ANURAG RPORT

8/3/2019 ANURAG RPORT

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About J.B.S ENTERPRISE

J.B.S. ENTERPRISE has installed a 220kV Substation receiving 2

incomer lines from Ambheti delivering total power of 230MW to

different areas. The data collected from this substation are sent to

WRLDC (WESTERN REGIONAL LOAD DISPATCH CENTRE). The mainpurpose of J.B.S. ENTERPRISE are Engineering, Procurement,

Construction, Testing, Commissioning, Operation and Maintenance

services for the Extra High Voltage Substations, Switch Yards and

Power Transformers.

Following subjects were studied during the period of training.

220/66KV & 66/11KV Power Transformers & various outdoor

protections.

Various tests on Transformers.

Tan Delta values.

Lightening Arrestors.

Surge Monitors

Current Transformers & Potential Transformers & their testing.

Capacitive Voltage Transformers (CVT)

Off load Isolators

220KV & 66KV SF6 Circuit Breakers, 11KV Vaccum Circuit Breakers.

Control Room, Control, Relay & RTCC Panels.

Relays, types of relays & their testing.

AC & DC Distribution, Battery Chargers

DC/CT/PT/Control/Protection/Indication/Alarm DC Supply


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B.H.E.L. 220/66 kV 50MVA POWER TRANSFORMER

RATING

Type of Cooling ON AN ON AF

Rating HV (MVA) 37.5 50

Rating LV (MVA) 37.5 50

No-Load Voltage HV(kV)

220

No-Load Voltage LV(kV)

66

Frequency (Hz) 50

Line Current HV (A) 131.22

Line Current LV (A) 437.39

Temperature(oC) 50

Temerature Rise(oC) 55

B.H.E.L. 220/66 kV 100MVA POWER TRANSFORMER

RATING

Type of Cooling ON AN ON AF OFAF

Rating HV (MVA) 60 80 100

Rating LV (MVA) 60 80 100

No-Load Voltage HV(kV)

220

No-Load Voltage LV(kV)

66

Frequency (Hz) 50


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Temperature(o

C) 50

TemeratureRise(oC)

55

B.H.E.L. 66/11 Kv 16MVA POWER TRANSFORMER

RATING

Type of Cooling ON AN ON AF

Rating HV (MVA) 12.5 16

Rating LV (MVA) 12.5 16

No-Load Voltage HV (kV) 66

No-Load Voltage LV (kV) 11

Frequency (Hz) 50



Temperature(oC) 50

Temerature Rise(oC) 55

Phase 3

Connection Symbol YNyn0

Core & Winding (Kg) 12500

Weight of Oil (Kg) 9900

Total Weight (Kg) 39100

Oil Quantity (L) 11120

Untanking Weight (Kg) 14700


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Outdoor Protections of Power Transformers

Pressure Relief Valve (PRV)

Buchholz Relay

Oil Surge Relay(OSR)

Magnetic Oil Gauge(MOG)

OLTC Motor drive for tap changing

OSR

OSR is an oil surge relay, similar to the buchholz surge flap usually placed on

the oil pipe between the OLTC and its oil tank to respond to rushes of oil from

the former indicative of an internal fault. It is used to protect against faults in

the cable termination box that would result in a rush of oil to the small

conservator.

PRV

Pressure Relief Valve. The pressure relief valve (PRV) is a type of valve used

to control or limit the pressure in transformers which can build up by a oil

heating, instrument or equipment failure, or fire.

Buchholz Relay

Buchholz relay is a gas- actuated relay installed in oil-immersed transformersfor protection against all kind of faults. It is used to gives an alarm in case ofslow developing faults or incipient faults in the transformer and to disconnectthe transformer from the supply in the event of severe internal faults. It isinstalled in the pipe between the conservator and main tank. This relay isused in oil-immersed transformers of rating above 750 kVA.


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Construction

Fig shows the constructional details of buchholz relay. It consists of a domedvessel placed in the pipe between the conservator and main tank of thetransformer. The device has two elements. The upper element consists of amercury type switch attached to a float. The lower element contains amercury switch mounted on a hinged type flap located on the direct path offlow of oil from the transformer to the conservator. The upper element closesan alarm circuit during slow developing faults whereas the lower element isarranged to trip the circuit breaker in case of severe internal faults.


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Operation:

The operation of buchholz relay is as follows:1. In case of slow developing faults within the transformer, the heat due tothe faultcauses decomposition of some transformer oil in the main tank. Theproducts of decomposition mainly contain 70 % of hydrogen gas. Thehydrogen gas being light tries to go into the conservator and in the processgets trapped in the upper part of the relay chamber. When a predetermined


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amount of gas gets accumulated, it exerts sufficient pressure on the float tocause it to tilt and close the contacts of mercury switch attached to it. Thiscompletes the alarm circuit to sound an alarm.

2. If serious fault occur in the transformer, an enormous amount of gas isgenerated in

the main tank. The oil in the main tank rushes towards the conservator viathe buchholz relay and in doing so it tilts the flap to close the contacts ofmercury switch. This completes the trip circuit to open the circuit breakercontrolling the transformer.

MOG

Magnetic Oil Gauge is used to show the level of Oil inside the

conservator tank of the transformer. Additionaly it can also give an

alarm when the oil level reduces below minimum and/or maximumadmitted level.

OLTC

On load Tap Changer (OLTC) is used with higher capacity transformerswhere HT side voltage variation is frequent and a nearly constant LT isrequired. It is placed on the HV side of the transformer to minimize thephysical size (resulting in smaller motor to operate or smaller forces toturn the crank handle manually) and cost of manufacturing since thecurrent is lower on that side and copper contacts will be smaller. OLTC

is fitted with the transformer itself. Multiple tappings from HV windingsare brought to the OLTC chamber and connected to fixed contacts.Moving contacts rotates with the help of rotating mechanism having aspindle. This spindle can be rotated manually as well as electricallywith a motor. Motor is connected in such a way that it can rotate inboth the directions so as to rotate the OLTC contacts in clockwise andanticlock-wise direction. Two push buttons are fitted on the LCP (localcontrol panel) to rotate the motor and hence the OLTC contacts inclockwise and anti-clockwise direction. This movement of contacts thuscontrols the output LV voltage of the transformer. So rotating of OLTCcontacts with spindle or push buttons in this way is a manual process.

In case this process of rotating the OLTC contacts and hencecontrolling the LV side voltage is to be done automatically then a RTCC(Remote Tap Changer Controller) is installed with the transformer HTPanel. The RTCC sends signals to LCP and LCP in turn rotates the motoras per the signals received from the RTCC.TESTING OF TRANSFORMER

The objective of testing is to:


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Ensure Quality

Ensure that the products manufactured have met therequirement of Customer Specification

Prevent accidents which may occur if a failed product is put into

service Ensure that the product is fit for its intended use

Safety Precautions

Cordoning off the test are.

Display of Danger boards.

Indication Lamps.

Use of Hand gloves & safety shoes.

Entry of unauthorized personnel to be restricted. Discharge the transformer after high voltage test.

Earth resistance.

Preparation for the Test

Earthing connection should be rigid to the transformer.

Check for oil level.

Release air from Buchholz Relay & the bushings.

Clean the bushings with non-fibrous dry cloth.

Ensure that all connections are tight.

Cables and shorting links should be of sufficient.

Ensure that the connections are made as per the circuit diagram.

Ensure that all instruments are properly earthed.

Tests on TransformersRoutine tests:

Turns/Voltage Ratio Test

Insulation Resistance Test.

Winding Resistance Test.

Oil Test. Vector Group/ Polarity Check.

No-Load Loss/Core loss Test.

Load Loss Test.

Impedance Test

Capacitance/ Tan Delta Test of Bushings.


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Voltage/ Turns Ratio Test:The Transformer Turns Ratio test(TTR) is used to make sure that the Turns Ratio between thewindings of the transformer is correct. This ratio decides whatthe output voltage of the transformer will be with respect to theinput voltage. The ratio is calculated under no-load conditions,

with ratios calculated at the tap positions for each winding andfor the winding as a whole. A voltage is applied to one windingand the voltmeters connected to both low voltage and highvoltage windings are read simultaneously. The transformer ratiois the ratio of the HV voltmeter and the LV voltmeter readings.When ratio tests are being made on three-phase transformers,the ratio is taken on one phase at a time, and the measured ratioshould be compared with the ratio calculated using nameplatevoltages. Any variation should be within .5%.

OC Testing of a Distribution Transformer:

1. Apply 440V 3Phase AC Supply on HV Side.2. Keep LV Side open & measure the following voltages:

RY 384 ry 14 rn 7YB 384 yb 14 yn 7BR 382 br 14 bn 7

Insulation Resistance Test: The winding insulation resistancetest (also known as the Meggar test) is a measure of quality ofinsulation within the transformer. It can vary due to moisturecontent, cleanliness and the temperature of the insulation parts.

All measurements are corrected to 20'C for comparisonpurposes. It is recommended that tank and core are alwaysgrounded when this test is performed. Each winding should beshort-circuited at the bushing terminals. Resistances are thenmeasured between each winding and all other windings andground (for 2 winding transformer - H-LG, L-HG and HL-G andthree winding transformer H-LTG, L-HTG, T-HLG, HL-TG, HT-LG,LT- HG and HLT-G ).

Meggering of a Distribution Transformer:1. Make sure transformer is disconnected & discharged from

supply.2. Connect the terminals of Meggar between following

terminals & note the resistance (in Mega Ohms):

HV side to earth.

LV side to earth.

HV side to LV side.


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Winding Resistance :The resistance of a transformer windingcan be measured after current has not passed through thetransformer for several hours, allowing it to reach the same

temperature as its surroundings. Winding resistance is calculatedby measuring the voltage and current simultaneously, with thecurrent as close to the rated current as possible. Calculating thewinding resistance can be helpful as it lets you calculate andcompensate for I2R losses, a major component of load losses asa whole. Winding resistance measurements can be made todetermine if any changes have occurred in the current carryingpath. The winding resistance measurements should be madewith a Wheatstone bridge, Kelvin bridge or similar bridge capableof measuring fractional ohms accurately. For Wye connectedvalues, measurements should be made between each pair of

bushings, then summed and multiplied by three-halves to get thecomparison value.

Testing of Winding Resistance using Micro-ohm Meter:Connect the terminals of micro-ohm meter to HV side & LV sideand measure the resistance. Resistance above few micro-ohmsindicate that the winding may be damaged & should beinspected by the Manufacturer.

Oil Test: A sample of insulating oil from a transformer in servicecan reveal much information about what is taking place inside

the transformer. There are three basic enemies to insulating oil -oxidation, contamination and excessive temperature. Thefollowing tests can be done:

o Dielectric breakdown

o Power factor

o Moisture content

o Interfacial tension

o Acid Number

Polarity/Vector Group Test: The polarity of atransformer is either additive or subtractive. In order to find outthe polarity of a transformer, a voltage is applied between theprimary bushings. If the resultant voltage between thesecondary bushings is greater than the applied voltage thatmeans that the transformer has additive polarity. If it is lower,the transformer has subtractive polarity. Polarity is not importantfor a single connected distribution transformer, but it is a vital


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concern if transformers are to be paralleled or bank connected.Three phase transformers are also checked for polarity by thesame means.

Polarity Check of a transformer:

1. Short R-r taken from HV & LV side.2. Apply 440V 3Phase AC Supply on HV Side.3. Measure the following Voltages & verify against the vector

group addition:

Rn + Yn = RY

RY 384 ry 14 Rn/rn 7

YB 384 yb 14 Yn/yn 7BR 382 br 14 Bn/bn 7

No-Load/Core Loss Test: Under no-load conditions,a transformer will continue to drain sources of electrical energy.The chief source of this drain is core loss, which occurs in themagnetic core through a combination of hysteresis and eddycurrent loss, among others. Core-loss is calculated by applyingthe rated voltage and frequency to a transformer under no-loadconditions. The resultant current is then measured, from whichthe loss of energy can be extrapolated.

No-Load loss/Inrush Current/Magnetizing current test:

1. Apply 440V 3Phase AC Supply on HV Side.2. Keep LV side open & measure the following current:

Ir = 2.1 mAIy = 2.0 mAIb = 3.6 mA


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Load Loss Test: Load loss is a combination of I2Rlosses, stray losses and eddy losses, all of which contribute tothe loss of electrical energy that is seen as current transferredfrom one winding to another. Load loss changes with themagnitude of the load: that is to say, higher loads see higherrates of loss. The load loss is therefore generally calculated forthe rated load, while the transformer is under full-loadconditions. It can be measured by applying a voltage to onewinding while the other winding is short-circuited. The voltage isadjusted until the current flowing through the circuit is the sameas the rated current. The power loss measured at this time is theload loss.

Load Loss/SC Testing of a Transformer:

1. Apply 440V 3Phase AC Supply on HV Side.2. Short LV side & measure the following current:

R 3.34 A Y 3.48 A B 3.95 Ar 98 A y 95 A b 90 An 5.1 A

Impedance Test: Impedance is a measure of the

resistance that leads to the loss of electrical energy in atransformer at full load, causing the ratio of the input and outputvoltages to differ from the Turns Ratio. It can be measured at thesame time as load loss. Impedance is found by measuring thevoltage required to pass the rated current through one windingof the transformer, while the other winding is short-circuited.This voltage is called the impedance voltage.

Tan Delta Test: Tan Delta, also called Loss Angle or DissipationFactor testing, is a diagnostic method of testing cables to determinethe quality of the cable insulation. This is done to try to predict theremaining life expectancy and in order to prioritize cable replacementand/or injection. It is also useful for determining what other tests maybe worthwhile.

Working


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In a perfect capacitor, the voltage and current are phase shifted 90degrees and the current through the insulation is capacitive. If thereare impurities in the insulation, like those mentioned above, theresistance of the insulation decreases, resulting in an increase inresistive current through the insulation. It is no longer a perfect

capacitor. The current and voltage will no longer be shifted 90degrees. It will be something less than 90 degrees. The extent towhich the phase shift is less than 90 degrees is indicative of the levelof insulation contamination, hence quality/reliability. This Loss Angleis measured and analyzed. Below is a representation of a cable. Thetangent of the angle is measured. This will indicate the level ofresistance in the insulation. By measuring IR/IC (opposite overadjacent the tangent), we can determine the quality of the cableinsulation. In a perfect cable, the angle would be nearly zero. Anincreasing angle indicates an increase in the resistive current throughthe insulation, meaning contamination. The greater the angle, the

worse the cable.

Whether using partial discharge or tan delta techniques, the point ofthe test is to grade all cables tested on a scale from high quality tolow. The point in the testing is to help a utility prioritize cablereplacement or injection. Again, comparative testing will show whichcables are worse than others and will, over time, permit the user todevelop their own in-house guidelines, unique to their situation.

Tan Delta () = IR/ ICAlso for applied voltage(V)Tan Delta () =Watt-loss (Active) Power

Reactive Power


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Lightening Arrestor:A Lightening arrestor is defined as the device that limits the surgevoltage on equipment by discharging or bypassing surge current; itprevents the continuous flow of 60Hz follow current to ground & is

capable of repeating this function as specified. A Surge arrestor is avoltage sensitive device that operates as a principle of a nonlinearresistor. To normal system voltage, it has very high impedance, and tovoltage & current produced by lightening it has very low impedance.

Surge Monitor:For system voltages above approx. 100kV, surge Monitors/Countersare often installed in series with the lightening arrestor. The mainreason for the use of surge counter on modern gapless ZnO arrestors isto check if a particular transmission line or phase suffers fromexceptional high number of overvoltages leading to arrestor operation

lightening faults on a line. A sudden increase in the counting ratemay also indicate an internal arrestor fault.

Current Transformer:A Current transformer (CT) is used for measurement of electriccurrents. Current transformers, together with voltagetransformers (VT) (potential transformers (PT)), are knownas instrument transformers. When current in a circuit is too high todirectly apply to measuring instruments, a current transformer

produces a reduced current accurately proportional to the current inthe circuit, which can be conveniently connected to measuring andrecording instruments. A current transformer also isolates themeasuring instruments from what may be very high voltage in themonitored circuit. Current transformers are commonly used inmetering and protective relays in sub-station.

Care must be taken that the secondary of a currenttransformer is not disconnected from its load while current isflowing in the primary, as the transformer secondary willattempt to continue driving current across the effectively

infinite impedance. This will produce a high voltage across theopen secondary (into the range of several kilovolts in somecases), which may cause arcing.

The CURRENT ratio of a standard magnetic-core transformer, ignoringlosses, is defined by the equation:

I2 = (N1/N2) x I1


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Where I1 = Input current, I2 = Output currentN1 = Number of turns of primary coilN2 = Number of turns of secondary coil

Knee-point voltage

The knee-point voltage of a current transformer is the magnitude ofthe secondary voltage after which the output current ceases to followthe input current. This means that the one-to-one or proportionalrelationship between the input and output is no longer within ratedaccuracy. The output current increases abruptly even with smallincrement in the input, if the voltage across the secondary terminalsexceeds the knee-point voltage. The knee-point voltage is notapplicable for metering current transformers, the concept of knee pointvoltage is pertinent to protect current transformers only since they arenecessarily exposed to high currents during faults.

BurdenThe burden, in a CT metering circuit is the(largely resistive) impedance presented to its secondary winding.Typical burden ratings for IEC CTs are 1.5 VA, 3 VA, 5 VA, 10 VA, 15 VA,20 VA, 30 VA, 45 VA & 60 VA. Items that contribute to the burden of a

current measurement circuit are switch-blocks, meters andintermediate conductors. The most common source of excess burdenin a current measurement circuit is the conductor between the meterand the CT. Often, substation meters are located significant distancesfrom the meter cabinets and the excessive length of small gaugeconductor creates a large resistance.


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220kV C.T. Ratio (1200-800-400/1-1-1-1-1)

Cores Terminals Ratio RatingKpv/Ex Amp./Sec. Ohms

at 75oCPurpose

VA Class

CORE1

1S1-1S21S1-1S31S1-1S4

400/1800/11200/1

---

PS600v/30mA/345v/2.0

ohms1380v/30mA/690v/4.0

ohms2070v/30mA/1035v/6.0

ohms

Distance/ffe rentiaProtectio

CORE2

2S1-2S22S1-2S32S1-2S4

400/1800/11200/1

30--

5P20 OC & EFProtectio

CORE3

3S1-3S2

(Join A)3S1-3S3(Join A)3S1-3S4(Join A)

400/1

800/1

1200/1

20

-

-

0.5 Metering

CORE4

4S1-4S2 1200/1 - PS 720v/30mA/360v/6.0ohms

Bus BaProtectio

CORE5

5S1-5S2 1200/1 - PS 720v/30mA/360v/6.0ohms

BackupProtectio

Potential Transformer:

Potential Transformer works on the same principle as that of an idealtransformer.i.e. The secondary voltage is substantially proportional tothe primary voltage and differs in phase from it by an angle which isapproximately zero for an appropriate direction of the connections.When an alternating (AC) voltage is applied to the primary winding of apotential transformer, an alternating magnetic field is generated that issensed by the secondary coil. The secondary coil then generates an ACvoltage whose waveform is the same as the waveform of the primaryvoltage. The amplitude of the AC voltage generated by the secondarycoil depends on the ratio of primary to secondary turns, often knownas the turns ratio. It also depends on the core material, the driving

frequency and coupling.

The VOLTAGE ratio of a standard magnetic-core transformer, ignoringlosses, is defined by the equation:

E2 = (N2/N1) x E1Where E1 = Input Voltage, E2 = Output Voltage

N1 = Number of turns of primary coilN2 = Number of turns of secondary coil


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220kV P.T. RATING

Insulation Level (Kv) 460/1050Neutral EARTHED

Primary Voltage (V) 220,000/1.732

Secondary Winding-1(1a-1n)

VOLT 110/1.732VA 200

CLASS 3P


VOLT 110/1.732VA 200

CLASS 3P


VOLT 110/1.732VA 100

CLASS 0.5

Capacitive Voltage Transformer:Capacitive Voltage Transformers (CVTs) are common in high-voltage

transmission lineapplications. These same applications require fast, yet secureprotection. However, as the requirement for faster protective relaysgrows, so does the concern over the poor transient response of someCVTs for certain system conditions.Solid-state and microprocessor relays can respond to a CVT transientdue to their high operating speed and increased sensitivity .

Poor CVT transient response and the distance element overreach itcauses are a serious concern for high-speed line protection. For faultsthat cause very depressed phase voltages, the CVT output voltage may

not closely follow its input voltage due to the internal CVT energystorage elements. Because these elements take time to change theirstored energy, they introduce a transient to the CVT output following asignificant input voltage change.

General CVT Structure


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When a fault suddenly reduces the line voltage, the CVT secondaryoutput does notinstantaneously represent the primary voltage. This is because theenergy storage elements, such as coupling capacitors and thecompensating reactor, cannot instantaneously change their charge orflux. These energy storage elements cause the CVT transient. CVTtransients differ depending on the fault point-on-wave (POW) initiation.The CVT transients for faults occurring at voltage peaks and voltagezeros are quite distinctive and different. Also, notice that the CVToutput does not follow the ideal output until 1.75 cycles after fault

inception.


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Power line carrier communication (PLCC) :Power line carrier communication (PLCC) is mainly usedfor telecommunication, tele-protection and tele-monitoringbetween electrical substations through power lines at high voltages,such as 110 kV, 220 kV, 400 kV. PLCC integrates the transmission of

communication signal and 50/60 Hz power signal through the sameelectric power cable. The major benefit is the union of two importantapplications in a single system.In a PLCC system the communication is established through the powerline. The audio frequency is carried by a carrier frequency and therange of carrier frequency is from 50 kHz to 500 kHz. The modulationgenerally used in these system is amplitude modulation. The carrierfrequency range is allocated to include the audio signal, protection andthe pilot frequency. The pilot frequency is a signal in the audio rangethat is transmitted continuously for failure detection.The voice signal is converted/compressed into the 300 Hz to 4000 Hz

range, and this audio frequency is mixed with the carrier frequency.The carrier frequency is again filtered, amplified and transmitted. Thetransmission of these HF carrier frequencies will be in the range of 0 to+32db. This range is set according to the distance betweensubstations.

Line Traps:When the carrier signal is coupled to the power line it can propagate intwo directions, either to the remote line terminal or into the station busand onto other lines. If the signal goes into the station bus much of itsenergy will be shunted to ground by the bus

capacitance. Also some of this energy would propagate out on otherlines thus transmitting the signal to a large portion of the system. Thisis undesirable since the same frequency may be used on another line.Because of these problems, a device is needed to block the energyfrom going back into the bus and direct it toward the remote lineterminal. This device is called a line trap.

The general design of a line trap is that of a parallel LC circuit. Thistype of a circuit presents a high impedance to the carrier signal at itsresonant frequency. Thus if the parallel LC circuit were placed in serieswith the transmission line, between the bus and the coupling capacitor,

then the carrier signal would propagate toward the remote terminal.The line trap must be capable of providing a very low impedance pathto the power frequency current. The inductor in the trap provides thispath, and it isdesigned to carry the large currents required. Another importantfunction of the line trap is to isolate the carrier signal from changes inthe bus impedance, thus making the carrier circuit more independentof switching conditions.


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Line Traps are connected in series with HV transmission lines. The highimpedance limits attenuation of the carrier signal within the powersystem by preventing the carrier signal from being: dissipated in the substation

grounded in the event of a fault outside the carrier transmission path dissipated in a tap line or a branch of the main transmission path.

Interposing CT's (ICT's) :Interposing CT's are used when the ratio of transformation is very high.It is also used to correct for phase displacement for differentialprotection of transformers.To enable a comparison to be made, the differential scheme should bearranged so that the relay will see rated current when the full loadcurrent flows in the protected circuit.In order to achieve this, the line current transformers must be matched

to the normal full load current of the transformer. Where this is not thecase it is necessary to use an auxiliary interposing current transformerto provide amplitude correction. The connection of the line CTs shouldcompensate for any phase shift arising across the transformer.Alternatively the necessary phase correction may also be provided bythe use of an interposing CT.

In all above applications it must be borne in mind that an I.C.T. imposes burden on the main C.t. if this burden is required tobe restricted to a certain value, the value must be specified toenable the manufacturer to design the I.C.T. suitably. If this

value of burden (expressed in VA) is too low, it may result intoan uneconomical design of the I.C.T.

Equipment Earthing:The function of an earthing and bonding system is to provide anearthing system connection to which transformer neutrals or earthingimpedances may be connected in order to pass the maximum faultcurrent. The earthing system also ensures that no thermal ormechanical damage occurs on the equipment within the substation,thereby resulting in safety to operation and maintenance personnel.

The earthing system also guarantees eqipotential bonding such thatthere are no dangerous potential gradients developed in thesubstation.

In designing the substation, three voltage have to be considered.1. Touch Voltage:This is the difference in potential between the

surface potential and the potential at an earthed equipmentwhilst a man is standing and touching the earthed structure.


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2. Step Voltage:This is the potential difference developed when aman bridges a distance of 1m with his feet while not touchingany other earthed equipment.

3. Mesh Voltage:This is the maximum touch voltage that isdeveloped in the mesh of the earthing grid.

Earthing Requirement:1. Conductors: Bare copper conductor is usually used for the

substation earthing grid. The copper bars themselves usuallyhave a cross-sectional area of 95 square millimeters, and theyare laid at a shallow depth of 0.25-0.5m, in 3-7m squares. Inaddition to the buried potential earth grid, a separate aboveground earthing ring is usually provided, to which all metallicsubstation plant is bonded.

2. Connections: Connections to the grid and other earthing jointsshould not be soldered because the heat generated during fault

conditions could cause a soldered joint to fail. Joints are usuallybolted, and in this case, the face of the joints should be tinned.

3. Earthing Rods:The earthing grid must be supplemented byearthing rods to assist in the dissipation of earth fault currentsand further reduce the overall substation earthing resistance.These rods are usually made of solid copper, or copper cladsteel.

4. Switchyard Fence Earthing:The switchyard fence earthingpractices are possible and are used by different utilities. Theseare:

Extend the substation earth grid 0.5m-1.5m beyond the

fence perimeter. The fence is then bonded to the grid atregular intervals.

Place the fence beyond the perimeter of the switchyardearthing grid and bond the fence to its own earthing rodsystem. This earthing rod system is not coupled to themain substation Earthing grid.

Circuit Breakers:A circuit breaker is an automaticallyoperated electrical switch designed to protect an electrical circuit fromdamage caused by overload or short. Its basic function is to detect afault condition and, by interrupting continuity, to immediatelydiscontinue electrical flow. Unlike a fuse, which operates once and thenhas to be replaced, a circuit breaker can be reset (either manually orautomatically) to resume normal operation. Circuit breakers are made


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in varying sizes, from small devices that protect an individualhousehold appliance up to large switchgear designed to protect highvoltage circuits feeding an entire city.

Operation:

The circuit breaker must detect a fault condition; in low-voltage circuitbreakers this is usually done within the breaker enclosure. Circuitbreakers for large currents or high voltages are usually arrangedwith pilot devices to sense a fault current and to operate the tripopening mechanism. The trip solenoid that releases the latch is usuallyenergized by a separate battery, although some high-voltage circuitbreakers are self-contained with current transformers, protectionrelays, and an internal control power source.

Once a fault is detected, contacts within the circuit breaker must opento interrupt the circuit; some mechanically-stored energy (using

something such as springs or compressed air) contained within thebreaker is used to separate the contacts, although some of the energyrequired may be obtained from the fault current itself. Small circuitbreakers may be manually operated; larger units have solenoids to tripthe mechanism, and electric motors to restore energy to the springs.

When a current is being interrupted, an arc is generated. This arc mustbe contained, cooled, and extinguished in a controlled way, so that thegap between the contacts can again withstand the voltage in thecircuit. Different circuit breakers use vacuum, air, insulating gas,or oil as the medium in which the arc forms.

Arc Interpretation:Miniature low-voltage circuit breakers use air alone to extinguish thearc. Larger ratings will have metal plates or non-metallic arc chutes todivide and cool the arc. Magnetic blowout coils or permanentmagnets deflect the arc into the arc chute.In larger ratings, oil circuit breakers rely upon vaporization of some ofthe oil to blast a jet of oil through the arc.

Gas (usually SF6) circuit breakers sometimes stretch the arc using amagnetic field, and then rely upon the dielectric strength of the sulfur

hexafluoride (SF6) to quench the stretched arc.

Circuit Breakers are classified according to operating mechanism & arcquenching Media.Classification :

1. By mechanism

Manual Closing Trip Switches


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Spring Charge Helical Spring Charged or Leaf SpringCharged.

Hydraulic Pressure

Pneumatic Operated.

2. Arc Quench Media: Bulk Oil Circuit Breaker (B.O.C.B.)

Minimum Oil Circuit Breaker (M.O.C.B.)

Air Circuit Breaker (A.C.B.)

Air Blast Circuit Breaker (A.B.C.B.)

SF6 Circuit Breaker

Vacuum Circuit Breaker (V.C.B.)Relays:Continuous & Short Time ratings

All relays carry current- and/or voltage-coil ratings as a guide to theirproper application.For relays complying with present standards, the continuous ratingspecifies what a relay will withstand under continuous operation in anambient temperature of 40oC. Relays having current coils also carry a1-second current rating, since such relays are usually subjected tomomentary Overcurrents. Such relays should not be subjected tocurrents in excess of the 1-second rating without the manufacturersapproval because either thermal or mechanical damage may result.Overcurrents lower than the 1 second-rating value are permissible forlonger than 1 second, so long as the I2t value of the 1-second rating is

not exceeded.It is not always safe to assume that a relay will withstand anycurrent that it can get from current transformers for as long asit takes a circuit breaker to interrupt a short circuit after therelay has operated to trip the circuit breaker. Also, should arelay fail to succeed in tripping a circuit breaker, thermaldamage should be expected unless back-up relays can stop theflow of short-circuit current soon enough to prevent suchdamage.

Contact Ratings

Protective-relay contacts are rated on their ability to close and to openinductive or non-inductive circuits at specified magnitudes of circuit current and ac ordc circuit voltage.protective relays that trip circuit breakers are not permitted tointerrupt the flow of trip-coil current, and hence they require only acircuit-closing and momentary current carrying rating. If a breaker failsto trip, the contacts of the relay will almost certainly be damaged. The


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circuit-opening rating is applicable only when a protective relaycontrols the operation of another relay, such as a timing relay or anauxiliary relay; in such a case, the protective relay should not have aholding coil or else it may not be able to open its contacts once theyhave closed.

BURDENS:The impedance of relay-actuating coils must be known to permit oneto determine if therelays voltage- or current-transformer sources will have sufficientcapacity and suitableaccuracy to supply the relay load together with any other loads thatmay be imposed on the transformers. These relay impedances arelisted in relay publications. This subject will be treated further when weexamine the characteristics of voltage and current transformers.

TIME CHARACTERISTICS:

A typical time curve for a high-speed relay is shown in Fig. It will benoted that this is aninverse curve, but that a 3-cycle (60-cycle-per-second basis) operatingtime is achieved only slightly above the pickup value, which permitsthe relay to be called High speed(HS).

Fig.: Time Characteristics of High Speed

One should not rely on the operation of any relay when the magnitudeof the actuatingquantity is only slightly above pickup, because the net actuating forceis so low that anyadditional friction may prevent operation, or may increase theoperating time. Eventhough the relay closes its contacts, the contact pressure may be solow that contamination of the contact surface may prevent electrical


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contact. This is particularly true in inverse-time relays where there maynot be much impact when the contacts close.

Reset Time:For accurate data, the manufacturer should be consulted. The reset

time will vary directly with the time-dial adjustment. The method ofanalysis described under TimeCharacteristics for estimating the amount of disc travel during shorttime intervals,combined with the knowledge of reset time, will enable one toestimate the operation of inverse-time relays during successiveapplication and removal of the actuating quantity, as when a motor isPlugged or when a circuit is tripped and then automatically reclosedon a fault several times, or during power surges accompanying loss ofsynchronism.

Differential Relay:Differential relays take a variety of forms, depending on the equipmentthey protect. The definition of such a relay is one that operates whenthe vector difference of two or more similar electrical quantitiesexceeds a predetermined amount.Most differential-relay applications are of the current-differential type.

Fig.: differential Relay ApplicationThe dashed portion of the circuit of Fig. represents the system elementthat is protected by the differential relay. This system element mightbe a length of circuit, a winding of a generator, a portion of a bus, etc.A current transformer (CT) is shown in each connection to the systemelement. The secondaries of the CTs are interconnected, and the coilof an overcurrent relay is connected across the CT secondary circuit.

Now, suppose that current flows through the primary circuit either to a

load or to a short circuit located at X. The conditions will be as in Fig. Ifthe two current transformers have the same ratio, and are properlyconnected, their secondary currents will merely circulate between thetwo CTs as shown by the arrows, and no current will flow through thedifferential relay.


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But, should a short circuit develop anywhere between the two CTs, theconditions ofFig. will then exist. If current flows to the short circuit from both sidesas shown, thesum of the CT secondary currents will flow through the differentialrelay. It is not necessary that short-circuit current flow to the fault fromboth sides to cause secondary current to flow through the differentialrelay. A flow on one side only, or even some current flowing out of oneside while a larger current enters the other side, will cause adifferential current. In other words, the differential-relay current will beproportional to the vector difference between the currents enteringand leaving the protected circuit; and, if the differential currentexceeds the relays pickup value, the relay will operate.

Restricted earth fault relays:These relays are differential relays connected to provide sensitiveprotection for equipment against ground faults. The most commonusage is for the protection of delta-wye transformers with a resistancegrounded neutral.One CT in the transformer neutral is balanced against the neutral ofthree phase CTs in the transformer output connection (or bushing CTsin the transformer circuit breaker) and connected in parallel. The REFrelay is connected across the two CT leads - this relay is a highimpedance type, operating on fault voltage developed across the CTleads for an internal fault. For an external earth fault, the zerosequence current in both sets of CTs is balanced and there is no faultvoltage developed on the CT leads.The REF scheme is a high speed differential protection, withinstantaneous operation for internal faults and high stability

(depending on setting) for external faults.

Over fluxing Relay:Over fluxing is a dangerous situation in which the magnetic fluxdensity increases to extremely high levels. The high flux density caninduce excessive eddy currents in the windings and in otherconductive parts inside the transformers. The heat generated by these


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eddy currents can damage the windings and the insulation. The highflux density also causes magnetostriction inside the transformer coreand produces noise.

The powerful magnetostrictive forces can also cause damage. Thewinding temperatures may also increase due to the heat produced.

The magnetic flux density is dependent on the current flowing throughthe primary windings in a transformer. This current is dependent onthe voltage applied across the windings and the winding impedance.The impedance is dependent on the frequency of the applied voltage.Over fluxing can be prevented by the use of a Over fluxing relay. Anover fluxing is an adaptation of an over voltage relay. The PT voltage isconnected across a resistor and a capacitor in series. The voltagesensing relay is connected across the capacitor. The relay operates inthe event of an over fluxing and isolates the transformer.

IDMT Relays:An inverse time relay is one in which the operating time is

approximately inversely proportional to the magnitude of the actuatingquantity. Fig. show the time current characteristics of an inversecurrent relay. At values of current less than pickup, the relay neveroperates. At higher values, the time of operation of the relay decreasessteadily with the increase of current. The inverse-time delay can beachieved by associating mechanical accessories with relays.
http://3.bp.blogspot.com/_jbHyppMByw8/Srt7DOV9GHI/AAAAAAAAD0M/ExowzILTCzU/s1600-h/overfluxing.JPG


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The inverse-time characteristic is obtained by connecting a time-limitfuse in parallel with the trip coil terminals. The shunt path formed bytime-limit fuse is of negligible impedance as compared with therelatively high impedance of the trip coil. Therefore, so long as the fuseremains intact, it will divert practically the whole secondary current ofthe CT from the trip Coil. When the secondary current exceeds thecurrent carrying capacity of the fuse will blow and the whole currentwill pass through the trip coil, thus opening the circuit breaker. Thetime lag between the incidence of excess current and the tripping ofthe breaker is governed by the characteristics of the fuse. Careful

selection of fuse can give the desired inverse-time characteristics.

Current Setting:It is often desirable to adjust the pickup current to any value. This isknow as current setting and is usually achieved by the use of tappingon the relay coil. The taps are brought out to a plug bridge, whichpermits to alter the number of turns on the relay coil, this changes thetorque on the disc and the hence the time of operation of the relay.

Pickup current = Rated secondary current of CT x Current setting

Plug setting multiplayer (P.S.M):

It is the ratio of fault current in the relay coil to the pick up current.

P.S.M = Fault current in relay coil


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pick up current

= fault current in relay coilrated secondary current of CT x current setting

Time- Setting Multiplier(T.M.S):A relay is generally provided with control to adjust the time ofoperation. This adjustment is known as time setting multiplier the timesetting dial is calibrated from 0 to 1 in steps 0.05. These figures aremultipliers to be used to convert the time derived from time/p.s.mcurve into the actual operating time. The actual time of operation iscalculated by multiplying the time setting multiplier with the timeobtained from time/ p.s.m curve of the relay.

NUMERICAL RELAYS:

MIT 114:MIT has three pole version with two phase & one earth elementand four pole version with 3 phase and one earth element.MIT 114-3 O/C + 1E/F with Highset.

The MIT 103/104/113/114 Protection unit consists of the followingmodules within its compact dimensions. Input Module Power Supply and Output Relay module Measuring Module Front Fascia

The three modules viz. Input, Power supply and Measuring modules areplugged into the Front fascia which houses switches, LEDs and LEDdisplay for the human machine interface. The relay has 3 or 4 inputcurrent transformers. The output from the currenttransformer is transformed to an equivalent voltage and sampled atthe rate of 16 samples per cycle and digitized by means of analog todigital converter. The basic relay has output relay with 2 N/O and1 N/C contacts asstandard:Trip 1 N/OAlarm 1 N/O

Protection unhealthy 1 N/C Additional 5 output contacts can be given as follows:Starter 1 C/OIDMTL Phase fault 1 N/OIDMTL Earth fault 1 N/OHighset Phase fault 1 N/OHighset Earth fault 1 N/O


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CT Input Rating:1A/5AFrequency: 50HzAuxiliary supply 48/110/220V DC/110V AC or 24/30/48/110V DCSettings:

Phase fault 5% to 250% insteps of 1%Earth fault 5% to 250% insteps of 1%Highset for Phase fault 50% to 3000% insteps of 50%, OFFHighset for Earth fault 50% to 3000% insteps of 50%, OFFTime multiplier for Phase fault 0.025 to 1.0 insteps of 0.001Time multiplier for Earth fault 0.025 to 1.0 insteps of 0.001Reset delay 0 to 60sec insteps of 1sec

Inverse Characteristics:Operating time can be calculated as follows:

T= K xTm

[I/Is]-1

where I=fault current, Is=current setting, Tm=time multiplier,SI3 - k = 0.14, = 0.02SI1 - k = 0.0613 = 0.02VI - k = 13.5, = 1.0EI - k = 80.0, = 2.0LTI - k = 120.0, = 1.0Definite Time relayFor DTL t=0 to 20sec insteps of 0.01sec

Burden:AC Current Input (Phase/Earth):5 A Rating 0.4 VA1 A Rating Auxiliary input 0.05 VAQuiescent (typical)5 W (DC)12VA (AC)


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MIB 202 (DIFFERENTIAL RELAY):The MIB202 is micro-controller based Numerical Biased Differential

Protection Relay with inbuilt Current Amplitude and Vector GroupCompensation features and also with Instantaneous DifferentialHighset Element for two winding Power Transformer andAutotransformers. MIB202 relay, which can be used to operate forinternal faults, likephase to phase, phase to earth and inter turn faults in theTransformers. The same relay, we can use for 1A or 5A CT input onboth LV & HV side.


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Working:The currents entering and leaving the transformer are measured,taking in to the Power Transformer vector grouping and

transformation ratio. Software interposing current transformers can beapplied to each set of current inputs to correct for any magnitude andvector mismatch and to remove zero sequence components wherenecessary. They are then summed to form an operate signal which isapplied to a three part biased differential characteristic on a phase byphase basis.The relay is provided with triple slope characteristics.1. Initial Differential setting2. Differential Bias slope3. Differential Bias slope limit

INITIAL DIFFERENTIAL SETTING:This is the value of current, expressed as a percentage of the chosencurrent rating, at which the relay will operate with zero bias current. Itssetting would normally be the same as that for the differential biasslopevalue.Setting Range:I - 10% to 50% of In in steps of 5%.

DIFFERENTIAL BIAS SLOPE:

Some unbalance current will appear in the differential circuit of therelayfor predictable reasons, e.g. due to the transformer tap position and toCT errors. The current will increase with increasing load or throughfaultcurrent in the transformer so, to maintain stability, the biasing currentmust increase proportionately. The bias slope expresses the current tooperate the relay as a percentage of the biasing (restraint) current.Thedifferential bias slope setting chosen must be greater than themaximum predictable percentage unbalance.

Setting Range:bS - 10% to 70% of In in steps of 5%5x

DIFFERENTIAL BIAS SLOPE LIMIT:


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This setting defines the upper limit of the bias slope and is expressedinmultiples of nominal rated current. A setting value must be chosenwhich will cover the maximum through fault current of thetransformer.

This setting gives more stability during CT saturation for heavythroughfault.Setting Range:SL - 200% to 2000% of In in steps of 100%

INTERPOSING CT MULTIPLIER (HV AND LV SIDE):This range of settings enable the effective ratio of the HV & LV CT's tobe adjusted.Setting Range:Ah & Al - 0.50 to 2.50 in steps of 0.01

HV INTERPOSING CT CONNECTION:An equivalent interposing CT connection can be selected from thisrangeof settings. The settings define the LV and HV winding configuration.E.g. Yd, followed by the angular position of the LV phasor with respectto the HV phasor. The angular position is described by the hour - handposition on the twelve-hour clock face, e.g. Yd1 or Yd11. In eachsetting, this is followed by the same angular relationship expressed indegrees. The complete Yd1 setting will therefore read Yd1, -30 and

Yd11 will read Yd11, 30.Setting Range:Vh - Yy0, Yy2, Yy4, Yy6, Yy8, Yy10, Yd1, Yd3, Yd5, Yd7, Yd9, Yd11,Ydy0 and Ydy6

LV INTERPOSING CT CONNECTION:As the HV connection but now applied to the LV CT's.Setting Range:VL - Yy0, Yy2, Yy4, Yy6, Yy8, Yy10, Yd1, Yd3, Yd5, Yd7,Yd9, Yd11, Ydy0and Ydy6.


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TYPICAL RELAY SETTING CALCULATION:Power Transformer Details:

Voltage = 132 / 33KV

Rating = 60MVATap Changer = +5% - 15%Vector Group = Yd1

Current Transformer Details:CT RatioFor HV Side = 300/1For LV Side = 1200/1

Calculation:HV rated current = 60MVA / (132 * 1.732)= 262.4A.

CT ratio for HV side is 300/1LV rated current = 60MVA / (33* 1.732)= 1049.76ACT ratio for LV side is 1200/1Mean tap value = [(+5) + (-15)] / 2 = -5%HV current at 5% tap = (60MVA) / (1.732 * 132KV *0.95)= 276.2AHV Multiplier = 300 / 276.2 = 1.086 = 1.09LV CT secondary current = 1049.7 / 1200 = 0.87475ASo the LV multiplier = 1200 / 1049.7 = 1.143 = 1.14

Initial Setting = 200mA (20%) or 2 times of maximum spill currentwhichever is greater.

Bias setting = 20% 2 times of maximum tap change %Bias Slope Limit = 4 times of full load currentHV ICT vector connection = Yd1, 30LV ICT vector connection = Yy0, 0HV ICT multiplier = 1.09LV ICT multiplier = 1.14

Output relays


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1.Trip 1 N/O contacts (13 & 14, 15 & 16).This contact to be usedwhiles testing the Bias characteristic and Highset of the relay.2. Biased Differential - 1 N/O Contact (17 & 19) . This contact to beused whiles testing the Bias characteristic and of the relay.3. Differential Highset 1 N/O contact (17 & 20). This contact to be

used while testing the highset characteristic.4. Protection Unhealthy - 1 N/C contact (17 & 18).


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Energy Conservation:Ways to reduce loss of energy in transmission lines:

1. Feeder Balancing:The procedure must consider all possible combinations of phase

load changes at each 3-phase connection point for either single-phase spurs or loads. Consideration must be given to the order inwhich loads are considered so as not to exclude the bestcombinations of load phase connections when all selections havebeen made. The best set of connections will minimize theimbalance as far as possible for each 3-phase section of feederbetween spur/load connections.


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Balancing reduces feeder losses because any phase peakreduction affects the losses for the phases as the square of thecurrent magnitude. A feeder section with 1-ohm resistance thathas phase currents of 50A/100A/150A will have 35kW in losses.

When balanced at 100A/100A/100A, the loss reduces to 30kW.The same effect is even more evident in the reduction of reactivepower losses because the X/R ratio of most feeder sections isgreater than 1.Balancing improves voltage on a feeder by equalizing the voltagedrops in each phase along the feeder. Released feeder capacityprovides more reserve loading capacity for emergency loadingconditions. It is realistic to assume that the benefits in improveduse of feeder capacity and improved voltage quality are of moresignificance than the value of loss reduction except when loadingis already high.

2. Phase Balancing:Phase imbalance is calculated from line flows resulting from thePower Flow application. Phase balancing calculations can bebased on either power (kW) or complex power magnitude (kVA).

Phase balancing will run for multiple circuits.

The imbalance will not be calculated at the substation, butat each start of circuit component.

Therefore, the resulting imbalance at the substation maynot improve and could possibly be worse.

The Phase Balancing application recommends phase movementsof single and two-phase laterals for all user selected circuits,making use of all time points selected for analysis.

Typically, Phase Balancing should be run at the time ofpeak loading.

For multiple time point selections, analysis uses theaverage load over selected time points.

The lateral movements are to improve the balance at thethree-phase, grounded start of circuit component.

If the start of circuit component is ungrounded, i.e. deltaconnected, then phase balancing will improve the balance at allof the three-phase power transformers closest to substationsgrounded on the secondary side.

3. Energy Efficient Transformers:Most energy loss in dry-type transformers occurs through heat orvibration from the core. The new high-efficiency transformers


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minimize these losses. The conventional transformer is made upof a silicon alloyed iron (grain oriented) core. The iron loss of anytransformer depends on Energy Efficient Technologies inElectrical System Bureau of Energy Efficiency 187 Fluid Couplingthe type of core used in the transformer. However the latest

technology is to use amorphous material - a metallic glass alloyfor the core. The expected reduction in energy loss overconventional (Si Fe core) transformers is roughly around 70%,which is quite significant. By using an amorphous core- withunique physical and magnetic properties- these new type oftransformers have increased efficiencies even at low loads 98.5% efficiency at 35% load. Electrical distribution transformersmade with amorphous metal cores provide excellent opportunityto conserve energy right from the installation. Though thesetransformers are a little costlier than conventional iron coretransformers, the overall benefit towards energy savings will

compensate for the higher initial investment. At presentamorphous metal core transformers are available up to 1600kVA.

4. Energy Efficient Lighting:

Using CFL Lamps

LED Lamps

Calculated Illumination in specific areas

Control of Lighting:1. Manual Control of Switches.

2. Remote Switching.3. Proximity Switches.4. Photoelectric Sensors.5. Timer Switches.6. Computerized Switching.7. Door Switches.

Use of 3 phase load instead of 1 phase load for purpose ofexhaust fans, welding, tube well, pumps, etc.

Mechanical Balancing of Electricity driven equipments.

Use of Energy Efficient Lubricants.

Comparison of specifications incl. energy loss of rewindedmotor with standard motor.

Use of 1 phase preventer.

ANURAG RPORT

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