Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled into metal-enclosed switchgear line ups for indoor use, or may be individual components installed outdoors in a substation. Air-break circuit breakers replaced oil-filled units for indoor applications, but are now themselves being replaced by vacuum circuit breakers (up to about 35 kV). Medium voltage circuit breakers which operate in the range of 600 to 15,000 volts should be inspected and maintained annually or after every 2,000 operations, whichever comes first.
The above maintenance schedule is recommended by the applicable standards to achieve required performance from the breakers.
Maintenance procedures include the safety practices indicated in the ROMSS (Reclamation Operation & Maintenance Safety Standards) and following points that require special attention.
- Be sure the circuit breaker and its mechanism are disconnected from all electric power, both high voltage and control voltage, before it is inspected or repaired.
- Exhaust the pressure from air receiver of any compressed air circuit breaker before it is inspected or repaired.
- After the circuit breaker has been disconnected from the electrical power, attach the grounding leads properly before touching any of the circuit breaker parts.
- Do no lay tools down on the equipment while working on it as they may be forgotten when the equipment is placed back in service.
Maintenance Procedures For:
Medium Voltage Air Circuit Breakers
The following suggestions are for use in conjunction with manufacturer’s instruction books for the maintenance of medium voltage air circuit breakers:
- Clean the insulating parts including the bushings.
- Check the alignment and condition of movable and stationary contacts and adjust them per the manufacturer’s data.
- See that bolts, nuts, washers, cotter pins, and all terminal connections are in place and tight.
- Check arc chutes for damage and replace damaged parts.
- Clean and lubricate the operating mechanism and adjust it as described in the instruction book. If the operating mechanism cannot be brought into specified tolerances, it will usually indicate excessive wear and the need for a complete overhaul.
- Check, after servicing, circuit breaker to verify that contacts move to the fully opened and fully closed positions, that there is an absence of friction or binding, and that electrical operation is functional.
Medium Voltage Oil Circuit Breakers
The following suggestions are for use in conjunction with the manufacturer’s instruction books for the maintenance of medium-voltage oil circuit breakers:
- Check the condition, alignment, and adjustment of the contacts.
- Thoroughly clean the tank and other parts which have been in con tact with the oil.
- Test the dielectric strength of the oil and filter or replace the oil if the dielectric strength is less than 22 kV. The oil should be filtered or replaced whenever a visual inspection shows an excessive amount of carbon, even if the dielectric strength is satisfactory.
- Check breaker and operating mechanisms for loose hardware and missing or broken cotter pins, retain ing rings, etc.
- Adjust breaker as indicated in instruction book.
- Clean and lubricate operating mechanism.
- Before replacing the tank, check to see there is no friction or binding that would hinder the breaker’s operation. Also check the electrical operation. Avoid operating the breaker any more than necessary without oil in the tank as it is designed to operate in oil and mechanical damage can result from excessive operation without it.
- When replacing the tank and refilling it with oil, be sure the gaskets are undamaged and all nuts and valves are tightened properly to prevent leak age.
Medium Voltage Vacuum Circuit Breakers
Direct inspection of the primary contacts is not possible as they are enclosed in vacuum containers. The operating mechanisms are similar to the breakers discussed earlier and may be maintained in the same manner. The following two maintenance checks are suggested for the primary contacts:
- Measuring the change in external shaft position after a period of use can indicate extent of contact erosion. Consult the manufacturer’s instruction book.
- Condition of the vacuum can be checked by a hipot test. Consult the manufacturer’s instruction book.