Inspection, inspection and testing of transformers

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Inspection, inspection and testing of transformers


Installing a transformer is more than just wiring according to the wiring diagram. The first part of the installation process involves the initial inspection and testing of the transformer upon receipt from the factory or warehouse. Once these procedures have been successfully completed, the installation can begin.

Inspection, inspection and testing of transformers

When a transformer arrives at a factory or construction site, there are several things to check before accepting the shipment. For larger power transformers, some electrical tests must be performed to ensure that the transformer unit has been manufactured correctly and is in satisfactory condition. It is best to inspect and test the transformer before installation and before power-on for the first time to ensure that it is in good working order.

A complete drawing of the coils and regulator must be found on the nameplate (or nameplate) of the transformer. The nameplate provides the installer with all data related to the transformer, including ratings, impedances, primary and secondary voltages, phasing, permissible temperature rise, oil type (if used), weight and wiring diagrams. It also lists the manufacturer's name, model number, and serial number.

Inspection of the transformer

The first thing to do is to check for any damage that may have occurred during transportation. Bushings and insulators should be checked for cracks and chips. Exterior finishing needs to be checked. If the paint is worn or scraped off, it must be repaired. A device outdoors or in an aggressive environment will corrode and cause leaks. Cooling fins, if present, should be checked for dents that could affect the performance of the cooling system.

Inspection, inspection and testing of transformers

Insulation resistance

A relatively common problem with transformers is insulation failure between the coils. An insulation resistance test is a test performed to measure the leakage current through the insulation of a coil. When the leakage current is too high, the insulation can fail and short-circuit the coil. The insulation resistance test should be carried out with a high voltage tester or megohmmeter.

Inspection, inspection and testing of transformers

For best results when testing insulation resistance, several conditions must be met. The transformer must not be in operation and must not be connected to any circuits, switches, capacitors, etc. The temperature must be above the dew point of the ambient air to prevent moisture build-up on the surface of the insulation.

The test voltage must be at the required level. Too high voltage can cause overvoltage or damage to the insulation. Each winding should be tested individually, with all other windings being grounded. The transformer manual shall indicate the permissible voltage and the type of test required. A transformer rated at 600V or less can usually be tested with a 500V or 1000V megohmmeter to find leaks to earth or between the primary and secondary windings.

Capacitors are sometimes used for power factor correction and must be discharged or disconnected before testing the transformer. Temperature must be taken into account. The tests should be carried out at a temperature of about 20 ° C.

Winding resistance

A winding resistance test is a test performed to measure the electrical resistance of the windings of a transformer. If the resistance increases, additional heating of the wire that makes up the windings occurs. This could result in a coil fire if the temperature gets high enough to soften or melt the wire.

A precision ohmmeter is used for this test. All windings must be checked, therefore the tap changer of the on-load transformer windings must be switched to all possible positions. For a very large transformer, such as a 20 MVA unit in a substation, this test can take several hours.

The ratio of the turns of the windings

To ensure that all windings are connected and working correctly, a turn ratio test between the primary and secondary windings can be performed. Again, all windings must be checked, so the tap changer of the transformer windings under load must cycle through all its possible positions, since these devices effectively change the transformation ratio of the transformer.

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