Power Classroom | Maintenance and Fault Repair Methods of Transformers

A transformer is a device that uses the principle of electromagnetic induction to change the alternating voltage. Its main components are the primary coil, secondary coil, and iron core (magnetic core). Its main functions include voltage transformation, current transformation, impedance transformation, isolation, and voltage regulation (magnetic saturation transformer), etc. According to its purposes, it can be divided into power transformers and special transformers (such as electric furnace transformers, rectifier transformers, power frequency test transformers, voltage regulators, mine transformers, audio transformers, intermediate frequency transformers, high-frequency transformers, impulse transformers, instrument transformers, electronic transformers, reactors, instrument transformers, etc.).

The maintenance of transformers is the daily work that electricians and electricians who are also responsible for repairs must carry out to maintain the normal technical condition of transformers and extend their service life. The maintenance of transformers is an important part of electrical equipment management. If the maintenance work is done properly, it can not only reduce the equipment failure rate, save maintenance costs, and reduce costs, but also bring good economic benefits to the company and employees. So, what matters should we pay attention to in the maintenance and repair of transformers?

During normal operation, the electrical load borne by the transformer should be about 75-90% of the rated capacity of the transformer.

The thermometer installed on the transformer should be recorded at the same time when inspecting the transformer. For transformers without on-site operators, the voltage, current, and upper oil temperature of the transformer should be recorded during each regular inspection. In addition, for distribution transformers, the load of three phases should be measured during the period of maximum load. If the distribution is unbalanced, it should be redistributed. The measurement period should be specified in the on-site regulations.

Before a transformer is put into operation after installation or maintenance (usually after drying) and after a long-term outage, the insulation resistance of the coils should be measured, and the measured value and the oil temperature at the time of measurement should be recorded in the transformer's resume card.
A megohmmeter with a voltage of 1000-2500 volts should be used to measure the insulation resistance of the coils. The allowable value of the insulation resistance of the coils is not specified.
The ratio of the insulation resistance value measured during the operation of the transformer to the value measured before the transformer is put into operation after installation or major overhaul and drying is the main basis for judging the insulation status of the transformer during operation. The measurement of the insulation resistance should be carried out as far as possible at the same temperature using a megohmmeter with the same voltage.

The maximum unbalanced current at the low voltage side of the transformer shall not exceed 25% of the rated value; the allowable range of the change in the power supply voltage of the transformer is ±5% of the rated voltage. If this range is exceeded, the tap changer should be used for adjustment to make the voltage reach the specified range.
(The adjustment should be carried out when the power is cut off.) Usually, the voltage regulation is achieved by changing the position of the tapping taps of the primary winding. The device for connecting and switching the positions of the tapping taps is called a tap changer, which adjusts the transformation ratio by changing the number of turns of the high-voltage winding of the transformer.
A low voltage has no impact on the transformer itself, only reducing its output to some extent, but it has an impact on the electrical equipment; when the voltage increases, the magnetic flux increases, the iron core is saturated, the iron core loss increases, and the temperature of the transformer rises.

In special cases, the transformer can operate overload for a short time, but it shall not exceed 30% of the rated load in winter and 15% of the rated load in summer. In addition, the overload capacity of the transformer should be determined according to the temperature rise of the transformer and the regulations of the manufacturer.
Overload is divided into two situations: normal overload and accident overload. Normal overload occurs in normal power supply situations due to an increase in the power consumption of users. It will cause the temperature of the transformer to rise, leading to the accelerated aging of the insulation of the transformer and a reduction in its service life. Therefore, overload operation is generally not allowed.

There are many types of transformer faults, but the common fault phenomena are nothing more than short circuits, open circuits, and leakage. The reasons for these faults can be summarized into three aspects: first, there are design errors; second, the manufacturing quality is poor; third, the operating conditions exceed the design requirements.

Transformer Repair Method:
Step 1: Dismantle the Iron Core
Transformers are often filled with materials such as asphalt and wax. Before disassembly, it should be heated to make these materials melt and separate from the transformer. In addition, the transformer leads should be soldered off from the terminals, the fixing splints should be removed, the first piece of silicon steel sheet should be pried up with a screwdriver, and then the silicon steel sheets should be pulled out one by one with pliers. When taking out the silicon steel sheets, care must be taken not to damage the coil leads, and at the same time, avoid breaking and bending the silicon steel sheets. Then, collect the taken-out silicon steel sheets together and wrap them with paper to prevent loss and damage to the insulation.

Step 2: Dismantle the Coils
After taking out the iron core, first conduct an inspection of the appearance of the coils. If there is no visible damage, then use an insulation resistance tester again to check which coil the fault occurs in, so as to disassemble it purposefully. When disassembling, pay attention to recording the interlayer insulation, wire diameter, the number of turns per layer, and the number of layers, etc., for reference when rewinding. At the same time, attention should be paid to protecting the surface of the wire insulation paint for reuse; special attention should also be paid to finding and identifying the causes of coil short circuits and open circuits. After finding and dealing with the faulty part, it can be rewound according to the original winding method. After winding and repair, a comprehensive test and inspection should be carried out. It can be used continuously only after all indicators meet the requirements.