GAS INSULATED SUBSTATION (GIS) ARRANGEMENT CONSTRAINTS

The GIS arrangement is influenced by a number of important constraints. Certain of these are of more interest to the manufacturer while the others are of more interest to the user. The constraints are as follows:

a) Area. The required area for the GIS will be influenced by the selected clearance between breaker poles, by the mode of the circuit-breaker mounting (i.e., vertical or horizontal), and by the need for adequate maintenance clearances.

b) Height. For an outdoor installation, the GIS height may have important aesthetic considerations. For an indoor installation, the required headroom may be a factor determining the mode of the circuit-breaker mounting.

c) Bus and junctions. The arrangement may influence the length of the bus and the number of junctions required.

d) Line exits. The overall GIS dimensions will be influenced by the type of line exit selected. For EHV substations in particular, overhead exits require spreading the GIS arrangement to meet minimum phase-tophase clearances in air.

e) Position indicators. The arrangement should afford a clear view of as many mechanical position indicators for disconnecting and grounding switches, from as few locations as practical. All position indicators should be visible from the floor or a readily accessible platform.

f) Expansion. If expansion is foreseen, the arrangement should be such that expansion of the original installation can be accomplished with minimum GIS downtime.

g) Auxiliary connections. The length and the number of terminal points of control wiring, hydraulic, and SF6 gas connections should be minimized.

h) Control cabinets. The number and the location of control cabinets may be influenced by the arrangement.
i) Shipping units. It is essential to minimize the number of shipping splits in order to keep the installation time of GIS to a minimum.

j) Maintenance. The arrangement should afford maximum flexibility for routine maintenance. Equipment removal and SF6 gas handling should be accomplished with ease.

k) Operation. The ease of operation should be ensured. Operating handles should be accessible and grouped. All indicating devices and gauges should be clearly visible and easily accessible. Access to viewports should be convenient.

l) Power transformer. Location and type of electrical connection for the power transformers will affect arrangement (SF6-oil, SF6-air, or through an SF6-cable bushing).

m) Cable connections. Location and type of electrical connection for cables will affect arrangement (SF6-oil, SF6 to solid dielectric cable, or through an SF6-air bushing).

n) Surge arresters. The arrangement, and particularly the length, of bus connections may dictate whether surge arresters are required inside the GIS.

o) Layout. The modular design of GIS components offers a high degree of flexibility for any single line diagram specified. Considering the high reliability of today’s GIS components, with over 2000 operations before maintenance, simplified arrangements with less redundancy could be a way to reduce costs, particularly in stations with limited outage cost.

A ring bus arrangement instead of a one-and-a-half breaker scheme, or a single instead of a double bus bar, can considerably reduce the total cost of a GIS. Using cable connections to overhead lines and/or transformers, especially at voltage levels up to 242 kV, may considerably reduce the total space required for an installation without limiting the access for ease of maintenance.

Many different physical arrangements are possible for the same single-line diagram, balancing the
shortest connection to adjacent equipment with the best fit to a particular site.

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