CN109728597B - A method and system for fitting low-voltage ride-through characteristics of photovoltaic inverters - Google Patents

Abstract

The invention provides a photovoltaic inverter low-voltage ride through characteristic fitting method and system, which comprises the steps of obtaining electrical quantity according to a set low-voltage fault ride through test, and checking the consistency of the electrical quantity obtained by the same test and different tests; selecting key electrical quantity according to the consistency of the electrical quantity to perform low voltage ride through feature fitting; the electrical quantity includes: voltage steady-state values before and during a low-voltage fault steady-state interval, active current steady-state values and reactive current steady-state values before and during a low-voltage fault steady-state interval, and a current recovery rate during a low-voltage fault recovery period. The technical scheme provided by the invention is simple to operate, easy to realize, good in universality, capable of accurately fitting the low-voltage ride through external characteristics of the inverter and convenient to popularize.

Description

A method and system for fitting low voltage ride-through characteristics of photovoltaic inverters

technical field

The invention belongs to the field of power systems, and in particular relates to a method and system for fitting a low voltage ride-through characteristic of a photovoltaic inverter.

Background technique

The scale of photovoltaic power generation is developing rapidly. With the increasing capacity of photovoltaic power plants in the power grid, it is of great significance to master the low voltage ride-through characteristics of inverters for analyzing the impact of photovoltaic power plants on the power grid.

Photovoltaic inverter can realize independent control of active and reactive power, and has fast dynamic response speed. It is the core system of photovoltaic power generation system. The inverter is a typical power electronic system. Unlike traditional generators, the grid-connected characteristics of the inverter are mainly determined not by the physical structure of the inverter itself, but by its controller and parameters. The analysis of inverter low voltage ride through characteristics is a typical "gray box" problem. In practical projects, inverters of various brands usually use similar circuit topologies and control structures, but the specific control strategies and parameters are different, and they are not disclosed to the public. If the mechanism analysis method lacks control strategies and parameters, and only the test method is used, the necessary theoretical analysis is lacking. In the case of different strategies and limited known conditions, it is necessary to develop a low-voltage ride-through characteristic fitting method with good versatility and easy promotion.

SUMMARY OF THE INVENTION

The present invention proposes a method and system for fitting the low voltage ride-through characteristics of photovoltaic inverters. need.

The present invention provides a method for fitting a low voltage ride-through characteristic of a photovoltaic inverter, comprising:

Obtain the electrical quantity according to the set low-voltage fault ride-through test, and verify the consistency of the electrical quantity obtained from different tests in the same test;

Select key electrical quantities according to the consistency of electrical quantities to perform low voltage ride-through feature fitting;

The electrical quantities include: the voltage steady state value before the low voltage fault and the low voltage fault steady state interval, the active current steady state value and the reactive current steady state value before the low voltage fault and the low voltage fault steady state interval, the low voltage Current recovery rate during fault recovery.

The selection of key electrical quantities according to the consistency of electrical quantities includes:

The absolute value of voltage steady-state value deviation obtained from different tests is less than 0.05pu, the absolute value of active current and reactive current steady-state value deviation is less than 0.1pu, and the absolute value of current recovery rate deviation during low-voltage fault recovery is less than 0.2pu/s The electrical quantity of the test is the key electrical quantity.

The low voltage ride through characteristic fitting includes: characteristic fitting during low voltage fault and characteristic fitting during low voltage recovery.

The feature fitting during the low voltage fault includes:

curve fitting the steady state value of the reactive current and the voltage in the steady state interval of the low voltage fault;

Curve fitting is performed on the steady state value of the active current in the steady state interval of the low voltage fault and the voltage.

Curve fitting the steady-state value of reactive current and voltage in the steady-state interval of the low-voltage fault as follows:

I _q =min(K _{q_LV} (U _LV -U _term )+K _{Iq0_flag} I _q0 +I _{q0_LV} ,I _{qmax_LV} )

In the formula, I _q is the steady-state value of the reactive current in the steady-state interval of the low-voltage fault, K _{q_LV} is the reactive current support coefficient, U _LV is the voltage threshold for entering the low-voltage fault, U _term is the AC side terminal voltage, and K _{Iq0_flag} is the reactive current superposition flag coefficient, I _q0 is the steady state value of reactive current before the low voltage fault, I _{q0_LV} is the initial value of the reactive current of the low voltage fault, and I _{qmax_LV} is the maximum reactive power in the steady state interval of the low voltage fault current.

The formula for calculating the steady state value of reactive current I _q0 before the low voltage fault is as follows:

In the formula, i _q0 represents the reactive current value of the steady-state interval before the low-voltage fault, K _iq0Start represents the data serial number at the start of the steady-state interval before the low-voltage fault, and K _iq0End represents the data serial number at the end of the steady-state interval before the low-voltage fault. .

The formula for calculating the steady state value of reactive current I _q in the steady state interval of the low voltage fault is as follows:

where i _q represents the active current value of the low-voltage fault steady-state interval, K _iqStart represents the data serial number at the start of the low-voltage fault steady-state interval, and K _iqEnd represents the data serial number at the end of the low-voltage fault steady-state interval.

The steady-state value of the active current and the voltage in the steady-state interval of the low-voltage fault are curve-fitted as follows:

In the formula, I _p is the active current steady-state value in the low-voltage fault steady-state interval, P ₀ is the active power before entering the low-voltage fault, U _term is the AC side terminal voltage, and I _{max_FRT} is the maximum value of the low-voltage fault steady-state interval. Current, I _q is the reactive current in the steady-state interval of the low-voltage fault, I _p0 is the steady-state value of the active current before the low-voltage fault, K _{p1_FRT} and K _{p2_FRT are both} active current coefficients, and I _{p0_FRT} is the active current of the low-voltage fault The initial value, I _{p_flag} is the active current limit flag bit in the steady state interval of the low voltage fault.

The calculation formula of the active current steady-state value I _p0 before the low voltage fault is as follows:

In the formula, i _p0 represents the active current value of the steady-state interval before the low-voltage fault, K _ip0Start represents the data serial number at the start of the steady-state interval before the low-voltage fault, and K _ip0End represents the data serial number at the end of the steady-state interval before the low-voltage fault.

The calculation formula of the active current steady-state value I _p in the steady-state interval of the low-voltage fault is as follows:

where _ip represents the active current value of the low-voltage fault steady-state interval, K _ipStart represents the data serial number at the start of the low-voltage fault steady-state interval, and K _ipEnd represents the data serial number at the end of the low-voltage fault steady-state interval.

The feature fitting during the low voltage recovery period includes: calculating an average value of the current recovery rate during the voltage ride-through recovery period.

The formula for calculating the current recovery rate during the low voltage fault recovery period is as follows:

In the formula, dI _{p_LV} is the current recovery rate during low-voltage fault recovery, I _{p_LV2} is the active current at the end of low-voltage fault recovery, I _{p_LV1} is the active current at the start of low-voltage fault recovery, and t _{p_LV2} is the end of low-voltage fault recovery time, t _{p_LV1} is the start time of low voltage fault recovery.

Before setting the low voltage ride-through test, it also includes setting the initial operating condition of the inverter;

The operating conditions include high power, medium power and low power;

High power condition means that the output power of the inverter is 70% to 100% of the rated power;

The medium power condition means that the output power of the inverter is 40% to 60% of the rated power;

The low power condition means that the output power of the inverter is 10% to 30% of the rated power.

The set low voltage ride through test includes:

Make the AC side voltage of the inverter drop to the preset range of the rated voltage respectively;

The preset interval of the rated voltage includes: 0-20%, 20%-30%, 50%-60%, and 70%-90%.

The present invention provides a photovoltaic inverter low-voltage ride-through characteristic fitting system, comprising:

Consistency module: obtain the electrical quantity according to the set low-voltage fault ride-through test, and check the consistency of the electrical quantity obtained from different tests of the same test;

Fitting module: According to the consistency of electrical quantities, select key electrical quantities to perform low-voltage ride-through feature fitting;

The electrical quantities include: the voltage steady state value before the low voltage fault and the low voltage fault steady state interval, the active current steady state value and the reactive current steady state value before the low voltage fault and the low voltage fault steady state interval, the low voltage Current recovery rate during fault recovery.

The fitting module includes: a feature fitting sub-module during low voltage fault and a feature fitting sub-module during low voltage recovery.

The feature fitting sub-module during the low voltage fault includes: a first fitting unit for curve fitting the steady state value of reactive current and voltage in the steady state interval of the low voltage fault, and a first fitting unit for fitting the steady state value of the low voltage fault to the voltage. A second fitting unit that performs curve fitting between the steady-state value of the active current in the interval and the voltage.

The first fitting unit performs curve fitting on the steady state value of the reactive current and the voltage in the steady state interval of the low voltage fault as follows:

I _q =min(K _{q_LV} (U _LV -U _term )+K _{Iq0_flag} I _q0 +I _{q0_LV} ,I _{qmax_LV} )

In the formula, I _q is the steady-state value of the reactive current in the steady-state interval of the low-voltage fault, K _{q_LV} is the reactive current support coefficient, U _LV is the voltage threshold for entering the low-voltage fault, U _term is the AC side terminal voltage, and K _{Iq0_flag} is the reactive current superposition flag coefficient, I _q0 is the steady state value of reactive current before the low voltage fault, I _{q0_LV} is the initial value of the reactive current of the low voltage fault, and I _{qmax_LV} is the maximum reactive power in the steady state interval of the low voltage fault current.

The second fitting unit performs curve fitting on the steady-state value of the active current and the voltage in the steady-state interval of the low-voltage fault as follows:

In the formula, I _p is the active current steady-state value in the low-voltage fault steady-state interval, P ₀ is the active power before entering the low-voltage fault, U _term is the AC side terminal voltage, and I _{max_FRT} is the maximum value of the low-voltage fault steady-state interval. Current, I _q is the reactive current in the steady-state interval of the low-voltage fault, I _p0 is the steady-state value of the active current before the low-voltage fault, K _{p1_FRT} and K _{p2_FRT are both} active current coefficients, and I _{p0_FRT} is the active current of the low-voltage fault The initial value, I _{p_flag} is the active current limit flag bit in the steady state interval of the low voltage fault.

Compared with the closest prior art, the technical solution provided by the present invention has the following beneficial effects:

The technical solution provided by the present invention, by calculating the key electrical quantities of the inverter during the low voltage ride-through period, uses a formula to fit the low-voltage ride-through characteristics of inverters of different brands and models, and has the advantages of simple operation, easy implementation and good versatility , which can accurately fit the low-voltage ride-through external characteristics of the inverter, and is an engineering practical method that is easy to popularize.

Description of drawings

FIG. 1 is a flowchart of a method for fitting a low voltage ride-through characteristic of a photovoltaic inverter according to the present invention;

FIG. 2 is a schematic diagram of a low-penetration test system for an inverter in an embodiment of the present invention;

FIG. 3 is a basic flow chart of inverter low-voltage ride-through characteristic fitting in an embodiment of the present invention;

4 is a comparison diagram of a simulated value and an actual measured value under high-power operating conditions in an embodiment of the present invention;

5 is a comparison diagram of a simulated value and an actual measured value under low-power operating conditions in an embodiment of the present invention;

Detailed ways

The present invention will be further described below with reference to the accompanying drawings.

As shown in FIG. 1 , the present invention provides a method for fitting a low voltage ride-through characteristic of a photovoltaic inverter, including:

The present invention provides a method for fitting a low voltage ride-through characteristic of a photovoltaic inverter, comprising:

Obtain the electrical quantity according to the set low-voltage fault ride-through test, and verify the consistency of the electrical quantity obtained from different tests in the same test;

Select key electrical quantities according to the consistency of electrical quantities to perform low voltage ride-through feature fitting;

The electrical quantities include: the voltage steady state value before the low voltage fault and the low voltage fault steady state interval, the active current steady state value and the reactive current steady state value before the low voltage fault and the low voltage fault steady state interval, the low voltage Current recovery rate during fault recovery.

The selection of key electrical quantities according to the consistency of electrical quantities includes:

The absolute value of voltage steady-state value deviation obtained from different tests is less than 0.05pu, the absolute value of active current and reactive current steady-state value deviation is less than 0.1pu, and the absolute value of current recovery rate deviation during low-voltage fault recovery is less than 0.2pu/s The electrical quantity of the test is the key electrical quantity.

The low voltage ride through characteristic fitting includes: characteristic fitting during low voltage fault and characteristic fitting during low voltage recovery.

The feature fitting during the low voltage fault includes:

curve fitting the steady state value of the reactive current and the voltage in the steady state interval of the low voltage fault;

Curve fitting is performed on the steady state value of the active current in the steady state interval of the low voltage fault and the voltage.

Curve fitting the steady-state value of reactive current and voltage in the steady-state interval of the low-voltage fault as follows:

I _q =min(K _{q_LV} (U _LV -U _term )+K _{Iq0_flag} I _q0 +I _{q0_LV} ,I _{qmax_LV} )

In the formula, I _q is the steady-state value of the reactive current in the steady-state interval of the low-voltage fault, K _{q_LV} is the reactive current support coefficient, U _LV is the voltage threshold for entering the low-voltage fault, U _term is the AC side terminal voltage, and K _{Iq0_flag} is the reactive current superposition flag coefficient, I _q0 is the steady state value of reactive current before the low voltage fault, I _{q0_LV} is the initial value of the reactive current of the low voltage fault, and I _{qmax_LV} is the maximum reactive power in the steady state interval of the low voltage fault current.

The formula for calculating the steady state value of reactive current I _q0 before the low voltage fault is as follows:

In the formula, i _q0 represents the reactive current value of the steady-state interval before the low-voltage fault, K _iq0Start represents the data serial number at the start of the steady-state interval before the low-voltage fault, and K _iq0End represents the data serial number at the end of the steady-state interval before the low-voltage fault. .

The formula for calculating the steady state value of reactive current I _q in the steady state interval of the low voltage fault is as follows:

where i _q represents the active current value of the low-voltage fault steady-state interval, K _iqStart represents the data serial number at the start of the low-voltage fault steady-state interval, and K _iqEnd represents the data serial number at the end of the low-voltage fault steady-state interval.

The steady-state value of the active current and the voltage in the steady-state interval of the low-voltage fault are curve-fitted as follows:

In the formula, I _p is the active current steady-state value in the low-voltage fault steady-state interval, P ₀ is the active power before entering the low-voltage fault, U _term is the AC side terminal voltage, and I _{max_FRT} is the maximum value of the low-voltage fault steady-state interval. Current, I _q is the reactive current in the steady-state interval of the low-voltage fault, I _p0 is the steady-state value of the active current before the low-voltage fault, K _{p1_FRT} and K _{p2_FRT are both} active current coefficients, and I _{p0_FRT} is the active current of the low-voltage fault The initial value, I _{p_flag} is the active current limit flag bit in the steady state interval of the low voltage fault.

The calculation formula of the active current steady-state value I _p0 before the low voltage fault is as follows:

In the formula, i _p0 represents the active current value of the steady-state interval before the low-voltage fault, K _ip0Start represents the data serial number at the start of the steady-state interval before the low-voltage fault, and K _ip0End represents the data serial number at the end of the steady-state interval before the low-voltage fault.

The calculation formula of the active current steady-state value I _p in the steady-state interval of the low-voltage fault is as follows:

where _ip represents the active current value of the low-voltage fault steady-state interval, K _ipStart represents the data serial number at the start of the low-voltage fault steady-state interval, and K _ipEnd represents the data serial number at the end of the low-voltage fault steady-state interval.

The feature fitting during the low voltage recovery period includes: calculating an average value of the current recovery rate during the voltage ride-through recovery period.

The formula for calculating the current recovery rate during the low voltage fault recovery period is as follows:

In the formula, dI _{p_LV} is the current recovery rate during low-voltage fault recovery, I _{p_LV2} is the active current at the end of low-voltage fault recovery, I _{p_LV1} is the active current at the start of low-voltage fault recovery, and t _{p_LV2} is the end of low-voltage fault recovery time, t _{p_LV1} is the start time of low voltage fault recovery.

Before setting the low voltage ride-through test, it also includes setting the initial operating condition of the inverter;

The operating conditions include high power, medium power and low power;

High power condition means that the output power of the inverter is 70% to 100% of the rated power;

The medium power condition means that the output power of the inverter is 40% to 60% of the rated power;

The low power condition means that the output power of the inverter is 10% to 30% of the rated power.

The set low voltage ride through test includes:

Make the AC side voltage of the inverter drop to the preset range of the rated voltage respectively;

The preset interval of the rated voltage includes: 0-20%, 20%-30%, 50%-60%, and 70%-90%.

The closer the electrical quantities obtained from different tests in the same test, the better the consistency of the electrical quantities of the tests.

The present invention provides a photovoltaic inverter low-voltage ride-through characteristic fitting system, which may include:

Consistency module: obtain the electrical quantity according to the set low-voltage fault ride-through test, and check the consistency of the electrical quantity obtained from different tests of the same test;

Fitting module: According to the consistency of electrical quantities, select key electrical quantities to perform low-voltage ride-through feature fitting;

The electrical quantities include: the voltage steady state value before the low voltage fault and the low voltage fault steady state interval, the active current steady state value and the reactive current steady state value before the low voltage fault and the low voltage fault steady state interval, the low voltage Current recovery rate during fault recovery.

The fitting module includes: a feature fitting sub-module during low voltage fault and a feature fitting sub-module during low voltage recovery.

The feature fitting sub-module during the low voltage fault includes: a first fitting unit for curve fitting the steady state value of reactive current and voltage in the steady state interval of the low voltage fault, and a first fitting unit for fitting the steady state value of the low voltage fault to the voltage. A second fitting unit that performs curve fitting between the steady-state value of the active current in the interval and the voltage.

The first fitting unit performs curve fitting on the steady state value of the reactive current and the voltage in the steady state interval of the low voltage fault as follows:

I _q =min(K _{q_LV} (U _LV -U _term )+K _{Iq0_flag} I _q0 +I _{q0_LV} ,I _{qmax_LV} )

In the formula, I _q is the steady-state value of the reactive current in the steady-state interval of the low-voltage fault, K _{q_LV} is the reactive current support coefficient, U _LV is the voltage threshold for entering the low-voltage fault, U _term is the AC side terminal voltage, and K _{Iq0_flag} is the reactive current superposition flag coefficient, I _q0 is the steady state value of reactive current before the low voltage fault, I _{q0_LV} is the initial value of the reactive current of the low voltage fault, and I _{qmax_LV} is the maximum reactive power in the steady state interval of the low voltage fault current.

The second fitting unit performs curve fitting on the steady-state value of the active current and the voltage in the steady-state interval of the low-voltage fault as follows:

In the formula, I _p is the active current steady-state value in the low-voltage fault steady-state interval, P ₀ is the active power before entering the low-voltage fault, U _term is the AC side terminal voltage, and I _{max_FRT} is the maximum value of the low-voltage fault steady-state interval. Current, I _q is the reactive current in the steady-state interval of the low-voltage fault, I _p0 is the steady-state value of the active current before the low-voltage fault, K _{p1_FRT} and K _{p2_FRT are both} active current coefficients, and I _{p0_FRT} is the active current of the low-voltage fault The initial value, I _{p_flag} is the active current limit flag bit in the steady state interval of the low voltage fault.

The low-voltage recovery period feature fitting submodule performing feature fitting during the low-voltage recovery period includes: calculating an average value of the current recovery rate during the voltage ride-through recovery period. The consistency module includes a test sub-module for setting a low voltage ride-through test;

The low voltage ride-through test set by the test sub-module includes:

Make the AC side voltage of the inverter drop to the preset range of the rated voltage respectively;

The preset interval of the rated voltage includes: 0-20%, 20%-30%, 50%-60%, and 70%-90%.

The photovoltaic inverter low-voltage ride-through characteristic fitting system further includes a setting module for setting the inverter initial operating condition before setting the low-voltage ride-through test;

The operating conditions include high power, medium power and low power;

High power condition means that the output power of the inverter is 70% to 100% of the rated power;

The medium power condition means that the output power of the inverter is 40% to 60% of the rated power;

The low power condition means that the output power of the inverter is 10% to 30% of the rated power.

Example

Before fitting the low-voltage ride-through characteristics of the photovoltaic inverter, first build the inverter low-pass-through test system as shown in Figure 2, which mainly includes photovoltaic strings (square array)/simulated DC sources, inverters and power grids Disturbance generation system.

As shown in Figure 3, the basic process of fitting the inverter's low-voltage ride-through characteristics:

Step 1. By adjusting the maximum output of the controllable DC power supply, set the initial operating conditions of the inverter, and make the inverter operate in three operating conditions of high power, medium power and low power, wherein the high power operating conditions are: It means that the output power of the inverter is 70% to 100% of the rated power. The medium power condition means that the output power of the inverter is 40% to 60% of the rated power. The low power condition means that the output power of the inverter is the rated power. 10% to 30% of the power.

Step 2. Set up a low-voltage fault ride-through test to make the AC side voltage of the inverter drop to 0~20%, 20%~30%, 50%~60% and 70%~90% of the rated voltage respectively. The same test Repeat at least twice;

Step 3: Calculate the steady state values of active current and reactive current before the low voltage fault and during the steady state of the fault, calculate the current recovery rate during the fault recovery period, and verify the consistency of different test data in the same group of tests;

Step 4. Distinguish the working conditions and consider overcurrent protection as the boundary condition. The feature fitting during the low-voltage fault includes: curve fitting the relationship between the steady-state value of active/reactive current and the voltage in the steady-state interval of the low-voltage fault. combine.

The fitting formula of reactive current is

I _q =min(K _{q_LV} (U _LV -U _term )+K _{Iq0_flag} I _q0 +I _{q0_LV} ,I _{qmax_LV} ) (1)

where I _q is the reactive current during low voltage ride-through, K _{q_LV} is the reactive current support coefficient, U _LV is the voltage threshold for entering low voltage ride-through control, U _term is the AC side terminal voltage, and K _{Iq0_flag} is the reactive current superposition Flag coefficient, I _q0 is the reactive current before entering the low voltage ride-through control, I _{q0_LV} is the initial value of the reactive current of the low voltage ride-through, and I _{qmax_LV} is the maximum reactive current during the low voltage ride-through control.

The fitting formula of active current is

where I _p is the active current during the low voltage ride-through period, P ₀ is the active power before entering the low voltage ride-through control, U _term is the AC side terminal voltage, I _{max_FRT} is the maximum current during the low voltage ride-through control period, and I _q is Reactive current during low voltage ride-through, K _{p1_FRT} is the active current coefficient 1, K _{p2_FRT} is the active current coefficient 2, I _P0 is the active current before entering the low voltage ride-through control, I _{p0_FRT} is the active current starting value of the low voltage ride-through , I _{p_flag} is the active current limit flag during low voltage ride-through.

The formula for calculating the steady state value of active current before the low voltage fault is:

In the formula, i _p0 represents the active current value of the steady-state interval before the low-voltage fault, K _ip0Start represents the data serial number at the start of the pre-fault steady-state interval, and K _ip0End represents the data serial number at the end of the pre-fault steady-state interval.

The formula for calculating the steady-state value of reactive current before the low voltage fault is:

In the formula, i _q0 represents the reactive current value in the steady-state interval before the low-voltage fault, K _iq0Start represents the data serial number at the start of the pre-fault steady-state interval, and K _iq0End represents the data serial number at the end of the pre-fault steady-state interval.

The formula for calculating the steady-state value of the active current in the steady-state interval of the low-voltage fault is:

where _{ip represents the active current value of the low-voltage fault steady state interval, K ipStart represents} the data serial number at the start of the fault steady state interval, and K _ipEnd represents the data serial number at the end of the fault steady state interval.

The formula for calculating the steady state value of reactive current in the steady state interval of low voltage fault is:

where i _q represents the active current value of the low voltage fault steady state interval, K _iqStart represents the data serial number at the start of the fault steady state interval, and K _iqEnd represents the data serial number at the end of the fault steady state interval.

The current recovery rate during low voltage fault recovery is calculated as

In the formula, I _{p_LV2} is the active current at the end of fault recovery, I _{p_LV1} is the active current at the start of fault recovery, t _{p_LV2} is the end of fault recovery, and t _{p_LV1} is the start of fault recovery.

The feature fitting during low-voltage fault recovery includes: calculating the current recovery rate and taking the average value of multiple tests.

In the present invention, in order to verify the correctness of the proposed fitting method, the low voltage ride through characteristic of a certain type of photovoltaic inverter is fitted. Carry out the low voltage ride through test according to steps 1, 2 and 3, calculate the steady state value of reactive current and active current during the steady state of the low voltage fault, and calculate the active power/ Curve fitting is performed on the relationship between reactive current and voltage, and in formula (1), K _{q_LV} takes 2, U _LV takes 0.9, K _{Iq0_flag} takes 0, I _q0 takes 0, I _{q0_LV} takes 0, and I _{qmax_LV} takes 1.08 ; In formula (2), I _{p_flag} takes 2, K _{p1_FRT} takes 0, K _{p2_FRT} takes 0, I _{p0_FRT} takes 0.16, and I _{max_FRT} takes 1.1;

Therefore, the fitting formula of the low voltage ride-through characteristic of the inverter is obtained as follows:

I _q =min(2×(0.9-U _term ),1.08)

_Ip = 0.16.

The average current recovery rate during low-voltage fault recovery was measured to be 1.25 pu/s.

The inverter fitting formula and current recovery rate parameters are substituted into the electromechanical transient simulation model for electromechanical transient simulation. The simulation curve and the measured curve have good consistency, as shown in Figure 4 and Figure 5. By comparing the simulation data with the actual test data, it can be found that the low-voltage ride-through characteristic fitting method proposed by the present invention can well fit the practicability of the inverter regardless of whether it is a high-power or low-power condition. The correctness and effectiveness of the proposed fitting method.

As will be appreciated by those skilled in the art, the embodiments of the present application may be provided as a method, a system, or a 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, etc.) 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 present application. It will be understood that each flow and/or block in the flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to the processor of a general purpose computer, special purpose computer, embedded processor or other programmable data processing device to produce a machine such that the instructions executed by the processor of the computer or other programmable data processing device produce A system for implementing the functions specified in a flow or flow of a flowchart and/or a block or blocks of a block diagram.

These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable memory result in an article of manufacture comprising a system of instructions, the instructions The system implements the functions specified in the flow or flow of the flowchart and/or the block or blocks of the block diagram.

These computer program instructions can also be loaded on a computer or other programmable data processing device to cause a series of operational steps to be performed on the computer or other programmable device to produce a computer-implemented process such that The instructions provide steps for implementing the functions specified in the flow or blocks of the flowcharts and/or the block or blocks of the block diagrams.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit its protection scope. Although the application has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: After reading this application, those skilled in the art can still make various changes, modifications or equivalent replacements to the specific embodiments of the application, but these changes, modifications or equivalent replacements are all within the protection scope of the pending claims.

Claims (7)

1. A method for fitting a low-voltage ride-through characteristic of a photovoltaic inverter, comprising: Obtain the electrical quantity according to the set low-voltage fault ride-through test, and verify the consistency of the electrical quantity obtained from different tests in the same test; Select key electrical quantities according to the consistency of electrical quantities to perform low voltage ride-through feature fitting; The electrical quantities include: the voltage steady state value before the low voltage fault and the low voltage fault steady state interval, the active current steady state value and the reactive current steady state value before the low voltage fault and the low voltage fault steady state interval, the low voltage Current recovery rate during fault recovery; The low voltage ride-through feature fitting includes: feature fitting during low voltage faults and feature fitting during low voltage recovery; The feature fitting during the low voltage fault includes: curve fitting the steady state value of the reactive current and the voltage in the steady state interval of the low voltage fault; curve fitting the active current steady state value and the voltage in the steady state interval of the low voltage fault; Curve fitting the steady-state value of reactive current and voltage in the steady-state interval of the low-voltage fault as follows: I _q =min(K _{q_LV} (U _LV -U _term )+K _{Iq0_flag} I _q0 +I _{q0_LV} ,I _{qmax_LV} ) In the formula, I _q is the steady-state value of the reactive current in the steady-state interval of the low-voltage fault, K _{q_LV} is the reactive current support coefficient, U _LV is the voltage threshold for entering the low-voltage fault, U _term is the AC side terminal voltage, and K _{Iq0_flag} is the reactive current superposition flag coefficient, I _q0 is the steady state value of reactive current before the low voltage fault, I _{q0_LV} is the initial value of the reactive current of the low voltage fault, and I _{qmax_LV} is the maximum reactive power in the steady state interval of the low voltage fault current; The formula for calculating the steady state value of reactive current I _q0 before the low voltage fault is as follows:

In the formula, i _q0 represents the reactive current value of the steady-state interval before the low-voltage fault, K _iq0Start represents the data serial number at the start of the steady-state interval before the low-voltage fault, and K _iq0End represents the data serial number at the end of the steady-state interval before the low-voltage fault. ; The formula for calculating the steady state value of reactive current I _q in the steady state interval of the low voltage fault is as follows:

where i _q represents the active current value of the low-voltage fault steady-state interval, K _iqStart represents the data serial number at the start of the low-voltage fault steady-state interval, and K _iqEnd represents the data serial number at the end of the low-voltage fault steady-state interval; The steady-state value of the active current and the voltage in the steady-state interval of the low-voltage fault are curve-fitted as follows:

In the formula, I _p is the active current steady-state value in the low-voltage fault steady-state interval, P ₀ is the active power before entering the low-voltage fault, U _term is the AC side terminal voltage, and I _{max_FRT} is the maximum value of the low-voltage fault steady-state interval. Current, I _q is the reactive current in the steady-state interval of the low-voltage fault, I _p0 is the steady-state value of the active current before the low-voltage fault, K _{p1_FRT} and K _{p2_FRT are both} active current coefficients, and I _{p0_FRT} is the active current of the low-voltage fault The initial value, I _{p_flag} is the active current limit flag in the steady-state interval of the low-voltage fault; The calculation formula of the active current steady-state value I _p0 before the low voltage fault is as follows:

In the formula, i _p0 represents the active current value of the steady-state interval before the low-voltage fault, K _ip0Start represents the data serial number at the start of the steady-state interval before the low-voltage fault, and K _ip0End represents the data serial number at the end of the steady-state interval before the low-voltage fault; The calculation formula of the active current steady-state value I _p in the steady-state interval of the low-voltage fault is as follows:

where _ip represents the active current value of the low-voltage fault steady-state interval, K _ipStart represents the data serial number at the start of the low-voltage fault steady-state interval, and K _ipEnd represents the data serial number at the end of the low-voltage fault steady-state interval. 2 . The method for fitting a low voltage ride-through characteristic of a photovoltaic inverter according to claim 1 , wherein the selecting the key electrical quantities according to the consistency of the electrical quantities comprises: 2 . The absolute value of voltage steady-state value deviation obtained from different tests is less than 0.05pu, the absolute value of active current and reactive current steady-state value deviation is less than 0.1pu, and the absolute value of current recovery rate deviation during low-voltage fault recovery is less than 0.2pu/s The electrical quantity of the test is the key electrical quantity. 3 . The method for fitting a low voltage ride through characteristic of a photovoltaic inverter according to claim 1 , wherein the characteristic fitting during the low voltage recovery period comprises: calculating an average value of the current recovery rate during the voltage ride through recovery period. 4 . 4. The method for fitting a low voltage ride-through characteristic of a photovoltaic inverter according to any one of claims 1 or 3, wherein the calculation formula of the current recovery rate during the low voltage fault recovery period is as follows:

In the formula, dI _{p_LV} is the current recovery rate during low-voltage fault recovery, I _{p_LV2} is the active current at the end of low-voltage fault recovery, I _{p_LV1} is the active current at the start of low-voltage fault recovery, and t _{p_LV2} is the end of low-voltage fault recovery time, t _{p_LV1} is the start time of low voltage fault recovery. 5. The method for fitting a low voltage ride-through characteristic of a photovoltaic inverter according to claim 1, wherein before setting the low voltage ride-through test, it further comprises setting an initial operating condition of the inverter; The operating conditions include high power, medium power and low power; High power condition means that the output power of the inverter is 70% to 100% of the rated power; The medium power condition means that the output power of the inverter is 40% to 60% of the rated power; The low power condition means that the output power of the inverter is 10% to 30% of the rated power. 6. The method for fitting a low voltage ride-through characteristic of a photovoltaic inverter according to claim 5, wherein the set low voltage ride-through test comprises: Make the AC side voltage of the inverter drop to the preset range of the rated voltage respectively; The preset interval of the rated voltage includes: 0-20%, 20%-30%, 50%-60%, and 70%-90%. 7. A photovoltaic inverter low-voltage ride-through characteristic fitting system, comprising: Consistency module: obtain the electrical quantity according to the set low-voltage fault ride-through test, and verify the consistency of the electrical quantity obtained from different tests of the same test; Fitting module: According to the consistency of electrical quantities, select key electrical quantities to perform low-voltage ride-through feature fitting; The electrical quantities include: the voltage steady state value before the low voltage fault and the low voltage fault steady state interval, the active current steady state value and the reactive current steady state value before the low voltage fault and the low voltage fault steady state interval, the low voltage Current recovery rate during fault recovery; The fitting module includes: a feature fitting sub-module during low voltage fault and a feature fitting sub-module during low voltage recovery; The feature fitting submodule during the low voltage fault includes: a first fitting unit for curve fitting the steady state value of reactive current and voltage in the steady state interval of the low voltage fault, and a first fitting unit for fitting the steady state value of the low voltage fault to the voltage. a second fitting unit for curve fitting between the steady-state value of active current in the interval and the voltage; The first fitting unit performs curve fitting on the steady state value of the reactive current and the voltage in the steady state interval of the low voltage fault as follows: I _q =min(K _{q_LV} (U _LV -U _term )+K _{Iq0_flag} I _q0 +I _{q0_LV} ,I _{qmax_LV} ) In the formula, I _q is the steady-state value of the reactive current in the steady-state interval of the low-voltage fault, K _{q_LV} is the reactive current support coefficient, U _LV is the voltage threshold for entering the low-voltage fault, U _term is the AC side terminal voltage, and K _{Iq0_flag} is the reactive current superposition flag coefficient, I _q0 is the steady state value of reactive current before the low voltage fault, I _{q0_LV} is the initial value of the reactive current of the low voltage fault, and I _{qmax_LV} is the maximum reactive power in the steady state interval of the low voltage fault current; The second fitting unit performs curve fitting on the steady-state value of the active current and the voltage in the steady-state interval of the low-voltage fault as follows:

In the formula, I _p is the active current steady-state value in the low-voltage fault steady-state interval, P ₀ is the active power before entering the low-voltage fault, U _term is the AC side terminal voltage, and I _{max_FRT} is the maximum value of the low-voltage fault steady-state interval. Current, I _q is the reactive current in the steady-state interval of the low-voltage fault, I _p0 is the steady-state value of the active current before the low-voltage fault, K _{p1_FRT} and K _{p2_FRT are both} active current coefficients, and I _{p0_FRT} is the active current of the low-voltage fault The initial value, I _{p_flag} is the active current limit flag bit in the steady state interval of the low voltage fault.

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