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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