current tranformers edit

7/30/2019 Current Tranformers Edit

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Current Transformer


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About

Instrument transformers

Too high current, measurement, protection

ONLY AC currents

No effect on main circuit

Symbol


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Theory of Operation

Ideal

No different

Produce Iac in sec. proportional to current measured in primary

Reduce high currents(thousands) to lower value(5 or 1 amp)

Convenient ,Monitoring actual current in an AC TL using a standard

ammeter

secondary current ranges from 1 to 5 amps


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Construction

Unlike VT or PT

Primary

Only one or very few turns

primary wire size > secondary wire size

Secondary Large number of coil turns

Wound on a laminated core of low-loss magnetic material.

Has a large CSA so that the magnetic flux density is low

Using much smaller CSA wire, depending upon how much the current must be

stepped down. Number of 2ndry turns is multiple of the 1ry turns


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Equivalent Circuit


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Equivalent Exact circuit equations:

S: For Secondary P: For Primary


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Types of CTs

Wound current transformers Primary winding

Turns ratio

Insulation, non uniform Sec. winding, leakage, T class

i.e not accurate

Toroidal current transformers No primary windings Threaded line

Split core

Uniform Sec. winding, no leakage

Bar-type current transformers Bus bar

Single turn

Insulated against high voltage

Uniform Sec. winding, no leakage


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CT Polarity

Polarised

Bar-Type Primary

P1& P2/H1 & H2

P1 generator, P2 load

arrow to load

Secondary S1 & S2/X1 & X2

S1 current in, S2 out

connect correctly, wrong measurement

Toroidal current transformers

Arrow, Current direction S1 current in, S2 out


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CT Polarity

Wound CT Gen P1, Load P2

S1 current in, S2 out


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Current transformer cores

High precision current transformers request soft magnetic coreswith ideally

high saturation

high permeability

low remanence

Good temperature stability


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Classification

ANSI/IEEE Accuracy ClassesDepends on Application

o Metering accuracy CT

Very high accuracy, from FL to LL values

e.g. large capacity revenue billing CTso Relaying accuracy CT

Supply protective relays

Relays, not normal in normal loads

Reasonable accuracy, fault and high overload, 20 Ifl


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Classification

ANSI/IEEE Accuracy Classes:

1.Metering accuracy CT

- Available in Maximum Ratio Error Classes of: + 0.3% , + 0.6% , + 1.2%, +2.4%.

- For Burdens (Loads) of: 0.1, 0.2, 0.5, 0.9, 1.8 ohms.

- Which equals 2.5, 5.0, 12, 22-1/2, 45 volt-amperes ( va )


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Classification

2.Relaying Accuracy CTs Class C (Calculated) is low leakage reactance type - donut units - Formerly Class L

( L for Low Leakage)

Class T (Tested) is high leakage reactance type - bar-type units - Formerly Class H (

H for High Leakage)

Available secondary voltages: 10, 50, 100, 200, 400, 800.

Will support burdens of: 0.1, 0.2, 1.0, 2.0, 4.0, 8.0 ohms.


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Classification

IEC-60044-1 Primary:

10 12.5 15 20 25 30 40 50 60 75 A

and their decimal multiple or fractions

Underlined are preferred

Secondary:

1, 2 and 5 A

5 A is preferred Delta, 3


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CT selection

Ratio Selection

Relay Step Tap

Secondary Burden Calculation

Determining CT performance with ANSI / IEEE

i. T class CT

ii. C class CT

iii. C Class CT with CT performance curves


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1.Ratio Selection

For CT and the connected equipments

a. Thermal effect of systemb. Thermal limit

c. Common 5A CTs

d. Pick up Secondary current less than 5A

2. . Relay Step Tap

Minimum pick up and operational current values of the relay Should be higher than load

Calculate margin above load between load and relay (according to Relay characteristics and types

3.SecondaryBurden Calculation

Calculate Burden impedance Relay Impedance Lead impedance

Depends on relay type (Solid state, electromagnetic)

Solid state: low resistance mainly constant with Current variation

Electromagnetic: resistance varies with current variation


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4.Determining CT performance with

ANSI / IEEE

With T class CT: Performance curve

Match and pick

corresponding impedance

to the needed conditions


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4.Determining CT performance with

ANSI / IEEE Performance curves (Voltage, Current and CT ratio)

Match the desired condition to get the missing parameter from curve


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CT Ratings

Rated primary voltage (V)

Rated secondary voltage (V)

Rated frequency (HZ) Rated output of CT (VA)

Rated output of PT (VA)

Accuracy class

Rated insulation level KV


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Magnetic Saturation

Magnetic field strength H (A/m) in the coil is given by:

I = Current through the coil in Amperes (A)

n = Number of turns in the coil

L = Total circumference of the peripheral coil

Induced Magnetic flux density B ( in Wb/m2 or

Tesla) is given by:

= Magnetic permeability in (Wb/A.m) in the

medium (air or iron core) in the coil

H = Magnetic field strength


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CT features

A primary winding (usually a cable or other conductors by toroidal).

Set high primary currents to default value, usually 1 A or 5 A.

Maximum voltage for each winding is dependent on core material and diameter.

Secondary is almost shorted and operated far from max tension / saturation.

In short circuit conditions InP max lasts for up to 1 to 3 sec.

An open winding leads to a very hot and dangerous voltage and causes magnetic

saturation. Winding insulation is only rated for a 2 kV peak voltage!


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Accuracy

accuracy depends on some factors:o Burden

o Burden class/saturation class

o Rating factor

o Load

o External electromagnetic fieldso Temperature

o Physical configuration

o The selected tap, for multi-ratio CTs


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Applications

Metering

Protective Relays

Electrical Power Industry


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Cautions

CT secondary short circuitedo Unless connected burden (i.e. meters, relays, etc.), Secondary must be s.c

o Why? Low current, HV at terminals, arc, damage CT


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Thank You


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References

http://www.idc-online.com/technical_references/ http://www.facebook.com/photo.php?fbid=10200915737280017&set=a.102009

15530994860.1073741826.1362938322&type=1&theater

file://localhost/C:/Users/reem/Downloads/Desktop/arvind_srinivasan.pdf http://www.butlerwinding.com/store.asp?pid=28351 http://www05.abb.com/global/scot/scot235.nsf/veritydisplay/cb9b52d0823be86

6c125798400459d13/$file/ABB-DTR%20current%20transformers%20EN%2011-

01a.pdf

http://www.abb.com/product/db0003db002618/bb12a5314453ed00c12579ca004701fc.aspx

http://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.html
http://www.idc-online.com/technical_references/http://www.facebook.com/photo.php?fbid=10200915737280017&set=a.10200915530994860.1073741826.1362938322&type=1&theaterhttp://www.facebook.com/photo.php?fbid=10200915737280017&set=a.10200915530994860.1073741826.1362938322&type=1&theaterhttp://c/Users/reem/Downloads/Desktop/arvind_srinivasan.pdfhttp://www.butlerwinding.com/store.asp?pid=28351http://www05.abb.com/global/scot/scot235.nsf/veritydisplay/cb9b52d0823be866c125798400459d13/$file/ABB-DTR%20current%20transformers%20EN%2011-01a.pdfhttp://www05.abb.com/global/scot/scot235.nsf/veritydisplay/cb9b52d0823be866c125798400459d13/$file/ABB-DTR%20current%20transformers%20EN%2011-01a.pdfhttp://www05.abb.com/global/scot/scot235.nsf/veritydisplay/cb9b52d0823be866c125798400459d13/$file/ABB-DTR%20current%20transformers%20EN%2011-01a.pdfhttp://www.abb.com/product/db0003db002618/bb12a5314453ed00c12579ca004701fc.aspxhttp://www.abb.com/product/db0003db002618/bb12a5314453ed00c12579ca004701fc.aspxhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.directindustry.com/prod/chint-electric-co-ltd/three-phase-current-transformers-69106-585255.htmlhttp://www.abb.com/product/db0003db002618/bb12a5314453ed00c12579ca004701fc.aspxhttp://www.abb.com/product/db0003db002618/bb12a5314453ed00c12579ca004701fc.aspxhttp://www05.abb.com/global/scot/scot235.nsf/veritydisplay/cb9b52d0823be866c125798400459d13/$file/ABB-DTR%20current%20transformers%20EN%2011-01a.pdfhttp://www05.abb.com/global/scot/scot235.nsf/veritydisplay/cb9b52d0823be866c125798400459d13/$file/ABB-DTR%20current%20transformers%20EN%2011-01a.pdfhttp://www05.abb.com/global/scot/scot235.nsf/veritydisplay/cb9b52d0823be866c125798400459d13/$file/ABB-DTR%20current%20transformers%20EN%2011-01a.pdfhttp://www05.abb.com/global/scot/scot235.nsf/veritydisplay/cb9b52d0823be866c125798400459d13/$file/ABB-DTR%20current%20transformers%20EN%2011-01a.pdfhttp://www05.abb.com/global/scot/scot235.nsf/veritydisplay/cb9b52d0823be866c125798400459d13/$file/ABB-DTR%20current%20transformers%20EN%2011-01a.pdfhttp://www05.abb.com/global/scot/scot235.nsf/veritydisplay/cb9b52d0823be866c125798400459d13/$file/ABB-DTR%20current%20transformers%20EN%2011-01a.pdfhttp://www05.abb.com/global/scot/scot235.nsf/veritydisplay/cb9b52d0823be866c125798400459d13/$file/ABB-DTR%20current%20transformers%20EN%2011-01a.pdfhttp://www.butlerwinding.com/store.asp?pid=28351http://www.butlerwinding.com/store.asp?pid=28351http://c/Users/reem/Downloads/Desktop/arvind_srinivasan.pdfhttp://c/Users/reem/Downloads/Desktop/arvind_srinivasan.pdfhttp://www.facebook.com/photo.php?fbid=10200915737280017&set=a.10200915530994860.1073741826.1362938322&type=1&theaterhttp://www.facebook.com/photo.php?fbid=10200915737280017&set=a.10200915530994860.1073741826.1362938322&type=1&theaterhttp://www.facebook.com/photo.php?fbid=10200915737280017&set=a.10200915530994860.1073741826.1362938322&type=1&theaterhttp://www.idc-online.com/technical_references/http://www.idc-online.com/technical_references/http://www.idc-online.com/technical_references/http://www.idc-online.com/technical_references/

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