Documentation By

S.THIRUMAL

thirualeee@gmail.com

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Introduction

With ever increasing the power demand now we are gone the step up voltage

of 800kV and above level in AC system 500kV and above in DC system same kind we

are committed to ensure the land demand. Due to this system and requirements we are

shifted to the Gas Insulated Substation.

AIS H-GIS GIS

AREA5696

(100%)

3776

(66%)

882

(15%)

VOLUME227840

(100%)

151040

(66%)

12348

(5%)

In GIS parts are protected from deterioration of exposure atmospheric air,

moisture, pollution ect. And as such longer life more reliable and less maintenance In

current power industry nowadays, High Voltage Apparatus with gas (SF6) insulation is

the main player. Nearly all new main intake substations are using Gas Insulated

Switchgears (GIS). It seems that a lot of people in the industry tends to think that GIS is

better than liquid insulated switchgears. There are some who feels that liquid insulated

switchgears are already obsolete and GIS is the way of the future. I have asked some of

my colleague on which is the better insulation between SF6 and oil. Not suprisingly all of

them says SF6 has better insulation capability than oil. However this is not true. In the

previous post in Gas Insulation, Paschen's Law states that the breakdown voltage is the

function of pressure multiply by distance of gap (Vs=f(p x d). ). We know that liquid is

denser than gas which means that liquid has more pressure. When we relate this with

Paschen's Law, it is proven that liquid acts as a far better insulation than gas. Liquid also

quench the arc faster than gas and has the capability to dissipate the heat better than gas

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The picture below shows the movement of electron from cathode to anode in two

different types of insulation. In gas insulation medium, the distance between molecules of

the gas is wide apart so electron can move easily. So avalanche can easily occur which in

turn will create insulation breakdown. However in liquid insulation medium, the

configuration of molecules is very near and the number of molecules are much higher

than in gas insulation medium. This means that liquid is much denser than gas and leave

a little space for electron to move.

The concept is as same as when you are speeding on a highway like a Mat Rempit. If

there are a lot of cars on the highway, you cannot ride your motorcycle very fast. But if

there are only two or three cars on the highway, then you can speed like the highway is

owned by your father. A word of advice, please ride and drive carefully.

FIG-Electron movement in gas insulation FIG-2 Electron movement in

Liquid insulation

Inductive instrument transformers for metering and protection

For measurement and protection purposes inductive, single phase current and voltage transformers are used, sometimes also three phase modern current and voltage sensors. In general voltage transformers are located in a single enclosure, separated from the residual part of the bay with a barrier insulator. The current transformer is normally integrated in the circuit breaker. Alternatively, a solution in a separate enclosure can be selected. The primary sided insulation is provided by SF6-gas for both, current and voltage

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transformers. The terminals of the secondary windings are interfaced to the substation via a terminal box consisting of a multiple bushing disk and terminal blocks.Inductive current transformers

The current transformer is designed as a low voltage transformer. The available transformation ratios, apparent output power, accuracy classes, etc. of the transformers correspond to the normal requirements of modern protection and measurement technology. Current transformers are of toroidal core type. Depending on the protection concept they can be arranged in front of or behind the circuit breaker’s interrupting unit. Usually current transformers are integrated into the flanges of the circuit breaker enclosure. Depending on the apparent output power different separate enclosures with large and small flange connections are available as well.

Inductive voltage transformerBesides the standardized versions, the voltage transformer

portfolio contains variants with Ferro-resonance damping as well as voltage transformers with an integrated isolation device. In the latter case a motor operated isolation device and a manually operated isolation device are available. Voltage transformers with integrated isolation devices are usually applied, when a cable test with a high DC

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voltage on the feeder side is performed and the voltage transformer was connected to the feeder sided cable. On the secondary side of the voltage transformer, measurement windings and an open delta winding for earth fault detection can be provided.

The voltage transformer contains side by side windings. The layers loaded with high voltage are insulated from each other by a plastic film. The intermediate spaces are impregnated in a special process with SF6-gas.

Gas Insulated TransformerIntroduction

Since gas insulated transformer does not need the conservator, the height of

transformer room can be reduced. In addition, its non-flammability and non tank-

explosion characteristics can remove the fire fighting equipment from transformer room.

As a result, gas insulated transformer, gas insulated shunt reactor and GIS control panels

can be installed in the same room. With such arrangement, a fully SF6 gas insulated

substation can be recognized

Specifications and Ratings

Rising demand for electric power in large cities has encouraged large-scale substations to

be tucked away underground in overpopulated urban areas, leading to strong demand for

incombustible and non-explosive, large-capacity gas insulated transformers from the

view point of accident prevention and compactness of equipment. In line with this

requirement, several types of large-capacity gas insulated transformer have been

developed.

The gas-forced cooling type was considered to be available for up to approximately

60MVA, while all other gas insulated transformer with higher ratings are liquid cooled.

But the liquid cooling type has the disadvantage of a complex structure for liquid cooling.

Thus, TOSHIBA began development of gas forced cooling type gas insulated

transformer, making best use of accumulated experience, latest analyzing technique and

the results of innovative research activities. As a result, TOSHIBA has delivered 275kV-

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300MVA gas cooled and gas insulated transformer, of which its structure is as simple as

the oil immersed type and is the largest capacity gas insulated transformer in the world.

Realization of gas insulated transformer

Since heat capacity of SF6 gas is much smaller than that of insulating oil, the following

measures are taken into account.

1. Raise the SF6 gas pressure to 0.5MPa

2. Produce as large flow as possible by optimizing the layout of gas ducts in the windings

3. Develop high capacity gas blower with high reliability

4. Apply highly thermal-resistant insulating materials to raise the limit of winding

temperature rises

Internal structure of gas insulated transformer

ADVANTAGE OF GAS INSULATED TRANSFORMER

1. Non-flammability

Gas insulated transformers, using incombustible SF6 gas as a insulation and cooling

medium, enable to remove a fire fighting equipment from transformer room.

2. Tank-explosion Prevention

Pressure tank enables to withstand the pressure rise in case of internal fault.

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3. Compactness

By directly coupling with gas-insulated switchgear, substation space can be minimized as

the result of compact facilities.

4. Easy installation

Oil or liquid purifying process is not necessary in case of gas insulated transformer.

5. Easy inspection and maintenance work

Only SF6 gas pressure shall be basically monitored during periodically inspection.

6. Environmentally Friendly

The use of SF6 gas abolishes the risk of oil leakage.

Circuit breaker module

The central element of the gas-insulated switchgear is the three-pole circuit breaker module enclosureComprising the following two main components:

Interrupter unit Spring-stored energy operating mechanism

(single pole or common drive)The design of the interrupter unit and of the operating mechanism is based on proven and in most cases identical designs, which have often been applied for outdoor switchgear installations.

Operating mechanism

The spring-stored energy operating mechanism provides the force for opening and closing the circuit breaker. It is installed in compact corrosion free aluminum housing. The closing spring and the opening spring are arranged so as to ensure good visibility in the operating mechanism block. The entire operating mechanism unit is completely isolated from the SF6 gas compartments. Antifriction

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bearings and a maintenance free charging mechanism ensure decades of reliable operation. Proven design principles of Siemens circuit breakers are used, such as vibration-isolated latches and load free decoupling of the charging mechanism. The operating mechanism offers the following advantages:

Defined switching position which is securely maintained even if the auxiliary power supply fails

Tripping is possible irrespective of the status of the closing spring

High number of mechanical operations Low number of mechanical parts Compact design.

Interrupter unit

The interrupter unit used in the circuit breaker for arc-quenching operates on the self-compression principle. Owing to the low amount of drive energyrequired, the mechanical forces involved are minimal. This has a positive effect on the stressing of both the circuit breaker and the enclosure. The sameinterrupter unit is used for single pole and common drive.

The current path

In the case of a self-compression circuit breaker, the current path is formed by the contact support(1), the base (6), and the moving contact cylinder (5). In the closed condition, the operating current flows through the main contact (3). An arcingcontact (4) is connected in parallel to the main contact.

Interruption of operating current

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During the breaking operation, the main contact (3) opens first and the current commutates on the arcing contact (4), which is still closed. This avoids erosion of the main contact. As the breaking opera tion progresses, an arc develops between the contacts (4). Simultaneously, the contact cylinder (5) moves into the base (6) and compresses the remaining arc-quenching gas. The compressed arc quenching gas flows through the contact cylinder (5) into the contact gap and extinguishes the arc.

Interruption of fault currentIf the short circuit current is high, the arc-quenching gas at the

arcing contact is heated considerably by the arc’s energy. This leads to a pressure rise in the contact cylinder. Consequently, the energy required for producing the arc-quenching pressure does not have to be supplied by the operating mechanism. As the switching operation progresses, the fixed arcing contact releases the outflow from the nozzle (2). The gas now flows out of the contact cylinder and through the nozzle, thus extinguishing the arc.

Arc-quenching principle1. Contact support2. Nozzle3. Main contact4. Arcing contact5. Contact cylinder6. Base

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High-speed Earthing switchThe high-speed Earthing switch used is of the so-called “pin-

type”. In this type of switch, the Earthing pin at earth potential is pushed into the tulip-shaped fixed contact. The earthing switch is equipped with a spring-operated mechanism, charged by an electric

motor.

Control and monitoring – consistent and flexible control and protection

Proven switchgear control

All the elements required for control and monitoring are accommodated in a decentralized arrangement in the high-voltage

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switching devices. The switching device control systems are factory-tested and the Switchgear is usually supplied with bay-internal cabling all the way to the integrated local control cubicle. This minimizes the time required for commissioning and reduces the possibilities of error. By default, the control and monitoring system is implemented with electromechanical components. Alter natively, digital intelligent control and protection systems including comprehensive diagnostics and monitoring functions are available. More detailed information on condition of the substation state permits condition-based maintenance. This consequently reduces life cycle costs even further

Gas monitoring

Each bay is divided into functionally distinct gas compartments (circuit breaker, disconnector, voltage transformer, etc.). The gas compartments are constantly observed by means of density monitors with integrated indicators; any deviations are indicated as soon as they arrive at the defined response threshold. The optionally available monitoring system includes sensors that allow remote monitoring and Trend forecasts for each gas compartment.

Flexible and reliable protection in bay and substation control

Control and feeder protection are generally accommodated in the local control cubicle, which is itself integrated in the operating panel of the switchgear bay. This substantially reduces the amount of time and space required for commissioning. Alternatively, a version of the local control cubicle for installation separate from the switchgear is available. Thus, different requirements with respect to the arrangement of the control and protection components are easy to meet. The cabling between the separately installed local control cubicle and the high-voltage switching devices is effected via coded

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plugs, which minimizes both the effort involved and the risk of cabling errors. Of course we can supply high-voltage switchgear with any customary bay and substation control equipment upon request.

We provide uniform systems to meet your individual requirements.Neutral interfaces in the switchgear control allow interfacing

Conventional control systems with contactor interlocking and control panel

digital control and protection comprising user friendly bay controllers and substation automation with PC operator station (HMI)

Intelligent, uniformly networked digital control and protection systems with supplementary monitoring and telediagnostics functions.

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