Maintenance of Gas Insulated Substation
Channel: KashTech
Date: 2022-08-18

Operation and Maintenance of a Gas Insulated Substation
A major difference between conventional air insulated substations (AIS) and the gas
insulated substations (GIS) is that the SF6 gas insulated electrical components are
placed within an earthed, pressurized metallic container. This is a significant change
in operating the switchgear with circuit breakers, disconnectors, and earth switches
because there is no direct observation or visual contact of the open or closed
disconnect switch gap or the open or closed earthing switch status.
Besides this difference, other special operating characteristics are related to view
ports, cameras, endoscopes, induced currents in the metallic containers, alarms, local
control cubicles, remote control, mimic schemes, gas zone, unmanned substations,
etc.
The maintenance paragraphs give real life experiences from existing GIS technology
that has been in service for a number of years. It describes the nature and sources of
defects including their repair and sophisticated planning. Usual maintenance
methodologies on measures to determine the quality of the SF6 insulating gas, visual
and optical verifications, gas handling records, and leak detection routines are
presented. Special practices on interlocks, gas zones, spare parts, and special
equipment are discussed. Experiences and lessons learned on gas zone identification,
secondary interface, wiring methods are combined and presented. Preconstruction
staff training that involves continuous monitoring, GIS retrofit and bus leak
maintenance topics are also discussed.
This course presents the GIS repair process. Information is presented on the types of
anticipated defects, which demand repair of a GIS installation, how much time needs
to be expected, and how service continuity is related to the design.
Also course describes how a GIS installation can be extended after it has been
installed and used for a number of years. This course provides details about the
definition of the interface between two GIS installations of different dimensions. It also
presents the work that has to be completed when extension is not expected from the
initial design stage, how service continuously during extension work can be given, and
how the new interface can be verified after extension. Also, course explains retrofit for
GIS installation upgrade. Here the challenges of old switchgear installations are
discussed and details are provided to retrofit or upgrade a substation. A specific view
is taken on replacement of circuit breakers in GIS installation to increase switching
capacity. The situation of overloading GIS equipment is also described. The principle
of the GIS installation for continuous rating current is presented under the thermal and
dielectric conditions given in a substation design. The determination of the thermal
limits based on IEEE and IEC standards are also included. The maximum continuous
load current and the short-time overload capacity are discussed and equations for
overload calculations are shown.
Gas Insulated Substation Operation
Both AIS and GIS installations use circuit breakers, disconnect switches (isolators),
and earthing switches, and have different means of showing their status, either opened
or closed. Operation of a GIS substation involves the same principles as operating an
air insulated substation (AIS) even though the different active elements are physically
arranged differently. The first obvious difference is that the blades of the disconnect
switches and the earthing switches installed in a GIS substation are surrounded by
earthed metallic enclosures. This enclosure prevents the blades from being easily
visible to discover their fully opened or fully closed status. The second apparent
difference relates to the bus conductors being installed inside earthed metallic
enclosures. These enclosures prevent the bus from being earthed with portable
personnel grounds except at very discrete locations, such as air-to-gas bushing
terminations at transmission lines and transformer banks.
Typically, a GIS installation demands more extensive electrical interlocking between
the circuit breakers, disconnect switches (isolators), and earthing switches. The
specific detailed procedure of operating and interlocking is typically determined by the
GIS end user.