Analysis of Oil Immersed Transformer Fault

In today's industrialized society, power supply is closely related to the production of industrial facilities, the operation of national defense and military facilities, the development of scientific and technological research, and the normal conduct of people's lives. With the improvement of people's living standard, the demand for energy is increasing, so the transformer load increases with the increase of accidents. Transformer accident is harmful to the safe operation of power grid. so it is necessary to find fault and eliminate the fault in time, so that the power grid can be unblocked.

The operation condition of oil immersed transformer has direct influence on the power supply quality of the whole power system. In the actual operation, due to various factors, the transformer will inevitably appear in the course of a variety of faults. These failures lead to power outages, heavy equipment, equipment outages, and even major power accidents. Therefore, it is necessary to make a scientific analysis of the oil immersed transformer fault.

1.Fault analysis of abnormal sound of transformer

When the transformer is in normal operation, its sound mainly comes from two aspects: the transformer body and the cooling system. The vibration of iron core caused by magnetic and magnetic leakage of silicon steel sheet is the main cause of vibration of transformer body. When the following faults occur, the transformer will produce abnormal sound:

(1) The core bolt of the iron core is not firmly fixed, which will lead to loose core and cause irregular vibration between the silicon steel sheets, thus causing strong and uneven noise.

(2) When the winding flashover discharge or the grounding wire of the iron core is broken, it will cause the iron core to discharge the high voltage outer shell, and cause the discharge sound inside the transformer.

(3) A few parts of transformer is not firmly fixed, within the transformer there is iron tapping sound. 
(4) In the case of light load or empty load, some of the end of the laminated silicon steel sheet will vibrate, and there will be a mosquito flapping sound inside the transformer.

(5) If there is a breakdown in the transformer. There will be a big and small discharge sound.

2. Analysis of Oil immersed transformer oil level and oil temperature abnormal fault

(1) The oil level of transformer is too high or too low

The oil level of the transformer is affected by the following factors: a. internal oil content  b.oil temperature  c.transformer load  d.ambient temperature  e.transformer sealing and so on. Oil pillow volume is generally about 10% of the transformer volume. If the oil level is too high, easy to cause oil spill. If the oil level is too low, the transformer leads to the wiring part to be exposed to the air, which reduces the insulation effect and may cause an internal short circuit. At the same time, as the oil and air contact area becomes larger, the insulation strength of the oil decreases rapidly. In case of power failure or low temperature in winter, the oil level will drop rapidly, triggering the action of light gas relay. From the above analysis, we can see that the transformer must keep the normal oil level in operation. Operators must always check the level of oil level gauge, and take corresponding measures to maintain normal oil level, to ensure the safe operation of the transformer.

(2) Oil leakage fault of transformer

Transformer oil has a strong penetration force, usually after the infiltration of oil seal for poor sealing or welding point welding two categories.

To achieve the prevention of transformer oil, first, need to carefully check the record of the original data, and experience of all seal welds and check the transformer oil, according to find all leaks; second, the leakage point welding in time; third, material, try to choose the installation process of high quality seals and improved transformer.

(3) oil temperature continues to rise

Transformer in the daily operation of the oil temperature rising phenomenon, indicating that the transformer internal problems. The main reason is the iron core overheating or winding turn to turn short-circuit. The bolt clamping core insulation or eddy current caused by the destruction of the core overheating. The eddy current will keep the core temperature high for a long time, resulting in iron loss and rising oil temperature. Through that bolt insulation was destroyed will cause overheating of the screw. When the temperature rises to the extent that the iron core is fused together, the transformer should be cut off immediately so as to avoid a major accident such as fire or explosion.

3. Analysis of oil immersed transformer circuit fault

(1) Insulation material damage caused by failure

First, when the transformer is installed, the cover is checked and exposed to air, causing moisture. The vacuum treatment is not complete before oiling, so that the moisture is concentrated around the insulating parts. Second, maybe due to equipment manufacturing cycle is short, if the insulation factory is not fully dry, insulating paper and board out after running for a period of time, water absorption water, resulting in low voltage winding of the transformer insulation performance is reduced, resulting in DC leakage current increases. Third, poor sealing, easy to cause winding insulation part of the damp. Fourth, if the volume of hydrogen and carbon monoxide in the transformer oil is relatively high, the adsorption of the gas produced by the solid insulating material or stainless steel in the manufacturing process should be taken into account.

Insulation damp is one of the main causes of insulation failure, so it is necessary to strengthen the prevention and supervision work, do a good job of checking the insulation effect of the important components of the transformer, and find out and eliminate the fault as soon as possible. When installing and overhauling transformers, minimize the time that the transformer is exposed to the atmosphere. In the production of transformers, the standard of drying process should be strictly carried out, and the moisture content of insulating paper (sheet) shall be controlled within 0.3. The leakage points of the transformer body and accessories should be treated in time to avoid infiltration of moisture into the transformer. Improve design and manufacturing processes, and monitor material selection.

(2)Influence of discharge fault on transformer insulation

The influence of the discharge on the transformer insulation is divided into two kinds of damage: a. the discharge particle directly impacts the insulation and makes the insulation breakdown. b. ozone and nitrogen oxides generated by the discharge, chemical corrosion, insulation components, and finally lead to thermal breakdown.

The main form of aging of insulating materials is  a. the damage of chemical structure of insulating materials is caused by partial discharge. b.the thermal effect of the discharge point causes the thermal cracking of the insulation and accelerates the aging process. c.the ozone and nitrogen oxides generated by the discharge process corrode the insulator by chemical reaction and weaken the insulation performance. (d.during the discharge process, the transformer oil decomposes to produce bubbles, which make it difficult to heat the solid insulation and cause damage to the solid insulation.

(3)Partial discharge fault

Within the insulation structure, the phenomenon of non - penetrating discharge at the edge of the hole is known as partial discharge under the action of voltage. The reason is: a hole or cavity in transformer oil gas, gas of low dielectric constant reduces the compressive strength of insulating material, causing the discharge gap between the transformer oil; processing net, will make the bubble in the transformer oil caused by discharge; discharge between metal parts or conductive contact will cause the bad. Although the energy intensity of partial discharge is not large, but the potential harm can not be overlooked, if long-term partial discharge will cause breakdown or damage to equipment, is a serious safety hazard.

(4)analysis of automatic trip fault

In case of standby transformer, if the main transformer is automatic tripping, the working transformer should be deactivated immediately, and the standby transformer can be used at the same time. To check the cause of automatic tripping of the transformer. If internal trouble is found during inspection, internal inspection should be carried out immediately. If it is found that the tripping is caused by differential protection, the electrical equipment in the differential protection range shall be checked and power transmission shall be carried out after troubleshooting.

4.Epilogue

According to the above fault analysis, the staff should monitor the sound, the oil level and the circuit condition of the transformer. According to the characteristics of the relevant fault, make timely and ready analysis, and quickly formulate the corresponding repair plan, to repair the fault at the fastest speed. In short, when the oil immersed transformer after the failure, the staff should do the analysis according to the real-time fault situation, and formulate the corresponding countermeasures, eliminating the trouble in the bud, to ensure the safe and stable operation of transformer.

Matters that need attention and are about the operation and off-stream of power transformer

  Transformer is outage for one month or put aside and outage for more than 6 months. 

Transformer

What test should be done before it is put into use?

1. If the transformer is outage for one month, we should use the insulation resistance tester 

DC Winding Resistance Meter

to measure insulation before resumption of power.

2. If the transformer is outage for more than 6 months, we should do insulation resistance test and insulation oil pressure test.

3. If the special transformer of drainage and irrigation which is in a dry and cold area, the outage period may be appropriately extended, but not more than eight months.

What are the requirements for transformer operation and outage?

(1) The new transformer must be tested at rated voltage for 5 times, and after the overhaul, the impulse voltage test should be carried out at least 3 times.

500kV25KJ Impulse Voltage Generator 

(2) when the transformer is put into operation, the cooler should be put into use first, and the cooler runs for a period of time (about 15min).

(3) In the 110kV and above neutral grounding system, when the transformer is put into operation and shutdown, the neutral point must be grounded first.Transformer neutral point of the arc suppression coil should be returned and then investted. The neutral point of the two transformers shall not be connected to the neutral bus of an arc suppression coil at the same time.

(4) The power operation, stop the load side of the switch, and then stop switch power supply side (side power from low to high stop); the first pull transformer side switches, pull the bus side gate. Power supply operation is opposite.

(5) Putting into standby transformer should be based on the actual location of the device and meter instructions to determine whether it is carried out the load.  the load has been carried out in order to enable the operation of the transformer power outage. The transformer on the diagonal and the 3/2 connection, although the transformer has been cut off, but the heavy gas and differential protection of the transformer can still cause the closing of the switch. It shauled be based on the actual situation and site regulations to change the location of the gas protection signal or gas out.

(6) The station transformer is not allowed long-term coordination. It can be used to cut the low pressure side of the low voltage side of the circulation with high pressure knife gate cut off the transformer station.

How to dry transformer?

Transformer is the principle of electromagnetic induction to change the AC voltage of the device. The main components are the primary coil, secondary coil and iron core (magnetic core).

The purpose of drying transformer is to remove water from the transformer insulation material and increase the insulation resistance to improve its flashover voltage. Transformers above 3kV must be dried. The transformer body is mainly composed of an iron core , a coil and an insulating material. After assembled, transformer must through the drying process to remove the water and gas of insulating materials before joining the transformer oil. It is to ensure the transformer insulation strength and service life enough. For high voltage transformer, the insulation material should be less than 0.5%. 

1. Induction heating method

The instrument itself is placed in the tank, the use of fuel tank wall eddy current loss of heat to dry. At this time the wall temperature should not exceed 115-120 degrees, the body temperature should not exceed 90-95. In order to winding the coil of convenience, as far as possible the number of turns of the coil less or less current, the wire can be  35-50mm2 wire, the general current is 150A. The oil tank wall can be filled with a plurality of asbestos bars, and the wire is wound on the asbestos strip.

2. Hot-air drying

The transformer itself is placed in a dry room through the hot air drying. The inlet hot air temperature should be increased gradually, and the filter should be installed at the inlet of hot air to prevent the entry of Mars and dust. The maximum temperature should not exceed 95. Hot air blowing device not directly, as uniform as possible from the body of the wind in all directions, so that the moisture released by the box cover hole.

Problems should be paid attention to in transformer drying:

(1) If the drying chamber is not vacuum, the vent should be opened on the lid of the box or the throttle hole can be used to make the moisture release.

(2) When the oil is heated, the insulation layer should be installed outside the tank. The insulating layer can be made of insulating material such as asbestos cloth, glass cloth and so on.

(3) In order to improve the drying quality of windings, there are two major factors to be considered: the first is to control the drying temperature; the other is to improve the vacuum of the equipment. On the first point, the general drying equipment can meet the technical requirements, to second points, subject to a number of factors, must be taken into account, reasonable arrangements in order to achieve good drying effect.

(4) The process of vacuum drying, drying in low temperature stage, not in the vacuum or low vacuum drying conditions, otherwise it is not conducive to the elimination of core temperature and moisture. When the temperature rose to 70 to 80 to improve the vacuum degree. When drying is carried out from 1 to 2h, the water vapor in the oil tank is more, the heat radiation ability is improved, the internal temperature tends to be uniform, the water is gradually reduced, and the heat radiation ability is reduced.

      (5)A method of identification of insulation after drying and transformer technical specifications for the test.

 After the transformer is dry, it is necessary to use insulation resistance tester to make a comprehensive identification of its insulation performance to check its drying effect. Identification of the project, in addition to casing, the rest are the same as the hoisting of the heart of the test items.