Transmission-principle, structure and operation maintenance of transformer gas protection


Gas protection is the main protection of transformer internal faults, which can act sensitively to transformer inter-turn and inter-layer short circuit, iron core fault, internal bushing fault, winding internal disconnection, insulation deterioration and oil level drop and other faults.When the internal failure of oil-immersed transformer, due to the arc will make the insulation material decomposition and produce a lot of gas, flow from the tank to the oil pillow, its intensity is different with the severity of the fault, the reaction of this air flow and oil flow and action protection is called gas protection, also called gas protection.In the operation of the transformer, due to internal faults, sometimes we cannot identify and take measures in time, which is prone to cause some accidents. After adopting gas relay protection, similar events are avoided to a certain extent.What is gas?Gas is formed by the decomposition of cellulose and organic matter by anaerobic bacteria in the early stage of coal accumulation by ancient plants.In the environment of high temperature and high pressure, gas continues to be generated due to physical and chemical reactions while coal is formed.Gas is colorless and odorless, but sometimes it smells like apples, because aromatic hydrocarbons are emitted at the same time.The relative density of gas to air is 0.554. In the standard state, the density of gas is 0.716kg/m³, and the permeability of gas is 1.6 times that of air. It is difficult to dissolve in water and can neither help combustion nor maintain respiration.The main components of gas are alkanes, of which methane predominates, with small amounts of ethane, propane and butane. In addition, it generally contains hydrogen sulfide, carbon dioxide, nitrogen and water gas, as well as trace inert gases, such as helium and argon.Under standard conditions, methane to butane exists as a gas and pentane above as a liquid.In case of open fire, it can be burned and “gas” explosion occurs, which directly threatens the safety of miners.Two, the definition of gas protection in the gas protection relay, the upper part is a sealed buoy, the lower part is a metal baffle, both are equipped with sealed mercury contacts.The buoy and baffle can be rotated about their respective axes.In normal operation, the relay is filled with oil, the buoy is immersed in oil, in the floating position, and the mercury contact is disconnected;The baffle is sagging under its own weight, and its mercury contacts are broken.When there is a slight fault inside the transformer, the speed of gas generation is relatively slow. When the gas rises to the oil storage tank, it first accumulates in the upper space of the gas relay, so that the oil level drops, the buoy drops with it, and the mercury contact is closed and the delay signal is connected, which is the so-called “light gas”.When transformer internal serious fault occurs, a strong gas, the pressure in the fuel tank is instantaneous jump, produce very big oil flow in the direction of oil conservator impact, because the oil flow impact damper, damper to overcome the spring resistance, drive the magnet to the spring contact direction, the mercury contact closure, through to trip circuit, the circuit breaker tripped, this is the so-called “gas”.Heavy gas action, immediately cut off all power supply connected with the transformer, so as to avoid the expansion of the accident, play the role of transformer protection.Gas relay has buoy type, plate type, open cup type and other different models.Most use QJ-80 relay, its signal circuit is connected to the opening cup, trip circuit baffle.The so-called gas protection signal action refers to the closure of the signal circuit contact of the upper opening cup in the relay due to various reasons, and the light plate lights up.Regulations: for capacity of 800kVA and above the oil-immersed transformer and 400kVA and above the workshop oil-immersed transformer, should be installed with gas protection.3. Classification of gas protection Gas protection is generally divided into light gas and heavy gas.1, light gas protection: transformer internal overheating, or partial discharge, so that the transformer oil temperature rise, produce a certain amount of gas, collected in the relay, reach a certain amount of relay, trigger relay, send a signal.2. Heavy gas protection: after a serious short circuit occurs in the transformer, it will impact the transformer oil and make a certain oil flow rush to the baffle of the relay, acting in trip.Four, the scope of protection of gas protection is the main protection of the transformer, it can reflect all the faults in the tank.It includes: multiphase short circuit in the tank, short circuit between winding turns, short circuit between winding and iron core or shell, iron core failure, oil level drop or oil leakage, poor contact of tap switch or poor welding of wire, etc.Gas protection is quick, sensitive, reliable and simple in structure.However, it cannot reflect the failure of the external circuit of the tank (such as the lead line), so it cannot be used as the only protection device to protect the transformer from internal failure.In addition, gas protection is also prone to misoperation under the interference of some external factors (such as earthquake).The gas relay of on-load voltage regulating switch of transformer has the same function as the gas relay of main transformer, different installation positions and different models.The gas relay is installed on the connecting pipeline between the transformer and the oil storage tank. Attention should be paid to the following installation: 1. First, close the disc valve on the gas relay pipeline.If the disc valve is not properly closed or under other circumstances, the oil in the oil pillow can be discharged when necessary to prevent a large amount of oil from spilling out during work.2. Before the installation of the new gas relay, check whether the inspection certificate is available, whether the diameter and flow rate are correct, whether the internal and external parts are damaged, dismantle the internal parts if there is temporary binding, and finally check whether the action of buoy, baffle, signal and trip contact is reliable, and close the vent valve.3. The gas relay should be installed horizontally, and the arrow direction marked on the top cover points to the oil pillow. In engineering, the pipe axis direction of the relay should be slightly higher towards the oil pillow, but the tilt with the horizontal plane should not exceed 4%.4. Open the disc valve and fill the gas relay with oil. After filling with oil, deflate from the vent valve.If the oil pillow with capsules, we should pay attention to the method of filling oil and deflating, to minimize and avoid gas into the oil pillow.5, when carrying out protective wiring, should prevent wrong connection and short circuit, avoid live operation, at the same time to prevent the conductive rod rotation and small porcelain head oil leakage.6. Insulation testing and transmission test should be carried out before operation.6. Test Items of gas relay The following inspection items and test items should be made before installation and use: 1. General inspection items: glass window, vent valve, pin control and outlet terminal are complete without oil seepage, buoy, opening cup and glass window are complete without crack.2. Test Item 2.1. Sealing test: Leakage test shall be conducted at the overall refueling pressure (20mPa, duration 1h), and there shall be no leakage.2.2, terminal insulation strength test: the outlet terminal and the outlet terminal withstand power frequency voltage 2000V, lasting 1min, can also be used to test the insulation resistance of 2500V megohm meter, shaking test 1min instead of power frequency voltage, insulation resistance should be above 300m ω.2.3. Light gas action volume test: When 250 ~ 300cm3 air is accumulated in the shell, light gas should operate reliably.2.4. Heavy gas movement velocity test.Vii. Daily inspection Operation Regulations DL/T572-95 (hereinafter referred to as “regulations”) stipulates that in the daily inspection items of transformers, the presence or non-presence of gas in the gas relay shall be checked first, and the following points shall be paid attention to during gas inspection: 1. The valve on the connecting pipe of the gas relay shall be in the open position.2. The respirator of the transformer should be in normal working condition.3. The input of gas protection connecting piece should be correct.4. The oil level of the oil pillow should be in an appropriate position, and the relay should be filled with oil.5. Waterproof cover of gas relay must be firm.6. Relay wiring terminals should not leak oil, and should be able to prevent the invasion of rain, snow and dust. Power supply and its secondary loop should be waterproof, oil-proof and anti-freezing measures, and waterproof, oil-proof and anti-freezing inspection should be carried out in spring and autumn.Viii. When the gas protected operation transformer is in normal operation, the gas relay works without any abnormality.As for the running state of gas relay, the regulations have the following provisions: 1. When the transformer is running, the gas protection should be connected to the signal and trip, and the gas protection of the on-load tap switch should be connected to the trip.2.1 When two transformers are controlled by a circuit breaker, and one of them is switched to standby, the standby transformer should be switched to standby for heavy gas signal.2.2. When the oil filter, oil refill, submersible change oil pump or the adsorbent of the oil cleaner is replaced and the valve on the connecting pipe of the gas relay is opened and closed.2.3 when working in gas protection and its secondary circuit.2.4 Open the gas, oil discharge and oil intake valves in all other places except for the oil production sample and the vent valve at the upper part of the gas relay.2.5. When the oil level of the oil level meter rises abnormally or the suction system has abnormal phenomena, it is necessary to open the gas or oil discharge valve.3. During the earthquake warning period, the operation mode of heavy gas protection should be determined according to the specific situation of transformer and seismic performance of gas relay.Transformers that are out of operation due to heavy gas protection action caused by earthquake shall be checked and tested for transformer and gas protection before operation, and can be put into operation only after confirming no abnormality.1. Reasons for light gas operation 1.1. Air enters the transformer because the oil filtering, refueling or cooling system is not tight.1.2. The oil level is lower than the light gas buoy of gas relay due to temperature drop or oil leakage 1.3. A small amount of gas is generated due to transformer failure 1.4.Under the action of the through-fault current, the oil flow speed between the oil gap accelerates. When the pressure difference between the oil gap and the outside of the winding changes greatly, the gas relay may misoperate.When the fault current multiple is very large, the winding temperature rises rapidly, which makes the volume of oil expand, resulting in the misoperation of the gas relay.1.5. Gas relay or secondary loop failure.All the factors mentioned above may cause gas protection signal action.1. After the action of the transformer gas protection device, it should be carefully checked, carefully analyzed and correctly judged immediately, and treatment measures should be taken immediately.A. When the gas protection signal is activated, the transformer should be checked immediately to find out the cause of the action, whether it is caused by accumulated air, lower oil level, secondary loop fault or internal fault of the upper transformer.If there is gas in the gas relay, the amount of gas should be recorded, the color of the gas should be observed and whether the test is combustible, and the gas and oil samples should be taken for chromatographic analysis, and the fault nature of the transformer can be determined according to the relevant regulations and guidelines.Chromatographic analysis refers to the collected gas with the chromatograph of hydrogen, oxygen, carbon monoxide, carbon dioxide, methane, ethane, ethylene, acetylene and other gases for qualitative and quantitative analysis, according to the name and content of the component accurately determine the nature of the fault, development trend, and severity.B. If the gas in the gas relay is colorless, odorless and non-flammable and determined as air by chromatographic analysis, the transformer can continue to operate and the intake defect can be eliminated in time.If the gas in the gas relay is flammable and the dissolved gas chromatographic analysis results in the oil are abnormal, the transformer should be comprehensively determined to determine whether the operation is stopped.2. When the gas relay is tripped, the transformer shall not be put into operation before the cause is identified and the fault is eliminated.In order to find out the cause should focus on the following factors, make a comprehensive judgment.A. Whether breathing is obstructed or exhaust is not exhausted;B. Protection and dc secondary circuits are normal;C. Whether the appearance of the transformer is abnormal and obviously reflects the nature of the fault;D. Whether the gas accumulated in the gas relay is flammable;E. Chromatographic analysis results of gas in gas relay and dissolved gas in oil;F. Necessary electrical test results;G. Action of other relay protection devices of transformers.Eleven, the safety measures of gas protection gas protection action, light protection action signal, remind maintenance personnel to deal with the transformer immediately;As a result, the transformer will stop running immediately and the reliability of power supply cannot be guaranteed. Therefore, anti-accident measures for gas protection are put forward: 1. The lower buoy of gas relay is changed to baffle plate and the contact is changed to vertical, so as to improve the reliability of heavy gas operation.2. In order to prevent short circuit of gas relay due to water leakage, rain proof measures should be taken on its terminals and cable lead terminal box.3. Oil-proof wire should be used as the lead wire of gas relay.4. The lead wire and cable of the gas relay should be connected to the terminals in the cable lead terminal box respectively.

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