Study of Elements of a 22013233kv Substation

8/9/2019 Study of Elements of a 22013233kv Substation

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In our project we are going to study about the operation of different equipments in

substation. It includes study of transmission lines, bus bars, circuit breakers, isolators,

earth switches, various types of transformers such as power transformer, capacitor

voltage transformer, current transformers, lightning arresters, wave traps and grounding

system of substation. We will also discuss about the various protection scheme applied in

the substation for this equipments.

The protection system is designed to limit the effects of disturbances in power

system, which when allowed persisting, may damage the substation and interrupt the

supply of electrical energy. It covers various types of protection used in substation for

220/132/33 KV transmission lines such as bus bar protection relays, auto reclosing

schemes, etc.,

The present day electrical power system is AC i.e., electric power is generated,

transmitted and distributed in the form of alternating current. The electric power is

produced at the power stations which are located at favourable places, generally quite

away from the consumers. It is delivered to the consumers through a large network of

transmission and distribution. At many places in the line of the power, it may be desirable

and necessary to change some characteristics of power supply. This is accomplished by

suitable apparatus called Substation.

Generating voltage at the power station is stepped upto high voltage for

transmission of electric power. The assembly of apparatus used for this purpose is the

substation. Similarly,near the consumers localities, the voltage may have to be stepped

down to utilization level. This job is again accomplished by a suitable apparatus called

substation. The type of equipment needed in the substation will depend upon the servicerequirement.

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2.1 Introduction

An electrical substation is a subsidiary station of an electricity generation,

transmission and distribution system where voltage is transformed from high to low or

the reverse using transformers. Electric power flows through several substations between

generating plant and consumer changing the voltage level in several stages.

A substation that has a step-up transformer increases the voltage with decreasing

current, while a step-down transformer decreases the voltage with increasing the current

for domestic and commercial distribution. The word substation comes from the days

before the distribution system became a grid. At first substations were connected to only

one power station where the generator was housed and were subsidiaries of that powerstation.

2.2 Elements of Substation

Substations generally contain one or more transformers and have switching,

protection and control equipment. In a large substation, circuit breakers are used to

interrupt any short-circuits or overload currents that may occur on the network. Smaller

distribution stations may use re-closer circuit breakers or fuses for protection of branch

circuits. A typical substation will contain line termination structures, high-voltage

switchgear, one or more power transformers, low voltage switchgear, surge protection,

controls, grounding (earthing) system, and metering. Other devices such as power factor

correction capacitors and voltage regulators may also be located at a substation.

Substations may be on the surface in fenced enclosures, underground, or located in

special-purpose buildings.

High-rise buildings may have indoor substations. Indoor substations are usually

found in urban areas to reduce the noise from the transformers, to protect switchgear

from extreme climate or pollution conditions.

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2.3 Types of Substation

Substations are of three types. They are:

a) Transmission Substation

b) Distribution Substation

c) Collector Substation

a) Transmission Substation

A transmission substation connects two or more transmission lines. The simplestcase is where all transmission lines have the same voltage. In such cases, the substation

contains high-voltage switches that allow lines to be connected or isolated for fault

clearance or maintenance. A transmission station may have transformers to convert the

voltage from voltage level to other, voltage control devices such as capacitors, reactors or

Static VAR Compensators and equipment such as phase shifting transformers to control

power flow between two adjacent power systems. The largest transmission substations

can cover a large area (several acres/hectares) with multiple voltage levels, many circuit

breakers and a large amount of protection and control equipment (voltage and current

transformers, relays and SCADA systems). Modern substations may be implemented

using International Standards such as IEC61850.

b) Distribution Substation

A distribution substation transfers power from the transmission system to the

distribution system of an area. It is uneconomical to directly connect electricity

consumers to the high-voltage main transmission network, unless they use large amounts

of power. So the distribution station reduces voltage to a value suitable for local

distribution. The input for a distribution substation is typically at least two transmission

or sub transmission lines. Input voltage may be, for example, 220KV or whatever is

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common in the area. Distribution voltages are typically medium voltage, between 33 and

66 kV depending on the size of the area served and the practices of the local utility.

Besides changing the voltage, the job of the distribution substation is to isolate

faults in either the transmission or distribution systems. Distribution substations may also

be the points of voltage regulation, although on long distribution circuits (several

km/miles), voltage regulation equipment may also be installed along the line.

Complicated distribution substations can be found in the downtown areas of large

cities, with high-voltage switching and, switching and backup systems on the low-voltage

side. Most of the typical distribution substations have a switch, one transformer, and

minimal facilities on the low-voltage side.

c) Collector substation

In distributed generation projects such as a wind farm, a collector substation may

be required. It somewhat resembles a distribution substation although power flow is in

the opposite direction. Usually for economy of construction the collector system operates

around 35 KV, and the collector substation steps up voltage to a transmission voltage for

the grid. The collector substation also provides power factor correction, metering and

control of the wind farm.

2.4 Substation Transformer Type

Further, transmission substations are mainly classified into two types depending on

changes made to the voltage level. They are:

a) Step-Up Transmission Substations.

b) Step-Down Transmission Substations.

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a) Step-Up Transmission Substation

A step-up transmission substation receives electric power from a near by

generating facility and uses a large power transformer to increase the voltage for

transmission to distant locations.

There can also be a tap on the incoming power feed from the generation plant to

provide electric power to operate equipment in the generation plant.

b) Step-Down Transmission Substation

Step-down transmission substations are located at switching points in an electrical

grid. They connect different parts of a grid and are a source for sub transmission lines or

distribution lines.

2.5 General Considerations

The general considerations regarding the substation that are discussed are

functions,design and different layouts of the substation.

a) The Functions of the substation are:

i. To Change voltage from one level to another.

ii.To Regulate voltage to compensate for system voltage changes.

iii. To Switch transmission and distribution circuits into and out of the grid system.

iv. To Measure electric power quantity flowing in the circuits.

v. To Connect communication signals to the circuits.

vi. To Eliminate lightning and other electrical surges from the system.

vii. To Connect electric generation plants to the system.

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viii. To Make interconnections between the electric systems of more than one utility.

b) Design

The main issues facing a power engineer are reliability and cost. A good design

attempts to strike a balance between these two to achieve sufficient reliability without

excessive cost. The design should also allow easy expansion of the station, if required .

Selection of the location of a substation must consider many factors. Sufficient

land area is required for installation of equipment with necessary clearances for electrical

safety and for access to maintain large apparatus such as transformers. Where land is

costly such as in urban areas, gas insulated switchgear may save money overall. The site

must have room for expansion due to load growth or planned transmission additions.

Environmental effects of the substation must be considered such as drainage, noise and

road traffic effects. Grounding (earthing) and ground potential rise must be calculated to

protect passers-by during a short-circuit in the transmission system. And of course, the

substation site must be reasonably central to the distribution area to be served.

c) Different Layouts for Substation

i) Single Bus Bar: With this design, there is an ease of operation of the substation.

This design also places minimum reliance on signaling for satisfactory operation of

protection. Additionally there is the facility to support the economical operation of future

feeder bays.

Fig 2.1 shows single bus bar Substation

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Such a substation has the following characteristics.

a. Each circuit is protected by its own circuit breaker and hence plant outage does

not necessarily result in loss of supply.

b. A fault on the feeder or transformer circuit breaker causes loss of the transformer

and feeder circuit, one of which may be restored after isolating the faulty circuit

breaker.

c. A fault on the bus section circuit breaker causes complete shutdown of the

substation. All circuits may be restored after isolating the faulty circuit breaker.

d. A bus-bar fault causes loss of one transformer and one feeder. Maintenance of one

bus-bar section or isolator will cause the temporary outage of two circuits.

e. Maintenance of a feeder or transformer circuit breaker involves loss of the circuit.

ii) Mesh Substation

The general layout for a full mesh substation is shown in the schematic Fig2.2

The characteristics of such a substation are as follows

a. Operation of two circuit breakers is required to connect or disconnect a circuit,

and disconnection involves opening of a mesh.

b. Circuit breakers may be maintained without loss of supply or protection, and no

additional bypass facilities are required.

c. Bus-bar faults will only cause the loss of one circuit breaker. Breaker faults will

involve the loss of a maximum of two circuits.

d. Generally, not more than twice as many outgoing circuits as infeeds are used in

order to rationalise circuit equipment load capabilities and rating.

Mesh substation

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Fig 2.2 shows mesh substation.

2.6 Layout

a) Principle of Substation Layouts

Substation layout consists essentially in arranging a number of switchgear

components in an ordered pattern governed by their function and rules of spatial

separation.

b) Spatial Seperation

i. Earth Clearance: This is the clearance between live parts and earthed structures,

walls, screens and ground.

ii. Phase Clearance: This is the clearance between live parts of different phases.

iii. Isolating Distance: This is the clearance between the terminals of an isolator and

the connections.

iv. Section Clearance: This is the clearance between live parts and the terminals of a

work section. The limits of this work section, or maintenance zone, may be the

ground or a platform from which the man works .

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c) Separation of maintenance zones

Two methods are available for separating equipment in a maintenance zone that

has been isolated and made dead.

i. The provision of a section clearance

ii. Use of an intervening earthed barrier

The choice between the two methods depends on the voltage and whether horizontal

or vertical clearances are involved.

i. A section clearance is composed of the reach of a man taken as 8 feet plus an

earth clearance.

ii. For the voltage at which the earth clearance is 8 feet the space required will be the

same whether a section clearance or an earthed barrier is used.

2.7 Maintenance

Maintenance plays a major role in increasing the efficiency and decreasing the

breakdown. The rules and basic principle are discussed.

Separation by earthed barrier = Earth Clearance + 50mm for barrier + Earth Clearance

Separation by section clearance = 2.44m + Earth clearance

i. For vertical clearances it is necessary to take into account the space occupied by

the equipment and the need for an access platform at higher voltages.

ii. The height of the platform is taken as 1.37m below the highest point of work.

Maintenance is done through two ways:

a) By Establishing Maintenance Zones.

b) By Electrical Separations.

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a) Establishing Maintenance Zones

Some maintenance zones are easily defined and the need for them is self evident

as in the case of a circuit breaker. There should be a means of isolation on each side of

the circuit breaker, and to separate it from adjacent live parts when isolated either by

section clearances or earth barriers

b) Electrical Separations

Together with maintenance zoning, the separation, by isolating distance and phase

clearances, of the substation components and of the conductors interconnecting them

constitute the main basis of substation layouts.

There are at least three such electrical separations per phase that are needed in a

circuit:

i. Between the terminals of the bus bar isolator and their connections.

ii. Between the terminals of the circuit breaker and their connections.

iii. Between the terminals of the feeder isolator and their connections.

2.8 Conclusion:

We have studied in detail about the substation description and in the next chapter

we are going to discuss about the line diagram of shapurnagar 220/132/33KV substation.

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3.1 Introduction

We are going to discuss about the line diagram and number of feeders of 220KV

substation and the voltage that has been transmitted to other substations and inter

connection of 220 KV line and also about the lines that feeds this substation from

generating units.

3.2 Line diagram:

In power engineering, a one-line diagram or single-line diagram is a simplified

notation for representing a three-phase power system. The one-line diagram has its

largest application in power flow studies. Electrical elements such as circuit breakers,

transformers, capacitors, bus bars, and conductors are shown by standardized schematic

symbols. Instead of representing each of three phases with a separate line or terminal,

only one conductor is represented. It is a form of block diagram graphically depicting the

paths for power flow between entities of the system. Elements on the diagram do not

represent the physical size or location of the electrical equipment, but it is a common

convention to organize the diagram with the same left-to-right, top-to-bottom sequence as

the switchgear or other apparatus represented.

We are getting power supply from two thermal power plants one is KTPS1 and

the other from which have two lines, named as Malkaram1 & Malkaram 2.

The single line diagram of 220/132/33 kV SHAPUR NAGAR sub station is

shown at the end of this report.

3.3 The interconnection of 220 KV Grid Substations

The interconnection of 220KV to different grid substations is given below,220 KV SHAPURNAGAR - GACHIBOWLI circuit No. 1.

220 KV SHAPURNAGAR - GACHIBOWLI circuit No. 2.



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3.4 Feeders

Feeder circuits are the connections between the output terminals of a

distribution substation and the input terminals of primary circuits. The distribution feeder

circuit conductors leave the substation from a circuit breaker via underground cables,

called substation exit cables. The underground cables connect to a nearby overhead

primary circuit outside the substation. This eliminates multiple circuits on the poles

adjacent to the substations there by improving the overall appearance of the substation.

Fig.3.1 shows 3-phase distribution feeder bay

This substation has two types of feeder i.e. 132 KV and 33 KV feeder. They are

12 feeders of 132 KV which are basically collector substation and it has 16 feeders of33KV which are industries and for domestic user.

a) The interconnection of 132KV Grid Substations

The interconnection of 132KV to different grid substations is given below,

i. SHAPURNAGAR - MEDCHAL- I circuit No.1.ii. SHAPURNAGAR - MEDCHAL-I circuit No. 2.

iii. SHAPURNAGAR - R.C.PURAM.

iv. SHAPURNAGAR - NARSAPUR.

v. SHAPURNAGAR - ALER.vi. SHAPURNAGAR - GUMMADI DALA.

vii. SHAPURNAGAR - BHONIGIR.viii. SHAPURNAGAR - GUNROCK.

ix. SHAPURNAGAR - MOULALI.

x. SHAPURNAGAR - IDPL.xi. SHAPURNAGAR - SANATHNAGAR RAILWAY.

xii. SHAPURNAGAR - BOLLARAM.

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b) The interconnection of 33KV Grid Substations

The interconnection of 33KV to different substations is given below,

i. SHAPURNAGAR - SHAPURNAGAR

ii. SHAPURNAGAR - JEEDIMETLA circuit No.1iii. SHAPURNAGAR - JEEDIMETLA circuit No.2iv. SHAPURNAGAR - JEEDIMETLA circuit No.3

v. SHAPURNAGAR - JEEDIMETLA circuit No. 4

vi. SHAPURNAGAR - SATYAM circuit No. 1vii. SHAPURNAGAR - SATYAM circuit No. 2

viii. SHAPURNAGAR - JAIRAJ circuit No.1

ix. SHAPURNAGAR - JAIRAJ circuit No. 2x. SHAPURNAGAR - AIRFORCE ACADEMY circuit No. 1

xi. SHAPURNAGAR - AIRFORCE ACADEMY circuit No. 2

xii. SHAPURNAGAR - RCC

xiii. SHAPURNAGAR - B.PALLIYxiv. SHAPURNAGAR - H.A.L

xv. SHAPURNAGAR - IDPL

xvi. SHAPURNAGAR - H.M.T

3.5 Conclusion

We have discussed about the line diagram of 220 KV Shapurnagar substation and

interconnection of this substation with other grid and number of feeders that are

connected to this substation and in the next chapter we are going to discuss about the

transformers.

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4.1 Introduction

We are going to discuss about the basic principle of transformer, working,

construction, losses, application and the transformers used in substation and their tapping

details.

a) Definition

A transformer is a device that transfers electrical energy from one circuit to

another through inductively coupled conductors through the transformer's coils or

windings. Except for air-core transformers, the conductors are commonly wound around

a single iron-rich core, or around separate but magnetically-coupled cores. A varying

current in the primary winding creates a varying magnetic field in the core (or cores) of

the transformer. This varying magnetic field induces a varying electromotive force

(EMF) or voltage in the secondary winding. This effect is called mutual induction.

b) Basic Principle

The transformer is based on two principles. Firstly, an electric current can produce a

magnetic field (electromagnetism) and secondly that a changing magnetic field within

the a coil of wire induces a voltage across the ends of the coil (electromagnetic

induction). Changing the current in the primary coil changes the magnitude of the

magnetic field. The changing magnetic flux link with the secondary coil where a voltage

is induced across its ends.

Fig 4.1 shows step down transformer

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A simplified transformer design is shown in Fig 4.1. A current passing through

the primary coil creates a varying magnetic field. The primary and secondary coils are

wrapped around a core of very high magnetic permeability, such as iron, This ensures

that most of the magnetic field lines produced by the primary current are within the iron

core and pass through the secondary coil as well as the primary coil. Transformers are

essential for high voltage power transmission, which makes long distance transmission

economically practical.

c) Practical Considerations

i. Effect of frequency

The time-derivative term in Faraday's Law shows that the flux in the core is the

integral of the applied voltage. Hypothetically an ideal transformer would work with

direct-current excitation, with the core flux increasing linearly with time. In practice, the

flux would rise to the point where magnetic saturation of the core occurs, causing a huge

increase in the magnetizing current and overheating the transformer. All practical

transformers must therefore operate with alternating current.

ii. Transformer universal EMF equation

If the flux in the core is sinusoidal, the relationship for either winding between its

Voltage of the winding E, and the supply frequency f, number of turns N, core cross-

sectional area a and peak magnetic flux density B is given by the universal EMF

equation:

The EMF of a transformer at a given flux density increases with frequency. By

operating at higher frequencies, transformers can be physically more compact because a

given core is able to transfer more power without reaching saturation and fewer turns are

needed to achieve the same impedance.

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However properties such as core loss and conductor skin effect also increase with

frequency. Aircraft and military equipment employ 400 Hz power supply which reduce

core and winding weight.

iii. Energy Losses

An ideal transformer would have no energy losses, and would be 100% efficient.

In practical transformers energy is dissipated in the windings, core, and surrounding

structures. Larger transformers are generally more efficient, and those rated for electricity

distribution usually perform better than 98%.Experimental transformers using

superconducting windings achieve efficiencies of 99.85%, while the increase in

efficiency is small, when applied to large heavily-loaded transformers the annual savings

in energy losses are significant.

Transformer losses are divided into losses in the windings, termed copper loss, and

those in the magnetic circuit, termed iron loss. Losses in the transformer arise from:

i. Winding resistance

ii. Hysteresis losses

iii. Eddy currents

iv. Magnetostriction

v. Mechanical losses

vi. Stray losses

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4.2 Construction

The constructional details of the transformer are

a.) Cores

i Laminated steel cores

ii Solid cores

iii Toroidal cores

iv Air cores

b) Windings

Windings are usually arranged concentrically to minimize flux leakage.

Fig. 4.2(i) shows windings of transformer

The Fig 4.2 (i) shows Cut view through transformer windings. White: insulator.

Green spiral: Grain oriented silicon steel. Black: Primary winding made of oxygen-free

copper. Red: Secondary winding. Top left: Toroidal transformer. Right: C-core, but E-

core would be similar. The black windings are made of film.

Top: Equally low capacitance between all ends of both the windings. Since most

cores are at least moderately conductive they also need insulation at Bottom.

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c) Coolant

The oil reservoir is visible at the top. Radioactive fins aid the dissipation of heat

Fig 4.2(ii) shows coolant of transformer

High temperatures will damage the winding insulation. Power transformers rated

up to several hundred KVA can be adequately cooled by natural convective air-cooling,

sometimes assissted by fans. Some power transformers are immersed in transformer oil

that both cools and insulates the windings. The oil is a highly refined mineral oil that

remains stable at transformer operating temperature. The oil-filled tank often hasradiators through which the oil circulates by natural convection some large transformers

employ forced circulation of the oil by electric pumps, aided by external fans or water-

cooled heat exchangers.

Oil-filled transformers undergo prolonged drying processes to ensure that the

transformer is completely free of water vapuor before the cooling oil is introduced. This

helps to prevent electrical breakdown under load. Oil-filled transformers may be

equipped with Buchholz relays, which detect gas evolved during internal arcing and

rapidly de-energize the transformer to avert catastrophic failure.

Experimental power transformers in the 2 MVA range have been built with

superconducting windings which eliminates the copper losses, but not the core steel loss

but these are cooled by liquid nitrogen or helium.

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d) Tappings

No-load tap changers (NLTC) or load tap changers (LTC) can be obtained on

power transformers.

The addition of no-load taps in the primary of a substation transformer makes it

possible to adapt the transformer to a range of supply voltages (usually a 10 percent

overall range of which 5 percent is above nominal and 5 percent below nominal, usually

in 2.5 percent steps). Since no-load taps are not capable of interrupting any current

including transformer charging current, the transformers have to be de-energized when

the manual no-load tap position is changed. All taps should have full capacity ratings.

Any decision to use load tap changing transformers should be based on a carefulanalysis of the particular voltage requirements of the loads served and consideration of

the advantages and disadvantages including costs of alternatives such as separate voltage

regulators.

e) Terminals

Very small transformers will have wire leads connected directly to the ends of the

coils and brought out to the base of the unit for circuit connections. Larger transformers

may have heavy bolted terminals, bus bars or high-voltage insulated bushings made of

polymers or porcelain.

A large bushing can be of complex structure since it must provide careful control

of the electric field gradient without letting the transformer leak oil.

4.3 Types and Classification Factors

A wide variety of transformer designs are used for different applications though

they share several common features. Important common transformer types include:

a. Auto transformer

b. Poly Phase transformers

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c. Leakage transformer

d. Resonant transformers

Instrument transformers

Classification of Transformers is based on following factors.

i. By power capacity: from a fraction of a volt-ampere (VA) to over a thousand

MVA.

ii. By frequency range: power, audio, or radio frequency.

iii. By voltage class: from a few volts to hundreds of kilovolts.

iv. By cooling type: air cooled, oil filled, fan cooled, or water cooled.

v. By application: such as power supply, impedance matching, output voltage and

current stabilizer, or circuit isolation.

vi. By end purpose: distribution, rectifier, arc furnace, amplifier output.

vii. By winding turns ratio: step-up, step-down, isolating (equal or near-equal ratio),

and variable.

Among the above mentioned transformers only instrument transformers are widely

used in the sub station. Hence only instrument transformers are discussed in this

section.

4.3.1 Instrument Transformer:

Instrument transformers are used to step-down the current or voltage to

measurable values. They provide standardized, useable levels of current or voltage in a

variety of power monitoring and measurement applications.

Both current and voltage instrument transformers are designed to have

predictable characteristics on overloads.

Proper operation of over-current protection relays requires that current

transformers provide a predictable transformation ratio even during a short circuit.

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These are further classified into two types which are discussed below.

a) Current Transformers

b) Voltage Transformers

a) Current Transformers:

i. Principle of Operation

A current transformer is defined as as an instrument transformer in which the

secondary current is substantially proportional to the primary current (under normal

conditions of operation) and differs in phase from it by an angle which is approximatelyzero for an appropriate direction of the connections. This highlights the accuracy

requirement of the current transformer but also important is the isolating function, which

means no matter what the system voltage the secondary circuit need to be insulated only

for a low voltage.

The current transformer works on the principle of variable flux. In the ideal current

transformer, secondary current would be exactly equal (when multiplied by the turns

ratio) and opposite to the primary current.

But, as in the voltage transformer, some of the primary current or the primary

ampere-turns are utilized for magnetizing the core, thus leaving less than the actual

primary ampere turns to be transformed into the secondary ampere-turns. This naturally

introduces an error in the transformation. The error is classified into current ratio error

and the phase error.

ii. Definitions:

Typical terms used for specifying current transformer are,

Rated primary current: The value of current which is to be transformed to a

lower value. In CT parallence, the load of the CT refers to the primary current.

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Rated secondary current: The current in the secondary circuit and on which the

performance of the CT is based. Typical values of secondary current are 1 A or 5 A.

Rated burden:The apparent power of the secondary circuit in Volt-amperes

expressed at the rated secondary current and at a specific power factor.

Composite Error: The RMS value of the difference between the instantaneous

primary current and the instantaneous secondary current multiplied by the turns ratio,

under steady state conditions.

Accuracy limit factor:The value of primary current up to which the CT compiles

with composite error requirements. This is typically 5, 10 or 15, which means that the

composite error of the CT has to be within specified limits at 5, 10 or 15 times the rated

primary current.

Short time rating: The value of primary current (in kA) that the CT should be able

to withstand both thermally and dynamically without damage to the windings with the

secondary circuit being short-circuited. The time specified is usually 1 or 3 seconds.

Class PS/ X CT: In balance systems of protection, CT s with a high degree of

similarity in their characteristics are required. These requirements are met by Class PS

(X) CT s. Their performance is defined in terms of a knee-point voltage (KPV), the

magnetizing current (Image) at the knee point voltage or 1/2 or 1/4 the knee-point

voltage, and the resistance of the CT secondary winding corrected to 75C. Accuracy is

defined in terms of the turns ratio.

Knee point voltage: The point on the magnetizing curve where an increase of

10% in the flux density (voltage) causes an increase of 50% in the magnetizing force

(current).

Summation CT: When the currents in a number of feeders need not be

individually metered but summated to a single meter or instrument, a summation current

transformer can be used.

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The summation CT consists of two or more primary windings which are

connected to the feeders to be summated, and a single secondary winding, which feeds a

current proportional to the summated primary current. A typical ratio would be 5+5+5/

5A, which means that three primary feeders of 5 are to be summated to a single 5A

meter.

Core balance CT (CBCT): The CBCT, also known as a zero sequence CT, is used

for earth leakage and earth fault protection. The concept is similar to the RVT. In the

CBCT, the three core cable or three single cores of a three phase system pass through the

inner diameter of the CT. When the system is fault free, no current flows in the secondary

of the CBCT. When there is an earth fault, the residual current (zero phase sequence

current) of the system flows through the secondary of the CBCT and this operates therelay. In order to design the CBCT, the inner diameter of the CT, the relay type, the relay

setting and the primary operating current need to be furnished.

Interposing CTs (ICTs): Interposing CTs are used when the ratio of

transformation is very high. It is also used to correct for phase displacement for

differential protection of transformer.

iii. Standards: The Indian and international standard references for CT s are asgiven in the table 4.1.

Table 4.1 shows standard reference for CTs

iv. Typical specification for a 11 kV CT

System voltage:11 kV

StandardStandard Number Year

Indian IS 2705 1992

British BS 3938 1973

American ANSI C.57.13 1978

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Insulation level voltage (ILV) : 12/28/75 kV

Ratio: 200/1 - 1 - 0.577 A

Core 1: 1A, metering, 15 VA/class 1, ISF= 150 V,Img at Vk/2


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In a practical transformer, errors are introduced because some current is drawn for

the magnetization of the core and because of drops in the primary and secondary

windings due to leakage reactance and winding resistance. One can thus talk of a voltage

error which is the amount by which the voltage is less than the applied primary voltage

and the phase error which is the phase angle by which the reversed secondary voltage

vector is displaced from the primary voltage vector.

ii. Definitions

Typical terms used for specifying a voltage transformer (VT) are:

a. Rated primary voltage: This is the rated voltage of the system whose voltage is

required to be stepped down for measurement and protective purposes.

b. Rated secondary voltage: This is the voltage at which the meters and protective

devices connected to the secondary circuit of the voltage transformer operations.

c. Rated burden: This is the load in terms of volt-amperes (VA) posed by the

devices in the secondary circuit on the VT. This includes the burden imposed by

the connecting leads. The VT is required to be accurate at both the rated burden

and 25% of the rated burden.

d. Rated voltage factor: Depending on the system in which the VT is to be used,

the rated voltage factors to be specified are different. The table 4.2 below is

adopted from Indian and International standards.

Table 4.2 shows rated votage factor for VTs

Rated

voltage

factor

Rated

timeMethod of connecting primary winding in

system

1.2 Continuous Between phases in any network.

Between transformer star-point and earth in any

network.

1.2

1.5

Continuous Between phase and in an effectively earthed

neutral system.

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1.2

1.9

Continuous

for 30

seconds

Between phase and earth in a non-effectively

earthed neutral system with automatic fault

tripping.

1.2

1.9

Continuous

for 8 hours

Between phase and earth in an isolated neutral

system without automatic fault tripping or in aresonant earthed system without automatic fault

tripping.

e. Temperature class of insulation: The permissible temperature rise over the

specified ambient temperature. Typically, classes E, B and F.

f. Residual voltage transformer (RVT): RVTs are used for residual earth fault

protection and for discharging capacitor banks. The secondary residual voltage

winding is connected in open delta. Under normal conditions of operation, there is

no voltage output across the residual voltage winding. When there is an earth

fault, a voltage is developed across the open delta winding which activates the

relay. When using a three phase RVT, the primary neutral should be earthed, or

otherwise third harmonic voltages will appear across the residual winding. phase

RVTs typically have 5 limb constructions.

iii. Standards

The Indian and international standard references for VT s are as given in the table

below 4.3

Table 4.3 shows standard references for VTs

Standard

Standard

NumberYear

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Indian IS 3156 1992

British BS 3841 1973

American ANSI C.57.13 1978

Typical specification for a 11 kV VT

System voltage: 11 kV

Insulation level voltage (ILV): 12 /28/75 kV

Number of phases: Three

Vector Group: Star / Star

Ratio: 11 kV/ 110 V

Burden: 100 VA

Accuracy: Class 0.5

Voltage Factor: 1.2 continuous and 1.5 for 30 secondswith provision for fuse

c) Coupling capacitor voltage transformers:

Coupling capacitor voltage transformers, commonly termed capacitor voltage

transformers (CVTs), are devices used for coupling to a power line to provide low

voltage for the operation of relays and metering instruments.

Power line carrier accessories or provisions for future installation of carrier

accessories may be included in the base. Coupling capacitor voltage transformers are

commonly supplied without carrier accessories, especially at voltages above 11 kV, as a

more economical alternative to inductive voltage transformers. Coupling capacitor

voltage transformers can be provided with the same ratings and accuracy as inductive

voltage transformers

Fig 4.2(iii)showing Coupling capacitor voltage transformers

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However, because of the energy-storage capability of capacitors, sudden

reductions in the power line voltage may result in momentary distortion of the CCVT

secondary voltage. The amount of distortion is related to CCVT capacitance and the

burden (secondary load) value and configuration. Modern CCVT designs are available to

minimize this problem.

4.3.2 Power Transformers

Power transformers convert power-level voltages from one level or phase

configuration to another. They can include features for electrical isolation, power

distribution, and control and instrumentation applications

EHV power transformers are usually oil immersed with all three phases in one

tank. Auto transformers can offer advantage of smaller physical size and reduced losses.

The different classes of power transformers are:

i. O.N.: Oil immersed, natural cooling.

ii. O.B.: Oil immersed, air blast cooling.

iii. O.F.N.: Oil immersed, oil circulation forced.

iv. OF.A.: Oil immersed, oil circulation forced, air blast cooling.

Power transformers are usually the largest single equipment in a substation. For economy

of service roads, transformers are located on one side of a substation and the connection to

switchgear is by bare conductors. Because of the large quantity of oil, it is essential to take

precaution against the spread of fire. Hence, the transformer is usually located around a sump

used to collect the excess oil.

4.4 TestsA number of routine and type tests have to be conducted on VT s and CTs before

they can meet the standards specified above. The tests can be classified as:

i. Accuracy tests

ii. Dielectric insulation tests

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iii. Temperature rise tests

iv. Short circuit tests.

4.5 Commissioning

Once the unit is received and packing is opened first thing is to check whether

there are any transit damages.

In case of minor damages, such as loose screws or likewise, they can be attended

immediately. In case of major damages, the report for this is to be sent to the supplier

who can immediately attend these.

Once the unit is found to have received in good condition, the following need to be

checked

i. Check the primary terminals.

ii. Check the secondary terminals.

iii. Check Earthing.

iv. Check oil level

v. Check Insulation Resistance: For primary (H.T) winding it should be minimum

500m ohms with 1000V.D.C.Meggar and for secondary (L.T) winding. It should

be minimum 25M ohms with 500V.D.C Merger.

vi. Check Ratio- for this (a) Pass the rated primary current through primary

(b) Check the secondary current across the respective Terminals.

If everything is all right, put transformer into operation verification of terminal

markings and polarity

4.6 Applications and General Instructions

There are certain applications of transformers and general instructions for

erection, uses and maintenance.

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a) Applications

A major application of transformers is to increase voltage before transmitting

electrical energy over long distances through wires. Wires have resistance and so

dissipate electrical energy at a rate proportional to the square of the current through the

wire. By transforming electrical power to a high-voltage (and therefore low-current) form

for transmission and back again afterward, transformers enable economic transmission of

power over long distances.

b) General Instructions for erection

These instructions should be adhered to with all types of instrument transformers

regardless of their technical characteristics.

i. The transformer can be lifted and moved only in vertical direction by means of

transport equipment (crane, fork truck etc.).

ii. It is forbidden to move transformer grasping it from insulator, head or high

voltage connections.

iii. It is required to undertake all necessary steps to prevent any metal part of

transport equipment (ropes, chains and similar) from getting in touch with

insulator thus avoiding damaging of glaze or insulator itself.

iv. Transformers should be mounted on corresponding supports or base and firmly

tightened for this purpose.

v. Check up whether base to which transformer is fixed is in horizontal position.

vi. Connecting cables/conductors by means of which transformer is connected to

high voltage bus-bar or supply system should be correctly dimensioned placed

and mounted not to cause additional over stresses of transformer connections.

vii. Prior to the connection of transformer compare connection diagram with

indications on the transformer and carryout connection in compliance with

corresponding indications.

viii.Properly carryout earthing on all intended spots on boxes and or base frame of

transformers.

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ix. Upon completion of above check up prior to putting in operation if assembly

properly done.

x. Put connected transformer on line.

xi. Compare instrument indicated with operational condition in supply system.

c) General Instructions for use

i. Regular periodical inspection

ii. Check up of all sealed spots in order to ascertain oil leak, if any

iii. Cleaning of insulator and possible painting of transformer.

iv. Check up of all placement of diaphragm and oil level in oil level indicators.

v. In case of damage of diaphragm or if there is no oil level indicators,

transformer should be thoroughly checked up by the service mechanic since

probably more serious defect occurred. This should be carried out at least

once a year or in two.

vi. Check up of primary and secondary connections. their cleaning and tightening

is precaution.

vii. Check up of sealed places consists of detection of oil around connections,

flanges etc. no case transformer should be opened.

viii. All earthed parts should be checked and if required, they should be cleaned

and tightened.

ix. Painting of originally painted transformer parts is advisable if required during

regular check-ups.

x. Transformer should not be opened barring in service workshop.

d) General Instructions for Maintenance

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The maintenance of transformer is usually done in specialized workshops, but if

possible also on the spot.

After the maintenance,

i. Follow all steps as said under erection, commissioning & inspection.

ii Measure insulation resistance and loss angle after major maintenance.

4.7 Conclusion

We have discussed in detailed about the working and malignances of transformers

and in the next chapter we are going to discuss about the various instrument used in

substation for protection of substation.

5.1 Introduction

We are going to discuss about the various equipment used in the substation like

lightning arresters, Control and Relay panel ,Circuit Breakers, conductor systems ,DC

Battery and Charger ,Wave Taper ,Bus bar and Isolators and their working principle and

maintenances.

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5.2 Types of Instruments

a) Lightning Arresters

Surge arrester protects the costly outdoor electrical

equipment from over voltages caused by atmospheric

disturbances due to lightning and internal disturbances due to

switching surges.

i. Construction: The assembly consists of stack of Metal

Oxide elements with contact plates between discs and held

rigidly by a tie rod assembly. The striking aspect of this

arrester is its simplicity of construction with no grading

components, no gaps either in series or in parallel.

A system of silicone bumpers on each contact plate provides

dissipation of the heat generated in the elements for Temporary

Over Voltages and Transmission Line Discharges in addition to

rugged support to prevent damage in shipping. Doubling Gaskell

seal and pressure relief vents are provided as in convention

design.

Fig 5.2(i) lightning arresters

The Pressure Relief arrangement transfers the internal arc to outside in the remote event

of arrester failure.

ii. Installation of Lightning Arresters

Three simple rules to be followed in installing lightning arresters for the effective

protection of the equipment

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i. The arrester should be connected to a ground of low resistance for effective

discharge of the surge current.

ii. The arrester should be mounted close to equipment to be protected and connected

with shortest possible leads. On both the line and ground side to reduce the

inductive effects of the leads while discharging large surge currents.

iii. To protect the transformer windings. It is desirable to interconnect the ground

lead of the arrester with the tank and also the neutral of the secondary. This

interconnection reduces the stress imposed on the transformer winding by the

surge currents to the extent of the drop across the ground.

iii. Maximum Continuous Operating Voltage

Under actual service conditions METOVAR functions as insulators at the

maximum line to ground operating voltage. For each arrester rating there is a limit to the

magnitude of the voltage that may be continuously applied. There for M.C.O.V is the

designated maximum permissible power frequency voltage that may be applied

continuously across the arrester terminal.

iv. Caution

Under no circumstances, the Maximum Continuous Power Frequency Voltage

between phase and ground appearing the arrester should exceed the arrester M.C.O.V as

specified in the name plate.

v. Packing

Each arrester is packed in a wooden box with proper cushioning material. The

terminal connectors are also packed in the same wooden box taken to see that the arresterhousing is not damaged due to rough handling

b) Control and Relay Panel

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The control and relay panel is of cubical construction suitable for floor mounting.

All protective, indicating and control elements are mounted on the front panel for ease of

operation and control. The hinged rear door will provide access to all the internal

components to facilitate easy inspection and maintenance. Provision is made for

terminating incoming cables at the bottom of the panels by providing separate line-up

terminal blocks. For cable entry provision is made both from top and bottom.

The control and relay panel accepts CT, PT aux 230 AC and 220V/10V DC

connections at respective designated terminal points. 220V/10V DC supply is used for

control supply of all internal relays and timers and also for energizing closing and

tripping coils of the breakers. 230V AC station auxiliary supply is used for internal

illumination lamp of the panel and the space heater. Protective HRC fuse are providedwith in the panel for P.T secondary. Aux AC and battery supplies.

Each Capacitor Bank is controlled by breaker and provided with a line ammeter with

selector switch for 3 phase system & Over current relay (2 phase and 1 Earth fault for 3

ph system). Under voltage and over voltage Relays.

Neutral Current Unbalance Relays are for both Alarm and Trip facilities breaker

control switch with local/remote selector switch, master trip relay and trip alarms

acknowledge and reset facilities.

c) Protective Relaying

Protective relays are used to detect defective lines or apparatus and to initiate the

operation of circuit interrupting devices to isolate the defective equipment. Relays are

also used to detect abnormal or undesirable operating conditions other than those caused

by defective equipment and either operate an alarm or initiate operation of circuit-

interrupting devices. Protective relays protect the electrical system by causing the

defective apparatus or lines to be disconnected to minimize damage and maintain service

continuity to the rest of the system

There are different types of relays.

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i. Over current relay

ii. Distance relay

iii. Differential relay

iv. Directional over current relay

i. Over Current Relay

The over current relay responds to a magnitude of current above a specified value.

There are four basic types of construction: They are plunger, rotating disc, static, and

microprocessor type. In the plunger type, a plunger is moved by magnetic attraction when

the current exceeds a specified value. In the rotating induction-disc type, which is a

motor, the disc rotates by electromagnetic induction when the current exceeds a specified

value.

Static types convert the current to a proportional D.C mill volt signal and apply it to a

level detector with voltage or contact output. Such relays can be designed to have various

current-versus-time operating characteristics. In a special type of rotating induction-disc

relay, called the voltage restrained over current relay.

The magnitude of voltage restrains the operation of the disc until the magnitude of

the voltage drops below a threshold value. Static over current relays are equipped with

multiple curve characteristics and can duplicate almost any shape of electromechanical

relay curve. Microprocessor relays convert the current to a digital signal. The digital

signal can then be compared to the setting values input into the relay. With the

microprocessor relay, various curves or multiple time-delay settings can be input to set

the relay operation. Some relays allow the user to define the curve with points or

calculations to determine the output characteristics.

ii. Distance Relay

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The distance relay responds to a combination of both voltage and current. The

voltage restrains operation, and the fault current causes operation that has the overall

effect of measuring impedance. The relay operates instantaneously (within a few cycles)

on a 60-cycle basis for values of impedance below the set value. When time delay is

required, the relays energizes a separate time-delay relay or function with the contacts or

output of this time-delay relay or function performing the desired output functions.

The relay operates on the magnitude of impedance measured by the combination of

restraint voltage and the operating current passing through it according to the settings

applied to the relay. When the impedance is such that the impedance point is within the

impedance characteristic circle, the relay will trip. The relay is inherently directional. The

line impedance typically corresponds to the diameter of the circle with the reach of the

relay being the diameter of the circle.

iii. Differential Relay

The differential relay is a current-operated relay that responds to the difference

between two or more device currents above a set value.

The relay works on the basis of the differential principle that what goes into the

device has to come out .If the current does not add to zero, the error current flows tocause the relay to operate and trip the circuit.

The differential relay is used to provide internal fault protection to equipment such

as transformers, generators, and buses. Relays are designed to permit differences in the

input currents as a result of current transformer mismatch and applications where the

input currents come from different system voltages, such as transformers. A current

differential relay provides restraint coils on the incoming current circuits. The restraint

coils in combination with the operating coil provide an operation curve, above which the

relay will operate. Differential relays are often used with a lockout relay to trip all power

sources to the device and prevent the device from being automatically or remotely re-

energized. These relays are very sensitive. The operation of the device usually means

major problems with the protected equipment and the likely failure in re-energizing the

equipment

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iv. Directional Over current Relay

A directional over current relay operates only for excessive current flow in a given

direction. Directional over current relays are available in electromechanical, static, and

microprocessor constructions. An electromechanical overcorrect relay is made directional

by adding a directional unit that prevents the over current relay from operating until the

directional unit has operated. The directional unit responds to the product of the

magnitude of current, voltage, and the phase angle between them or to the product of two

currents and the phase angle between them. The value of this product necessary to

provide operation of the directional unit is small, so that it will not limit the sensitivity of

the relay (such as an over current relay that it controls). In most cases, the directional

element is mounted inside the same case as the relay it controls. For example, an overcurrent relay and a directional element are mounted in the same case, and the

combination is called a directional over current relay. Microprocessor relays often

provide a choice as to the polarizing method that can be used in providing the direction of

fault, such as applying residual current or voltage or negative sequence current or voltage

polarizing functions to the relay.

d) Circuit Breakers

A circuit breaker is an automatically-operated electrical switch designed to

protect an electrical circuit from damage caused by overload or short circuit. Its basic

function is to detect a fault condition and these by interrupting continuity, to immediately

discontinue electrical flow.

i. Principle of Operation

All circuit breakers have common features in their operation, although details vary

substantially depending on the voltage class, current rating and type of the circuit

breaker.

The circuit breaker must detect a fault condition in low-voltage circuit breakers this

is usually done within the breaker enclosure. Circuit breakers for large currents or high

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voltages are usually arranged with pilot devices to sense a fault current and to operate the

trip opening mechanism. The trip solenoid that releases the latch is usually energized by a

separate battery, although some high-voltage circuit breakers are self-contained with

current transformers, protection relays and an internal control power source.

Once a fault is detected, contacts within the circuit breaker must open to interrupt the

circuit. Some mechanically-stored energy (using something such as springs or

compressed air) contained within the breaker is used to separate the contacts, although

some of the energy required may be obtained from the fault current itself. The circuit

breaker contacts must carry the load current without excessive heating, and must also

withstand the heat of the arc produced when interrupting the circuit. Contacts are made of

copper or copper alloys, silver alloys and other materials. Service life of the contacts islimited by the erosion due to interrupting the arc. Miniature circuit breakers are usually

discarded when the contacts are worn, but power circuit breakers and high-voltage circuit

breakers have replaceable contacts.

When a current is interrupted, an arc is generated - this arc must be contained, cooled,

and extinguished in a controlled way, so that the gap between the contacts can again

withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating

gas, or oil as the medium in which the arc forms. Different techniques are used to

extinguish the arc including:

i. Lengthening of the arc

ii. Intensive cooling (in jet chambers)

iii. Division into partial arcs

iv. Zero point quenching

v. Connecting capacitors in parallel with contacts in DC circuits

Finally, once the fault condition has been cleared, the contacts must again be

closed to restore power to the interrupted circuit.

ii. Arc Interruption

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Miniature low-voltage circuit breakers use air alone to extinguish the arc. Larger

ratings will have metal plates or non-metallic arc chutes to divide and cool the arc.

Magnetic blowout coils deflect the arc into the arc chute.

In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast

a jet of oil through the arc.

Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the arc using a

magnetic field, and then rely upon the dielectric strength of the sulfur hexafluoride (SF 6)

to quench the stretched arc.

Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than

the contact material), so the arc quenches when it is stretched a very small amount (


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Miniature circuit breakers used to protect control circuits or small appliances may not

have sufficient interrupting capacity to use at a panelboard. These circuit breakers are

called "supplemental circuit protectors" to distinguish them from distribution-type circuit

breaker.

iv. High-voltage circuit breakers

400KV SF6 circuit breakers

Electrical power transmission networks are protected and controlled by high-voltage

breakers. The definition of "high voltage" varies but in power transmission work is

usually thought to be 72,500 V or higher according to a recent definition by the

International Electro technical Commission (IEC).

High-voltage breakers are nearly always solenoid-operated, with current sensing

protective relays operated through current transformers. In substations the protection

relay scheme can be complex, protecting equipment and busses from various types of

overload or ground/earth fault.

High-voltage breakers are broadly classified by the medium used to extinguish the

arc.

i. Bulk oil

ii. Minimum oil

iii. Air blast

iv. SF6

Fig.5.2 (iii) shows

circuit breaker

High-voltage circuit breakers used on transmission systems may be arranged to

allow a single pole of a three-phase line to trip, instead of tripping all three poles.For

some classes of faults this improves the system stability and availability.

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e) Conductor Systems

An ideal conductor should fulfill the following requirements:

i. Should be capable of carrying the specified load currents and short time currents.

ii. Should be able to withstand forces on it due to its situation. These forces comprise

self weight, and weight of other conductors and equipment, short circuit forces

and atmospheric forces such as wind and ice loading.

iii. Should be corona free at rated voltage.

iv. Should have the minimum number of joints.

v. Should need the minimum number of supporting insulators.

vi. Should be economical.vii. The most suitable material for the conductor system is copper or aluminum. Steel

may be used but has limitations of poor conductivity and high susceptibility to

corrosion.

Fig.5.2(iii) shows Conductor systems

In an effort to make the conductor ideal, three different types have been utilized,

and these include:

i. Flat surfaced Conductors.

ii. Stranded Conductors.

iii. Tubular Conductors.

f ) DC Power Supply

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i. DC Battery and Charger

All but the smallest substations include auxiliary power supplies. AC power is

required for substation building small power, lighting, heating and ventilation, some

communications equipment, switchgear operating mechanisms, anti-condensation heaters

and motors. DC power is used to feed essential services such as circuit breaker trip coils

and associated relays, supervisory control and data acquisition (SCADA) and

communications equipment. This describes how these auxiliary supplies are derived and

explains how to specify such equipment.

ii. Battery and Charger configurations

Capital cost and reliability objectives must first be considered before defining the

battery and battery charger combination to be used for a specific installation. The

comparison given in Table 5.1 describes the advantages and disadvantages of three such

combinations.

Table 5.1: Capital cost and reliability objectives must first be considered before defining

the battery/battery charger combination to be used for a specific installation. The

comparison given describes the advantages and disadvantages of three such combinations

Type Advantages Disadvantages

1. Single

100% battery

and 100%

charger

Low capital cost

No standby DC System outage for

maintenance Need to isolate

battery/charger combination from load

under boost charge conditions in order to

prevent high boost voltages appearing on

DC distribution system2. Semi-

duplicate

50% batteries

and

Medium capital cost Standby DC

provided which is 100% capacity

on loss of one charger Each

battery or charger can be

------------------------------

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Table 5.1: Capital cost and reliability objectives must first be considered before defining

the battery/battery charger combination to be used for a specific installation. The

comparison given describes the advantages and disadvantages of three such combinations

Type Advantages Disadvantages

100%

chargers

maintained in turn. Each battery

can be isolated and...

iii. 220V DC Battery

Make: Exide, Capacity: 300 AH at 27

No. of Cells: 110 No. , Date of installation: 06/200

Make: Universal, Sr. No. : BC 1020/82

Date of installation: 1983

Input Rating: Voltage: 415 V + 10 %

Frequency: 50 Hz. 3 Phase

Output Rating:

Float: 220 V, 10 Amp Fig.5.2(iv) shows 220V Battery Charger

Boost: 180 V, 30Amp

g) Wave Trapper

This is relevant in Power Line Carrier Communication (PLCC) systems for

communication among various substations without dependence on the telecom company

network. The signals are primarily teleportation signals and in addition, voice and data

communication signals. Line trap also is known as Wave trap.

h) Bus Bar

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A bus bar in electrical power distribution refers to thick strips of copper or

aluminum that conduct electricity within a switchboard, distribution board, substation, or

other electrical apparatus.

The size of the bus bar is important in determining the maximum amount of current

that can be safely carried. Bus bars are typically either flat strips or hollow tubes as these

shapes allow heat to dissipate more efficiently due to their high surface area to cross-

sectional area ratio. The skin effect makes 50-60 Hz AC bus bars more than about 8 mm

(1/3 in) thick inefficient, so hollow or flat shapes are prevalent in higher current

applications. A hollow section has higher stiffness than a solid rod of equivalent current-

carrying capacity, which allows a greater span between bus bar supports in outdoor

switchyards. A bus bar may either be supported on insulators or else insulation maycompletely surround it. Bus bars are protected from accidental contact either by a metal

enclosure or by elevation out of normal reach.

Neutral bus bars may also be insulated. Earth bus bars are typically bolted directly

onto any metal chassis of their enclosure. Bus bars may be enclosed in a metal housing,

in the form of bus duct or bus way, segregated-phase bus, or isolated-phase bus.

i. Protection

Bus bars are vital parts of a power system and so a fault should be cleared as fast

as possible. A bus bar must have its own protection, although they have high degrees

of reliability. Bearing in mind the risk of unnecessary trips, the protection should be

dependable, selective and should be stable for external faults, called 'through faults'.

The most common fault is phase to ground, which usually results from human

error.

There are many types of relaying principles used in bus bar.

A special attention should be made to current transformer selection since

measuring errors need to be considered.

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i) Isolators

Isolators are used to connect and disconnect high voltage power systems under no

load conditions.

These are essentially off load devices although they are capable of dealing with small

charging currents of bus bars and connections. The design of isolators is closely related to

the design of substations. Isolator design is considered in the following aspects:

i. Space Factor

ii. Insulation Security

iii. Standardization

iv. Ease of Maintenance

v. Cost

Some types of isolators include:

i. Horizontal Isolation types

ii. Vertical Isolation types

iii. Moving Bushing types

i. Properties

The isolators comprises three identical poles (in the case of the three phase system

only) each pole consisting of

i. A Galvanized Fabricated Base out of MS Channel having one supporting

insulation mounting stool.

ii. Three post insulators stacks one for mounting one the centre rotating stool and

other two stacks on both ends of the base channel.

iii. Moving contact assembly for mounting on the centre rotating insulator stack and

the fixed contact assembly with terminal pad or two outer insulator stacks.

iv. Tandem pipe for interlinking the three poles and operating down pipe to link the

tandem pipe with the bottom operating mechanism of 3 phase system.

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v. Bottom operating mechanism box.

vi. Earthing switch moving contact assembly

vii. Earthing switch fixed contact assembly for fixing to the main switch fixed

contacts.

viii. Earthing switch operating down pipe to link earth switch tandem pipe to the

bottom

ix. Bottom operating mechanism box

x. Mechanical interlock between main switch and earthling switch.

5.3 Conclusion

We have discussed about the various types of instrument used in substances and for

protection of substation in detailed. In the next chapter we are going to discusses about

the insulators and they importances in substation

6.1 Introduction

This chapter describes the different types of overhead line and substation

insulators, their design characteristics and their application. Conductors are attached to

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their support by means of an insulator unit. For overhead lines up to 33 kV and for

outdoor substation equipment, the insulator is typically of the post insulator type. For

overhead lines above 33 kV and substation aerial conductor bus bars, suspension or

tension cap and pin or long rod insulator units are employed. Insulators must be capable

of supporting the conductor under the most onerous loading conditions. In addition,

voltage flashover must be prevented under the worst weather and pollution situations

with leakage currents kept to negligible proportions

a) Principle

The principle dielectric used on overhead power lines is air at atmospheric

pressure. The air surrounding the bare high voltage threshold. It is however necessary to

attach the conductors at certain points onto the cross arms of the pylons. The problem of

reliably suspending the conductors of high voltage transmission lines has therefore been

with us since the turn of the century. The task is particularly complex, bearing in mind

the multiple extreme stresses present are mechanical, electrical and environmental

stresses.

6.2 Types of Insulators

a) Porcelain pin type

insulators

These were originally used for

telephone lines and lightning conductors, have

been adapted for power transmission and some

variations are still in use for medium voltage

systems. A pin-type insulator is shownschematically in figure 6.2(i)and 6.2(ii)

Fig. 6.2(i)

Porcelain Insulator

b) Cap and Pin Type Insulators

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The pin-type insulator is so called because in use it is screwed onto a galvanized

forged steel 'pin' which mounted vertically on a metal or wooden cross arm.

For low voltage systems, 6.6 to 11 kV, it is usual to have a one-piece insulatorshed in which the porcelain is loaded largely in compression. A typical pin-type insulator

is shown in Figure 6.2(ii). The sketches show that the top of the porcelain body is formed

into a groove into which the conductor is bound by means of wire or fixed with the aid of

special clips. Toughened glass pin-type insulators require a metal cap; this holds together

the 'diced' pieces of glass which result if the glass becomes shattered.

Fig.62 (ii): shows cap & pin insulator

c) Post Type Insulator

These insulators consist of a solid porcelain cylinder, corrugated to increase the

leakage length, with metalware on each end. They are used to support the high voltage

conductor and are mounted on pedestals or on the power line cross arms. Post insulators

are tall and are mainly used in substations. These insulators are Class A; the shortest

distance through the porcelain exceeds 50% of the shortest distance through air between

the electrodes. They are therefore un puncturable. A typical example of a post insulator is

shown schematically in figure 6.2(iii)

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Fig.6.2(iii)post insulator

d) Porcelain Long Rod Insulators

Long rod insulators are similar to post insulators but are lighter, slimmer and are

used as suspension insulators.

Long rod insulators have the apparent advantage over cap and pin insulators in

that metal fittings exist only at the ends of the insulators.

6.3 Bushings

Bushings are used to insulate the conductors of the high voltage terminals

of a transformer as is shown schematically in figure 5. 3 Traditionally,

transformer bushings are manufactured using porcelain. Capacitive grading, using

foil cylinders is often used to improve the axial and radial field distribution.

Fig.6.3 shows bushings

6.4 Terminology

When applying insulators, it is necessary to describe the insulator dimensions,

using the following terms:

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Creep agedistance: The shortest distance between the metal ware at the two ends of the

insulator, when following the contours of the insulator, excluding intermediate metal

fittings. This distance is easily measured by sticking masking tape to the insulator

surface.

Specific Creep age distance:The quotient of the creep age distance in mm and the line-to-

line rms. voltage of the three phase system in kV.

Connecting length: The axial length of the insulator between the end terminals.

Arcing distance:The distance between the metal ware, measured as the length of a tightly

pulled piece of string.

Inter shed spacing:The distance between corresponding points on adjacent sheds.

6.5 Pollution Deposition Process

Insulators exposed to the environment collect pollutants from various sources.

Pollutants that become conducting when moistened are of particular concern.

Two major sources are:

i. Coastal pollution: the salt spray from the sea or wind-driven salt laden solid

material such as sand collects on the insulator surface. These layers become

conducting during periods of high humidity and fog. Sodium chloride is the main

constituent of this type of pollution.

ii. Industrial pollution: substations and power lines near industrial complexes are

subject to the stack emissions from nearby plants. These materials are usually dry

when deposited; they may then become conducting when wetted. The materials

will absorb moisture to different degrees, and apart from salts, acids are also

deposited on the insulator.

a) The role of the weather

Wind is instrumental in the deposition process. High humidity, fog or light rain

cause wetting of the pollution layers. Heavy rain removes the pollution layer especially

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on the upper sides of the sheds

b) Air flashover versus pollution flashover

If the electric stress in air at atmospheric pressure exceeds 3 kV mm, ionization can

occur. Depending on the gap configuration, flashover may follow. The power flashover

voltage of a clean dry single cap and pin insulator with a 280 mm creep age distance is 72

kV. Leakage current flows over the insulator surface and the heating effect of the current

causes drying out of the layer at certain spots and the formation of dry bands. Arcs

occur across these bands and if the pollution is of sufficient severity, the insulator may

flash over

6.6 Failure Modes of InsulatorsFlashovers, caused by air breakdown or pollution, generally do not cause physical

damage to the insulators and the system can often be restored by means of auto closing.

Some other events, however cause ir-repairable damage to the insulators .

a) Puncture

As previously mentioned, porcelain pin-type and cap and pin insulators may suffer

punctures between the pin and the either the pin or the high voltage conductor.

These occurrences are usually caused by very steep impulse voltages, where the

time delay for air flashover exceeds that of puncture of porcelain. Punctures caused by

severe stress over dry bands also occur on composite insulators on sheds and through the

sheath. A puncture of the sheath is particularly serious as this exposes the glass fiber rod

to the environment .

b) Shattering

Glass insulators shatter when exposed to severe arcing or puncturing due to

vandalism. One advantage is that they retain their mechanical integrity.

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c) Erosion

Prolonged arcing of glass insulators leads to erosion of the surface layer of the

glass. This may lead to shattering of the glass discs - a result of the tempering process

used during manufacture. Arcing and corona over long periods may cause removal ofshed or sheath material in the case of polymeric insulators. Severe erosion may lead to

the exposure of the glass fiber core.

d) Tracking

Tracking occurs when carbonized tracks form because of arcing. These tracks are

conductive. This phenomenon only occurs in carbon-based polymers.

e) Brittle Fracture

Water entry into the glass fiber core of composite insulators, coupled with the

influence of weak acids, has been shown to lead to brittle fracture of the rod. The by-

products of partial discharges in the presence of water can lead to the formation of weak

acids. The integrity of the metal/polymer and glass/polymer interfaces is therefore

extremely important - especially if acid-resistant glass is not used.

6.7 Remedies

There are certain remedies provided for different equipments available in substation

from being damaged.

They are ,

a) Washing

b) Greasing

c) Choice of Creep age length

a) Washing

Substation or line insulators can be washed when de-energized or when

energized. Automatic washing schemes and helicopters have been used for this purpose.

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The costs are usually prohibitive. A thin layer of silicone grease, when applied

to ceramic insulators increases the hydro-phobicity of the insulators.

b) Greasing

Room temperature cured silicone rubber coatings are available to be used on

ceramic substation insulators. These coatings have good hydrophobic properties when

new. Research is still in progress to evaluate their aging processes. We face Pollution

particles that are deposited on the insulator surface are also encapsulated by the grease

and protected from moisture. The disadvantage of greasing is that the spent grease must

be removed and new grease applied, usually annually.

c) Choice of Creep age Length

When using non-ceramic insulators, it is advisable to use a shorter creepage length

especially in locations of severe pollution. Recent research indicates that under conditions

of severe research results or revised specifications are available it is considered a safe

approach to use IEC 815 for non-ceramic insulators as well.

6.8 Conclusion:

We have discussed about the insulator used in substation and various types ofinsulators.

7.1 Introduction

We are going to discusses about the various earthing techniques used in sub station

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a) Grounding

A properly designed and installed grounding system ensures reliable performance

of electrical substations .

Just how important is substation reliability? Fast clearing of faults, made possible

by good grounding, improves the overall safety and reliability of an electrical system.

Therefore, substation reliability must be as "built-in" as possible because of the high

available fault current levels present and unlikely occurrence of follow-up grounding

inspections.

Fig.7.1 shows grounding

7.2 Types and Methods of Grounding

There are different types and methods of grounding which ensures the reliable

performance of a substation.

a) Types

Grounding of earth may be classified as (i) Equipment grounding (ii) System

grounding and (iii) Neutral grounding.

Equipment grounding deals with earthing the non current carrying metal parts of

the electrical equipment. On the other hand, system grounding means earthing some part

of the electrical system e.g. earthing of neutral point of star connected system in

generating stations and substations.

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i. Equipment Grounding

The process of connecting non current carrying metal parts of the electrical

equipment to earth in such a way that in case of insulation failure, the enclosure

effectively remains at earth potential is called Equipment grounding.

ii. System Grounding

The process of connecting some electrical part of the power system (neutral point of

a star connected system, one conductor of the secondary of a transformer) to earth is

called System grounding.

iii. Neutral Grounding

The process of connecting neutral point of 3-phase system to earth either directly

or through some circuit element (e.g. resistance or reactance etc.) is called Neutral

grounding.

Neutral grounding provides protection to personal and equipment. It is because

during earth fault the current path is completed through the earthed neutral and the

protective devices operate to isolate the faulty conductor from the rest of the system.

b) Methods of Grounding

The methods commonly used for grounding the neutral point of a 3-phase system

are:

(i) Solid or effective grounding (ii) Resistance grounding

(iii) Reactance grounding (iv) Resonant grounding

i. Solid Grounding

When the neutral point of a 3-phase system is directly connected to earth through a

wire of negligible resistance and reactance is called Solid or Effective grounding. Under

fault conditions, the voltage of any conductor to earth will not exceed the normal phase

voltage of the system.

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Advantages:

a) The neutral is effectively held at earth potential.

b) No arcing phenomenon or over voltage condition can occur.

c) Permits the easy operation of earth fault relay.

Disadvantages:

a) It causes the system to become unstable.

b) The increased earth fault current results in greater interference in the neighboring

communication lines.

ii. Resistance Grounding

When the neutral point of a 3-phase system is connected to earth through a resistor,

it is called Resistance grounding. The value of R should be neither v

Documents

Study of Elements of a 22013233kv Substation