Current TransformerBy:

تاج / يحيى مهندسالدين

APPLICATIONCurrent transformers (CT,s) are instrument

transformers that are used to supply a reduced value of current to protective relays , meters and other instruments.

CT,s provide isolation from the high voltage primary , permit grounding of the secondary windings for safety , and step down the magnitude of the measured current to a value that can be safely handled by the instruments

RatioThe most common CT secondary full load current is

1A or 5A.

Example a 1000/1 A CT will produce 1A of secondary

current when 1000 A flows through the primary. As the primary current changes the secondary

current will vary accordingly.

CT ratio are expressed as a ratio of rated primary current to the rated secondary current .

POLARITY

All CT,s are subtractive polarity .

On subtractive polarity transformers the H1 primary lead and the X1 secondary lead will be on the same side of the transformer.

Polarity refers to the instantaneous direction of the primary current with respect to the secondary current and is determined by the way the transformer leads are brought out of the case

CT POLARITY

P1

S1

Terminals Marking

general rules

The terminal markings shall identify: the primary and secondary windings; the winding sections, if any; the relative polarities of windings and winding sections; the intermediate tapings, if any

Graphic symbols of current transformers

The main tasks of instrument transformer are:

• Insulate secondary circuits from the primary.

• To transform current, or voltages, from a high value to a value easy to handle for relays and instruments.

• permit the use of standard current ratings for secondary equipment.

1

2

2

1

N

N

I

I

CT Ring Type

CT

Summation CT

CT equivalent circuit

Excitation Curve

Non saturated - zone

intermediate - zone

TEST RESULTVolt (V)Current (mA)Volt (V)Current (mA)

8.70.959527.7

11.31.260328.8

15.11.462532.3

21.51.964535.8

30.22.465037.6

51.73.468043.7

103.365.669045.9

215.349.271053.3

26710.773060.6

301.4811.875096.7

387.614.5770150.5

430.6816785204.3

473.7518.3800287.0

516.820.6805379.8

56023.8820573.4

59527.78401111.0

60328.88602150

SECONDARY EXCITING CURRENTMAG CURVE

1

10

100

1000

0.1 1.0 10.0 100.0 1000.0 10000.0

MAG CURVE

A C.T consists essentially of an iron core with two windings. One winding is connected in the circuit whose current is to be measured.

The flow of current in the primary winding produces an alternating flux in the core and this flux induces an e.m.f in the secondary winding which results in the flow of secondary current when this winding is connected to an external closed circuit .

The magnetic effect of the secondary current , in accordance with fundamental principles , is in opposition to that of the primary and the value of the secondary current automatically adjust itself to such a value , that the resultant magnetic effect of the primary and secondary currents , produce a flux required to induce the e.m.f. necessary to drive the secondary current through the impedance of the secondary.

TERMS & SPECIFICATIONS

Thermal continuous current rating The thermal continuous current rating (r.m.s.value in operates) 1.2 times , or in

extended-range current transformers 1.2 or 2.0 times , the rated current.

Thermal short –time current IthIth is the value of current quoted on the

rating plate with a duration of 1 sec. whose heating effect the current transformer can withstand without damage with the secondary winding short circuited (r.m.s in KA)

Ith =

Ik =

(50/f) 0.05 (t Ik

Un* 3

Ssc

ExampleMVA SC = 5000 MVA

V = 380 KV

I k =

I k = 7.597 KA

Ith =

I dyn = 2.5 Ith

380* 3

5000

)0.05(50/60(1 Ik

I dyn is the h ighest amplitude of current whose mechanical effects the CT can withstand , with the secondary winding short circuited , without damage (peak value in KA)

I dyn = 2.5 … 3 I th

Dynamic current rating I dyn

Burden = The impedance of the secondary circuit in ohms and power factor. The burden is usually expressed as the apparent power (S) in volt-amperes absorbed at a specified power-factor at the rated secondary current.

Burden

RB

LB

BURDEN=

VA / I²

EXTERNAL BURDEN

To protect instrument and meters from high fault currents the metering cores must be saturated 10-40 times the rated current depending of the type of burden.

The instrument security factor “Fs”

sib

in FPP

PPn *

The main characteristics of protection CT cores are:

• Lower accuracy than for measuring transformer .

• High saturation voltage.

• Little , or no turn correction at all.

5P and 10P The error is then 5 and 10 at the specified ALF and at rated burden.

The Accuracy Limit Factor indicates the over current as a multiple times the rated current , up to which the rated accuracy (5P or 10P) is fulfilled (with the rated burden connected).

ALFPp

PPn

ib

in *

No. of primary turns = 1 turn

No. of secondary turns = N turn

Ip = N * Is

Ideal transformer for (Is) to flow through R there must be some potential Es = The E.M.F

Es = Is * R

Es is produced by an alternating flux in the core.

dt

dEs

Flux required to produce Es

BsCTss zIRIE **

AB*Where

B = Flux density in the coreA = cross-sectional area of core

NAfBEk ****44.4

LCTBss zzzIE Requiredsk EE

Equ. 1

Equ. 2

CT 2000/5 , Rs =0.31, Imax =40 KA , MaX Flux density =1.6 Tesla

Find maximum secondary burden permissible if no saturation is to occur.

Solution

N=2000/5 = 400Turns

Is max = 40000/400 = 100Amps

From Equ.1

Vk = 4.44*1.6*20*60*(400/10000) = 340 Volt

Max burden = 340/100 = 3.4 ohmsMax connected burden = 3.4 - 0.31 = 3.09

CT ratio are selected to match the maximum load current requirements.i.e. the maximum design load current should not exceed the CT rated current.

The CT ratio should be large enough so that the CT secondary current does not exceed 20 times rated current under the maximum symmetrical primary fault current.

It is customary to place CT,s on both sides of the breaker. So that the protection zones will overlap.

The protection Engineer can determine which side of the breaker is best for CT location .

All possibilities of fault position should be considered .

The overlap should occur across a C.B, so the C.B lies in both zones for this arrangement it is necessary to install C.Ts on both sides of the C.B.

C.T,s mounted on both sides of breaker no unprotected region

No region un protected

Current transformers mounted on C.B side only of breaker fault shown not cleared by bus bar protection.

Current transformers mounted on bus bar side only of breaker fault shown not cleared circuit protection.

C,B will open by line protection but fault will last.

PROTECTION IEC standard

RATIO 2000/5 APOWER 20 VACLASS 5P20

MEASURING IEC standard

RATIO 2000/5 APOWER 20 VACLASS 0.5SF5

CT Class X

THE FOLLOWING INFRMATION IS REQUIREDTurns RatioKnee Point VoltageMaximum Excitation Current Secondary Circuit Resistance

TPX, TPY AND TPZ Current Transformers

CTs of class P, models were developed for CTs of class TPX (closed-core), TPY and TPZ (nonclosed-core). All models are based on known rated values of the CTs. This is an advantage of the presented method, because no additional measurements of the parameters of the CTs are needed.

TPXHigh remanence type CTThe high remanence type has no limit for the remanence

flux. This CT has a magnetic core without any air gap and a remanence flux might remain for almost infinite time. In this type of transformers the remanence flux can be up to 70-80% of the saturation flux.

Typical examples of high remanence type CT are class P, TPS, TPX according to IEC,class P, X according to BS (British Standard) and non gapped class C, K according to

ANSI/IEEE.

TPY

Low remanence type CTThe low remanence type has a specified

limit for the remanence flux. This CT is made with a small air gap to reduce the remanence flux to a level that does not exceed 10% of the saturation flux.

The small air gap has only very limited influence on the other properties of the CT. Class TPY according to IEC is a low remanence type CT.

TPZNon remanence type CTThe non remanence type CT has practically

negligible level of remanence flux. This type of CT has relatively big air gaps in order to reduce the remanence flux to practically zero level. At the same time, these air gaps minimize the influence of the DC-component from the primary fault current.

The air gaps will also reduce the measuring accuracy in the non-saturated region of operation. Class TPZ according to IEC is a non remanence type CT.

As a matter of safety, the secondary

circuits of a current transformer should never be opened under load, because these would then be no secondary mmf to oppose the primary mmf, and all the primary current would become exciting current and thus might induce a very high voltage in the secondary.

General As a matter of safety, the secondary circuits

of a current transformer should never be opened under load, because these would then be no secondary mmf to oppose the primary mmf, and all the primary current would become exciting current and thus might induce a very high voltage in the secondary.

f

Ip

Ie Ze

XpRp e Rs

Sec

g

h

c

d

Pri

Is

EQUIVALENT DIAGRAM

Ve = EXCITATION VOLTAGE Vef Ie = CURRENT

Ze = IMPEDANCEVt = TERMINAL VOLTAGE Vgh

KNEE POINT OR EFFECTIVE POINT OF SATURATION

ANSI/IEEE: as the intersection of the curve with a 45° tangent line

IEC defines the knee point as the intersection of straight lines extended from non saturated and saturated parts of the excitation curve.

IEC knee is higher than ANSI - ANSI more conservative.

ANSI/IEEE KNEE POINT

Exc

itatio

n V

olts

Kne

e P

oint

Vol

ts

45° LINE

IEC KNEE POINT

ANSI/IEE KNEE POINT

EX: READ THE KNEE POINT VOLTAGE

RATIO CONSIDERATIONS

CURRENT SHOULD NOT EXCEED CONNECTED WIRING AND RELAY RATINGS AT MAXIMUM LOAD. NOTE DELTA CONNECTD CT’s PRODUCE CURRENTS IN CABLES AND RELAYS THAT ARE 1.732 TIMES THE SECONDARY CURRENTS

RATIO CONSIDERATIONS

SELECT RATIO TO BE GREATER THAN THE MAXIMUM DESIGN CURRENT RATINGS OF THE ASSOCIATED BREAKERS AND TRANSFORMERS.

RATIO CONSIDERATIONS

RATIOS SHOULD NOT BE SO HIGH AS TO REDUCE RELAY SENSITIVITY, TAKING INTO ACCOUNT AVAILABLE RANGES.

RATIO CONSIDERATIONS

THE MAXIMUM SECONDARY CURRENT SHOULD NOT EXCEED 20 TIMES RATED CURRENT. (100 A FOR 5A RATED SECONDARY)

RATIO CONSIDERATIONS

HIGHEST CT RATIO PERMISSIBLE SHOULD BE USED TO MINIMIZE WIRING BURDEN AND TO OBTAIN THE HIGHEST CT CAPABILITY AND PERFORMANCE.

RATIO CONSIDERATIONS

FULL WINDING OF MULTI-RATIO CT’s SHOULD BE SELECTED WHENEVER POSSIBLE TO AVOID LOWERING OF THE EFFECTIVE ACCURACY CLASS.

The core should be demagnetized as the final test before the equipment is put in service. Using the Saturation test circuit, apply enough voltage to the secondary of the CT to saturate the core and produce a secondary current of 3-5 amps. Slowly reduce the voltage to zero before turning off the variac.

Core Demagnetizing

TESTING

SaturationThe saturation point is reached when there is a rise in the test current but not the voltage.

Burden

TESTING

PolarityThis test checks the polarity of the CT

RatioInsulation test

SATURATIONAbnormal high primary currentHigh secondary burdenCombination of the above two factors will result in the creation of high flux density in the current transformer iron core.

When this density reaches or exceeds the design limit of the core , saturation results.

SATURATION

The accuracy of the CT becomes very poor.

The output wave form distorted.The result secondary current lower in magnitude.

The greatest dangerous is loss of protective device coordination

SATURATION

list of CT problems usually found at site:

• Shorted CT secondaries• Open-circuited CT secondaries• Miswired CTs• CTs that had not been wired• CTs installed backwards• Incorrect CTs• Defective CTs• CTs with incorrect ratios or on the wrong taps

THANKSتحياتى : مع

الدين / تاج يحيى م