TEXT 16. Gas Insulated Substations

A gas insulated substation (GIS) uses a superior dielectric gas, SF6 (sulfur hexaflouride), 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 substations (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 often. 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.

GIS was first developed in various countries between 1968 and 1972. After about 5 years of experience, the use rate increased to about 20% of new stations in countries where space is limited. In other countries with space easily available, the higher cost of GIS relative to AIS has limited use to special cases.

SF6 is an inert, non-toxic, colorless, odorless, tasteless, and non-flammable gas consisting of a sulfur atom surrounded by and tightly bonded to six fluorine atoms. It is about five times as dense as air. SF6 is used in GIS at pressures from 400 to 600 kPa absolute. SF6 has two to three times the insulating ability of air at the same pressure. SF6 is about one hundred times better than air for interrupting arcs. It is the universally used interrupting medium for high voltage circuit breakers, replacing the older mediums of oil and air. SF6 decomposes in the high temperature of an electric arc, but the decomposed gas recombines back into SF6 so well that it is not necessary to replenish the SF6 in GIS. The SF6 in the equipment must be dry enough to avoid condensation of moisture as a liquid on the surfaces of the solid epoxy support insulators because liquid water on the surface can cause a dielectric breakdown. Absorbants inside the GIS enclosure help keep the moisture level in the gas low.

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.

Circuit breaker. GIS uses essentially the same dead tank SF6 puffer circuit breakers used in AIS. Instead of SF6-to-air as connections into the substation as a whole, the nozzles on the circuit breaker enclosure are directly connected to the adjacent GIS module.

Current Transformers. CTs are inductive ring type installed either inside the GIS enclosure or outside the GIS enclosure. The GIS conductor is the single turn primary for the CT. CTs inside the enclosure must be shielded from the electric field produced by the high voltage conductor or high transient voltages can appear on the secondary through capacitive coupling. For CTs outside the enclosure, the enclosure itself must be provided with an insulating joint, and enclosure currents shunted around the CT. Both types of construction are in wide use.

Voltage Transformers. VTs are inductive type with an iron core. The primary winding is supported on an insulating plastic film immersed in SF6. The VT should have an electric field shield between the primary and secondary windings to prevent capacitive coupling of transient voltages. The VT is usually a sealed unit with a gas barrier insulator. The VT is either easily removable so the GIS can be high voltage tested without damaging the VT, or the VT is provided with a disconnect switch or removable link.

Disconnect and Ground Switches. Disconnect switches have a moving contact that opens or closes a gap between stationary contacts when activated by a insulating operating rod that is itself moved by a sealed shaft coming through the enclosure wall. The stationary contacts have shields that provide the appropriate electric field distribution to avoid too high a surface stress. The moving contact velocity is relatively low and the disconnect switch can interrupt only low levels of capacitive current (e.g., disconnecting a section of GIS bus) or small inductive currents (e.g., transformer magnetizing current). Ground switches have a moving contact that opens or closes a gap between the high voltage conductor and the enclosure. Fast-acting ground switches are frequently used at the connection point of the GIS to the rest of the electric power network, not only in case the connected line is energized, but also because the fast-acting ground switch is better able to handle discharge of trapped charge and breaking of capacitive or inductive coupled currents on the connected line.

Bus. To connect GIS modules that are not directly connected to each other, an SF6 bus consisting of an inner conductor and outer enclosure is used. Support insulators, sliding electrical contacts, and flanged enclosure joints are usually the same as for the GIS modules.

Direct Transformer Connections. To connect a GIS directly to a transformer, a special SF6-to-oil bushing that mounts on the transformer is used. The bushing is connected under oil on one end to the transformer’s high voltage leads. The other end is SF6 and has a removable link or sliding contact for connection to the GIS conductor. Because leakage of SF6 into the transformer oil must be prevented, most SF6-to-oil bushings have a center section that allows any SF6 leakage to go to the atmosphere rather than into the transformer.

Surge Arrester. Because the GIS conductors are inside in a grounded metal enclosure, the only way for lightning impulse voltages to enter is through the connections of GIS to the rest of the electrical system. Air insulated surge arresters in parallel with the SF6-to-air bushings usually provide adequate protection of the GIS from lightning impulse voltages.

 

1. Read and translate the text.

2. Are the following statements true or false? If false, explain why they are incorrect.

1). A GIS requires less space than an AIS.

2). SF6 possesses much less insulating ability than that of air.

3). The SF6 in GIS must be regularly replenished.

4). CTs are installed either inside or outside the GIS enclosure.

5). For CTs inside the enclosure, the enclosure itself must be provided with the shielding from the electric field produced by the conductor.

6). The VT can easily do without an electric field shield between primary and secondary windings.

7). The disconnect switch can break both low levels and medium levels of current.

 

3. Answer the questions about the text.

1). What does SF6 stand for?

2). What are the advantages of GIS compared to AIS?

3). What happens to SF6 after it decomposes in the high temperature of an electric arc?

4). What parts of typical GIS can you name?

5). How does a disconnect switch work?

6). What is the function of a ground switch?

7). What prevents leakage of SF6 into the transformer oil?

8). Where can lightning impulse voltages enter the GIS?

 

4. Complete the table. Use a dictionary if necessary. Do not fill in the shaded boxes.

Noun	Verb	Adjective	Adverb
	interrupt
		active
		capable
frequency
age
		necessary

 

 

5. Complete the sentences using the words from the box below.

varies internal experience capability standards

1). Service life of GIS has been shown by … to be more than 30 years.

2). The pressure of the SF6 gas … with temperature.

3). The insulating and interrupting … of the SF6 gas depends on the density of the SF6.

4). … specify how the tests must be done.

5). Experience has shown that the … parts of GIS are so well protected inside the metal enclosure that they do not age.

 

6. Make a written annotation to the text.