NEPCO National Electrical Power Company ETC تم رفعه بواسطة صفحة : الهندسه...

National Electrical Power Company

بواسطة رفعه تمعلم : ” الكهربيه الهندسه صفحة

وفن“ / حمادى : محمد عالء م أدمن

INSTRUMENT TRANSFORMERS

An instrument transformer is a device used to reduce the current or voltage values into values can be handled by other equipments.

Is a device to transform the power system current and voltage to lower magnitudes, and provide an isolation between the power network and the relay and other instruments connected to the transformer secondary winding

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-Reduce the voltage or current into values that other equipment can handle it.

-Isolate the equipments from the power network(i.e. we need more insulation)

-To provide possibilities of a standardization of instruments and relays to a few rated currents and voltages.

Instrument Transformers Why is it Needed?

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Classification of Instrument Transformers

-OIL IMMERSED

-CAST RESIN

-Epoxy

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Types Of Instrument Transformers 1-Current transformers

2-Voltage transformers

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CURRENT TRANSFORMER

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CURRENT TRANSFORMER

A current transformer is used to transform a primary current quantity in terms of its magnitude and phase to a secondary value such that in normal conditions the secondary value is substantially proportional to the primary value.

IEC 60044

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Construction of Current Transformer

A current transformer is a transformer has its primaryprimary winding is connected in series with the power circuit with thick windings and few turns , usually one turn. and the secondary with thin windings with many turns. Equation of current transformer:

Ip/Is = Ns/Np

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CT Equivalent Circuit

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Referred to secondary

Wound primary type

Types Of Current Transformers

The wound type has a separate primary and secondary winding mounted on a laminated iron core. It is used for auxiliary current transformers and for many low or moderate ratio current transformers used in switchgear of up to 11kV Rating.

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The wound primary is used for the smaller currents, but it can only be applied on low fault level installations due to thermal limitations as well as structural requirements due to high magnetic forces.

This type of current transformer is designed so that the primary winding consists of one or more turns of heavy wire connected in series in the circuit The secondary winding consists of a larger number of turns of relatively smaller wires.And is connected to instruments or control devices

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Bushing type:

would be mounted in a transformer or circuit breakerFor currents greater than 100 A,

This consists of a cylindrical ring core built up of thin iron

laminations. Around the core is wound copper wire which forms

the secondary winding. The primary winding is formed by the

bushing conductor

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Window current transformer

If we enclose the current transformer in the bushing type In a molded case, we obtain another type of current transformer called the window type

Through the core and secondary winding, there is an insulated hole through which the user can place his own conductor.

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The rating of a CT has its own standards. the rating is 5 A , 2 A and 1 A

Current Transformer Rating

IEC 60044-1

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The current transformer is designed to give the secondary a current that is proportional to the primary current

Examples: CTR = 200/5 = 40CTR = 1200/5 = 240CTR = 400/1 = 400

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CT errors results from the excitation current, so in order to check the ct function its essential to measure the excitation curve,

The magnetizing current of a CT depends on the cross section and the length of the magnetic circuit, the number of turns and the magnetic circuit material

This curve is the best method of determining a CT’s performance. It is a graph of the amount of magnetizing current required to generate an open-circuit voltage at the terminals of the unit.

CT Characteristics

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Current Transformer Knee-point

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If the load and the magnitude of the primary current are large enough to saturate the CT, neither the flux nor the induced voltage are sinusoidal. The excitation current can take large magnitudes and the resulting error will be large.

The secondary current is no longer proportional to the primary current. The current error which corresponds to the magnetization current increases significantly

CT response in saturated state

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CT Secondary Current transformers generally work at a low flux density. Core is then made of very good metal to give small magnetizing current the e.m.f, induced in the secondary winding isthat required to drive the secondary current through the total impedance of the secondary circuit,

And that the core flux inducing this e.m.f, on open-circuit, secondary impedance now becomes infinite and the core saturates. This induces a very high voltage in the primary up to approximately system volts and the corresponding volts in the secondary will depend on the number of turns, multiplying up by the ratio (i.e. volts/turn × no. of turns).

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Or we can say.With CT’s the increase of CT secondary burden will also increase the secondary voltage. This is maintain the correct magnitude of secondary current.

As the burden decreased will result in decreasing the secondary voltage as well.This voltage increase will increase the secondary burden, supposing that we have infinite secondary burden, theoretically the secondary voltage is infinite, but for practical reasons, the voltage will be very high. In terms of hundreds of kilovolts. This voltage will cause to break down the insulation between secondary and primary voltage, or between the secondary coils, so the secondary of CT should never be opened.

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Since CT normally has much more turns in secondary compared to the primary, the voltage generated on the open-circuited CT will be much more than the system volts, leading to flashovers.

this peak value may be as low as a few hundred volts in a small measuring c.t. with a 5A secondary winding, but it might reach many kilovolts in the case of, say a 2000/1A protective c.t. with a large core section.

open circuit voltage at CT is almost:V=(3.5*Zb*Ip/Ns)^.5

THEY CAN KILL YOU!

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WYE CONNECTION:

Current Transformer Connection

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DELTA CONNECTION:

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Current Transformer Grounding

The grounding of CTs is important to both safety and the correct operation of protective relays.

To assure safe and reliable operation, the neutral of the CT secondary should have a single ground location for each circuit.

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The grounding of CTs is important to both safety and the correct operation of protective relays. To assure safe and reliable operation, the neutral of the CT secondary should have a single ground location for each circuit.

The recommended method of grounding is to install a single ground point at the first point of application (switchboard or relay panel) of the CT secondary circuit. The grounding is done only at one point. Multi point may introduce some problems…….?

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the grounding is done at the instrument transformer location at

the site or at the first panel application. Grounding the ct’s at the

site location may cause a problems:

Another grounding may be done at the panel without knowing that there

is a grounding at the location

The grounding is done mainly at the common point,

*NEPCO grounding standard :

The CTs are grounded at the place where the metering or relaying

*other standard. GE, ABB ….. ETC

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The unused CT secondaries:

The unused secondary of the current transformers as there may be multi secondaries , and no all the secondaries are used, these un used secondaries must be grounding. The grounding practice as we mentioned later at the instrument transformers location or at the first point of application.

In case of multi ratio secondaries:One of these connection are applicable

Sometimes shorting terminals are available at the transformer secondaies.

CT Standards

There are primarily two standards that are used to specify the performance of CTs for protection applications.

ANSI and the IEC

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DIFFERENCES BETWEEN THE ANSI & THE IEC

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A Ct can be separated into 2 distinct groups

1-Metering Type

2-Protection Type

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Current Transformers Specifications:

Rated primary currents:10 - 12.5 - 15 - 20 - 25 - 30 - 40 - 50 - 60 – 75 and their multiple factors

Accuracy power: this also called the “BURDEN” and it’s the maximum load can be connected to the CT secondary for the rated secondary current.Standard values: (1 - 2.5 - 5 - 10 - 15 -30) VA.

Accuracy class: this defines the guaranteed transformation ratio and phase displacement error limits under specific power and current conditions.

National Electric Power Company

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Metering current transformers according to the IEC -60044-1 the metering Ct must have the accuracy for the rated current.

The standardized IEC accuracy classes are: 0.1 - 0.2 - 0.5 - 1 - 3 - 5.Classes 0.5 and 1 are used in the majority of cases.Class 0.2 is only used for precision metering.Classes 0.1 - 3 - 5 are never used in medium voltage.

Metering symbols :B, Fs or sometimes there is no symbol

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Metering CTs In general, the following appliesCLASS:

• 0.1 or 0.2 for precision measurements

• 0.5 for high grade kilowatt hour meters for commercial grade kilowatt hour meters

• 3 for general industrial measurements

• 3 or 5 for approximate measurements

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Accuracy limit factor:ALF represents the value of the primary current at which the accuracy class still presents.

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EXAMPLE:500/1 A 15 VA cl 0.5

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Burden

Is the amount of the impedance connected to the ct secondary and rated in ohms or VA. The burden can be expressed in two ways. The burden can be expressed as the total impedance in ohms of the circuit or the total volt-amperes and power factor at a specified value of current or voltage and frequency. Ex: the burden of a CT is 25 VA, with 5A secondary.So the burden in ohms :25= VI = I*R *I = I²R25=5*5*RR= burden in ohms = 1 Ohm

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BURDEN (depending on pilot lead length)

• Moving iron ammeter 1-2VA

• Moving coil rectifier ammeter 1-2.5VA

• Electrodynamics instrument 2.5-5VA

• Maximum demand ammeter 3-6VA

• Recording ammeter or transducer 1-2.5VA

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CT’s for Protection:

Protection symbols V,H,L T and P1.IEC 60044-1 Accuracy is defined by the accuracy class.The IEC accuracy classes are 5P and 10PThe IEC ALF values are: 5 - 10 - 15 - 20 - 30.

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Example:100/1 A 15 VA 5P10

Protection CT’s In general the following applies:

• Instantaneous over current relays & trip coils - 2.5VA Class 10P5• Thermal inverse time relays - 7.5VA Class 10P10• Low consumption Relay - 2.5VA Class 10P10• Inverse definite min. time relays (IDMT) over current - 15VA Class 10P10/15• IDMT Earth fault relays with approximate time grading - 15VA Class 10P10• IDMT Earth fault relays with phase fault stability or accurate time grading required - 15VA Class 5P10

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BS 3938 (class X)BS 3938 specifically defines current transformers designed for protection under the heading class X.

This defines the rated knee-point voltage VK . . This voltage, when applied to the terminals of the secondary increased by 10%, causes a maximum increase in magnetizing current of 50%.

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Class PX: Class X CTs are special CTs used mainly in balanced protection systems (including restricted earth fault) where the system is sensitively dependenton CT accuracy. Further to the general CT specifications, the manufacturer needs to know:

• Vkp - Voltage knee point• Io - Maximum magnetizing current at Vkp• Rs - Maximum resistance of the secondary winding

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The "X" indicates that they do not belong to any standardized class. They are customized transformers therefore it is necessary to specify the individual characteristics of the current transformers such as the turns ratio, rated primary and secondary current, Voltage knee point, the maximum magnetizing current at the knee point and the Maximum resistance of the secondary winding.

Class X CTs are generally used where high knee points are require to prevent operation at higher currents without saturation.

Class X CTs are further divided into Class A and Class B CTs.

Class A CTs are more expensive and are designed to operate even at maximum fault current without saturating. Class B CTs cost less and are used in high-impedance applications. They tend to saturate during transient conditions.

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ERRORS IN CT

1-Ratio errorrepresent the difference between the rated ratio and the measured ratio

%E=(actual turns ratio-rated turns ratio)/rated turns ratio *100%

2-Phase error:represents the phase difference between the primary and the secondary

currents

(delta= θ (Ip)- θ (Is

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Primary P1 P2

Secondary:

* single core secondary S1 S2

* single core multi ratio secondary S1-S2,S3

* multi core secondary 1S1, 1S1-2S2,2S2 ......

CT Terminal Marking

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According to IEC 60044-1National Electrical Power Company

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CT Symbols

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CT'S USED IN NEPCO

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-800/400/1 A for distance protection and fault location equipments

-800/400/1 A for directional overcurrent, earth fault, sensitive earth fault protection and fault recorder

-800/400/1A for Busbar zone protection

For 132 KV Line Circuit

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For 132 KV Transformer Circuit HV Side

-100/1A for restricted earth fault protection

-100/1A overcurrrent protection

-1200/600/1A for busbar zone protection

-100/1A transformer differential protection

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For 33KV Feeder Circuit

-400/200/1A for over current , earth fault and sensitive earth fault protection

-400/200/1 for metering

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For 132KV Transformer Circuit MV Side

-800/400/1A for directional overcurrent and earth fault protection

-800/400/1A for transformer differential protection and restricted earth fault protection

-400/200/1A for metering

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CT’s Used In EDCO

Current Transformer Nameplate

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1-polarity test:

Each transformer should be tested in order to verify the polarity of the current transformer, we perform this test to check the polarity of a CT.

such that we connect a dc battery to the primary "almost 12 Vdc". and we connect an ammeter to the secondary winding.

a low voltage battery is used to energize the primary winding, on closing the push button the dc ammeter should give a positive flick, and at open a negative flick,

Current Transformer Tests

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According to BS 3938 states that at the instant when current is flowing from P1 to P2 in primary, then current, in secondary must flow from S1 to S2 through the external circuit

Connect battery –ve terminal to the current transformer P2 primary terminal.

This arrangement will cause current to flow from P1 to P2 when +ve terminal is connected to P1 until the primary is saturated. If the polarities are correct, a momentary current will flow from S1 to S2.

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Connection Diagram Of Polarity Test

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2-magnetizing current curve.

Several points should be tested on each current transformer magnetization curve. this can be done energizing the secondary winding from local main supply through a variable autotransformer, while the primary circuit remains open.

The magnetizing current is measured on the ammeter, an the secondary voltage on the voltmeter. the applied voltage should be raised slowly till the magnetizing current is seen to rise very rapidly for a small increase of voltage. this indicates the approximate knee-point or saturation flux level of the current transformer.

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Connection Diagram Of Magnetizing Curve Test

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3-Insulation TestIn this test we will measure the insulation resistance between the primary winding and ground And between secondary winding and ground Between secondary and primary

P1 with E P2 with E S1 and E S2 and EP1 and S1 P2 and S1 P2 and S1 P2 and S2

4-Secondary resistance test

5-continuity Test:Determine if there is no broken conductor within the transformer winding, we will use an ohm meter and connection cord, test resistance between primary windings terminals,test resistance between secondary winding terminals.

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6-Ratio Test:

Determine the transformation ratio, we will use a primary injection set, amps meters, connection cords.

we inject a current into the primary then we record the secondary and find the ratio.

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7-dc Winding resistance Test:The CT resistance must be checked during the routine maintenance.A dc voltage is applied then a voltmeter measure the voltage , then the current is measured .

The resistance is then R= V/I

But this resistance must be compared with the manufacturer dataThe temperature conversion must be done to the 75 degrees….. ?

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The following factors affect CT prices:

1.Specifying a higher VA or CLASS than necessary usually results in a higher cost

2.The cost generally increases as the CT's internal diameter increases

13.A CT’s are usually more expensive than 5A

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VOLTAGE TRANSFORMER

I s a device used to decrease the network voltage into values can be handled by other equipments.

Equation of VT:

Vp/Vs=Np/Ns

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Secondary Of Voltage Transformer

IEC 60044-2

Rated secondary voltage 100, 100/sqrt3, 110110/sqrt3 volts depending on the type of connection

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Voltage Transformer ErrorsPrimary source of errors is overloadingthe transformer.

Ratio error: Represents the difference between the primary voltage multiplied by the transformation ratio and the secondary voltage.

E%=(actual ratio-rated ratio)/rated ratio *100%

Phase Error: Represents the phase difference between the primary side and the secondary side.

Delta= θ(Vp)-θ(Vs)

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VT ConnectionVoltage transformers are commonly used in three-phase groups,

generally in star–star configuration

WYE connection

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OPEN DELTA Connection

It is common to detect earth faults in a three-phase system using the displacement that occurs in the neutral voltagewindings connected in a broken delta.

The residual voltage (neutral displacement voltage, polarizing voltage) for earth fault relays can be obtained from a three-phase set of VTs, which have their primary winding connectedphase to earth and one of the secondary

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To prevent secondary circuits from reaching dangerous potential, the circuits should be earthed. Earthing should be made at only one point of a VT secondary circuit

Secondary earthing of voltage transformers

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Voltage Transformer Accuracy:

Accuracy Grade : represents the ratio and phase error in the transformer.

Voltage Factor: represents the transformer withstand voltage for 30 sec or for some VT's continuous. maximum primary voltage can the VT withstand depends on earthling system and VT primary connection.

IEC specifies

the voltage factors:1.9 for systems not being solidly earthed.1.5 for solidly earthed systems.

VT Accuracy Class

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Accuracy power: or the burden is the rated load that the VT can supply at the rated voltage

The standardized values are:10 - 15 - 25 - 30 - 50 - 75 - 100 - 150 - 200 - 300 - 400 - 500 - VA.

Voltage transformers used for measuring

The standardized IEC accuracy classes are: 0.1 - 0.2 - 0.5 - 1 - 3.– Classes 0.1 and 0.2 are only used for laboratory devices.– Classes 0.5 and 1 are used in the majority of cases.– Class 3 is used very little.

Example:

Voltage transformer for protection:

The IEC accuracy classes are 3P and 6P. In practice, only class 3P is used

Types Of Voltage Transformers

shell type

dry type oil type

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VT must never be short-circuited on the secondary, because the power supplied increases and the transformercan be damaged by the resulting heat rise

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Voltage drop in voltage transformers

The voltage drop in the secondary circuit is of importance. The voltage drop in thesecondary fuses and long connection wires can change the accuracy of the measurement.

The voltage drop in the leads from the VT to the associated equipment must be considered as this, in practice, can be alarming mainly in case of measuring circuits. This is the one that separates the metering circuits (with low burden) from protective circuits (with higher burdens)

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Voltage Transformer Nameplate

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Voltage Transformer Testing

Polarity Test

Insulation Resistance Test

Ratio Test

continuity Test

High voltage test

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In general, the size of an inductive VT is proportional to its nominal voltage and, for this reason, the cost increases in a similar manner to that of a high voltage transformer. One alternative, and a more economic solution, is to use a capacitor voltage transformer . A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down extra- high voltage signals and provide low voltage signals either for measurement or to operate a protective relay. In its most basic form the device consists of three parts: two capacitors across which the voltage signal is split, an inductive element used to tune the device to the supply frequency and a transformer used to isolate and further step-down the voltage for the instrumentation or protective relay.

Capacitor Voltage Transformer

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CVT Equivalent Circuit

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This device is effectively a capacitance voltage divider, and is similar to a resistive divider in that the output voltage at the point of connection is affected by the load

in fact the two parts of the divider taken together can be considered as the source impedance which produces a drop in voltage when the load is connected

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CVT Nameplate

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VT’S Used In NEPCO

VT’S Used In EDCO

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THE END

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Appendix

References supporting these papers:1.SEL protection Instrument Transformers part 7

2.ETC Instrument transformers3.IEC 60044-1, 60044-2

4 .Current Transformer Grounding, netaworld.org by Jim wenPowell Electrical ,Manufacturing Co

5 .C&C Ltd Catalogue & Price List May 2006NOTES ON CURRENT TRANSFORMERS6. Current transformers for HV protection by Michel Orlhac7.Shnider protection guide

8 .Current transformer the basics, NK Technologies 9. Protection of Electrical Networks, Christophe Prévé

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