The Complete Guide to Power Transformer Failure:Reasons and Methods

Power transformers may encounter their own accidents during operation, which will lead to power transformer failures. This article mainly describes how to prevent foreign matter from entering the power transformer, prevent insulation damage, and prevent overvoltage breakdown.

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Prevent foreign objects from entering the power transformer

In addition to the special regulations of the manufacturer, the oil tank should be inspected and cleaned when installing the power transformer.

If necessary, check and remove foreign objects at the bottom of the tank.

For power transformers with oriented cooling, care should be taken to remove foreign objects in the oil inlet pipe and header.

When installing or replacing the cooler of the power transformer, the oil pipeline, the cooler, and the inside of the submersible pump must be repeatedly flushed with qualified insulating oil until the flushed oil test is qualified and there is no foreign matter.

If there are too many foreign objects, further inspection and treatment should be carried out.

It is necessary to prevent the activated alumina or silica gel powder in the oil purifier device from entering the power transformer.

Oil purifiers should not be used for fully enclosed power transformers.

The operating unit should check the filter screen and replace the adsorbent regularly.

The submersible pump should use E-class bearings with good wear resistance, and non-class bearings are prohibited.

When possible, the upper bearing should be changed to a centripetal thrust ball bearing.

It is recommended to use a low-speed oil pump with a speed not greater than 1000r/min.

The high-speed oil pump with a speed of 3000r/min should be replaced.

For the disc motor oil pump, attention should be paid to the adjustment of the gap between the stator and the rotor to prevent the surface friction of the iron core.

In case of abnormalities such as overheating, vibration, noise, and severe oil leakage during operation, the operation should be shut down for maintenance.

Localities should reasonably arrange regular inspections and repairs of submersible pumps based on the actual operating conditions of the equipment.

After the internal fault of the power transformer is tripped, the oil pump should be cut off to prevent foreign matter such as free carbon and metal particles generated by the fault from entering the non-faulty parts of the power transformer.

When installing, overhauling the hanging cover, or entering and inspecting, in addition to shortening the time the body is exposed to air, it is also necessary to prevent foreign objects such as tools and materials from being left in the power transformer.

When performing vacuum oil treatment, prevent metal powder or foreign matter from entering the power transformer due to wear of the bearings of the vacuum oil filter or damage to the filter screen.

In order to prevent the vacuum pump from lubricating oil from being sucked into the power transformer body when the vacuum pump is out of service or failure, the vacuum system should be equipped with a check valve or buffer tank.

When the oil flow relay indicator is abnormal during operation, pay attention to check whether the oil flow relay baffle plate is damaged or falling off.

Prevent insulation damage

The insulation damage of the power transformer should be prevented during the suspension inspection and internal inspection.

When installing power transformer cable bushings, the lead wires should be prevented from kinking, and the lead wires should not be lifted with excessive force.

If the lead is too long or too short, find out the cause and deal with it.

It is strictly forbidden to step on the lead wire and insulating support during maintenance.

When insulators need to be replaced during installation or maintenance, materials, and components that meet the requirements of the manufacturer and passed inspection shall be used and dried.

Prevent high coil temperature, insulation deterioration, or burnout

The overload operation of power transformers should be implemented in accordance with GB/T5164-94 “Guidelines for the Load of Oil-immersed Power Transformers” and DL/T572-95 “Power Transformer Operation Regulations”.

If the overload operation time is short, for example, less than 5min, it can be treated as an emergency.

The manufacturer should be required to provide the load capacity table of the power transformer.

The load capacity table will be executed after the user checks.

The hot spot temperature of the power transformer in operation shall not exceed the limit and specific limits specified in GB/T15164-94.

In actual operation, the load current and top oil temperature should be monitored.

When the heat dissipation effect of the radiator (cooler) is reduced due to the external dirt of the radiator (cooler) and the decrease in the efficiency of the oil pump, the allowable overload time shall be appropriately shortened.

The air cooler of the power transformer is externally flushed with compressed air or water every 1-2 years to ensure the cooling effect.

When the power transformer is defective or the insulation is abnormal, it shall not exceed the rated current to operate and strengthen the operation monitoring.

For power transformers whose load capacity is suspected or modified, temperature rise tests should be carried out if necessary to determine their load capacity.

When it is suspected that the coil is locally overheated (such as caused by insulation expansion or blockage of the oil passage), the limit output can be reduced as appropriate.

When the cooling system of a strong oil circulating power transformer fails, the allowable load level, duration, and top oil temperature of the power transformer shall comply with the power transformer operating regulations or the manufacturer’s regulations.

The cooling system with strong oil circulation must have two independent power sources and be equipped with automatic switching devices.

To switch tests regularly. The signal device should be complete and reliable.

The fan blades of the cooler should be balanced and the angle should be adjusted.

Pay attention to regular maintenance to ensure normal operation.

The fan motor with large vibration and severe wear should be replaced.

For power transformers with a long-running, high-temperature rise or long-term overload, the furfural content in the oil can be measured to determine the degree of insulation aging.

If necessary, paper samples can be taken to measure the degree of polymerization for insulation aging identification.

Power Transformer failure

Prevent over-voltage breakdown accidents

The neutral point ungrounded power transformer operating in the neutral point effective grounding system must be installed with a reliable overvoltage in order to prevent the neutral point displacement overvoltage during the commissioning, shutdown, and accidental tripping process. protect.

When switching the no-load power transformer, the neutral point must be reliably grounded.

The power transformer should be protected by a zinc oxide lightning arrester.

For power transformers connected to GIS through long cables (or gas-insulated cables), in order to avoid damage to the inter-turn insulation at the first end of the high-voltage coil due to VFTO (VFTO), in addition to requiring the manufacturer to take relevant measures (Such as increasing the equivalent capacitance at the entrance of the transformer, etc.), the operation mode of “live cold standby” should be adopted during operation (that is after the circuit breaker is opened, its bus-side switch is kept in the closed state) to reduce the empty switching.

The probability of VFTO generated by the load bus.

The neutral point of the power transformer shall have two grounding down conductors connected to different locations of the main grounding grid, and each grounding down conductor shall meet the thermal stability requirements.

Prevent breakdown accidents under working voltage

For newly installed power transformers with voltage levels of 220kV and above, partial discharge tests should be carried out in accordance with national standards or industry standards during the handover test, and the test voltage applied to the turns and the main insulation is 1.5 times the maximum voltage of the equipment.

Newly installed power transformers with 110kV voltage levels can be implemented by comparison.

A partial discharge test should also be carried out on power transformers that have been overhauled and replaced with insulating parts or partial coils and dried.

In the test, the limit level of the partial discharge amount should be used as another control index of the test.

It should not only be required by whether the test voltage meets the voltage value specified in the “Preventive Test Regulations for Power Equipment” (DL/T596-1996).

In order to avoid the expansion of defects due to the excessively high applied voltage.

When the oil chromatogram of the power transformer in operation is abnormal and the equipment is suspected of discharging faults, a partial discharge test can be carried out. The principle of the test is the same as 1.6.2.

When acetylene appears in the insulating oil of 500kV (including 330kV) power transformers and shunt reactors, the monitoring period should be shortened immediately and the trend of monitoring changes should be tracked.

For 500kV (including 330kV) shunt reactors, when the combustible gas in the oil increases and a small amount of acetylene is produced, but the acetylene content tends to stabilize, it can be treated differently, and the operating limit can be appropriately relaxed.

However, the reasons should be found out and attention should be paid to the changes in the gas content in the oil.

For three-phase power transformers with voltage levels of 220kV and above, based on operating experience and monitoring results, if there is suspected of a dendritic discharge fault in the enclosure, the enclosure should be unwrapped and inspected visually when the ceiling is overhauled.

For thin-insulated power transformers, technical supervision can be carried out as general power transformer equipment.

If a serious defect is found, the power transformer body should not be renovated and overhauled, and the replaced thin-insulated power transformer should not be relocated and installed.

Prevent the protection device from malfunctioning or refusing to move

The protection device of the power transformer must be complete and reliable, and it is strictly forbidden to put the power transformer and the low-voltage side equipment of the power transformer into operation without protection. When the protection device is stopped for a short time due to work needs, corresponding safety protection measures should be formulated, and the power transformer protection device should be restored to use immediately after the work is completed.

The gas relay should be installed and adjusted correctly and checked regularly to eliminate the malfunctioning factors caused by the short circuit of the contact. If a rain cover is installed, the contact point can be prevented from being moved by damp.

Prevent the improper installation of the oil conservator system from causing fuel injection, false oil surface, or malfunction of the protection device.

The action contact of the pressure relief valve should be connected to the signal circuit. When connecting the action contact of the pressure release valve to the trip circuit as required, there should be a complete anti-misoperation measure: for example, the action contacts of two pressure release devices on the same equipment are connected in series, and the contact box is equipped with moisture-proof measures.

The action contact of the coil thermometer and the top oil thermometer should be connected to the signal, and it is not suitable to be connected to the trip.

The main power transformer of 220kV and above should be equipped with a fault recorder to record the main transformer current, voltage, phase, duration, and other parameters under fault conditions to improve the quality of accident analysis and provide a reliable basis for formulating preventive measures.

The high- and low-voltage side backup protection of 220kV and above power transformers should be powered by different DC power sources to prevent the backup protection from being completely paralyzed when the DC is lost for some reason, and continuous failures for a long time and expanding the consequences of the accident.

About Bin Dong

Hello, I am Bin, General manager of Daelim which is a leading transformer manufacturer. If you have problems when you are looking for the equipment, what you need to do is tell us.

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