SF6 Gas Insulated Switch Gear (GIS)

SF6 Gas Insulated Switchgear (GIS)

N.MAHESH

D.ANIL KUMAR

09261A0240

09261A0216

¾ E.E.E, B.TECH

¾ E.E.E,B.TECH

MGIT , GANDIPET

MGIT,GANDIPET

Mobile No. 7416380626

Mobile No.9000179052

[email protected]

[email protected]

Abstract:   In 1960 the first gas insulated metalenclosed 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.

The range of application of SF ₆ 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 SF ₆ 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, circuitbreakers, instrument transformers, cable terminations and joints are contained in earthed enclosures filled with sulphur hexafluoride gas (SF ₆).

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.

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.

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.

AIS Substation

Comparison of Required Area between 220 kV GIS S/S and AIS S/S

GIS

Comparison between GIS & AIS

GIS Substation

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.

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

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.

2. Disconnectors 3. Earthing Switch 4. Current Transformer 5. Voltage Transformers 6. Cables Compartment 7. Control Cubical

Basic components of the circuit breaker are shown in the figure below as follow: 1. Cover 2. Support insulator

Circuit Breaker :

3. Barrier insulator 4. Interrupter

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

5. Cast enclosure 6. Pressure relief devices 7. Fixed contact assembly 8. Bottom plate

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.

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

Earthing Switch Two different types of  earthing switch are normally used, the slowoperating earthing switch and the fastclosing (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 high-voltage 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 springclosing device.

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.

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 lowcapacitive 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.

The advantages of the voltage Transformer as follow:

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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.

enclosure. They are preferably placed outside on the SF6 busing or on cables.

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 gas-tight bushings for secondary connections.  Variable length for optimal.

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

Cables compartment:

It is sealing end box for underground cable termination which is suitable for SF6 gasfilled 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.

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.

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).

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

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.

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.

2. Temperature Rise Test

3. Operating and Mechanical Endurance Test

Tests of GIS: : 1. Capacitive Current Switching Test

4. Water Burst Test

5. Dielectric Test 3. Surface treatment ( cleaned and painted )

4. Voltage Transformer HV Test 1. Supply of Casting

2. Checking the Dimensions 5. Pressure and Tightness Test

1. HV Site Test

2. HV Test Equipment , Control Panel

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