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

Abstract: Due to the extremely large area of the urban power grid, the complex operating environment, and various human factors, electrical faults are difficult to completely avoid. These issues can have serious consequences if not properly managed.

Electricity is a critical part of modern life, and any accident in the power system can significantly impact its operation. To ensure the stable functioning of the urban power distribution system, it is essential to install and maintain relay protection devices correctly.

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 certain period. In the context of relay protection, reliability means that the device should act when a fault occurs within its designated range and must not operate when it shouldn't. Failure to act (rejection) or unintended operation (malfunction) can cause severe damage to the power system.

The consequences of rejection or malfunction vary depending on the system's configuration. For example, in systems with sufficient reserve capacity and strong interconnections, a misoperation may have limited effects. However, in systems with weak connections or limited reserves, even a minor malfunction can lead to widespread outages or system instability. Therefore, the importance of improving reliability varies based on the specific scenario.

2. Evaluation Indexes of Protective Devices

2.1 Common States of Relay Protection Devices

Relay protection devices can be in several states: normal operation, maintenance, correct operation, malfunction, rejection, and repair. Understanding these states helps assess the performance and reliability of the device.

2.2 Key Evaluation Indicators

2.2.1 Correct Action Rate: This is the percentage of times the relay operates correctly during a given period. It helps identify weak points in the system and compare different protection schemes.

2.2.2 Reliability (r(t)): The probability that the device remains in a normal state up to time t.

2.2.3 Availability (a(t)): The probability that the device is working normally at time t, regardless of previous states.

2.2.4 Failure Rate (h(t)): The likelihood of failure per unit time, given that the device has been functioning up to time t.

2.2.5 Mean Time Between Failures (MTBF): The average time between failures, indicating the device’s overall durability.

2.2.6 Repair Rate (m(t)): The rate at which the device is repaired after a failure.

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

3. 10kV Power System Relay Protection

3.1 Operating Conditions of the 10kV Power Supply System

Normal operation involves all equipment running within rated parameters. Faulty conditions threaten the system’s safety, while abnormal operations indicate potential issues that need attention.

3.2 Tasks of Relay Protection Devices in 10kV Systems

During normal operation, the device monitors equipment status and provides data for operators. In case of a fault, it quickly isolates the affected area to prevent further damage. For abnormal conditions, it alerts personnel so they can take corrective actions promptly.

3.3 Analysis of Common Current Protection Methods

3.3.1 Inverse Time Overcurrent Protection: The response time depends on the short-circuit current. While 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 via a time relay. It is commonly used in ungrounded 10kV systems and consists of current transformers, relays, and signal components.

Setting the operating current involves avoiding the maximum load current to ensure the device only activates during actual faults.

4. Conclusion

Improving the reliability of relay protection—both in terms of non-rejection and non-malfunction—is crucial for ensuring the safe and stable operation of power systems. With the increasing complexity of urban power grids, accurate settings and proper maintenance of protective devices are more important than ever.

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