Maintenance Of Meduim Voltage Circuit Breakers
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.
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Safety Practices
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.
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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.
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Maintenance Procedures For 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.
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Maintenance Procedures For 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.
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SOURCE: MAINTENANCE OF POWER CIRCUIT BREAKERS by HYDROELECTRIC RESEARCH AND TECHNICAL SERVICES GROUP
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Maintenance Of Low Voltage Circuit Breakers
The deterioration of low voltage circuit breaker is normal and this process begins as soon as the circuit breaker is installed. If deterioration is not checked, it can cause failures and malfunctions. The purpose of an electrical preventive maintenance and testing program should be to recognize these factors and provide means for correcting them.
A good organized maintenance program can minimize accidents, reduce unplanned shutdowns and lenghten the mean time between failures of electrical equipment.
Benefits of good electrical equipment maintenance can be reduced cost of process shutdown (caused by circuit breaker failure), reduced cost of repairs, reduced downtime of equipment, improved safety of personnel and property.
Frequency Of Maintenance
Low-voltage circuit breakers operating at 600 volts alternating current and below should be inspected and maintained very 1 to 3 years, depending on their service and operating conditions. Conditions that make frequency maintenance and inspection necessary are:
- High humidity and high ambient temperature.
- Dusty or dirty atmosphere.
- Corrosive atmosphere.
- Frequent switching operations.
- Frequent fault operations.
- Older equipment.
A breaker should be inspected and maintained if necessary whenever it has interrupted current at or near its rated capacity.
Maintenance Procedures
Manufacturer’s instructions for each cir cuit breaker should be carefully read and followed. The following are general pro cedures that should be followed in the maintenance of low-voltage air circuit breakers:
- An initial check of the breaker should be made in the TEST position prior to withdrawing it from to enclosure.
- Insulating parts, including bushings, should be wiped clean of dust and smoke.
- The alignment and condition of the movable and stationary contacts should be checked and adjusted according to the manufacturer’s instruction book.
- Check arc chutes and replaces any damaged parts.
- Inspect breaker operating mechanism for loose hardware and missing or broken cotter pins, etc. Examine cam, latch, and roller surfaces for damage or wear.
- Clean and relubricate operating mechanism with a light machine oil (SAE-20 or 30) for pins and bearings and with a nonhardening grease for the wearing surfaces of cams, rollers, etc.
- Set breaker operating mechanism adjustments as described in the manufacturer’s instruction book. If these adjustments cannot be made within the specified tolerances, it may indicate excessive wear and the need for a complete overhaul.
- Replace contacts if badly worn or burned and check control device for freedom of operation.
- Inspect wiring connections for tightness.
- Check after servicing circuit breaker to verify the 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.
Much of the essence of effective electrical equipment preventive maintenance can be sumarrized by four rules:
- Keep it DRY
- Keep it CLEAN
- Keep it COOL
- Keep it TIGHT
SOURCES:
- MAINTENANCE OF POWER CIRCUIT BREAKERS by HYDROELECTRIC RESEARCH AND TECHNICAL SERVICES GROUP
- ELECTRICAL POWER EQUIPMENT MAINTENANCE AND TESTING – By Paul Gill
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