The importance of relay protection in the power system

Relay protection refers to the study of power system failures and abnormal operating conditions that jeopardize safe operation in order to explore countermeasures against accident automation. In the course of its development, it was mainly used relays with contacts to protect the power system and its components (generators, transformers, transmission lines, etc.) from damage. The basic task is: When the power system has a fault or an abnormal condition, the faulty device is automatically removed from the system within the shortest possible time and within the smallest area that can be achieved, or a signal is sent to eliminate the cause of the abnormal condition by the on-duty personnel to reduce or Avoid damage to the equipment and the impact on the power supply to adjacent areas.

After the failure of the power system, the main features of changes in the frequency of electrical power are:

(1) The current increases. In the event of a short circuit, the current between the fault point and the power supply on the electrical equipment and on the transmission line will increase from the load current to much more than the load current.

(2) Voltage drop. When phase-to-phase short circuit and ground short circuit fault occur, the phase-to-phase voltage or phase voltage at each point of the system decreases, and the closer to the short-circuit point, the lower the voltage.

(3) The phase angle between current and voltage changes. In normal operation, the phase angle between current and voltage is the power factor angle of the load, which is generally about 20°. When the three-phase short circuit occurs, the phase angle between the current and the voltage is determined by the impedance angle of the line, which is generally 60°. 85°, and when the three-phase short circuit in the opposite direction of protection, the phase angle between the current and voltage is 180°+(60°~85°).

(4) Measuring impedance changes. The impedance is measured as the ratio of voltage to current at the point of measurement (protection installation). In normal operation, the measured impedance is the load impedance; in the case of a metallic short-circuit, the measured impedance is converted into the line impedance, and the measured impedance significantly decreases after the fault, and the impedance angle increases.

The importance of relay protection 1. The relay protection device must have the following five basic properties: (1) Security. No malfunction should occur when it is not to be actuated. (2) Reliability. In this action, no Reject action (3), rapidity. Can eliminate faults or anomalies with the shortest time limit. (4) Selectivity. Cut off the fault in the smallest possible interval to ensure maximum power supply to the fault-free part. (5) Sensitivity. The ability to reflect failures, usually expressed as a sensitivity factor.

2. Power Regulations: No power equipment is allowed to operate without relay protection. All operating equipment must have two sets of AC and DC input and output circuits independent of each other, and respectively control the relay protection devices of different circuit breakers for protection. When any relay protection device or any set of circuit breakers refuses to operate, another relay protection device can be used to operate another set of circuit breakers to cut off the fault. In all cases, it is required that the DC power taken by the two protective relays and circuit breakers be supplied with different fuses. It can be seen that although relay protection is not a primary device of the power system, it plays an indispensable role in ensuring the safe operation of the primary device.

3. Relay protection and automatic devices play a key role in ensuring the safe and economical operation of the power system and preventing the occurrence and expansion of accidents. Due to the particularity of the power system, the occurrence of electrical faults is unavoidable. Once local grids and equipment accidents occur and are not effectively controlled, it will result in stable damage to the grid and widespread power outages. The modern large-scale power grid is more dependent on relay protection and has higher requirements on its correct rate of action.

4, regular preventive tests. The basic purpose of periodical overhaul protection should be "to ensure that the entire relay protection system is in good condition and to ensure the safety and reliability of the operation." Therefore, in principle, the inspection project should be clearly distinguished from the new installation project, and only a few targeted tests can be conducted. Attention should be focused on items that have a major impact on the safety and reliability of the protective action, and maintenance should be avoided to obtain the maximum return on investment for regular inspections.

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