The technology of high-voltage-substation components like circuit breakers, surge arresters and transformers has improved markedly in recent years but with very little impact on external dimensions. So the basic layout of high-voltage substations has essentially remained the same.

However all this may be about to change thanks to a new substation technology called PASS (Plug and Switch System), the first major application of which is at a just-commissioned substation in Australia.

The basic innovation of PASS, developed by ABB High Voltage Technologies Ltd, is to integrate circuit breaker, disconnectors, earthing switch, current and voltage sensors and bushings within one metal-encapsulated compartment filled with gas (SF6). PASS also benefits considerably from digital control, which reduces wiring to a minimum and allows adoption of an open system architecture for monitoring and diagnostics. These features further reduce operating costs and increase substation availability. Also, thanks to continous self monitoring by intelligent control systems, which trigger alarms in the control centre when intervention is needed, time-based maintenance requirements are almost zero and predictive maintenance is used instead.

Other benefits, as well as space savings, include reductions in materials requirements for manufacture, site preparation, network connections and cabling. In a typical double-busbar layout, conventional switchgear uses 1080 m2, whereas PASS needs only 420 m2 – a saving of around 60 per cent.

PASS – the basic design

PASS uses designs of circuit breaker, disconnector and earthing switch well proven in conventional GIS (gas insulated systems). The bushings are made of composite material. An epoxy-resin-impregnated glass fibre tube provides the required mechanical characteristics, while a housing of moulded silicone rubber provides electrical functions such as creepage distance, as well as environmental protection.

The function of traditional high voltage current and voltage instrument transformers is carried out by advanced-generation high performance current and voltage sensors, which combine both functions in a single component. This device handles control, measurement. protection and revenue metering with high resolution and accuracy. The compactness of this sensor technology allows additional units to be installed on the opposite side of the equipment should they be needed for protection and/or point-on-wave switching.

A fibre-optic digital bus (MVB) provides the data transfer for control and monitoring of each PASS pole. The digital interface is the PISA (Process Interface for Sensors and Actuators) system. The PISAs continously supervise and regulate the operating system for the circuit breakers and disconnector/earthing switches and process signals from current/voltage sensors as well as from position, gas-tightness and temperature sensors.

A single plugable cable, containing the fibre optics as well as the power supply for the drive mechanism, connects each PASS pole to the substation control & protection system – resulting in reduced installation and commissioning times.

The intelligent control system means that a very wide range of monitoring options are available (eg relating to circuit-breaker condition and remaining life).

Applying the technology

The possibilities for retrofit, extension or uprating of existing AIS (air-insulated switchgear) installations are often limited due to lack of available space. But the compactness achievable with PASS can often get round these problems. It can also be a useful building block in new substations, reducing the need for structures and foundations.

An example of an extension project using PASS is the Bottmingen substation (photo above) in Basle, Switzerland. ATEL, the Olten-based Aare-Ticino power generation company, decided to extend its Bottmingen switchyard with a bus coupler bay as part of its Power System Planning 2000 strategy. Among the requirements for this project which led to the choice of PASS were: unmanned operation with minimum maintenance; lowest possible profile (AIS technology would have extended structures above the existing busbar, requiring planning permission); minimal outage for installation; optimal use of available space.

When it comes to new substations, Powerlink of Queensland, Australia, is proving an enthusiastic supporter of the technology, with the following installations, using, or projected to use, the technology:

Blackwall, 275 kV, switching station, 14 bays for eight feeders and an SVC (Static Var Compensator) – commissioning November 1999. This facility is a critical element in electricity supply to Brisbane.

Bulli Creek, 330 kV switching station, eight bays for four feeders and four shunt reactors – commissioning November 2000.

Braemar, 275/330 kV transformation station, with six 330 kV bays and eleven 275 kV bays, including provision for an SVC – commissioning June-November 2000. Along with Bulli Creek, this is part of the NSW-Queensland interconnector.

Powerlink has extensive experience with conventional air-insulated switchgear, having built many installations over the years, and has recently completed a 14 bay 275 kV AIS substation at Belmont. It has also looked at the possibility of building gas-insulated substations (GIS). But a 275 kV GIS is estimated to have a capital cost 2-2.5 times the cost of a substation using AIS, with a much higher life cycle cost. So GIS is not justified on the Powerlink system unless special site constraints exist, which is not the case for the three new substation projects mentioned above.

However, after careful evaluation Powerlink concluded that PASS technology was optimal for these three sites. Among the factors leading to this conclusion were: reduced maintenance; simpler construction and installation, with one factory-tested module per phase; reduction in site area; virtual elimination of cabling; and substitution of the single large control building by a number of lower cost prefab modular control buildings.

Assessing the benefits

Powerlink has assessed the cost of PASS substations relative to AIS and concluded that, while the total cost is very similar on an initial capital cost basis, total life cycle costs of PASS are around 18 per cent less – although this neglects benefits of PASS such as higher availability, better performance, increased safety, environmental benefits and potential for future introduction of automated switching.

The Powerlink management recognised that there were some risks inherent in choosing new technology, particularly as at the time of the decision some elements of PASS were still under development. However the risks were considered to be outweighed by the significant potential benefits.

Powerlink also reports that the adoption of PASS has had a major impact on how it goes about the substation design process, with the abandonment of requirements formerly seen as essential, and is proving to be a major catalyst for improvement and change. Among the benefits are: drastic reduction in the number of drawings per substation; radical changes to substation layout; elimination of line disconnectors; adoption of modular distributed control buildings; acceptance of a new approach to switching, isolation and access; and a radically dfferent approach to maintenance.

The company is now looking at further improvements to maximise the benefits from PASS, and is for example investigating the possibilities of automated switching.