Ibrahim Ahmed Saad Al-Shamrani Engineering & Design Department – EHV Division KSA, Dammam –HQ Office Saudi Electricity Company
SF6 Gas Insulated Switchgear (GIS) August 16, 2009
Contents Abstract Acknowledgements …………………………………………………..….. 3 Introduction …………………………………….…………………...………..4 Comparison between GIS & AIS...…………………………….…………...5 GIS Components
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Circuit Breaker…….………………………………………………….9 Disconnectors ……………………………………………………….11 Earthing Switch ……………………………………………………..13 Voltage Transformer ………………………………………………..15 Current Transformer ………………………………………………..17 Cables Compartment ……………………………………………….19 Local Control Cubicles LCC ………………………………………..21
SF6 Gas Sulphur Hexafluoride …………………………………………….23 Relief Device ………………………………………………………….…….24 Tests of GIS …………………………………………………………………25
Type Test ……………………………………………………………25 Routine Test ………………………………………………………...27 Site Test ……………………………………………………………..30
Conclusion …………………………………………………………………31 Reference ………………………………………………………………….32
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Abstract In 1960 the first gas insulated metal-enclosed switchgear went into service in the German 110 kV network. Today GIS technology is successfully used for voltages up to 800 kV. GIS is an attractive alternative to conventional air insulated substations (AIS) because of the increased availability, reliability and the reduced life cycle costs. This report discusses the following aspects of SF6 GIS: enclosures; circuit breakers; disconnectors and earthing switches; voltage and current transformers; connection modules; type testing.
The high voltage GIS is highly reliable, three-phase encapsulated GIS equipment dedicated to 72.5 kV up to 145 kV applications. It can be assembled on site very quickly, and it’s compact and flexible design saves space as well as reduces civil work.
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 non-magnetic and corrosion-resistance cast aluminum or welded aluminum sheet. After general study and search this report represented the most aspects of GIS 145 kV Type respect to the Manufacturers Manual and International Standards.
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Acknowledgements I would like to express my gratitude to all those who gave me the possibility to complete this report. I want to thank the Engineering & Design – Extra High voltage Division for giving me permission to commence this report in the first instance, and to do the necessary research work and to use departmental data and for their corporation and friendly assistance.
This was a very nice experience for me to have a training course on the Areva Factory – GIS 145 kV Yard, and I am sure it will be useful for my future career. Appreciate the effort of my colleagues from several Departments those attend with me this course for their commitment and assist to deliver the material of this course to be useful for all of us. I will not forget to give all my respect to our instructor Mr. Moisés FORTES - Project Manager of Gas Insulated Substation-Areva - Araua, Switzerland for his help and kind behave.
I would like to express special thanks to Mr. Abdulaziz Al-Wayel, EHV Division Manager, not only because he is my manager but because he is helping all engineers to achieve their successes and I am really appreciating his infinite support and cooperation for offering me this opportunity to attend this training course.
I am deeply indebted to my supervisor Mr. Mohammed Al Mubark whose help, stimulating, suggestions and encouragement helped me in all the time of research for and writing of this report.
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Introduction The increasing demand of electrical power in cities and industrial centers necessitates the installation of a compact and efficient distribution and transmission network. High voltage gas insulated switchgear (GIS) is ideal for such applications. The range of application of SF₆ Gas Insulated Switchgear extends from voltage ratings of 72.5 kV 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 loadcenter 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).
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Gas Insulated Switchgear (GIS) that uses compresses sulfur hexafluoride (SF6) gas overcomes many of the limitations of the conventional open type AIS, as it offers the following advantages: 1. The space occupied by the switchgear is greatly reduced. 2. It is totally unaffected by atmospherically conditions such as polluted or saline air in industrial and coastal areas, or desert climates.
3. It possesses a high degree of operational reliability and safety to personal. 4. It is easier to install in difficult site conditions ( e.g. on unstable ground or in seismically active areas) 5. In addition to having a dielectric strength much greater than that of air, SF6 has the advantages of being nontoxic and non flammable.
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Comparison between GIS & AIS GIS Substation
AIS Substation
Comparison of Required Area between 220 kV GIS S/S and AIS S/S
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GIS Components In GIS all live parts are enclosed in compressed gas system which is divided into a number of compartments. This division enables the isolation of one compartment for maintenance or repair purpose while the other compartments remain pressurized. In figure below the single-line diagram of a double bus-bar arrangement is shown.
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In figure below the diagram shows a typical gas GIS circuit breaker Bay. Basic components that make up any one GIS bay are as follow: 1. Circuit Breaker 2. Disconnectors 3. Earthing Switch 4. Current Transformer 5. Voltage Transformers 6. Cables Compartment 7. Control Cubical
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Circuit Breaker Circuit breakers differ from switches in that they not only manually make and break the circuit while carrying their normal currents, but are also capable of making and breaking the circuit under the severest system conditions. Breaking or making the circuit under load conditions represents no real problem for a circuit breaker since the interrupted current is relatively low and the power factor is high. Under short circuit conditions, however, the current may reach tens of thousands of amperes at a power factor as low as 0.1. It is duty of a circuit breaker to interrupt such currents as soon as possible to avoid equipment damage. Loss of system stability is consequence of slow fault clearance. Fault clearance time has been reduced during the last 50 years due to the high technology adopted in circuit breaker design and the use of static relays.
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Basic components of the circuit breaker are shown in the figure below as follow: 1. 2. 3. 4. 5. 6. 7. 8.
Cover Support insulator Barrier insulator Interrupter Cast enclosure Pressure relief devices Fixed contact assembly Bottom plate
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Disconnectors Disconnectors are made up from insulators, enclosures, and conductors of different geometrical shapes to give an optimum layout. They are equipped with copper contacts that are spring loaded to give the disconnector high electrical efficiency and high mechanical reliability. Disconnectors must be carefully designed and tested to be able to break small charging current without generating too-high overvoltage, otherwise flashover to earth may occur.
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The operation mechanisms of the disconnectors and earthing switches are of the same design for most GIS. The main features are motorized or manual operation, electrical interlocking against incorrect operation, and mechanically lockable end positions
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Earthing Switch Two different types of earthing switch are normally used, the slow-operating earthing switch and the fast-closing (high speed) earthing switch. Slowoperating earthing switch are used for protection purpose when work is being done in the substation, but are operated only when it is certain that the highvoltage system is not energized. The fast-closing earthing switch can close against full voltage and short circuit power. The high speed earthing switch is achieved by means of a spring-closing device.
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The advantages of Make-proof earthing switch (high speed) are as follows: High speed closing and opening by mechanical spring charged mechanism. Induced current switching of overhead lines. Insulated by removable earth link for measurement on individual phases of the primary circuit without gas handling.
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Voltage Transformer The most commonly used voltage transformer VT is of the inductive type. In three-phase enclosed GIS designed, three voltage transformers are placed in one enclosure. It is also possible to design a voltage transformer consisting of a low-capacitive voltage divider connected to an electronic amplifier. The capacitance between the inner conductor and a concentric measuring electrode near the enclosure is then used as the high-voltage capacitor. This design is suitable only for the highest system voltages.
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The advantages of the voltage Transformer as follow: Variable location on feeder and busbars. Integrated disconnecting facility for GIS and power cable testing without dismantling and gas handling. Flexible gas compartment allocation for optimal service oriented gas supervision.
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Current Transformer In the single-phase enclosed GIS; the core of a current transformer is located outside the enclosure, thus ensuring a completely undisturbed electrical field between the enclosure and the conductor. The return current in the enclosure is broken by an insulating layer. In three-phases enclosed GIS design the cores of the current transformers are normally located inside the enclosure. They are preferably placed outside on the SF6 busing or on cables.
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The advantages of the three-phase Current Transformer as follow: Cores in air outside the SF6 Gas compartment to reduce access of moisture and to suppress gastight bushings for secondary connections. Variable length for optimal.
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Cables Compartment It is sealing end box for underground cable termination which is suitable for SF6 gas-filled for accommodating XLPE or LPOF single core copper cable terminations. The boxes shall be designed to accept the cable along with its terminations cone from below. The scope shall also include necessary cable supports and cable terminations suppliers shall be made for proper electrical and mechanical interface.
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Cables compartments have some features as follow: 1. Optimized solution for IEC dry-type (plug-in type) power cable connection. 2. Adjustable support structures for minimum requirements for the GIS floor. 3. Fixation to the GIS floor by cemented anchor bolts, no need for special foundation (steel beams….etc).
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Local Control Cubicles “LCC” LCC is the interface cubicles to all secondary systems of a substation which are represent a station control and protection. LCC includes control and alarm functions as well as the correct distribution of auxiliary power supply for the relevant GIS bay. it is in charge for a safe and reliable operation and monitoring of all switchgear elements.
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Main functions realized in the local control cubicles: AC-Supply Motor drives, heating, panel lighting, and panel sockets. DC-Supply Control and alarm indication General control functions Remote & local control, interlocking bypass, and double action prevention of the mechanisms. Control of Disconnectors & Earthing switches ON/OFF control, supervision of operating times and intermediate positions. Control of Circuit Breakers Control of the closing and tripping coils, anti-pumping, interface to synchronizing devices, interface to protection devices, and supervision of spring charging time. Arc Detection System Interlocking Interlocking between the high voltage mechanisms on the GIS Human Machine Interface (HMI) Mimic equipped with control switches and position indictors. Alarm indication & Signalization Alarm unit for alarm indication and signalization. Supervision of SF-6 Gas Compartments Supervision of the gas pressure (stage 1, stage 2) Terminal Interface 1. Remote control 2. Interlocking 3. Metering 4. Measuring 5. protection
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SF6 Gas (sulphur hexafluoride) The properties of SF6 gas are many and this is why GIS now being commonly used more than AIS because pure SF6 gas is chemically very stable, inert, almost water insoluble, non-inflammable, non-poisonous, odorless, colorless and heavier than air. Electrical discharges and arcs will decompose SF6 gas. On cooling, a large part of the decomposed gas recombines. Reactions may. However, also occur with design material (e.g. with vaporizing arcing contact material). This result in the formation of gaseous sulphur fluoride and solid metallic fluoride are powder and in the presence of water or moisture also in the development of hydrogen-fluoride and sulphur dioxide. Some of these decomposition products are conspicuous through their unpleasant piercing odour.
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Relief Device It is a device which fitted in each gas compartment including GIS surge arrester to relieve over pressure created by internal arcing faults automatically and instantaneously.
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Tests of GIS Type Test 1. Capacitive Current Switching Test
2. Temperature Rise Test
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3. Operating and Mechanical Endurance Test
4. Water Burst Test
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5. Dielectric Test
Routine (Production) Test 1. Supply of Casting
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2. Checking the Dimensions
3. Surface treatment ( cleaned and painted )
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4. Voltage Transformer HV Test
5. Pressure and Tightness Test
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Site Test 1. HV Site Test
2. HV Test Equipment , Control Panel
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Conclusion The GIS comprising of busbars, control gears (CB, DS/ES), CTs, VTs and other protective devices (rupturing disc, pressure gauges, etc) enclosed in metallic enclosure and filled with pressurized SF6 gas.
The new optimized gas-insulated switchgear unites progressive and futuristic concepts and techniques. The use of vacuum circuit breakers ensures unrestricted mechanical and electrical functionality throughout the service life of the system.
Both appropriately rated sensors and conventional instrument transformers can be used for the monitoring of current and voltage. In conjunction with the computerized, digital bay control and protection system, operator convenience, safety and system reliability are increased, and project planning, production and commissioning are simplified. The plug-in technology reduces assembly times at site to minimum.
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Reference 1. TRANSMISSION MATERIALS STANDARD SPECIFICATION 32-TMSS-02 (Revision no.0) 2. Gas-Insulated Switchgear GIS Areva Manual – Switzerland
3. CIGRE Working Group 15-03 CIGRE paper 15/23-01, 1992. 4. IEC 60044 “ Instrument Transformers” 5. IEC 60376 “Specification of Technical Grade Sulfur Hexafluoride (SF6) for use in Electrical Equipment”
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