How should the protection configuration of high-voltage transformers be properly implemented?
1. Install surge arresters to prevent lightning overvoltage: For the lightning protection of distribution transformers, install gapless metal oxide surge arresters as overvoltage protection to prevent insulation breakdown inside the transformer caused by high-voltage lightning waves penetrating from high- and low-voltage lines, thereby preventing short circuits and eliminating lightning damage accidents.
When using surge arresters for transformer protection: First, purchase qualified products through proper channels, and conduct rigorous testing before commissioning to ensure they meet operational requirements; second, regularly perform preventive tests on operating equipment, and promptly replace any products whose leakage current exceeds standard values; third, regularly test the transformer grounding resistance. For distribution transformers of 100 KVA and above, the grounding resistance must be within 4 Ω, while for transformers below 100 KVA, the grounding resistance must be within 10 Ω.
If the test values are not within the specified range, measures such as extending grounding wires, adding grounding bodies, and applying physical or chemical methods should be taken to meet the required values. Grounding resistance should be re-tested twice a year in April and July to prevent disconnection at welding points or resistance exceeding standards due to environmental or other factors. If the transformer grounding resistance exceeds limits, lightning current cannot flow into the ground but instead applies high voltage to the transformer low-voltage side through the grounding line, potentially damaging the transformer; fourth, appropriate installation locations should be chosen. High-voltage surge arresters should be installed near the high-voltage bushing of the transformer to minimize the chance of direct lightning entering the transformer. Low-voltage surge arresters should be installed near the low-voltage bushing to ensure proper operation before the lightning wave enters the transformer, following electrical equipment installation standards to avoid losing protective function due to improper installation.
2. Install instantaneous and overcurrent protection to ensure selective fault line disconnection: Short-circuit and overload protection of distribution transformers is realized by fuses installed on the transformer's high-voltage side and leakage circuit breakers on the low-voltage side (the device provides leakage protection and low-voltage overcurrent protection). To effectively protect the transformer, it is necessary to correctly select the fuse element (fuse wire, fuse blade, etc.) and set the low-voltage overcurrent protection. The high-voltage fuse should ensure it blows during short-circuits inside or at transformer bushings. Fuse selection principles: (1) For transformers of 100kVA or below, select high-voltage fuses at 2–2.5 times the rated current; (2) For transformers above 100kVA, select fuses at 1.5–2 times the rated current.
The overcurrent trip value of the low-voltage leakage breaker is set at 1.3 times the rated value of the transformer's low-voltage side. The overcurrent protection set values for each low-voltage branch should not exceed the main protection's trip value and should be less than the transformer's low-voltage rated current. Generally, the overcurrent value is chosen based on the conductor's carrying capacity to ensure timely action when a short circuit occurs in branch circuits or the load exceeds limits, disconnecting the load and faulty lines to protect the transformer.
At the same time, satisfy the selectivity requirements of each protection level. In the event of a low-voltage branch short-circuit, the branch circuit acts while the leakage circuit breaker overcurrent protection does not operate. In case of low-voltage total circuit fault or short-circuit, the leakage breaker overcurrent reacts, but the high-voltage fuse does not blow. In the event of an internal transformer fault, the high-voltage fuse blows, while the higher-level substation high-voltage line protection does not trip, ensuring correct graded operation of the distribution network protection. The high-voltage fuse protection material must be selected according to standards, strictly prohibiting the use of copper, aluminum, or other conductors as a replacement for fuse elements.
