Method for identifying system fault direction, directional element and relay protection device
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Solution Overview
Problem
Conventional negative-sequence directional elements in electric power systems fail to accurately determine fault direction when negative-sequence voltage is very small, leading to incorrect identification due to noise interference and reliance on system impedance settings that vary with system changes.
Innovation Solution
A negative-sequence impedance directional element based on compensation voltage, which uses a compensation point within the transmission line to ensure non-zero compensation voltage, allowing for fault direction determination using both amplitude and angle information without requiring system-specific threshold settings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a phase angle based negative-sequence directional element is used to determine fault direction, then the directional element can work effectively under normal conditions, but it cannot function normally when the negative-sequence voltage is very small (e.g., in super-strong systems or high impedance faults)
Solution Approach 1:
The patent introduces an intermediary mechanism by using zero-sequence voltage and current to calculate compensation voltage (Uc = U0 - I0*Zq0) and compensation impedance (Zc = Z0 - Zq0). This intermediary calculation allows the directional element to determine fault direction through the relationship between negative-sequence voltage and the calculated compensation impedance, rather than directly relying on weak negative-sequence voltage signals. The intermediary compensation parameters bridge the gap between available measurements and reliable direction determination.
Solution Approach 2:
The patent changes the measurement parameters from directly using negative-sequence voltage and current phase angle to using a composite parameter approach: calculating zero-sequence compensation voltage and impedance from three-phase measurements, then comparing the negative-sequence voltage phase angle with the compensation impedance angle. This parameter transformation enables reliable operation even when negative-sequence voltage is very small, as the compensation parameters provide a stable reference framework.
2Device complexity
If traditional negative-sequence directional elements are used, then the device structure is simple, but the device lacks resistance to noise interference when negative-sequence voltage is very small
Solution Approach 1:
The patent introduces intermediary calculations using zero-sequence components to generate compensation voltage and impedance parameters. These intermediary parameters serve as noise-resistant references that can be compared against negative-sequence measurements. The compensation impedance Zc = Z0 - Zq0 acts as a mediator that filters out noise effects, providing a stable basis for direction determination even in high-noise environments with weak negative-sequence signals.
Solution Approach 2:
The patent performs preliminary calculations of zero-sequence compensation parameters (Uc and Zc) before the actual direction determination. By pre-calculating these compensation parameters from the measured three-phase voltages and currents, the system prepares noise-resistant reference values in advance. This preliminary action ensures that when noise interference occurs during fault conditions, the directional element has already established stable reference parameters for accurate comparison and decision-making.
3Adaptability or versatility
If conventional directional elements rely on system impedance settings, then the element can be configured for specific systems, but the threshold parameters must be adjusted when system changes occur
Solution Approach 1:
The patent implements self-service by automatically calculating compensation parameters (Zq0 and Uc) from the measured three-phase voltages and currents during system operation. Rather than requiring manual configuration of threshold parameters based on system impedance, the system autonomously derives the compensation parameters needed for direction determination. This self-calibrating approach eliminates the need for operators to adjust threshold parameters when system conditions change, as the compensation parameters adapt automatically to current system state.
Solution Approach 2:
The patent introduces dynamic parameter calculation where compensation impedance Zc = Z0 - Zq0 and compensation voltage Uc are continuously updated based on real-time measurements of three-phase voltages and currents. This dynamic approach replaces static threshold settings with adaptive parameters that automatically adjust to system changes. The directional element maintains accuracy under varying system conditions because the compensation parameters are recalculated in real-time, eliminating the need for manual threshold reconfiguration.
Data Source
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AI summary
The present invention relates to a method for identifying a fault direction in an electric power system, the sequence impedance directional element, and a relay protection device including the element. The electric power system comprises two power supply networks (Vs, Vr) and an electric power transmission line, with a relay protection device (REL1) being arranged at the first endpoint of the electric power transmission line. The method comprises the steps of: measuring the three-phase current flowing through the relay protection device and the three-phase voltage (S101); determining the sequence voltage and sequence current at the relay protection device based on the measured three-phase current and voltage (S102); calculating the sequence impedance at a predetermined compensation point in the electric power transmission line based on the determined sequence voltage and sequence current at the relay protection device (S103); and comparing the calculated sequence impedance at the predetermined compensation point with a reference threshold (S104), and determining the direction of a fault occurring in the electric power system relative to the relay protection device based on the comparison result (S105). The method and the directional element of the present invention can accurately determine the fault direction even under the condition that the negative-sequence voltage at the relay protection device is very small, and they possess a good anti-interference capability. Fig. 8