Maintenance of measuring voltage transformers

Purpose of measuring voltage transformers

Measuring voltage transformers are used to convert high voltage to low standard values, which is used to power measuring instruments and various relays for protection and automation. They are the same as current transformers isolate measuring devices and relays from high voltage, ensuring the safety of their service.

Device for measuring voltage transformers

According to the principle of the device, the connection scheme and the features of operation, voltage transformers practically do not differ from power transformers. The power of voltage transformers does not exceed tens or hundreds of volt-amperes. At low power, the operating mode of the voltage transformers approaches the idle mode. Opening the secondary winding does not lead to dangerous consequences.

Connection diagrams of voltage measuring transformers

At voltages of 35 kV and below, voltage transformers, as a rule, are connected through fuses, so that if the voltage transformer is damaged, they do not cause the development of accidents. For the safety of personnel, one of the terminals of the secondary winding of the voltage transformers must be grounded.

Operation of voltage measuring transformers

Support voltage transformers and their secondary circuits are carried out by personnel and consist of monitoring the operation of the voltage transformers themselves and monitoring the health of the secondary voltage circuits.

The operation of voltage transformers is controlled during equipment inspections. At the same time, attention is paid to the general condition of the voltage transformer, the presence of oil in them, the absence of discharges and cracking inside the voltage transformer, the absence of overlap traces on the surface of insulators and porcelain covers, the degree of pollution of insulators, the absence of cracks and chips in the insulation, as well as the condition of the reinforcement joints. If cracks are found in the porcelain, the voltage transformers should be disconnected and subjected to detailed inspection and testing.

Voltage transformers for 6 ... 35 kV with a small oil volume do not have expanders and oil indicators. They do not add oil to the cover with 20 ... 30 mm. The resulting space above the oil surface acts as an expander. The detection of traces of oil leakage from such voltage transformers requires immediate removal from service, checking the oil level and eliminating the leak.

During operation, it is necessary to ensure that the safety connections are selected correctly. The reliability of the fuses is guaranteed if the rated current of the fuse is 3 ... 4 times less than the short-circuit current at the most distant point from the voltage transformers of the secondary circuits.

On control panels, it is necessary to systematically monitor the presence of voltage from voltage transformers with the help of voltmeters and signaling devices (panels, signal lamps, bell).

If the secondary voltage disappears due to blown low voltage fuses, they must be replaced and the switched off automatic devices must be switched on.