Stephen Trotter, division head of ABB Power Systems UK, explains how hybrid switchgear modules that combine the virtues of AIS and GIS technology can offer greater flexibility for substation design
The term ‘hybrid' refers to the combination of both conventional air insulated switchgear (AIS) and the newer metal-clad gas insulated (GIS) switchgear. This hybrid solution, as found in ABB's Pass MO design - rated up to 170 kV, uses existing, tried and trusted GIS components together with a conventional and extremely reliable AIS bus to connect the various hybrid modules. All the necessary substation switchgear bay functions, including a circuit breaker, one or more combined disconnector/earthing switches, bushings for connection to single or double busbar systems and a current transformer are integrated in one compact module, eliminating the need for separate pieces of equipment for each function.
The advantages of the hybrid switchgear design include:
- AIS busbar: The AIS busbar is relatively inexpensive while offering proven reliability.
- All live contacts in SF6: Experience has shown AIS disconnector switch contacts require relatively high levels of maintenance, while experience with GIS is exactly the opposite. The use of SF6 technology makes the hybrid switchgear virtually maintenance free, this combines with a high level of reliability to ensure a lower global life cycle cost.
- Fewer switching elements: Use of GIS technology allows rationalisation of switching elements.
- Factory pre-assembled and tested: The hybrid modules are fully pre-assembled and tested in the factory. This ensures a higher quality of finished bay than if it is assembled under site conditions, minimises installation time on site - typically two days per bay, reduces the possibility of delay due to adverse site conditions and there is less need for skilled resources on site.
- Monitoring and on-line diagnostics: The integrated nature of the plant facilitates the use of electronic monitoring and on-line remote diagnostics.
- Substation modularisation: A modular approach to substation design offers cost and time savings during the design and construction phases. The use of standardized components reduces the number of possible variations and hence the risk of design errors. More predictable costs also offers a higher level of confidence in the project estimation process.
- Space saving and reduced civil works: The hybrid design can save up to 70% of the space normally required for a conventional AIS substation, while also reducing the need for civil works such as foundations, steelwork and cable trenching operations
- Combined disconnector/earthing switch: Pass MO is equipped with a combined disconnector/earthing switch. The mechanism has a minimal number of mechanical components and is intrinsically reliable and maintenance-free.
- Circuit breaker: The Pass MO circuit breaker is a single pressure interrupter that operates by means of the well known selfblast principle. The energy for interrupting currents is partly supplied by the arc itself, this reduces the energy the operating mechanism needs to provide by around 50% compared with a conventional puffer type circuit breaker.
- Versatility: The Pass MO range offers a series of modules for HV substations including: single bus bar (SBB); double bus bar (DBB); double circuit breaker (DCB). It can also be installed as a high voltage bay on a mobile truck for use in emergencies or if work has to be carried out on existing HV bays.
- Transportation: The Pass MO fits into a standard truck container and does not require any packaging. No special arrangements are needed for shipping and transportation, and once on site just a simple 30° rotation of the outer poles is needed for the final layout.
Tight fit for Breamish Street substation
Well over 2,000 Pass MO bays have been installed worldwide. Following its approval by the ENA (Energy Networks Association) one of the first UK projects to feature the range was CE Electrics UK's new 66/11 kV Breamish Street substation on a brownfield, urban site in Newcastle-upon-Tyne.
The new primary substation is helping CE Electric UK to deliver an additional 18 MVA of firm capacity to meet the growing demand for additional power, and need for load transfer, created by the significant urban redevelopment programmes on the north bank of the River Tyne. The restricted space available presented a particular technical challenge, since the Breamish Street site is not only compact in size, it is also hemmed in on all four sides by a hotel, a pharmacy, a residents association and the 18th century St Ann's church.
The space-saving capability of the design has been utilised to construct a new substation comprising two 66/11 kV 15/30 MVA CER transformers, a 66 kV in/out unit and a 13 panel 11 kV switchboard. A 66 kV feeder unit has also been installed at Fossway, the closest CE Electric UK substation.
Providing vital construction space at Reading
Hybrid switchgear has also provided an innovative interim solution for the new Scottish and Southern Energy (SSE) 132 kV indoor GIS substation at Reading, currently under construction by an ABB and Balfour Beatty consortium. The site presented a particular challenge as it was already completely full with time expired AIS switchgear that needed to remain in service until the circuits could be transferred to the new substation.
At first, it appeared the only possibility would be to extend the site onto the local, heavily wooded, green space to offer the additional room needed for the construction of the new indoor GIS building. However, extending the site would have involved considerable planning time and expense and significant project delays.
An innovative alternative was found by using ABB switchgear as a temporary measure to provide additional space to enable the new GIS building to be built within the existing site footprint. Firstly, ABB dismantled the generator circuit breakers that used to serve the old North Earley power station, which was demolished some years ago. This freed up just enough space to install the Pass MO modules to take over the operation of the AIS circuits at the far end of the site. This enabled the old AIS switchgear to be dismantled to make room for the new GIS substation. After all the circuits have been transferred to the new substation the Pass MO modules will be removed.