Gas-insulated switchgear

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gas-insulated switchgear

A gas-insulated switchgear ( English , GIS gas-insulated switchgear ) is - in contrast to the air-insulated switchgear ( English air-insulated switchgear, AIS ) - completely gas-tight encapsulated switching installation for high-voltage and medium-voltage , which for isolating the electrical conductor or conductors with sulfur hexafluoride ( SF 6 ) as a protective gas. In contrast to air-insulated switchgear, compact switchgear can be implemented in tight spaces.

Basics

Control panel of a GIS for 110 kV
Cutaway model of a GIS with cable termination, isolator, current transformer and circuit breaker

Gas-insulated switchgear are much more compact than air-insulated switchgear because SF 6 has a dielectric strength three to four times higher than air at normal pressure. In addition, SF 6 supports the extinguishing of spark gaps more effectively than air. Gas-insulated switchgears are usually designed for indoor use, but can easily be converted for outdoor use. GIS have almost completely displaced air-insulated switchgear in the area of ​​indoor applications due to their smaller size. Air-insulated switchgear in medium-voltage networks up to approx. 20 kV have long been cheaper in terms of acquisition and maintenance costs, but remotely controllable gas-insulated switchgear systems have been much cheaper to operate since the turn of the millennium and, depending on the manufacturer, are maintenance-free.

The insulation gas is kept under pressure of 5 bar to 10 bar in order to ensure the insulation capability. The reason for the gas pressure, which is higher than normal pressure, is that the mean path length of the free electrons in the gas, described by Paschen's law , is reduced. Due to the electric field strength, electrons cannot be accelerated as strongly at high gas pressure on average as at normal pressure and hit the SF 6 molecules.

Because SF 6 is the most harmful of all greenhouse gases , around 23,000 times stronger than CO 2 , this market development was temporarily halted due to the impending ban. In the meantime, however, there are exemptions for gas-insulated switchgear, which were justified with small amounts of escaping gas and orderly disposal . In addition, various gas mixtures such as nitrogen (N 2 ) and carbon dioxide (CO 2 ) can reduce the absolute SF 6 content in the insulating gas. In the case of gas-insulated pipelines (GIL) without switching operations in the pipe sections, the SF 6 content can be reduced to around 5%. However, these mixed gases are more sensitive to impurities. Impurities lead to a reduction in the insulation strength and require a higher gas pressure, which increases the mechanical effort and system costs.

As an alternative to SF 6 , ABB launched a new insulating gas Air + in 2015 . The new gas mixture consists of fluoroketones (C 5 F 10 O) and carbon dioxide (CO 2 ) or nitrogen. In August 2015, the first Air + -insulated switchgear with 170 kV and 24 kV was put into operation by the Zurich City Electricity Company (ewz).

Designs

GIS are generally not sold as a single breaker , but rather as panels. A bay is understood to mean the circuit breaker or switch disconnector , the busbar and line (cable) disconnectors, the line (cable) grounding disconnector, the current and voltage transformers , the cable connections or outdoor bushings and the circuit breaker drive .

Behavior in the event of internal fault

GIS in an open-air system with a cable transfer station for 420 kV

In the event of a fault inside the switchgear, an internal arc can occur. This can result in very high overpressures and heat build-up, which mechanically and thermally stress the switchgear and can destroy it under certain circumstances. To prove the suitability of a switchgear with regard to personal protection in the event of an arc fault, arc fault tests must be carried out. Since February 1, 2007, according to IEC 62271-200, the following criteria must be observed in the event of an accident:

  • Properly secured doors and covers will not open.
  • The housing does not tear open and no parts with a mass of more than 60 g may be thrown away from the system.
  • There are no holes in the accessible sides of the switchgear.
  • The indicators are not ignited by hot gases.
  • The enclosure of the switchgear remains connected to its earthing point.

All medium-voltage switchgears built from February 1, 2007 must meet the new IEC 62271-200 standard. Existing switchgear can continue to be operated in accordance with IEC 60298 (the old standard). As a result, there is grandfathering for all switchgear that was put into operation before this deadline.

An internal fault in high-voltage electrical systems, including arcing faults, can, in extreme cases, also endanger the system building, analogous to a gas explosion . In order to control the overpressure in the switchgear room caused by arcing faults, measures such as absorber elements in the switchgear, pressure relief ducts and pressure relief openings in the walls of the room are installed to secure the switchgear rooms.

Manufacturer

Web links

Commons : Gas Insulated Switchgear  - Collection of Images

Individual evidence

  1. ↑ Gas-insulated switchgear . wienernetze.at. Archived from the original on February 27, 2016. Info: The archive link was automatically inserted and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved February 27, 2016. @1@ 2Template: Webachiv / IABot / www.wienernetze.at
  2. ↑ a b H. Rebholz, W. Köhler, S. Tenbohlen: Dielectric strength of different gases in GIS . University of Stuttgart, 2005 ( Online [PDF; 386 kB ]).
  3. ↑ Breakthroughs in switchgear technology with eco-efficient gases as an alternative to SF6. (PDF) ABB, accessed on August 4, 2020 .