GAS INSULATED SUBSTATION CONSTRUCTION AND SERVICE LIFE BASIC INFORMATION

GIS is assembled of standard equipment modules (circuit breaker, current transformers, voltage transformers, disconnect and ground switches, interconnecting bus, surge arresters, and connections to the rest of the electric power system) to match the electrical one-line diagram of the substation.

A cross section view of a 242-kV GIS shows the construction and typical dimensions (Figure 2.1). The modules are joined using bolted flanges with an “O” ring seal system for the enclosure and a sliding plug-in contact for the conductor.

Internal parts of the GIS are supported by cast epoxy insulators. These support

insulators provide a gas barrier between parts of the GIS, or are cast with holes in the epoxy to allow gas

to pass from one side to the other.

Up to about 170 kV system voltage, all three phases are often in one enclosure (Figure 2.2). Above 170 kV, the size of the enclosure for “three-phase enclosure,” GIS becomes too large to be practical. So a “single-phase enclosure” design (Figure 2.1) is used.

There are no established performance differences between three-phase enclosure and single-phase enclosure GIS. Some manufacturers use the single phase enclosure type for all voltage levels.

Enclosures today are mostly cast or welded aluminum, but steel is also used. Steel enclosures are painted inside and outside to prevent rusting. Aluminum enclosures do not need to be painted, but may be painted for ease of cleaning and a better appearance. The pressure vessel requirements for GIS enclosures are set by GIS standards (IEEE Std. C37.122-1993; IEC, 1990), with the actual design, manufacture, and test following an established pressure vessel standard of the country of manufacture.

Because of the moderate pressures involved, and the classification of GIS as electrical equipment, third-party inspection and code stamping of the GIS enclosures are not required.

Conductors today are mostly aluminum. Copper is sometimes used. It is usual to silver plate surfaces that transfer current. Bolted joints and sliding electrical contacts are used to join conductor sections. There are many designs for the sliding contact element. In general, sliding contacts have many individually sprung copper contact fingers working in parallel. Usually the contact fingers are silver plated.

A contact lubricant is used to ensure that the sliding contact surfaces do not generate particles or wear out over time. The sliding conductor contacts make assembly of the modules easy and also allow for conductor movement to accommodate the differential thermal expansion of the conductor relative to the enclosure.

Sliding contact assemblies are also used in circuit breakers and switches to transfer current from the moving contact to the stationary contacts. Support insulators are made of a highly filled epoxy resin cast very carefully to prevent formation of voids and/or cracks during curing.

Each GIS manufacturer’s material formulation and insulator shape has been developed to optimize the support insulator in terms of electric field distribution, mechanical strength, resistance to surface electric discharges, and convenience of manufacture and assembly. Post, disc, and cone type support insulators are used.

Quality assurance programs for support insulators include a high voltage power frequency withstand test with sensitive partial discharge monitoring. Experience has shown that the electric field stress inside the cast epoxy insulator should be below a certain level to avoid aging of the solid dielectric material.

The electrical stress limit for the cast epoxy support insulator is not a severe design constraint because the dimensions of the GIS are mainly set by the lightning impulse withstand level and the need for the conductor to have a fairly large diameter to carry to load current of several thousand amperes. The result is space between the conductor and enclosure for support insulators having low electrical stress.

Service life of GIS using the construction described above has been shown by experience to be more than 30 years. The condition of GIS examined after many years in service does not indicate any approaching limit in service life.

Experience also shows no need for periodic internal inspection or maintenance. Inside the enclosure is a dry, inert gas that is itself not subject to aging. There is no exposure of any of the internal materials to sunlight. Even the “O” ring seals are found to be in excellent condition because there is almost always a “double seal” system. The lack of aging has been found for GIS, whether installed indoors or outdoors.

ECONOMICS OF GAS INSULATED SUBSTATION (GIS) BASIC INFORMATION

A gas-insulated substation (GIS) uses a superior dielectric gas, SF6, at moderate pressure for phase-to phase and phase-to-ground insulation. The high voltage conductors, circuit breaker interrupters, switches, current transformers, and voltage transformers are in SF6 gas inside grounded metal enclosures.

The atmospheric air insulation used in a conventional, air-insulated substation (AIS) requires meters of air insulation to do what SF6 can do in centimeters. GIS can therefore be smaller than AIS by up to a factor of 10.

A GIS is mostly used where space is expensive or not available. In a GIS the active parts are protected from the deterioration from exposure to atmospheric air, moisture, contamination, etc. As a result, GIS is more reliable and requires less maintenance than AIS.

The equipment cost of GIS is naturally higher than that of AIS due to the grounded metal enclosure, the provision of an LCC, and the high degree of factory assembly. A GIS is less expensive to install than an AIS.

The site development costs for a GIS will be much lower than for an AIS because of the much smaller area required for the GIS. The site development advantage of GIS increases as the system voltage increases because high voltage AIS take very large areas because of the long insulating distances in atmospheric air.

Cost comparisons in the early days of GIS projected that, on a total installed cost basis, GIS costs would equal AIS costs at 345 kV. For higher voltages, GIS was expected to cost less than AIS. However, the cost of AIS has been reduced significantly by technical and manufacturing advances (especially for circuit breakers) over the last 30 years, but GIS equipment has not shown any cost reduction until very recently.

Therefore, although GIS has been a well-established technology for a long time, with a proven high reliability and almost no need for maintenance, it is presently perceived as costing too much and is only applicable in special cases where space is the most important factor.

Currently, GIS costs are being reduced by integrating functions as described in the arrangement section above. As digital control systems become common in substations, the costly electromagnetic CTs and VTs of a GIS will be replaced by less-expensive sensors such as optical VTs and Rogowski coil CTs.

These less-expensive sensors are also much smaller, reducing the size of the GIS and allowing more bays of GIS to be shipped fully assembled. Installation and site development costs are correspondingly lower. The GIS space advantage over AIS increases. GIS can now be considered for any new substation or the expansion of an existing substation without enlarging the area for the substation.

GAS INSULATED SUBSTATION BASIC INFORMATION AND TUTORIALS

What Are Gas Insulated Substations?

High-voltage gas-insulated substations have been in service since the early 1960s. Operation of 800-kV equipment has proved successful since the end of 1979. Prototype testing of 1100 through 1600-kV substation equipment proved the feasibility of this equipment at the next generation of voltage levels.

The basic principle of gas-insulated equipment is that the high-voltage current-carrying parts are within a metal enclosure and are held in a concentric configuration by cast epoxy spacer insulators. The space between the conductor and the enclosure is filled with sulfur hexafluoride gas under moderate pressure.

Medium-voltage to 170-kV equipment is available in three phases in one enclosure; for higher voltages, it is generally in a single-phase enclosure arrangement. The equipment can be installed indoors or outdoors, and it can be designed for any bus scheme.

Depending on the voltage level, bus scheme, and whether connecting lines are installed underground or overhead, the land area required for gas-insulated equipment is 10% for 800 kV to 20% for 145 kV of the space required for comparable air-insulated equipment.

Because of its smaller size and enclosed current-carrying parts, this equipment is excellently suited for installation where real estate is at a premium, where the environmental constraints dictate a minimum of visual exposure, and where the continuity of service may be threatened by airborne contamination.

The dielectric medium is the sulfur hexafluoride (SF6) gas, which became commercially available in 1947. SF6 has been used as an insulating medium in electronic devices, power apparatus, and HVDC converter stations. Its excellent properties make it ideally suited both as an insulating and as an arc-quenching agent. SF6 gas is colorless, odorless, chemically inert, nontoxic, nonflammable, and noncorrosive.

Its dielectric strength is greatly superior to that of air, and it is close to 100 times as effective as air in quenching an electric arc. Pure SF6 is heavier than air, which causes it to settle in low areas, thus diluting oxygen in air. It is therefore necessary to learn proper safety rules before entering any area where pockets of SF6 could accumulate.

Although the gas is self-restoring, during its exposure to an electric arc it will yield decomposition by products. In the presence of moisture, which is especially the case in failed and ruptured equipment, these by-products will hydrolyze, and all resulting reaction products must be considered hazardous.

The level of gas pressure at which the equipment will operate to meet specified ratings is a function of the relationship between diameters of the conductor and the enclosure (the size of the gap), and the temperature at which the equipment will operate. At the higher pressures, the gas would liquefy at higher temperatures.

At lower pressures, dielectric strength and arc-quenching qualities of the gas would be reduced. Therefore, the gas-insulated equipment operating pressure is usually between 0.35 and 0.52 MPa (50 and 75 lb/in2, gage).

Environmental effects of SF6 that might be released to the atmosphere from GIS have been thoroughly studied. SF6 does not affect the earth’s ozone layer, but it is a strong greenhouse gas. Relative to CO2, it has a global warming potential of 23,400 due to its infrared absorption and emission characteristics and very long life in the atmosphere (half-life is projected to be 3200 years).

Fortunately, the concentration of SF6 in the atmosphere is very low, and with proper handling, leak checking, and recycling, the contribution of SF6 to anthropogenic global warming due to its use in electrical equipment can be kept below 0.1%.