Electrical and Physical Arrangement, Grounding, Testing For GIS

Three-phase Enclosure GIS

Electrical and Physical Arrangement:

For any electrical one-line diagram there are usually several possible physical arrangements. The shape of the site for the GIS and the nature of connecting lines and/or cables should be considered.

Figure 1: One-and-One-Half Circuit Breaker Layouts

Figure 1 compares a “natural” physical arrangement for a breaker and a half GIS with a “linear” arrangement. Most GIS designs were developed initially for a double bus, single breaker arrangement Figure 2.

Figure 2: Three-phase Enclosure GIS

This widely used approach provides good reliability, simple operation, easy protective relaying, excellent economy, and a small footprint. By integrating several functions into each GIS module, the cost of the double bus, single breaker arrangement can be significantly reduced. An example is shown in Figure 3.

Figure 3: Integrated (Combined Function) GIS.

Disconnect and ground switches are combined into a “three-position switch” and made a part of each bus module connecting adjacent circuit breaker positions. The cable connection module includes the cable termination, disconnect switches, ground switches, a VT, and surge arresters.

Grounding:

The individual metal enclosure sections of the GIS modules are made electrically continuous either by the flanged enclosure joint being a good electrical contact in itself or with external shunts bolted to the flanges or to grounding pads on the enclosure.

While some early single-phase enclosure GIS were “single point grounded” to prevent circulating currents from flowing in the enclosures, today the universal practice is to use “multipoint grounding” even though this leads to some electrical losses in the enclosures due to circulating currents.

The three enclosures of a single-phase GIS should be bonded to each other at the ends of the GIS to encourage circulating currents to flow.

These circulating enclosure currents act to cancel the magnetic field that would otherwise exist outside the enclosure due to the conductor current. Three-phase enclosure GIS does not have circulating currents, but does have eddy currents in the enclosure, and should also be multipoint grounded.

With multipoint grounding and the resulting many parallel paths for the current from an internal fault to flow to the substation ground grid, it is easy to keep the touch and step voltages for a GIS to the safe levels prescribed in IEEE 80.

Testing:

Test requirements for circuit breakers, CTs, VTs, and surge arresters are not specific for GIS and will not be covered in detail here. Representative GIS assemblies having all of the parts of the GIS except for the circuit breaker are design tested to show that the GIS can withstand the rated lightning impulse voltage, switching impulse voltage, power frequency overvoltage, continuous current, and short-circuit current.

Standards specify the test levels and how the tests must be done. Production tests of the factory-assembled GIS (including the circuit breaker) cover power frequency withstand voltage, conductor circuit resistance, leak checks, operational checks, and CT polarity checks.

Components such as support insulators, VTs, and CTs are tested in accordance with the specific requirements for these items before assembly into the GIS. Field tests repeat the factory tests. The power frequency withstands voltage test is most important as a check of the cleanliness of the inside of the GIS in regard to contaminating conducting particles, as explained in the SF6 section above.

Checking of interlocks is also very important. Other field tests may be done if the GIS is a very critical part of the electric power system, when, for example, a surge voltage test may be requested.

Source: ‘Electric Power Substations Engineering’