CT Saturation Tutorial

CT Saturation Tutorial

Presented by

Tony Giuliante

Physical Properties of Core

Length L

Area A

B-H Characteristic

H

Flux to Volts per Turn

Flux to Volts per Turn

V

=

ω

_B

_A

N

Electric Field to Ampere Turns

Convert B-H Characteristic

V/N vs. NI

CT Exciting Characteristic

V

I

2000:5

300:5

Simplified Bushing CT Circuit

N

I

I_S

Simplified Bushing CT Circuit

N

I

Simplified Bushing CT Circuit

Flux vs Voltage

d

Φ

dt

V

=

_N

Φ

=

_N

∫

V

_dt

1

Φ

=

_N

∫

I

R

_dt

1

Flux vs Voltage

Φ

=

_N

∫

I

R

_dt

1

Voltage Demand

I

_S

R

_TB

Voltage & Flux Waveforms

I

R

Flux Design Limits

+

Φ

-

Φ

Φ

Secondary Current

No Saturation

Increased Voltage Demand

Five times I

_S

R

_TB

5*I

R

Flux for Ideal CT

No Saturation

Current Output for Ideal CT

No Saturation

Amperes

Time (Seconds)

Primary Current Secondary Current

Flux Design Limits

+

Φ

-

Φ

Flux Design Limits

+

Φ

-

Φ

Flux Excursion

Current vs Flux

Φ

AC Saturation

• Large Fault Current

• Large Burden

• Low CT Kneepoint Voltage

AC Saturation

Relay Applications

• Large Fault Current

87UAT

Unit Auxiliary Transformers

DC Offset

Offset Current vs Flux

Time (Seconds)

Primary Current Flux

Sec. Amperes or Flux Density Primary Current Secondary Current

Secondary Current

Secondary Current

Observations

• 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

2 Magnetic Flux Density (B) I_SEC

Time (Seconds) Amperes

Secondary Current Primary Current

Differential Current

Large Differential Current

DC Saturation Factors

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 Percentage in % of saturation of cts

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

Primary Current Secondary Current

50% Remanent Flux

75% Remanent Flux

Remanent Flux Results

Remanent flux Time-to-saturation

0 % 1+ cycles

50% 1/2 cycle

75% 1/3 cycle

IEEE Guide for the Application of

Current Transformers Used for

CT Classification

CT Accuracy Class

• ANSI defines accuracy rating classes by a

letter and number

C100, C800 or T100, etc.

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

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

2000:5

300:5 45o _Tangent

A B

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

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

Example - Equivalent Circuit

Example - continued

– V_B (voltage to the burden)

Examples

• Determine Accuracy Class

• Selecting CT Ratings

• Calculating Time to Saturation

V

> I

· Z

(1 + X/R)

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

Transient Response of

Current Transformers

Power Systems Relaying Committee

VK I F TBR ( I - K ) =_R T -_{CT TS} TCT - t TS - t

e

- e

}

{

V

I

V

_K

Saturation Parameters

V

I

V

_M

& I

_M

CT Inductance

DC Offsets

DC Offset Current

• Depends on where in the voltage wave the

FIA

Voltage Waveform

FIA

Voltage Waveform

Power System

Z =

√ R

_{+ X}

θ =

(

ω

L / R)

G

R

L

Power System

R

L

Fault at FIA =

θ

No Offset

G

R

L

Current Waveform

No Offset FIA = θ

Fault at FIA =

θ ± 90

Max Offset

G

R

L

Current Waveform

0 10 20 30 40 50 -3 -2 -1 0 1 2 Time - Milliseconds Cu rren t

Total Current

Equations

v(t) = Vmax * sin ( Wt + Close_Ang ) i (t) = i ss (t) + i trans (t)

Power System

Time Constants

L/R (MS) X/R Ang (deg) Power System

1 0.377 20.66 High Fault Resistance

2 0.754 37.02 5 1.885 62.05 Distribution Lines 10 3.770 75.14 Subtransmission Lines 30 11.310 84.95 EHV Lines 100 37.699 88.48 Transformers 200 75.398 89.24 Generators 400 150.796 89.62 1000 376.991 89.85 Large Generators 0 1 2

Subtransmission Line

L/R = 10 ms

0 10 20 30 40 50 -3 -2 -1 0 1 2 Time - Milliseconds Cu rren t

Generator

L/R = 200 ms

EHV Line

L/R = 30 ms

0 10 20 30 40 50 -3 -2 -1 0 1 2 Time - Milliseconds Cu rren t

Distribution Line

L/R = 5 ms