11.2 Sf6-gas-insulated Switch Gear (gis)

Triple busbars A

Bus coupling SSI/II/III

A I II III

B

I II III

I II III

Section- and bus coupling for all possible ties between the 6 sections A-B

B

US

Bus coupling SSI/II/III for A or B Bypass coupling SSI/II/III to bypass (US) for A or B

A

LTr

B

I II III

US

Section coupling for A-B, Bus coupling SSI/II/III via LTr, Bypass coupling A SSI/II/III to bypass, Bypass coupling B/ bypass via LTr

11.2 SF6 gas-insulated switchgear (GIS)

The range of application of SF6 gas-insulated switchgear extends from voltage ratings of 72.5 up to 800 kV with breaking currents of up to 63 kA, and in special cases up to 80 kA. Both small transformer substations and large load-centre substations can be designed with GIS technology. The distinctive advantages of SF6 gas-insulated switchgear are: compact, low weight, high reliability, safety against touch contact, low maintenance and long life. Extensive in-plant preassembly and testing of large units and complete bays reduces assembly and commissioning time on the construction site. GIS equipment is usually of modular construction. All components such as busbars, disconnectors, circuit-breakers, instrument transformers, cable terminations and joints are contained in earthed enclosures filled with sulphur hexafluoride gas (SF6). The “User Guide for the application of GIS” issued by CIGRÈ WG 23-10 includes comprehensive application information. Up to ratings of 170 kV, the three phases of GIS are generally in a common enclosure, at higher voltages the phases are segregated. The encapsulation consists of nonmagnetic and corrosion-resistant cast aluminium or welded aluminium sheet. Table 11-1 shows an overview of the various sizes.

497

11

11.2.1 General

Table 11-1 Rating data and dimensions of the GIS range from 72.5 to 800 kV Range Service voltage in kV Lightning impulse voltage Breaking current in kA Load current in A Bay width in m Bay height in m Bay depth in m Bay weight in t

ELK-04

ELK-14

72.5 – 123 550 40 2 500

EXK-01

145 – 170 750 40 – 50 3150

245 – 300 1050 40 – 63 4000

ELK-34 362 – 550 1550 40– 63 6300

ELK-4 800 2000 40 – 50 6300

0.8/1.0 2.3 3.2 2.5

1.2 3.0 4.6 3.7

1.7 3.5 5.1 7.0

2.7 4.8 6.0 11.0

4.5 7.5 8.0 14.0

11.2.2 SF6 gas as insulating and arc-quenching medium Sulphur hexafluoride gas (SF6) is employed as insulation in all parts of the installation, and in the circuit-breaker also for arc-quenching. SF6 is an electronegative gas, its dielectric strength at atmospheric pressure is approximately three times that of air. It is incombustible, non-toxic, odourless, chemically inert with arc-quenching properties 3 to 4 times better than air at the same pressure, see also Section 10.4.4. Commercially available SF6 is not dangerous, and so is not subject to the Hazardous Substances Order or Technical Regulations on Hazardous Substances (TRGS). New SF6 gas must comply with IEC 60376 (VDE 0373 Part 1). Gas returned from SF6 installations and apparatus is dealt with in IEC 60480 (VDE 0373 Part 2). SF6 released into the atmosphere is considered a greenhouse gas. With its contribution to the greenhouse effect below 0.1%, the proportion of SF6 is low compared to that of the better known greenhouse gases (carbon pressure dioxide, methane, nitrous oxide etc.). To prevent density kg/m3 102 kPa any increase of SF6 in the atmosphere, its use should in future be confined to closed systems. Devices suitable for processing and storing SF6 gas are available for this purpose. The gas pressure is monitored in the individually sealed gas compartments and in the circuit-breaker housing. The low gas losses (below 1 % per year) are taken into account with the first gas filling. Automatic make-up facilities are not necessary. The isolating gas pressure is generally 350 to 450 kPa at 20 °C. In some cases this can be up to 600 kPa. The quenching gas pressure is 600 to 700 kPa. Outdoor apparatus exposed to arctic conditions contains a mixture of SF6 and N2, to prevent the gas from liquefying. The pressure-temperature relationship of pure SF6 gas is shown in Fig. 11-1. Fig. 11-1 p/t diagram of pure SF6 gas 498

temperature

Arcing causes the decomposition of very small amounts of SF6 gas. The decomposition products react with water, therefore the gas’s moisture content, particularly in the circuit-breaker, is controlled by drying (molecular) filters. Careful evacuation before first gas filling greatly reduces the initial moisture content. Fig. 11-2 illustrates the conversion of water vapour content into dewpoint, see also Section 15.5.2.

Fig. 11-2

11

Conversion of water vapour content into dewpoint

11.2.3 GIS for 72.5 to 800 kV

SF6 switchgear type EXK/ELK For voltages from 72.5 to 800 kV ABB has five graduated module sizes of the same basic design available. The modular construction offers the advantages of quantity production, standard components, simple stocking of spares and uniform performance. By combining the various components of a module size, it is possible to assemble switching installations for all the basic circuit configurations in Section 11.1.2.They are thus able to meet every layout requirement. As a general recommendation, the intended location for totally enclosed equipment should comply with the requirements of DIN VDE 0101 for indoor switchgear installations. The buildings can be of lightweight construction, affording some protection against the outdoor elements. With minor modifications, GIS apparatus can also be installed outdoors.

499

Components The busbars are segregated by barrier insulators at each bay and form a unit with the busbar disconnectors and the maintenance earthing switches. The circuit-breaker operates on the self-blast principle. Conventional puffer-type breakers use the mechanical energy of the actuator to generate the breaker gas stream while the self-blast breaker uses the thermal energy of the short-circuit arc for this purpose. This saves up to 80% of the actuation energy. Depending on their size, the breakers have one to four breaker gaps per pole. They have single- or triple-pole actuation with hydraulic spring mechanisms, see also Section 10.4.4 and 10.4.5. Switch-disconnectors are used in smaller distribution substations. These are able to switch load currents and connect and disconnect transformers as well as unloaded lines and cables. They are able to close onto short-circuit currents and carry them for a short time. They also work on the single-pressure puffer principle and have a motordriven spring operating mechanism. The current transformers for measuring and protection purposes are of the toroidalcore type and can be arranged before or after the circuit-breaker, depending on the protection concept. Primary insulation is provided by SF6 gas, so it is resistant to ageing. Iron-free current transformers using the Rogowski coil principle are used with SMART-GIS. They allow quantized evaluation of short-circuit currents and so make it possible to create a contact erosion image of the circuit-breaker. Voltage transformers for measurement and protection can be equipped on the secondary side with two measuring windings and an open delta winding for detecting earth faults. Inductive voltage transformers are contained in a housing filled with SF6 gas. Foilinsulated voltage transformers are used, with SF6 as the main insulation. Capacitive voltage transformers can also be employed, usually for voltages above 300 kV. The high-voltage capacitor is oil-insulated and contained in a housing filled with SF6 gas. The low-voltage capacitors and the inductive matching devices are placed in a separate container on earth potential. Capacitive tappings in conjunction with electronic measuring amplifiers are also available. Electro-optical voltage transformers using the Pockels principle are also used with SMART-GIS. The cable sealing end can accommodate any kind of high voltage cable with conductor cross-sections up to 2000 mm2. Isolating contacts and connection facilities are provided for testing the cables with d.c. voltage. If there is a branch disconnector, it is sufficient to open this during testing. Plug-in cable sealing ends for cross-linked polyethylene cables are available for voltages of up to 170 kV. They consist of gas-tight plug-in sockets, which are installed in the switchgear installation, and prefabricated plugs with grading elements of silicone rubber. Plug-in cable sealing ends do not have insulating compound. They are half as long as the standard end seal. The make-proof earthing switch can safely break the full short-circuit current. A storedenergy mechanism with a motorized winding mechanism gives it a high closing speed. It may also be manually actuated.

500

Maintenance earthing switches, which may be required during servicing, are usually placed before and after the circuit-breaker. Normally mounted on or integrated in the isolator housing, they are operated by hand or motor only when the high-voltage part is dead. The maintenance earthing switch after the circuit-breaker may be omitted if there is a high-speed earthing switch on the line side. SF6 outdoor bushings allow the enclosed switchgear to be connected to overhead lines or the bare terminals of transformers. To obtain the necessary air clearances at the outdoor terminals, the bushings are splayed using suitably shaped enclosure sections. SF6 oil bushings enable transformers to be connected directly to the switchgear, without outdoor link. The bushing is bolted straight to the transformer tank. A flexible bellows takes up thermal expansion and erection tolerances and prevents vibration of the tank due to the power frequency from being transmitted to the switchgear enclosure. SF6 busbar connections are chiefly suitable for transmitting high powers and currents. They can be used for large distances, e.g. from an underground power plant or transformer station to the distant overhead line terminal, also refer to Section 11.2.7. The surge arresters are generally of the gap-less type and contain metal oxide resistors. If the installation is bigger than the protected zone of the line-side arrester, arresters can also be arranged inside the installation. It is generally advisable to study and optimize the overvoltage protection system, particularly with distances of more than 50 m. Each bay has a control cubicle containing all the equipment needed for control, signalling, supervision and auxiliary power supply.

Barrier insulators divide the bay into separate gas compartments sealed off from each other. This minimizes the effects on other components during plant extensions, for example, or in case of faults, and also simplifies inspection and maintenance. The flanged joints contain non-ageing gaskets. Any slight leakage of gas can pass only to the outside, but not between the compartments. The circuit-breaker in Fig. 11-3 has one extinction chamber per phase, that in Fig. 11-6 has three. Depending on the breaking capacity, a pole can have up to four extinction chambers connected in series. As shown in Table 11-1, the breakers can handle breaking currents of up to 63 kA. In branches where only load currents have to be switched, up to a rated voltage of 362 kV switch-disconnectors can be used instead of circuit-breakers for economic reasons. Each switching device is provided with an easily accessible operating mechanism (arranged outside the enclosure) with manual emergency operation. The contact position can be seen from reliable mechanical position indicators.

501

11

The gastight enclosure of high-grade aluminium is of low weight so that only light foundations are required. The enclosure surrounds all the live parts, which are supported on moulded-resin insulators and insulated from the enclosure by SF6 gas at a pressure of 350 to 450 kPa.

Fig. 11-3 SF6 GIS for 123 to 170 kV, section through a bay, double busbar and cable branch 1 Busbar with combined disconnector / maintenance earthing switch, 2 Circuit-breaker, 3 Current transformer, 4 Voltage transformer, 5 Combined disconnector / maintenance earthing switch with cable sealing end, 6 High-speed earthing switch, 7 Control cubicle

11.2.4 SMART-GIS A characteristic of SMART-GIS is replacement of conventional secondary technology, such as transformers, contactors and auxiliary switches with modern sensor technology and actuators. Inductive proximity switches and rotary transducers detect the position of the switching devices; the SF6 gas density is calculated from the gas pressure and temperature. Actuators control the trip solenoids and the electric motors of the mechanisms. Specially designed sensors detect current and voltage. Rogowski coils and electro-optical voltage transformers without ferromagnetic components are generally used for this purpose. To ensure secure transmission of signals, fibre-optic cables instead of the conventional hard-wired connections are used within the bay and for connection to the station control system. The process is controlled and monitored by decentralized distributed computersupported modules (PISA = Process Interface for Sensors and Actuators), which communicate with one another and with higher-order control components via a process bus. All sensors and the entire electronics for data processing and communications are selfmonitoring and software routines continuously check the hardware in use. Timer controls can be set for important data. Critical states can be avoided before they affect operation and maintenance. This results in a reduced reserve and redundancy requirement in the system and improved economy of operation.

502

11.2.5 Station arrangement Gas supply All enclosed compartments are filled with gas once at the time of commissioning. This includes allowance for any leakage during operation (less than 1 % per year). All the gas compartments have vacuum couplings, making gas maintenance very easy, most of which can be done while the station remains in operation. The gas is monitored by density relays mounted directly on the components. Electrical protection system A reliable protection system and electrical or mechanical interlocks provide protection for service staff when carrying out inspections and maintenance or during station extension, and safeguard the equipment against failure and serious damage. The fast-response busbar protection system is recommended for protecting the equipment internally.

7

2 4

1

11

6

3

Fig. 11-4 SMART-GIS Type EXK-01 for 72.5 to 123 kV, section through a switchbay with double busbar and cable feeder, 1 Busbar with combined disconnector and earthing switch, 2 Circuit-breaker, 3 Current sensor (Rogowski coil), 4 Electro-optical voltage transformer, 6 Make-proof earthing switch, 7 Control cubicle

503

Earthing Being electrically connected throughout, the switchgear enclosure acts as an earth bus. It is connected at various points to the station earthing system. For inspection or during station extension, parts of the installation can be earthed with suitably positioned maintenance earthing switches. Protective earthing for disconnected cables, overhead lines or transformers is provided by short-circuit make-proof earthing switches located at the outgoing feeders. By short-circuiting the insulation between earthing switch and metal enclosure during operation, it is possible to use the earthing switch to supply low-voltage power or to measure switching times and resistances. Thus there is no need to intervene inside the enclosure. Erection and commissioning Only lightweight cranes and scaffolding are required. Cranes of 5000 kg capacity are recommended for complete bays, lifting gear of 2000 to 4000 kg capacity is sufficient for assembling prefabricated units. Cleanliness on site is very important, particularly when erecting outdoors, in order to avoid dirt on the exposed parts of joints. The completely installed substation undergoes a voltage test before entering operation. This is done with eighty per cent of the rated power-frequency test voltage or impulse withstand voltage. If a test transformer of suitable size is available, testing is done with a.c. voltage. Resonance test equipment or generators for oscillating switching surges are commonly used with rated voltages above 245 kV.

11.2.6 Station layouts The modular construction of SF6 switchgear means that station layouts of all the basic circuit configurations shown in Section 11.1 are possible. For layout engineering, attention must be paid to DIN VDE 0101. Sufficiently dimensioned gangways must allow unhindered access to the components for erection and maintenance. Minimum gangway distances must be observed even when the cubicle doors are open. A somewhat larger floor area, if necessary at the end of the installation, facilitates erection and later extensions or inspection. A separate cable basement simplifies cable installation and distribution. Where outdoor lines terminate only at one side of the building, the required clearances between bushings determine the position of the SF6-switchgear bay. These are usually at intervals of three to four bays. If overhead line connections are brought out on both sides of the building or are taken some distance by means of SF6 tube connections, the respective feeder bays can be next to each other. Installations of the model ranges EXK-01 for 72.5/123 kV and ELK-0 for 145/170 kV as shown in Fig. 11-5 are extremely compact because of the three-phase encapsulation of all components. Combining busbar, disconnector and earthing switch into one assembly reduces the depth of the building.

504

c)

3.0

3.6

5.0

a)

Bay width1.2

7.0

11

b)

Fig. 11-5 SF6 switchgear type ELK-04 for 123 to 170 kV with double busbar (dimensions in m) a) Section at cable bay, b) Section at overhead line bay, c) Circuit and gas diagram at a) 1 Barrier insulator, 2 Busbar gas compartment, 3 Feeder gas compartment, 4 Circuitbreaker gas compartment, 5 Voltage transformer

Installations for rated voltages of 245 kV or more are single-phase encapsulated. This makes the components smaller and easier to handle. Busbar and busbar disconnector are combined in one assembly. The busbars are partitioned at each bay so that if access to the busbar compartment is necessary (e.g. for station extension) only small amounts of gas have to be stored. Partitioning each bay avoids damage to adjacent bays in the event of a fault. 505

a)

b)

Fig. 11-6 SF6 switchgear installation type ELK-14 for rated voltage 245 to 300 kV (dimensions in m) a) Cable feeder, b) Overhead Line branch

The structural type with standing breaker is preferred in all installation layouts. This allows the interrupter chambers to be easily removed from the circuit-breakers with a crane or lifting gear. Single busbars, formerly used only for small installations, have become more important owing to the high reliability of the apparatus and its outstanding availability. Plant operation has become less complicated by dividing the station into sections by means of bus-ties. 506

Bypass buses with their disconnectors add another busbar system to stations with single or double busbars. The bypass bus enables any circuit-breaker to be isolated without interrupting the feeders. A special form of the single busbar is the H connection or double H connection. It is employed chiefly for load centres in urban and industrial areas. These stations often have switch-disconnectors instead of circuit-breakers. Combined busbars: In GIS stations with double busbars the second busbar is increasingly used as a bypass bus with the aid of an additional disconnector, resulting in a so-called combined busbar. This greatly improves the station availability at little extra cost.

11.2.7 SF6-insulated busbar links SF6-insulated busbar links are particularly suitable for transmitting high power. They complement the usual cables and overhead lines for voltages above 72.5 kV, see Table 11-2. They have the following advantages over cable links: greater transmission capacity with smaller losses, low charging power, non-ageing oil-free insulation, earthed enclosure with full earth-fault current carrying capacity. Large differences in height are easily overcome. Bridging considerable distances is possible without shunt reactors. SF6-insulated tie links are often left exposed, particularly for shorter distances or in walkable, covered ducts. Owing to the low ohmic losses, extra cooling is generally unnecessary. Table 11-2 Rating data and dimensions of the SF6 insulated busbar connections type CGI (typical values) kV

72.5

123

145

245

420

550

800

Transmission output above ground underground

MVA 175 MVA 125

450 250

525 300

1200 650

3250 1600

4800 2200

7400 3300

1000 1200 1200 1500

2400

11

Service voltage

Rated current, underground

A

2100

2300

Losses at rated current, 3ph

W/m 115

105

120

148

154

180

Weight with SF6 gas, 1ph

kg/m 13.2

14.5 14.5 30.9

44.7

50.3

59.3

Gas pressure at 20 °C

kPa

420

420

420

420

420

420

420

External diameter

mm

165

240

240

310

470

510

620

Centre-to-centre distance of phases

mm

305

370

370

460

660

710

810

Right-of-way width

mm

1200 1300 1300 1500

2100

2300

2600

105

507