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I. Composition of relay protection
The relay protection system is mainly composed of the following parts:
1. Measuring element
-Measures the electrical parameters of the power system, such as current, voltage, frequency, etc. The measuring element converts the actual high voltage and high current signals into small signals suitable for the relay protection device to process through the mutual inductor.
2. Comparison element
-Responsible for comparing the measured electrical parameters with the set protection setting to determine whether they exceed the allowable range. When an abnormality is detected, the protection action is triggered.
3. Execution element
-When a fault occurs, the relay protection device cuts off the faulty part by controlling the circuit breaker, switch and other equipment. The execution element is the last link of the relay protection system action to ensure that the fault area is quickly isolated.
4. Auxiliary equipment
-Including power module, communication module and display module, etc., used to ensure the normal operation of the relay protection device and the control and monitoring of the operator.
2. History of relay protection
Relay protection has evolved with the development of power systems and has gone through several important stages:
1. Early mechanical relays (early 20th century)
- The earliest relay protection devices were relays based on mechanical principles, which used electromagnetic effects to sense electrical quantities in power systems and trigger protection actions.
- Features: These devices have simple structures, but slow action speed and low sensitivity, and are suitable for simple power systems.
- Disadvantages: Relatively low reliability and accuracy, and are easily affected by mechanical wear and environmental factors.
2. Static relays (1950s)
- Static relays use electronic components (such as transistors, capacitors, etc.) to replace traditional mechanical components, greatly improving the response speed and sensitivity of relays.
- Features: There are no mechanical moving parts, which reduces wear and false operation, and improves reliability and action accuracy.
- Disadvantages: Despite the improved performance, these relays still rely on analog circuits and have relatively simple functions.
3. Digital relay protection (1970s to present)
- With the development of computer technology, digital relay protection devices have begun to be used. It uses a microprocessor or computer to monitor and control the power system in real time.
- Features: It has high sensitivity and reliability, and can realize complex protection logic and self-diagnosis functions. Digital relays can record data and analyze faults, which is convenient for system maintenance and optimization.
- Advantages: It has strong programmability, can be flexibly adjusted according to different application scenarios, and is easy to upgrade and maintain.
4. Intelligent relay protection (now and in the future)
- With the development of smart grids, relay protection devices have entered the intelligent stage, and can interact with other devices through communication networks to form systematic protection and control.
- Features: It has extensive communication capabilities, real-time monitoring and adaptive functions, and can improve the intelligence and self-learning ability of protection through artificial intelligence technology.
III. Basic performance requirements of relay protection
Relay protection devices must meet the following basic performance requirements to ensure that they can reliably play a protective role in the event of a fault:
1. Selectivity
- When the relay protection system fails, it must only cut off the faulty part and keep the other parts running normally. Selective protection reduces the impact on the overall system and ensures the continuity of power supply.
2. Speed
- The relay protection device should be able to respond quickly after a fault occurs to minimize the damage to the power equipment and system. Generally speaking, the shorter the relay protection action time, the better the protection effect.
3. Sensitivity
- The relay protection device should have sufficient sensitivity to detect minor faults or abnormal signals in the system and take timely measures. In the case of insufficient sensitivity, the fault may not be identified, resulting in equipment damage or the expansion of the accident.
4. Reliability
- The relay protection device must not malfunction during normal operation and operate reliably when the system fails. High reliability is a key indicator to ensure the safe operation of the power system.
5. Anti-interference
- The relay protection device must have strong anti-interference ability to avoid malfunction when affected by external electromagnetic interference or other factors.
IV. Classification of relay protection
According to different working principles, protection objects and functions, relay protection can be divided into the following categories:
1. Classification by working principle
- Current protection: judge the fault according to the change of current value, such as overcurrent protection.
- Voltage protection: Faults are determined based on changes in voltage values, such as overvoltage and undervoltage protection.
- Differential protection: Faults are determined by comparing the current difference at both ends of the protection area.
- Distance protection: Fault distance is determined by measuring the impedance from the fault point to the relay, and is mainly used for the protection of transmission lines.
2. Classification by protection object
- Generator protection: used for overcurrent, overvoltage, undervoltage, differential and other protection of generators.
- Transformer protection: used for differential protection, overload protection, etc. of transformers.
- Transmission line protection: used for overcurrent protection, distance protection, etc. of transmission lines.
- Motor protection: used for short circuit, overload, overtemperature and other protection of motors.
3. Classification by protection function
- Main protection: directly protect electrical equipment or lines, and take priority in case of faults, such as differential protection and distance protection.
- Backup protection: when the main protection fails, the backup protection provides support, and usually takes a longer time to operate, such as overcurrent protection.
4. Classification by action time
- Instantaneous protection: It acts almost immediately after the fault occurs, without time delay.
- Delayed protection: It acts after a delay of a period of time after the fault is detected, usually used to avoid false action or coordinate with other protection.
In a word,
- Development history: Relay protection has evolved from mechanical relays to intelligent relay protection, and its performance and functions have been continuously improved.
- Composition: The relay protection system includes measuring elements, comparison elements, actuators and auxiliary equipment.
- Performance requirements: Relay protection must have basic performance such as selectivity, speed, sensitivity, reliability and anti-interference.
- Classification: Relay protection is classified according to working principle, protection object, function and action time to meet the protection needs of different power systems and equipment.
