800 kV GIS

An ultra-high voltage substation made super compact

08/20/2015 - 11.59 am

GIS
Grid reliability & efficiency
UHV

Taking advantage of latest design techniques as well as innovative concepts, the newly developed 800 kV GIS substation succeeds in combining compactness, high reliability and performance in compliance with latest IEC standards.

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In large countries such as India, China or Brazil, conventional and new sustainable power sources are often located in regions remote from load centres and need large-scale AC transmission systems. One solution to reduce the power losses and improve the transmission capacity of AC links is to increase the system voltage up to 800–1,200 kV AC. For these high voltage transmission systems, the use of gas-insulated substations (GIS) is extremely advantageous, as this substation type is highly reliable and requires less maintenance than air-insulated substations (AIS) because all active parts are protected from environmental hazards. In addition, a GIS’s inherent compactness (owing to the superior insulating properties of SF6 compared with air) reduces bay dimensions and overall substation footprint and height, which is very important in minimising seismic impact.

However, “at 800 kV, existing GIS are based on technologies from the 1990s, so dimensions are still relatively large and 800 kV GIS are mostly installed outdoors,” explains Mathieu Bernard, Alstom Grid’s R&D project manager for bay apparatus and substations. “Responding to a growing demand for greater compactness (notably from the Indian utilities), our main objective was therefore to develop an 800 kV GIS compact enough to be installed in a small building. One key point was the circuit breaker architecture, as it represents a large part of the substation.”

1_Innovative circuit breaker architecture with smaller footprint and reduced height

“We tried various circuit breaker configurations combining two 420 kV breaking chambers in series,” says Nicolas Garbi, Alstom Grid’s R&D project manager for the circuit breaker. “The optimal solution was found by positioning the two chambers vertically, side by side, in a single tank, with an oblique conductor in between. This enables us to have the two breakers as close as possible to each other – the shortest distance between them is less than 5 cm. The chambers can also be equipped, where necessary, with closing resistors without greatly increasing the dimensions of the enclosure.”

Circuit-breaker section view

With this innovative circuit breaker design, optimised substation architecture and other innovations (see below), Alstom’s new 800 kV GIS not only achieves the reduced footprint required but also, with a maximum height of 5 metres as for the standard architecture, can be easily installed inside a building. Moreover, even though it is the most compact 800 kV GIS, the unit still offers “exceptional access to all components and viewports: the highest drive position is at 3 metres, readily viewable from the floor without requiring specific – and heavy – catwalks.”

Typical diameter arrangement inside a building

Another important objective was to ensure reliability under any and all service conditions, particularly with respect to earthquakes. “As India – a major developing network using 800 kV GIS – can be subject to serious seismic events, one of our R&D missions was to take this risk into consideration in the very early stages of the design of our new substation,” says Bernard. Having a reduced height is already a good point where a high level of seismic withstand is required. In addition, all the most massive equipment is close to the floor (hence a low centre of gravity), and seismic calculations have been conducted jointly with design studies to ensure optimum behaviour of the substation. “As a result, Alstom’s 800 kV GIS offers top-class safety with regard to seismic constraints of 0.3 G and more.”

2_Performance and reliability

Development of equipment for such high voltage ratings cannot be made by simply applying a size-factor ratio from lower voltage products. The characteristics of UHV overhead lines and substation schemes demanded the continuation of fundamental studies and the application of innovative solutions to achieve maximum reliability for the equipment.

For instance, when the service voltage rises, the bus charging current switching (BCCS) capability of a disconnector has to be increased. As a consequence, managing BCCS implies a better understanding of the complete phenomenon. “When the circuit breaker opens, the load current is interrupted and only a small capacitive current can flow through the closed disconnector,” Bernard explains. “During the opening operation, multiple restrikes can be observed between contacts. The main issue in disconnector development is to ensure that no flashover between the two electrodes will propagate and reach the enclosure. An innovative solution has been found, applied and validated: a specific characteristic of the electrode, which includes mobile parts, allows the gap to be reduced during the closing operation. The reliability of the disconnector in terms of very fast transient overvoltage (VFTO) phenomenon is therefore increased.” At the same time, bus transfer performance has also been studied in depth, for all voltage levels, in order to be able to comply with IEC standard requirements (and even beyond, as some customers may demand). “To do so, we developed an innovative concept of mobile arcing contact, combining fast translation and rotating displacement.” This new solution, which is patented, has been tested and validated on a prototype 800 kV GIS rolled out for the production systems.

Disconnector section view

The kinematics of the equipment was also a topic of concern. A multi-domain simulation programme was used to model many possible kinematics combinations and to optimise connecting rods, crank handles and hydraulic drive to have minimum energy consumption for the required opening and closing speeds. “The kinematics of the circuit breaker is driven by a single hydraulic command, even in the case of a circuit breaker connected with pre-insertion resistor (PIR). The actuation system is installed at the bottom of the circuit breaker tank and moves two different shafts, one for the chambers and one for the PIR,” adds Garbi.

3_Fully IEC compliant, even for the most constraining performance

Particular efforts have been made to ensure that the 800 kV GIS meets foreseeable reliability requirements. It has been successfully subjected to all IEC-type tests: dielectric testing and temperature rise, bus charging current switching and bus transfer for the disconnector, making test for high speed earthing switch, terminal faults, short line faults and capacitive switching for the circuit breaker. The result of this complex development project, which required enhanced international collaborative R&D work in France, China and India, is a cutting-edge 800 kV GIS that is super compact, highly reliable and easily maintainable. Manufacturing and after-sales service will mainly be ensured by the GIS manufacturing site in Chennai, India.

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