1

11 KV Substation.

Pole-Mounted Sub-Station:

1. It is a distribution sub-station placed overhead on a pole. It is the cheapest form of substation as it does not involve any building work. Fig (i) shows the layout of pole-mounted sub-station

2. Whereas Fig (ii) shows the schematic connections. The transformer and other equipment are mounted on H-type pole (or 4-pole structure).

3. The 11 kV line is connected to the transformer (11kV / 400 V) through gang isolator and fuses.

4. The lightning arresters are installed on the H.T. side to protect the sub-station from lightning strokes.

5. The transformer steps down the voltage to 400V, 3-phase, 4-wire supply. The voltage between any two lines is 400V whereas the voltage between any line and neutral is 230 V.

6. The oil circuit breaker (O.C.B.) installed on the L.T. side automatically isolates the transformer from the consumers in the event of any fault.

7. The pole-mounted sub-stations are generally used for transformer capacity up to *200 kVA.

8. The following points may be noted about pole-mounted sub-stations :

A. There should be periodical check-up of the dielectric strength of oil in the transformer and

O.C.B.

B. In case of repair of transformer or O.C.B., both gang isolator and O.C.B. should be shut

off.

Distribution substation consists of:

1. Pin type insulator

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2. AB switch

3. Lightning Arrester

4. Circuit Breaker

5. Distribution Transformer

6. Earthing

7. DO Fuse

8. Stay Wire

9. MV cable

10. LV cable

➢ Fig shows the single line diagram of 11KV/440V substation DP structure. The single line

diagram contains the 11KV distribution line, lightning arrester, A B switches, drop out

fuse, 11KV/440V transformer, LT CB, etc.

➢ First the 11KV supply is coming from State Electricity Board to the DP structure through

cable via metering unit at the AB switch then drop out fuse then pin type insulator and then

it goes to the transformer HT bushing.

➢ In this structure the lightening arrestors are connected at the top. It is used to protect the

substation equipment from lightening strokes.

➢ AB switch is use to isolate the supply from system. If AB switch is open, then contacts are

open and supply could not come to the transformer. Hence, if fault occur at consumer side

then by opening the AB switch it can safely repair the fault. But before operating a fault,

care should be taken that the line should be discharge properly.

➢ AB switch works as an isolator. After that it contains Drop Out fuse.

➢ Then supply come to H T bushing of transformer. The transformer is 11KV/440V deltastar

connected.

➢ A step down transformer which step down the 11KV to 440V to main panel of the college

or industry. From this main panel the supply is distributed the main area through cables,

MCB and other switches.

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B. Lightning Arrester:

A lightning arrester is a device used on Electrical power systems from the

damaging effects of Lightning. The typical lightning arrester has a high-voltage

terminal and a ground terminal. When a lightning surge (or switching surge, which is

very similar) travels along the power line to the arrester, the current from the surge is

diverted through the arrestor, in most cases to earth.

If protection fails or is absent, lightning that strikes the electrical system introduces

thousands of kilovolts that may damage the distribution lines, and can also cause severe damage

to transformers and other electrical devices. Lightning-produced extreme voltage spikes in

incoming power lines can damage electrical appliances.

C. Air Break Switch:

A. Pin Type Insula tor :

A pin insulator consists of a non - conducting

material such as porcelain, glass, plastic, polymer,

or wood that is formed into a shape that will

isolate a wire from a physical support on a utility

pole or other structure, provide a means to hold the

insulat or to the pin, and provide a means to secure

the conductor to the insulator. By contrast to a

Strain insulator, the pin insulator is directly

connected to the supporting pole. The pin insulator

is designed to secure the conductor to itself.

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An air break switch disconnectors are the vital part of any overhead line network, providing crucial

points of isolation. Most overhead line network designed so that when a fault occurs or

maintenance work needs to be carried out it is relatively simple, by means of a systematic series

switching operations, to isolate the certain section of overhead line. When this switching process

is carried out it is absolutely imperative that the air break switch disconnector is reliable and

effective.

D. Distribution Transformer:

A distribution transformer is a transformer that provides the final voltage

transformation in the electric power distribution system, stepping down the voltage

used in the distribution lines to the level used by the customer. The invention of a

practical efficient transformer made AC power distribution feasible; a system using

distribution transformers was demonstrated as early as 1882.

Distribution transformers normally have ratings less than 500 kVA, although some national

standards can describe up to 5000 kVA as distribution transformers. Since distribution

transformers are energized for 24 hours a day (even when they don't carry any load), reducing iron

losses has an important role in their design. As they usually don't operate at full load, they are

designed to have maximum efficiency at lower loads. To have a better efficiency, voltage

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regulation in these transformers should be kept to a minimum. Hence they are designed to have

small leakage reactance.

E. Drop Out Fuse:

What are they

In the utilities industry, a fuse cutout is a combination of a fuse and a switch. These units are used

primarily on overhead feeder lines and are designed to protect distribution transformers from any

current spikes or surges that can overload equipment. A cutout consists of three major components:

Body:

The frame which supports the fuse tub/blade and is mounted to the cross arm or bracket. The

insulator body on this frame can be either polymer or porcelain material. The live connector parts

are also mounted to the ends of this frame. Fuse Holder:

Known as the fuse tube or “door” that contains the fuse link. This piece acts as a simple switch.

When the fuse operates, the fuse holder will drop open disengaging the switch from the line. This

ensures any downstream circuits are electrically isolated.

Fuse Link:

Also known as an element is the replaceable portion of the product that extinguishes due to higher

than normal current transfers.

How do they work?

A current surge from a customer circuit or a transformer will cause the fuse inside the tube to

expand and melt. Once the fuse reaches maximum current capacity it breaks and this energy is

thrown out of the bottom of the tube and disconnects the transformer from the line by the tube

dropping out of the upper contact and swinging down on the hinge. The physical indication that

the fuse has been extinguished and needs to be replaced is seen when the tube swings open and

remains in a downward orientation.

Instrument transformers:

The lines in sub-stations operate at high voltages and carry current of thousands of amperes.

The measuring instruments and protective devices are designed for low voltages (generally 110 V)

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and currents (about 5 A). Therefore, they will not work satisfactorily if mounted directly on the

power lines. This difficulty is overcome by installing instrument transformers on the power lines.

The function of these instrument transformers is to transfer voltages or currents in the power lines

to values which are convenient for the operation of measuring instruments and relays. There are

two types of instrument transformers viz. (i) Current transformer (C.T.) (ii) Potential transformer

(P.T.)

Current transformer (C.T.):

A current transformer in essentially a step-up transformer which steps down the current to

a known ratio. The primary of this transformer consists of one or more turns of thick wire

connected in series with the line. The secondary consists of a large number of turns of fine wire

and provides for the measuring instruments and relays a current which is a constant fraction of the

current in the line. Suppose a current transformer rated at 100/5 A is connected in the line to

measure current. If the current in the line is 100 A, then current in the secondary will be 5A.

Similarly, if current in the line is 50A, then secondary of C.T. will have a current of 2·5 A. Thus

the C.T. under consideration will step down the line current by a factor of 20.

Voltage transformer:

It is essentially a step down transformer and steps down the voltage to a known ratio. The

primary of this transformer consists of a large number of turns of fine wire connected across the

line. The secondary winding consists of a few turns and provides for measuring instruments and

relays a voltage which is a known fraction of the line voltage. Suppose a potential transformer

rated at 66kV/110V is connected to a power line. If line voltage is 66kV, then voltage across the

secondary will be 110 V.

Metering and Indicating Instruments:

There are several metering and indicating instruments (e.g. ammeters, voltmeters, energy

meters etc.) installed in a sub-station to maintain watch over the circuit quantities. The instrument

transformers are invariably used with them for satisfactory operation.

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PURPOSE OF SUBSTATION EARTHING SYSTEM

The object of an earthing system in a substation is to provide under and around the

substation a surface that shall be at a uniform potential and near zero or absolute earth potential as

possible. The provision of such a surface of uniform potential under and around the substation

ensure that no human being in the substation subject in shock of injury on the occurrence of a short

circuit or development of other abnormal conditions in the equipment installed in the yard.

The primary requirements of a good earthing system in a substation are:

1. It is stabilize circuit potentials with respect to ground and limit the overall potential rise.

2. It is protect life and property from over voltage.

3. It is provide low impedance path to fault currents to ensure prompt and consistent operation

of protective devices during ground faults.

4. It is keep the maximum voltage gradient along the surface inside and around the substation

within safe limits during ground fault.

Grounding or Earthing:

The process of connecting the metallic frame (i.e. non-current carrying part) of electrical

equipment or some electrical part of the system (e.g. neutral point in a star-connected system, one

conductor of the secondary of a transformer etc.) to earth (i.e. soil) is called grounding or

earthing.

It is strange but true that grounding of electrical systems is less understood aspect of power

system. Nevertheless, it is a very important subject. If grounding is done systematically in the line

of the power system, we can effectively prevent accidents and damage to the equipment of the

power system and at the same time continuity of supply can be maintained. Grounding or earthing

may be classified as: (i) Equipment grounding (ii) System 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

sub-stations

Equipment Grounding:

The process of connecting non-current-carrying metal parts (i.e. metallic enclosure) of the

electrical equipment to earth (i.e. soil) in such a way that in case of insulation failure, the enclosure

effectively remains at earth potential is called equipment grounding.

We are frequently in touch with electrical equipment of all kinds, ranging from domestic

appliances and hand-held tools to industrial motors. We shall illustrate the need of effective

equipment grounding by considering a single-phase circuit composed of a 230 V source connected

to a motor MM as shown in Fig. Note that neutral is solidly grounded at the service entrance. In

the interest of easy understanding, we shall divide the discussion into three heads viz. (i)

Ungrounded enclosure (ii) enclosure connected to neutral wire (iii) ground wire connected to

enclosure.

(i) Ungrounded enclosure: Fig. shows the case of ungrounded metal enclosure. If a person

touches the metal enclosure, nothing will happen if the equipment is functioning correctly. But if

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the winding insulation becomes faulty, the resistance Re between the motor and enclosure drops

to a low value (a few hundred ohms or less). A person having a body resistance Rb would complete

the current path as shown in Fig.

If Re is small (as is usually the case when insulation failure of winding occurs), the leakage current

IL through the person’s body could be dangerously high. As a result, the person would get severe

*electric shock which may be fatal. Therefore, this system is unsafe.

(ii) enclosure connected to neutral wire: may appear that the above problem can be solved

by connecting the enclosure to the grounded neutral wire as shown in below Fig. Now the leakage

current IL flows from the motor, through the enclosure and straight back to the neutral wire (See

Fig). Therefore, the enclosure remains at earth potential. Consequently, the operator would not

experience any electric shock.

The trouble with this method is that the neutral wire may become open either accidentally or due

to a faulty installation. For example, if the switch is inadvertently in series with the neutral rather

than the live wire (See in below Fig), the motor can still be turned on and off. However, if someone

touched the enclosure while the motor is off, he would receive a severe electric shock See in below

Fig), It is because when the motor is off, the potential of the enclosure rises to that of the live

conductor.

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(iii) Ground wire connected to enclosure: To get rid of this problem, we install a third wire,

called ground wire, between the enclosure and the system ground as shown in below Fig The

ground wire may be bare or insulated. If it is insulated, it is coloured green.

System Grounding

The process of connecting some electrical part of the power system (e.g. neutral point of a

starconnected system, one conductor of the secondary of a transformer etc.) to earth (i.e. soil) is

called system grounding.

The system grounding has assumed considerable importance in the fast expanding power system.

By adopting proper schemes of system grounding, we can achieve many advantages including

protection, reliability and safety to the power system network. But before discussing the various

aspects of neutral grounding, it is desirable to give two examples to appreciate the need of system

grounding.

1. Below Fig. (i) shows the primary winding of a distribution transformer connected between

the line and neutral of a 11 kV line. If the secondary conductors are ungrounded, it would

appear that a person could touch either secondary conductor without harm because there is

no ground return. However, this is not true. Referring to Fig. 26.5, there is capacitance C1

between primary and secondary and capacitance C2 between secondary and ground. This

capacitance coupling can produce a high voltage between the secondary lines and the ground.

Depending upon the relative magnitudes of C1 and C2, it may be as high as 20% to 40% of

the primary voltage. If a person touches either one of the secondary wires, the resulting

capacitive current IC flowing through the body could be dangerous even in case of small

transformers [See in below Fig.(ii)]. For example, if IC is only 20 mA, the person may get a

fatal electric shock.

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If one of the secondary conductors is grounded, the capacitive coupling almost reduces to

zero and so is the capacitive current IC. As a result, the person will experience no electric

shock. This explains the importance of system grounding.

2. Let us now turn to a more serious situation. Shown in below Fig. (i) shows the primary

winding of a distribution transformer connected between the line and neutral of a 11 kV line.

The secondary conductors are ungrounded. Suppose that the high voltage line (11 kV in this

case) touches the 230 V conductor as shown in below Fig. (i). This could be caused by an

internal fault in the transformer or by a branch or tree falling across the 11 kV and 230 V

lines. Under these circumstances, a very high voltage is imposed between the secondary

conductors and ground. This would immediately puncture the 230 V insulation, causing a

massive flashover. This flashover could occur anywhere on the secondary network, possibly

inside a home or factory. Therefore, ungrounded secondary in this case is a potential fire

hazard and may produce grave accidents under abnormal conditions.

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If one of the secondary lines is grounded as shown in Fig. (ii), the accidental contact

between a 11 kV conductor and a 230 V conductor produces a dead short. The shortcircuit

current (i.e. fault current) follows the dotted path shown in Fig. (ii). This large current will

blow the fuse on the 11 kV side, thus disconnecting the transformer and secondary

distribution system from the 11 kV line. This explains the importance of system grounding

in the line of the power system.

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How Insulation Resistance is measured

You have seen that good insulation has high resistance; poor insulation, relatively low

resistance. The actual resistance values can be higher or lower, depending upon such factors as the

temperature or moisture content of the insulation (resistance decreases in temperature or moisture).

With a little record-keeping and common sense, however, you can get a good picture of the

insulation condition from values that are only relative.

The Megger insulation tester is a small, portable instrument that gives you a direct reading

of insulation resistance in ohms or megaohms. For good insulation, the resistance usually reads in

the megohm range. The Megger insulation tester is essentially a high-range resistance meter

(ohmmeter) with a built-in direct-current generator. This meter is of special construction with

current and voltage coils, enabling true ohms to be read directly, independent of the actual voltage

applied. This method is nondestructive; that is, it does not cause deterioration of the insulation.

The generator can be hand-cranked or line-operated to develop a high DC voltage which

causes a small current through and over surfaces of the insulation being tested (Fig. 2). This current

(usually at an applied voltage of 500 volts or more) is measured by the ohmmeter, which has an

indicating scale. Below Fig. shows a typical scale, which reads increasing resistance values from

left up to infinity, or a resistance too high to be measured.

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Types of Insulation Resistance Tests

Short-Time or Spot-Reading Test

In this method, you simply connect the Megger instrument across the insulation to be tested and

operate it for a short, specific time period (60 seconds is usually recommended). As shown

schematically in Fig. 6, you’ve simply picked a point on a curve of increasing resistance values;

quite often the value would be less for 30 seconds, more for 60 seconds. Bear in mind also that

temperature and humidity, as well as condition of your insulation affect your reading.

Time-Resistance Method

This method is fairly independent of temperature and often can giveyou conclusive

information without records of past tests. It is based on the absorption effect of good insulation

compared to that of moist or contaminated insulation. You simply take successive readings at

specific times and note the differences in readings (see curves, Fig.). Tests by this method are

sometimes referred to as absorption tests. Note that good insulation shows a continual increase in

resistance (less current – see curve A) over a period of time (in the order of 5 to 10 minutes). This

is caused by the absorption current we spoke of earlier; good insulation shows this charge effect

over a time period much longer than the time required to charge the capacitance of the insulation.

If the insulation contains much moisture or contaminants, the absorption effect is masked by a high

leakage current which stays at a fairly constant value, keeping the resistance reading low

(remember: R = E/I).

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The time-resistance test is of value also because it is independent of equipment size. The

increase in resistance for clean and dry insulation occurs in the same manner whether a motor is

large or small. You can, therefore, compare several motors and establish standards for new ones,

regardless of their horsepower ratings.

IR Value of Transformers:

Voltage Test voltage LV side Test voltage HV side IR Value

415V 500V 2.5KV 100MΩ

Up to 6.6KV 500V 2.5KV 200MΩ

6.6KV to 11KV 500V 2.5KV 400 MΩ

11KV to 33KV 500V 5KV 600 MΩ

33KV to 66KV 1000V 5KV 600 MΩ

66KV to 132KV 1000V 5KV 600 MΩ

Steps for measuring the IR of Transformer:

1. Shut down the transformer and disconnect the jumpers and

lightning arrestors.

2. Discharge the winding capacitance. 3. Thoroughly clean all

bushings

4. Short circuit the windings.

5. Guard the terminals to eliminate surface leakage over terminal bushings.

6. Record the temperature.

7. Connect the test leads (avoid joints).

8. Apply the test voltage and note the reading. The IR. Value at 60 seconds after application

of the test

9. Voltage is referred to as the Insulation Resistance of the transformer at the test temperature.

10. The transformer Neutral bushing is to be disconnected from earth during the test.

11. All LV surge diverter earth connections are to be disconnected during the test.

12. Due to the inductive characteristics of transformers, the insulation resistance reading shall

not be taken

13. Until the test current stabilizes.

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14. Avoid meggering when the transformer is under vacuum.

Factors affecting on IR value of Transformer

The IR value of transformers are influenced by

1. Surface condition of the terminal bushing

2. Quality of oil

3. Quality of winding insulation

4. Temperature of oil

5. Duration of application and value of test voltage

Measurement of Earth Electrode Resistance:

The most commonly used method of measuring the earth resistance of an earth electrode is the 3-

point measuring technique shown in Figure.

This method is derived from the 4-point method – which is used for soil resistivity

measurements.

The 3-point method, called the “fall of potential” method, comprises the Earth Electrode to be

measured and two other electrically independent test electrodes, usually labeled P (Potential) and

C (Current). These test electrodes can be of lesser “quality” (higher earth resistance) but must be

electrically independent of the electrode to be measured.

An alternating current (I) is passed through the outer electrode C and the voltage is

measured, by means of an inner electrode P, at some intermediary point between them.

The Earth Resistance is simply calculated using Ohm’s Law: Rg = V/I.

Other more complex methods, such as the Slope Method or the Four Pole Method, have

been developed to overcome specific problems associated with this simpler procedure, mainly for

measurements of the resistance of large earthing systems or at sites where space for locating the

test electrodes is restricted.

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When performing a measurement, the aim is to position the auxiliary test electrode C far enough

away from the earth electrode under test so that the auxiliary test electrode P will lie outside the

effective resistance areas of both the earth system and the other test electrode (see Figure 2).

• If the current test electrode, C, is too close, the resistance areas will overlap and there will

be a steep variation in the measured resistance as the voltage test electrode is moved.

• If the current test electrode is correctly positioned, there will be a ‘flat’ (or very nearly

so) resistance area somewhere in between it and the earth system, and variations in the

position of the voltage test electrode should only produce very minor changes in the

resistance figure.

Many of these methods have been designed in an attempt to alleviate the necessity for excessive

electrode separations, when measuring large earth systems, or the requirement of having to know

the electrical centre of the system.

Three such methods are briefly described below. Specific details are not given here, but instead

the reader is referred to the relevant technical paper where these systems are described in detail.

1. The slope method

2. The star-delta method

3. The four-potential method (Wenner method)

Methods of Reducing Resistivity of Soil

Types of soil resistivity:

Sl. No. Type of soil Resistivity in Ohm-cm

1 Loamy garden soil 500 - 5000

2 Clay 800 - 5000

3 Clay, Sans and Gravel mix 4000 - 25000

4 Sand and Gravel 6000 - 10000

5 Slates, Slab sand stone 1000 - 50000

6 Crystalline Rock 20000 - 100000

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Soil Treatment:

When the soil resistance is high, even the multiple electrodes in large number may also fail to

produce low resistance to earth.

To reduce the resistivity of soil immediately surrounding the electrode some salt substances

are made available as a solution with water. The substances are used salt sodium chloride

(NaCl), Calcium chloride (CaCl2), Sodium carbonate (Na2CO3), copper sulphate (CuSO4) and

soft cock and charcoal in suitable proportion. General practice to treat the soil surrounding the

ground electrode with common salt, charcoal and soft cock in order to bring down the earth

resistance.

Use of Bentonite in Soil Treatment

Bentonite is clay with excellent electrical properties. It swells to several times its original

volume when suspended in water. It binds the water of crystallization and the water absorbed

during the mixing process is retained over a long period.

Bentonite suspension in water when used to surround the earth electrode virtually increases the

electrode surface area. Even during the summer months, bentonite suspension retains the

moisture where as the natural soil dries up. Bentonite may be used to advantage in rocky

terrain.

Use of fly ash in soil treatment

As per CPRI studies reveals that fly ash from thermal stations has equivalent chemical

composition and hence can be used for the electrical installations in areas of high ground

resistivity. Fly ash can also be used as a chemical treatment material to reduce soil resistivity

Use of fly ash in soil treatment

As per CPRI studies reveals that fly ash from thermal stations has equivalent chemical

composition and hence can be used for the electrical installations in areas of high ground

resistivity. Fly ash can also be used as a chemical treatment material to reduce soil resistivity

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Maintenance schedule of substaion: Daily

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Monthly schedule:

20

21

Quarterly schedule:

22

Half yearly schedule:

23

Yearly schedule:

24

25

26

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Safety practices followed during preventive, routine and

breakdown maintenance:

1. Ensure all safety arrangement while working on electrical installation.

2. Ensure that all tools & tackles are in good & working condition.

3. Check and thoroughly investigate the transformer whenever any alarm or protection is

operated.

4. Check the protection system periodically.

5. Ensure every employee is familiar with the instructions for restoration of persons suffering

from electric shock.

6. Trained the staff in operating the fire-fighting equipment.

7. Always avoid un-balance loading on phase.

8. Do earthing of all points before starting maintenance.

9. Keep all spares away from dirt.

10. Work with full confidence.

11. Ensure thorough and full cleaning of insulators, since partial cleaning is worse than no

cleaning.

12. Ensure perfect isolation of supply before commencement of maintenance work.

13. Put a caution board when on work.

Use of fly ash in soil treatmentMaintenance schedule of substaion: Daily