What are the most common types of protection for transformers?
Whether the transformer operates normally or not directly affects the user's production and domestic electricity consumption, and is related to the safety of the equipment, resulting in huge economic losses. This paper discusses in detail the protection types and precautions of distribution transformers in daily use.
A transformer is a static electrical appliance in the power distribution system that exchanges voltage and current according to the law of electromagnetic induction to transmit alternating current energy. It is usually installed in utility poles, benches or power distribution stations, and the voltage of 6~10 kV is generally reduced to about 100 volts and input to the user.
Lightning protection is the most common protection item for transformers. Lightning protection is to prevent overvoltage from lightning strikes. Valve-type arresters and tubular arresters to protect transformers are required to be installed as close to the transformer as possible. The grounding wire should be connected to the metal casing of the distribution transformer and the neutral point of the low-voltage side to be grounded together. Because once a lightning strike damage accident occurs, it will cause a power outage.
In addition to lightning protection, transformer protection can be divided into:
1. Body protection: It is a non-electrical protection, including body light and heavy gas protection, on-load pressure regulating light gas, heavy gas protection and pressure release. The principle is that when the transformer fails, it often affects the insulating oil of the transformer, resulting in the generation of gas. At this time, the Buchholz relay of the transformer acts, and the body protection is to trip or alarm according to the actions of different Buchholz relays.
2. Differential protection: It reflects the internal fault of the transformer (including the cables between the CTs on three sides or on both sides). Taking three turns as an example, the currents on three sides of the transformer are collected. Under normal circumstances, according to the KCL theorem, the current flowing into the transformer is equal to the current flowing out of the transformer, that is, the differential current is zero; if there is an internal fault in the transformer, there must be a relatively large current on one side, resulting in the differential current not being 0 and the protection action. If it is an external fault, the current flowing into the transformer is still equal to the current flowing out of the transformer, and the protection will not operate. Common differentials include differential flow, ratio differential, etc. The two circles are the same.
3. Distance protection: It is very rarely used at present, but it is useful. Simply speaking, it collects voltage and current, calculates the impedance, and then judges whether to act according to the calculated impedance. Not recommended.
4. Overcurrent protection: used for transformer backup protection. It reflects transformer faults, busbar faults and feeder faults. Generally, it is not only used as transformer backup, but also as bus backup, feeder protection backup, etc. Overcurrent protection includes: phase current, negative sequence, zero sequence; as well as definite time, inverse time, etc.
5. Gap overcurrent and gap overvoltage protection: collect the power in the discharge gap of the transformer to deal with the ground fault
6. Restrictive grounding protection: a lot of external use. Using zero-sequence current to judge, it seems that there is also called zero-sequence differential protection.
7. Overload: including alarm, starting air cooling, blocking on-load voltage regulation, etc.
8. Overheating load: According to the heat accumulation characteristics provided by the transformer, the heat accumulation of the transformer is calculated according to the load current, which is divided into two stages: alarm and trip.
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