CT Saturation Tutorial

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Copyright ATG Consulting 2003 1 CT Saturation Tutorial Presented by Tony Giuliante N Turns I P I S Bushing CT

Transcript of CT Saturation Tutorial

Page 1: CT Saturation Tutorial

Copyright ATG Consulting 2003 1

CT Saturation Tutorial

Presented byTony Giuliante

N TurnsIP

IS

Bushing CT

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Physical Properties of Core

Length L

Area A

B-H Characteristic

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B-H Characteristic

B

H

Flux to Volts per Turn

Φ = ∫s B • dA

Φ = B • A sin (ωt)

dΦ = ω • B • A cos (ωt)dt

V =N

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Flux to Volts per Turn

V = ω • B • AN

Electric Field to Ampere Turns

ΝΙ = ∫ H • dL

ΝΙ = H • L

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Convert B-H Characteristic

B

H

V = ω • B • AN

ΝΙ = H • L

V/N vs. NI

V

N

NI

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CT Exciting Characteristic

VS

IS

2000:5

300:5

Simplified Bushing CT Circuit

NIP

IS

REB

REB = RLEADS + RDEVICES

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Simplified Bushing CT Circuit

IM

XMNIP

IS

REB

RCT

Simplified Bushing CT Circuit

IM

XMV = IS RTB

NIP

IS

RTB

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Flux vs Voltage

dΦdtV = N

Φ = ∫ V • dtN1

Φ = ∫ IS RTB • dtN1

Flux vs Voltage

Φ = ∫ IS RTB • dtN1

Flux equals the AREA under the Voltage

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Voltage DemandIS RTB

Voltage & Flux Waveforms

Φ

IS RTB

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Flux Design Limits

+ ΦS

- ΦS

Φ

Secondary CurrentNo Saturation

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Increased Voltage DemandFive times IS RTB

5*IS RTB

Flux for Ideal CTNo Saturation

Φ

5*IS RTB

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Current Output for Ideal CTNo Saturation

Amperes

Time (Seconds)

Primary Current Secondary Current

Flux Design Limits

+ ΦS- ΦS

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Flux Design Limits

+ ΦS- ΦS

Flux Excursion

Φ

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Current vs Flux

Φ

AC Saturation

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AC Saturation

• Large Fault Current

• Large Burden

• Low CT Kneepoint Voltage

AC SaturationRelay Applications

• Large Fault Current– Unit Auxiliary Transformers

• Large Burden– High Impedance Bus Differentials

• Low CT Kneepoint Voltage– Compact Distribution Switchgear

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87UAT

Unit Auxiliary Transformers

G

DC OffsetTransformer

Fault current includes a dc component, or offset, that makes the current asymmetrical.

L/R = 100 ms X/R = 37.7

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Offset Current vs Flux

Time (Seconds)

Primary Current Flux

Sec. Amperes

or

Flux Density

Time (Seconds)

Amperes

Primary Current Secondary Current

Secondary Current

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Secondary CurrentObservations

• Secondary current is distorted due to the core flux saturation

• Secondary current distorts after a short time (time-to-saturation)

• Distortion slowly dissipates as primary dc offset decays

0 1 2 3 4 5 6 7-50

0

50

100Secondary Current

Am

ps

0 5 6 7-1

0

1

2Magnetic Flux Density (B)

Tesl

a

1 2 3 4Cycles

I SEC

I PRIM

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Time (Seconds)

Amperes

Secondary Current Primary Current

Differential Current

Large Differential Current

DC Saturation Factors

• Large DC Time Constant

• Large Burden

• Low CT Kneepoint Voltage

• High Remanent Flux

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Remanent Flux• Trapped magnetic flux in core if a previous

offset current is interrupted before reaching a symmetrical state

• High X/R ratios make remanent flux more likely due to the slow decay rates of offset current

Remanent Flux Survey

Remanent flux Percentagein % of saturation of cts

0 - 20 3921 - 40 1841 - 60 1661 - 80 27

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Remanent Flux Example

• CT data– 1200:5, C800, burden = 1.6 +j 0.7 ohm

• Fault current 24,000 amps with dc offset• X/R ratio = 19• Display ct secondary output current for

remanence of 0%, 50% and 75% of saturation

Time (Seconds)

Amperes

Primary Current Secondary Current

0% Remanent Flux

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50% Remanent Flux

Time (Seconds)

Amperes

Primary Current Secondary Current

75% Remanent Flux

Time (Seconds)

Amperes

Primary Current Secondary Current

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Remanent Flux ResultsRemanent flux Time-to-saturation

0 % 1+ cycles50% 1/2 cycle75% 1/3 cycle

IEEE Guide for the Application of Current Transformers Used for

Protective Relaying Purposes

C37.110-1996

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CT Classification

CT Accuracy Class

• ANSI defines accuracy rating classes by a letter and numberC100, C800 or T100, etc.

• Letter designates how the accuracy can be determined

• Number designates the minimum secondary terminal voltage under a standard burden

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Accuracy Class Letter

• “C” means by Calculation– non-gapped cores with negligible leakage flux,

such as bushing cts• “T” means by Test

– cts with leakage flux, such as cts with wound primaries

• Old classes “H” and “L”H T and L C

Accuracy Class Number

• Minimum secondary terminal voltage produced– at 20 times rated current– into a standard burden– without exceeding a 10% ratio correction factor

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What is a Standard Burden?

• IEEE Standard Requirements for Instrument Transformers C57.13-1993 the standard relaying burdens are 1, 2, 4 and 8 ohms at a lagging 0.5 p.f.

• 20 times rated secondary current of 5 A is 100 A, and 100 A times the standard burdens yield C ratings of 100, 200, 400 and 800 V

VS

IS

2000:5

300:5

45o Tangent

A B

CT Knee Point Voltages

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Knee Point Definitions

• Point A is the ANSI knee point voltage– point tangent to 45 degree slope line

• Point B is the IEC knee point– where a 10% increase in voltage causes a 50 %

increase in current• IEC knee point is higher than ANSI knee

point

CT Excitation Impedance

• Excitation curve represents the exciting impedance in terms of voltage and current

• The ANSI knee point (A) represents the point of maximum permeability of the iron core

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Examples

• Determine Accuracy Class

• Selecting CT Ratings

• Calculating Time to Saturation

Example - Find Accuracy Class

• Find the approximate ct accuracy class from the excitation curve– the C class is defined for a 10% ratio correction

factor at 20 times rated current• 10% of (20 X 5 A) is 10 A• for IE = 10 A, use the excitation curve to find VS as

about 500 V

– next find the ct terminal voltage by subtracting the internal voltage drop from VS

» (continued)

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Example - Equivalent Circuit

IP IS = 100 A

VS = 500IE = 10 A

VB = ? ZB

Example - continued– VB (voltage to the burden)

VB = VS - (IS X RS)VB = 500 - (100 X 0.61)VB = 439 V

– The approximate ct accuracy class is the next lowest ANSI class number (C400)

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Examples

• Determine Accuracy Class

• Selecting CT Ratings

• Calculating Time to Saturation

VX > IS · ZTB (1 + X/R)

VX = saturation voltage

IS = secondary current

ZTB = total ct secondary burden

Avoiding CT Saturation

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CTs for Generator Differentials• For generators, typically cts cannot be sized

to avoid saturation because of:– high fault current– high X/R ratio

• Common applications would:– select adequate ct primary rating– select highest practical C class– match manufacturer and types of cts

Examples

• Determine Accuracy Class

• Selecting CT Ratings

• Calculating Time to Saturation

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Transient Response of Current Transformers

Power Systems Relaying Committee

VKI F TBR

( I - K ) =RT -CT TS

TCT

- tTS

- t

e - e + 1TCT TSω }{

Time to Saturate Equation

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VS

IS

2000:5

300:5

TangentsIntersect

VK

VK

Saturation ParametersR = R + R +RTB CT LEADS DEVICES

I = FAULT CURRENT(SEC RMS AMPS)

F

K = .5 - .75 IRON CORE.1 AIR GAP

R

ω = 377

TCT =LMRTB

{

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VS

IS

2000:5

300:5

45o Tangent

VM & IM

VM

IM

CT Inductance

TCT =M

RTB

L

X =MVMI M

L =MXMω

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DC Offsets

DC Offset Current

• Depends on where in the voltage wave the fault occurs.

• Fault time is defined as:F I A => Fault Initiation Angle

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FIA

Voltage Waveform

0 45 90 135 180 225 270 345 360 Degrees

0 4.16 8.33 12.5 16.67 Time ms 60 Hz

FIA

Voltage Waveform

0 45 90 135 180 225 270 345 360 Degrees

0 5 10 15 20 Time ms 50 Hz

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Power System

Z = √ R2 + X2

θ = ARCTAN (ω L / R)G

R L

Power System

θ = Characteristic AngleG

R L

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Fault at FIA = θNo Offset

G

R L

0 10 20 30 40 50-3

-2

-1

0

1

2

Time - Milliseconds

Cur

rent

Current WaveformNo Offset FIA = θ

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Fault at FIA = θ ± 90Max Offset

G

R L

0 10 20 30 40 50-3

-2

-1

0

1

2

Time - Milliseconds

Cur

rent

Current WaveformMax Offset FIA = θ + 90

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0 10 20 30 40 50-3

-2

-1

0

1

2

Time - Milliseconds

Cur

rent

Total Current

Equations

v(t) = Vmax * sin ( Wt + Close_Ang )

i (t) = i ss (t) + i trans (t)

i ss (t) = [ Vmax / Z ] * sin ( Wt + Alpha )

Alpha = Close_Ang - ArcTan ( WL/R )

i trans (t) = [ e^ (-R/L) t ] * [ -Vmax / Z ] * sin ( Alpha )

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Power SystemTime Constants

L/R (MS) X/R Ang (deg) Power System1 0.377 20.66 High Fault Resistance2 0.754 37.025 1.885 62.05 Distribution Lines

10 3.770 75.14 Subtransmission Lines30 11.310 84.95 EHV Lines100 37.699 88.48 Transformers200 75.398 89.24 Generators400 150.796 89.62

1000 376.991 89.85 Large Generators

0 10 20 30 40 50-3

-2

-1

0

1

2

Time - Milliseconds

Cur

rent

Subtransmission LineL/R = 10 ms

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0 10 20 30 40 50-3

-2

-1

0

1

2

Time - Milliseconds

Cur

rent

GeneratorL/R = 200 ms

0 10 20 30 40 50-3

-2

-1

0

1

2

Time - Milliseconds

Cur

rent

EHV LineL/R = 30 ms

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0 10 20 30 40 50-3

-2

-1

0

1

2

Time - Milliseconds

Cur

rent

Distribution LineL/R = 5 ms

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