CN114069709A - Low voltage ride through comprehensive control method for virtual synchronous machine - Google Patents

Low voltage ride through comprehensive control method for virtual synchronous machine Download PDF

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CN114069709A
CN114069709A CN202111410080.4A CN202111410080A CN114069709A CN 114069709 A CN114069709 A CN 114069709A CN 202111410080 A CN202111410080 A CN 202111410080A CN 114069709 A CN114069709 A CN 114069709A
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CN114069709B (en
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毛行奎
潘佳清
董纪清
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Xiamen Lijing New Energy Technology Co ltd
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Fuzhou University
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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
    • H02J3/381—Dispersed generators
    • 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
    • H02J3/001—Methods to deal with contingencies, e.g. abnormalities, faults or failures
    • H02J3/00125—Transmission line or load transient problems, e.g. overvoltage, resonance or self-excitation of inductive loads
    • 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
    • H02J3/01—Arrangements for reducing harmonics or ripples
    • 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
    • H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
    • H02J3/16—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load by adjustment of reactive power
    • 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
    • H02J3/26—Arrangements for eliminating or reducing asymmetry in polyphase networks
    • 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
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
    • 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
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/30—Reactive power compensation
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/40—Arrangements for reducing harmonics
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/50—Arrangements for eliminating or reducing asymmetry in polyphase networks

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  • Supply And Distribution Of Alternating Current (AREA)
  • Inverter Devices (AREA)

Abstract

The invention relates to a virtual synchronous machine low voltage ride through comprehensive control method, when a voltage drop fault occurs in a power grid, a power grid voltage positive sequence d-axis component is fed forward to a VSG reactive power excitation voltage regulation equation without a differential link, so that VSG output voltage can quickly follow the drop of a power grid voltage amplitude; increasing the VSG output reactive power instruction value according to the voltage drop depth, and simultaneously reducing the VSG output active power instruction value to avoid VSG steady-state output overcurrent; when the VSG output inductance current exceeds a set threshold value, a virtual resistor is put into the VSG output inductance current to restrain transient overcurrent; when the three-phase voltage of the power grid falls off asymmetrically, VSG three-phase output current balance, VSG output active power double-frequency ripple suppression and VSG output reactive power double-frequency ripple suppression are achieved by setting different current instruction compensation values of d and q axis current loops of positive and negative sequences. The method is beneficial to improving the low voltage ride through capability of the VSG and improving the output power quality of the VSG when the voltage of the power grid fails.

Description

Low voltage ride through comprehensive control method for virtual synchronous machine
Technical Field
The invention belongs to the technical field of grid-connected inverter control, and particularly relates to a low voltage ride through comprehensive control method for a virtual synchronous machine.
Background
As a link between distributed energy and a power distribution network, a traditional grid-connected inverter is composed of power electronic devices, has high response speed and almost no inertia, and as the permeability of a distributed power supply is continuously improved, more and more inverters are incorporated into a large power grid, so that the equivalent total damping and the equivalent total inertia provided by a traditional generator in the large power grid are greatly reduced, and the development of new energy is limited to a certain extent. Therefore, a concept of a Virtual Synchronous Generator (VSG) is provided for scholars, the VSG is used as a control method and simulates a mathematical model of a synchronous generator, so that the traditional grid-connected inverter has inertia and damping characteristics similar to those of the traditional synchronous generator, and the adverse effect of distributed power supply grid connection on a large power grid is reduced.
When a voltage drop fault occurs in a power distribution network, a traditional grid-connected inverter is equivalent to a current source after grid connection, so that the current instruction value can be directly subjected to amplitude limiting treatment to ensure that the inverter is not disconnected or damaged due to overcurrent, and therefore, the current type VSG is easier to control in low voltage ride-through. Currently, there are two main categories of VSG low voltage ride through technologies, the first category is smooth switching between voltage source control and current source control: switching the VSG control to current source control during a grid voltage fault; the second major category is that the VSG is still controlled as a voltage source during a grid fault; the first broad category of controls changes the attributes of the VSG during a grid voltage fault; most of the second category of control currently discusses the grid voltage symmetrical or asymmetrical faults singly, and lacks a comprehensive and complete VSG low-voltage ride-through control scheme.
Disclosure of Invention
The invention aims to provide a comprehensive control method for low voltage ride through of a virtual synchronous machine, which is beneficial to improving the low voltage ride through capability of a VSG and improving the output power quality of the VSG when the voltage of a power grid fails.
In order to achieve the purpose, the invention adopts the technical scheme that: a virtual synchronous machine low voltage ride through comprehensive control method comprises the following steps:
1) all three-phase intersection flow is converted from a three-phase static ABC coordinate system to a two-phase rotating dq0 coordinate system through park, and the amplitude of an alternating current component is oriented on a d axis;
2) VSG output voltage, VSG output current and power grid voltage are collected in real time, and positive sequence components and negative sequence components of collected quantity are obtained by adopting a positive-negative sequence separation method to participate in VSG loop control;
3) when the VSG output current is detected to exceed the current threshold value, outputting a first voltage command value E of the VSG reactive power loopmSubtracting the product value of the positive sequence d-axis component and a positive coefficient of the VSG output inductive current as a second voltage command value U of the positive sequence d-axis voltage loop+ drefThe product value of the VSG positive sequence q-axis voltage loop first voltage command value 0 minus the positive sequence q-axis component of the VSG output inductive current and a positive coefficient is used as the positive sequence q-axis voltage loop second voltage command value U+ qref；
4) When detecting that the positive sequence d-axis component of the power grid voltage is smaller than the set threshold value 0.9pu, firstly, outputting a rated value U of the VSG output voltage in the VSG reactive power excitation voltage regulation control equationnPositive sequence d-axis component U substituted for grid voltage+ pdCalculating the VSG output reactive power instruction value Q according to the grid voltage drop depthrefCalculating the VSG active power command value P according to the rated capacity of the VSG and the reactive power command at the momentref；
5) After the voltage drop of the power grid is detected, if the voltage of the power grid is in an asymmetric drop fault, three control targets of VSG three-phase output current balance, VSG output active power double-frequency ripple suppression and VSG output reactive power double-frequency ripple suppression are achieved by assigning different values to the VSG current inner loop current instruction value.
Further, in step 2), the outer ring of the control system is an active power frequency modulation link and a reactive power voltage regulation link, the middle ring is a positive sequence d-axis voltage ring and a positive sequence q-axis voltage ring, and the inner ring is a positive sequence d-axis component I controlling the VSG output inductive current+ LdCurrent loop and positive sequence q-axis component I+ LqThe current loop of (1), namely a positive sequence d-axis current loop and a positive sequence q-axis current loop, is added with a negative sequence d-axis component I for controlling the VSG output inductive current- LdCurrent loop and negative sequence q-axis component I- LqI.e. negative sequence d-axis current loops and negative sequence q-axis current loops.
Further, in step 3), the current threshold is set according to the rated current value of the VSG, a positive coefficient multiplied by a positive sequence d-axis component and a positive sequence q-axis component of the VSG output inductor current is equivalent to a virtual resistance value, the addition of the virtual resistance is equivalent to an increase of an equivalent impedance between the VSG ac output side and the grid-connected point, a total impedance is obtained by dividing a maximum voltage variation amount which may occur in the grid voltage by the rated current of the VSG, and the virtual resistance value is calculated from the total impedance and the equivalent impedance between the VSG ac output side and the grid-connected point.
Further, in step 5), the calculation methods of the current inner loop command values under the three control targets are as follows: calculating an expression of VSG output active power and reactive power when the voltage of a power grid falls asymmetrically, wherein the output active power and the reactive power are composed of a direct current component, a double power frequency sine component and a double power frequency cosine component, and when VSG three-phase output current balance control is to be realized, the current instruction values of a negative sequence d-axis current loop and a negative sequence q-axis current loop are set to be 0; when VSG active power double-frequency ripple suppression control is to be realized, the VSG active power double-frequency sine ripple component and cosine ripple component are enabled to be 0, the grid voltage amplitude and the VSG output voltage amplitude are oriented under a d axis under a two-dimensional rotating coordinate system, so that the q axis component of the voltage is 0, and then command values of a VSG output inductive current positive sequence d axis component, a VSG output inductive current positive sequence q axis component, a VSG output inductive current negative sequence d axis component and a VSG output inductive current negative sequence q axis component are obtained according to the two conditions; when VSG output reactive power double-frequency ripple suppression control is to be realized, the VSG output reactive power double-frequency sine ripple component and cosine ripple component are made to be 0, and command values of a VSG output inductive current positive sequence d-axis component, a VSG output inductive current positive sequence q-axis component, a VSG output inductive current negative sequence d-axis component and a VSG output inductive current negative sequence q-axis component are obtained.
Further, the calculation method of the VSG output inductor current command value under the three control targets is as follows: when the voltage of the power grid is normal, the VSG outputs a current instruction value I of a positive sequence d-axis component and a positive sequence q-axis component of the inductive current+ drefAnd I+ qrefFor the output value of the VSG intermediate control loop voltage-saving ring, the positive sequence d-axis component current instruction value is approximate to the VSG output active power instruction value PrefDivide by the positive sequence d-axis component U of the VSG output voltage+ 0dAnd 1.5, the positive sequence Q-axis component current command value is approximate to VSG output reactive power command value QrefDivide by the positive sequence d-axis component U of the VSG output voltage+ 0dAnd 1.5, subtracting the current instruction value output by the VSG intermediate control loop voltage-saving ring from the current instruction value of the control target to be realized to obtain compensation values of instruction values required to be added to the VSG positive sequence d-axis and q-axis current inner rings, and subtracting 0 from the calculated negative sequence d-axis and q-axis current instruction values to obtain the compensation values of the instruction values on the VSG negative sequence d-axis and q-axis current inner rings.
Compared with the prior art, the invention has the following beneficial effects: the virtual synchronous machine low voltage ride through comprehensive control method is flexible in control, simple in logic and strong in practicability, and can be realized by the method: 1) in the event of a grid voltage sag, a low voltage ride through of the VSG can be achieved, without changing the characteristics of the VSG during a grid fault, the inverter still being controlled as a voltage source. 2) During the grid voltage drop fault, the output current of the VSG is always controlled below the rated current, and the safety of the VSG is reliably ensured. 3) When the voltage of the power grid has an asymmetric drop fault, smooth switching among three kinds of control of VSG three-phase output current balance, VSG output active power double-frequency ripple suppression control and VSG output reactive power double-frequency ripple suppression control can be realized.
Drawings
FIG. 1 is a system block diagram of an embodiment of the present invention;
FIG. 2 is a detailed control block diagram of a control method according to an embodiment of the present invention;
FIG. 3 is a flow chart illustrating a control method according to an embodiment of the present invention;
fig. 4 is a comparison waveform of VSG output inductor current when the three-phase of the grid voltage drops symmetrically to 0.5pu in the embodiment of the present invention, and no measure is taken;
FIG. 5 is a VSG output power comparison waveform when the three-phase symmetrical voltage of the power grid falls to 0.5pu and no measure is taken and the method is adopted in the embodiment of the invention;
fig. 6 is a simulation waveform of VSG output inductor current, output active power, and output reactive power when the three-phase of the grid voltage drops asymmetrically to 0.2pu and no measures are taken in the embodiment of the present invention;
fig. 7 is a simulation waveform of VSG inductive current, output active power, and output reactive power when the three-phase asymmetry of the grid voltage drops to 0.2pu and the method is adopted in the embodiment of the present invention.
Detailed Description
The invention is further explained below with reference to the drawings and the embodiments.
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 example 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.
The embodiment provides a low voltage ride through control method suitable for a VSG, which includes the following specific implementation methods:
modifying the traditional VSG reactive power excitation voltage regulation control equation as shown in formula (1) into: the VSG reactive power instruction value is subtracted from the VSG output reactive power, the obtained difference value passes through a proportional integral link, and then the output value of the proportional integral link is superposed to the VSG output voltage rated value UnThe value of (d) is used as a first voltage command electric value E of the positive sequence d-axis voltage loopmAnd the modified VSG reactive power excitation voltage regulation control equation is shown as the formula (2).
Figure BDA0003373422190000041
Figure BDA0003373422190000042
Add one-level voltage ring behind active power ring and the reactive power ring in the VSG control, contain positive sequence d axle voltage ring and positive sequence q axle voltage ring, add one-level electric current inner ring behind the voltage ring, the electric current inner ring includes: a positive sequence d-axis current loop, a positive sequence q-axis current loop, a negative sequence d-axis current loop, and a negative sequence q-axis current loop; the current instruction value of the positive sequence d-axis current loop is the positive sequence d-axis voltage loop output quantity, the current instruction value of the positive sequence q-axis current loop is the positive sequence q-axis voltage loop output quantity, and the current instruction value of the negative sequence d-axis current loop and the current instruction value of the negative sequence q-axis current loop are not fixed and are determined according to a control target to be realized when the power grid voltage is subjected to asymmetric drop. The system structure diagram in the embodiment of the invention is shown in fig. 1, the detailed control block diagram of the low voltage ride through control method is shown in fig. 2, and the flow diagram of the control method is shown in fig. 3.
When the power grid fails, the positive sequence d-axis component of the power grid voltage is fed forward to a VSG reactive power excitation voltage regulation equation, and the VSG output at the moment is calculated according to the power grid voltage drop depthThe calculation formulas of the reactive component current value and the active component current value (the reactive component current value, namely the q-axis component of the VSG output inductance current in the coordinate system of the two-phase rotation dq0, and the active component current value, namely the d-axis component of the VSG output inductance current in the coordinate system of the two-phase rotation dq 0) in the inductance current are shown as (I) in formula (3) and formula (4)NRated current, U, of VSG+ pdFor positive sequence d-axis component of the grid voltage, UnAnd the voltage amplitude of the rated phase of the VSG) is obtained, and the VSG output reactive power instruction value and the VSG output active power instruction value can be calculated by using the reactive component current and the active component current of the VSG output inductive current. At the moment, the VSG output reactive power instruction value is increased to help the grid voltage to recover to normal, and the VSG output active power instruction value is reduced to avoid steady-state overcurrent of the VSG during the grid voltage drop.
Figure BDA0003373422190000051
Figure BDA0003373422190000052
When the VSG output current is detected to exceed the current threshold value, the VSG reactive power loop is enabled to output a first voltage instruction value EmSubtracting the product value of the positive sequence d component and a positive coefficient of the VSG output inductive current as a second voltage command value U of the positive sequence d-axis voltage loop+ drefThe product value of the positive sequence q component of the VSG output inductive current and a positive coefficient subtracted from the first voltage command value 0 of the positive sequence q-axis voltage loop is used as the second voltage command value U of the positive sequence q-axis voltage loop+ qrefThe calculation formulas are shown in formula (5) and formula (6).
U+ dref＝Em-I+ LdRv (5)
Figure BDA0003373422190000061
Asymmetry occurring at the network voltageWhen the fault occurs, different current compensation values can be assigned to the current instruction values of the positive sequence d-axis current loop, the positive sequence q-axis current loop, the negative sequence d-axis current loop and the negative sequence q-axis current loop to realize three control targets of VSG three-phase output current balance, VSG output active power double-frequency ripple suppression and VSG output reactive power double-frequency ripple suppression. The expression of the current compensation value is shown in formulas (7), (8) and (9) ((
Figure BDA0003373422190000062
And
Figure BDA0003373422190000063
respectively a positive sequence d-axis current loop current instruction compensation value, a positive sequence q-axis current loop current instruction compensation value, a negative sequence d-axis current loop current instruction compensation value and a negative sequence q-axis current loop current instruction compensation value, U+ 0dAnd U- 0dPositive sequence d-axis component and negative sequence d-axis component) m of the VSG output voltage is 0, the VSG three-phase output current balance control is carried out, and three-phase asymmetry and distortion rate of grid-connected current can be reduced; when m is 1, and k is 1, the VSG outputs active power and double-frequency ripple suppression control can be performed, and double-frequency ripple existing in the output active power can be reduced; when m is 1 and k is-1, the control device is used for controlling the VSG to output the double-frequency ripple suppression of the reactive power, and the double-frequency ripple existing in the output reactive power can be reduced.
Figure BDA0003373422190000064
Figure BDA0003373422190000065
Figure BDA0003373422190000066
Figure BDA0003373422190000067
Figure BDA0003373422190000068
According to the VSG low-voltage ride-through comprehensive control method provided by the invention, a traditional VSG reactive power excitation voltage regulation control equation is improved into a control equation without a differential link, and when a voltage drop fault occurs in a power grid, a d-axis component of a positive sequence of the power grid voltage is fed forward to the VSG reactive power excitation voltage regulation equation, so that the VSG output voltage can quickly follow the drop of the amplitude of the power grid voltage; positive and negative sequence components of VSG output voltage, output current and power grid voltage are obtained by adopting a positive and negative sequence separation method, when the voltage of a power grid drops, the VSG output reactive power instruction value is increased according to the voltage dropping depth, and meanwhile, the VSG output active power instruction value is reduced to avoid VSG steady-state output overcurrent; setting a VSG output inductive current threshold, and putting a virtual resistor to inhibit transient overcurrent when the VSG output inductive current exceeds the current; particularly, when three-phase voltage of a power grid falls off asymmetrically, three control targets of VSG three-phase output current balance, VSG output active power double-frequency ripple suppression and VSG output reactive power double-frequency ripple suppression are achieved by setting different current instruction compensation values of current loops of a positive sequence d axis, a positive sequence q axis, a negative sequence d axis and a negative sequence q axis. The method does not need extra hardware to restrain the VSG overcurrent, reduces cost and loss, and is suitable for the symmetric drop and asymmetric drop faults of the power grid voltage.
Fig. 1 is a block diagram of a system according to an embodiment of the present invention. And (3) adopting MATLAB/Simulink simulation software to carry out simulation. The main parameters used by the simulation model are shown in table 1:
table 1 simulation principal parameters
Figure BDA0003373422190000071
When the voltage of the power grid is normal, the virtual synchronous machine is in grid-connected operation, normal energy exchange is carried out with the power grid, VSG output voltage, output current and the voltage of the power grid are collected in real time, and positive and negative sequence components of the collected quantity are obtained by adopting a positive and negative sequence separation method for control. The control system comprises an outer ring, an intermediate ring, a negative sequence d-axis current ring, a positive sequence q-axis current ring, a positive sequence d-axis current instruction compensation value, a positive sequence q-axis current instruction compensation value, a negative sequence d-axis current instruction compensation value and a negative sequence q-axis current instruction compensation value, wherein the outer ring of the control system is an active power frequency modulation ring and a reactive power voltage regulation link, the intermediate ring is a positive sequence d-axis voltage ring and a positive sequence q-axis voltage ring, and the inner ring is a positive sequence d-axis current ring and a positive sequence q-axis current ring.
When the grid voltage is dropped asymmetrically, if three-phase output current balance of the VSG needs to be realized to reduce the three-phase asymmetry and distortion rate of the grid-connected current, m in the equations (7), (8), (9) and (10) may be set to 0.
If it is necessary to implement VSG output active power double frequency ripple suppression control to reduce the double frequency ripple present in the output active power, m may be 1, and k may be 1 in equations (7), (8), (9) and (10).
If it is necessary to implement VSG output reactive power double frequency ripple suppression control to reduce the double frequency ripple present in the output reactive power, m may be 1, and k may be-1 in equations (7), (8), (9) and (10).
After the four current loop current compensation values in the current inner loop in the VSG control are determined, the virtual resistance value added in the VSG control and shown in the formulas (5) and (6) is determined next, the voltage difference value between the VSG grid-connected point voltage and the VSG output voltage caused by grid three-phase voltage drop is set to be delta U, the equivalent impedance between the VSG grid-connected point and the VSG output LC filter capacitor is very small, and the equivalent impedance value is 0.14 ohm in the detailed embodiment, therefore, the maximum grid voltage drop variation is set to be 0.8pu, when the grid voltage rated value is 311V, the Δ U is 248.8V, and assuming that the VSG operates under the rated current condition, the VSG output current increment Δ I is allowed to be 0.3pu under the condition of ensuring the safety of the VSG prototype, and as in the detailed embodiment, the VSG rated current is 12.86A, therefore, the Δ I is 3.85A, and the required virtual resistance R is calculated according to the formula (12).vTo 64.62 ohms, set the power to add to the virtual resistanceThe current threshold is 1.3 times the rated current.
Figure BDA0003373422190000081
Three-phase modulation wave U is obtained by reverse park conversion of positive sequence d-axis and positive sequence q-axis current loop output quantities of VSG output inductive current+ ra、U+ rb、U+ rcThe output quantity of the current loop of the negative sequence d axis and the negative sequence q axis of the VSG output inductive current is converted by reverse park to obtain a three-phase modulation wave U- ra、U- rb、U- rc，U+ ra、U+ rb、U+ rcAnd U- ra、U- rb、U- rcAnd adding corresponding phase sequences to obtain the three-phase modulation wave of the VSG.
VSG is in grid-connected operation with the rated output power of 6kW, and the amplitude of the grid phase voltage is 311V at the moment; when t is t2, the grid voltage symmetrically drops to 0.5pu, as shown in fig. 4, at this time, the transient peak value of the VSG output current reaches 5.8pu, and the steady-state output current reaches 2.1 pu; as shown in fig. 5, the VSG is not able to output reactive power to assist in the grid voltage recovery. The method provided by the invention comprises the following steps: when the drop fault of the power grid is detected, firstly, a virtual resistor is switched according to whether the inductive current exceeds a set current threshold value of 12.86A or not, the virtual resistor has the effect of limiting VSG output transient overcurrent, and a VSG output current reactive component instruction value I related to the voltage drop depth of the power grid is calculated according to the expression (3) and the expression (4)qrefAnd a value of the active component instruction IdrefThe VSG is changed to output an active power instruction value and an idle power instruction value so as to limit overcurrent during steady-state operation of the VSG; as shown in figure 4, when the method is adopted, the VSG inductive current is well limited below 1.3pu, the VSG is ensured not to be disconnected due to overcurrent when the voltage of the power grid drops to faults, and meanwhile, as can be seen from figure 5, the VSG can also output 1.8kvar of reactive power according to a theoretical calculated value to help the voltage of the power grid to recover, the method provided by the invention is verified to symmetrically drop the three-phase voltage of the power gridEffectiveness at fault.
As shown in fig. 6, the voltage of the a-phase of the power grid drops to 0.2pu at the time t-t 2, and the voltage of the a-phase of the power grid returns to normal at the time t-t 5, as shown in fig. 6, if no control is applied, the output inductive current of the three phases of the VSG is asymmetric and has serious distortion, the current peak value far exceeds the rated current of the prototype at this time, and very large double power frequency ripple components exist in the output active power and the output reactive power of the VSG, which seriously reduces the quality of electric energy. As shown in fig. 7, when the method of the present invention is used, m is 0 in the time period t from t2 to t3, and according to the foregoing, VSG three-phase output current balance control is used, and at this time, the VSG three-phase output inductive current is three-phase-symmetric, and the power frequency ripple component of the output active power and the output reactive power is reduced to some extent; when t is t 3-t 4, m is 1, k is 1, VSG is adopted to output active power double-frequency ripple suppression control, and compared with VSG three-phase output current balance control, VSG three-phase inductive current symmetry is reduced, VSG output active power double-frequency ripple quantity is further reduced, and VSG output reactive power ripple quantity is increased; when t is t 4-t 5, m is 1, k is-1, and at this time, VSG output reactive power double-frequency ripple suppression control is adopted, and at this time, compared with the VSG three-phase output current balance control, the VSG three-phase output inductive current symmetry is reduced, the output reactive power double-frequency ripple quantity is further reduced, and the output active power double-frequency ripple quantity is increased. However, in the period of t-t 2-t 5, the VSG three-phase output inductance current is always controlled to be close to the rated current, and the three kinds of control can improve the power quality and realize smooth switching.
The control method provided by the invention can ensure that VSG output does not overflow no matter when the three-phase symmetrical fault or the three-phase asymmetrical fault occurs in the power grid, can output specified reactive power to help the voltage recovery of the power grid, and can realize the free switching of three controls, namely VSG three-phase output current balance, VSG output active power double-frequency ripple suppression control and VSG output reactive power double-frequency ripple suppression control to improve the power quality particularly when the three-phase asymmetrical fault occurs in the voltage of the power grid.
The foregoing is directed to preferred embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. However, any simple modification, equivalent change and modification of the above embodiments according to the technical essence of the present invention are within the protection scope of the technical solution of the present invention.

Claims (5)

1. A virtual synchronous machine low voltage ride through comprehensive control method is characterized by comprising the following steps:
1) all three-phase intersection flow is converted from a three-phase static ABC coordinate system to a two-phase rotating dq0 coordinate system through park, and the amplitude of an alternating current component is oriented on a d axis;
2) VSG output voltage, VSG output current and power grid voltage are collected in real time, and positive sequence components and negative sequence components of collected quantity are obtained by adopting a positive-negative sequence separation method to participate in VSG loop control;
3) when the VSG output current is detected to exceed the current threshold value, outputting a first voltage command value E of the VSG reactive power loopmSubtracting the product value of the positive sequence d-axis component and a positive coefficient of the VSG output inductive current as a second voltage command value U of the positive sequence d-axis voltage loop+ drefThe product value of the VSG positive sequence q-axis voltage loop first voltage command value 0 minus the positive sequence q-axis component of the VSG output inductive current and a positive coefficient is used as the positive sequence q-axis voltage loop second voltage command value U+ qref；
4) When detecting that the positive sequence d-axis component of the power grid voltage is smaller than a set threshold value, firstly, outputting a rated value U of the VSG output voltage in the VSG reactive power excitation voltage regulation control equationnPositive sequence d-axis component U substituted for grid voltage+ pdCalculating the VSG output reactive power instruction value Q according to the grid voltage drop depthrefCalculating the VSG active power command value P according to the rated capacity of the VSG and the reactive power command at the momentref；
5) After the voltage drop of the power grid is detected, if the voltage of the power grid is in an asymmetric drop fault, three control targets of VSG three-phase output current balance, VSG output active power double-frequency ripple suppression and VSG output reactive power double-frequency ripple suppression are achieved by assigning different values to the VSG current inner loop current instruction value.
2. The method as claimed in claim 1, wherein in step 2), the control system comprises an outer loop including an active power frequency modulation link and a reactive power voltage regulation link, an intermediate loop including a positive sequence d-axis voltage loop and a positive sequence q-axis voltage loop, and an inner loop including a positive sequence d-axis component I for controlling the output inductor current of the VSG+ LdCurrent loop and positive sequence q-axis component I+ LqThe current loop of (1), namely a positive sequence d-axis current loop and a positive sequence q-axis current loop, is added with a negative sequence d-axis component I for controlling the VSG output inductive current- LdCurrent loop and negative sequence q-axis component I- LqI.e. negative sequence d-axis current loops and negative sequence q-axis current loops.
3. The method as claimed in claim 1, wherein in step 3), the current threshold is set according to a rated current value of the VSG, a positive coefficient multiplied by a positive sequence d-axis component and a positive sequence q-axis component of the VSG output inductor current is equivalent to a virtual resistance value, the virtual resistance is added equivalent to an increase in an equivalent impedance between the VSG ac output side and the grid-connected point, a total impedance is obtained by dividing a maximum voltage variation that may occur in the grid voltage by the rated current of the VSG, and the virtual resistance value is calculated from the total impedance and the equivalent impedance between the VSG ac output side and the grid-connected point.
4. The integrated control method for the low voltage ride through of the virtual synchronous machine according to claim 1, wherein in the step 5), the calculation methods of the current inner loop command values under the three control targets are as follows: calculating an expression of VSG output active power and reactive power when the voltage of a power grid falls asymmetrically, wherein the output active power and the reactive power are composed of a direct current component, a double power frequency sine component and a double power frequency cosine component, and when VSG three-phase output current balance control is to be realized, the current instruction values of a negative sequence d-axis current loop and a negative sequence q-axis current loop are set to be 0; when VSG active power double-frequency ripple suppression control is to be realized, the VSG active power double-frequency sine ripple component and cosine ripple component are enabled to be 0, the grid voltage amplitude and the VSG output voltage amplitude are oriented under a d axis under a two-dimensional rotating coordinate system, so that the q axis component of the voltage is 0, and then command values of a VSG output inductive current positive sequence d axis component, a VSG output inductive current positive sequence q axis component, a VSG output inductive current negative sequence d axis component and a VSG output inductive current negative sequence q axis component are obtained according to the two conditions; when VSG output reactive power double-frequency ripple suppression control is to be realized, the VSG output reactive power double-frequency sine ripple component and cosine ripple component are made to be 0, and command values of a VSG output inductive current positive sequence d-axis component, a VSG output inductive current positive sequence q-axis component, a VSG output inductive current negative sequence d-axis component and a VSG output inductive current negative sequence q-axis component are obtained.
5. The comprehensive control method for the low voltage ride through of the virtual synchronous machine according to claim 4, wherein the calculation method of the VSG output inductive current command value under three control targets comprises the following steps: when the voltage of the power grid is normal, the VSG outputs a current instruction value I of a positive sequence d-axis component and a positive sequence q-axis component of the inductive current+ drefAnd I+ qrefFor the output value of the VSG intermediate control loop voltage-saving ring, the positive sequence d-axis component current instruction value is approximate to the VSG output active power instruction value PrefDivide by the positive sequence d-axis component U of the VSG output voltage+ 0dAnd 1.5, the positive sequence Q-axis component current command value is approximate to VSG output reactive power command value QrefDivide by the positive sequence d-axis component U of the VSG output voltage+ 0dAnd 1.5, subtracting the current instruction value output by the VSG intermediate control loop voltage-saving ring from the current instruction value of the control target to be realized to obtain compensation values of instruction values required to be added to the VSG positive sequence d-axis and q-axis current inner rings, and subtracting 0 from the calculated negative sequence d-axis and q-axis current instruction values to obtain the compensation values of the instruction values on the VSG negative sequence d-axis and q-axis current inner rings.
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