Utility Asset Managers all know how critical their power transformers are and understand the risks and consequences of any failure. These managers are now focusing on the fact that high voltage circuit breakers are often just as critical. When a transformer problem is detected, protection relays and circuit breakers are relied upon to isolate it from the grid and a breaker’s “failure to operate” is not a palatable option. With circuit breaker fleets worldwide averaging more than 25 years of age and operating budgets shrinking yearly, many Asset Managers are becoming acutely aware of the key drivers for monitoring these core pieces of infrastructure.
- Dependability is key: Circuit Breakers (CB) are critical pieces of equipment
The ability of a CB to operate when there is a fault is critical. If there is a major problem with a transformer and you need to isolate it, protection relays will rely on CB’s. Failure of a CB to open will cause equipment damage down the line, greater grid disturbances, and a fall in a utility’s System Average Interruption Frequency Index (SAIFI) reliability figures. The goal for any operator is to be able to reliably depend on the operation of the CB.
- Mitigating the consequences: Avoiding failure to operate rather than saving the asset
Transformers need to be protected because of their high replacement cost. CB’s – while the next most expensive asset in the substation – are at least an order of magnitude less expensive than transformers, and therefore, the key purpose for monitoring is not to try and “save” the CB, but to avoid “failure to operate” and protect against its consequences. This could be the expensive transformer behind the CB or not being able to connect a windfarm to the grid.
- Reducing maintenance costs: Regular maintenance of a CB is required
CB’s have moving parts that travel quickly (in milliseconds) with high acceleration and vibration, and regular timing tests, inspections, overhaul, lubrication, contact replacement, gas refills etc. are required. Utilities want to move to Condition Based Maintenance to increase asset availability (less off-line periods for inspections and tests). They are also looking to reduce lifetime costs and resource usage, by stretching maintenance intervals and reducing the number of checks/interventions.
- Reliably extending operating life: CB fleets are becoming old and less reliable
Many CBs are getting old and using older technology (oil CB, hydraulic, pneumatic, and so on), making them less reliable and requiring more maintenance than newer CBs. Utilities cannot afford to replace them all and need to extend the operating life, just when the likelihood of issues are increasing. Operators want peace of mind that these older CBs will operate when asked to. An efficient, factual, “risk-based” replacement strategy also needs to be supported.
- Managing the environment: Reducing SF6 gas emissions
Most CB’s use SF6 gas to extinguish the electrical arc. SF6 gas is expensive to buy and studies have shown this gas is harmful for the environment with 28,000x the warming potential of CO2. There are now targets (both government or corporate) in place for SF6 gas emission and in some cases penalties apply for releasing SF6 to the atmosphere, so leaks can be costly. With agreed fleet SF6 leak targets reducing yearly, control and prevention of gas leaks in older CB’s which are less gas tight is becoming a priority.
- Integrating with Asset Performance Management (APM): Supporting the operator’s strategy
Most utilities are now moving toward some form of APM (Asset Performance Management) to understand in real time the state of their asset base, implement “Reliability Centred Maintenance” and prioritise operational spending (OPEX) and asset replacement (CAPEX). This approach started with transformers, but it is now expanding to other critical assets and CB’s are next on the list.
Most Common Causes of Circuit Breaker faults
The Cigré Working Group A3.06 covered service experience totalling 281,090 CB-years from 83 utilities of 26 countries in the years 2004–2007, limited to SF6 technology, and found that the most common cause of faults where that the CB does not open and close on command or is locked in the open or closed position (due to insufficient SF6 gas pressure):
Harnessing CBWatch3 to monitor CBs
CBWatch3 is an online monitoring solution suitable for most high voltage CBs. It records information using non-invasive sensors and monitors key diagnostic parameters during the CB operation. It looks for significant changes in performance and assesses the CB against the most common causes of failure. The CBWatch3 highlights the timely need for mechanical maintenance or arcing contact replacement, reducing the need for fixed interval inspections and overhauls. It enables instead a more cost effective and less reactive “as needed” maintenance approach.
Key benefits of the CBWatch3:
- Compact, modern online monitoring device – ready for the digital substation and small enough to fit into a number of CB control cabinets
- Modular – only get the functionalities you want
- For any brand and most types of HV circuit breakers
- Measures and checks key parameters every time the CB operates
- Continuously monitors other items in between operations
- Field experience – over 4,800 poles monitored by previous generations of monitors
How it helps your organisation:
- Continuously monitor asset health: Ensure your CB is always ready to operate
- Optimise your maintenance: Only when needed and with advance warnings
- Develop smarter asset replacement strategies: Decisions backed by facts and data
- Foster greener substations: Track and minimise SF6 leakage.
Acknowledgement: This article is based on content created by Dominique Legrand, GE Product Line Manager for M&D products.