CN111769588A - VSG low-voltage ride-through control method and system based on power grid unbalance fault - Google Patents
VSG low-voltage ride-through control method and system based on power grid unbalance fault Download PDFInfo
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Abstract
The invention provides a VSG low-voltage ride-through control method based on a power grid unbalance fault. The method comprises the following steps: when the power grid has an unbalanced drop fault, the positive and negative sequence double current control of the inverter is carried out based on a preset unbalanced fault control strategy to obtain the positive sequence current and the negative sequence current which are required to be output by the normal grid-connected operation of the VSG system of the inverter, and the positive sequence current and the negative sequence current signals are superposed to obtain a modulation signal. According to the technical scheme provided by the invention, aiming at the unbalanced drop fault of the power grid, the positive-sequence current and negative-sequence current controllers are adopted, the negative-sequence current is inhibited, and the positive-sequence current reactive compensation control is added, so that the low-voltage ride-through fault is realized while the normal work of the inverter is ensured.
Description
Technical Field
The invention belongs to the field of power distribution networks, and particularly relates to a VSG low voltage ride through control method and system based on a power grid unbalance fault.
Background
In the actual operation of the power grid, the unbalance of the power grid can be caused by the unbalance of three-phase loads, single-phase or multi-phase grounding short circuits, interphase short circuits and other faults, and in the actual operation of the power grid, the probability (95%) of the unbalanced drop fault of the power grid voltage is far greater than the probability (5%) of the balanced drop fault of the power grid voltage.
The conventional inverter control design is generally realized under the condition of three-phase grid balance, and when the grid voltage is unbalanced and fails, if the inverter still uses the control strategy in the balanced state, the following effects are caused: when the voltage of the power grid is unbalanced, the electromotive force of the power grid has a negative sequence component, so that alternating negative sequence current is generated, second harmonic is generated in the direct current side through pulse width modulation, and after the harmonic current is filtered by a direct current capacitor and a load resistor, the direct current voltage generates second harmonic voltage, so that the input power of the direct current side has larger second harmonic.
Although the harmonic amplitude can be reduced to a certain extent by increasing the capacitance, the operation performance of the inverter is affected, the efficiency is reduced, and the loss is increased. The second harmonic in the dc voltage is modulated to generate a third harmonic current on the ac side. The existence of the negative sequence current can cause the current on the alternating current side of the three-phase inverter to be asymmetric, and the operation of the inverter is seriously influenced. And a large amount of low-order harmonics can cause the distortion of the voltage of a power grid, so that the loss of the filter inductor and the transformer on the alternating current side is increased, and the like.
Disclosure of Invention
In order to solve the problems of loss caused by power grid unbalance and influence on power grid voltage in inverter control design, the invention provides a VSG low-voltage ride-through control method based on power grid unbalance fault.
The technical scheme provided by the invention is as follows:
a VSG low voltage ride through control method based on a power grid unbalance fault comprises the following steps:
when the power grid has an unbalanced drop fault, the positive and negative sequence double current control of the inverter is carried out based on a preset unbalanced fault control strategy, and the positive sequence current and the negative sequence current which are required to be output by the normal grid-connected operation of the inverter VSG system are obtained;
and superposing the positive sequence current signal and the negative sequence current signal to obtain a modulation signal, and modulating the modulation signal to obtain a switching signal of each bridge arm of the inverter.
Preferably, the preset unbalance fault control strategy comprises:
carrying out coordinate transformation on the unbalanced voltage to obtain a voltage direct current component and an alternating current component which changes at twice angular frequency;
filtering the alternating current component changing at twice angular frequency by using a wave trap, and reserving a direct current component;
controlling the positive sequence current of the direct current component according to a preset balance fault control strategy to obtain reactive current supporting the voltage of the power grid and corresponding active current;
and controlling the negative sequence current of the direct current component to be zero by taking the balance of the output voltage as a target.
Further, the coordinate transformation of the unbalanced voltage to obtain a voltage direct current component and an alternating current component varying at twice the angular frequency includes:
performing clark conversion on the unbalanced voltage to obtain a positive sequence component and a negative sequence component of the unbalanced voltage in a two-phase static coordinate system;
and carrying out positive dq conversion and negative dq conversion on the positive sequence component and the negative sequence component to obtain a direct current component of the unbalanced voltage in a dq coordinate system and an alternating current component changing at twice angular frequency.
Further, the controlling the positive sequence current of the dc component according to a preset balance fault control strategy to obtain the reactive current supporting the grid voltage and the corresponding active current includes:
acquiring the current grid-connected point voltage;
when the voltage of the grid-connected point is greater than the lowest threshold and less than the maximum drop threshold, calculating a dynamic reactive current value and an active current value to be output according to the voltage of the grid-connected point, the rated voltage and the rated current;
and when the voltage of the grid-connected point is greater than the maximum drop threshold and less than the maximum threshold, calculating the dynamic reactive current value and the active current value to be output according to the rated current.
Further, the set lowest threshold value and the set highest threshold value are specified multiples of the rated voltage;
the maximum drop threshold is the difference between the rated voltage and the maximum depth of the drop allowed by the grid-connected point voltage;
and lowest threshold < maximum drop threshold < highest threshold.
Further, the controlling the negative-sequence current of the dc component to be zero with the output voltage balance as a target includes:
setting reference instruction signals of negative sequence active current and reactive current of the direct current component to be zero;
and adjusting the active current and the reactive current of the negative sequence current of the direct current component to be zero by adopting a PI (proportional integral) regulator according to the reference instruction signal.
Further, the performing positive dq conversion and negative dq conversion on the positive sequence component and the negative sequence component to obtain a dc component of the unbalanced voltage in a dq coordinate system and an ac component varying at twice the angular frequency includes:
the grid voltage vector is subjected to positive dq coordinate transformation to obtain a positive sequence direct current component and a negative sequence alternating current component which changes at twice angular frequency in a dq coordinate;
and transforming the grid voltage vector by a negative dq coordinate to obtain a negative sequence direct current component and a positive sequence alternating current component which changes at twice angular frequency in the dq coordinate.
A VSG low voltage ride through control system based on a grid imbalance fault, the system comprising:
the judging module is used for acquiring the current grid-connected point voltage and judging whether the power grid has an unbalanced drop fault;
the fault control module is used for carrying out positive and negative sequence double-current control on the inverter based on a preset unbalanced fault control strategy when the unbalanced drop fault occurs in the power grid so as to obtain positive sequence current and negative sequence current which are required to be output during normal grid-connected operation of the inverter VSG system;
and the modulation output module is used for superposing the positive sequence current signal and the negative sequence current signal to obtain a modulation signal and modulating the modulation signal to obtain a switching signal of each bridge arm of the inverter.
The fault control module includes: the device comprises a coordinate transformation unit, a filtering unit, a positive sequence current control unit and a negative sequence current control unit;
the coordinate transformation unit is used for carrying out coordinate transformation on the unbalanced voltage to obtain a voltage direct-current component and an alternating-current component which changes at twice angular frequency;
the filtering unit is used for filtering the alternating current component changing at twice angular frequency by adopting a wave trap and reserving the direct current component;
the positive sequence current control unit is used for controlling the positive sequence current of the direct current component according to a preset balance fault control strategy to obtain reactive current supporting the voltage of a power grid and corresponding active current;
and the negative sequence current control unit is used for controlling the negative sequence current of the direct current component to be zero by taking the output voltage balance as a target so as to eliminate the power grid imbalance.
The modulation output module includes: a modulation unit and an output unit;
the modulation unit is used for superposing the positive sequence current signal and the negative sequence current signal to obtain a modulation signal, and modulating the modulation signal to obtain a switching signal of each bridge arm of the inverter;
and the output unit is used for outputting the switching signals of each bridge arm of the inverter.
Compared with the prior art, the invention has the beneficial effects that:
the invention provides a VSG low voltage ride through control method based on a power grid unbalance fault, when the power grid unbalance drop fault occurs, the positive and negative sequence double current control of an inverter is carried out based on a preset unbalance fault control strategy, and the positive sequence current and the negative sequence current which are required to be output by the normal grid-connected operation of an inverter VSG system are obtained; and superposing the positive sequence current signal and the negative sequence current signal to obtain a modulation signal, and modulating the modulation signal to obtain a switching signal of each bridge arm of the inverter. According to the technical scheme provided by the invention, when the power grid has an asymmetric fault, the positive and negative sequence double-current controllers are adopted, the positive sequence current reactive compensation control is added while the negative sequence current is inhibited, and the low-voltage ride-through fault is realized while the normal work of the inverter is ensured.
The technical scheme provided by the invention is used for accurately separating positive and negative sequence components according to different coordinate transformations, and adopts a filtering method, and the positive and negative sequence components of the voltage of the asymmetric power grid are accurately extracted by using the wave trap, and simultaneously, double frequency components influencing the voltage balance of the power grid are filtered.
The technical scheme provided by the invention aims at controlling the positive sequence current according to the control idea under the balanced fault, controlling the negative sequence current to be zero by taking the output voltage balance as the target, controlling the negative sequence component to be zero, controlling by making the negative sequence current reference value to be zero, and finally generating the switching signal of the inverter by adding the modulation signals generated by the positive and negative sequence control, thereby improving the operation performance of the inverter, improving the operation efficiency, reducing the loss and avoiding the serious distortion of the grid-connected current.
Drawings
FIG. 1 is a flowchart illustrating an embodiment of a VSG low voltage ride through control method based on a grid imbalance fault according to the present invention;
fig. 2(a) is a vector diagram of unbalanced voltage coordinate clark transformation in the embodiment of the present invention;
FIG. 2(b) is a vector diagram of transformation of an unbalanced voltage coordinate dq according to an embodiment of the present invention;
FIG. 3 is a block diagram of the positive and negative sequence separation control of unbalanced voltage by the wave trap method in the embodiment of the present invention;
fig. 4 is an amplitude-frequency characteristic of the wave trap at ω 200rad/s according to the embodiment of the present invention;
FIG. 5 is a block diagram of low voltage ride through control under an unbalanced condition in an embodiment of the present invention;
fig. 6 is a schematic structural diagram of a VSG low voltage ride through control system based on a grid imbalance fault according to the present invention.
Detailed Description
For a better understanding of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings and examples. It is to be understood that the embodiments described are only a few embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The invention provides a VSG low voltage ride through control method based on a power grid unbalance fault, and relates to control of low voltage ride through capability of an inverter system in the field of power distribution networks. In the actual operation of a power grid, the probability (95%) of the unbalanced drop fault of the power grid voltage is far greater than the probability (5%) of the balanced drop fault of the power grid voltage, and when the power grid voltage has the unbalanced fault, if the inverter still uses a control strategy in a balanced state, the three-phase output current at the alternating current side of the inverter is unbalanced, and meanwhile, harmonic current is injected into the power grid, so that the grid-connected current is seriously distorted. The low voltage ride through control strategy under the unbalanced fault provided by the invention adopts a control method of inhibiting negative sequence current by a positive sequence current controller and a negative sequence current controller, and simultaneously adds reactive compensation control, thereby realizing the ride through fault while the inverter works normally.
Example 1:
the embodiment of the invention provides a VSG low voltage ride through control method based on a power grid unbalance fault, the specific implementation process of which is shown in FIG. 1, and the method comprises the following steps:
s101: when the power grid has an unbalanced drop fault, the positive and negative sequence double current control of the inverter is carried out based on a preset unbalanced fault control strategy, and the positive sequence current and the negative sequence current which are required to be output by the normal grid-connected operation of the inverter VSG system are obtained;
s102: and superposing the positive sequence current signal and the negative sequence current signal to obtain a modulation signal, and modulating the modulation signal to obtain a switching signal of each bridge arm of the inverter.
Specifically, in step S101, when an unbalanced drop fault occurs in the power grid, the positive and negative sequence double current control of the inverter is performed based on a preset unbalanced fault control strategy to obtain a positive sequence current and a negative sequence current that need to be output during normal grid-connected operation of the inverter VSG system, and the specific implementation process includes:
step S101-1, coordinate transformation is carried out on the unbalanced voltage to obtain a voltage direct current component and an alternating current component which changes at twice angular frequency, and the method comprises the following steps:
s101-1-1, carrying out clark transformation on the unbalanced voltage to obtain a positive sequence component and a negative sequence component of the unbalanced voltage in a two-phase static coordinate system;
step S101-1-2, performing positive dq conversion and negative dq conversion on the positive sequence component and the negative sequence component to obtain a direct current component of the unbalanced voltage in a dq coordinate system and an alternating current component changing at twice angular frequency;
step S101-2, filtering the alternating current component changing at twice angular frequency by using a wave trap and reserving the direct current component;
step S101-3, controlling the positive sequence current of the direct current component according to a preset balance fault control strategy to obtain the reactive current supporting the voltage of the power grid and the corresponding active current, and specifically comprising the following steps:
step S101-3-1, determining a preset maximum drop threshold U according to the voltage of the grid-connected pointCThe method specifically comprises the following steps:
step S101-3-1-1, determining the maximum value K of the preset parameter Kmax：
The preset parameter K is calculated as follows:
wherein iqReactive current, i, output for the inverter systemdFor outputting active current, U, of the inverter systemATo set the lowest threshold, UBM is a set current coefficient and is less than or equal to 1.1;
the maximum value K of the preset parameter K can be obtained according to the formula (1)maxCalculated as follows:
when m is 1.1, the compound is,
step S101-3-1-2, determining the maximum depth delta U of the allowed drop of the voltage of the grid-connected point when the active current output of the inverter is all rated currentmaxCalculated as follows:
wherein, KsFor a predetermined parameter, KmaxIs the maximum value of the preset parameter K, m is the set current coefficient and m is less than or equal to 1.1;
when m is 1.1, Ks≤1.571；
Get Ks1.5, then
Step S101-3-1-3, determining a preset maximum drop threshold U according to the voltage of the grid-connected pointCCalculated as follows:
UC＝UN-ΔUmax
wherein, UCIs a preset maximum drop threshold, Δ UmaxThe maximum allowable drop depth of the voltage of the grid-connected point is U when the active current output of the inverter is all rated currentNIs a rated voltage;
according to step S101-3-1-2,. DELTA.Umax＝0.307UNThen, there are:
UC＝UN-0.307UN＝0.593UN；
step S101-3-2, when the voltage of the grid-connected point is greater than the lowest threshold and smaller than the maximum drop threshold, calculating a dynamic reactive current value and an active current value to be output according to the voltage of the grid-connected point, the rated voltage and the rated current, and respectively calculating by using the following formulas:
Wherein iqtDynamic reactive current i to be output by a photovoltaic inverter systemdtThe active current to be output by the photovoltaic inversion system is U, which is the current voltage of the grid-connected pointNTo rated voltage, INFor rated current, m is a set current coefficient, UATo set the lowest threshold, UBTo set the highest threshold, UCIs a preset maximum drop threshold;
when m is 1.1, the formula is:
step S101-3-3, when the voltage of the grid-connected point is larger than the maximum drop threshold and smaller than the maximum threshold, calculating a dynamic reactive current value and an active current value to be output according to the rated current, and calculating by using the following formulas respectively: :
Wherein iqtDynamic reactive current i to be output by a photovoltaic inverter systemdtIs light ofActive current, U, to be output by a photovoltaic inverter systemATo set the lowest threshold, UBTo set the highest threshold, UCM is a preset maximum drop threshold value and is a set current coefficient;
when m is 1.1, the formula is:
s101-3-4, when the voltage of the grid-connected point is smaller than the lowest threshold value, the inverter is disconnected and cannot provide support; when the voltage of the grid-connected point is greater than the highest threshold value, the active current is taken from an outer ring instruction, and the reactive current output is zero;
step S101-4, controlling the negative sequence current of the direct current component to be zero by taking output voltage balance as a target, and eliminating power grid unbalance, specifically comprising:
s101-4-1, setting reference command signals of negative sequence active current and reactive current of the direct current component to be zero;
and S101-4-2, adjusting the active current and the reactive current of the negative sequence current of the direct current component to be zero by adopting a PI (proportion integration) regulator according to the reference instruction signal.
Example 2:
when the voltage of the power grid is unbalanced, the fundamental voltage can be decomposed into positive sequence fundamental voltage EPNegative sequence fundamental voltage ENAnd zero sequence fundamental voltage E0. The general distributed grid-connected system adopts a three-phase three-wire system, so that the components of the unbalanced grid voltage do not contain a zero-sequence component E0I.e. the unbalanced grid voltage can be expressed as:
E＝EP+EN(1)
when each phase voltage is expressed as an instantaneous value, the following is made
In the formula:
ea、eb、ec-grid voltages eachA phase instantaneous value;
EPm-the fundamental amplitude of the positive sequence component of the grid voltage;
ENm-the fundamental amplitude of the negative sequence component of the grid voltage;
Clark conversion is carried out on the formula (2) to obtain the component of the asymmetric voltage in the two-phase static coordinate system
The network voltage vector can be seen from the formula (3)
When three phases are unbalanced, the three-phase imbalance can be regarded as a positive sequence component E rotating anticlockwise at fundamental angular frequencyPAnd a negative sequence component E rotating clockwise at the fundamental angular frequencyNThe synthesis is shown as a formula (4). Resultant voltage vector at this time
The angular frequency of rotation is no longer fixed and the amplitude also changes with time, and figure 2(a) shows the voltage vector
Vector decomposition in a coordinate system.
Then, the partial is converted into a two-phase rotating coordinate system rotating counterclockwise at an angular frequency omega by thetadThe initial phase angle of the rotating coordinate system can be expressed as
At this time, the reverse rotation vector
The projected component in the dq coordinate system is a direct current quantity, and the voltage vector is rotated clockwise
The projected component in the dq coordinate system is an alternating component that varies at twice the angular frequency, as shown in fig. 2 (b).
Similarly, if the dq coordinate system rotates clockwise at the angular frequency ω, the grid voltage positive sequence component EPThe projection component in the dq coordinate system is an alternating component varying at twice the angular frequency, and the negative-sequence component E of the voltageNIt appears as a direct current in the dq coordinate system. In order to distinguish between two dq coordinate systems with different rotation directions, a coordinate system rotated counterclockwise is referred to as a dq rotation coordinate system, and a coordinate system rotated clockwise is referred to as a negative dq rotation coordinate system.
Filtering methods generally use traps that can eliminate signals at fixed frequencies, but do not work for signals at any frequency other than this. After the grid voltage vector is subjected to positive dq coordinate conversion, a positive sequence component of the grid voltage vector is a direct current component, and a negative sequence component of the grid voltage vector is a fixed double-frequency component. Similarly, the positive sequence component is also a double-frequency component after the negative dq coordinate conversion, so that the double-frequency component in the positive and negative coordinate systems can be conveniently filtered by adopting the wave trap, so that the positive and negative sequence voltage and current are separated, and the positive and negative sequence separation block diagram is shown in fig. 3.
In fig. 3, the transfer function of the trap is shown in equation (6), and when the damping coefficient ξ is 0.1, it can be seen from fig. 4 that the trap is at ω0When the amplitude attenuation is 200 pi rad/s, the amplitude attenuation can reach-295 dB, and the separation requirement is completely met.
When the power grid is unbalanced, the inverter is required not to be disconnected from the power grid and not to be overflown, and simultaneously, active power and reactive power are required to be transmitted to the power grid. When the power grid is in asymmetric fault, the control method of inhibiting negative sequence current by adopting the positive and negative sequence double-current controllers is adopted, and reactive compensation control is added at the same time, so that the fault ride-through is realized while the inverter works normally.
The control process comprises the following steps: firstly, the current of a grid-connected point is detected, when the power grid has an unbalanced fault, the positive and negative sequence separation is carried out on the current by using a wave trap method, meanwhile, a state control module carries out state switching from 1 to 2, the subsequent control strategy is as shown in figure 5, the upper half part is adopted for positive sequence current control, the lower half part is adopted for negative sequence current control, and instruction signals i of negative sequence current are respectively setdref-and iqref-，idref- (0) and iqref-＝0。
As shown in fig. 5, the positive-sequence current control is controlled according to the control concept under the balanced fault, and the negative-sequence current control is controlled by making the negative-sequence current reference value zero with the output voltage balance as the target. And finally, generating a switching signal of the inverter by adding the modulation signals generated by the positive and negative sequence control.
Example 3:
based on the same inventive concept, the present invention further provides a VSG low voltage ride through control system based on the grid imbalance fault, as shown in fig. 6, the system includes:
the judging module is used for acquiring the current grid-connected point voltage and judging whether the power grid has an unbalanced drop fault;
the fault control module is used for carrying out positive and negative sequence double-current control on the inverter based on a preset unbalanced fault control strategy when the unbalanced drop fault occurs in the power grid so as to obtain positive sequence current and negative sequence current which are required to be output during normal grid-connected operation of the inverter VSG system;
and the modulation output module is used for superposing the positive sequence current signal and the negative sequence current signal to obtain a modulation signal and modulating the modulation signal to obtain a switching signal of each bridge arm of the inverter.
The fault control module includes: the device comprises a coordinate transformation unit, a filtering unit, a positive sequence current control unit and a negative sequence current control unit;
the coordinate transformation unit is used for carrying out coordinate transformation on the unbalanced voltage to obtain a voltage direct-current component and an alternating-current component which changes at twice angular frequency;
the filtering unit is used for filtering the alternating current component changing at twice angular frequency by adopting a wave trap and reserving the direct current component;
the positive sequence current control unit is used for controlling the positive sequence current of the direct current component according to a preset balance fault control strategy to obtain reactive current supporting the voltage of a power grid and corresponding active current;
and the negative sequence current control unit is used for controlling the negative sequence current of the direct current component to be zero by taking the output voltage balance as a target so as to eliminate the power grid imbalance.
The modulation output module includes: a modulation unit and an output unit;
the modulation unit is used for superposing the positive sequence current signal and the negative sequence current signal to obtain a modulation signal, and modulating the modulation signal to obtain a switching signal of each bridge arm of the inverter;
and the output unit is used for outputting the switching signals of each bridge arm of the inverter.
As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flow diagrams and/or block diagrams, and combinations of flows and/or blocks in the flow diagrams and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
The present invention is not limited to the above embodiments, and any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention are included in the scope of the claims of the present invention which are filed as the application.
Claims (10)
1. A VSG low voltage ride through control method based on a power grid unbalance fault is characterized by comprising the following steps:
when the power grid has an unbalanced drop fault, the positive and negative sequence double current control of the inverter is carried out based on a preset unbalanced fault control strategy, and the positive sequence current and the negative sequence current which are required to be output by the normal grid-connected operation of the inverter VSG system are obtained;
and superposing the positive sequence current signal and the negative sequence current signal to obtain a modulation signal, and modulating the modulation signal to obtain a switching signal of each bridge arm of the inverter.
2. The method for controlling VSG low voltage ride-through under grid imbalance fault according to claim 1, wherein the preset imbalance fault control strategy comprises:
carrying out coordinate transformation on the unbalanced voltage to obtain a voltage direct current component and an alternating current component which changes at twice angular frequency;
filtering the alternating current component changing at twice angular frequency by using a wave trap, and reserving a direct current component;
controlling the positive sequence current of the direct current component according to a preset balance fault control strategy to obtain reactive current supporting the voltage of the power grid and corresponding active current;
and controlling the negative sequence current of the direct current component to be zero by taking the balance of the output voltage as a target.
3. The method for controlling the VSG low voltage ride through under the grid imbalance fault according to claim 2, wherein the step of performing coordinate transformation on the imbalance voltage to obtain a voltage direct current component and an alternating current component which changes at twice the angular frequency comprises the following steps:
performing clark conversion on the unbalanced voltage to obtain a positive sequence component and a negative sequence component of the unbalanced voltage in a two-phase static coordinate system;
and carrying out positive dq conversion and negative dq conversion on the positive sequence component and the negative sequence component to obtain a direct current component of the unbalanced voltage in a dq coordinate system and an alternating current component changing at twice angular frequency.
4. The method according to claim 2, wherein the controlling the positive sequence current of the dc component according to a preset balance fault control strategy to obtain a reactive current supporting a grid voltage and a corresponding active current comprises:
acquiring the current grid-connected point voltage;
when the voltage of the grid-connected point is greater than the lowest threshold and less than the maximum drop threshold, calculating a dynamic reactive current value and an active current value to be output according to the voltage of the grid-connected point, the rated voltage and the rated current;
and when the voltage of the grid-connected point is greater than the maximum drop threshold and less than the maximum threshold, calculating the dynamic reactive current value and the active current value to be output according to the rated current.
5. The method for controlling VSG low voltage ride through under grid imbalance based on claim 4,
the set lowest threshold value and the set highest threshold value are designated multiples of rated voltage;
the maximum drop threshold is the difference between the rated voltage and the maximum depth of the drop allowed by the grid-connected point voltage;
and lowest threshold < maximum drop threshold < highest threshold.
6. The method according to claim 2, wherein the controlling the negative sequence current of the dc component to be zero with the aim of output voltage balancing comprises:
setting reference instruction signals of negative sequence active current and reactive current of the direct current component to be zero;
and adjusting the active current and the reactive current of the negative sequence current of the direct current component to be zero by adopting a PI (proportional integral) regulator according to the reference instruction signal.
7. The method for controlling the VSG low voltage ride through under the grid imbalance fault according to claim 3, wherein the step of subjecting the positive sequence component and the negative sequence component to positive dq conversion and negative dq conversion to obtain the DC component of the imbalance voltage in a dq coordinate system and the AC component varying at twice angular frequency comprises:
the grid voltage vector is subjected to positive dq coordinate transformation to obtain a positive sequence direct current component and a negative sequence alternating current component which changes at twice angular frequency in a dq coordinate;
and transforming the grid voltage vector by a negative dq coordinate to obtain a negative sequence direct current component and a positive sequence alternating current component which changes at twice angular frequency in the dq coordinate.
8. A VSG low voltage ride through control system based on under grid unbalance fault is characterized by comprising:
the judging module is used for acquiring the current grid-connected point voltage and judging whether the power grid has an unbalanced drop fault;
the fault control module is used for carrying out positive and negative sequence double-current control on the inverter based on a preset unbalanced fault control strategy when the unbalanced drop fault occurs in the power grid so as to obtain positive sequence current and negative sequence current which are required to be output during normal grid-connected operation of the inverter VSG system;
and the modulation output module is used for superposing the positive sequence current signal and the negative sequence current signal to obtain a modulation signal and modulating the modulation signal to obtain a switching signal of each bridge arm of the inverter.
9. The system of claim 8, wherein the fault control module comprises: the device comprises a coordinate transformation unit, a filtering unit, a positive sequence current control unit and a negative sequence current control unit;
the coordinate transformation unit is used for carrying out coordinate transformation on the unbalanced voltage to obtain a voltage direct-current component and an alternating-current component which changes at twice angular frequency;
the filtering unit is used for filtering the alternating current component changing at twice angular frequency by adopting a wave trap and reserving the direct current component;
the positive sequence current control unit is used for controlling the positive sequence current of the direct current component according to a preset balance fault control strategy to obtain reactive current supporting the voltage of a power grid and corresponding active current;
and the negative sequence current control unit is used for controlling the negative sequence current of the direct current component to be zero by taking the output voltage balance as a target so as to eliminate the power grid imbalance.
10. The system of claim 8, wherein the modulation output module comprises: a modulation unit and an output unit;
the modulation unit is used for superposing the positive sequence current signal and the negative sequence current signal to obtain a modulation signal, and modulating the modulation signal to obtain a switching signal of each bridge arm of the inverter;
and the output unit is used for outputting the switching signals of each bridge arm of the inverter.
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