the voltage source converter under the condition of weak grid connection according to claim 6, wherein the current reference value is added to the current loop reference value to compensate the loop gain, and specifically comprises:
wherein s represents a laplacian operator; the superscripts "gf" and "cf" represent electrical signals in the main circuit coordinate system and the control coordinate system, respectively; Δ i cfsdref 、Δi cfsqref 、Δi gfsd And Δ i gfsq Respectively representing the d-axis current reference value and the q-axis current reference value under a reference coordinate system and the response of the d-axis current and the q-axis current under a main circuit coordinate system; delta theta pll Outputting phase angle disturbance for the phase-locked loop; g CL Representing a current loop transfer function; i is d0 、I q0 Respectively representing the steady-state components of the d-axis current and the q-axis current; r eq 、L eq Representing the equivalent resistance and the equivalent inductance of the AC system, G dcd 、G dcq The control current reference values for the d-axis and q-axis, respectively, compensate the loop gain.
8. A terminal device comprising a processor and a memory, the processor being arranged to implement instructions; the memory is used for storing a plurality of instructions, wherein the instructions are suitable for being loaded by the processor and executing the grid-connected control method of the voltage source converter under the weak grid connection condition according to any one of claims 1-5.
9. A computer readable storage medium having stored therein a plurality of instructions, wherein the instructions are adapted to be loaded by a processor of a terminal device and to execute the method for grid-connected control of a voltage source converter under weak grid connection conditions according to any one of claims 1 to 5.
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