GIS and hybrid switchgear technology



The need for higher reliability, more compactness and insensitivity to environmental factors are the key drivers for the evolution of SF6 gas insulated switchgear (GIS). The higher the voltage the more applicable is the use of GIS switchgear. With today’s ever-increasing land and civil cost for substations, the ability to accommodate more and more feeders in a compact space is valuable.

GIS and hybrid switchgear technology provide flexibility for substation design and commissioning. This article provides an insight into the approach followed in terms of operational features, safety and interlocks to ensure safe and reliable closed-door operation. The move towards ultra-compact switchgear without sacrificing safety and reliability is achieved by reducing the number of variables in the system prone to failure which could present danger to operation service personnel, improving the product quality, while reduction cost.

Fig. 1: General arrangement for TAMCO’s 36 kV type GV3N GIS (Front termination).

Why hybrid switchgear

SF6 gas issues

  • SF6 gas has the GWP of 23 500.
  • Annual consumption of SF6 gas in switchgear industry is 10 000 t per annum.
  • Switchgear industry is accountable for 80% usage of SF6.
  • Handling of SF6 gas on site is a nuisance, which is not desired by most of the utilities. Thus, elimination of gas handling on site will help in wastage of SF6 gas.

Approach

  • Wherever possible do not use SF6 gas: In line with this approach the company has developed air-insulated fixed type switchgear which can be rated at 12 kV, 25 kA, switching up to 2000 A.
  • Wherever possible minimise the use of SF6 gas: Air-insulated fixed type switchgear remains compact up to 12 kV range but at higher voltages it is not possible to achieve compactness using air insulation. The company has developed SF6 insulated hybrid switchgear to minimise the use of SF6 gas.

For example, TAMCO’s GV3N 36 kV hybrid GIS switchgear uses only 25% SF6 gas compared to conventional 36 kV GIS switchgear.

Hybrid SF6 switchgear

Hybrid switchgear combines the components of traditional air-insulated switchgear (AIS) and SF6 gas-insulated switchgear (GIS) technologies. It is characterised by a compact and modular design, which encompasses several different functions in one module.

Due to the introduction of vacuum technology the need for withdraw ability has come under question. This is because withdraw ability was mandatory for maintaining the breaker operating mechanism and the power path of circuit breaker. However, with the continuous evolution of superior vacuum technology there is hardly anything which can be maintained in the power path. Thus, if nothing is to be maintained then why withdraw the circuit breaker at all? This resulted in a fixed breaker design with a disconnecting switch in series performing the isolating function.

The hybrid switchgear described in this article comprises a vacuum interrupter housed inside a gas filled sealed tank. This modular approach is based on flexibility and customisability. The hybrid module can be used for extension or substitution in any traditional substation which uses an air-insulated busbar.

Fig. 2: General arrangement for TAMCO’s 36 kV type GV3N GIS (Rear termination).

Hybrid GIS switchgear advantages

  • Air Insulated busbar: The air insulated busbar is relatively inexpensive while offering proven reliability. Screened busbars can be quickly and easily assembled at site. No SF6 handling making it environment friendly.
  • All live contacts in SF6: All switching units including the interconnecting conductors are insulated in SF6. The use of SF6 technology makes the hybrid switchgear virtually maintenance free, this combines with a high level of reliability to ensure a lower life cycle cost.
  • Factory pre-assembled and tested: The hybrid modules are fully pre-assembled and tested in the factory. This ensures a higher quality of finished bay than if it is assembled under site conditions, minimises installation time on site. Reduces the possibility of delay due to adverse site conditions and there is less need for skilled resources on site.
  • Substation modularisation: A modular approach to substation design offers cost and time savings during the design and construction phases. The use of standardised components reduces the number of possible variations and hence the risk of design errors. More predictable costs also offer a higher level of confidence in the project estimation process.
  • Space saving and reduced civil works: The hybrid design can save up to 70% of the space normally required for a conventional AIS substation, while also reducing the need for civil works such as foundations, steelwork and cable trenching operations.

Design approach

Safety

Safety is the primary requirement in any switchgear product design as it involves human for operation. The factors that need to be considered are:

  • Internal arc safe with independent duct for release of hot gases.
  • Closed door safe operation.
  • Safe and fool-proof interlocks.
  • Operation guide via mimic diagram.
  • Mechanical indicators.

Fig. 3: Typical construction and SLD for TAMCO’s 36 kV type GV3N GIS.

Reliability

Reliability is the second design factor that provides confidence in building reliable and uninterrupted electrical networks. The factors that provide reliability are:

  • Mechanical repeatability.
  • Robust and proven mechanism.
  • Lesser components in linkages.
  • Minimal degradation of performance with time.

Compactness

As cities expand due to population growth, the availability of land becomes constrained. Hybrid panels are designed for a compact floor space without compromising safety and reliability.

Design aspect

Construction

The units are made of high grade, pickled in oil mild steel sheets, punched and folded on numerically controlled machines and painted by an advanced cathodic electro-deposition (CED) process which provides optimum protection against corrosion and weathering. Cubicle parts are fitted with gas sealed compartments for vacuum circuit.

The cubicle comprises two or three HV compartments and a control circuit compartment:

  • Busbar compartment in air with touch-proof (screened) busbars system.
  • Cable compartment in air with touch-proof (screened) cable termination system.
  • VCB and DS compartment in SF6 gas.
  • Low voltage compartment in air.
Table 1: Key for Fig. 4.
1. POE key.
2. Disconnector and ESW indicator.
3. Cable chamber access selector.
4. Disconnector and ESW manual operation handle slot.
5. Open/close lever for disconnector and ESW shutter.
6. POI selector.
7. VCB manual operation handle slot.
8. VCB shutter open/close lever for manual operation.
9. Pressure gauge for GIS tank.
10. VCB manual trip/close switch.
11. VCB on/off indicator.
12. VCB TNC switch.
13. Local/remote operation selector switch.

The above approach requires minimal consumption of SF6 gas as only VCB and DS is only in SF6 gas unlike conventional GIS switchgear where all HV compartments are in SF6. By using this approach, the SF6 consumption is reduced to 1,5 kg for up to 24 kV and 4,5 kg for 36 kV switchgear respectively. This is a strike reduction in SF6 gas consumption by up to 75% in comparison to conventional GIS.

Interlocks

The company’s 36 kV GIS is designed to maximise safety in installation, operation and maintenance and complies with the latest IEC standards and fitted with all mandatory interlocks as recommended by these international standards.

Features of hybrid GIS technology

  • SF6 gas insulation with VCB as short circuit current interruption.
  • Compact floor area, low carbon footprint.
  • Minimum maintenance and sealed for life contact system.
  • Touch proof cable termination in air.
  • Ample cable termination height.
  • Plug-in type power cable termination and cable test facilities.
  • Fully modular design with extension on both sides.
  • Design for reliability with minimal maintenance.
  • All fixed references for VI thereby ensuring repeated mechanical parameters.
  • All switches are pad-lockable on customer demand.
  • Utilising an “on”, “off”, “earth” disconnector switch that is fully mechanically-interlocked with VCB.

Fig. 4: Typical panel user interface for non-motorised DS (See key in Table 1).

Conclusion

Sustainability is the ability to anticipate the future and react accordingly to what will determine success. Since the product will be used as a protective device for the system as well as keeping operating and service personnel safe, the technology has aligned its growth and success to the needs of economic, environmental and social aspects in an equitable and inclusive way.

Any adverse effect on environment is minimised by the development and deployment of clean technology during the course of project. The project aimed to demonstrate the project/business growth can go hand-in-hand with an underlying commitment to the environment and society. The use of cutting-edge technology has generated multiple advantages. Hybrid GIS is a most reliable and economical solution for new installations or extensions and retrofit of existing substations minimising the faulted area and outage period in case of failure.

Acknowledgement

This article first appeared in APT’s Transmission & Distribution magazine and is republished here with permission.

Contact Coert Scholtz, Malaysian Switchgear, Tel 011 298-1800, coert@malaysianswitchgear.co.za

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