The Last 10 to 15 Years have Seen a Significant Increase in the Range of Equipment specifically designed for continuous on-line condition monitoring of high-voltage substation equipment. Today, the opportunity exists for continuous on-line monitoring to assist network providers in meeting new industry demands.

The introduction of electricity markets has imposed operational demands on electricity networks in an environment of cost reduction, coupled with increased risks and liabilities. Utilities are now using several asset-management drivers:

• Financial drivers of revenue and cost

• Service standards that include voltage quality, system reliability and availability

• The minimisation and alleviation of market constraints

• Safety and environment.

On the East Coast of Australia, two of the major transmission network service providers (TNSPs) — TransGrid and Powerlink Queensland, whose transmission systems are interconnected — have adopted a similar strategic approach to on-line monitoring in order to realise the potential benefits. Working independently, the two TNSPs — following similar patterns of reviewing existing practices, technology evaluation and field trials in the 1990s — have established asset-management practices that are closely linked to the cost benefits of on-line condition monitoring.

TRANSGRID'S STRATEGY DEVELOPMENT

In 2002, TransGrid formalised its policy regarding on-line condition monitoring for the most significant items of high-voltage substation equipment, namely circuit breakers, power transformers and instrument transformers. TransGrid developed its strategy by relating the justification for installing the on-line monitoring to the cost benefits. This exercise involved assessing potential plant failures, the equipment available on the Australian market and the management of on-line monitoring information. Table 1 offers an example of the rationalised cost-benefit analysis used for each item of equipment identified for possible on-line monitoring.

In addition, TransGrid considered several other factors based on system operational and equipment reliability statistics, experience, geographical location and the cost of on-line monitoring equipment compared to replacing the existing capital asset.

TransGrid initially concentrated its continuous on-line monitoring efforts at new substations, where the technologies could be included in the overall design for the substation and supporting infrastructure such as telecommunications links already existed.

POWERLINK'S STRATEGY DEVELOPMENT

In the late 1990s, Powerlink embraced significant changes to its existing policy by adopting new high-voltage technologies. With the availability of hybrid switching plants and the condition-monitoring technologies that were emerging, Powerlink examined the potential savings plant-monitoring technologies offered. It conducted field trials that incorporated on-line monitoring devices from major plant manufacturers and third-party equipment suppliers.

The range of devices installed included circuit-breaker monitoring, dissolved gas analysis equipment for power transformers and insulation-pollution monitoring. Coupled with a significant investment in the development of an operational wide-area network (WAN), the field-monitoring devices enabled Powerlink to collect a large volume of data for processing and analysis at its remote engineering and operations offices.

However, to maximise the benefits from its investment in technology, Powerlink changed its organisational structure and established an asset-monitoring team (AMT).

Powerlink uses on-line monitoring as both a predictive model and a reactive model, with the former being used to optimise condition-based maintenance and the latter being used to improve post-fault diagnosis to minimise plant outage periods. Using these models for managing real-time faults and long-term plant condition monitoring, Powerlink developed a monitoring standard for use in reactive and predictive modes for all primary plant, secondary system and communication network assets. Table 2 shows the standard that Powerlink developed for its SF₆ circuit breakers.

SUBSTATION EQUIPMENT OUTCOMES

After several years of developing their strategies, Powerlink and Transgrid have identified some key outcomes of using on-line condition monitoring for their substation equipment. The substation equipment outcomes fall under three categories:

1. Circuit breakers

   Circuit-breaker on-line monitors are now available for all forms of monitoring, from basic equipment to multifunctional microprocessor-based monitoring devices, some of which also include control functions. Since 1998, Powerlink has been installing circuit breakers equipped by the manufacturer with either monitoring or control/monitoring devices. To date, 4.5% (39 of 883 units) of conventional air-insulated switchgear (AIS) circuit breakers and 96% (198 of 206 units) of hybrid gas-insulated switchgear (GIS) circuit breakers include some form of monitoring. Presently, Powerlink has decided not to retrofit monitoring devices to older equipment until the economic benefits of on-line monitoring are established.

   Similarly, in 1998, TransGrid decided to purchase new circuit breakers with on-line monitoring equipment fitted by the switchgear manufacturer. Today, 7.5% (104 of 1391 units) of AIS and 45% (18 of 40 units) of GIS circuit breakers are fitted with original equipment manufacturer (OEM) monitoring devices.

2. Instrument transformers

   Current-transformer monitoring systems are available for insulation leakage current and tan delta (ä) measurement, the latter of which was installed at six TransGrid substations following a particular type of current-transformer fault. For many years, both TNSPs have installed simple secondary-voltage comparators on capacitor-voltage transformers to monitor the porcelain-insulated high-voltage capacitor stack. Although Powerlink now routinely uses composite insulators for this equipment, the benefits of monitoring are still just as relevant. Powerlink is installing analog-density monitoring on all-new SF₆ gas-insulated instrument transformers in addition to conventional alarm switches.

3. Power transformers and reactors

   Monitoring devices are available for power transformers and reactors to monitor the level and condition of the insulating oil, partial discharge, tap-changer operation and temperature. Both utilities use the full range of devices available with varying degrees of reliability. Dissolved gas analysis, tap-changer performance and tan delta are the predominant characteristics monitored.