current tranformers edit
TRANSCRIPT
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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/