As utilities look to cut costs, the industry is moving toward a new approach to maintenance–a predictive approach. At Grant County Public Utility District in Washington state, maintenance costs were a significant expenditure. The utility serves 40,000 customers and has 410 miles of 115-kV and 230-kV transmission lines. Customers are thinly spread over a large area, resulting in a relatively high per-customer maintenance bill for the district.
In a drive to cut costs without negatively affecting reliability, the district embarked on a $30 million upgrade of its transmission system. It also cut back on electric system maintenance staff and looked to expand its monitoring systems as a cost-effective way of ensuring equipment performance and safety.
As part of the upgrade, the utility built a new 115-kV and 230-kV switchyard. The yard needed eight circuit breakers, and, in response to environmental and maintenance concerns, the district’s engineers introduced a gas-leak clause as part of the tender. Under this clause, tendering manufacturers were obliged to ensure an SF6 gas leakage rate in the circuit breaker of no more than 0.5 percent per year, and no more than 2.5 percent over the first five years. Breakers failing this rate would be repaired free of charge and guaranteed for the next five years with the same maximum leakage rates. Repaired breakers that were still leaking at the end of 10 years would be replaced free of charge. The clause was not popular with manufacturers, but nonetheless many bid for the contract.
ALSTOM’s CBWatch-1 installed on a Grant County PUD dead-tank circuit breaker.
To ensure the clause was adhered to, the district considered circuit-breaker monitoring to monitor the SF6 leakage rate. An important criteria was to ensure that using and installing the monitoring solution was not more trouble than the information gained was worth. The district decided to buy equipment solely to monitor SF6 gas density, as other operational features could be monitored by protective relays, with minimal hardware changes at a later date.
ALSTOM’s CBWatch-1 was chosen for the installation. The eight monitoring units not only detect gas leaks but can calculate the leakage rate, which enables the utility to improve its maintenance program. An extremely slow leak can wait while maintenance staff deal with more pressing matters, but more serious leaks can be dealt with quickly. CBWatch-1 allowed the district to move from a regime of periodically checking circuit breakers toward one of only carrying out maintenance when it is strictly needed.
As Joe White, senior apparatus engineer with Grant Country PUD, explained, “We don’t have the maintenance staff to carry out continual checking of circuit breakers. With CBWatch-1, we can continue the move away from planned maintenance programs, toward predictive ones.”
Another advantage is that the utility now has a reliable way to monitor circuit breaker performance and thereby enforce the conditions of the guarantee.
The CBWatch-1 is a digital device that measures the pressure and temperature of the gas in the circuit breaker and then calculates the SF6 density using the Beattie and Bridgeman algorithm, which is the physical law closest to experimental results. This information is then transmitted digitally to the control board.
Moving over to predictive maintenance is becoming an important aspect of cost-control for utilities. The monitors at Grant County PUD have been in place for slightly more than one year and are operating efficiently. “When we looked at CBWatch-1, the costs were acceptable,” White said. “It was good value for money.”