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Interphase fault between transformer leads and adjacent busbars
Published on.
2024-02-22
Currently, transformers in operation in China's power system typically use three-winding transformers or autotransformers, and the vast majority are single-phase transformer banks. Phase-to-phase backup protection is configured to clear phase-to-phase faults between transformer leads and adjacent busbars. Because the transformers are single-phase transformer banks, and both the transformer and line main protections are configured in a dual-redundant manner, the transformer's phase-to-phase backup protection is configured according to simplified principles. The high-voltage and medium-voltage side phase-to-phase backup protection methods generally use directional impedance protection with offset characteristics, and are configured identically; the low-voltage side generally uses current or voltage protection. In most cases, the transformer's impedance protection can cooperate with the transformer's main protection and the first stage of the line impedance protection, meeting the sensitivity requirements under various operating modes, but its operation and maintenance are relatively complex. Operational practice has proven that the transformer impedance protection has a relatively high maloperation rate. For each maloperation, a careful analysis should be conducted to find solutions. Its operating principle is that when the switch is in the closed position, the SOTF protection is automatically exited through the auxiliary contacts of the switch tripping position. Its action requires two conditions: the switch is in the open position and within 250 ms of the switch closing; the impedance element operates or the current and voltage values reach their inherent set values within the device. After the main transformer operated again, no abnormalities were found in the inspection of the main transformer body, and the oil sample test was also normal. Measures Because the SOTF element in the impedance protection has very high sensitivity, it is generally used only when charging the transformer, it is recommended to withdraw it from operation or change the wiring method. It is recommended to replace the float-type gas relay with a baffle type. If the original Toshiba factory equipment is still used, a sealing test should be performed on the float. The protection devices in substations mostly use imported protection devices, but imported protection has the problem of multiple functions and low utilization rate (usually only some functions are used). Therefore, technicians should carefully study and analyze each function of the device so that it can be correctly used to serve the safe operation of the power system.
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