GIS Substation: SF6, Testing and Common Failures

Short answer

A GIS (gas-insulated switchgear) substation encloses circuit breakers, disconnectors, instrument transformers and busbars inside earthed metal enclosures filled with an insulating gas, normally SF6. It takes up a fraction of the area of a conventional substation and protects the live parts from the environment, but it demands strict control of the gas, cleanliness during assembly and on-site high-voltage tests.

What a GIS substation is

A GIS (gas-insulated switchgear) substation groups the switchyard equipment inside earthed metal enclosures filled with a pressurized insulating gas. Traditionally that gas is sulfur hexafluoride (SF6), whose dielectric strength and arc-quenching capability are far higher than those of air. As a result, the clearances between live parts are reduced and the substation takes up a fraction of the area of an air-insulated substation (AIS). The parts and functions common to any substation are covered in electrical substation.

GIS is chosen when space is scarce or expensive (cities, expansions inside existing sites), when the environment is aggressive (coast, industrial pollution, areas of heavy rainfall) or when the installation is underground, for example in the powerhouses of hydroelectric plants.

Main parts

Each GIS bay is built from modules that couple together:

Power circuit breaker with its operating mechanism (spring, hydraulic or hydraulic-spring, depending on the manufacturer).
Disconnectors and earthing switches. There are maintenance earthing switches and fast-acting earthing switches, able to close onto an energized circuit.
Current transformers, almost always ring-core around the conductor, and encapsulated inductive voltage transformers or voltage sensors.
Busbars inside enclosures, in single-bus, double-bus or breaker-and-a-half arrangements.
Connection interfaces: gas-to-air bushings to overhead lines, terminations for power cables and direct connections to the transformer.
Encapsulated surge arresters, when the insulation coordination study calls for them.
Barrier insulators, which divide the GIS into independent gas compartments, and support insulators that let the gas through.
Density monitors in each compartment, which measure temperature-compensated gas density.
Local control cabinet (LCC), with the controls, indications, interlocks and the interface to the bay controller.

How it works

The conductor runs centered inside the enclosure and the gas insulates between the two. The enclosure is earthed at many points, so personnel are never exposed to live parts. Compartmentalization means a leak or an internal fault affects only a limited volume of gas and makes maintenance by sections easier.

Density monitors normally have two stages: a low-density alarm and a blocking or trip stage. Interlocks between circuit breaker, disconnectors and earthing switches are critical, because in a GIS the state of the contacts cannot be seen at a glance; manufacturers provide mechanical position indicators and, on some models, viewing windows.

SF6 is a very potent greenhouse gas. For that reason its handling is regulated, and every filling and recovery is measured and recorded, and some manufacturers offer alternatives using gas mixtures or dry air with vacuum interruption. Before specifying one of these alternatives, check with the manufacturer which voltage and current levels it covers.

Types

Extra-high and high voltage. GIS with a rated voltage above 52 kV is governed by IEC 62271-203 (IEEE C37.122 in the IEEE world). At high voltages single-phase encapsulation (one phase per enclosure) is usually used; at lower voltages three-phase encapsulation is common.
Medium voltage. Gas-insulated switchgear up to 52 kV is governed by IEC 62271-200 and competes with air-insulated medium-voltage switchgear. It is common in industry, mining, oil and solar plants because of its size and low maintenance needs.
Hybrid or compact. Modules that combine an encapsulated circuit breaker and disconnectors with air-insulated busbars. They are used in expansions and in outdoor substations where space is limited.
Indoor or outdoor. Most are installed in a building, but there are designs for outdoor use.

Associated protection

A GIS carries the same protection as any substation (distance on lines, transformer differential, overcurrent on feeders), with some points of emphasis:

Busbar differential protection 87B is almost always necessary. An internal fault in a GIS produces an arc inside the enclosure; the faster it is cleared, the lower the damage and the lower the risk of burn-through.
Breaker failure 50BF must be well tested, because repairing a GIS circuit breaker takes time and requires isolating neighboring compartments.
Gas density signals are integrated into the control scheme: the second stage blocks closing and opening of the circuit breaker or trips it, depending on the project philosophy.
The location of the current transformers defines the protection zones. Check on the drawings that the busbar and line zones overlap across the circuit breaker and that no dead zones are left.

Commissioning and maintenance tests

At commissioning, the on-site tests aim to confirm that shipping and assembly did not damage the equipment. The most common are:

Gas tightness of each compartment and gas quality: moisture (dew point), purity and decomposition products. The specifications for new gas and reclaimed gas are in IEC 60376 and IEC 60480.
Main circuit resistance on each current path.
Circuit breaker times, simultaneity and travel, and operation of disconnectors and earthing switches. See GIS circuit breaker tests at EHV, HV and MV.
On-site applied voltage test, normally with a resonant source, accompanied by partial discharge measurement, as explained in partial discharge in GIS.
Verification of density monitors, interlocks, indications and control.
Tests on current and voltage transformers, and protection tests with secondary injection.

In maintenance, the trend of gas density, the number of operations and the accumulated interrupted current of the circuit breaker are monitored, along with the mechanisms and, in critical installations, online partial discharge monitoring. You can see the full scope in power circuit breaker tests.

Partial discharge in GIS is measured with the UHF method using equipment such as the UHF 800, and SF6 leaks are located with detectors such as the WIKA GIR-10.

Common failures

Gas leaks at flanges, gaskets, density monitors or pipe joints. Slow trends are detected by comparing temperature-compensated readings.
Particles and contamination left during assembly, which produce partial discharge and can end in an internal arc.
Moisture in the gas, from poorly done fillings or saturated desiccants.
Mechanism problems: springs, charging motors, coils and auxiliary contacts.
Very fast transient overvoltages (VFTO) when operating disconnectors, which stress the insulation and the connected equipment.
External corrosion of flanges and structures in aggressive environments.
Cable termination failures from poor installation of the accessory.

We support you with your GIS

At Tiza Group we do testing, commissioning and protection configuration in GIS substations at medium, high and extra-high voltage, for utilities and industry. Field work is done in Colombia, or elsewhere on request; protection settings and event analysis can be done remotely. If you are about to energize a GIS or need to diagnose an anomaly, write to us.

References

IEC 62271-1: high-voltage switchgear and controlgear, common specifications.
IEC 62271-203: gas-insulated metal-enclosed switchgear for rated voltages above 52 kV.
IEC 62271-200: AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.
IEC 60376: specification of technical grade sulfur hexafluoride (SF6).
IEC 60480: specifications for the re-use of SF6.
IEC 60270: high-voltage test techniques, partial discharge measurements.
IEEE C37.122: standard for gas-insulated substations rated above 52 kV.

Frequently asked questions

What happens if SF6 pressure drops in a GIS compartment?

The compartment's density monitor gives an alarm first and, if the density keeps falling, a second stage that blocks circuit breaker operation or trips it, depending on the project philosophy. With less gas, dielectric strength and interrupting capacity drop, so the equipment must not be operated until the leak is found and repaired.

Why is a GIS given an on-site high-voltage test if it was already tested at the factory?

Because the GIS is shipped in modules and assembled on site, where particles or moisture can get in and misalignments can occur. The on-site voltage test, ideally with partial discharge measurement, confirms that assembly left no dielectric defects.

Does a GIS need busbar differential protection?

In practice it almost always has it, because an internal fault in a GIS must be cleared very fast to limit damage to the enclosure and repair time, which is usually long. The final decision depends on the busbar arrangement and the owner's protection philosophy.