Shahid D12 Grid

7/31/2019 Shahid D12 Grid

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Lab Report

Visit to 132(Kv) Grid Station

The Department of Electrical Engineering

Swedish College of Engineering and Technology

Wah Cantt

Prepared by:

M.SHAHID MANZOOR2k9-SCET-EE/64

Objective:

To observe the high voltage transmission and distribution with protection system of grid station

and how grid station is helpful for transmission and distribution and how we protect the entire system

safely.

About this Report:

This report is an overview of the principles and theory of grid station and operation. The

objective is to provide grid station personnel with a basic understanding of the major components of the

grid station and the role of each component in achieving grid station efficiency.

This report is mainly about the electrical engineering. i have tried to explore our knowledge as

more as we could but we also understand that there is a lot of things left in this report. This is my first


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attempt to present any kind of report at professional level, so this can also be one of the reasons for the

weak points in this report.

As my report mainly concern to the electrical point of view, so we have discussed almost

everything related to electrical, not in detail but we have touched almost all. Like there is knowledge

about Switch yard, Transformer, Generator. We have also discussed about the protection system of

Switch yard, Transformer, control room and switching room.

About Grid Station:

Grid stations are the station which reduce the High Voltage to Low Voltage and increase the

Current, the Current is Low and Voltage is high at the Generation Plant to reduce the energy losses

during transmission of Electricity.

We visit to 132 Kv grid stations which located in D-12 Islamabad. Its an outdoor grid station and

distributes electricity to Islamabad.

There are two transmission lines comes from Treble Dam & Mangla Dam. In this grid station use

single circuit breaker scheme. It is connected with Ring system

Inside the switch yard:Mainly there are different but most important things for the protection, measurement, metering

and for the other purposes.

1. Transformer2. Isolator3. Insulator4. Insulator strings5. Current transformer (C.T.)6. Potential transformer (P.T.)7. Relays8. Circuit Breaker (SF 6)9. Bus Bars10.Step down Transformer11.Lightening Arrester12.Earthing13.Earth Switch

Transformer:

In this grid station use step down transformer and its cooling type is oil natural and air natural

(O/N & A/N).

Its an one to one transformer and transformer ratio is 20/26 MVA. Its input is 132 KVA and

output is 11.5 KVA


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Transformer consists of following parts.

1. Power fans2. Condenser type bushing3. Oil conservator tank4. Bucholz relay5. Air drier6. Pressure relief valve7. Winding temperature controller8. Thermostat and thermometer9. Iron and oil temp detector10.Current transformers11.Tap changer12.Fire protection cooling system13.Automatic voltage regulator

Isolator:

These are used for the protection of the any unit. This is a mechanical device and information

about the opening and closing of the isolator is send to the C.C.R.

Its main purpose is to isolate one unit from the other unit. Mainly two isolators are used for the

protection of breaker.

Insulation String:

Its main purpose is to provide the insulation between the transmission line, substation,

transformer etc.

This is such an important device that without it there is not any concept of the transmission.

Current Transformer (C.T):

There are five coils of CT and these are basic purposes of using of CT.

Measurement of the current. Protection of the relays and breakers etc. Metering For the Breaker failure protection.As very high amount of current is flowing through the line so we cant measure such a big amount of

current by simple ammeters. If we want to design an ammeter for this purpose, then the size and the

cost of that meter will be very big costly respectively.


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So, to overcome this problem, we use CT which gives output in ratio like 2400/1 or 600/1 or 24/1

which means that if suppose 2400 amperes current is flowing then it will give the output as 1 ampere.

Similarly we can say that if 1 amp current is flowing in secondary of CT then its mean that 2400 or 600

amp are flowing in the primary of it.

Potential transformers (P.T):

It is used to measure the voltages and it has been installed with the bus bar to measure the

voltage across the bus bar.

We are checking for high voltage and low voltage but carefully for the low voltage because the

auxiliaries which we are running from this voltage will draw very high current if the voltage is very low

and this can damage our equipment.

So, we keep on checking for the low voltage and the high voltage. We have to use P.T. because

we cant design such a big voltmeter.

Relays:

These are operated by taking information from C.T. and P.T. Each and every thing of the system

even in the switch yard is being protected by relays because they are made very much sensitive for the

faults. They sense every fault for which it has been designed and then some other important devices are

operated from the signal of the relay.

There are many types of the relays,

Differential Protection relay.

Over current & Over voltage protection relay. Under voltage relay. Directional over current relay. Buchholz relaySimilarly there are many other types of the relays which we will discuss later.

Differential Protection Relay:

Differential protection is a very reliable method of protecting generators, transformers, buses, and

transmission lines from the effects of internal faults.


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Figure: Differential Protection of a Generator

In a differential protection scheme in the above figure, currents on both sides of the equipment are

compared. The figure shows the connection only for one phase, but a similar connection is usually used

in each phase of the protected equipment. Under normal conditions, or for a fault outside of the

protected zone, current I1 is equal to current I2 . Therefore the currents in the current transformers

secondarys are also equal, i.e. i1 = i2 and no current flows through the current relay.

If a fault develops inside of the protected zone, currents I1 and I2 are no longer equal, therefore i1 and i2

are not equal and there is a current flowing through the current relay.

Buchholz relay

Two ball-shaped floats and two glass-enclosed reed switches are visible inside this cutaway view

of a Buchholz relay

In the field ofelectric power distribution and transmission, a Buchholz relay is a safety device mounted

on some oil-filled power transformers and reactors, equipped with an external overhead oil reservoircalled a conservator. The Buchholz Relay is used as a protective device sensitive to the effects of

dielectric failure inside the equipment.

Depending on the model, the relay has multiple methods to detect a failing transformer. On a

slow accumulation of gas, due perhaps to slight overload, gas produced by decomposition ofinsulating

oil accumulates in the top of the relay and forces the oil level down. A float switch in the relay is used to

initiate an alarm signal. Depending on design, a second float may also serve to detect slow oil leaks.

If an arc forms, gas accumulation is rapid, and oil flows rapidly into the conservator. This flow of oil

operates a switch attached to a vane located in the path of the moving oil. This switch normally will

operate a circuit breaker to isolate the apparatus before the fault causes additional damage. Buchholz

relays have a test port to allow the accumulated gas to be withdrawn for testing. Flammable gas found

in the relay indicates some internal fault such as overheating or arcing, whereas air found in the relaymay only indicate low oil level or a leak

Overvoltage & Over current Protection

In grid station, high voltage power supply spikes with durations ranging from a few

microseconds to hundreds of milliseconds are commonly encountered. The electronics within these

systems must not only survive transient voltage spikes, but in many cases also operate reliably

throughout the event. In systems where power is distributed over long wires severe transients are

generated by load steps. Corroded connections between a power source and load can lead to an abruptinterruption of current flow, and a high value of dI/dt.

The best example of this condition is automotive load dump; where there is a sudden break in

the connection to the battery caused by vibration and corroded terminals. These transients pose a

difficult challenge for engineers trying to protect sensitive electronics
http://en.wikipedia.org/wiki/Reed_switchhttp://en.wikipedia.org/wiki/Electric_power_distributionhttp://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Reactance_%28electronics%29http://en.wikipedia.org/wiki/Dielectrichttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Transformer_oilhttp://en.wikipedia.org/wiki/Transformer_oilhttp://en.wikipedia.org/wiki/Float_switchhttp://en.wikipedia.org/wiki/Electric_archttp://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Arcinghttp://en.wikipedia.org/wiki/Arcinghttp://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Electric_archttp://en.wikipedia.org/wiki/Float_switchhttp://en.wikipedia.org/wiki/Transformer_oilhttp://en.wikipedia.org/wiki/Transformer_oilhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Dielectrichttp://en.wikipedia.org/wiki/Reactance_%28electronics%29http://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Electric_power_distributionhttp://en.wikipedia.org/wiki/Reed_switch


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Circuit Breakers (SF6):

Sulfur hexafluoride (SF6) gas is an alternative to air as an interrupting medium. SF6 is a

colorless nontoxic gas, with good thermal conductivity and density approximately five times that of air.

SF6 is chemically inert up to temperature of 150 C and will not react with metals, plastics, and other

materials commonly used in the construction of high voltage circuit breakers.

In this grid station the pressure of SF6 gas is 6.4 bar.

Principal

The principle of operation is similar to the air blast breakers, except that the SF6 gas is not

discharged into the atmosphere. A closed circuit completely sealed and self-contained construction is

used. The equipment consists of a compressor, a storage container, a blast valve that admits gas to the

interrupting chamber, and a filter through which the exhaust gas is returned to the compressor. This iscalled the double pressure breaker design. Improvement on the double pressure design is the puffer

design, also sometimes called the single pressure design. SF6 gas is normally under constant pressure of

about 5 ATM. During the opening operation the gas contained inside a part of the breaker is

compressed by moving cylinder that supports the contacts or by a piston. This forces the SF6 through

the interrupting nozzle.

By connecting several interrupting heads in series, SF6 breakers can be constructed for voltages of up to

765 kV

Sequence of Opening Operation in SF6 Puffer Breaker:

Figure 17 Sequence of Opening Operation in SF6 Puffer Breaker


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Earthing Switch:

Earthing switch connect the live parts/ line conductors and earth. This switch is normally open.

Earthing switch is used to earth the live parts during maintenance and during testing.

During maintenance although circuit is open still there are some voltages on line , due to

which capacitance between line and earth is charged. Before proceeding to maintenance work the

voltage s discharged to earth, by closing the earth switch.

Maintenance Earthing Switch:

These are two or three pole units with a manual operating mechanism.

High Speed Earthing Switch:

These are operated by spring energy. Spring is charged by motor-mechanism.

Battery Room:

PURPOSE:

The purpose of the battery room is to provide D.C. supply needed for the relay action (mostly

for protection purposes).

They are also source of excitation in case of blackout thud have vital use as

D.C backup supply.

Control Room:

Component of Control Room

Protection Relays panels Auto Transformer Bank (ATB) Panel Bus bar Panel Shunt Reactor Panel Rectifier Extinguisher
http://electrical-engineering-tutorials.blogspot.com/2010/09/earthing-switch.htmlhttp://electrical-engineering-tutorials.blogspot.com/2010/09/earthing-switch.html

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Shahid D12 Grid