This breaker uses SF6 or sulfur hexafluoride as arc quenching or arc extinguisher medium. Although SF6 has very good dielectric property. But like minimum oil circuit breaker, the SF6 CB only utilizes the extinguishing property of the SF6 gas. Since, this page is about the maintenance of SF6 circuit breaker, let us go to the topic directly.
The main problem with a SF6 circuit breaker is its leakage. A SF6 circuit breaker may suffer from gas leakage problem. Because of that the pressure of the gas inside the extinguisher chamber decreases. The arc quenching capability of the breaker may deteriorates if pressure of the gas decreases. So to avoid this, regular checking of a SF6 CB, for SF6 leakage is quite necessary.
We can check this by watching the pointer of gas density monitor on regular basis. If the pointer shows a decrease in SF6 pressure, we must go through SF6 gas leakage test.
SF6 Gas Leakage Test
We can do SF6 gas leakage test either by soap water or a SF6 detector. But the latter is more preferable.
If we detect any leakage, we must rectify the same preferably in presence of competent engineer of the manufacturer.
At the same time we can fill up the CB with the new SF6 gas to makeup the pressure loss.
Again sometimes, the leakage may be so small, that we cannot detect it even with the help of a gas detector. At that time, we may cover each of the joints of CB poles as well as the joints of the SF6 gas piping network with polyethylene sheets. Since the SF6 gas is heavier than air, it will not diffuse away in the air easily. Instead it will accumulate inside the polythene sheets slowly.
After 24 to 48 hours, we check with a detector the space inside the poly sheet enclosure. Then only we may be able to arrest the leakage.
Other Steps for Maintenance of SF6 Circuit Breaker
- If the CB has grading capacitors, then, we need to check the oil leakage from grading capacitors on monthly basis. If we found any leakage, we must immediately piling it.
- The dew point measurement of SF6 CB may be carried out once in every 3 years. Because it is good for knowing the health of the CB.
- We also need to check the duty cycle of the CB once in a year.
- If the pressure of the SF6 gas becomes so low, that it may be unable of proper arc quenching, there is a lock out in the CB. During lock out the CB does not operate.
Lockout Checking during Maintenance of SF6 Circuit Breaker
If lock out does not come correctly at low gas pressure condition, there may be huge arcing in the CB contacts during operation. Due to this the CB may blast out with collapsing of the power system.
So, we can understand, how important is the lock out scheme of a SF6 Circuit Breaker.
The lockout signal initiates from the NO (or NC as per scheme) contacts of the density monitor. Hence, we must check the lockout scheme of the CB by manually shorting the NO contact of the density monitor. We must do it once in a year.
More Steps for Maintenance of SF6 Circuit Breaker
- In addition to the above mentioned actions, also we need to clean the insulators, mechanism, kiosk, control terminals, and auxiliary switch of the CB preferably once in 6 months.
- Like maintenance of minimum oil circuit breaker, we have also to lubricate the gears assembly and moving mechanism of SF6 CB.
- Also we need to check the locking pins and elements as when required.
- Another important part of maintenance of SF6 circuit breaker is its DCRM test.
Dynamic Contact Resistance Measurement (DCRM)
DCRM means, Dynamic Contact Resistance Measurement. Not only for SF6 Circuit Breakers, may we measure contact existence of all types of Circuit Breakers. Generally, we perform this test on an early basis. Not only dynamic resistance, we can also measure static contact resistance of the CB with its main current carrying paths.
We require, CB operation analyzer, with DCRM kit and micro ohm meter.
If we find that there is a variation of result the measurements 10% in compared to previous result, we need to contact with manufacturer for rectification.
In case of no CB, we can do the rectification ourselves.
At that condition, for no CB, we must check the extinguisher chambers, fixed contacts, tightness of moving contact tips, contact surface etc. By properly maintaining these we can re-establish the proper contact resistance of the CB.
Operating Times Measurement of SF6 CB
There is an important test to know the performance of the CB during its operating condition. And this is operation time measurement of the CB. We need CB operation analyzer for that. Also we can use CB operating time measurement kit if the analyzer is not readily available.
If the test results differ by more than 5% from its last year test result, with the same kit, the we should take a proper care for the maintenance.
These results may differ due to either defective operating coils, low voltage appearing across the operating coils, or increases or decreases of contact gap between fixed and moving contacts. We should perform this test during any major maintenance of SF6 Circuit Breaker.
In addition to those, we need to check the current of closing and tripping coil during closing tripping operation of the CB.
Also the healthiness of the counter must be checked once in a year.
Maintenance of Hydraulic Operating Mechanism
If the CB has hydraulic operating mechanism, the following additional steps, we should take
- We must check the pressure in accumulator once in every half year.
- Also we need to check the oil level of the compressor monthly. Then top up the oil (recommended graded mobil) if required.
- Also we need to check the oil leakage of the compressor if found, we have to rectify it immediately.
- Yearly once we have to check the oil pressure drop of the compressor during the duty cycle operation of the circuit breaker.
- Auto start and stop of the oil pump must also be checked during the scheduled maintenance of SF6 circuit breaker.
Maintenance of Pneumatic Operating Mechanism
Also some high voltage circuit breaker may have pneumatic operating mechanism. For that we need to take the following actions.
- There is always some water accumulated in the air reservoir. This is due to condensation of moisture vapor of the compressed air. So we need to release this water from the air reservoir on daily basis. We can do this by opening the water release valve fitted at the bottom of the compressed air reservoir.
- Also we need to check the oil level of compressor and dryer.
- Also we need to check the air pressure drop in the reservoir during duty cycle of the circuit breaker once in a year.
- We perform the overheating of compressor and air filter once in a year.