Basic Working Principle and Setting Method of Gas Protection for Power Transformer
1. Basic working principle of gas protection
When a fault occurs inside the power transformer oil tank (including the grounding short circuit through arc resistance caused by slight turn to turn short circuit and insulation damage), due to the action of fault point current and arc, transformer oil (usually 25# insulating oil) and other insulating materials will decompose due to local heating to produce gas. Because the gas is relatively light, they will flow from the oil tank to the upper part of the conservator. When the fault is serious, the oil will expand rapidly and produce a large amount of gas. At this time, violent gas mixed with oil flow will rush to the upper part of the conservator to make the contact of the relay act, connect the specified control circuit, and send a signal in time or automatically cut off the transformer to form a protection device that acts in response to the above gas, which is called gas protection.
Gas relay is the main component of gas protection, such as fj3-80 and QJ series. It is installed on the connecting pipe between the oil tank body and the conservator. In order to increase the sensitivity and reliability of gas protection, all the gas generated by faults in the transformer must pass through the gas relay smoothly, Therefore, when installing the transformer, the oil tank body shall be inclined to the conservator by 1 1.5%, and the oil pipe shall be inclined to the conservator by 2 4%.The operation of the old gas relay (float mercury contact relay) is not stable enough, and its anti vibration ability is poor, which is easy to cause misoperation due to mercury flow due to vibration. The open cup baffle gas relay has high vibration resistance and reliable action. Therefore, the open cup baffle gas relay is widely used in China's power system.
During normal operation, the relay is filled with transformer oil. In case of slight internal fault of the transformer, a small amount of gas generated by decomposition of the transformer rises and accumulates in the air chamber at the upper part of the gas relay, forcing the oil level to rotate and fall, so that the upper magnetic steel is close to a pair of dry spring contacts and sends a light gas fault signal after action. If it is a serious fault, a large amount of gas will be generated. At the same time, the oil temperature will rise, the hot oil will expand, and the pressure in the tank will increase sharply, resulting in the rapid impulse of oil and air flow to the lower baffle of the relay, resulting in the lower magnetic steel close to another pair of dry spring contacts and acting on tripping.
2. Setting of gas relay
The setting of gas relay can be carried out through differential pressure flow rate test or oil pump oil speed test equipment. The starting value is set according to the oil flow speed. Generally, 0.5 1.5m/s is used for heavy gas. When the transformer has forced oil circulation device, the range of 1 1.5m/s is used; when there is no forced oil circulation device, the range of 0.6 1m / S is used; For light gas, the volume of the space occupied by the gas in the relay is set, and the signal is sent when the range is 250 300CM3.
