Analysis of the steps and precautions for live-load testing of relay protection devices

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Abstract: Relay protection is of great significance for the safe and reliable operation of power equipment and substations; therefore, it is important to verify the relay devices in power systems to ensure that their protective functions can be fully utilized. To improve the calibration level of relay devices, this paper analyzes the specific steps and precautions for on-line compliance calibration based on practical work experience, for reference. I. The role of live load testing Live load testing is an essential task that must be carried out during the construction of power systems. Only through load testing can it be determined effectively whether the completed transmission projects and newly installed power equipment are operating properly. During the load testing process, ensuring that relay devices operate in a reliable and safe manner is a prerequisite for enabling the primary equipment in power engineering to be put into use. It is also an important way to assess the operational quality of secondary equipment. Furthermore, during the construction of power infrastructure, load verification must also be carried out. Only by verifying the live loads can the wiring methods and protection device designs within the power system be effectively examined, which facilitates the timely identification of incorrect wiring methods and the improvement of protection device designs. II. Analysis of the steps and precautions for live-load testing of relay protection devices 1. Live-load testing of bus differential protection The buses in power plants and substations are important components of the power system. If a bus fault is not quickly isolated, it will lead to the escalation of the accident, disrupt the stable operation of the power system, and result in its collapse. Therefore, the bus differential protection must be in operation whenever it is functioning properly. However, when installing a new circuit breaker, the bus differential protection should be disabled before the circuit breaker is charged; after it is under load, the protection circuit should be reactivated only after confirming that the current polarity is correct. Therefore, the bus differential protection circuit should be activated only after the current polarity is correct. Therefore, the load-tested verification of bus differential protection is carried out as follows: ① Deactivate the bus differential protection. ②Perform the charging operation. ③After the circuit breaker is loaded, relay protection personnel carry out inspection work. ④After verifying that the current polarity of the protection circuit is correct, the bus differential protection is activated. Precautions for loading verification of bus differential protection: ① The method of disabling the bus differential protection must be correct. First, the bus differential protection circuit breaker outlet connector should be disabled, and then the protection DC power supply should be turned off. When removing the fuse from the DC power supply, the positive terminal should be removed first, followed by the negative terminal; alternatively, it is possible to keep the protective DC power supply in operation depending on the requirements on site. ②During load testing, in addition to measuring the currents in the three-phase circuit and the differential circuit, it is necessary to measure the unbalanced current in the neutral wire to ensure the integrity and correctness of the circuit. ③After verification, the operations performed for the bus differential protection must be correct. First, connect the DC power supply; after confirming that the entire protection device is functioning properly, use a high-impedance voltmeter to check that there is no voltage across the terminals of the outlet connections. Once it is confirmed that there is no voltage there, proceed to connect the outlet connections of each circuit breaker one by one. ④Activate the bus differential protection correctly according to the operation mode of the bus and the type of bus differential protection. Special attention should be paid to the switching of the circuit breaker voltage circuit and the switching of the outlet connection pieces for bus differential failure protection. When automatic switching is achieved using an isolating switch reset relay, it is necessary to check the status of the reset relay to prevent it from failing to be energized or from not returning to its normal state.