The principle, structure and operation and maintenance of transformer gas protection

Gas protection is the main protection for transformer internal faults. It can act sensitively to transformer inter-turn and inter-layer short circuits, iron core faults, internal bushing faults, internal winding disconnection, insulation deterioration and oil level drop. When a fault occurs inside the oil-immersed transformer, the arc will decompose the insulating material and generate a large amount of gas, which flows from the oil tank to the oil pillow, and its intensity varies with the severity of the fault, reflecting the air flow and oil flow The action protection is called gas protection, also called gas protection. During the operation of the transformer, due to internal faults, sometimes we cannot identify and take measures in time, which is easy to cause some accidents. After the protection of the gas relay, the occurrence of similar incidents can be avoided to a certain extent.

October 05, 2022

1. What is gas?

    Gas is formed by the decomposition of cellulose and organic matter by the action of anaerobic bacteria in the early stage of coal accumulation by ancient plants. In a high temperature and high pressure environment, gas continues to be generated due to physical and chemical effects while coal is being formed. Gas is a colorless, odorless gas, but sometimes you can smell an apple-like aroma, which is due to the gushing of aromatic hydrocarbons and gas at the same time. The relative density of gas to air is 0.554, and the density of gas in standard state is 0.716kg/m³. The permeability of gas is 1.6 times that of air. It is insoluble in water, does not support combustion, and cannot maintain breathing. It can cause people to suffocate due to lack of oxygen, and can burn or explode.

The main components of gas are alkanes, of which methane accounts for the vast majority, and a small amount of ethane, propane and butane. In addition, it generally contains hydrogen sulfide, carbon dioxide, nitrogen and moisture, as well as trace amounts of inert gases such as helium and argon. Wait. Under standard conditions, methane to butane exists in gaseous state, and pentane and above are liquid. In case of an open fire, it can burn, and a "gas" explosion will occur, which directly threatens the safety of miners.

2. Definition of Gas Protection

In the gas protection relay, the upper part is a sealed float, the lower part is a metal baffle, and both are equipped with sealed mercury contacts. The pontoons and baffles are rotatable about their respective axes. During normal operation, the relay is filled with oil, the buoy is immersed in the oil, in the floating position, and the mercury contact is disconnected; the baffle sags due to its own weight, and its mercury contact is also disconnected. When there is a slight fault inside the transformer, the gas is generated slowly, and the gas first accumulates in the upper space of the gas relay on the way to the oil conservator, causing the oil level to drop, and the buoy to drop to close the mercury contact, and the on-time delay Signal, this is the so-called "light gas"; when a serious fault occurs inside the transformer, a strong gas gas is generated, and the pressure in the oil tank increases instantaneously, resulting in a large oil flow impacting the direction of the oil pillow, because the oil flow impacts the baffle , The baffle overcomes the resistance of the spring, drives the magnet to move in the direction of the spring contact, closes the mercury contact, turns on the trip circuit, and trips the circuit breaker. This is the so-called "heavy gas". Heavy gas action, immediately cut off all the power connected to the transformer, so as to avoid the expansion of the accident, and play the role of protecting the transformer.

There are different types of gas relays such as float type, baffle type and open cup type. Most of them use QJ-80 relays, the signal circuit is connected to the open cup, and the trip circuit is connected to the baffle. The so-called gas protection signal action means that the signal circuit contact of the upper opening cup in the relay is closed due to various reasons, and the light plate light is on.

Regulations stipulate: For oil-immersed transformers with a capacity of 800kVA and above and oil-immersed transformers with a capacity of 400kVA and above, gas protection should be installed.

3. Classification of gas protection

Gas protection is generally divided into two categories: light gas and heavy gas.

1. Light gas protection: The transformer is overheated or partially discharged, causing the oil temperature of the transformer to rise, generating a certain amount of gas, which is collected in the relay, and when a certain amount is reached, the relay is triggered and a signal is sent.

2. Heavy gas protection: After a serious short circuit occurs in the transformer, it will have an impact on the transformer oil, causing a certain oil flow to rush to the baffle of the relay and tripping.

4. The scope of protection of gas protection

    Gas protection is the main protection of the transformer, it can reflect all faults in the tank. Including: multi-phase short circuit in the fuel tank, short circuit between winding turns, short circuit between the winding and the iron core or the casing, iron core failure, oil level drop or oil leakage, poor contact of the tap changer or poor wire welding, etc. The gas protection action is fast, sensitive and reliable, and the structure is simple. However, it cannot reflect the fault of the external circuit of the oil tank (such as the lead wire), so it cannot be used as the only protection device to protect the internal fault of the transformer. In addition, gas protection is also prone to malfunction under the interference of some external factors (such as earthquakes).

The Buchholz relay of the transformer on-load voltage regulating switch has the same function as the Buchholz relay of the main transformer, with different installation positions and different models.

Five, the installation method of protection

   The Buchholz relay is installed on the connecting pipeline from the transformer to the oil conservator. During installation, attention should be paid to:

1. First, close the butterfly valve on the gas relay pipeline tightly. If the butterfly valve is not tightly closed or there are other conditions, the oil in the oil pillow can be drained if necessary to prevent a large amount of oil from overflowing during work.

2. Before installing the new gas relay, check whether there is an inspection certificate, whether the caliber and flow rate are correct, whether the internal and external parts are damaged, if there is temporary binding inside, it should be disassembled, and finally check the action of the buoy, baffle, signal and trip contact. Is it reliable, and close the bleed valve.

3. The gas relay should be installed horizontally, and the direction of the arrow marked on the top cover points to the oil pillow. In the project, the line axis direction of the relay is allowed to be slightly higher at the end of the oil pillow, but the inclination from the horizontal plane should not exceed 4%.

4. Open the butterfly valve to fill the gas relay with oil, and then release the air from the air release valve after filling with oil. If the oil pillow has a capsule, attention should be paid to the method of filling and deflating the oil to minimize and avoid gas entering the oil pillow.

5. When carrying out protection wiring, it is necessary to prevent wrong connection and short circuit, avoid live operation, and at the same time prevent the conductive rod from rotating and the small porcelain head from leaking oil.

6. Before putting into operation, insulation shaking test and transmission test should be carried out.

6. Test items of gas relays

The following inspection items and test items should be done before the gas relay is installed and used:

1. General inspection items:

The glass window, air release valve, control needle and lead-out terminal are completely without oil leakage, and the float, open cup, glass window, etc. are completely free of cracks.

2. Test items

2.1. Sealing test: the overall oil pressure (pressure is 20mPa, duration is 1h) leak test, there should be no leakage.

2.2. Terminal insulation strength test: The power frequency voltage between the outgoing terminal and the outgoing terminal is 2000v for 1min. The insulation resistance can also be measured by a 2500v megohmmeter. The insulation resistance should be above 300mΩ.

2.3. Light gas action volume test: when 250∽300cm3 of air is accumulated in the shell, the light gas should act reliably.

2.4. Flow velocity test of heavy gas action.

7. Daily inspection

Power transformer operation regulations DL/T572-95 (hereinafter referred to as "regulations") stipulates that in the daily inspection of transformers, the first thing to check is whether there is gas in the gas relay, and the inspection of gas should pay attention to the following points:

1. The valve on the connecting pipe of the gas relay should be in the open position.

2. The respirator of the transformer should be in normal working condition.

3. The gas protection connecting piece should be put in correctly.

4. The oil level of the oil pillow should be in a proper position, and the relay is full of oil.

5. The waterproof cover of the gas relay must be firm.

6. The terminals of the relay should not seep oil, and should be able to prevent the intrusion of rain, snow and dust. The power supply and its secondary circuit should have waterproof, oil-proof and anti-freezing measures, and should be waterproof and oil-proof in spring and autumn. and antifreeze checks.

Eight, the operation of gas protection

When the transformer is in normal operation, the Buchholz relay works without any abnormality. Regarding the operating state of the Buchholz 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-changer should be connected to the trip.

2. When the transformer performs the following work during operation, the heavy gas protection should be reconnected to the signal:

2.1. When using one circuit breaker to control two transformers, when one of them is transferred to standby, the standby transformer should be reconnected to the signal.

2.2. When filtering oil, replenishing oil, replacing submersible oil pump or replacing adsorbent of oil purifier and opening and closing the valve on the connecting pipe of gas relay.

2.3. When working on gas protection and its secondary circuit.

2.4. Except for the oil sample and the air release valve at the upper part of the Buchholz relay, open the air release, oil drain and oil inlet valves in all other places.

2.5 . When the oil level of the oil level gauge rises abnormally or the suction and suction system has abnormal phenomena, it is necessary to open the air release or oil discharge valve.

3. During the earthquake forecast period, the operation mode of heavy gas protection should be determined according to the specific conditions of the transformer and the seismic performance of the gas relay. Transformers that are out of service due to heavy gas protection action due to earthquakes should be checked and tested before they are put into operation, and they can be put into operation only after confirming that there is no abnormality.

9. Action reasons for gas protection

1. Reasons for light gas action

1.1. Air enters into the transformer due to poor oil filter, refueling or cooling system.

1.2. The oil level is lower than the gas relay light gas buoy due to temperature drop or oil leakage

1.3. A small amount of gas is produced by transformer failure

1.4. The transformer has a traversing short-circuit fault. Under the action of the penetrating fault current, the oil flow speed between the oil gaps is accelerated. When the pressure difference between the oil gap and the outside of the winding changes greatly, the gas relay may malfunction. The traversing fault current causes the winding action to heat up. When the fault current multiple is large, the winding temperature rises rapidly, which causes the volume of the oil to expand, causing the gas relay to malfunction.

1.5, gas relay or secondary circuit failure.

The above factors may cause the gas protection signal to act.

10. Treatment after gas protection action

1. After the transformer gas protection device operates, it should be carefully checked, carefully analyzed, and correctly judged, and measures should be taken immediately.

       A. When the gas protection signal is activated, check the transformer immediately to find out the cause of the action, whether it is caused by the accumulation of air, the reduction of oil level, the fault of the secondary circuit or the internal fault of the upper transformer. If there is gas in the gas relay, record the amount of gas, observe the color of the gas and test whether it is flammable, and take gas and oil samples for chromatographic analysis. The fault nature of the transformer can be determined according to relevant regulations and guidelines. Chromatographic analysis refers to the qualitative and quantitative analysis of hydrogen, oxygen, carbon monoxide, carbon dioxide, methane, ethane, ethylene, acetylene and other gases contained in the collected gas using a chromatograph. Accurately determine the nature, development trend, and severity of failures.

      B. If the gas in the gas relay is colorless, odorless and non-flammable, and the chromatographic analysis determines that it is air, the transformer can continue to operate and eliminate the air intake defect in time.

       If the gas in the gas relay is flammable and the chromatographic analysis result of the dissolved gas in the oil is abnormal, a comprehensive judgment should be made to determine whether the transformer is out of service.

      2. When the Buchholz relay trips, the transformer shall not be put into operation until the cause is identified and the fault is eliminated. In order to find out the reasons, the following factors should be mainly considered and a comprehensive judgment should be made.

a. Whether breathing is poor or exhausted;

b. Whether the secondary circuits such as protection and DC are normal;

c. Whether the appearance of the transformer has any abnormal phenomenon that clearly reflects the nature of the fault;

d. Whether the gas accumulated in the gas relay is flammable;

e. The chromatographic analysis results of the gas in the gas relay and the gas dissolved in the oil;

f. Necessary electrical test results;

g. The action of other relay protection devices of the transformer.

11. Safety measures related to gas protection

The gas protection action, the light one sends a protection action signal to remind the maintenance personnel to deal with the transformer immediately; the serious one trips the transformer switch, which causes the transformer to stop running immediately, which cannot guarantee the reliability of the power supply. The anti-accident measures for gas protection are put forward. :

1. Change the lower buoy of the gas relay to the baffle type and the contact to the vertical type to improve the reliability of heavy gas action.

2. In order to prevent the gas relay from being short-circuited due to water leakage, rainproof measures should be taken on its terminals and cable lead terminal boxes.

3. The lead wire of the gas relay should be an oil-proof wire.

4. The lead wires and cables of the gas relay should be connected to the terminals in the cable lead terminal box respectively.

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