CN115313483A - Grid-connected control method and system for voltage source type converter under weak power grid connection condition - Google Patents

Abstract

The invention discloses a grid-connected control method and system of a voltage source converter under the condition of weak grid connection. Simplify and simplify to obtain the output current small-signal mathematical model of the voltage source converter grid-connected system; for the output current small-signal mathematical model, extract the control current reference value to compensate the loop gain; add the current reference value to the current loop reference value The value of the compensation loop gain is used to decouple the grid-connected system of the voltage source converter into a first-order loop only related to the current loop parameters, so as to realize the grid-connected control of the voltage source converter. By adding feedforward control to the reference value of the current loop of the grid-connected system of the converter, the present invention can compensate for the dynamic error introduced by the phase-locked loop dynamics and the impedance of the AC system, thereby realizing d-axis control and q-axis control of the grid-connected system of the converter. The decoupling of control effectively improves the stability of the grid-connected system.

Description

Grid connection control method and system for voltage source converter under weak grid connection conditions

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 for a voltage source type converter under a weak grid connection condition.

Background technique

The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

A weak grid refers to a system with an excessively large equivalent impedance of the AC system, and a short-circuit ratio is usually used. It is generally considered that the AC system with a short-circuit ratio of 2 to 3 is a weak AC system, also known as a weak grid; the AC system with a short-circuit ratio lower than 2 is an extremely weak AC system, also known as an extremely weak grid. When the voltage source converter is connected to the weak AC system, there is a strong interaction between its typical control links and the weak AC system, which will lead to grid-connected system instability and endanger the safe and stable operation of the grid-connected system in severe cases.

The phase-locked loop is the key control link that leads to the instability of the voltage source converter connected to the AC system. By optimizing the control parameters and control structure to weaken or compensate the output error of the phase-locked loop after the grid-connected system is disturbed, the grid-connected system can be effectively improved. stability. Existing studies generally propose control parameters and structure optimization methods to enhance the stability of grid-connected systems from three aspects: control parameter optimization, phase-locked loop structure optimization, and additional feedforward compensation control.

The core idea of the control parameter optimization method is to reshape the output characteristics of the converter by optimizing the parameters of the control link, thereby enhancing the grid-connected stability of the voltage source converter system. Closed-loop control strategy and phase-locked loop control structure, but the relationship between the response speed and stability of the control system needs to be balanced and the relationship between the various 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 converter system by reshaping the impedance characteristics of the phase-locked loop control link or designing a reasonable additional control. Among them, the method of reshaping the output characteristics of the phase-locked loop mainly improves the overall stability of the grid-connected system by enhancing the self-stability of the phase-locked loop. Synchronous control and other realizations have the advantage of only needing to readjust the control parameters of the phase-locked loop without changing the overall structure of the double closed-loop control; The stability of the grid-connected system is used to compensate the dynamics of the phase-locked loop, but because the additional control link changes the control structure of the grid-connected system, the control parameters of the grid-connected system need to be redesigned.

The main problems of the existing feed-forward compensation control method are: high control design complexity and relatively complicated parameter design. The existing additional feedforward compensation control methods generally change the control structure of the system, resulting in changes in the amplitude-frequency characteristics of the entire control loop, increasing the complexity of the control system, and requiring redesign of the control parameters of the grid-connected system.

Contents of the invention

In order to solve the above problems, the present invention proposes a voltage source converter grid-connected control method and system under the condition of weak grid connection. By adding a feed-forward link to the current loop reference value to compensate the dynamic error of the phase-locked loop, it can directly Compensate the feedback branch error caused by the dynamics of the phase-locked loop and the impedance of the AC system, and then decouple the grid-connected system of the voltage source converter into two independent first-order systems to meet the safety requirements when the converter is connected to a weak AC system. stable operation requirements.

In some embodiments, the following technical solutions are adopted:

A voltage source converter grid-connected control method under weak grid connection conditions, including:

Establishing a mathematical model according to the typical topology of the voltage source converter grid-connected system, linearizing and simplifying the mathematical model, and obtaining the output current small-signal mathematical model of the voltage source converter grid-connected system;

For the small-signal mathematical model of the output current, extract the control current reference value to compensate the loop gain;

Add the current reference value to the current loop reference value to compensate the loop gain, decouple the voltage source converter grid-connected system into a first-order loop only related to the current loop parameters, and realize the voltage source converter grid-connected control .

As an optional implementation, a small-signal mathematical model of the output current of the voltage source converter grid-connected system is obtained, specifically:

Among them, s represents the Laplacian operator; the superscripts "gf" and "cf" represent the electrical signals in the main circuit coordinate system and the control coordinate system respectively; Δi _cfsdref , Δi _cfsqref , Δi _gfsd and Δi _gfsq represent the reference coordinate system The d, q axis current reference value and the d, q axis current respond in the main circuit coordinate system; Δθ _pll is the phase angle disturbance of the phase locked loop output; G _CL represents the current loop transfer function; I _d0 and I _q0 represent the d axis respectively Current and q-axis current steady-state components; R _eq and L _eq represent the equivalent resistance and equivalent inductance of the AC system.

As an optional implementation manner, for the output current small-signal mathematical model, the control current reference value is extracted to compensate the loop gain, specifically:

Among them, K _pgsc and K _igsc represent the proportional coefficient and integral coefficient of the current loop respectively, s represents the Laplacian operator; _Req represents the equivalent resistance of the AC system, I _d0 and I _q0 represent the d-axis current and the q-axis current stability State components.

As an optional implementation manner, the current reference value is added to the current loop reference value to compensate the loop gain, specifically:

Among them, s represents the Laplacian operator; the superscripts "gf" and "cf" represent the electrical signals in the main circuit coordinate system and the control coordinate system respectively; Δi _cfsdref , Δi _cfsqref , Δi _gfsd and Δi _gfsq represent the reference coordinate system The d, q axis current reference value and the d, q axis current respond in the main circuit coordinate system; Δθ _pll is the phase angle disturbance of the phase locked loop output; G _CL represents the current loop transfer function; I _d0 and I _q0 represent the d axis respectively Current and q-axis current steady-state components; _{Req, L eq represent} the equivalent resistance and equivalent inductance of the AC system, G _dcd , G _dcq are the control current reference value compensation loop gain of the d-axis and q-axis respectively.

As an optional implementation, before establishing the mathematical model according to the typical topology of the voltage source converter grid-connected system, the following settings are made:

The filter time constant of the voltage feedforward link and the equivalent delay of the converter control link are very small, and the transfer function of the voltage feedforward filter link and the equivalent link of the converter delay are G _f ≈ 1, G _d ≈ 1;

Since the control bandwidth of the outer loop is generally much smaller than the control bandwidth of the inner loop, it can be considered that the outer loop control of the grid-connected converter has not responded within the time scale of the current loop bandwidth, so the influence of the outer loop dynamics on the grid-connected system is ignored.

In other embodiments, the following technical solutions are adopted:

The voltage source converter grid-connected control system under weak grid connection conditions, including:

The mathematical modeling module is used to establish a mathematical model according to the typical topology of the voltage source converter grid-connected system, linearize and simplify the mathematical model, and obtain the output current small signal mathematical model of the voltage source converter grid-connected system Model;

The compensation loop gain calculation module is used to extract the control current reference value compensation loop gain for the output current small-signal mathematical model;

The grid-connected control module is used to add the current reference value to the current loop reference value to compensate the loop gain, decouple the voltage source converter grid-connected system into a first-order loop related only to the current loop parameters, and realize the voltage source type converter grid-connected control.

In other embodiments, the following technical solutions are adopted:

Add the current reference value to the current loop reference value to compensate the loop gain, specifically:

Among them, s represents the Laplacian operator; the superscripts "gf" and "cf" represent the electrical signals in the main circuit coordinate system and the control coordinate system respectively; Δi _cfsdref , Δi _cfsqref , Δi _gfsd and Δi _gfsq represent the reference coordinate system The d, q axis current reference value and the d, q axis current respond in the main circuit coordinate system; Δθ _pll is the phase angle disturbance of the phase locked loop output; G _CL represents the current loop transfer function; I _d0 and I _q0 represent the d axis respectively Current and q-axis current steady-state components; _{Req, L eq represent} the equivalent resistance and equivalent inductance of the AC system, G _dcd , G _dcq are the control current reference value compensation loop gain of the d-axis and q-axis respectively.

In other embodiments, the following technical solutions are adopted:

A terminal device, which includes a processor and a memory, the processor is used to implement various instructions; the memory is used to store a plurality of instructions, and the instructions are suitable for being loaded by the processor and executing the above-mentioned voltage source type conversion under the weak grid connection condition A method for grid-connected control of converters.

In other embodiments, the following technical solutions are adopted:

A computer-readable storage medium, in which a plurality of instructions are stored, and the instructions are suitable for being loaded by a processor of a terminal device and executing the above-mentioned grid-connected control method of a voltage source converter under a weak grid connection condition.

Compared with prior art, the beneficial effect of the present invention is:

(1) The present invention can compensate the dynamic error introduced by the phase-locked loop dynamics and the impedance of the AC system by adding feed-forward control to the reference value of the current loop of the converter grid-connected system, and then realize the d-axis control of the converter grid-connected system The decoupling from the q-axis control effectively improves the stability of the grid-connected system.

At the same time, the current reference value feedforward compensation control also has the advantage of enhancing the stability of the grid-connected system without parameter design, and only needs to add compensation branches. Great significance.

Other features and advantages of additional aspects 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.

Description of drawings

Figure 1 is a schematic diagram of a typical voltage source converter grid-connected system;

Figure 2 is a small signal block diagram of the current inner loop;

Fig. 3 is a small signal block diagram of a feedforward compensation decoupling method grid-connected system in an embodiment of the present invention.

Detailed ways

It should be pointed out that the following detailed description is exemplary and intended to provide further explanation to the present application. 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 should be noted that the terminology used here is only for describing specific implementations, and is not intended to limit the exemplary implementations according to the present application. As used herein, unless the context clearly dictates otherwise, the singular is intended to include the plural, and it should also be understood that when the terms "comprising" and/or "comprising" are used in this specification, they mean There are features, steps, operations, means, components and/or combinations thereof.

Embodiment one

In one or more embodiments, a voltage source converter grid-connected control method under weak grid connection conditions is disclosed. For the voltage source converter grid-connected system, an analytical transfer function model is established and the output current is small The signal model extracts the feedback loop gain caused by the phase-locked loop dynamics and the impedance coupling of the AC system, and adds a feed-forward compensation loop to the reference value to enhance the stability of the grid-connected system.

The method of this embodiment specifically includes the following processes:

(1) Establish a mathematical model according to the typical topology of the voltage source converter grid-connected system, linearize and simplify the mathematical model, and obtain the output current small-signal mathematical model of the voltage source converter grid-connected system;

A typical voltage source converter grid-connected system is shown in Figure 1. The 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. Among them, the phase-locked loop module is used to extract the AC system frequency; the current loop is used to generate the modulation signal to realize the control target of the grid-connected system; the modulation module generates the trigger pulse from the modulation signal, and controls the voltage prototype converter to output the AC voltage.

Before establishing the mathematical model according to the typical topology of the converter grid-connected system, the following assumptions need to be made, specifically:

The filter time constant of the voltage feedforward link and the equivalent delay of the converter control link are very small, and it is approximately considered that the transfer function of the voltage feedforward filter link and the equivalent link of the converter delay are G _f ≈ 1, G _d ≈ ¹ ;

Since the control bandwidth of the outer loop is generally much smaller than the control bandwidth of the inner loop, it can be considered that the outer loop control of the grid-connected converter has not responded within the time scale of the current loop bandwidth, so the influence of the outer loop dynamics on the 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 easily obtained:

Among them, s represents the Laplacian operator; the superscripts "gf" and "cf" represent the electrical signals in the main circuit coordinate system and the control coordinate system respectively; Δi _cfsdref , Δi _cfsqref , Δi _gfsd and Δi _gfsq represent the reference coordinate system The d, q axis current reference value and the d, q axis current respond in the main circuit coordinate system; Δθ _pll is the phase angle disturbance of the phase locked loop output; G _CL represents the current loop transfer function; I _d0 and I _q0 represent the d axis respectively Current and q-axis current steady-state components; R _eq and L _eq represent the equivalent resistance and equivalent inductance of the AC system.

According to the output current small-signal model of the converter grid-connected system, the corresponding current closed-loop small-signal block diagram can be obtained, as shown in Figure 2.

It can be seen from Figure 2 that if the reference value of the current loop fluctuates Δi _cfsdref and Δi _cfsqref , the disturbance signal passes through the current loop transfer function link and the main circuit parameters, and the AC system impedance and the phase-locked loop respectively pass through the A loop and B loop feedback and input signal link.

This is because the phase-locked loop will have a phase angle tracking error after the system is disturbed, and there will be a relative phase angle error between the control coordinate system established by the phase angle output of the phase-locked loop and the main circuit coordinate system established by the actual voltage phase angle of the grid-connected point . Under the effect of the equivalent impedance of the AC system on the output current of the grid-connected system, the coordinate system offset caused by the phase angle output error of the phase-locked loop will act on the corresponding current control loop through the dq axis feedback loop A and feedback loop B respectively. The smaller the strength of the AC grid, the greater the equivalent impedance of the corresponding AC grid. Therefore, under the same control parameters, the influence of the current feedback branch on the output current is greater, which is also the reason why the interaction of the phase-locked loop control link is easy to cause the system instability when the impedance of the AC system is large.

(2) For the output current small-signal mathematical model, extract the control current reference value to compensate the loop gain;

In this embodiment, according to the output current model of the grid-connected converter, the dq-axis control current reference value compensation loop gains G _dcd and G _dcq of the converter grid-connected system are calculated respectively, and the voltage source converter under the weak grid connection condition is obtained Feedforward compensation control model:

Among them, K _pgsc and K _igsc represent the current loop proportional coefficient and integral coefficient respectively.

(4) Combined with Fig. 3, the current reference compensation loop gains G _dcd and G _dcq are respectively added to the input d-axis current reference value and q-axis current reference value to directly compensate the coupling caused by the interaction between the AC system and the phase-locked loop function, and then decouple the control link of the voltage source converter grid-connected system into a first-order decoupling loop that is only related to the current loop transfer function G _CL and the equivalent resistance of the converter, inductance R _eq , and L _eq .

Specifically, adding the current reference value to the current loop reference value to compensate the loop gain, specifically:

Since the last two terms in the above two equations are essentially the same, they cancel out. After adding the current reference value compensation loop gain to the current loop reference value, it becomes:

When the operating state of the grid-connected system is known, the voltage source converter grid-connected system can be decoupled into a first-order loop that is only related to the current loop parameters by adding feedforward compensation branches to the dq axis control loop. Enter

Without redesigning the parameters of the control system, it can effectively improve the stability of the grid-connected system.

Embodiment two

In one or more embodiments, a voltage source converter grid-connected control system under weak grid connection conditions is disclosed, including:

The mathematical modeling module is used to establish a mathematical model according to the typical topology of the voltage source converter grid-connected system, linearize and simplify the mathematical model, and obtain the output current small signal mathematical model of the voltage source converter grid-connected system Model;

The compensation loop gain calculation module is used to extract the control current reference value compensation loop gain for the output current small-signal mathematical model;

The grid-connected control module is used to add the current reference value to the current loop reference value to compensate the loop gain, decouple the voltage source converter grid-connected system into a first-order loop related only to the current loop parameters, and realize the voltage source type converter grid-connected control.

In this embodiment, the current reference value is added to the current loop reference value to compensate the loop gain, specifically:

Among them, s represents the Laplacian operator; the superscripts "gf" and "cf" represent the electrical signals in the main circuit coordinate system and the control coordinate system respectively; Δi _cfsdref , Δi _cfsqref , Δi _gfsd and Δi _gfsq represent the reference coordinate system The d, q axis current reference value and the d, q axis current respond in the main circuit coordinate system; Δθ _pll is the phase angle disturbance of the phase locked loop output; G _CL represents the current loop transfer function; I _d0 and I _q0 represent the d axis respectively Current and q-axis current steady-state components; _{Req, L eq represent} the equivalent resistance and equivalent inductance of the AC system, G _dcd , G _dcq are the control current reference value compensation loop gain of the d-axis and q-axis respectively.

Embodiment Three

In one or more embodiments, a terminal device is disclosed, including a server, the server includes a memory, a processor, and a computer program stored on the memory and operable on the processor, and the processor executes the The program realizes the grid-connected control method of the voltage source converter under the weak grid connection condition in the first embodiment. For the sake of brevity, details are not repeated here.

It should be understood that in this embodiment, the processor can be a central processing unit CPU, and the processor can also be other general-purpose processors, digital signal processors DSP, application specific integrated circuits ASIC, off-the-shelf programmable gate array FPGA or other programmable logic devices , discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor may be a microprocessor, or the processor may be any conventional processor, or the like.

The memory may include read-only memory and random access memory, and provide instructions and data to the processor, and a part of the memory may also include non-volatile random access memory. For example, the memory may also store device type information.

In the implementation process, each step of the above method can be completed by an integrated logic circuit of hardware in a processor or an instruction in the form of software.

Embodiment Four

In one or more embodiments, a computer-readable storage medium is disclosed, in which a plurality of instructions are stored, and the instructions are suitable for being loaded by a processor of a terminal device and executing the weak grid connection described in Embodiment 1 The grid-connected control method of voltage source converter under the conditions.

Although the specific implementation of the present invention has been described above in conjunction with the accompanying drawings, it does not limit the protection scope of the present invention. Those skilled in the art should understand that on the basis of the technical solution of the present invention, those skilled in the art do not need to pay creative work Various modifications or variations that can be made are still within the protection scope 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.

Images