A method for distance element blocking of a sending line of a photovoltaic power plant

Abstract

The application discloses a method for distance three-section protection blocking of a sending line of a photovoltaic power station. Firstly, when the distance three-section protection is started, a phase current component of the protected line is extracted, the third harmonic content of the phase current component and the ratio of the third harmonic to the second harmonic are calculated, when the third harmonic content of the phase current component exceeds 0.5 and the ratio of the third harmonic to the second harmonic exceeds 1.5, it is judged that the inrush current blocking distance three-section protection is generated, otherwise, it is judged that the fault current opening distance three-section protection is generated. The method can correctly distinguish the inrush current and the excitation inrush current generated by the no-load closing of a booster transformer of the photovoltaic power station, and avoids incorrect protection action of the distance three-section protection due to slow transient inrush current decay time.

Description

Technical fields:

[0001] This invention belongs to the field of power systems and relates to the field of distance protection technology for photovoltaic power plant transmission lines, specifically to a method for blocking distance elements for photovoltaic power plant transmission lines. Background technology:

[0002] Distance protection is one of the main and backup protection systems for transmission lines, offering advantages such as being unaffected by grid operating conditions and providing fault location information. When a fault occurs on the transmission line, the distance element activates when both the current and voltage at the protection installation point reach the threshold value, performing zero-sequence direction determination. When both zero-sequence direction elements on both sides of the transmission line determine a positive direction fault, the protection trips.

[0003] Currently, transient inrush currents generated during the switching of main transformers in photovoltaic power plants have numerous adverse effects on relay protection. Distance protection, as one of the mainstream backup protection systems for power lines, plays a crucial role in the safe operation of photovoltaic power plant transmission lines. To meet grid dispatch and plant power demand, as well as the fluctuation characteristics of natural energy, main transformers in photovoltaic power plants mostly switch between operating and shutdown states. During this period, closing a main transformer under no-load conditions causes a sudden change in its terminal voltage, leading to core flux saturation and the generation of excitation inrush current. Coupled with the weak feedback characteristics of the renewable energy system, the main transformer operating in parallel generates a similar inrush current. These two types of inrush currents alternate over time. Due to their slower decay rate, the excitation inrush current of the closed transformer also decays more slowly. When the sum of these two types of inrush currents flows into the transmission line, the transmission line current exhibits an increased amplitude and a longer decay period. Therefore, after a transient inrush current occurs, the distance protection element based on the time gradient setting may prematurely meet the conditions and malfunction. This makes the operation of the distance protection backup protection principle extremely unstable when the main transformer is frequently switched on and off in a photovoltaic power station. Summary of the Invention:

[0004] To address the problems existing in the prior art, the purpose of this invention is to provide a method for blocking distance elements in the transmission lines of photovoltaic power plants. This method can correctly determine whether a transient inrush current occurs in the main transformer of the photovoltaic power plant and block the distance protection in a timely manner, thereby preventing the distance protection from malfunctioning due to the long attenuation period of the distance element during inrush current.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] ① After the distance protection device is activated, it extracts the three-phase current of the protected line;

[0007] ② Extract the third harmonic I of the three-phase current using the Fourier algorithm. 3rd Second harmonic I 2nd With fundamental wave Ifund ;

[0008] ③ Determine whether the third harmonic ratio is greater than k. 31 And whether the ratio of the third harmonic to the second harmonic is greater than the threshold value k. 32 When the distance exceeds the set threshold value, the distance element is locked; otherwise, the distance protection is enabled.

[0009] The method of this invention requires extracting the three-phase current of the protected line after the distance protection is started, and calculating the third harmonic ratio and the third-to-second harmonic ratio of the current. The method adds a blocking condition for the distance element in the distance protection, so that the distance element, which is prone to erroneous operation, will be blocked after a transient inrush current is detected in the main transformer in the photovoltaic power station. This effectively improves the operation performance of the distance protection when applied to the transmission line of the photovoltaic power station. Attached Figure Description

[0010] Figure 1 This is a flowchart of the method for implementing the present invention.

[0011] Figure 2 This is a schematic diagram of a circuit applicable to the method of the present invention. Detailed implementation method:

[0012] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0013] like Figure 1 As shown, this invention is a method for three-stage protection interlocking of transmission lines from photovoltaic power plants, comprising the following steps:

[0014] Step 1: After the distance protection device is activated, extract the three-phase current of the protected line;

[0015] Step 2: Extract the third harmonic I of the three-phase current using the Fourier algorithm. 3rd Second harmonic I 2nd With fundamental wave I fund ;

[0016] Step 3: Determine if the third harmonic ratio is greater than k. 31 And whether the ratio of the third harmonic to the second harmonic is greater than the threshold value k. 32 When the distance exceeds the set threshold value, the distance element is locked; otherwise, the distance protection is enabled.

[0017] Example

[0018] like Figure 2 As shown, L1 is a 50MW photovoltaic power plant transmission line, with distance protection devices installed on both sections, including distance elements in the protection devices. The following description uses the protection on the photovoltaic power plant side of the transmission line as an example to illustrate the solution of this invention.

[0019] The protection device on the photovoltaic power station side of the transmission line collects the three-phase current. After the protection device is activated, the third harmonic of the three-phase current is extracted using the Fourier algorithm. 3rd Second harmonic I 2nd With fundamental wave I fund When the third harmonic I 3rd With fundamental wave I fund The ratio exceeds 0.5 and the third harmonic I 3rd With second harmonic I 2nd When the ratio exceeds 1.5, the distance protection element on the photovoltaic power station side is locked; otherwise, the distance protection element is unlocked.

[0020] By adopting the above-mentioned distance element blocking scheme, when the current of the photovoltaic power station's transmission line changes suddenly, it can correctly determine whether the main transformer in the substation generates a transient inrush current, thereby blocking the distance protection element in a timely manner and effectively improving the reliability of the distance protection when applied to the transmission line of the photovoltaic power station.

Claims

1. A method for three-stage protection interlocking of transmission lines from photovoltaic power plants, characterized in that, Includes the following steps: ①After the three-stage distance protection is activated, the three-phase current components of the protected line are extracted; ② Extract the third harmonic of the three-phase current using the Fourier algorithm. Second harmonic with fundamental wave ; ③ Identify the third harmonic with fundamental wave Is the ratio greater than the threshold value? And the third harmonic With second harmonic Is the ratio greater than the threshold value? When the third harmonic with fundamental wave The ratio is greater than the threshold value And the third harmonic With second harmonic The ratio is greater than the threshold value When the blocking distance is in the specified state, the three-stage protection is applied; otherwise, the three-stage protection is applied to the open distance.

2. The method for three-stage protection interlocking of transmission lines from photovoltaic power plants as described in claim 1, characterized in that: The threshold value Set to 0.5, threshold value Set to 1.5.

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