CN111769589A - VSG low-voltage ride-through control method and system based on power grid balance fault - Google Patents

VSG low-voltage ride-through control method and system based on power grid balance fault Download PDF

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CN111769589A
CN111769589A CN201910261504.1A CN201910261504A CN111769589A CN 111769589 A CN111769589 A CN 111769589A CN 201910261504 A CN201910261504 A CN 201910261504A CN 111769589 A CN111769589 A CN 111769589A
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voltage
grid
current
threshold
connected point
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宋振浩
吴鸣
吕志鹏
孙丽敬
郑楠
徐斌
王刘芳
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State Grid Corp of China SGCC
China Electric Power Research Institute Co Ltd CEPRI
Electric Power Research Institute of State Grid Anhui Electric Power Co Ltd
State Grid Anhui Electric Power Co Ltd
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State Grid Corp of China SGCC
China Electric Power Research Institute Co Ltd CEPRI
Electric Power Research Institute of State Grid Anhui Electric Power Co Ltd
State Grid Anhui Electric Power Co Ltd
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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
    • 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
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    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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    • Y02E40/30—Reactive power compensation

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Abstract

The invention provides a VSG low-voltage ride-through control method and system based on a power grid balance fault. The method comprises the following steps: and when the grid-connected point voltage is between the lowest threshold and the highest threshold, calculating and determining a dynamic reactive current value and a corresponding active current value which are required to be output by the photovoltaic inverter system and used for supporting the grid-connected point voltage according to the grid-connected point voltage based on a preset maximum drop threshold. According to the technical scheme provided by the invention, when the voltage drops to a certain range, the control strategy of the distributed inverter system is changed to enable the distributed inverter system to output certain reactive current, and the voltage of a grid connection point is improved to a certain extent to realize low voltage ride through.

Description

VSG low-voltage ride-through control method and system based on power grid balance fault
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 balance fault.
Background
Various faults may occur during the operation of the power grid, of which voltage dips are one. The cause of voltage drop mainly refers to short-circuit faults of a power grid, and the short-circuit faults are divided into single-phase earth faults, two-phase earth faults, phase-to-phase faults and three-phase earth short-circuit faults.
With the increasing permeability of distributed power generation such as photovoltaic power generation and wind power generation in a power system, the influence of the distributed power generation on a power grid is increasingly prominent. Therefore, the influence of various operating states of distributed power generation such as photovoltaic power generation and wind power generation on the stability of the power grid when the power grid fails must be considered. Taking photovoltaic power generation as an example, the access of a high-density photovoltaic distributed inverter system will have a profound influence on the safe and stable operation of a power grid, and especially when the voltage of the power grid drops due to a short-circuit fault, if the photovoltaic distributed inverter system is suddenly disconnected from the power grid, impact is caused to the main grid, so that the fault is enlarged.
In order to ensure that an electric power system can safely and stably operate, power grid companies of various countries put strict technical requirements on a photovoltaic grid-connected system according to various national conditions, wherein the strict technical requirements comprise Low Voltage Ride Through (LVRT) capability, and the photovoltaic power station and the wind power plant grid-connected system are required to have the Low Voltage Ride Through (LVRT) capability and the capability of providing dynamic reactive support for the power grid during the LVRT period.
At present, the scheme for realizing the low voltage ride through capability mainly adopts a rotor short-circuit protection technology or introduces a novel topological structure, and the schemes need to modify a system hardware structure.
Disclosure of Invention
In order to improve the low voltage ride through capability of a photovoltaic grid-connected system, meet the national and international technical requirements, simultaneously, no additional hardware equipment is added, and the power generation cost is reduced, the invention provides a VSG low voltage ride through control method based on the power grid balance fault.
The technical scheme provided by the invention is as follows:
a VSG low voltage ride through control method based on a power grid balance fault comprises the following steps:
acquiring the current grid-connected point voltage, and judging whether the grid-connected point voltage value is between a set lowest threshold value and a set highest threshold value of the inverter capable of grid-connected operation;
when the grid-connected point voltage is between the lowest threshold and the highest threshold, calculating and determining a dynamic reactive current value and a corresponding active current value which are required to be output by the photovoltaic inverter system and used for supporting the grid-connected point voltage according to the grid-connected point voltage based on a preset maximum drop threshold;
adjusting the photovoltaic inverter system based on the dynamic reactive current value and the active current value;
the maximum drop threshold is between the lowest threshold and the highest threshold.
Preferably, when the voltage of the grid-connected point is between the set lowest threshold and the set highest threshold, based on a preset maximum drop threshold, calculating and determining a dynamic reactive current value and a corresponding active current value, which are required to be output by the photovoltaic inverter system and used for supporting the voltage of the grid-connected point, according to the voltage of the grid-connected point, includes:
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, when the voltage of the grid-connected point is greater than the lowest threshold and smaller than the maximum drop threshold, the dynamic reactive current value and the active current value which need to be output are calculated according to the voltage of the grid-connected point, the rated voltage and the rated current, and are respectively calculated by the following formulas:
Figure BDA0002015408590000021
UA＜U≤UCand m is less than or equal to 1.1
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.
Further, when the voltage of the grid-connected point is greater than the maximum drop threshold and less than the maximum threshold, the dynamic reactive current value and the active current value which need to be output are calculated according to the rated current, and the calculation is respectively carried out by the following formulas:
Figure BDA0002015408590000022
UC＜U≤UBand m is less than or equal to 1.1
Wherein iatDynamic reactive current i to be output by a photovoltaic inverter systemdtActive current, U, to be output by the photovoltaic inverter systemATo set the lowest threshold, UBTo set the highest threshold, UcM is a preset maximum drop threshold value, and m is a set current coefficient.
Further, in the above-mentioned case,
the set minimum threshold is 0.2UN；
The set maximum threshold value is 0.9UN；
Wherein, the UNIs a rated voltage.
Further, the preset maximum drop threshold is calculated according to the following formula:
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.
Further, the maximum depth DeltaU of the allowed drop of the grid-connected point voltagemaxCalculated as follows:
Figure BDA0002015408590000031
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.
Further, the preset parameter K is calculated according to the following formula:
Figure BDA0002015408590000032
wherein iqReactive current, i, output for the inverter systemdFor outputting active current, U, of the inverter systemATo set the lowest threshold, UBIs the set highest threshold.
Further, the maximum value K of the preset parameter KmCalculated as follows:
Figure BDA0002015408590000033
a VSG low voltage ride through control system based on a grid balancing fault, the system comprising:
the voltage and current acquisition module is used for acquiring the voltage, the active current and the reactive current of a grid-connected point in real time and recording data;
the state control module is used for calculating and determining dynamic reactive current and corresponding active current which are required to be output by the photovoltaic inversion system and used for supporting the voltage of the grid-connected point according to the voltage of the grid-connected point, the lowest threshold value, the highest threshold value and the maximum drop threshold value;
and the adjusting module is used for adjusting the photovoltaic inversion system based on the dynamic reactive current value and the active current value.
The voltage acquisition module includes: the device comprises a voltage acquisition unit and a current acquisition unit;
the voltage acquisition unit is used for acquiring the voltage of the grid-connected point and recording data;
and the current acquisition unit is used for acquiring active current and reactive current and recording data.
The state control module includes: the device comprises a judging unit, a selecting unit, a reference current calculating unit under a steady state, a reference current calculating unit under a fault and an output unit;
the judging unit is used for judging the size relationship between the grid-connected point voltage and a minimum threshold, a maximum threshold and a maximum drop threshold and determining the running state of the power grid;
the selection unit is used for selecting the control mode of the inverter system according to the output result of the judgment unit and switching to the corresponding calculation unit; when the power grid normally operates, switching to a reference current calculation unit under a steady state, and when the power grid has voltage drop, switching to a reference current calculation unit under a fault;
the reference current calculating unit under the steady state is used for calculating active current and reactive current under the non-voltage dropping state;
the under-fault reference current calculating unit is used for calculating active current and reactive current which are respectively output when the voltage of the grid-connected point is between a lowest threshold value and a maximum drop threshold value and between the maximum drop threshold value and a highest threshold value according to the real-time voltage, the rated voltage and the rated current;
and the output unit is used for outputting the calculated active current and reactive current data.
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 balance fault, which is used for acquiring the voltage of a current grid-connected point and judging whether the voltage value of the grid-connected point is between a set lowest threshold and a set highest threshold; when the grid-connected point voltage is between a set lowest threshold and a set highest threshold, calculating and determining a dynamic reactive current value and a corresponding active current value which are required to be output by a photovoltaic inverter system and used for supporting the grid-connected point voltage according to the grid-connected point voltage based on a preset maximum drop threshold; and adjusting the photovoltaic inverter system based on the dynamic reactive current value and the active current value. The technical scheme provided by the invention judges the voltage drop range and the power grid operation state, determines the active output current and the reactive output current in different voltage drop ranges, realizes the fault ride-through while the inverter system works normally, and improves the low-voltage ride-through capability of the inverter system.
The technical scheme provided by the invention is that the voltage drop range of the grid-connected point is 0.2UNTo 0.9UNAnd meanwhile, the control strategy of the distributed inverter system is changed to enable the distributed inverter system to output certain reactive current, and the voltage of a grid connection point is improved to a certain extent to realize low voltage ride through.
The technical scheme provided by the invention only changes the control strategy of the inverter system and does not additionally increase hardware equipment, so that the power generation cost can be reduced.
Drawings
FIG. 1 is a flowchart illustrating an embodiment of a VSG low voltage ride through control method based on a power grid balancing fault according to the present invention;
FIG. 2 is a flow chart of a reactive support based low voltage ride through control strategy in an embodiment of the present invention;
fig. 3(1) is a voltage sag vector diagram of a three-phase ground fault in the embodiment of the present invention;
fig. 3(2) is a single-phase short-circuit fault voltage sag vector diagram in the embodiment of the present invention;
fig. 3(3) and fig. 3(4) are voltage sag vector diagrams of interphase short-circuit faults in the embodiment of the invention;
fig. 3(5), fig. 3(6), and fig. 3(7) are graphs of voltage sag vectors of two-phase ground faults in the embodiment of the present invention;
FIG. 4 shows the low voltage ride through requirements of the photovoltaic power station in China in the embodiment of the invention;
FIG. 5 is a graph of the relationship between the reactive current and the voltage sag level in Germany according to the embodiment of the present invention;
fig. 6 is a block diagram of low voltage ride through control of the distributed inverter system under a balanced fault in the embodiment of the present invention;
FIG. 7 is a schematic structural diagram of a VSG low voltage ride through control system based on a grid balancing 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 balance fault, relates to control of low-voltage ride-through capability of an inverter system in the field of a power distribution network, and particularly relates to a distributed power generation system which is required to have the fault ride-through capability when the voltage of the power grid drops or rises and can provide a supporting function for the power grid in order to reduce the influence degree of the distributed power generation system on the power grid to the minimum in the distributed power generation system. The low voltage ride through mode provided by the invention only changes the control strategy of the inverter system, does not additionally increase hardware equipment, reduces the power generation cost, improves the low voltage ride through capability of the inverter system, and reduces the faults of the distributed power generation system.
Example 1:
the embodiment of the invention provides a VSG low voltage ride through control method based on a power grid balance fault, the specific implementation process of which is shown in FIG. 1 and comprises the following steps:
a VSG low voltage ride through control method based on a power grid balance fault comprises the following steps:
acquiring the current grid-connected point voltage, and judging whether the grid-connected point voltage value is between a set lowest threshold value and a set highest threshold value of the inverter capable of grid-connected operation;
when the grid-connected point voltage is between the lowest threshold and the highest threshold, calculating and determining a dynamic reactive current value and a corresponding active current value which are required to be output by the photovoltaic inverter system and used for supporting the grid-connected point voltage according to the grid-connected point voltage based on a preset maximum drop threshold;
adjusting the photovoltaic inverter system based on the dynamic reactive current value and the active current value;
the maximum drop threshold is between the lowest threshold and the highest threshold.
Specifically, in step S101, the set minimum threshold is 0.2UN(ii) a The set maximum threshold value is 0.9UN(ii) a Wherein, the UNIs a rated voltage;
specifically, in step S102, when the grid-connected point voltage is between the lowest threshold and the highest threshold, based on a preset maximum drop threshold, a dynamic reactive current value and a corresponding active current value, which are required to be output by the photovoltaic inverter system and used for supporting the grid-connected point voltage, are determined by calculating according to the grid-connected point voltage, and the method includes:
step S102-1, determining the maximum value K of the preset parameter KmaxThe specific process comprises the following steps:
step S102-1-1, calculating a preset parameter K according to the following formula:
Figure BDA0002015408590000061
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;
step S102-1-2, according to the step S102-1-1, presetting the maximum value K of the parameter KmaxCalculated as follows:
Figure BDA0002015408590000062
id＝0
when m is 1.1, the compound is,
Figure BDA0002015408590000063
step S102-2, determining the maximum depth delta U of the allowed drop of the grid-connected point voltagemaxCalculated as follows:
Figure BDA0002015408590000071
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
Figure BDA0002015408590000072
Step S102-3, 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 S102-2, Δ Umax＝0.307UNThen, there are:
UC＝UN-0.307UN＝0.593UN；
step S102-4, 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:
Figure BDA0002015408590000073
UA＜U≤UCand m is less than or equal to 1.1
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:
Figure BDA0002015408590000074
step S102-5, 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 respectively calculating by using the following formulas:
Figure BDA0002015408590000075
UC＜U≤UBand m is less than or equal to 1.1
Wherein iqtDynamic reactive current i to be output by a photovoltaic inverter systemdtActive current, U, to be output by the 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:
Figure BDA0002015408590000081
step S102-5, when the voltage of the grid-connected point is smaller than the lowest threshold value, the inverter is disconnected and cannot provide support; and when the voltage of the grid-connected point is greater than the highest threshold value, the active current is taken from the outer ring instruction, and the reactive current output is zero.
Example 2:
the distributed inverter system generally operates with unit power under the normal condition of grid-connected operation, and the reactive current reference value is 0. When the voltage of the power grid drops, the normal operation control outer ring is disconnected, the reference value of the reactive current takes value according to the formula (1),
Figure BDA0002015408590000082
if the current on the AC side is limited not to exceed 1.1 times of rated current when the voltage drops, namely
Figure BDA0002015408590000083
Figure BDA0002015408590000084
When the voltage drops to the distributed inversion system, the worst grid-connected operation working condition can be kept (when the voltage drops to 20% of rated voltage), if all the output of the inversion system is reactive current, the delta U is (0.9-0.2) UN＝0.7UNFrom the above formula
0.7KIN≤1.1IN(2)
Then there is
K≤1.571 (3)
When the active current output of the inverter is id＝INWhen the maximum reactive current allowed to be output is
Figure BDA0002015408590000085
In this case, if K is 1.5, i is not decreaseddIn the case of (2), the maximum allowable drop depth of the grid-connected point voltage obtained by the equation (1) is
ΔU≈0.307UN(5)
That is, when the voltage drops below 0.593UN, the active current output must be reduced if the grid-connected current amplitude requirement is to be met. When the active current reference value is
Figure BDA0002015408590000091
Wherein the content of the first and second substances,
Figure BDA0002015408590000092
when the voltage drops to 0.593UNIn the above case, in order to make the inverter system output reactive current as much as possible, the active output current can be fixed as
idref＝IN(8)
iqref＝0.46IN(9)
The idea of the above control strategy is: when the voltage drop range of the grid-connected point is 0.2UN to 0.9UN, the control strategy of the distributed inverter system is changed to enable the distributed inverter system to output certain reactive current, and the voltage of the grid-connected point is improved to a certain extent to realize low-voltage ride through, and the flow chart is shown in fig. 2. The low voltage ride through mode is only to change the control strategy of the inverter system, and no additional hardware equipment is added, so that the power generation cost can be reduced, and the control strategy block diagram of the system is shown in fig. 6.
Example 3:
based on the same inventive concept, the present invention further provides a VSG low voltage ride through control system based on a grid balancing fault, as shown in fig. 7, the system includes:
the voltage and current acquisition module is used for acquiring the voltage, the active current and the reactive current of a grid-connected point in real time and recording data;
the state control module is used for calculating and determining dynamic reactive current and corresponding active current which are required to be output by the photovoltaic inversion system and used for supporting the voltage of the grid-connected point according to the voltage of the grid-connected point, the lowest threshold value, the highest threshold value and the maximum drop threshold value;
and the adjusting module is used for adjusting the photovoltaic inversion system based on the dynamic reactive current value and the active current value.
The voltage acquisition module includes: the device comprises a voltage acquisition unit and a current acquisition unit;
the voltage acquisition unit is used for acquiring the voltage of the grid-connected point and recording data;
and the current acquisition unit is used for acquiring active current and reactive current and recording data.
The state control module includes: the device comprises a judging unit, a selecting unit, a reference current calculating unit under a steady state, a reference current calculating unit under a fault and an output unit;
the judging unit is used for judging the size relationship between the grid-connected point voltage and a minimum threshold, a maximum threshold and a maximum drop threshold and determining the running state of the power grid;
the selection unit is used for selecting the control mode of the inverter system according to the output result of the judgment unit and switching to the corresponding calculation unit; when the power grid normally operates, switching to a reference current calculation unit under a steady state, and when the power grid has voltage drop, switching to a reference current calculation unit under a fault;
the reference current calculating unit under the steady state is used for calculating active current and reactive current under the non-voltage dropping state;
the under-fault reference current calculating unit is used for calculating active current and reactive current which are respectively output when the voltage of the grid-connected point is between a lowest threshold value and a maximum drop threshold value and when the voltage of the grid-connected point is between the maximum drop threshold value and a highest threshold value according to real-time voltage, rated voltage and rated current;
and the output unit is used for outputting the calculated active current and reactive current data.
Example 4:
fig. 3(1) to 3(7) are voltage sag vector diagrams in which a dotted line indicates a sag front-phase voltage and a solid line indicates a sag back-phase voltage. Wherein, fig. 3(1) is a three-phase ground short fault, belonging to symmetric dropping, and the others belonging to asymmetric dropping; FIG. 3(2) shows a single-phase short-circuit fault; FIG. 3(3), FIG. 3(4) are interphase short-circuit faults; fig. 3(5), fig. 3(6), and fig. 3(7) show two-phase ground faults.
FIG. 4 shows the low voltage ride through requirements of the photovoltaic power station in China:
(1) low voltage ride through requirement
And the moment when the voltage drop is caused by the grid fault is 0 moment, and the photovoltaic inverter keeps normal grid-connected operation before the moment. It can be seen that:
1) when the voltage of a grid-connected point of the photovoltaic power station falls to 0p.u. (by taking a rated voltage as a reference), the photovoltaic power station can continuously run for 0.15s without being disconnected; for wind power, the minimum voltage drop bearing value is 0.2p.u., and the wind power is operated for 0.625s without being disconnected from a grid.
2) When the voltage of the grid-connected point of the photovoltaic power station falls below the red curve, the photovoltaic power station can be cut off from the power grid.
3) The photovoltaic power plant can recover to 90% of the rated voltage, i.e. 0.9p.u., within 2s of the voltage drop during the occurrence of a fault.
(2) Dynamic reactive support capability
According to the regulations in the national grid-connected standard of photovoltaic power stations, when a power system has short-circuit fault and voltage drops, the dynamic reactive current injected into a power grid by the photovoltaic power stations should meet the following requirements:
(1) from the moment when the voltage of the grid-connected point drops, the response time of the dynamic reactive current is not more than 30 ms;
(2) dynamic reactive current I of photovoltaic power station injected into power system from response of dynamic reactive circuit to voltage recovery to 0.9p.uqThe voltage change of the grid-connected point should be tracked in real time, and the following conditions are met:
Figure BDA0002015408590000111
wherein:
upccthe voltage per unit value of the grid-connected point of the photovoltaic power station;
INrated installed capacity of photovoltaic power station/(/) (
Figure BDA0002015408590000112
× grid-connected point nominal voltage).
According to the low voltage ride through technical standard, a corresponding low voltage ride through control strategy of the distributed power generation system can be formulated so as to meet the grid connection technical requirement of the distributed power generation system.
Fig. 5 is a graph relating reactive current to voltage sag in germany, where the injected current is all reactive current when the voltage sag exceeds 50% of the rated voltage. If the grid voltage is recovered to 90% of the rated voltage, the grid-connected inverter system still needs to keep the voltage support for 500ms according to the LVRT characteristic.
As can be seen from fig. 5, when the voltage drop amplitude exceeds 10%, the pv grid-connected inverter must inject reactive current into the grid to support the grid-connected point voltage, and the proportion of the reactive current in the total current increases by at least 2% for every 1% voltage drop. The reactive current required to be injected can be expressed as
Figure BDA0002015408590000113
In the formula:
k is a constant; Δ U-Voltage sag depth; u shapeN-a nominal voltage; i isN-rated current.
The reactive current required to be injected when the voltage of the power grid drops within 20ms can be calculated by the formula.
FIG. 6 is a block diagram of a low-voltage ride-through control strategy of a distributed inverter system under a power grid balance fault, and the control idea is that when a voltage drop range of a grid-connected point is 0.2UNTo 0.9UNAnd meanwhile, the control strategy of the distributed inverter system is changed to enable the distributed inverter system to output certain reactive current, and the voltage of a grid connection point is improved to a certain extent to realize low voltage ride through.
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 (12)

1. A VSG low voltage ride through control method based on a power grid balance fault is characterized by comprising the following steps:
acquiring the current grid-connected point voltage, and judging whether the grid-connected point voltage value is between a set lowest threshold value and a set highest threshold value of the inverter capable of grid-connected operation;
when the grid-connected point voltage is between the lowest threshold and the highest threshold, calculating and determining a dynamic reactive current value and a corresponding active current value which are required to be output by the photovoltaic inverter system and used for supporting the grid-connected point voltage according to the grid-connected point voltage based on a preset maximum drop threshold;
adjusting the photovoltaic inverter system based on the dynamic reactive current value and the active current value;
the maximum drop threshold is between the lowest threshold and the highest threshold.
2. The method for controlling VSG low voltage ride through under grid balancing fault according to claim 1, wherein the step of calculating and determining a dynamic reactive current value and a corresponding active current value, which are required to be output by a photovoltaic inverter system to support a grid-connected point voltage, according to the grid-connected point voltage based on a preset maximum droop threshold when the grid-connected point voltage is between a set minimum threshold and a set maximum threshold comprises:
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.
3. The method for controlling VSG low voltage ride-through under grid balance fault according to claim 2, wherein when the grid-connected point voltage is greater than the minimum threshold and less than the maximum droop threshold, the dynamic reactive current value and the active current value to be output are calculated according to the grid-connected point voltage, the rated voltage and the rated current, and are respectively calculated by the following formulas:
Figure FDA0002015408580000011
and m is less than or equal to 1.1
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.
4. The method as claimed in claim 2, wherein when the grid-connected point voltage is greater than the maximum droop threshold and less than the maximum threshold, the grid-connected point voltage is based on a rated value
And calculating the dynamic reactive current value and the active current value which need to be output by constant current, and respectively calculating by using the following formulas: :
Figure FDA0002015408580000021
and m is less than or equal to 1.1
Wherein iqtDynamic reactive current i to be output by a photovoltaic inverter systemdtActive current, U, to be output by the photovoltaic inverter systemATo set the lowest threshold, UBTo set the highest threshold, UCM is a preset maximum drop threshold value, and m is a set current coefficient.
5. The method for controlling the VSG low voltage ride through based on the grid balancing fault according to claim 1,
the set minimum threshold value is 0.2UN；
The set maximum threshold value is 0.9UN；
Wherein, the UNIs a rated voltage.
6. The method for controlling the VSG low voltage ride-through under the grid balance fault according to claim 1, wherein the preset maximum drop threshold is calculated according to the following formula:
UC＝UN-ΔUmax
wherein, UCIs a preset maximum drop threshold, Δ UmaxWhen the active current output of the inverter is totally ratedMaximum allowable drop depth, U, of grid-connected point voltage at constant currentNIs a rated voltage.
7. The method as claimed in claim 6, wherein the maximum depth AU of the grid-connected point voltage sag is larger than the maximum depth AU of the grid-connected point voltage sagmaxCalculated as follows:
Figure FDA0002015408580000022
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.
8. The method according to claim 7, wherein the preset parameter K is calculated according to the following formula:
Figure FDA0002015408580000023
wherein iqReactive current, i, output for the inverter systemdFor outputting active current, U, of the inverter systemATo set the lowest threshold, UBIs the set highest threshold.
9. The method according to claim 8, wherein the maximum value K of the preset parameter K is set as KmaxCalculated as follows:
Figure FDA0002015408580000031
10. a VSG low voltage ride through control system based on grid balance fault is characterized by comprising:
the voltage and current acquisition module is used for acquiring the voltage, the active current and the reactive current of a grid-connected point in real time and recording data;
the state control module is used for calculating and determining dynamic reactive current and corresponding active current which are required to be output by the photovoltaic inversion system and used for supporting the voltage of the grid-connected point according to the voltage of the grid-connected point, the lowest threshold value, the highest threshold value and the maximum drop threshold value;
and the adjusting module is used for adjusting the photovoltaic inversion system based on the dynamic reactive current value and the active current value.
11. The system of claim 10, wherein the voltage acquisition module comprises: the device comprises a voltage acquisition unit and a current acquisition unit;
the voltage acquisition unit is used for acquiring the voltage of the grid-connected point and recording data;
and the current acquisition unit is used for acquiring active current and reactive current and recording data.
12. The system of claim 10, wherein the state control module comprises: the device comprises a judging unit, a selecting unit, a reference current calculating unit under a steady state, a reference current calculating unit under a fault and an output unit;
the judging unit is used for judging the size relationship between the grid-connected point voltage and a minimum threshold, a maximum threshold and a maximum drop threshold and determining the running state of the power grid;
the selection unit is used for selecting the control mode of the inverter system according to the output result of the judgment unit and switching to the corresponding calculation unit; when the power grid normally operates, switching to a reference current calculation unit under a steady state, and when the power grid has voltage drop, switching to a reference current calculation unit under a fault;
the reference current calculating unit under the steady state is used for calculating active current and reactive current under the non-voltage dropping state;
the under-fault reference current calculating unit is used for calculating active current and reactive current which are respectively output when the voltage of the grid-connected point is between a lowest threshold value and a maximum drop threshold value and when the voltage of the grid-connected point is between the maximum drop threshold value and a highest threshold value according to real-time voltage, rated voltage and rated current;
and the output unit is used for outputting the calculated active current and reactive current data.
CN201910261504.1A 2019-04-02 2019-04-02 VSG low-voltage ride-through control method and system based on power grid balance fault Pending CN111769589A (en)

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