11_Currenttransformer

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Current Transformers

Protection Application Handbook

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Current TransformersIntroduction

Main tasks of current transformer are

– Measurement of Current

– Measurement of Power

– Isolation between High voltage and Low Voltage

– Inputs to Relays & Protection Systems

I2

I1

I1 N1

N2I2

I1 = N2

I2 = N1

IEC 44-1 and IEC 44-6 covers instrument transformers

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CTs and VTs

Current transformer

I1/I2 = N2/N1

Magnetizing current is negligible

Voltage transformer

E1/E2 = N1/N2

Voltage drop by magnetizing current is negligible

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Sec. Cores

P1

P2

Bottom TankTerminal Box

Insulator

Pri. ConductorPri. Insulation

Conn. Head Pri. Terminal

Expan. TankOil Level Ind.

Nitrogen Gas

CT – Construction

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Vector diagram of current transformer

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Measuring errors

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Current TransformersMeasuring errors

Ratio 1:1 assumed

P1

P2

S1

S2Zb

RctIm

IsIp

Is = Ip - Im

CT error = Im

relative CT error = Im / Is

since Is Ip

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Current TransformersFactors influencing CT output and magnetising current

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Current Transformer output

Metering and instrument

High accuracy for 25-100 % of rated burden

Protection and DR

Lower accuracy but high capability to transform high fault current. Protection classes 5P and 10P are acc to IEC 44-1 and cores for transient behavior are acc to IEC 44-6

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• Metering Core

– VA Burden, Accuracy, ISF

– e.g. 15 VA, 0.5 Cl., ISF < 10

Metering

Protection

Types of current transformers

Protection Core

VA Burden, Accuracy, ALF

e.g. 15 VA, 5P20

PS Core

Vk, Io, Rct

e.g. Vk > 400 V, Io < 50 mA at Vk/2, Rct < 5 Ohms

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E / B

I0 /H

Eknee 1.5

E = 4.44BmAfN2 Volts

2 Tesla Saturated CRGO

x Bn

Bs

0.8 Tesla Saturated Mumetal

Bnx

Metering Core:CRGO – 0.4 to 0.8 Tmetal – 0.3 to 0.35 TProtection Core:CRGO – 1.5 to 1.6 T

Working Flux Density

B – H Characteristics

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Metering cores

Metering cores must saturate before 10-40 times rated current depending on burden. This is defined by Fs instrument security factor. At lower burdens the saturation value can be given by

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Current TransformersMetering cores : ratio and angle errors for different classes acc to IEC 44-1

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100 12050205

Phase displacement (min)

90

60

30

10

Class 0.2

Class 0.5

Example:Plotted curves for class 0.5

Rated primary current (%)

+ 0.2

0.5

1.0

+ 1.5

100 1205020

0.75

0.5

0.75

- 1.5

- 0.2

1.0

Current error (%)

Class 0.2

Class 0.5

25% of rated burden

100% of rated burden

5

Accuracy Curves

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Protection cores

Main characteristics Lower accuracy than measuring core

High saturation voltage

Little or no turns correction

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Current TransformersProtection cores

Examples: 30VA, 5P20 40VA, 10P20 20VA, 10P10 20VA, 5P40

30VA, 5P20

30VA rated burden at rated secondary current5 total error in % at accuracy limit and rated burdenP protection class20 minimum multiple of rated secondary current at which

the accuracy limit is reached with rated burden

Rct is often not noted and has to be measured.For new orders, Rct has to be specified.

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Current TransformersCT Effective accuracy limiting factor n'

n'

204 30

4 2113

CT's are normally not fully burdened, the effective accuracy limiting factor n' is therefore higher then the rated n

P1

P2

S1

S2

Rct=4

Im

IsIp

Ual

Ual = (Rct + Rbn) · Isn · n =(Rct + Rbeffective) · Isn · n'

Example: 30VA, 5P20, Rct = 4 n = 201000/1A effective burden = 2 n' = 113

Rbeffecitive =2

Rbn =30

n' nRct Rbn

Rct Rbn

Pct Pn

Pct Pbeffective effective

Example:

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Current TransformersCT Class X according BS 3938

Class X is widely usedClass X (or TPS) is imperative for high impedance circulating current

protections and is also suitable for most other protection schemes.

Specify: Knee point voltageSecondary winding resistance at 75°Magnetising current at voltage specifiedmost frequently: full or half knee point voltage

Class X impliesLow stray reactanceclosed core and equally distributed secondary turns, among other parameters

No secondary winding turn correction# of secondary turns = theoretical #

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A CT in class 5Pn with Sn [VA]is approximately equal to a CT in class X with identical Rct andVk BS = ( Sn / Isn + Isn · Rct ) · n / 1.3

Factor is 1.3 due to different definitions of Ualf (= the knee point voltage)A CT in class X with Vk [V]is approximately equal to a CT in class 5Pn with identical Rct andSn [VA] = ( 1.3 VkBS / n - Isn · Rct ) · Isn

[V] ·

Example 1 Cl. 5P20 converted to Cl. X1000/1 A, 40VA, 5P20, Rct= 6 equals approximately to a CT in class X withVk = (40VA / 1A + 1A · 6 ) · 20 / 1.3 = 707V 1000/1 A, Vk=710V, Rct=6

Example 2 Cl. X converted to Cl. 5P201000/1 A, class X, Vk= 710 V; Rct= 6 equals approximately to a class 5P20 withSn = (1.3 · 700V / 20 - 1A · 6 ) · 1A = 40 VA 1000/1 A, 40 VA, 5P20, Rct=6

·

CT Conversion class X to 5Pn and vice versa

·

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Ual = Accuracy limiting voltage (secondary voltage)Former designation, still very popular:

Vk = knee-point voltage

Definitions according different standards:

BS Voltage at which an additional 10% increase in voltage requires an increase of 50% of magnetizing current

ANSI Voltage at which the slope is 45° for plots on log-log paper

IEC 5P Voltage at which the total error = 5% of current applied

CT Definitions of knee-point voltage

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Current TransformersCT Definitions of knee-point voltage

+10%

+50%

U [V]

Im [A]

CT-ratio = 1200/1A

100

1000

10000

0.001 0.01 0.1 1.0 10

ANSI

Slope = 45°

5 P 20BS

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Useful tips in selecting CT cores

Select primary rated current close to object rated current. Gives high resolution of metering CT.

For protection core have highest possible CT ratio. Gives least requirement of core.

With secondary taps output gets reduced

1 A CT is better. Cable burden reduces.

Do not overdimension burden of metering core. Security factor increases.

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Select current security factor and ALF depending on type of equipment connected

Do not specify higher accuracy requirements than necessary. Cost increases.

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Limit Rct specially for 1A CT ratio CTs to get best of CT output .Always have internal resistance much less than rated burden. Have Rct <0.2 – 0.5 ohms per 100 turns. Bigger value for bigger core and smaller value for smaller cores.

Cores are considered big when 1-2 V for 100 turns and medium when 0.5-1V per 100 turns

In 1 A core keep core size down to keep secondary resistance low

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Short circuit current in a typical fault loop

Short circuit Current

L R

G

i = 2 Ipsc e-t/Tp Cos - Cos (t +)

DC component is zero when = 90 deg

DC component is maximum when = 0 deg

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Tp

-1

-0.5

0

0.5

1

1.5

2

0 10 20 30 40 50 60 70 80 90 100

time

cu

rren

t

ms

Short circuit Current with DC offset

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Current TransformersEquivalent circuit of a CT

L2

R2

U1

UR

L0

UL

i1 i2

L0 is magnetizing Inductance of CTL2 is inductance of CT secondaryR2 is Resistance of CT secondaryTs = (Lo+L2)/R2 is Secondary time constant

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CT primary current

CT secondary current

current

time

CT Saturation due to DC-offset (Transient saturation)

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By analyzing the circuit we come to the conclusion that the CT needs to be overdimensioned by a factor Ktot

Ktot the total overdimensioning factor is defined as

Ktot = Kssc .Ktf . Krem.

Total overdimensioning factor

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The symmetrical short circuit current factor Kssc is defined

as ratio of rms value of short circuit current (Ipsc ) and rated

current ( Ipn )

Kssc = Ipsc / Ipn

Symmetrical short circuit current factor Kssc

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The transient dimensioning factor is the ratio of total flux in the CT core to peak value of AC component of flux

Ktf = Tp Ts . Cos / (Tp – Ts) . e-t/Tp - e-t/Ts + Sin .e-t/Ts - Sin (t +)

Considering Sin (t +) = -1.

The transient dimensioning factor Ktf can be expressed as

Ktf = Tp Ts . Cos / (Tp – Ts) . e-t/Tp - e-t/Ts + Sin .e-t/Ts + 1

Transient dimensioning factor Ktf

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Remanance factor ( Kr ) is ratio of remanant flux to saturation flux

Krem the remanance dimensioning factor is defined as

Krem = I/(1-Kr)

Remanance dimensioning factor Krem.

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Current TransformersInfluence of Ts on Ktf

Influence of Ts

10 s

5 s

3 s

2 s

1 s

0 .5 s

10.2 0.4 0.6 0.8

5

10

15

20

K tf

Ts =

T im e in seconds

Except for CTs with big airgaps Ts is few or several seconds

Influence of this on Ktf is relatively small

During first 100 ms

Tp considered is 60 ms

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Current TransformersInfluence of Tp on Ktf

Ktf increases when

Tp increases

Influence of Tp

300 m s

30 m s

200 m s

100 m s

10.5 1.5

20

40

60

80

K tf

50 m s

Tp =

T im e in seconds

Ts considered here is 3Secs

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Current TransformersInfluence of tsat on Ktf

25 50 75 100

2 m s

3

K tf

4

1

2

10 m s

8 m s

6 m s

4 m s

tsat

Max DC and reduced DC

Tp in m sTp in m s50 100 150 200

20 m s

20

K tf

8

4

12

16

Max DC and reduced DC

70 m s

50 m s

40 m s

30 m s

tsat

tsat > 15msTs=3S,Full DC

tsat < 15msTs=3S

To be able to specify the CT requirements and the required Ktf factor one should

know the maximum time to saturation required by the protective relay viz tsat .

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Current TransformersCT Transient performance classes

TPS complementary to class X (low leakage flux)TPX closed coreTPY core with anti remanence air gapsTPZ core wirh considerable air gaps, linearised core

Application examples:

TPS high impedance circulating current protectionTPX most commonTPY line protection with autoreclosingTPZ special applications (differential protection of large generators)

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Current TransformersTP classes Error limits

At rated primary At accuracy limitcurrent condition

Class Ratio error Phase displacement Maximum peakin % instantaneous

Minutes Centirad error in %

TPX ± 0.5 ± 30 ± 0.9 = 10

TPY ± 1.0 ± 60 ± 1.8 = 10

TPZ ± 1.0 180 ± 18 5.3 ± 0.6 ac = 10

Remanent flux for TPS and TPX no limit specified (approx. 80% max.)for TPY < 10%for TPZ negligible

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Current TransformersCT Small air gaps reduce remanence (TPY)

Drawing not to scale

Remanence

closed core, type TPS, TPX

core with small air gap(s) to reduce remanence type TPYTypically used for lines with autoreclosing

Im

flux

Reduced remanence

typical 80%

typical <10%

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When the CT requirements are specified for dependability

No need to consider remanance.

Risk of short delay is often acceptable .

When the CT requirements are specified for security (as for example for differential relays)

Consider remanance

Recommendations for Remanance

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