Power system relay protection basic knowledge learning - Database & Sql Blog Articles

Abstract: Due to the vast scale of urban power grids, the complex operating environment, and various human factors, electrical faults are difficult to completely avoid. Any accident in the system can have a significant impact on the operation of the entire power system. To ensure the stable functioning of the urban power distribution system, it is essential to properly configure relay protection devices.

Keywords: Power system, 10kV, Power system relay protection

1. Basic Concept of Relay Protection

Reliability refers to the ability of a component, device, or system to perform its intended function under specified conditions for a defined period. In the context of relay protection, reliability means that when a fault occurs within the device’s designated range, it should operate correctly, and should not malfunction when no action is required. The failure or incorrect operation of relay protection devices can cause serious damage to the power system.

The trade-off between improving reliability and reducing failure rates is often challenging. Depending on the structure and load characteristics of the power system, the consequences of both refusal to act and malfunctions vary. For instance, in systems with sufficient rotating reserve capacity and multiple transmission lines, a malfunction might cause minimal disruption. However, if a relay fails to operate during a fault, it could lead to severe equipment damage or system instability. Therefore, in such cases, improving the reliability of non-rejection becomes more critical.

2. Evaluation Indexes of Protective Devices

2.1. States of Relay Protection Devices

Relay protection devices are considered repairable components. Their operational states include: normal operation, inspection status, correct operation, malfunction, rejection, and fault repair. Understanding these states helps assess the performance and reliability of the system.

2.2. Common Evaluation Indicators

2.2.1. Correct Action Rate: This is the percentage of times the relay protection device operates correctly within a certain period (e.g., one year). It helps identify weak links in different protection systems, such as 220kV vs. 500kV systems.

2.2.2. Reliability r(t): The probability that a component remains in a normal state from time 0 to t without failure.

2.2.3. Availability a(t): The probability that a component is working normally at time t.

2.2.4. Failure Rate h(t): The probability of failure per unit time at time t, given that the component was in good condition up to that point.

2.2.5. Mean Time Between Failures (MTBF): The average time between two consecutive failures.

2.2.6. Repair Rate m(t): The probability of repairing a failed component per unit time after failure.

2.2.7. Mean Time to Repair (MTTR): The average time required to restore the device after a failure.

3. Relay Protection in 10kV Power Systems

3.1. Operating Conditions of the 10kV Power System

Normal operation refers to the system running under rated conditions. Faults occur when equipment or lines pose a risk to safety. Abnormal operation means the system has been disrupted but hasn’t reached the failure stage.

3.2. Tasks of Relay Protection Devices in 10kV Systems

During normal operation, the device must monitor equipment status and provide reliable data for operators. When a fault occurs, it should isolate the faulty section quickly and selectively. In abnormal conditions, it should alert the personnel promptly.

3.3. Analysis of Common Current Protection Methods

3.3.1. Inverse Time Overcurrent Protection: The operation time depends on the short-circuit current. The higher the current, the faster the response. Although simple in wiring, it is complex internally and less accurate than electromagnetic-based systems.

3.3.2. Time-Limited Overcurrent Protection: The operation time is fixed and set by a time relay. It is commonly used in 10kV ungrounded systems and consists of current transformers, relays, and signal devices. The setting of the operating current must avoid maximum load currents.

4. Conclusion

Improving the reliability of relay protection—both in avoiding failures and preventing malfunctions—is crucial for the safe operation of urban power systems. With numerous electrical devices interconnected, proper configuration and accurate setting of protection values are essential to maintain system stability.

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Bai Ruishun (Beijing) Electric Co., Ltd.