Needs for Maintenance of Minimum Oil Circuit Breaker (MOCB)
The bulk oil circuit breaker (BOCB) had become obsolete many years ago. Nowadays, the minimum oil circuit breaker (OCB) has also become almost obsolete. But there are many oil circuit breakers still exist in different power systems. Although the authorities are replacing those with SF6 or vacuum circuit breakers time to time. But as long as MOCBs remain in the systems, the maintenance of minimum oil circuit breaker remains essential. So, we should need to know about the matter.
Overview of MOCB
The oil is an insulating and arc quenching medium. In a conventional bulk oil circuit breaker, the oil servers as both insulating and arc quenching medium. But a minimum oil circuit breaker, only utilizes, the arc quenching properly of the oil. The solid core ceramic insulator provides necessary insulation to the circuit breaker. The circuit breaker consists of an oil dashboard. This dashboard is smaller oil chamber. This chamber contains required oil for quenching of arc between the contacts of the CB.
The minimum oil circuit breaker has two portions in its pole. The top portion is a porcelain chamber, contains the oil. The making and breaking of the contacts of the CB occur in this chamber. Whereas the bottom porcelain chamber is hollow, only supports the top chamber. Also, the moving contact lever runs through this bottom chamber.
The main advantage of a minimum oil circuit breaker is that, it uses less quantity of oil than a BOCB. Hence, the risk of fire hazard is low in a MOCB.
Steps for Maintenance of Minimum Oil Circuit Breaker
The following actions are necessary for proper maintenance of minimum oil circuit breaker.
Checking of Breaker Operation
If any doubt arises regarding its operation, we should check the breaker operation . In that case we must check the operation breaker from both remote and local. For that we must operate the circuit breaker electrically from remote (i.e. Control Room) and from local (i.e. switch yard). We check the proper operation of the breaker locally also by pushing its mechanical on off push buttons or on off levers.If we found any coating developed on the sliding surface, we must remove it before we put the CB in the service again.
Checking the Contacts
Also after a doubtful operation of the MOCB, we should check the contacts of the MOCB, for any burning sign. Also we should not forget to loosen and tighten the tip before we finally tight it.
Checking the Arc Quenching Chamber
In addition, we should check the condition of arc quenching chamber. If we found any burning spot on the chamber body, we must remove it from the breaker. Then we rinse it with clean transformer oil. After that, only we refit the chamber inside the breaking unit. But if the extinguisher, chamber is in quite bad condition, we must replace it.
Cleaning the Circuit Breaker
We should clean the entire CB, including its mechanism box, gear assembly, insulators, breaker heads, power contacts, etc with non fluffy cloths. If there is any rust in any portion of the mechanism, we must remove it. After cleaning the gears and moving assemblies, we must properly lubricate them with proper grade grease. Also, if there is any carbon deposition on the insulator body, we should clean it with trichloro ethylene or acetone. During lubricating, we should remember to avoid over greasing of gear wheels. Also we must keep the friction clutch, free from lubrication. Also, we must lubricate the sliding parts, catches, rollers and operating magnet with lubricating oil not by grease. Lastly, we must also follow the greasing and lubricating procedures provided in the user manual of the MOCB.
Checking and Replacing Locking Elements
We must check all the locking elements of tie rod assembly as well as of the other portions of the breaker mechanism. If we find any damaged or slightly damaged locking element, we must immediately replace it with new one. Although after every overhauling of the MOCB, we must replace all the locking elements, even these are not damaged.
Tightening Nuts Bolts
During operations of circuit breaker, there may always be a mechanical shaking. These mechanical shaking may cause loosing of foundation bolt and other hardware bolts. So, we should check the tightness of all nuts and bolts during the maintenance of minimum oil circuit breaker. If there is any loose bolt, we need to immediately tight it.
Checking Power Connections
After that we must check the power connections of the CB. If there any oxidation, we should remove the oxide before re-tight it. We must always use spring washers where there is a chance of loose connection during heavy electrical over impulses.
Checking the Auxiliary Switch
We must also check cleanliness of auxiliary switch during routine maintenance. If any broken part is found we should replace it. Also we need to ensure the proper configuration of NOs and NCs after maintenance.
Checking and Replacing Motor Brushes
The brushes of spring charging motor may get corroded due to excessive friction. We must replace these brushes.
Checking the Oil Level
We need to check the oil level the MOCB poles, on the oil level glass fitted at the top head of the poles, on monthly basis. Also we need to search for any visible oil leakage on the CB pole. If the oil level becomes low, we must top up with new oil. Again, if there is any visible oil leakage, we must plug it with proper action.
Checking Paintings and Gaskets
Quarterly we inspect the MOCB for any defects in paintings and door gaskets of mechanism kiosk. If we ignore these, there may be permanent holes in the kiosk. Also, we must replace or repair the damaged gasket without neglecting it.
Checking Duty Cycles
In addition to all above mentioned steps, we should also check the duty cycle of the breaker operation at yearly maintenance of minimum oil circuit breaker.