Major components you can spot while looking at HV/EHV GIS (Gas-insulated switchgear)

Introduction to GIS sections / bays

Gas-insulated switchgear (GIS) is a piece of high voltage equipment that is being constantly developed day by day. The basics of GIS technology is more or less the same, but everything else under the hood is improved a lot comparing to just a few years ago. This article explains major GIS components and their characteristics.

GIS are available internationally, covering the complete voltage range from 11 kV to 800 kV. The thermal current-carrying capacities and the fault-withstanding capabilities are tailored to meet all the substation requirements. More than 200,000 GIS bays have been in service all over the world since the introduction of such substation systems in the transmission and distribution field.

Figure 1 shows a single line diagram of a section at a substation identifying different components. Single busbar, double busbar and 3/2 circuit breaker are popular configurations at substations.

In GIS, the modular components are assembled together to form a desired arrangement for a section or a bay. Figure 2 shows a cross-section of a double bus GIS section. Here, the constituent components are assembled side by side. The porcelains and connections (ACSR conductors), as required in a yard substation, are totally eliminated in this new configuration.

The high voltage conductors (bus bars) are supported on simple disc insulators.

Where typical double busbar feeder components are:

1. Circuit-breaker interrupter unit
2. Stored-energy spring mechanism
3. Circuit-breaker control unit
4. Busbar I
5. Busbar disconnector I
6. Busbar II
7. Busbar disconnector II
8. Work-in-progress earthing switch
9. Work-in-progress earthing switch
10. Outgoing-feeder disconnector
11. Make-proof earthing switch (high-speed)
12. Current transformer
13. Voltage transformer
14. Cable sealing end

GIS components

The following are the principle gas insulated modules for a substation:

1. Busbar
2. Disconnecting switch
3. Circuit breaker
4. Current transformer and
5. Earth switch
6. Accessories

The auxiliary gas insulated module or accessories, excluding control panel, that are required to complete a substation are terminations, instrument voltage transformer and surge and lightning arrester.

1. Busbar

The busbar is one of the most elementary components of the GIS system. Co-axial busbars are common in isolated-phase GIS as this configuration results in an optimal stress distribution. Busbars of different lengths are used in GIS to cater to the requirement of circuit or the bay formation.

Various sizes of the bus enclosures exist nowadays.

1.1 Connectors

The high voltage and high current electrical connections from one module to another in a gas insulated substation system are carried out with the help of the spring loaded plug-in contacts. Plug-in contact systems impart the maximum flexibility during assembly and dismantling. These contacts offer plug-in features and are suitable for tubular conductors.

The connections made are reliable without the need for any additional hardware to secure their location.

1.2 Insulating Materials and Insulators

The following insulating materials are commonly used in low tension (LT) and air insulated substation applications:

1. Sheet moulding compound (SMC),
2. Dow moulding compound (DMC),
3. Glass fibre reinforced plastics,
4. Compression and thermo-setting plastics, and
5. Refractory-based in-sulating materials (like cordrite and alumina)

Of these insulations, glass/silica-based systems are generally found unsuitable for SF6 applications due to their weak resistance to hydrofluoric acid (a by-product of moisture and decomposed SF6). Large shrinkage and instability at higher working temperatures prohibit the use of plastics in GIS.

Stable polymers like PTFE (poly tetra fluoroethane) are selectively used in GIS and associated accessories.

Insulating materials like PTFE (teflon) with very high volume resistivity retain electrical charges for long durations. This material property is sometimes undesirable and causes a deterioration in the performance of GIS (critically for direct current applications).

Alumina-filled epoxy matrix is a common insulating material for GIS-related applications. The filler alumina offers good resistance to decomposed SF6 products like hydrofluoric acid (HF) as compared to silica or felspar (common fillers used with epoxy).

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2. Disconnectors

Disconnectors (or disconnect switches) are placed in series with the circuit breaker to provide additional protection and physical isolation. In a circuit, two disconnectors are generally used, one on the line side and the other on the feeder side. Disconnect switches are designed for the interruption of small currents, induced or capacitively coupled.

During the closing operation, this gap is bridged by the moving contact. The moving contact is attached to a suitable drive, which imparts the desired linear displacement to the moving contact at a pre-determined design speed.

A firm contact is established between the two contacts with the help of spring-loaded fingers or the multi-lam contacts. The isolation gap is designed for the voltage class of the isolator and the safe dielectric strength of the gas.

Figure 4 shows a cross-section of an isolated-phase GIS diconnector.

An insulator is used to drive the moving contact and to isolate the drive from the high voltage components of the disconnector. The shape and size of the insulator are controlled by the electrical and mechanical requirements of the isolator. In three-phase ac systems, the individual phase isolators are ganged together to operate simultaneously.

Leak-tight rotary seals are used in gas insulated isolators for transferring motion from external drive to the gas. Disconnectors in high voltage GIS operate at SF6 pressures of 0.38 MPa to 0.45 MPa.

This is how disconnectors are operated inside SF6 filled switchgears (GIS).

The operating speed of the disconnector moving contact ranges from 0.1 to 0.3 m/sec. The design of electrostatic shields on two fixed contacts and the earth side of the drive insulator plays an important role in ensuring the satisfactory performance of a gas insulated diconnector.

Note that there are a lot of variations of disconnectors and that visually they could be visually different.

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3. Circuit breaker

The circuit breaker is the most critical part of a gas insulated substation system. The circuit breaker in a gas insulated system is metal-clad and utilises SF6 gas, both for insulation and fault interruption.

The SF6 gas pressure in a circuit breaker is around 0.65 MPa. The circuit breaker is directly connected to either current transformers or the isolators in gas. A barrier is maintained between the circuit breaker and the other connected equipment, operating at lower gas pressure, to maintain a pressure difference.

Spring, spring-hydraulic and pure hydraulic are the preferred drives for the circuit breakers of gas insulated substations.

Hydraulic drives are reliable, robust and compact as compared to their spring counterparts. Hydraulic drives can be interfaced to the circuit breaker directly without any intermediate motion seals and linkages. The spring drives are relatively cheaper and can be used only with the state-of-the-art self-blast or hybrid circuit breakers.

Opening speeds in the range of 6.0-8.0 m/sec and operating energies in the range of 4500-8500 Nm are common for operating the GIS circuit breakers. Note that mentioned values can be different depending on the manufacturer.

The circuit breaker enclosure also serves as the main support element for the individual GIS bay. The GIS circuit breakers are oriented both in horizontal and vertical configurations, depending on the system requirements and ease of installation.

A cross-section of a GIS circuit breaker is given in Figure 5 above.

Testing the timing of a GIS circuit breaker

Testing the timing of a circuit breaker within a gas-insulated switchgear (GIS). Using this measurement method, both sides of the breaker remain grounded throughout the test.

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4. Current transformer

The conventional substations use either live-tank or dead-tank type current transformers with oil/SF6 insulation. A porcelain insulator is used to insulate the low potential section of the current transformer from the high voltage zone.

Ribbon or cut silicon steel cores are used for the magnetic circuit of the current transformer for obtaining the desired ratio and accuracy. Hairpin shaped primary conductor is the standard geometry for a dead-tank type current transformer. The current transformers in gas insulated systems are essentially in-line current transformers.

Gas insulated current transformers, with classical coaxial geometry, consist of the following parts:

1. Tubular primary conductor
2. Electrostatic shield
3. Ribbon-wound toroidal core and
4. Gas-tight enclosure

A ribbon-wound silicon steel core (formed in toroidal shape) is used for the magnetic circuit of the current transformer. A coaxial electrostatic shield, at ground potential, is placed between the high voltage primary and the toroidal magnetic core of the current trans-former for ensuring zero potential at the secondary of the current transformer.

The electrostatic shield also helps in generating a perfect coaxial geometry and uniform electrical field in the gas gap.

The length of the current transformer module thus changes with the number and types of current transformers specified. The magnetic core and the secondary winding assembly of the current transformer are supported in gas by an enclosure or a grounded support enveloping the core and the winding.

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5. Earth switch

Fast earth switch and maintenance earth switch are the two types of earth switches used for gas insulated substation systems. The maintenance earth switch is a slow device used to ground the high voltage conductors during maintenance schedules, in order to ensure the safety of the maintenance staff.

In such a situation, the use of a fast earth switch provides a parallel (low resistance) path to drain the residual static charge quickly, thereby protecting the instrument voltage transformer from the damages that may otherwise be caused. The basic construction of these earth switches is identical.

The earth switch is the smallest module of a gas insulated substation system. The module is made up of two parts:

1. Fixed contact, which is located at the live bus conductor and which forms a part of the main gas insulated system;
2. Moving contact system mounted on the enclosure of the main module and aligned to the fixed contact.

SF6 fast earthing switch operation

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6. Accessories

Incomer and feeder connections are the main accessories of a substation. At the incomer, the supply is received from a higher level substation or from a ring main.

The power is received and delivered through either the underground cables or the overhead lines, at a substation. If they are economically viable, underground cables are also employed for other similar power installations. In either case, interfaces are required to receive/deliver the power.

Beyond this voltage level, conventional terminations, with capacitive foil grading and liquid insulation, are employed. For air-to-gas termination, the use of composite insulator for bushings has been gaining importance because they are light-weight, and offer better mechanical and seismic performances. Figure 5.16 shows a gas-to-air bushing featuring a composite insulator.

An instrument voltage/potential transformer, used for metering and protection, forms a part of the GIS and is gas insulated. This equipment is directly mounted and connected to GIS, at times with an isolator /disconnector in series.

In exceptional cases, lower kV class substations are also equipped with surge arresters to provide additional safety and reliability. The conventional yard surge/lighting arresters are used for gas insulated substation systems, where overhead lines are used to source/deliver the energy.

6.1 Control panel

Both local and remote control panels are used in GIS. The local control panel (LCP) provides an access to the various controls and circuit parameters of an individual GIS bay. The local control panel facilitates the monitoring of gas pressures, status of the switchgear element and operating fluid pressures, of oil, SF6 and air.

A dedicated local control panel for each bay is a common specification. The local control panel essentially features interlocks, operating buttons and a single line diagram.

The operator can verify the status of the circuit through a glass panelled clear door, containing the mimicked single line diagram, indicators and push buttons. The circuit operations are possible only by the authentication and authorisation process based on physical issuance of the ‘clear door key’ by the concerned authority.

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Sources:

1. Switchgears book by BHEL – Bharat Heavy Electricals Limited
2. Gas-insulated switchgear up to 550 kV, 63 kA, 5000 A, Type 8DQ1 – SIEMENS