A short description about the different components of Switching Yard

Its all about the different components of a switching yard in a sub-station.


The outdoor busbars are either rigid type or strain type.

In the rigid type, pipes are used for busbar and also for making connections among the various equipments wherever required. The busbar and the connections are supported on pedestal insulators. Since the busbars are rigid, the clearances are fixed. Due to large diameter of pipes, the corona loss is also substantially less. However this type of busbar is very costly and it also requires a larger area.

The strain type busbars are overhead system of wires strung between 2 supporting structures (known as Gantries) and supported by strain type insulators. The strain must be limited to 500-1000 Kg depending upon the size of the conductors used.

The Howrah substation has 3 busbar systems. They are:-
1. The main bus
2. The transfer bus
3. The third bus

In the substation, each circuit is connected either to the main bus or third bus through an isolator. If any problem arises in the main bus, the entire load can be transferred to the third bus via bus coupler and the transfer bus without disrupting the continuity of the service. The two main buses are connected by a tie-bus with a bus protection arrangement. One of the main utilities of the third bus is to reduce the current load of the main bus.

The materials in common use for busbars and connections of the strain type are:-
a) ACSR (Aluminium Conductor Steel Reinforced)
b) AAAC (All Aluminium Alloy Conductor)

According to the current carrying capacity of the conductors, they are classified as follows:-

Type/ Name of Conductor Maximum Current Carrying Capacity
Panther 450-500 amp
Deer 500-550 amp
Zebra 550-560 amp
Moose 800-900 amp

Clearance Table:-

Nominal System Voltage 220 KV 132 KV 33 KV
Phase to Phase 2400 mm 1600 mm 400 mm
Phase to Earth 2100 mm 1380 mm 300 mm

For example:-


1. Conductor Size/ Detail:-

Voltage Main Bus Transfer Bus Jack Bus Equipment Connection
220 KV Twin Moose Single moose Single moose 3 inch Aluminium pipe bus
132 KV Twin Moose Single moose Single moose 2.5 inch Aluminium pipe bus
33 KV Twin Moose Single moose Single moose 1.5 inch Aluminium pipe bus

2. Main Bus:-

22/132/33 KV system shall be of twin moose ACSR with inter conductor spacing of 300 mm. Maximum tension/conductor shall be 1000 kg.

3. Transfer Bus:-

220/132/33 system KV shall be single moose ACSR. Maximum tension/conductor shall be 1000 kg.

4. Insulator Strings:-

220/132/33 KV system shall be single tension/ single suspension type.
Voltage Single Tension Single Suspension
220 KV 16/15 15/14
132 KV 11/10 10/9
33 KV 3 3

5. Equipment Dropper:-

For all bays of 220 KV, system shall be ACSR moose conductor.
For all bays of 132 KV, system shall be ACSR moose conductor.
For all bays of 33 KV, bus dropper shall be ACSR moose conductor.
For all bays of 33 KV, equipment dropper shall be ACSR moose except for the following:-
Equipment connector for 100 KVA and 630 KVA transformers shall be ACSR panther conductor;
Equipment dropper for 33 KV lightning arrester of all circuits shall be ACSR panther conductor.

6. Wave Trap:-

For 132 KV D.C line (double circuit), wave trap shall be only on 'B' phases of both lines.
For 220 KV D.C line (double circuit), wave trap shall be only on 'B' phases of both lines.

7. Capacitive voltage transformer (CVT):-

For 132 KV D.C line (double circuit), CVT shall be only on 'B' phases of both lines.
For 220 KV D.C line (double circuit), CVT shall be only on 'B' phases of both lines.

8. Shield Wire:-

For 132 KV, line shall have tension of 800kg/wire.
For 220 KV, line shall have tension of 1000kg/wire.


The purpose of interlocks in substation is to ensure the safety of equipment and operating personnel and to prevent unauthorized or inadvertent operation of equipment.
The present practice for 220 KV substations is to provide electrical interlocking for routine switching operations such as on-load busbar changeover and key interlocks for maintenance purposes such as circuit isolation and earthing.


Before 1960's the design criteria of substation earthing system was "low earth resistance" (E.R < 0.5 O for high voltage installations). During the 1960's, the new criteria for design and evaluation of substation earthing system were evolved, particularly EHV AC and HVDC substations. The new criteria are:-
1. Low Step Potential
2. Low Earth Potential
3. Low Touch Potential

The conventional "low earth resistance" criteria and low current earth resistance measurements continue to be in practice for substation and power station upto and including 220 KV.

To meet these requirements normally the earthing system comprises of an earthing mat buried at a suitable depth below the ground and provided with ground rods at suitable points. All the current carrying and non-current carrying parts of the equipments in the substation are connected to this grid so as to make sure that none of these parts are at higher potential than the grounding grid under fault conditions.

In all substations, there should be provision for earthing of the following:-
1. The neutral point of each separate system should have an independent earth which in turn should be connected to the station grounding mat.
2. Lightning arresters should have independent earth which in turn should be connected to the station grounding mat.
3. Equipment framework and other non-current carrying parts.
4. All extraneous metallic frameworks not associated with equipment.

Earthing in a substation must conform to the requirements of the Indian Electricity Rules and follow the directives laid down by Sections 1 and 3 of IS:3043-1966. The factors which influence the design are:-

a) Duration of fault
b) Magnitude of fault current
c) Resistivity of underlying strata
d) Resistivity of surface material
e) Material of electrode

The size of the earthing conductor is determined by the consideration of potential difference and temperature rise.

The approximate formula for determining the size is given by:-

A/I = [4050?at/dslog {1+?m+?e}] ½

Where, A= cross-section in circular mills
I = current in amperes
? = resistivity of material in micro ohm-cm
a = temperature coefficient of resistance of material in /°C
t = time for which current is applied in seconds
d = density of the material in gm/cc
s = specific heat of the material in cal/gm/°C
?m = maximum allowable temperature in °C
?e = ambient temperature in °C

The values of various constants as applicable for steel are given as:-
? = 15 micro-ohm
a = 0.00423/°C
d = 7.86 gm/cc
s = 0.114 cal/gm/°C
?m = 900°C for brazed joints
500°C for bolted joints
?e = 40°C (say)

Substituting the values of the constants, the size of the conductor is given by:
A = 26 x I x (t) 1/2 for bolted joints
= 26.1 x I x (t) 1/2 for welded joints

Permissible values of touch and step potentials in the design of grounding grids in a substation can be calculated from the following formula:-

Estep = 165 + ?/ (T) ½
Estep = 165 + 0.25?/ (T) ½

where ? = resistivity in ohm-m just beneath the feet of the person
T = time in seconds to clear the fault by the connected breaker


1. Earthing Mat:-

40 mm dia, 2m to 3m length per piece mild steel rod are welded to get straight lengths and are placed in horizontal x-y formation with mesh spacing 2m to 3m at 0.6m depth in soil. Joints between x and y arc are welded. The end points are further driven down to about 3m deep. The mat is at least 800-1000 mm below ground level.

2. Earthing Electrodes (spikes):-

Earth Electrodes are basically of 2 types:-

I) Driven Electrode:
a) Pipe
i) Galvanised (internal diameter 36 mm or 1.5")
ii) Cast Iron (diameter 100 mm or 4")
b) Rod
i) G.I (diameter 16 mm or 5/8")
ii) Copper (diameter 12.5 mm or ½")

II) Buried Electrode:
a) Sheet
i) G.I (6'x 3'x 16" gauge)
b) Strip
i) Copper (25 mm x 1.6 mm OR 1" x 3/32")
c) Plate
i) G.I (Thickness 60cm x 60 cm x 6.3 mm OR 2' x 2' x ¼')
ii) Copper (Thickness 60 cm x 60 cm x 3.15 mm)

30 to 40 mm dia GI pipes, 3m long (maximum length 4 m) are driven in the soil vertically (z-direction) and welded x-y rods of earth mesh via horizontal earthing rod.

Note: - a) Depth of electrode strictly depends on soil resistivity
b) Main earthing must be 1.25 m below the ground level and as per I.E rule, the minimum length of the electrode should be 3'.
c) Plate electrode must be 1.5 m below the ground level and it should be tilted at an angle of 30° under the earth.
d) Strip electrode must not be less than 1.25 m at a minimum depth of 0.5 m.

3. Riser:-

40 mm dia vertical rods welded to earthing mat are brought up to the structures to be earthed. Alternatively, 75x10 mm and 45x8 mm GI plates are welded to the earth mat and taken up vertically for bolting/ welding with the point to be earthed.

4. Earthing Strips or Flexible Stranded wires:-

75x10 mm galvanized iron plates/copper plates are welded/bolted to the nearest riser and attached to the point to be earthed. Flexible Stranded ACSR cables are connected between the overhead shielding wires and tower footings. Tower footings are connected to the earthing system.


Isolators (disconnecting switches) are used to transfer load from one bus to another and also to isolate equipments for maintenance. Although there is a large variety of Isolators available, the most critical factor is whether vertical break type or horizontal break type. Horizontal break type normally takes more width than vertical break type. In case of horizontal break type, space requirement can be reduced by the use of double break and a central rotating pillar.

Types of Isolators used in Howrah 220KV substation:-

1. Main Bus – Horizontal type centre rotating double break
2. Transfer Bus – Vertical/Horizontal type single break

Maker's names:-

1) Associated Electrical Industries - 800 Amp
2) KRUGG Engineering - 800 Amp
3) Alliance Industries - 800-1200 Amp
4) Bharat - 800-1200 Amp
5) G.Nandy - 1200-1600 Amp


A transformer is the largest and the costliest piece of equipment in a substation and it is thus the most important from the point of view of substation layout. On the account of large dimension it is generally not possible to install two transformers in adjacent bays. Another problem is the installation of radiators which makes the width of the transformer much more than the bay width. In order to reduce the chances of fire, large transformers are provided with a soaking pit of adequate capacity to contain the total quantity of oil.

One of the important factors governing the layout of a substation is whether the transformer is three-phase or a bank of three single-phase transformers. The space requirement in case of single banks is less than that for a three-phase transformer. Besides in case of single-phase banks, it is usual to provide one spare transformer which is kept on service bay and is used in case of fault or maintenance of any one of the single-phase units.


The Howrah substation uses 3 three-phase power transformers:-

1)160 MVA transformer (Crompton Greaves)
2)160 MVA transformer (B.H.E.L)
3)150 MVA transformer (Crompton Greaves)

In spite of the transformers being of different MVA ratings, they are operated in parallel. This has been done to increase the flexibility of the substation. However they have identical voltage rating and percentage impedance ratio.


The core of the transformer may be of various shapes i.e. core, shell, distributed core or spiracore. It is made of varnished laminated silicon of 35 mm thickness. Laminations are made in steps to give circular cross-section. The laminations are secured by bolts and nuts. Core is placed on channels or I-beam provided at the bottom of the tank. Small tanks are constructed from wielded sheet steel and large tanks from boiler sheet steel. Windings are made of insulated copper conductor. HV windings are separated from LV winding by series of ducts and bakelite cylinder or barrels. LV winding is put nearer to the core, for better embracing two windings are wounded on same limb.

As transformer is a static device sufficient arrangement is to be made for its cooling. It is dipped in oil for cooling and insulating purpose. Mineral oil having paraffin and bituminous base and some synthetic oils like chlorinated diphenyl is used as oil. A good transformer oil should have low viscosity, high dielectric strength, high flash point, high fire point, low acidity, less sulphur content and less sludge forming tendency.

3. BUSHING (H.V & L.V.)
These are kept on the top of the transformer and through these connections are made to the external circuit in case of big transformer. These are made of porcelain or ebonite. Above 33KV oil filled terminal bushing is used.

It is an air tight cylinder metal drum generally placed on the top of the transformer tank. It is practically filled with oil. To reduce the air-oil contact surface the tank is completely filled with oil. The expansion and contraction of oil with the change of temperature is taken up by conservator.

It is a tube like structure connected with conservator and contains some dehydrating agent like calcium chloride, phosphorus pentoxide, silica gel, etc. At the day time due to the rise of temperature, oil is expanded, so it comes out to the atmosphere and at night time oil gets contracted and thus moist air enters into the conservator. The moist air contains 2/3 rd of air .This is called breathing of transformer. Breather helps to absorb the moisture.

It is a drum like structure mounted on transformer wall, which reduces aching during tap changing operation.

This relay is used to protect the oil immersed transformer only. It mainly consists of two hinged floats. Any broken joints or any other faults produces heat and causes the evolution of gas. This gas is collected in chamber and press the smaller float to sink first. This closes the alarm circuit .If no attention is paid to it the gas collection will be more and the bigger float will sink. Thus closing another circuit which will cut out the transformer from the line. Now-a-days two mercury containers are used in place of floats.

It is of small thickness and large diameter plates placed vertically and used for heat dissipation of oil during operation. Larger diameter means larger surface area and better cooling.

There are two temperature indicators on the transformer tank, one for the oil temperature measurement and the other for air temperature measurement. General maximum temperature rise is 45 deg Celsius and overall ambient of 45/55.

The auto transformers have only one winding and there are different tap pointers. Upper hand of the trap indicates primary side voltage and rest indicate secondary voltage. The tap changing is generally done on HV side because of low current which reduce flashing during tap changing.

Several cooling fans are placed beside the radiator tube, which are used for oil cooling and they starts operation automatically, but when needed it can also be started manually.


The Howrah substation supplies traction power to S.E Railways at 25 KV through 2 circuits. The substation has 2 traction transformers for this purpose.


A substation has to be guarded against direct lightning strokes either by provision of overhead earth wires (known as shield wires) or spikes. This equipment is essential irrespective of the isoceraunic level of the area due to serious consequences and damage to costly equipments in case substation is hit by a direct stroke. The choice between these two methods depends upon several factors, economy being the most important one. Both methods are sometimes used in the same station, like in the Howrah substation of WBSEB. Generally, the spikes method requires taller structures than the alternative using earth wires. Generally, an angle of shield of about 45° for the area between two spikes and earth mats or ground wires and 30° for other areas is considered adequate for the design of lightning protection system.

Besides direct strokes, the substation equipments has also to be protected against traveling waves due to lightning strokes on the lines entering the substation. The apparatus most commonly used followed this purpose is the lightning arrester. There are lightning arresters provided to protect the feeders entering the substation and leaving it.

The most important and costly equipment in a substation is a transformer. The normal practice is to install a lightning arrester as close to the transformer as possible.

The fixing up off installation level of the different equipments within a substation requires a detailed study of insulation coordination with lightning arrester as the focal point. While providing protection to the equipment, the lightning arrester itself has to be protected so that it does not fail due to power frequency over voltages exceeding the rating of the arrester. In the EHV range, there is also the problem switching over voltages as the life of the arrester maybe considerably reduced due to frequent operations on such over voltages. Sometimes it is not possible to locate the lightning arrester very near to the transformer. However there is no problem as long as the transformer is within permissible distance from the lightning arrester.

At EHV, there are three types of lightning arresters:-
1) Silicon Carbide
2) Zinc Oxide
3) Gapless

Besides protecting the transformer, the lightning arrester also provides protection and the equipments on the bus side located within certain permissible distance. But in case of very large substations, where the lightning arrester for the transformer does not provide adequate protection to the other equipments, additional lightning arresters are placed on either side of the bus or on various lines as done in the Howrah 220 KV substation.
That type of lightning arrester used at Howrah is gapless Zinc Oxide lightning arrester. The unique nonlinear resistance property of zinc oxide is used to great effect here. During surge, ZnO offers a very low resistance path to the surge frequency current, thus providing easy earthing. But it provides high resistance to power frequency current thus preventing short circuit condition during normal operation.

Follow current:-
It is defined to be the normal system current which passes alongwith surge frequency current through the lightning arrester doing surge. The maximum limit of follow current is 300 amps.

Operating time:-
2 cycles; the lightning arresters used at Howrah are mostly provided with Grading Ring. The purpose of the grading ring is to stabilize the voltage gradient across the different lightning arrester units in a single integrated lightning arrester so as to protect the unit closest to the bus-bar.


A circuit breaker is a piece of equipment which can
1. Make or break a circuit either manually or automatically under normal conditions
2. Brake a circuit automatically under fault conditions
3. Make a circuit either manually or by remote control under fault condition

The circuit breaker essentially consists of fixed and moving contacts (known as electrodes). Under normal conditions these contacts remain closed. When a fault occurs on any part of the system, the trip coils of the circuit breaker get energized and the moving contacts are pulled apart by some mechanism ( mostly air pressure but nowadays spring charging is used), thus opening the circuit. When the contacts of the circuit breaker are separated under fault condition, an arc is struck between them. The main principle of working of a circuit breaker is to increase the dielectric strength of the medium between the two parting contacts so that it can withstand the rate of rise of voltage (RRV). In early times when the system voltage was low, only air was used as arc quenching medium. Gradually with the state the increase in system voltage pressurized year, oil began to be used as arc quenching medium. But with present day system voltage being as high as 400KV (in few cases 760 KV as well), the earlier air blast circuit breaker or oil circuit breaker are not used anymore. They have been replaced by SF6 circuit breaker which uses Sulphur hexafluoride gas as arc quenching medium. The dielectric strength of SF6 is much more than air or oil, thus giving more reliable operation. Vacuum circuit breaker also gives highly reliable operation but the manufacturing cost of the vacuum tube where the actual arcing takes place is uneconomical for voltages as high as 132 KV or more. Thus SF6 circuit breaker finds extensive use in voltage systems above 66 KV, while for voltages ranging up to 33 KV, vacuum circuit breaker is used.

Switch gear:-
It is the combination of circuit breaker, indicating, integrating, measuring and relay instruments. Its different parts are:-
1. Controlling arrangement
2. Regulating arrangement
3. Measuring arrangement
4. Protecting arrangement

Switch gear are of 2 types:-
1. Cubical type
a) fixed type
b) track type
2. Metal clad type
a) horizontal draw out
b) vertical draw out
SF6 circuit breakers:-

Working principle:

The SF6 circuit breaker uses Sulphur hexafluoride gas as arc quenching medium. In the closed position of the breaker, the contacts remain surrounded by SF6 gas at the pressure of about 2.8 kg/cm2. When the breaker operates, the moving contact is pulled apart and an arc is struck between the contacts. The movement of the contact is synchronized with the opening of a valve which permits SF6 gas at 14 kg/cm2 pressure from the reservoir to the arc interruption chamber. The high pressure flow of SF6 gas rapidly absorbs the free electrons in the arc path to form immobile negative ions which are ineffective as charge carriers. The result is that the medium between the contacts quickly builds up dielectric strength and causes extinction of the arc.

Spring operating mechanism:

The energy for operating the breaker is provided by the type BM spring operating mechanism. The energy for operation is stored in a spring for closing which during the closing operation Compresses a spring for the trip operation. An electrical motor provides with the energy for recompressing Closing spring. From the breaker closed position, the mechanism will perform an open-close-open (known as the duty cycle of the breaker) operation with the energy available in the closing spring. The motor will recharge the closing spring within 15 seconds for additional operations.

Sulphur hexafluoride gas:
In the breaker these gas is used both for arc interruption and electrical insulation. A chemical breakdown of the gas occurs when it is exposed to very high temperature or electric arc as in the breaker. The decomposition produces toxic gas, strong irritants and attacks the respiratory system.
Arced SF6 gas is accompanied by a strong and irritating odour indicating toxic decomposed products. The products are injurious and exposure to them should be avoided. However the desired arced gas containing decomposition can be cleaned and reused. In this case the gas should be circulated through molecular sieve filters to remove the active products.

Caution: All freshly activated absorbents short be cooled to ambient temperature before introducing SF6 gas to avoid exothermic reaction. Large accumulation of powders (solid decomposition products), resulting from abnormal arcing conditions can be neutralized by mixing in a bucket of water and bicarbonate of soda solution.


The SF6 gas system is a sealed, fixed volume filled with a specific quantity of gas. This results in the gas system pressures at a constant density, changing with temperature variations. A switch is provided to monitor the density of the gas system. The switch compensates for changes in pressure do to temperature and responds only to changes (gas leaks) in the density of the gas system.

Caution: Do not operate the circuit breaker by operating mechanism when the SF6 gas pressure in the pole unit is below the lockout pressure.


Instrument transformers are used in conjunction with ammeters, overcurrent relays etc.
There are two types of instrument transformers in use in the substation. They are:-

A. Current transformer.
B. Potential transformer.


Current transformers step down current from a very high value to a very low value. Their current ratio is substantially constant for given range of primary current and phase angle error is within specified limit. The V.A. rating of CT in small as compared to that of PT.
The location of CT determines the boundaries of protective zones.CT for bus protection are generally arranged such that circuit breakers are also covered by the protection and are not left unprotected.
CT secondaries should not be open circuited, and there should be no open circuit in continuity of pilots. For this purpose an alarm relay is provided to monitor the continuity.
If discontinuity occurs, the alarm relay gets actuated and gives alarm, after some delay it may trip the bus circuit breaker.
CTs are either bushing type or wound type. The bushing type CTs are normally accommodated within the breaker and the wound types are invariably separately mounted.

Current Transformers are mainly used for three purposes:
1. Metering
2. Summation
3. Protection.

Error in Instrument Transformer and type of error:-

This type of transformer, used in delicate purpose for measuring and protection, it is desirable that two vector quantities current and voltage must have a linear magnitude and transformation as well as shifting of phase shall be exactly 180 degree or exactly in phase opposition to primary quantities along same line to maintain accuracy of measurement and protection without any error. The main cause is the exciting current which makes the core active but introduces errors as such, which are unavoidable

Nameplate of a Current Transformer:

1. VA: Burden of current transformer or impedance of secondary side. (As per IS ,CTs to be used with electro-mechanical relays shall have specified VA 2.5,5,10,15,30 VA)
2. Class: Class of accuracy. It is usually denoted by P.( The accuracy class of a measuring CT is designated by the highest permissible ratio error at rated current for that accuracy class . The standard value 0.1, 0.2, 0.5,1,5 A. If a CT is specified as 30/0.5 it means having an output 30VA and highest ratio error ± 0.5%.
3. V.K.: Minimum voltage of secondary side (The knee point voltage referring to the B-H curve) . The expression for V.K. is given by -
V.K.= V*Is + ( Rct + Rb )
Where Is=secondary current;Rct=resistance of coil;Rb=impedance of coil
The V.K. for measuring CT is low while that for protective CT is high.

Errors in Current Transformer:

1. Phase angle error: This affects metering.
2. Ratio error: This affects protection.


Voltage transformers are used for measurement and protection .They may be either single phase or three phase unit. Voltage transformers are necessary for voltage, direction, and distance protection. The primary of voltage transformer is connected to power circuit between phase and ground. The volt ampere rating of voltage transformers is smaller as compared with that of power transformer.
There are two types of construction:-
1. Electromagnetic potential transformer: In it primary and secondary are wound on magnetic core, like that of usual transformer.
2. Capacitor potential transformer: In it the primary voltage is applied to a series capacitor group. The voltage across one of the capacitor is taken to auxiliary voltage transformer. The secondary of auxiliary voltage transformer is taken for measurement or protection.


1. For PT for protection purposes it is a common practice to measure the primary and secondary volts in term of line to line. In other words, 110 volts is generally L-L voltage in terms of the secondary. The line to neutral voltage of the coils is generally at 110/v3 volts.
2. Protective relays operate under system fault conditions. As the faults are associated with voltage drops, a protective voltage transformer is required to maintain its accuracy within the specified limits from 55 of rated voltage to voltage factor time the rated voltage.


A protective relay is a device that detects the fault and initiates the operation of circuit breaker to isolate the defective element from the rest of the system. The electrical quantities which may change under fault condition are voltage, current, frequency , phase angel. The relays detect the abnormal condition in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault condition.

Basic Operating Principle:
A typical relay circuit consists of-
1. The primary winding of current transformer which is connected in series with the line to be protected.
2. Secondary winding of a current transformer and the relay operating coil.
3. The tripping circuit which may be either dc or ac .It consists of a source of supply, the trip coil of the circuit breaker and the relay stationary contacts.

When a short circuit occurs on transmission line,then the current flowing in line increases to an enormous value. This results in a heavy flow of current through the relay coil, causing the relay to operate by closing its contacts. This in turn closes the contacts of the circuit breaker, making the breaker open and isolating the faulty section from rest of the system.

Types of Relays used at HOWRAH sub station:

1. Auxiliary relay
2. Over current relay
3. Tripping relay
4. Fuse failure relay
5. Definite time over current relay
6. Instantaneous earth fault relay
7. Percentage differential relay
8. Earth fault relay
9. Surge proof interposing interlocking relay
10. Inter tripped send relay
11. Biased differential relay
12. Under frequency relay
13. Neutral shift relay
14. Restricted earth fault relay

Microprocessor based Digital Relaying

Few months back the electromagnetic relays, described earlier, were replaced with Microprocessor based digital relays made by ABB (2 nos.) and ZiCom .
The block diagram of a microprocessor based digital programmable static relay is shown-

Basic principle of operation:

The stable states of a relay are 'ON' or 'OFF' i.e. conducting or non-conducting. The output of a logic circuit is a function of its inputs .The five basic functions performed by the basic logic blocks are AND, OR, NOT, NAND and NOR. Any switching function can be described as a combination of these functions.

In its basic principles the relays receives analog current and /or voltage signals and are processed in Analog to Digital converters such that corresponding digital signal is available for the digital relays.

Digital signals are in form of coded square pulses which represent discrete data. The signals are in binary form. The Microprocessor, being set with the recommended values compares the dynamic inputs and 'decides' accordingly to generate trip/alarm signal to the output device.


D.C. auxiliary supply is required for closing and tripping of circuit breakers, emergency lighting, and control board indications etc. During normal operation, motor-generator sets or rectifiers provide required D.C. supply. However, to take care of failure of motor-generator sets or the rectifier, a storage battery of adequate capacity is provided to meet the requirements. Normally, the storage battery merely keeps floating in the direct current system and supplies only in the case of failure of motor generator set or the rectifier unit.

The capacity of the battery should be adequate to supply:
1. Momentary currents required for the operation of the switchgears.
2. The continuous load of indicating lamps, holding coils for relays contractors etc.
3. Emergency lighting load.

For determining the current required for the operation of switchgear, it is usually assumed that not more than one breaker will close at a time. However there may be occasions when all breakers trip simultaneously. The maximum ampere-hour rating is usually based on one-minute operation. In so far continuous and emergency loads are considered the battery should be able to supply these loads for pre-determined time without drop in voltage below 1.75 volt per cell.

Generally the D.C. voltage required for a 220KV substation is 220 volts and for 132KV substation is 125 volts.


Guest Author: Abdul rahim13 Mar 2013

Hi I intend to know about insulating oil used in PTs,CTs and 132 Kv Circuit Breakers. I need its chemical composion / analysis, may we can use transformer oil as a breaker oil. Kindly reply soon. Abdul rahim

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