Wednesday, 7 June 2017

Dialy lesson-Introduction to Substation chapter-2









In this substation the incoming sources are six. The incoming power is in “3-phase” from that the sequence of devices are to be connected. Firstly, lightning arrestors, these are used protect from the lightning strokes and diverting the surges. After this CVT’s are connected in order to measure the supply voltage. After this line isolators are connected for isolation of supply, they operate under no load conditions. After this current transformer is connected, it is installed for measuring current and protection purpose using protection relays. After this CB’s are connected these are used to isolate the faulty part of the power system in case of abnormal conditions. After this bus isolator are connected. After this a 220kv parallel bus is employed.

This parallel bus is used for equal load sharing. After this, a step down transformer is connected to step down the voltage from 220kv to 132kv and further step-down of voltage from 132kv to 33kv.







  • Bus bars

    • Insulators

    • Isolating Switches

    • Circuit Breaker

    • Power Transformers

    • Instrument Transformers

    • Protective relays

    • Lightening arrestors

    • Metering & Indicating Instruments

    • Capacitor bank

    • Wave trap

    • Miscellaneous equipment

    • Batteries

2.3.1. BUS-BAR

The following are the important bus-bar arrangements used in sub-station.
When number of lines operating at the same voltage levels needs to be connected electrically then we use bus bars. Bus bars are conductors made of copper or aluminum, with very low impedance and high current carrying capacity. Busbars are the important components in a sub-station. The choice of a particular arrangement depends upon various factors such as system voltage, position of sub-station, degree of reliability, cost etc.

  • Single bus bar

  • Rigid bus bars

  • Multi bus bars


The over headlines & conductors should be supported on the poles or towers in such a way that currents from conductors do not flow to earth through supports i.e., line conductors must be properly insulated from supports. This is achieved by securing line conductors to supports with the help of insulators. In general, the insulators should have the following desirable properties.

  • High mechanical strength in order to withstand conductor load, wind load etc.

  • High electrical resistance of insulator material in order to avoid leakage currents to earth.

  • High relative permittivity of insulator material such that dielectric strength is high.

  • The insulator material should be non-porous, free from impurities and cracks otherwise, the permittivity will be lowered.

  • High ratio of puncture strength to flashover.







An isolator switch is part of an electrical circuit and is most often found in industrial applications. The switch does exactly what its name suggests in that it electrically isolates the circuit or circuits that are connected to it. Such a switch is not used normally as an instrument to turn on/off  of the circuit in the way that a light switch does. Either the switch isolates circuits that are continually powered or is a key element which enables an electrical engineer to safely work on the protected circuit.

Isolator switches may be fitted with the ability for the switch to padlock such that inadvertent operation is not possible (see: Lock and tag). In some designs the isolator switch has the additional ability to earth the isolated circuit thereby providing additional safety. Such an arrangement would apply to circuits which inter-connect power distribution systems where both end of the circuit need to be isolated.

Major difference between isolator and circuit breaker is that isolator is an off-load device, whereas circuit breaker is an on-load device.



Circuit breaker plays a significant role in substation protection. These are mechanical devices designed to  close  or  open  contact  members , thus  closing  or  opening  of  an  electrical  circuit  under  normal  or  abnormal  conditions .

Of  all  the  protecting  equipment  circuit  breakers  has  its own  importance .Automatic  circuit  breakers  which  are  usually  employed  for  the  protection  of  electrical  circuits  are  equipped  with  a  trip  coil  connected  to  a  relay  or  other  means , designed  to  open  the  circuit  breaker  automatically  under  abnormal  conditions , such  as  over  current . It performs the following functions.

  1. It makes and breaks  the  short  circuit  currents  of  magnitude  up to  which  it  is  designed  for.

  2. It opens and closes the circuit on load.

  3. It makes and breaks the normal operating current.



  1. Oil circuit breaker.

  2. Air blast circuit breaker.

  3. Vacuum circuit breaker.

  4. SF6 circuit breaker. 6 CIRCUIT BREAKER :







It is made up of three poles each activated by a spring operating mechanism. The pole is composed of

  • An interrupting chamber in a ceramic chamber laid out vertically.

  • A hallow insulating with ceramic support column through which passes the insulating tie rod and which is attached to the interrupting chamber moving contact.

  • At the base of column, a housing consisting the lever and crank assembling and which operates the moving contact as well as the opening spring.

  • The SF6 filling and monitoring device is also situated on the housing.

  • The operating device is a spring operating mechanism of the FKF 1-2 type.

  • Marshalling cubicle is attached to the frame of the central pole.



  • The current chopping tendency is minimum by using the gas SF6 at low pressure and low velocity.

  • The closed circuit gas cycle and low velocity operation elimination the moisture problem and gives noise less operation.

  • Low maintenance and auxiliary equipment.

  • There is no risk of fire.

  • Because of the outstanding arc quenching  properties of SF6 the arcing time is small, contact erosion is less.

  • No carbon particle is formed during  arcing and there is no reduction in dielectric strength of gas.

  • The dielectric strength of SF6 is 2 to 3 times than air it can interrupt much large circuits.



  • These are costly due to high cost of SF6

  • Sf6 GAS has to be reconditioned after every operation of the breakers additional equipment is required.

Nine indicating lights on front of compartments:

GREEN                               Breaker Open

RED                                     Breaker Closed

AMBER                               Auto Trip

WHITE                                Spring charged

BLUE Trip                           Circuit Healthy

RED & GREEN                   Earth Switch Closed & Open

YELLOW &BLUE              Breaker Test & Service Position

  1. Lamps used above shall be low watt LED Cluster type. Lamp and lens shall be replaceable from the front.

  2. A common audible alarm for each switchgear line-up shall be provided to alert the operator that circuit breaker has tripped.

  3. Means shall be provided for silencing the audible alarm whilst leaving it free to sound when any other alarm is initiated but the associated alarm indications shall continue until cancelled.



Sf6 circuit breakers are developed for voltages from 115KV to 230KV.

Power ratings 10MVA to 20MVA and interrupted time less than three cycles. VACCUM CIRCUIT BREAKERS:-
OPERATING MECHANISM:operating mechanism of a circuit breaker is as follows.

1.The  lower  end  of  a  breaker  is  fixed  to  a  spring  operated  or  solenoid operated  mechanism  so  that  the  metallic  bellows  inside  the  chamber  are moved  upward  and  downward  during  closing  and  opening  operations  respectively .

  1. The contact moving should be such that to avoid bounce.

  2. 3. It is worth noting  that  the  operating  mechanism  should  provide  sufficient  pressure  for  a  good  connection  between  contacts .

  3. 4. The pressure in the vacuum interrupt at the time of sealing off is kept about 10^-6 ton.

  4. 5. The interrupting rating is between 250 and 1000 MVA. The normal current carrying capacity for single interrupter is 800-3000A ,4.2KV-7.6KV.




Advantages of vaccum circuit breaker:

  1. It is self-contained and does not require filling of oil or gas.

  2. They do not need auxiliary air system, oil handling System. There is no need of periodic oil filing.

  3. Rapid recovery of very high dielectric



The earthing switch consists of a conductor bar. When earthing switch is to be closed, these bars swing and connect the contact on line unit of isolator to earth.

Earthling switch is connected between the line conductor and the earth normally open. When the line is disconnected the earthing switch is closed so as to discharge the voltage trapped on the line after the CB and isolators are opened. Though the line opened there is some voltage on the line to which the

capacitance between line and earth is charged (HV). Before proceeding to maintenance work these voltages have to be discharged to earth by closing the earth switch.



While opening:

  1. Open CB

  2. Open isolator

  3. Close earth switch

While closing:

  1. Open ear thing switch

  2. Close isolator

  3. Close CB



The lines in sub-station operate at high voltages and carry current of thousands of amperes. The measuring instruments and protective devices are designed for low voltages (generally 110v) and currents (about 5A). Therefore, they will not work satisfactory 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

  1. 1. Current transformer (C.T)

  2. 2. Potential transformer (P.T)


A current transformer (CT) is a type of instrument transformer designed to provide a current in its secondary winding proportional to the alternating current flowing in its primary. They are commonly used in metering and protective relaying in the electrical power industry where they facilitate the safe measurement of large currents, often in the presence of high voltages. The current transformer safely isolates measurement and control circuit from the high voltages typically present on the circuit being measured.

CT’s can be termed as (based on application)

  • Protective CT’s  used with relays and tripping circuits

  • Measuring CT’s  used with metering equipment

  • Intermediate CT’s suit the current ratings of devices designed to operate

Primary connection: - 

Primary of CT can be in parallel or series modes and hence 2 CT ratios are possible.

Secondary Connection: -

CT’s are provided with secondary taps for requirement of different CT ratio’s Burden is connected across required terminals and rest leads are left open or bypassed through resistors restricting short time current factor.


Potential Transformer or 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 transformer4 rated at 66V/110V is connected to a power line. If line voltage is 133KV, then voltage across the secondary will be 110V.





2.7. CAPACITOR VOLTAGE TRANSFORMER: -                                                   CVT is an Electrostatic type voltage transformer. CVT work on potential divider principle across a series connected capacitors. CVT is nothing but an HV Capacitors enclosed in porcelain bushing. CVT’s are more economic for voltages > 66 kV CVT can be employed for measurement purpose and also in power line carrier communication (PLCC). Here they avoid additional coupling capacitor. CVT is directly connected between ground and line. The performance of CVT is effected by supply frequency, switching transients and burden. Errors in CVT can be reduced by reducing the burden. For multipurpose CVT combines function of Voltage Transformer & Coupling capacitor. CVT’s are more economical at higher system voltages but a minor problem of introducing harmonics in secondary voltages.                                          


Accurate transformation of primary system voltage into secondary output voltages is suitable for

  • Features

  1. Relaying / Protection

  2. Ferro resonance Suppression

  3. Compact Structure

  4. Excellent Hermetically sealed

  5. Thermal Burden

  6. Maintenance Free Cast Aluminum Base Box

2.8 Power Transformer

A power transformer is used in a sub-station to step-up or to step-down the voltage. Except at the power station, all the subsequent sub-stations use step-down transformers to gradually reduce the voltage of electric supply and finally deliver it at utilization voltage. The modern practice is to use 3-phase transformer in sub-stations; although 3 single banks of transformers can also be used. The use of 3-phase transformer (instead of 3 single phase bank of transformers) permits two advantages.


Firstly, only one 3-phase load-tap changing mechanism can be used. Secondly, its installation is much simpler than the three single phase transformers.

The power transformer is generally installed upon lengths of rails fixed on concrete slabs having foundations 1 to 1.5m deep. For ratings up to 10MVA, naturally cooled, oil immersed transformers are used. For higher ratings, the transformers are generally air blast cooled.

A Power Transformer is a vital link in power transmission system and transfers power from one circuit to another circuit without changing frequency.

A Transformer fault will cause a large interruption in power supplies and the impacts more serious than a transmission line outage and also cause damage to power system stability Three Phase Power Transformers are constructed by winding three single phase transformers on a single core. These transformers are put into an enclosure which is then filled with dielectric oil. The dielectric oil performs several functions. Since it is a dielectric, a nonconductor of electricity, it provides electrical insulation between the windings and the case. It is also used to help provide cooling and to prevent the formation of moisture, which can deteriorate the winding insulation.



TEMPARATURE RISE IN OIL                                :             50C (MIN)   85(MAX)

TEMPARATURE RISE IN WINDING                     :             55C (MIN)   88(MAX)

RANGE                                                                       :             20/31.5MVA

TYPE OF COOLING                     : ONAN                   ONAF (Oil natural air forced)

KVA                                                : 20,000                             31,500

Volts at no load: HV                          132000                           132000

LV                                                    33000                                     33000

CURRENT                             HV     87.5A                               137.8A

LV    350A                                351A

PHASES                                 HV     3                                      3

LV     3                                       3

Impedance Voltage                           8.11%                              12.77%

INSULATION                                                             :             HV 550PEAK

NEUTRAL                                                                   :             38

LV                                                                                :             170PEAK

FREQUENCY                                                             :             50HZ

VECTOR SYMBOL                                                   :             YNY no

Volume of OLTC                                                         :             1350LITERS

Volume of main tank                                                    :             13450liters

Mass of oil                                                                    :             11700kgs

Un taking mass                                                             :             22800kgs

Total mass                                                                     :             48000kgs

No of taps                                        17n                                    0.937kv

Normal tapings                                                             :             5

No load losses                                                              :             16.19kw

Load losses                                                                   :             115.18kw







Following properties of transformer oil are choosen to determine the service ability

Electrical Parameters: Dielectric Strength, Specific Resistance

Chemical Parameters: Water Content, Acidity

Physical Parameters: Viscosity, Flash Point



Dielectric Strength: -

It is also known as Breakdown Voltage (BDV). BDV of oil is the ac voltage, which causes spark between two electrodes placed in the oil under test at a standard distance (Generally 2.5 mm as per most standards). 2kV/s in case of (IEC-156).


Specific Resistance: 

It is also called resistivity or DC resistance of volume of oil of unit cross sectional area and unit length. Unit is ohm-cm. It is desirable to have specific resistance of oil as high as possible. Resistivity of oil varies greatly with temperature of oil, an increase in temperature reduces the resistivity.

Resistivity of oil reduces considerably due to presence of moisture, acidity and solid contaminants.

Useful additional information can be obtained by measuring resistivity at both ambient temperature and at 90°C.

Good oils should have tan-delta as less as possible. A high tan delta is an indication of presence of contaminants such as resins. There is generally a relationship between tan delta and resistivity, both being affected by same contaminants. A decrease in resistivity is coupled with an increase in tan delta. In fact IEC-422 indicates that both the tests are normally not required to be conducted on same oil.



2.10. RELAYS: -

An electric power system is a combination of generators, transformers, transmission lines & distributions etc. the main objective of the power system is to supply the power to the customer without any interruption and maintain the quality. Due to the protective systems to the power systems the reliability and qualities of the supply is increased and the life of the power system is also increased. The short circuits are associated with heavy currents and it will damage the equipment. If we may not use a suitable protective system. An automatic protective device is needed to isolate the faulty element as early as possible to keep the healthy section of the system in normal operation.

Types of the relays can be used are:

Over current relay

Earth fault relay

Under voltage relay

Over voltage relay

Flash over voltage relay

H.V tri pole over current relay

L.V tri pole over current relay

Trip circuit super vision relay

Auxiliary relay

High speed inter tripping relay

Earth fault relay

To design the protective scheme, it is necessary to have an idea or intimate knowledge of faults. A fault can be detected by particular type of protection equipment and some of the protection equipment’s are more sensitive than the other.



Protective relays limit the damage in case of fault and monitors to prevent the fault. Therefore, fast and reliable protective relays should be used.
















It is a gas actuated relay. When a fault takes place in a transformer the oil of the tank may get over heated and gases are formed.  The generation of gases in the transformer take may be slow or fast depending on the whether a fault is minor or incipient one or heavy short circuit. The generation of gas in the transformer tank is used as a mean of fault detection. The Buchholz Relay is mainly used for the protection of the transformer against all internal faults and this relay does not respond for external faults.

Buchholz relay is the simplest form of protection of all transformers provided with conservators.

The type of the fault is taking place inside the transformer is detected by the formation of gases such as

  1. C2H2, CH4&H2 shows arcing from some deterioration of phenolic insulation.

Ex: fault in tap changer

  1. C2H4, C3H6, H2&CO2 show a hot spot in the winding.

  2. CH4, C2H4&H2 indicates hot spots in core joints.

  3. C2H2&H2 shows arcing in between constructional parts.



  1. The merit of this relay indicate incipient fault.

  2. This relay gives an alarm if the oil level reduces below a certain level due to leakage of oil from transformer.

  3. It gives an audible warning which informs operators there is some fault in the operation.

  4. Insulation failure of the transformer can also be detected y testing the gas.


This type of relay can be used only for transformers with conservators

  • The relay does not operate when oil is in above oil level

  • Due to economic considerations relay operate for 500kva

  • Does not protect the cables associated with transformer

  • Slow operating time



Impedance relays are used whenever overcurrent relays do not provide adequate protection. They function even if the short circuit current is relatively low. The speed of operation is independent of current magnitude.

Impedance relays monitor the impedance between the relay location and the fault. If the impedance falls within the relay setting, the relay will operate. The basic construction for impedance relays on which the principle of operation is easily explained is the balanced beam.

 Principle of Impedance Relay

The relay consists of a balanced beam. At each end of the balanced beam is a coil that exerts a force on the beam at that end. One coil is connected to a current from a current Transformer, the other coil is connected to a potential transformer. The voltage coil functions as a restraining coil, the current coil functions as an operating coil. Under normal conditions, the contact of the relay is kept open. During a fault, the voltage drops, and the current rises. The torque due to the current coil overpowers the torque due to the voltage coil, and the relay closes its contact.

Zones of Protection

In general, distance protection includes three steps of protection, with each step reaching a fixed preset distance and operating in a pre-set time.

Zone 1: reaches 80 - 90% of the protected line. The tripping is instantaneous.

Zone 2: extends beyond the protected line up to about 50% of the adjacent line. The tripping has a time delay, usually set to a value between 0.3 s to 0.5 s.

Zone 3 covers the protected line, the adjacent line, and up to 25% of the line next to the adjacent line. Tripping is delayed between 0.6 s to 1.0 s.

An Impedance relay measures the Impedance of the line at the relay location when a fault occurs on the protected line section, the measured Impedance is the Impedance of the line section and location of the fault it is proportional to the length of the line this Impedance has directional feature and will operate irrespective of the direction of the current it is usual to make maximum torque angle smaller than line Impedance angle this is done in order to reduce the effect of arc resistance on the reach of the relay .

The zone 3 element controls the operation of the timer and that is starting unit that the zone1 element operates without any intentional time delay but still it has some finite time of operates t1 where as z2 z3 operates with a time delay of t2 t3 respectively.

Let fault in zone1 all three units will start but since the operating time of unit 1 is smallest this will operate time of unit is smallest this will operate and the faulty section will be isolated from the source.

In case the fault is in second zone the units z2 z3 will start but until z2 will operate in timer t2 and isolate the faulty section from the source.


A mho relay measures a component of admittance but its character sticks when plotted on R-X diagram is a circle passing through the origin it is inherently a directional relay as it detects the faults in the forward direction only. Zone 3 mho relay operates as a starting relay or for controlling the line timer will also be replaced by an offset mho unit which has definite merit under close up conditions as it encloses the origin unit operation.






Distance Protection schemes shall have …

  • Three independent zones

  • Separate measurements for all Phase to Phase & Phase to ground faults.

  • Single and three pole tripping capability.

  • Directional characteristics for Zone-1, Zone-2 and Zone-3.

  • Accuracy of better than or equal to 5% of set value for reach measurement in Zone-1 & Better than equal to 10% of set value for Zone-2 & Zone-3.

  • Accuracy of better than or equal to 5% of set value for time measurement of Zone-2 & Zone-3.

  • Suitable fuse failure protection to monitor all types of fuse failure and block the protection.




Fault current If = E/(Zs + Zl)

Conventional over-current protection may not be always adequate for line protection as the reach of over current relay is function of Source Impedance which varies considerably, making it difficult to get fast and selective tripping.




The following points are to be consider when applying the Differential Protection:


  • The CT ratios must choose to suit the main transformer ratio so that the differential currents will be zero during normal operation and where the main transformer ratio is variable by means of tap changing.

  • Effect of magnetizing inrush current.




  • The Protective zone of a Differential relay includes faults in Transformer, faults on Buses or cables between CT and transformer and then rapidly initiate disconnection of the supply to the main transformer. Then damages as well as non-selective tripping of other protective relays are prevented.


  • The Transformer Differential relay must be able to cope with the following conditions:


  • When energizing the transformer after the fault, it is possible to obtain a large inrush current in the exciting winding. The magnitude and duration of the inrush current depends on

    • Design of transformer

    • Type of transformer connection

    • Method of neutral grounding

  • Generally, the magnitude of the inrush current can be 5-10 times the rated current when switching in to high voltage side and 10-20 times the rated current when switching to low voltage side. When fault current occurs, a voltage is generated rapidly across the relay coil. To prevent this voltage from low to high value, a non-linear resistor is connected in parallel.

  • The protection system for a three-phase distribution transformer immersed in a liquid dielectric contained in a tank, at least two of the three phases being equipped on the high voltage side of the transformer with respective current-limiting fuses. On each of the two phases equipped with current-limiting fuses, a protective micro-fuse is also connected in series with the current-limiting fuses and which operates faster than the current-limiting fuse. In addition, each micro-fuse is associated with a striker which, in turn, is connected to a mechanical trigger system which causes a three-phase short-circuiting device connected across the three phases to close in the event of a micro-fuse operating. Furthermore, the system includes at least one fault detector detecting one of pressure in the tank or the level of the dielectric. In particular, the three-phase short-circuiting device is situated on the high voltage side of the transformer between the current-limiting fuses and the high voltage windings.





Lightning is the phenomenon which accompanies the discharge of atmospheric charges from cloud to cloud or from cloud to earth. As lightning seeks the path of least resistance, it naturally tends to follow the shortest course between cloud and earth, such as building or towering projections. As illustrated positive electrical charges gather in the clouds and negative charges gather in the ground. When the attraction between these two charges are strong enough they come together in the form of lightning.

Lightning arrester equipment, properly manufactured and installed dissipates these charges. In temperate climates a large majority of lightning is negative downwards lightning, as the reduce interference between stations negatively-charged cloud-base discharges to the ground.








Lightning arrestors protects the equipment from lightning over voltages and switching over voltages. Switching over voltages is predominant in EHV system. Lightning arrester acts as a nonlinear resister. allows high resistance for normal voltages and low resistance for high voltages. These are the safety devices like pressure relief valves. We can say that, it is an over voltages protection device or surge protection device. It is located at the entrance of the transmission line into the Substations and as near as possible to the power Transformer Terminals.

Lightning arrestors will be provided on the support insulators to facilitate leakage current measurement and to count the number of surges discharged through the L.A.

L.A bottom flange will be earthed via leakage. Ammeter and surge counter. Leakage current is to be recorded periodically. If the leakage current enters in to the red range from the green range, the L.A is prone for failure hence it is to be replaced.

There should be independent earth pit for L.A in each phase so as to facilitate fast discharging and to avoid the raise in earth potential.

Surge counter readings are to be recorded at least once in a fort night.

L.A s is to be monitored periodically. If there is more than one block per LA, individual blocks shall be merged.

Lightning arrester also termed as surge arresters are use in HV and EHV systems for protection of apparatus insulation from lightening surges. These are usually connected between phase and ground terminals

Its basic function is to discharge the current impulse surges to earth. It consists of resistor blocks  which offer low resistance to high voltage surge and divert to the ground.

Lightning & Surge protection:-

Protection against the destructive   effects should be considered globally.
To avoid the fire of a building we will use the “external”  lightning protection system such as lightning conductor or meshed cage.
In the same way, we will install Surge Arrester or surge protector (also called SPD, Surge Protective Device) in the Electrical main board to protect sensitive electrical equipment against surges generated by lightning strokes fell close or directly on the building. In other words, a surge protector (SPD) must be coordinated with a safety protective device that disconnects from the network when the SPD is in short circuit. The cause of this development is mainly shot-circuit failure of the neutral or permanent over voltage which unfortunately appear on the electrical networks (from current or renewable energy).



An automatic power factor correction unit is used to improve power factor, in industrial networks. By increasing the power factor to near 1, current is reduced in the power system and usually utility charges for electrical power are reduced. A power factor correction unit usually consists of a number of capacitors that are switched by means of contactors. These contactors are controlled by a regulator that measures power factor in an electrical network. To be able to measure 'power factor', the regulator uses a CT (Current transformer) to measure the current in one phase.

Depending on the load and power factor of the network, the power factor controller will switch the necessary blocks of capacitors in steps to make sure the power factor stays above 0.9 or other selected values (usually demanded by the energy supplier).


2.14. METERING & Indicating Instrument: There are several metering and indicating instruments (e.g. ammeters, voltmeters, energy meters etc...) installed in sub-station to maintain watch over the circuit quantities. The instrument transformers are invariably used with them for satisfactory operation.



In addition to above, there may be following equipment in a sub-station:

  1. 1. Fuses

  2. 2. Carrier current equipment

  3. 3. Substation auxiliary supplies

2.15.1. FUSE:-

Fuse is perhaps the simplest and cheapest device used for interrupting an electrical circuit under short circuit or excessive over load current magnitude.

Fuse is based on heating effect.

The time of blowing out of fuse depends upon the magnitude of excessive current

The materials used to make the fuse have the following the features

  1. Low melting point

  2. Low Ohmic loss

  3. High conductivity

  4. Less deterioration

The materials used to make fuse are to be

  1. Cheepest from protection available

  2. There is no need of maintenance

  3. Interrupt enormous short circuit currents without noise flame gas or smoke

  4. Fuse carry normal working current safely without heating

  5. It breaks the circuit when the current exceeds the limit

  6. The protection against the oxidation copper and tin are used

  7. Zinc is a good of a fuse with considerable time lag is required

  8. The present trend is to used silver fuse

  9. Silver having melting point is 980deg cen

  10. Specific resistance 16 micro ohm-cm


Such equipment is installed in sub stations for communication, relaying, telemeter, or for supervisory control. This equipment is suitable mounted in a room known as carrier room and connected to high voltage power.


In substations only a small amount of power for electric lightning during regular periods of inspection, maintains and repair is required.

In regional substations the electric power is required for the auxiliaries –the lighting circuits, air blast fans of power transformers, battery charging sets, oil servicing facilities, compressors units in case of air blast circuit breakers, ventilating fans of the substation buildings, water supply and heating system etc.

In large substations it is wide practice to connect two transformers to the 11kv main bus bars   for supply of the auxiliaries at a voltage of 400v/230v.




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