CN114069709B - Virtual synchronous machine low voltage ride through comprehensive control method - Google Patents
Virtual synchronous machine low voltage ride through comprehensive control method Download PDFInfo
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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
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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
- H02J3/001—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
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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
- H02J3/01—Arrangements for reducing harmonics or ripples
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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
- H02J3/12—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
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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
- H02J3/26—Arrangements for eliminating or reducing asymmetry in polyphase networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- 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
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- 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
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- 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
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- 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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Abstract
The application relates to a virtual synchronous machine low voltage ride through comprehensive control method, when a voltage drop fault occurs in a power grid, a positive d-axis component of the power grid voltage is fed forward to a VSG reactive power excitation voltage regulation equation without a differential link, so that the VSG output voltage can quickly follow the drop of the power grid voltage amplitude; increasing the VSG output reactive power command value according to the voltage drop depth, and simultaneously reducing the VSG output active power command value to avoid the VSG steady-state output overcurrent; when the VSG output inductance current exceeds a set threshold value, a virtual resistor is put into the circuit to inhibit transient overcurrent; when the three-phase voltage of the power grid is in asymmetric drop, VSG three-phase output current balance, VSG output active power double frequency pulsation suppression and VSG output reactive power double frequency pulsation suppression are realized by setting current command compensation values of different positive and negative sequence d and q axis current loops. The method is beneficial to improving the capability of VSG low voltage ride through and improving the output power quality of VSG when the power grid voltage fails.
Description
Technical Field
The application belongs to the technical field of grid-connected inverter control, and particularly relates to a low-voltage ride through comprehensive control method of a virtual synchronous machine.
Background
As a tie between the distributed energy source and the distribution network, the traditional grid-connected inverter is composed of power electronic devices, has high response speed and almost no inertia, and as the permeability of the distributed power source is continuously improved, more and more inverters are integrated into a large power grid, so that the equivalent total damping and the equivalent total inertia provided by the traditional generator in the large power grid are greatly reduced, and the development of new energy sources is limited to a certain extent. Therefore, a scholars put forward the concept of a virtual synchronous machine (VSG, virtual synchronous generator), and the VSG is used as a control method, and simulates a mathematical model of a synchronous generator, so that a traditional grid-connected inverter has inertia and damping characteristics similar to those of the traditional synchronous generator, and the adverse effect of distributed power grid connection on a large power grid is reduced.
When a voltage drop fault occurs in the power distribution network, the traditional grid-connected inverter is equivalent to a current source after grid connection, so that the inverter can be directly subjected to amplitude limiting treatment on a current command value, the grid-off or damage of the inverter can be avoided due to overcurrent, the current-type VSG is easier in low-voltage ride-through control, the distributed energy permeability can be continuously improved, the voltage-type VSG can provide stable voltage and frequency under the off-grid condition, the application of the voltage-type VSG is wider, the voltage-type VSG is connected in parallel with two voltage sources with small connecting impedance due to the equivalent model of the power grid after grid connection, and the low-voltage ride-through technology of the voltage-type VSG cannot be directly used for referencing the low-voltage ride-through technology of the traditional inverter. Currently, there are two main categories regarding VSG low voltage ride through technology, the first category is smooth switching between voltage source control and current source control: switching the VSG control to a current source control during a grid voltage fault; the second broad category is that the VSG is still controlled to be the voltage source during grid faults; the first broad class of control changes the properties of the VSG during a grid voltage fault; the second type of control is currently mainly single in discussion of symmetric or asymmetric faults of the power grid voltage, and lacks a comprehensive and complete VSG low-voltage ride-through control scheme.
Disclosure of Invention
The application aims to provide a comprehensive control method for low voltage ride through of a virtual synchronous machine, which is beneficial to improving the capability of VSG low voltage ride through and improving the output power quality of VSG in the case of grid voltage failure.
In order to achieve the above purpose, the application adopts the following technical scheme: a virtual synchronous machine low voltage ride through comprehensive control method comprises the following steps:
1) All three-phase alternating current quantities are transformed from a three-phase static ABC coordinate system to a two-phase rotation dq0 coordinate system through park, and the amplitude of an alternating current component is oriented on a d axis;
2) Collecting VSG output voltage, VSG output current and grid voltage in real time, and adopting a positive-negative sequence separation method to obtain positive sequence components and negative sequence components of the collection quantity to participate in VSG loop control;
3) When detecting that VSG output current exceeds a current threshold value, outputting a first voltage command value E of a VSG reactive power loop m Subtracting the product value of the positive sequence d-axis component of the VSG output inductor current and a positive coefficient as a second voltage command value U of the positive sequence d-axis voltage ring + dref Taking the product value of the positive q-axis component of VSG output inductance current subtracted from the first voltage command value 0 of the positive q-axis voltage loop of VSG and a positive coefficient as the second voltage command value U of the positive q-axis voltage loop + qref ；
4) When the positive sequence d-axis component of the grid voltage is detected to be smaller than the set threshold value of 0.9pu, firstly, the VSG output voltage rating U in the VSG reactive power excitation voltage regulation control equation is used for n Positive sequence d-axis component U replaced by grid voltage + pd Calculating VSG output reactive power instruction value Q according to power grid voltage drop depth ref Calculating the VSG active power command value P according to the rated capacity of the VSG and the reactive power command at the moment ref ；
5) After the grid voltage drop is detected, if the grid voltage is an asymmetric drop fault, three control targets of VSG three-phase output current balance, VSG output active power double frequency pulsation suppression and VSG output reactive power double frequency pulsation suppression are realized by assigning different values to the VSG current inner loop current command value.
Further, in step 2), the outer loop of the control system is an active power frequency modulation link and a reactive power voltage regulation link, the middle loop is a positive sequence d-axis voltage loop and a positive sequence q-axis voltage loop, and the inner loop is a positive sequence d-axis component I controlling the VSG output inductance current + Ld Current loop and positive sequence q-axis component I of (2) + Lq The current loops of the (a) positive-sequence d-axis current loop and positive-sequence q-axis current loop are added with a negative-sequence d-axis component I for controlling VSG output inductance current - Ld Current loop and negative sequence q-axis component I of (2) - Lq Namely a negative sequence d-axis current loop and a negative sequence q-axis current loop.
Further, in step 3), the current threshold is set according to the rated current value of the VSG, the positive coefficient multiplied by the positive sequence d-axis component and the q-axis component of the VSG output inductor current is equivalent to a virtual resistance value, the addition equivalent of the virtual resistance is that the equivalent resistance between the VSG ac output side and the grid-connected point becomes large, the total resistance is obtained by dividing the 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 resistance and the equivalent resistance between the VSG ac output side and the grid-connected point.
Further, in step 5), the calculation method of the current inner loop command value under the three control targets is as follows: calculating an expression of VSG output active power and reactive power when the voltage of the 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 the balance control of VSG three-phase output current 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 made to be 0; when the VSG output active power double frequency pulsation suppression control is to be realized, enabling the VSG output active power double frequency sine pulsation component and the cosine pulsation component to be 0, and orienting the power grid voltage amplitude and the VSG output voltage amplitude under the d axis under a two-dimensional rotating coordinate system, so that the q axis component of the voltage is 0, and then obtaining command values of the positive sequence d axis component of the VSG output inductor current, the positive sequence q axis component of the VSG output inductor current, the negative sequence d axis component of the VSG output inductor current and the negative sequence q axis component of the VSG output inductor current according to the two conditions; when the VSG output reactive power frequency doubling pulsation suppression control is to be realized, enabling the VSG output reactive power frequency doubling sine pulsation component and the cosine pulsation component to be 0, and obtaining command values of a VSG output inductance current positive sequence d-axis component, a VSG output inductance current positive sequence q-axis component, a VSG output inductance current negative sequence d-axis component and a VSG output inductance current negative sequence q-axis component.
Further, the calculation method of the VSG output inductance current command value under three control targets comprises the following steps: when the power grid voltage is normal, VSG outputs current command value I of positive sequence d-axis component and positive sequence q-axis component of inductance current + dref And I + qref For the output value of the voltage ring of the VSG intermediate control loop, the direct sequence d-axis component current command value is approximately equal to the VSG output active power command value P ref Positive sequence divided by VSG output voltaged-axis component U + 0d And 1.5, the positive sequence Q-axis component current command value is approximately VSG output reactive power command value Q ref Positive sequence d-axis component U divided by VSG output voltage + 0d And 1.5, subtracting the current command value output by the voltage ring of the VSG intermediate control loop from the current command value of the control target to be realized to obtain the compensation value of the command value on the VSG positive sequence d-axis and q-axis current inner loop, and subtracting 0 from the calculated negative sequence d-axis and q-axis current command value to obtain the compensation value of the command value on the VSG negative sequence d-axis and q-axis current inner loop.
Compared with the prior art, the application has the following beneficial effects: the virtual synchronous machine low voltage ride through comprehensive control method which is flexible to control, simple in logic and strong in practicability is provided, and can be realized by the method: 1) When the grid voltage drops, the low voltage ride through of the VSG can be realized, the characteristics of the VSG are not changed during grid faults, and the inverter is still controlled to be a voltage source. 2) During the grid voltage drop fault, the VSG output current is always controlled below the rated current, and the safety of the VSG is reliably ensured. 3) When the power grid voltage has an asymmetric drop fault, the three controls of VSG three-phase output current balance, VSG output active power doubling pulse suppression control and VSG output reactive power doubling pulse suppression control can be smoothly switched.
Drawings
FIG. 1 is a system block diagram of an embodiment of the present application;
FIG. 2 is a detailed control block diagram of a control method according to an embodiment of the present application;
FIG. 3 is a flow chart of a control method according to an embodiment of the present application;
FIG. 4 is a comparative waveform of the VSG output inductor current when the grid voltage falls to 0.5pu in three-phase symmetry without any measure and with the method according to the embodiment of the application;
FIG. 5 is a comparative waveform of VSG output power when the grid voltage falls to 0.5pu in three-phase symmetry without any measure and with the method according to the embodiment of the present application;
FIG. 6 is a simulated waveform of the VSG output inductor current, output active power and output reactive power when the grid voltage three-phase asymmetry drops to 0.2pu in an embodiment of the application without any action;
fig. 7 is a simulation waveform of VSG inductor current, output active power and output reactive power when the method is adopted in an embodiment of the present application, where the three-phase asymmetry of the grid voltage drops to 0.2 pu.
Detailed Description
The application will be further described with reference to the accompanying drawings and examples.
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the 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 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 is also intended to include the plural unless the context clearly indicates otherwise, and furthermore, it is to be understood that the terms "comprises" and/or "comprising" when used in this specification are taken to specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof.
The embodiment provides a low voltage ride through control method suitable for VSG, which comprises the following specific implementation steps:
modifying a traditional VSG reactive power excitation voltage regulation control equation as shown in formula (1) into: the VSG reactive power command 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 superimposed on the VSG output voltage rated value U n The upper value is used as a first voltage command electric value E of a positive sequence d-axis voltage ring m The modified VSG reactive power excitation voltage regulation control equation is shown in formula (2).
Adding a first-stage voltage ring behind an active power ring and a reactive power ring in VSG control, including a positive-sequence d-axis voltage ring and a positive-sequence q-axis voltage ring, adding a first-stage current inner ring behind the voltage ring, wherein the current inner ring comprises: positive sequence d-axis current loop, positive sequence q-axis current loop, negative sequence d-axis current loop and negative sequence q-axis current loop; the current command value of the positive sequence d-axis current loop is positive sequence d-axis voltage loop output quantity, the current command value of the positive sequence q-axis current loop is positive sequence q-axis voltage loop output quantity, the current command value of the negative sequence d-axis current loop and the current command value of the negative sequence q-axis current loop are not fixed, and the current command value is determined according to a control target which needs to be realized when the power grid voltage is subjected to asymmetric dip. The system structure diagram in the embodiment of the application is shown in fig. 1, the detailed control block diagram of the proposed 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 d-axis component of the power grid voltage is fed forward to a VSG reactive power excitation voltage regulation equation, the reactive component current value and the active component current value (the reactive component current value is the q-axis component of the VSG output inductor current under the two-phase rotation dq0 coordinate system) in the VSG output inductor current at the moment are calculated according to the power grid voltage drop depth, and the calculation formulas (I) of the active component current value is the d-axis component of the VSG output inductor current under the two-phase rotation dq0 coordinate system are shown as formula (3) and formula (4) N For the rated current of VSG, U + pd Is the positive sequence d-axis component of the power grid voltage, U n The voltage amplitude of the VSG rated phase) and the reactive power command value and the active power command value of the VSG output can be calculated by the reactive component current and the active component current of the VSG output inductance current. At this time, the VSG output reactive power command value may be increased to help the grid voltage recover, and the VSG output active power command value may be decreased to avoid steady-state over-current of the VSG during the grid voltage dip.
When the VSG output current is detected to exceed the current threshold value, outputting a first voltage command value E by a VSG reactive power loop m Subtracting the product value of the positive sequence d component of the VSG output inductor current and a positive coefficient as a second voltage command value U of the positive sequence d-axis voltage ring + dref Taking the product value of the positive sequence q component of VSG output inductance current subtracted from the positive sequence q axis voltage loop first voltage command value 0 and a positive coefficient as a positive sequence q axis voltage loop second voltage command value U + qref The calculation formulas are shown as formula (5) and formula (6).
U + dref ＝E m -I + Ld R v (5)
When the power grid voltage has an asymmetric fault, different current compensation values can be given to 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 pulsation suppression and VSG output reactive power double frequency pulsation suppression. The current compensation value expression is shown as formulas (7), (8) and (9)And->The current command compensation 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 are respectively, U + 0d And U - 0d Positive and negative sequence d-axis components of the VSG output voltage, respectively) When m=0, the control is VSG three-phase output current balance control, so that the three-phase asymmetry and distortion rate of grid-connected current can be reduced; when m=1 and k=1, the active power doubling pulse suppression control is output for VSG, so that the doubling pulse existing in the output active power can be reduced; when m=1 and k= -1 is VSG output reactive power doubling pulse suppression control, the doubling pulse existing in the output reactive power can be reduced.
According to the VSG low-voltage ride through comprehensive control method, a traditional VSG reactive power excitation voltage regulation control equation is improved to a control equation without a differential link, and when a voltage sag fault occurs in a power grid, a positive d-axis component of the power grid voltage is fed forward to the VSG reactive power excitation voltage regulation equation, so that VSG output voltage can quickly follow the sag of the voltage amplitude of the power grid; the positive and negative sequence components of VSG output voltage, output current and grid voltage are obtained by adopting a positive and negative sequence separation method, when voltage sag occurs in the grid, a VSG output reactive power instruction value is increased according to the voltage sag depth, and meanwhile, a VSG output active power instruction value is reduced to avoid VSG steady-state output overcurrent; setting a VSG output inductance current threshold value, and inputting a virtual resistor to inhibit transient overcurrent when the VSG output inductance current exceeds the current; and particularly, when the three-phase voltage of the power grid is in asymmetric drop, three control targets of VSG three-phase output current balance, VSG output active power double frequency pulsation suppression and VSG output reactive power double frequency pulsation suppression are realized by setting current instruction compensation values of different positive sequence d-axis, positive sequence q-axis, negative sequence d-axis and negative sequence q-axis current loops. The method does not need extra hardware to inhibit VSG overcurrent, reduces cost and loss, and is suitable for grid voltage symmetrical drop and asymmetrical drop faults.
A system architecture block diagram of an embodiment of the present application is shown in fig. 1. And simulating by adopting MATLAB/Simulink simulation software. The main parameters used by the simulation model are shown in table 1:
TABLE 1 simulation principal parameters
When the voltage of the power grid is normal, the virtual synchronous machine runs in a grid-connected mode, normal energy exchange is carried out with the power grid, VSG output voltage, output current and power grid voltage are collected in real time, and positive and negative sequence components of the collected quantity are obtained through a positive and negative sequence separation method to control. The outer ring of the control system is an active power frequency modulation ring and a reactive power voltage modulation link, the middle ring is a positive sequence d-axis voltage ring and a positive sequence q-axis voltage ring, the inner ring is a positive sequence d-axis current ring and a positive sequence q-axis current ring, a negative sequence d-axis current ring and a negative sequence q-axis current ring are added on the basis, and a positive sequence d-axis current ring current instruction compensation value, a positive sequence q-axis current ring current instruction compensation value, a negative sequence d-axis current ring current instruction compensation value and a negative sequence q-axis current ring current instruction compensation value need to be determined according to the difference of three control targets.
When the grid voltage falls asymmetrically, if the VSG three-phase output current balance needs to be implemented to reduce the three-phase asymmetry and distortion rate of the grid-connected current, m=0 in the formula (7), the formula (8), the formula (9) and the formula (10) can be made.
If it is necessary to implement VSG output active power doubling ripple suppression control to reduce the doubling ripple present in the output active power, m=1, k=1 in expression (7), expression (8), expression (9) and expression (10) may be made.
If it is necessary to implement VSG output reactive power doubling ripple suppression control to reduce the doubling ripple present in the output reactive power, m=1, k= -1 in equation (7), equation (8), equation (9) and equation (10) may be made.
After determining the four current loop current compensation values in the current inner loop in the VSG control, determining the virtual resistance value added in the VSG control as shown in formulas (5) and (6), setting the voltage difference between the VSG grid-connected point voltage and the VSG output voltage caused by the three-phase voltage drop of the power grid as Deltau, setting the equivalent impedance between the VSG grid-connected point voltage and the VSG output LC filter capacitor as small as 0.14 ohm in the detailed embodiment, so that the equivalent impedance value is approximately ignored, setting the maximum variation of the power grid voltage drop as 0.8pu, setting Deltau as 248.8V when the rated voltage of the power grid is 311V, and allowing the VSG output current increment Deltai to be 0.3pu when the VSG is operated under the rated current condition, under the condition of guaranteeing the safety of the VSG model machine, as the VSG rated current as 12.86A, and Deltai being 3.85A, calculating according to the formula (12) to obtain the required virtual resistance R v The current threshold at the time of adding the dummy resistor was set to be 1.3 times the rated current for 64.62 ohms.
The current loop output quantity of the positive sequence d-axis and the positive sequence q-axis of the VSG output inductive current is subjected to reverse park transformation to obtain a three-phase modulation wave U + ra 、U + rb 、U + rc The negative sequence d-axis and negative sequence q-axis current loop output quantities of the VSG output inductive current are subjected to inverse park transformation to obtain a three-phase modulation wave U - ra 、U - rb 、U - rc ，U + ra 、U + rb 、U + rc And U - ra 、U - rb 、U - rc And adding the corresponding phase sequences to obtain the three-phase modulation wave of the VSG.
VSG toRated output power is 6kW, and grid-connected operation is performed, and the phase voltage amplitude of the power grid is 311V; when t=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 2.1pu; as shown in fig. 5, the VSG is not able to output reactive power to assist in grid voltage recovery at this time. The method provided by the application comprises the following steps: when detecting that the grid has a drop fault, firstly switching a virtual resistor according to whether the inductance current exceeds a set current threshold value of 12.86A, wherein the virtual resistor has the effect of limiting VSG output transient overcurrent, and calculating a VSG output current reactive component instruction value I related to the voltage drop depth of the grid according to the formulas (3) and (4) qref And an active component instruction value I dref The VSG output active power command value and the output reactive power command value are changed, so that the VSG is limited from overcurrent during steady-state operation; when the method is adopted, the VSG inductance current is well limited below 1.3pu, so that the VSG is ensured not to be disconnected due to overcurrent when the power grid voltage drops down, meanwhile, the VSG can also output reactive power of 1.8kvar according to a theoretical calculation value to help the power grid voltage recovery according to the theory calculation value in the method shown in fig. 5, and the effectiveness of the method in the power grid three-phase voltage symmetrical drop down fault is verified.
As shown in fig. 6, when the voltage of the phase a of the power grid drops to 0.2pu at the time t=t2, the voltage of the phase a of the power grid returns to normal at the time t=t5, as shown in fig. 6, if no control is added, the current of the three-phase output inductor of the VSG is asymmetric and severely distorted, the current peak value is far beyond the rated current of the prototype, and very large double power frequency pulsation components exist in the output active power and the output reactive power of the VSG, so that the power quality is severely reduced. As shown in fig. 7, when the method of the present application is adopted, m=0 is set in the time period from t=t2 to t3, and according to the foregoing, VSG three-phase output current balance control is adopted, and at this time, VSG three-phase output inductor current is three-phase symmetrical, and the double power frequency pulsation component of the output active power and the output reactive power is reduced; when t=t3 to t4, m=1, k=1, and at the moment, VSG output active power doubling pulse suppression control is adopted, and at the moment, compared with VSG three-phase output current balance control, the symmetry of VSG three-phase inductance current is reduced, the VSG output active power doubling pulse quantity is further reduced, and the VSG output reactive power pulse quantity is increased; when t=t4 to t5, m=1 and k=1, at this time, VSG output reactive power doubling pulse suppression control is adopted, and at this time, compared with VSG three-phase output current balance control, VSG three-phase output inductance current symmetry is reduced, output reactive power doubling pulse quantity is further reduced, and output active power doubling pulse quantity is increased. However, in the period of t=t2 to t5, the VSG three-phase output inductance current is always controlled near the rated current, which proves that all three controls can improve the power quality and can realize smooth switching.
The control method provided by the application can ensure that VSG output cannot overflow no matter when a three-phase symmetrical fault or a three-phase asymmetrical fault occurs in the power grid, can output appointed reactive power to help the recovery of the power grid voltage, and can realize the free switching of three controls, namely VSG three-phase output current balance, VSG output active power double frequency pulsation suppression control and VSG output reactive power double frequency pulsation suppression control, particularly when the three-phase asymmetrical fault occurs in the power grid voltage, so as to improve the electric energy quality.
The above description is only a preferred embodiment of the present application, and is not intended to limit the application in any way, and any person skilled in the art may make modifications or alterations to the disclosed technical content to the equivalent embodiments. However, any simple modification, equivalent variation and variation of the above embodiments according to the technical substance of the present application still fall within the protection scope of the technical solution of the present application.
Claims (3)
1. The comprehensive control method for the low voltage ride through of the virtual synchronous machine is characterized by comprising the following steps of:
1) All three-phase alternating current quantities are transformed from a three-phase static ABC coordinate system to a two-phase rotation dq0 coordinate system through park, and the amplitude of an alternating current component is oriented on a d axis;
2) Collecting VSG output voltage, VSG output current and grid voltage in real time, and adopting a positive-negative sequence separation method to obtain positive sequence components and negative sequence components of the collection quantity to participate in VSG loop control;
3) When the VSG output current is detected to exceed the current threshold value, subtracting a product value of a positive sequence d-axis component of the VSG output inductance current and a positive coefficient from a first voltage command value Em output by the VSG reactive power loop to serve as a second voltage command value U of the positive sequence d-axis voltage loop + dref Taking the product value of the positive q-axis component of VSG output inductance current subtracted from the first voltage command value 0 of the positive q-axis voltage loop of VSG and a positive coefficient as the second voltage command value U of the positive q-axis voltage loop + qref ；
4) When the positive sequence d-axis component of the grid voltage is detected to be smaller than a set threshold value, firstly replacing a VSG output voltage rated value Un in a VSG reactive power excitation voltage regulation control equation with the positive sequence d-axis component U of the grid voltage + pd Calculating VSG output reactive power instruction value Q according to power grid voltage drop depth ref Calculating the VSG active power command value P according to the rated capacity of the VSG and the reactive power command at the moment ref ；
5) After detecting that the power grid voltage drops, if the power grid voltage is an asymmetric drop fault, three control targets of VSG three-phase output current balance, VSG output active power double frequency pulsation suppression and VSG output reactive power double frequency pulsation suppression are realized by assigning different values to the VSG current inner loop current command value;
in step 5), the method for calculating the current inner loop command value under three control targets comprises the following steps: calculating an expression of VSG output active power and reactive power when the voltage of the 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 the balance control of VSG three-phase output current 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 made to be 0; when the VSG output active power double frequency pulsation suppression control is to be realized, enabling the VSG output active power double frequency sine pulsation component and the cosine pulsation component to be 0, and orienting the power grid voltage amplitude and the VSG output voltage amplitude under the d axis under a two-dimensional rotating coordinate system, so that the q axis component of the voltage is 0, and then obtaining command values of the positive sequence d axis component of the VSG output inductor current, the positive sequence q axis component of the VSG output inductor current, the negative sequence d axis component of the VSG output inductor current and the negative sequence q axis component of the VSG output inductor current according to the two conditions; when the VSG output reactive power frequency doubling pulsation suppression control is to be realized, enabling the VSG output reactive power frequency doubling sine pulsation component and the cosine pulsation component to be 0, and obtaining command values of a VSG output inductance current positive sequence d-axis component, a VSG output inductance current positive sequence q-axis component, a VSG output inductance current negative sequence d-axis component and a VSG output inductance current negative sequence q-axis component;
the calculation method of the VSG output inductance current command value under three control targets comprises the following steps: when the power grid voltage is normal, VSG outputs current command value I of positive sequence d-axis component and positive sequence q-axis component of inductance current + dref And I + qref The direct sequence d-axis component current command value is the output active power command value P of VSG ref Positive sequence d-axis component U divided by VSG output voltage + 0d And 1.5, the positive sequence Q-axis component current command value is VSG output reactive power command value Q ref Positive sequence d-axis component U divided by VSG output voltage + 0d And 1.5, subtracting the current command value output by the voltage ring of the VSG intermediate control loop from the current command value of the control target to be realized to obtain the compensation value of the command value on the VSG positive sequence d-axis and q-axis current inner loop, and subtracting 0 from the calculated negative sequence d-axis and q-axis current command value to obtain the compensation value of the command value on the VSG negative sequence d-axis and q-axis current inner loop.
2. The method for comprehensively controlling low voltage ride through of a virtual synchronous machine according to claim 1, wherein in the step 2), an outer ring of the control system is an active power frequency modulation link and a reactive power voltage modulation link, an intermediate ring is a positive-sequence d-axis voltage ring and a positive-sequence q-axis voltage ring, and an inner ring is a positive-sequence d-axis component I for controlling an output induction current of a VSG + Ld Current loop and positive sequence q-axis component I of (2) + Lq The current loops of the positive sequence d-axis current loop and the positive sequence q-axis current loop are added and controlled to output inductive current of VSGNegative sequence d-axis component I of (2) - Ld Current loop and negative sequence q-axis component I of (2) - Lq Namely a negative sequence d-axis current loop and a negative sequence q-axis current loop.
3. The method for comprehensively controlling low voltage ride through of a virtual synchronous machine according to claim 1, wherein in the step 3), a 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 q-axis component of an output inductor current of the VSG is equivalent to a virtual resistance value, the addition of the virtual resistance is equivalent to an equivalent resistance between an ac output side of the VSG and a grid-connected point becoming large, a total impedance is obtained by dividing a maximum voltage variation amount of a 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 ac output side of the VSG and the grid-connected point.
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