Voltage support method suitable for considering voltage drop degree under asymmetric fault of power grid

Abstract

The invention provides a voltage support method suitable for considering a voltage drop degree under an asymmetric fault of a power grid. The method comprises the following steps: S1, determining a topological structure of a new energy grid-connected system; s2, constructing a relation model of the grid-connected point voltage amplitude of the new energy grid-connected system and the injected power grid current; s3, constructing a voltage maximum amplitude model and a voltage minimum amplitude model of a grid-connected point of the new energy grid-connected system; substituting a maximum value and a minimum value of grid-connected voltage allowable fluctuation of the current new energy grid-connected system into a voltage maximum amplitude model and a voltage minimum amplitude model of the grid-connected point to determine a positive sequence voltage amplitude and a negative sequence voltage amplitude of the grid-connected point; s4, substituting the positive sequence voltage amplitude and the negative sequence voltage amplitude of the grid-connected point into the grid-connected point voltage amplitude and power grid current relation model to determine a positive sequence reactive current amplitude and a negative sequence reactive current amplitude required by the power grid; and S5, determining a maximum current peak value for carrying out voltage support on the power grid according to the positive sequence reactive current amplitude and the positive sequence active current amplitude, judging whether the current maximum current peak value is smaller than a set threshold value or not, and if so, controlling an inverter of the new energy grid-connected system to output reactive current according to the positive sequence reactive current amplitude determined in the step S4.

Description

Technical Field

[0001] The present invention relates to a power grid voltage support method, and in particular to a voltage support method that takes into account the degree of voltage drop under asymmetric power grid faults. Background Art

[0002] The rapid development of renewable energy sources such as photovoltaics and wind power (also known as new energy systems) has led to the widespread application of photovoltaics and wind power. The efficient utilization and large-scale grid connection of new energy sources such as photovoltaics and wind power have gradually become an important part of the power system.

[0003] However, renewable energy sources such as photovoltaics and wind power exhibit significant volatility and randomness. This can lead to frequent load fluctuations and instability in the power system when large-scale renewable energy is integrated into the grid, impacting the safe operation of the grid. Furthermore, when asymmetric faults occur in the grid, voltage fluctuations and current imbalances can further complicate the integration of renewable energy and even lead to power system instability. Therefore, inverters, as key devices connecting renewable energy generation equipment to the grid, play a crucial role. Grid-connection standards require inverters to have low-voltage ride-through (LVRT) capability, particularly in the event of asymmetric faults, to maintain stable operation and avoid large-scale power outages or power system collapse. Ensuring good LVRT capability and supporting grid voltage requires accurate control of the inverter's reactive current output. The positive-sequence reactive current primarily contributes to grid voltage support, but accurately determining the inverter's positive-sequence reactive current remains a technical challenge.

[0004] Therefore, in order to solve the above technical problems, it is urgent to propose a new technical means. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a voltage support method that takes into account the degree of voltage drop under asymmetric faults in the power grid, and determines the positive-sequence reactive current amplitude output by the inverter of the new energy grid-connected system according to the voltage amplitude of the grid-connected point of the new energy grid-connected system and the grid voltage, thereby effectively ensuring the low-voltage ride-through capability of the inverter, providing voltage support for the power grid and improving the voltage support efficiency, thereby effectively ensuring the stable and safe operation of the entire power grid system.

[0006] The present invention provides a voltage support method that takes into account the degree of voltage drop under asymmetric fault conditions in a power grid, comprising the following steps:

[0007] S1. Determine the topology of the new energy grid-connected system;

[0008] S2. Construct a model for the relationship between the voltage amplitude at the grid connection point and the current injected into the grid for a new energy grid-connected system;

[0009] S3. Construct a maximum voltage amplitude model and a minimum voltage amplitude model for the grid-connected new energy system. Substitute the maximum and minimum values ​​of the grid-connected voltage fluctuation allowed by the current new energy system into the maximum voltage amplitude model and the minimum voltage amplitude model to determine the positive-sequence voltage amplitude and the negative-sequence voltage amplitude at the grid-connected point.

[0010] S4. Substituting the positive-sequence voltage amplitude and negative-sequence voltage amplitude of the grid-connected point into the relationship model between the grid voltage amplitude and the grid current to determine the positive-sequence reactive current amplitude and negative-sequence reactive current amplitude required by the grid;

[0011] S5. Determine the maximum current peak for supporting the voltage of the power grid based on the positive-sequence reactive current amplitude and the positive-sequence active current amplitude, and judge whether the current maximum current peak is less than the set threshold. If so, control the inverter of the new energy grid-connected system to output reactive current according to the positive-sequence reactive current amplitude determined in step S4.

[0012] Furthermore, the relationship model between the voltage amplitude at the grid connection point and the current injected into the grid for the new energy grid-connected system is constructed as follows:

[0013]

[0014] Among them, U + Indicates the positive sequence voltage amplitude of the grid connection point of the new energy grid-connected system, U - Indicates the negative sequence voltage amplitude of the grid connection point of the new energy grid connection system, Indicates the positive sequence amplitude of the grid voltage, Indicates the negative sequence amplitude of the grid voltage, and They represent the positive sequence reactive current amplitude and negative sequence reactive current amplitude injected into the grid, ω represents the inverter output angular frequency of the new energy grid-connected system, L g is the equivalent inductance of the grid converted to the low-voltage side.

[0015] Furthermore, the maximum voltage amplitude model and the minimum voltage amplitude model of the grid connection point of the new energy grid connection system are constructed, specifically including:

[0016]

[0017] Among them: U max Indicates the maximum voltage amplitude at the grid connection point of the new energy grid-connected system, U min Indicates the minimum voltage amplitude at the grid connection point of the new energy grid-connected system, θ + and θ - They represent the initial phases of the positive-sequence voltage and negative-sequence voltage at the grid connection point respectively.

[0018] Furthermore, determining the maximum current peak value for supporting the grid voltage based on the positive-sequence reactive current amplitude and the positive-sequence active current amplitude specifically includes:

[0019]

[0020] Where: I peakmax Indicates the maximum current peak, I + and I - Represent the positive sequence current amplitude and negative sequence current amplitude respectively, and They represent the amplitude of the positive-sequence active current of the power grid and the amplitude of the negative-sequence active current of the power grid respectively.

[0021] The beneficial effects of the present invention are as follows: Through the present invention, the positive-sequence reactive current amplitude output by the inverter of the new energy grid-connected system is determined according to the voltage amplitude of the grid-connected point of the new energy grid-connected system and the grid voltage, thereby effectively ensuring the low-voltage ride-through capability of the inverter, providing voltage support for the grid and improving the voltage support efficiency, thereby effectively ensuring the stable and safe operation of the entire grid system. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0023] Figure 1 Flowchart of the present invention.

[0024] Figure 2 This is a topological diagram of the new energy grid-connected system of the present invention.

[0025] Figure 3 These are the function images of cos(δ), cos(δ-120°), and cos(δ+120°) in the present invention. DETAILED DESCRIPTION

[0026] The present invention is further described in detail below:

[0027] The present invention provides a voltage support method that takes into account the degree of voltage drop under asymmetric fault conditions in a power grid, comprising the following steps:

[0028] S1. Determine the topology of the new energy grid-connected system; Figure 2 As shown, Figure 2 In the figure, DC / AC refers to the inverter of the new energy grid-connected system, and PCC point refers to the grid-connected point of the new energy grid-connected system;

[0029] S2. Construct a model for the relationship between the voltage amplitude at the grid connection point and the current injected into the grid for a new energy grid-connected system;

[0030] S3. Construct a maximum voltage amplitude model and a minimum voltage amplitude model for the grid-connected new energy system. Substitute the maximum and minimum values ​​of the grid-connected voltage fluctuation allowed by the current new energy system into the maximum voltage amplitude model and the minimum voltage amplitude model to determine the positive-sequence voltage amplitude and the negative-sequence voltage amplitude at the grid-connected point.

[0031] S4. Substituting the positive-sequence voltage amplitude and negative-sequence voltage amplitude of the grid-connected point into the relationship model between the grid voltage amplitude and the grid current to determine the positive-sequence reactive current amplitude and negative-sequence reactive current amplitude required by the grid;

[0032] S5. Determine the maximum current peak value for grid voltage support based on the positive-sequence reactive current amplitude and the positive-sequence active current amplitude, and determine whether the current maximum current peak value is less than a set threshold. If so, control the inverter of the new energy grid-connected system to output reactive current according to the positive-sequence reactive current amplitude determined in step S4. Through the above method, the positive-sequence reactive current amplitude output by the inverter of the new energy grid-connected system is determined based on the voltage amplitude at the grid connection point of the new energy grid-connected system and the grid voltage, thereby effectively ensuring the low voltage ride-through capability of the inverter, providing voltage support for the grid and improving voltage support efficiency, thereby effectively ensuring the stable and safe operation of the entire grid system.

[0033] In this embodiment, the relationship model between the voltage amplitude at the grid connection point and the current injected into the grid for the new energy grid-connected system is constructed as follows:

[0034]

[0035] Among them, U + Indicates the positive sequence voltage amplitude of the grid connection point of the new energy grid-connected system, U - Indicates the negative sequence voltage amplitude of the grid connection point of the new energy grid connection system, Indicates the positive sequence amplitude of the grid voltage, Indicates the negative sequence amplitude of the grid voltage, and They represent the positive sequence reactive current amplitude and negative sequence reactive current amplitude injected into the grid, ω represents the inverter output angular frequency of the new energy grid-connected system, L g is the equivalent inductance of the grid converted to the low-voltage side.

[0036] The maximum and minimum voltage amplitude models for the grid connection points of the new energy grid-connected system are constructed as follows:

[0037]

[0038] Among them: U max Indicates the maximum voltage amplitude at the grid connection point of the new energy grid-connected system, U minIndicates the minimum voltage amplitude at the grid connection point of the new energy grid-connected system, θ + and θ - They represent the initial phases of the positive sequence voltage and negative sequence voltage at the grid connection point, respectively. For example, according to the current operating conditions, the maximum voltage amplitude and the minimum voltage amplitude of the current grid connection point can be determined. Substituting them into formula (2), we can get U + and U - , and then substitute this value into formula (1), we can get and Among them, the functions of cos(δ), cos(δ-120°), and cos(δ+120°) are as follows Figure 3 shown.

[0039] Specifically, determining the maximum current peak value for supporting the voltage of the power grid based on the positive-sequence reactive current amplitude and the positive-sequence active current amplitude includes:

[0040]

[0041] Where: I peakmax Indicates the maximum current peak, I + and I - Represent the positive sequence current amplitude and negative sequence current amplitude respectively, and They represent the positive sequence active current amplitude and the negative sequence active current amplitude of the power grid respectively, which are determined by formula (1) and Substitute into formula (3) to determine I peakmax , and judge I peakmax Is it less than the set threshold? If so, the inverter is controlled to output Because during operation, the voltage support is provided by the positive sequence reactive current, among which, and The determination is made through existing methods, which will not be described in detail here.

[0042] In practice, the output currents of the inverter's α and β phases in the stationary coordinate system are determined by the following formula, and then the positive sequence reactive current output by the inverter is controlled, where:

[0043] Inverter current output model of new energy grid-connected system:

[0044]

[0045] Solving the formula and Substituting into formula (4), we get I α and I β , where I α and Iβ They represent the output currents of the inverter's α-phase and β-phase in the stationary coordinate system respectively.

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A voltage support method that takes into account the degree of voltage drop under asymmetric fault conditions in a power grid, characterized by: The following steps are involved: S1. Determine the topology of the new energy grid-connected system; S2. Construct a model for the relationship between the voltage amplitude at the grid connection point and the current injected into the grid for a new energy grid-connected system; S3. Construct a maximum voltage amplitude model and a minimum voltage amplitude model for the grid-connected new energy system. Substitute the maximum and minimum values ​​of the grid-connected voltage fluctuation allowed by the current new energy system into the maximum voltage amplitude model and the minimum voltage amplitude model to determine the positive-sequence voltage amplitude and the negative-sequence voltage amplitude at the grid-connected point. S4. Substituting the positive-sequence voltage amplitude and negative-sequence voltage amplitude of the grid-connected point into the relationship model between the grid voltage amplitude and the grid current to determine the positive-sequence reactive current amplitude and negative-sequence reactive current amplitude required by the grid; S5. Determine the maximum current peak for supporting the voltage of the power grid based on the positive-sequence reactive current amplitude and the positive-sequence active current amplitude, and judge whether the current maximum current peak is less than the set threshold. If so, control the inverter of the new energy grid-connected system to output reactive current according to the positive-sequence reactive current amplitude determined in step S4.

2. The voltage support method according to claim 1, which is suitable for considering the voltage drop degree under asymmetric fault conditions in a power grid, is characterized in that: The relationship model between the voltage amplitude at the grid connection point and the current injected into the grid for the new energy grid-connected system is as follows: Among them, U + Indicates the positive sequence voltage amplitude of the grid connection point of the new energy grid-connected system, U - Indicates the negative sequence voltage amplitude of the grid connection point of the new energy grid-connected system, Indicates the positive sequence amplitude of the grid voltage, Indicates the negative sequence amplitude of the grid voltage, and They represent the positive sequence reactive current amplitude and negative sequence reactive current amplitude injected into the grid, ω represents the inverter output angular frequency of the new energy grid-connected system, L g is the equivalent inductance of the grid converted to the low-voltage side.

3. The voltage support method according to claim 2, which is suitable for considering the voltage drop degree under asymmetric fault conditions in a power grid, is characterized in that: The maximum and minimum voltage amplitude models for the grid connection points of the new energy grid-connected system are constructed as follows: Among them: U max Indicates the maximum voltage amplitude at the grid connection point of the new energy grid-connected system, U min Indicates the minimum voltage amplitude at the grid connection point of the new energy grid-connected system, θ + and θ - They represent the initial phases of the positive-sequence voltage and negative-sequence voltage at the grid connection point respectively.

4. The voltage support method according to claim 3, which is suitable for considering the voltage drop degree under asymmetric fault conditions in a power grid, is characterized in that: The maximum current peak value for supporting the grid voltage is determined by the positive sequence reactive current amplitude and the positive sequence active current amplitude. Specifically, it includes: Among them: I peakmax Indicates the maximum current peak, I + and I - Represent the positive sequence current amplitude and negative sequence current amplitude respectively, and They represent the amplitude of the positive-sequence active current of the power grid and the amplitude of the negative-sequence active current of the power grid respectively.

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