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Page 1: Main Report1

1Project Report 2010



















Dept of EEE MBC Peemade

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2Project Report 2010

SYNOPSISThe project is completed at KMML, world's first fully integrated Titanium dioxide pigment plant. The electrical system of KMML includes 110 kV substation. The project aims to design the protective system of the substation along with setting up a trip circuit.

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Protection is significant to the everyday life of human beings inseveral ways. The electrical energy has become part of man's life. The advancingmodern world comes to standstill without electrical energy supply. The power systemprotection is essential in assuring continuous, reliable energy supply in the form ofelectricity to the consumers. Failure of supply system due to surges or other causeshinders the delivery of electrical power to the loads. This undesirable conditionsresults in huge damages to the system with loss of resources.

The "PROTECTIVE SYSTEM" is a collection of equipments engaged inperceiving and secluding faults. Faults are abnormal states of any system. Theprotective system of the entire power system is beyond the scope of this project, soa substation is chosen. Substations are points on the power system that formcenters of power distribution. Substations handle numerous equipments for variouspurposes. Sufficient protection need to be provided for the apparatus so thatdamages due to faults are reduced. The level of protection provided and protectivescheme designated depends mainly on the voltage grading of the substation.

The functioning of protective system is relaying and circuitbreaking. Relaying is the perception and circuit breaking is seclusion. The relays andcircuit breakers along with other switch gears avert the consequences of fault.Faults, though rare events, cannot be completely excluded from the system, but canbe minimised through better design and effective protection unit.

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Located on the coast of the Arabian Sea, one of the richest mineraldeposits in the world, Kerala Minerals and Metals Ltd. (KMML) is among thepioneers in the field of mineral sand industry in India. The mineral separationindustry, MIs F.X. Perira and sons Pvt. Ltd., established in 1932, turned KMML in1972. Today KMML has variety of products like iIImenite, silimenite, monozite,synthetic rutile, zircon, leucoxene, titanium dioxide pigment, etc. in finest quality.

Two main plants of KMML are:-

1. Mineral Separation Plant

2. Titanium dioxide Pigment Plant.

The integrated Titanium dioxide Pigment Plant consists of:-

IBP - IIImenite Beneficiation Plant

ARP - Acid Regeneration Plant

U100/200 - Chlorination Plant

U300 - Oxidation Plant

U400/500 - Treatment and sand Milling Final Product Plant

The Plants U100 - 500 forms together the Pigment Production Plant (PPP). Utilities available are:-

Boiler Water Treatment Plant Air Compressor Station Oxygen Plant.

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Plant Process

The Mineral Separation Plant mines and separates iIImenite, rutile andleucoxene. Since the availability of rutile (95% Ti02) is limited, illmenite is the rawmaterial for the Ti02 Production. The raw illmenite is beneficiated to 90-92% Ti02 byreduction and leaching in the ISP. The spent Hydrochloric acid (leach liquor) isregenerated in the ARP for minimum pollution. Finally, the PPP converts thebeneficiated illmenite to the Titanium dioxide Pigment.

The Pigment Production Plant first purifies the beneficiated illmenite bychlorination in presence of petroleum coke. The titanium tetrachloride (tickle) vaporis condensed and further purified a by distillation. The condensed tickle is vaporised,preheated and oxidised to obtain raw Ti02 which is formed into slurry with water.The slurry is then treated with various chemicals and agitated with san,d in the sa'ndmiller. After surface treatment it is filtered and washed to remove salt. The dried,michronised Ti02 powder is then packed in hags of 25kg in the bagging machine.The packets are stacked on pallets by the palletiser.

The installed capacity of KMML is 22000 tonnes of Ti02 pigment peryear. The ISO 9002:2000 certified company sets up a Titanium sponge plant, inaddition to expanding the present capacity of the titanium dioxide Plant from 50,000 tonnes per annum

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Products and their application

TITANIUMDIOXIDE PIGMENT(RUTILE) Paints, Printing inks, Plastic, paper, rubber, Textiles, Ceramics.

TITANIUMTETRA CHLORID Rutile grade Titanium Dioxide Pigment, titanium sponge Imetal, titanium salts butyl titanate, titanium Oxychlorides.

ILLMENITE Synthetic rutile, Welding Electrodes, , Titanium Tetra Chloride, Titanium Dioxide Pigment, Ferro Titanium Alloys, Titanium Salt.

RUTILE Welding Electrodes, Titanium Dioxide Pigment Titanium Components, Titanium Tetrachloride, Titanium metal/sponge

LEUCOXENE Welding Electrodes, Titanium Tetrachloride, titanium Dioxide Pigment, Titanium Components.

ZIRCON Ceramics, Zirconium, Chemical, Foundries, Zirconium Metals, Refractory, Nuclear Technology

SILLIMENITE High temperature refractory, Ceramic industry

IRONOXIDE BRICK As building material

The wide range of products offered by KMML are exported to well-knowncompanies in Korea, Italy, Mauritius, China, West Africa, Turkey, Sri Lanka,Singapore, Bahrain, UAE and Thailand. These high quality chemicals from KMMLare used in the manufacture of dress, cosmetics, rubber products, plastics, paints,newsprints, tablets, enamels, emulsions and, printing ink.

The eco-friendly and socially committed company maintains highStandards of performance in every phase of production.

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The KMML is an EHT consumer of KSEB,supplied by the Edappon-Kundra substations 110kv. This 110kv is stepped down to 6.6kv using two 110/6.6kv,lO/12.5 MVA step-down transformer in the main H.T substation, from here the supply is fed to H.T Substation, lads and other five in plant L.T substation (6.6/0.433kv) through cables. The diesel generator (OG) sets of 500 KAV (x2) and 1OKAV UPS system are used for emergency supply during power failure. The KMML power system like the ring system of distribution as shown the figure

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The 11OkVyard contains a pair of 3-phase lines incorporating

lightning Arresters


Earth switches

Gas Circuit Breaker (SF6 Breaker)

Instrument Transformers

Power Transformers

Power Transformer

The yard includes two power transformers one an each line, steps down11Okvto 6.6kV for further distribution, with specification:

10/12,5MVA, 110/6.6kV, 3<1>50 Hz, Y-Y-Δ ONAN/ONAF with on load tapchanger OLTC, primary rigidly earthed and secondary earthed through NGR

ONAN/ONAF indicates type of cooling - Oil Natural Air Natural and Oil Natural AirForced. Above 10MVA transformers are air-blast cooled using fans. The on loadtap changer is provided for regulating the secondary voltage at 6.9 kV. The value ofNGR is 9.5Ω

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Main Step Down Station (MSDS)

The 110kv tapped from the Edk-2 line is stepped down to 6.9kV usingthe power transformers. This power is fed to 13HT motors and 5 inplant substations.The MSDS facilitates the control of the yard derives - isolators, GCB, earth switch.Various relays are installed to sense the faults on both the 110kv and 6.9kv sides.The circuit breakers on the 6.9kV side. MOCBs are set up on the control panel forthe HT motors. The Remote Tap Changing Centre (RTCC) , monitors and controlsthe on load tap changer.

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Implant Substation

There are fine 6.6/.433kV substations erected at the site of different plants.The implant substations consists two 6.6/.433kV transformers. The .433V supply isfed to the PowerControlCentre(PCC)through bus ducts and then to Motor ControlCenters (MCC) that feed the LT motors.

Power Control Centre (PCC)

PCC distributes power to different MCCs and PDBs through protective relaysand circuit breakers. The secondary of the two transformers at each implantsubstation is loaded through the PCC. The two sections from the two transformerscan be connected through bus-coupler when any section fails.

Motor Control Centre (MCC)

Every Motor in the industry is controlled from the 17MCCs. The motors areoperated using the local start and stop buttons from the MCC. MCC module for eachmotor consists of DOL starter. Overload Relay (OLR), Contactor and the switch fuseunits. The module has provisions for protection and isolation of the motors.

In. addition to the above features, the j3lectricalsystem involvescapacitor banks for power factor improvement. Capacitor banks are provided at theMSDS and all the 5 substations. The 110V D.C. supply requirements are met withthe 49 x 2.2V Lead-Acid battery unit. The Power Distribution Boards supply thepower for domestic purposes, to plug points, lights, etc. Metering equipments,protective relays and circuit breakers are installed wherever necessary. Air circuitbreakers are used for the operation of LT motors at 433V level.

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The electrical supply management system maintains the energysupply to all consumers instantly, automatically and safety with required quality, at alltimes. The substations ensure the service reliability and continuity of the system.The flow of electrical energy takes place through the substations and they servemany purposes:

Sources of energy supply for the local area of distribution. Points of control and protection of the supply system. Centers of power factor improvement. Voltage regulators on the feeders. Street lighting control and switching.

Basically, the substations are an assemblage of devices to change somecharacteristic of the electric supply. The main purpose of the substations is tochange the voltage level, with provisions for control and protection of the powerlines. Substations are thus relevant to all regions of the power system. The majorsubstation equipments are briefed below:

Bus bars

Copper or Aluminum rods kept at a constant voltage, form bus-bars. Theelectrical energy is tapped to and from the bus-bars. The equipments of thesubstations are connected to the bus-bar conductors. Various bus-bar arrangementsare possible depending on the facilities and utilities.

Power Transformers

Power transformers perform voltage transformations at the substation.Generally, three-phase transformers are used rather than 3 - Single phase bank oftransformers so that 3 - phase, load tap changing mechanism can be used and theinstallation is simple.

Circuit Breakers

Circuit breakers are operated to make or break any circuit manually undernormal conditions and automatically on fault. Air circuit breaker, Oil circuit breaker,Vacuum circuit breaker and SF6 breakers are commonly used. Relays are providedto inform the breaker about the fault.

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In order to disconnect a part of the system for repair and maintenanceisolators are employed. They are operated on no load, after opening the circuitusing the circuit breaker. For reconnection, isolators are closed and then circuitbreaker is turned.

Instrument transformers

For the purpose of measuring and monitoring the current and voltage in theline Instrument transformers; current transformers (C.T) and Potential transformer(P.T) are used. Current transformers are step - up transformers lowering the currentlevel proportionately. Potential transformers are step - down transformers with alower secondary voltage. In addition to metering, instrument transformers areimportant to protective units.

lightning Arresters

lightning Arresters are provided on the three lines for protection againstsurges due to lightning. The over voltages produced are passed to the earth withoutinterfering the system operation.

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In electrical engineering view, the fault is a defect which diverts current fromits intended path in the circuit. The causes of faults and types of faults arenumerous. Generally, breaking of conductors or failure of insulation leads to faultyconditions of the system. Causes of failures may be broadly classified as below:

Breakdown at normal voltage due

1. deterioration or ageing of insulation

2. damages caused by unpredictable incidents (blowing, ofheavy winds, trees falling across the lines, vehicles collidingwith towers or poles, birds shorting lines, line breaks etc.)

Breakdown due to abnormal voltages caused by switchingsurges or lightning.

Faults lead to the damaging of the equipment and the entire installationas the faulty current level is excessive. During faults, voltages of the three phasesbecome unbalanced and the power flow is towards the fault. In power system theprobability of occurrence of abnormality is more on power lines.

For the purpose of analysis ac faults can be classified as:


Single line to ground fault (L-G)

Double line to ground fault (L-L-G)

Three phase fault (L-L-L)

Line to line fault (L-L)

Simultaneous fault

Three phase to ground fault (L-L-L-G)

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The above mentioned are the most common and dangerous faults occurringin the power system and are together called short - circuit or shunt faults. They area result of the breakdown of insulation.

These short circuit faults have the following harmful effects on power system.

Excessive heating due to heavy currents resulting in fire orexplosion.

In the form of are, short circuits damage the elements of powersystem.

Introduce unbalance in symmetrical circuit

Stability is disturbed

Marked reduction in voltage

Loads such as motor turn sources of fault power

The other abnormal conditions in ac systems include Voltage and current unbalance

Over voltages

Reversal of power

Power swings

Under frequency

Temperature rise

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Open circuit etc

Some faults are not serious enough to call for the tripping ofcircuit breakers. In such cases protective relaying is arranged for giving an alarm. Inmore serious cases, the continuation of abnormal condition can be harmful. In suchcases the faulty part should be disconnected without any delay.

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The "System Protection" is pertinent to the undisturbed operation of electricalsystem; to safeguard the equipment and the operating personnel from the hazards ofarising abnormalities. Electrical supply management systems employ protectivedevices to ensure the continuity and reliability of service. Protective systems isolatethe faulty section from the healthy part. instantly, without delay.

Every electrical system must operate continuously under normal service conditions and must withstand short-time over currents and over voltages during abnormal conditions. The abnormal state, fault, diverts current from its intended path. The faulty impedance is negligible and heavy current flows. An uncleared fault may result in fire which not only damages the equipment of its origin but may spread in the system and cause total failure. If the faulty section is not isolated soon, the entire installation will be destroyed.

Faults can be minimized by improved system design, better operationand high quality maintenance but cannot be eliminated completely. Erection ofefficient protective systems is therefore highly important. Essential qualities ofprotection are reliability, sensitivity, selectivity, discrimination and fastness ofoperation.

The system reliability is improved by the protective system and so itsreliable operation is essential. The protective system must be ready to functioncorrectly at all times, under every conditions of faults of the system for which it hasbeen designed. The protective reliability is influenced by the simplicity inconstruction. The reliability of simple protective schemes is greater.

The peculiar nature of the protective system lies in their selectiveand discriminative operation. The selectivity is the ability to detect the fault positionand disconnect the flaw from the healthy system. Also, the protective system mustbe able to discriminate the faulty situation for prompt operation, time - delayedoperation or no operation. A well designed protective system must have properties -selectivity and discrimination, otherwise unnecessary tripping and excess isolationresults.

Any protective system must be sufficiently sensitive so that thesmallest fault current is detected and cleared. The faulty section must bedisconnected quickly and therefore fast operation of the protective system playsimportant role. In short, the protective system must prevent any chances of failuredue to flaws while ensuring desirable operations.

The protective system must differentiate the normal and abnormalsituations and act accordingly. The smallest fault must be detected and cleared

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quickly, with minimum disruption to the system. The property of selectivity allows theisolation of flaw with the healthy equipments intact. Basically reliable protection withless chances of failure is ensured.

Zones of Protection

The formation of zones of protection assigns an area of responsibility under aparticular protection system. The entire power system is covered by severalprotective zones, neighboring zones allowed to overlap so that no part is leftunprotected. Usually, not more than two elements come under a certain zone, Thezone boundaries are defined by the circuit breakers. The zonal protection respondsonly to the faults within its area of concern and does not identify through faults.There are protections without exact zone boundary. The concept of protective zoneshighlights effective system protection.

Back - up protection

The essential protection provided to any machine is the primary or mainprotection. In the event of failure of primary protection, the , back-up protection takesover the job of clearing the fault. Back-up protection is a precautionary protection toact when any element of the primary protection fails. The extent of back-upprotection depends on economical and technical considerations. The systemprotection is made more flexible by the back-up.

During the installation of electrical system, protection schemes are designedbased on the system model. Sufficient protection is provided to the system afterexamining the economical and technical aspects, to avoid wastage of resources.

The system protection is accomplished by protective relaying and circuitbreaking. The fault clearing process begins with the relays sensing the situation as aheavy current across the secondary of the CT to which they are connected. Theinformation is passed to the circuit breakers by closing the trip circuit. The fault isisolated as the circuit breaker contacts are separated and the arc drawn betweenthem is extinguished.


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The voltage surges in the power system caused by lightning and switchingresonance is shielded by

Use of overhead ground wires

Low tower footing resistance

Use of lightning arresters

The over voltages caused in the system is diverted to the earth by reducingthe resistance of the earth path using the above devices. The elements of powersystem should withstand the over voltages without insulation failure. The insulationlevel and quality is graded, so that the damage due to overvoltage is minimum andequipment insulation is economical.

Overvoltage caused during switching operations depend on the equivalentinductance, capacitance, resistance of the system and other local conditions. Thesurges on opening the circuit breakers are reduced using opening resistor acrosscircuit breaker interrupters. Pre-closing resistors are incorporated to preventswitching surges on closing the circuit breakers.

Protection of Transformers

The transformer is major equipment at every levels of power system. Powertransformers are static devices, totally enclosed and usually oil immersed with rarechances of fault occurrence. But the consequences of abnormalities are severeunless the transformer is quickly disconnected from the system. Fast clearing offaults reduces damage to the equipment and the interruption in the power servicecaused by drop in voltages and instability.

The protective equipments for transformer protection are selectedbased on the following factors:

1. Particulars of transformer

KVA rating

Voltage ratio

Connections of windings

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Percentage reactance

Neutral point earthing resistance

Value of system earthing resistance

Whether indoor or outdoor, dry or oil filled with or without conservator

2. Length and cross section of connecting leads between CTs and relay panel

3. Fault level at the power transformer terminals

4. Position of transformer and load characteristics

The transformer protective scheme includes gas relays which givealarm on incipient faults, differential system of protection which gives protectionagainst phase to phase faults and phase to ground faults, other protective relays andsurge arresters which gives protection to insulation from high voltage surges.

The faults in transformer can be caused by the failure of insulating material due to dust, moisture, voids, weakening of winding due to external short circuits. Athrough fault is one which is beyond the protected zone of the transformer, but fedthrough the transformer. Internal faults are within the protected zone of thetransformer, phase to phase or phase to ground short circuits. Incipient faults areinitially minor and gradually become serious.

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Power Transformer Protection

Abnormal Condition Protection RemarksIncipient faults belowoil level resulting indecomposition of oil,faults between phasesand between phase andground.

Buchholz relay soundsalarm(Gas actuated relay)Sudden pressure relayPressure relief valve

Buchholz relay used fortransformers of rating500 kVA and above.

Large internal faults phase-to-phase, phase to- ground, below oil level.Faults in tap-changer.

1. Buchholz relay tripsthe circuit-breaker.2. Percentage differentialprotection.3. High speed high setover current relay.

Buchholz relay too slowand less sensitive.Buchholz relay for tap changeralso.Percentage differentialprotection used fortransformers of andAbove 5 MVA.

Earth faults 1. Differential protection.2. Earth fault relay.

For transformers of andabove 5 MVA.(a) InstantaneousR.E.F. Relay.(b) Time lag E.F.Relay.

Through faults 1.Graded time lagover current relay.2.HRC Fuses

Protection of distribution transformers.Small distributiontransformers up to500 kVA.

Overloads 1.Thermaloverload relay2.Temperature relayssound alarm

Generally temperatureindicators are providedon the transformers.Temp. increase isindicated on control'board also. Fans startedat certain temp.

High voltage surges 1. Surge arresters2. R-C Surge suppressors

In addition to arrestersfor incoming lines.

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The faults in transformer can be caused by the failure ofinsulating materials due to dust, moisture, voids, weakening of winding due toexternal short circuits. A through fault is one which is beyond the protected zone ofthe transformer, but fed through the transformer. Internal faults are within theprotected zone of the transformer, phase to phase or phase to ground short circuits.Incipient faults ,are initially minor and gradually become serious.

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Being the functions of protection units, relaying and circuitbreaking efforts to avoid damages due to faults in the system. The protectivedevices respond to the change in the circuit to prevent adverse effects. The systemprotection is accomplished by a number of subsystems.

The subsystems are:-


Circuit breakers


The process of fault - clearing involves, operation of relay on fault, opening of theCircuit breaker and disconnection of the faulty portion.

Protective relays sense the abnormality and sends tripping commandto the circuit breaker, to isolate the section from the healthy system. The normal andabnormal situations are differentiated and the device responds to the faultautomatically. On sensing the fault, the relay closes its contacts, thereby completingthe trip circuit of the circuit-breaker. The circuit-breaker is a manual switch designedto operate automatically on fault. When the trip coil is energized by protectiverelaying, circuit breaker opens and disconnects the faulty part.Relays are protective electrical devices initiating the action of faultclearance. Interposed between the main circuit and the circuit breaker, they decideto signal the tripping circuit. Different types of relays are available. The conventionalrelays are electromechanical type having movable assemblies and operating on theprinciples of

electromagnetic induction ( inverse-time over current relays)

Electromagnetic attraction (instantaneous over current relays)

Thermal effect (bimetallic relay)

Gas pressure effect (buchholz relay)

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Static relays have stationary electronic circuits for measuring and are beingincreasingly used for various applications. Recently, programmable relays havebeen developed. Some relays of interest to this project are discussed below:

Overcurrent Relays

Overcurrent relays are provided for overcurrent protection, where the relaypicks up when the current exceeds the pick up level. Overcurrents occur due toshort circuits caused by the phase faults, earth faults or winding faults. Dependingon the relay time, overcurrent relays are categorized as:

Instantaneous overcurrent relay

Inverse time overcurrent relay

The relays designed to act instantaneously without time delay, on faults arethe instantaneous overcurrent relays. These are simple and fast electromagneticattraction type relays. Instantaneous overcurrent relay functions to clear the suddenexcessive fault current. The contacts of the relay close immediately, the currentexceeds the set value. Each relay is set for a particular value of current.

The time delay relay, Inverse time overcurrent relay has definite timedelay set on requirements. The fault is cleared within the time set, such that it variesinversely to the amount of fi:lUlt current. The characteristic of such relays varieswidely, depending on the point of core saturation in the electromagnetic inductionrelays.

The major characteristics include:

definite characteristic, l0t= k

Inverse characteristic, 11t= k

More- inverse characteristic, ,In t= k

Inverse Define Minimum time (IDMT) Characteristic

Definite time overcurrent relays have constant time of operation

over the working range. The core saturates early and the relays act after thespecified time irrespectively of the value of fault current. Some relays, initially followthe inverse characteristic near the pick up then follows the definite time curve slightlyabove pick up; such relays are IOMT relays. The inverse time relays have an

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inverse time current relationship. The more inverse relays may be of very inverse orextremely inverse.

The Indian standards (IS: 3231-1965) specify only the IDMT curve. Theextremely inverse relays are more elective than IOMT relays, since it's possible toachieve accurate distinction between faults and surges after an outage.

Principles of operation

An induction type over current relay operates on the principles ofelectromagnetic induction with a mechanism similar to that of the energy meter.

The relay consists of an Aluminum disc allowed to rotate between twoelectromagnets energized from the secondary of the CT. The primary winding of theupper electromagnet is tapped and connected to the plug setting bridge. Thenumber of turns in use can be adjusted by the plug setting, giving the desired currentsetting. The pick up current, the current at which the disc commences to rotate isthe product of rated CT current and the current setting value.

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The secondary winding is connected in series with the winding of the lowermagnet. This winding is energized by induction from the primary, and together theyset up the rotational torque on the suspended aluminum disc. The torque iscontrolled by a spiral spring and a brake magnet on the disc. The rotating discspindle carries a moving contact that bridges the fixed contact. The angle of rotationof the disc can be set by the time setting multiplier. The time multiplier is not theoperating time but is a multiplier to obtain the actual time from the name plate curveof the relay.

The induction disc type relays are crafted with plug setting and time-settingmultiplier to obtain wide operating range. Each relay has associated time-currentcharacteristic; refer figure (5) the abscissa and ordinate being the plug settingmultiplier and operating time. The time-PSM curve corresponding to the timemultiplier setting equal to one is always provided on the relay name plate.

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Attracted Armature relays are commonly used overcurrent relays, based on the electromagnetic attraction. These relays are simplest in construction and operation. The relay consists of an electromagnet energised by the actuating quantity. The plunger coupled to the electromagnet is subjected to the magnetic attraction when the coil is energised. The action of the plunger closes the contacts of the relay. These are fast relays and the operating time does not depend of the value of the actuating quantity.

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Differential Protection Relay

The differential protection relay responds to the phasor differencebetween two electrical quantities. Most differential relays are current differential,based on the Merz-price circulating current principle. The relay senses theunbalance in the current entering and leaving the protected zone. The differentialprotection is generally employed for the transformer, generators large motors andtransmission lines.

In power transformers, differential protection is provided against theinternal faults. The faults disturb the balancing of the primary and secondarycurrents. When the actuating current exceeds certain limit. The differentialprotection relay must be set up along with provisions to overcome, difficulties causedby different CT characteristics, tap-changing and inrush of magnetsing current.Biased differential relay is the most common type of the relay.

The currents on the two sides of the power transformer are unequal.To equalize them interposing CTs are used between the transformer and the relay.Interposing CTs have different turns ratio but the currents flowing from them to therelay will be equal. The phase-correction of the two currents is achieved by theappropriate CT connection. As a general rule, CTs on any star winding are deltaconnected and CTs on ay delta winding are connected in star. The different CTratios result in circulating currents under through fault conditions. The undesirableoperation of the relay is likely to occur during tap-changing and energising of thetransformer.

The biased differential relay has two coils operating coil and the restrainingcoil. The relay operates whenever the torque produced by the operating coil isgreater than that produced by the restraining coil. Under normal conditions therestraining coil produces the higher torque and the operating coil carries no current.The mal operation during through load conditions, tap-changing and inrush ofmagnetizing current is prevented by the restraining component in the percentagedifferential relay

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Buchholz Relay

Buchholz Relay exclusively protects the transformer from internalfaults. It is universally used on all oil-immersed transformers and is located betweenthe main tank and the conservator tank. The gas - actuated relay is constructedwith two mercury switches attached to hinged floats in a metallic chamber. The pipeconnection between the conservator tank and main tank passes through the mainchamber as indicated in the figure . The upper float is connected to the alarmcircuit and the lower float is connected to the trip circuit.

The Buchholz Relay indicates incipient faults so that thetransformer can be disconnected before the severe fault. The gas accumulated isreleased by the cock provided on the top of device. The testing of the oil givesinformation about the type of the insulation failure. The relay is ignorant to faultsabove the oil level. Also, these relays are slow with minimum operating time of 0.1 second, average time 0.2 second. The mercury switch may operate falsely due tohigh sensitivity.

Earth fault relay

Faults involving flow of current on the earth return path are earthfaults. Normal current in the earthed neutral wire is zero. On earth faults currentspass through the neutral wire to the ground. Over current relays engaged in earthfault protection are earth fault relays. The setting is done independent of the loadcurrent since earth fault currents are low compared to phase fault currents.

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Over current earth fault relay may be connected in the residual CTcircuit. The three CTs on each line are paralleled and the relay is provided acrossthem. Currents flow to the relays only when fault involving earth occurs. The earthfault protection may also be provided using a single CT on the neutral wire attachedto the relay. The neutral CT carries a current only when the fault current escapes tothe earth through neutral to earth connection. These methods have indefinite zones,sensing the earth faults anywhere in the system. Such arrangements areunrestricted earth fault protection.

The restricted earth fault protection employs 3 line CTs and a neutral CTto achieve the purpose. The secondary of the line CTs and neutral CT are paralleledwith the relay. Ideally the output of the CTs is proportional to the sum of 4zerosequence currents in the line and neutral earth connection if the latter is within theprotected zone. The relay current for external faults is either absent or summed tozero. For internal faults, the relay current is equal to twice the total fault current asthe sum of zero sequence currents in the CTs is greater than zero. Thus theprotection is restricted to the particular zone.

Motor protection Relays

The Motor Protection relays take care of complete protection of motors. Thefaults likely to occur in motors may be summed under the five heads:-



Over current

Over Voltage

Earth fault

The motor protection relays comprehend the entire motor protection. Theserelays are exclusively used in motor protection and are of different types. The choicedepends on the size and capacity of the motors.

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Static Motor Protection Relays

The response in a static relay is developed by static circuits - electronic /magnetic I optical without mechanical motion of components. However, the outputstage is compared of electromechanical relay units to operate the breaker. Thestatic motor protection relay provides accurate protection under all operatingconditions. The relay is designed to operate for both balanced and unbalanceconditions.

The type CTMM relay has five independent units to sense the variousfaults.

Thermal unit: Acts when the current exceeds 105% of the relay settings current. Theunit is provided against auxiliary supply failures.

Instantaneous 3 phase over current unit: To sense the balanced over current to themotor. The unit is adjustable and time delay of operation is provided to allow transient starting current.

Time delayed 3-phase over current unit: Provides protection during starting andprolonged starting conditions.

Instantaneous unbalance and single phasing element: Set up to trip instantly againstUnbalancing against single phasing.

Instantaneous earth fault element: Prevents the motor from earth fault adversities.

Microprocessor based Motor Protection relay (type MM30)

With technological advancements, static programmable relays OT microprocessor based relays developed. The digital technique combines supervision control and protection into a single unit. The microprocessor based relays are preferred for complex protection and control systems. These relays can be used to protect both HT and LT motors.In addition to providing protection the relay performs data acquisition, dataprocessing and data monitoring. The display unit of the relay monitors the currentvalues of the various system parameters. To indicate a faulty condition and the typeof fault, LEOs are mounted on the front panel. A bright LED indicates a particularfault. Five buttons are provided to program the relay, i.e., to enter the settings of therelay.

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Circuit Breakers

Circuit Breakers are mechanical devices employed in closing or opening ofan electrical circuit. The functioning circuit breaker carries full load currentcontinuously with provision of making or breaking the operating current. Infault clearing, circuit breaker interprets information from the relay by isolating thefaulty part.

To receive the signal from the fault sensor the circuit breaker is equippedwith trip coils. The switching and current interruption is performed by the fixed andmoving contacts. When the breaker is closed these contacts touch each other andcarry the current, under normal conditions. The contacts, called electrodes engageeach other under the pressure of a spring. Whenever fault occurs on any part of thesystem the trip coils get energized and the moving contacts are pulled apartby some mechanism, opening the circuit. During normal operation, station operatoropens or closes the breaker for the purpose of maintenance and switching.

The operation of circuit breaker is initiated by exciting the trip coils andinvolves separation of the current carrying contacts. This action leads to arcingbetween the contacts. The circuit is opened only when the discharge, ceases, i.e current interrupted. The arcing continuous for a brief period after the contacts areparted, allowing gradual transition from current carrying to isolating states of thebreaker. In case of fault isolation, the circuit needs to be opened quickly. Thephenomenon of arcing delays the current interruption, also generates huge amountsof heat, damaging the system or / and breaker itself. Therefore circuit breaker mustbe able to extinguish the arc within the shortest possible time.

Usually, circuit breakers are constructed with the contacts in an insulatingfluid. The insulating fluid functions to extinguish the arc drawn between the contacts,when the breaker opens and provides insulation between the contacts and fromeach contact to earth. The circuit breaking medium must have high dielectricstrength, non-inflammability, thermal stability, arc extinguishing ability, chemicalstability and commercial availability at moderate cost.

Circuit breakers are classified on the basis of the insulating or arc-quenchingmedium. The several types are:

Air circuit Breaker (ACB)

Oil Circuit Breaker (OCB)

Minimum Circuit Breaker (GCB)

Vacuum Circuit Breaker (VCB)

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In the ACB the contact separation and arc extinction takes place in air.The arc is extinguished by cooling, lengthening and splitting the arc. The highresistance principle increases the voltage drop across the arc above the systemvoltage and the arc is extinguished at current zero of the ac wave. This is theprinciple of operation of Air break type breaker in which the arc is split by thearc-chute. In Air-blast breakers, compressed air in admitted into the arc extinctionchamber and the air blast removes the ionized gases extinguishing the arc.


The circuit breaking contacts are placed in insulting oil medium. The heatgenerated by the arc struck between the contacts evaporates the surrounding oil anddissociates it into hydrogen gas at high pressure. The hydrogen gas cools the arcdue to its high heat conductivity and forces the oil into the space between thecontacts after arc interruption at current zero.

The MOCB is so called as it uses minimum oil for the purpose,


The gas circuit Breakers use sulphur hexafluoride (SF6) gas with good arcquenchingproperties and high dielectric strength as the insulating fluid. SF6 issuperior to oil and air chemically and physically.

Unlike in ACB and MOCB, the arc is reduced diametrically in SF6breakers.SF6gas is blown axially along the arc and heat is removed by axial convection andradial dissipation. Consequently, the arc diameter becomes small during currentzero, when turbulent flow of the gas is introduced for extinguishing the arc. The SF6regains it dielectric strength immediately after arc extinction.


The make-break contacts of the VCB are placed in vacuum. Ideal vacuumis a perfect dielectric medium in which arc cannot persist. However, the separationof contacts give rise to plasma, the vapor released from the contact surfaces, thevapor is of positive ions released from the metallic contacts. After current zero therate of release of vapor falls and the medium regains dielectric strength.

The successful operation of the circuit breaker is essential for the

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stable and reliable operation of the power system; protection of the system. Thecircuit breaker must operate under varying situation. The making breaking capacitiesare to be maintained always.

The transducers are current and potential transforms. They reduce thecurrent or voltage of the system to a level acceptable to the relays. The lower levelinput to the relays ensure that the physical hardware of the relays will be quite smalland less expensive. Also, the personnel working with the relays will be in a safeenvironment.

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The scenario of protection in a substation depends on its essentialequipments an,d operating voltage. Adequate protection is provided to everyequipment based on their importance, location, type rating, cost, probability andstatus of errors etc. Consequently the strategy for the 110kV substation of KMMLinvolves the protection of power transformers and HT motors. The scheme isdepicted in figure.

The primary side of the power transformer is provided with an SF6

breaker (GCB) to open the incoming 110kV lines when signaled by a relay ormanually operated by the personnel. The breaker is followed by 3 phase currenttransformer. The 3 cores of the CT serve the purposes of metering, overcurrentprotection and differential protection. Similarly, the low voltage side of thetransformer has an MOCB and a 3-phase current transformer

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On the 11OkV side, the core 1 is connected to an ammeter (0-100A)via an ammeter selector switch (ASS) for monitoring the current through the lines.The other cores carry the relays to protect the transformer and equipments on linefrom the adverse effects of faults. The relays placed on the 110kV side arediscussed below.

Core 2 Relays

The core 2 relays alias 50, 51 and 51-G detect the overcurrent resulting fromshort circuits. They act through the master trip relay 86-2 to open the GCB. Thefaults resulting in overcurrent on the lines are cleared by the action of these relays.


Name : Instantaneous Overcurrent Relay

Type : CDG

Set point : 850%

Design . Primary current = 67.5A

Allowable current = 900% of 67.5A

So the safe operating point is arbitrarily chosen at 850% of the normal primarycurrent.

When huge currents result from faults instant tripping must be ensured, forwhich the instantaneous relay is best suited. The operating point is set at a highvalue to allow the flow of heavy starting currents. The relay features with flagindication for each phase and 3 x 2 NO contacts. One of the 2 NO is connected tothe master relay coil and the other goes for flag indication.


Name : Inverse time Overcurrent Relay

Type : CDG 31

Time multiplier setting = 0.2

Plug setting . = 0.75

Design . Relay current = 0.675A

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Allowable current = 9 x 0.675 = 6.08A

Plug setting range = 50 - 200%

Select Plug setting . 75% = 0.75

When plug setting multiplier (PSM) = 2

The relay operating time = 10 seconds from the graph in figure (2)

For the relay to operate in 2 seconds keep the time multiplier at 0.2

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The CDG 31 relay is an inverse time overcurrent relay with definiteminimum time. The non-directional, disc type current relay has two normally open(NO) contacts. Whenever the relay energies due to overcurrent these contacts close;one giving flag indication and the other completes the circuit to the master trip relay.

51- G

Name : Earth Fault relay (very INV.)Type : CDG13Plug setting : 0.8Time multiplier : 0.1

The CDG13 is an overcurrent relay with very inverse characteristics. Thehigh selectivity of the relay made it to be used for the earth fault protection. Therelay is placed on the neutral-earth point of the CT.These relays are currentactivated relays set up to detect the short circuit faults; line - line (L-L) fault, lineline-line (L-L-L) fault, line-line-earth (L-L-G) fault, line-earth (L-G) fault. All the relaysare of CDG type, non-directional disc type relays. The actuating quantity, current isconverted to voltage and then used to energise the electromagnet. The inducedcurrent in the suspended disc rotates it to make a contact that is connected to theNO contacts related to the indication and trip circuit.

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Core 3 Relay (87)

The core 3 is exclusively for differential protection. The differential protectionrelay is numbered 87. Two interposing CTs from the primary and secondary sidesconnect the main CT to the differential relay. Any unbalance between the currentson the two sides is detected and the relay energises.

Name : Differential Protection RelayType : DTH 31Bias setting : 30%

The DTH 31 is a high speed biased differential relay designed to protectthree- phase power transformers against internal faults. Type DTH 31 is applicablefor two - winding transformers. The relay uses second harmonic restraint to preventoperation by magnetising inrush currents during transformer energising.Maloperation under over excited condition is prevented by the fifth harmonic by pass circuit. The high-set circuit instantly clears heavy internal faults. The transactors provide low burden to the relay. Static circuitry is employed throughout with output from a single attracted armature unit.

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Referring to the block diagram, the currents 11and b from the 2 sidesof the power transformer are vectorialy added in the centre tapped bias transactorT1. The bias setting of 15%, 30% and 45% can be obtained by 3 taps in each half ofthe transactor primary. The output of T1 is rectified and smoothed to obtain therestraint bias voltage VB. The centre tap is connected to the differential circuit T2, T3and T4 in series. The difference between 11and 12flows through the primaries of T2the harmonic restraint transactor, T3the operating transactor and T4 high-set currenttransformer.

The tuned circuit in the secondary of T3 is arranged to resonate at the secondharmonic frequency whose output is rectified to obtain harmonic restraint voltage Vh.The outputs of T3 and T4 are rectified and smoothed to obtain differential voltage VDand high-set voltage Va respectively. The greater of the restraining voltage levels VBand VH is compared with the differential operating voltage level Vp. When theoperating voltage level exceeds the restraining voltage by more than a presetamount, the second comparator produces an output to drive the common relay. Thehigh-set voltage Va operates the relay if differential current exceeds 10 times therated current.

The attracted armature output unit has 3 NO contacts, one for each phaseand a flag indication. When the differential fault is sensed on any phase thecorresponding NO contact closes and the relay is flagged. The closed contactcompletes the circuit to the master trip relay 86-1, which is then energised to openthe GCB on 110kV side and the MOCB on the 6.6 kV incomer.

Miscellaneous Relays

Various other relays are provided to sense the faults that are not the concernof above mentioned relays. Also, supervisor relays are employed to handle trippingof breakers.


Name : Buchholz Alarm Relay

Type : VAA 12

The relay 63-X is attached to the mercury switch, associated with the alarm ofthe Buchholz relay mounted on the power transformer. The incipient faults within thetransformer are sensed by the Buchholz relay and the alarm is initiated by the63 - X. Voltage activated, attracted armature type auxiliary unit is used.

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Name : Oil temperature Alarm Relay

Type : VAA 12

The relay is connected to the temperature sensor of the transformer oil.Whenever the oil temperature exceeds 6SoCthe relay energises and rings the alarm.Auxiliary relay type VAA is used. The relay is to be reset manually after cooling.


Name : Buchholz Oil temperature flow oil trip relay

Type : VAA 12

As the name indicates the relay senses major faults in the power transformer,high oil temperatures and low oil levels. The relay is reset manually after clearingthe fault. The 63-Y is the trip relay for the faults indicated by the alarms of 49 - Xand 63 - X. The relay is connected to the breaker through 86 -1.

86 -1

Name : Tripping Relay

Type : VAJ

The tripping relay 86-1 is the Master trip relay for 63Y, 8F and 50 R. The NOcontact of the relay closes whenever the 63Y, 87 and 50R is energized. Thepeculiarity of 86-1 is that it opens both the GCB and MOCB. The MOCB is thebreaker on the secondary side of the transformer. Thus the transformer is isolatedfrom either side. The fault sensed by these relays calls for the tripping of the primaryand secondary sides. The relay supervises the clearing of serious faults.

86 -2

Name : Tripping relay

Type : VAJ

The relays 50,51 and 51-G opens the GCB through the master trip relay, alsocalled as the lock out relay, 86-2.lnstead of connecting each of the three relaysdirectly to the GCB, a single relay, 86-2 connects them to the GCB.

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Name : Trip circuit supervision relay

Type : VAX

The entire relay circuit is under the supervision of the 97. The relay indicatesthe failure of the ?c supply to the relays. The relay is set up to act when the primaryprotection fails.

The relays on the primary sides are discussed above. On the secondary side thesimilar scheme is followed. The incomer on the 6.6 kV side of the transformer isoperated by the MOCB. The breaker is released manually by operating personnelunder normal conditions of switching. Upon fault the relay sense the fault and passthe information to the trip circuit of the circuit breaker.

The control panel for the HT motors is located in the MSDS. Every motor hasassociated MOCB and the motor protection relays- The CTMM or the MM-30. TheMM-30 is the programmable relay, set up with the LCD display. CTMM relaysemploy static circuitry in discriminating the faults and electromagnetic operation fortripping.

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Fortesque theorem states that any unbalanced three phase system of sinusoidal quantities can be resolved into three balanced systems of phasors called symmetrical components of the unbalanced system. The balanced systems are the

Positive sequence system

negative sequence system

zero sequence system

The symmetrical components differ in their phase sequence. The positivephase sequence system is that system in which the voltages or currents attainthe maximum in the cyclic order as in normal supply system. Assumingcounter - clockwise rotation the positive sequence components take the orderRBY as in figure below

The negative phase sequence components attain maximum in the reverseorder while the zero phase sequence components are in phase with each other. The3 sets of sequence components have their magnitudes same but differ in the phaseangle between the elements of each set.

The positive sequence currents are the normal current in the 3 lines of thesupply system. The flow of only the positive sequence currents occurs for normaloperation and during a 3 - phase fault. For a 3- phase fault, the magnitude of thecurrent increases to a high value, their phase sequence remaining the same asunder normal conditions. Such faulty situations are due to balance 3 - phase faults.

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The negative sequence components arise during an unbalanced state of thesystem, i.e., short circuiting and phase failures. These faults disturb the balance ofthe system and the currents experience change in magnitude and phase. The zerosequence currents emerge when faults involving earth occurs. When only negativesequence or positive sequence currents flow their sum will be equal to zero.

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Current Transformer

The three phase current transformer reduces the current level in the linesaccording to the ratio 100:1. The trip circuit is connected in the secondary of thecurrent transformer. The rating of the CT is selected in association with thecontinuous current possible through the circuit. The primary of the CT has a singleturn while the secondary is multi-turn winding. It is a step up transformer, steppingdown the current to a suitable value.

Sequence Filter

Any unbalanced current in the system is resolved into 3 sequence components - negative sequence, positive sequence and zero sequence. The sequence filtering is accomplished by the connection of the transducers across the CT secondary. The transducer 1 is between the B-phase and R-phase such that the current through it is the differential B-R current. When only positive sequence current flows the upper have of the centre-tapped transducer 1 is effective. The current in any phase at that time is equal to all others and only a pair of lines needs to be used. During unbalanced conditions both negative and positive sequencecurrent flows for which the additional transducer is provided between the remaininglines.


Transducers are devices that convert one form of energy into anotherform of energy .The current to voltage transducer, used here, are a transformer withmore number of turns on the secondary. The step up transformer is designed withhigh voltage and, very low current on the secondary. The reduction in current isachieved by using high resistance copper wire. So secondary winding is made out ofthinner wires than the primary winding. The two centre tapped transducers are rated8 mVJ2V,7.5VAThe primary winding has the same number of turns as the secondaryof the CT so that the CT secondary current is same as the primary current of thetransducer.

Squaring circuit

The squaring circuit comprises of two capacitors and a choke. Thefunction of the circuit is to provide the output as a boosted input voltage. The chokedoubles the voltage superimposed over it by one of the capacitor. The choke voltagetogether with the voltage across the other capacitor gives the boosted output. Theboosting is provided only to the negative sequence element for quick action of therelay. The negative sequence element thus acts for a lower value of current than thepositive sequence element.

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Bridge Rectifier

The bridge rectifier uses four diodes for the rectification of the acvoltage in the circuit. The coil chosen operates on dc voltage, but the entire circuit isset up on ac. The rectifier forms an interface between the dc coil and the circuit.Since a differential quantity is the actuator, the direction of power flow cannot bepredicted. Therefore bridge rectifier is essential at this stage.


Separate relay units are provided for the positive sequence and negativesequence elements. The positive sequence element is slower, neglecting thetransients. The reactance of the coil is 140 ohms, operating voltage 10V. Thenegative sequence relay coil is 945 ohms, 10V. The negative sequence element hasa higher impedance to handle the huge voltage due to boosting.

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The circuit consists of two relay units, one for positive sequence tripping(1+)and the other for negative sequence tripping (I-). The positive sequence elementis directly connected to the secondary of the transducer (TR1). When a three phasefault occurs, which is a rare event, the voltage across the transducer rises to a highvalue and energizes the 1+element directly. The coil operates at 10V but theresistors R1, R2 and R3 causes the relay to catch only at 24V.

The negative sequence is activated through the boosting network(squaring circuit) comprising capacitors C1, C2 and chokes C. Under normalconditions the capacitors remain charged and resistors R4, R5 and R6 prevent theoperation of the relay. On fault the voltage rises and the capacitor C1 charges theinductor. The choke coil doubles the voltage and feeds it to the capacitor C2. Theboosted voltage from the C2 energizes the relay coil of element 1-closing thecontacts. The unit is designed to operate at 8V.


The circuit is designed to operate the relay when the input current exceeds12A and 4A for the positive sequence and negative sequence elements respectively.The values are referred to the secondary of the CT.

CT ratio = 100: 1

Voltage rating of the transducer = 8mV/2V

The transducer is chosen to have its primary characteristics as that of CTsecondary.

Voltage on the transducer secondary equivalent to 1A CT current = 2V

Turns ratio of the transducer = V2N1 = 250

Secondary current I2 = I1.V1N2 = 4mA

Positive sequence element (I+)

The I+ must operate at V2 = 24V. Select R1 = 1.5k, R2 = 3k , R3 = 8000 .

Voltage drop across 1.5k = 1500*.004 = 6V

Voltage drop across 800ohm = 800*0.004 = 3V

Voltage drop across 3k = 3000*0.004=12V

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Total voltage = 6 + 3 + 12 = 21V

The 1+operates at an early stage V2 = 21V, i.e. I1 = 10.5A

Negative sequence element (I-)

The 1-must operate at V2 = 8 V.

For a resonating network , XL = XC.

Select the time period of the faulty pulse t = 11 ms.

Resonating frequency fr = 1/0.011 = 91 HZ.

Fr = 1/2*3.14 * (L C)^.5

Let C = 1*10 Then L = 3.2 H

The choke is tapped at an inductance of 3.2 H.

The reactance offered XL =2 * 3 14 * f * L = 1000 Q Voltage drop = 1000* 0.004 =4 V Boosted voltage level at 1A input = 4+2 =6V Boosted voltage level at 4A input = 8 + (1000 * 0.016) = 24 V Select R4 = 1 k, R5 = 3k, R6 = 470 Ω

Voltage drop across 1k = 1000 * 0.004 = 4V Voltage drop across 3k= 3000 * 0.004 = 12V

Voltage drop across 470 ohm = 470 * O.OO4 = 2V

Total voltage = 4 + 12 + 2 = 18V.

The 1-operates at an early stage V2 = 6V.

The 1+operates at a later stage for allowing transients.

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1. A Course in Electrical Power - J.B.Gupta

2. Elements of Power system Analysis - WilliamD. Stevenson, Jr.

3. Switchgear Protection and Power Systems - Sunil S. Rao

4. Principles of Power System - V.K.Mehta , Rohit Mehta

5. A Basic Course in Electrical Engineering - Francis M.Fernandez

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REFERENCES1 , J,B. Gupta, "A Course in Electrical Power ", NGR, pg.482, 12thedition

2 Dr, P.S. Bimbhra, " Electrical Machinery", On load tap changer,pg.102, 7thedition

3 Sunil. S, Rao, "Switchgear Protection and Power Systems", Stalling, pg.779, 11th edition

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The system protection is an important concern of a power engineer. Theexpansion of power system has ensued in and is ensuing from the rapid industrialprogress leading to improvement of standard of living of people. The relevance ofoperation of power system, without intervention of faults, is thus realized with nodoubt. Interruption of supply services by faults is prevented by protective systems.We have designed a protective scheme covering all the basic protection requirements of a substation that assists in the infallible service of the supply system. Objectionable situations, faults are to be screened under any circumstances. Different protective schemes prevail, we add ours to them.

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