# ct saturation tutorial

Post on 27-Apr-2015

620 views

Embed Size (px)

TRANSCRIPT

CT Saturation TutorialPresented by Tony Giuliante

Bushing CT

IS

IP

N Turns

Copyright ATG Consulting 2003

1

Physical Properties of CoreLength L Area A

B-H Characteristic

Copyright ATG Consulting 2003

2

B-H CharacteristicB

H

Flux to Volts per Turn=

s B

dA

= B A sin (t) V = d = B A cos (t) N dt

Copyright ATG Consulting 2003

3

Flux to Volts per Turn

V = BA N

Electric Field to Ampere Turns =

H

dL

= H L

Copyright ATG Consulting 2003

4

Convert B-H CharacteristicB V = BA N

= H L

H

V/N vs. NIV N

NI

Copyright ATG Consulting 2003

5

CT Exciting Characteristic2000:5

300:5

VS

IS

Simplified Bushing CT CircuitIS

IPN

REB

REB = RLEADS + RDEVICES

Copyright ATG Consulting 2003

6

Simplified Bushing CT CircuitRCT IS

IPN

XM IM

REB

Simplified Bushing CT CircuitIS

IPN

XM IM

V = IS RTB

RTB

Copyright ATG Consulting 2003

7

Flux vs VoltageV = N d dt 1 = N 1 = N

V

dt

IS RTB

dt

Flux vs Voltage1 = N

IS RTB

dt

Flux equals the AREA under the Voltage

Copyright ATG Consulting 2003

8

Voltage Demand IS RTB

Voltage & Flux WaveformsIS RTB

Copyright ATG Consulting 2003

9

Flux Design Limits+ S

- S

Secondary CurrentNo Saturation

Copyright ATG Consulting 2003

10

Increased Voltage Demand Five times IS RTB5*IS RTB

Flux for Ideal CTNo Saturation

5*IS RTB

Copyright ATG Consulting 2003

11

Current Output for Ideal CTNo SaturationPrimary Current Secondary Current

Amperes

Time (Seconds)

Flux Design Limits+ S - S

Copyright ATG Consulting 2003

12

Flux Design Limits+ S - S

Flux Excursion

Copyright ATG Consulting 2003

13

Current vs Flux

AC Saturation

Copyright ATG Consulting 2003

14

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

Copyright ATG Consulting 2003

15

Unit Auxiliary TransformersG

87UAT

DC OffsetTransformer

Fault current includes a dc component, or offset, that makes the current asymmetrical. L/R = 100 ms X/R = 37.7

Copyright ATG Consulting 2003

16

Offset Current vs FluxPrimary Current Flux Sec. Amperes or Flux Density

Time (Seconds)

Secondary CurrentPrimary Current Secondary Current

Amperes

Time (Seconds)

Copyright ATG Consulting 2003

17

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

Secondary Current 100 Amps 50 0 -50 0 2 Tesla 1 0 -1 0 1 2 3 4 Cycles 5 6 7 I SEC 1 2 3 4 5 6 Magnetic Flux Density (B) 7 I PRIM

Copyright ATG Consulting 2003

18

Large Differential CurrentSecondary Current Primary Current

Amperes

Differential Current

Time (Seconds)

DC Saturation Factors Large DC Time Constant Large Burden Low CT Kneepoint Voltage High Remanent Flux

Copyright ATG Consulting 2003

19

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 SurveyRemanent flux in % of saturation 0 - 20 21 - 40 41 - 60 61 - 80 Percentage of cts 39 18 16 27

Copyright ATG Consulting 2003

20

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

0% Remanent FluxPrimary Current Secondary Current

Amperes

Time (Seconds)

Copyright ATG Consulting 2003

21

50% Remanent FluxPrimary Current Secondary Current

Amperes

Time (Seconds)

75% Remanent FluxPrimary Current Secondary Current

Amperes

Time (Seconds)

Copyright ATG Consulting 2003

22

Remanent Flux ResultsRemanent flux 0% 50% 75% Time-to-saturation 1+ cycles 1/2 cycle 1/3 cycle

IEEE Guide for the Application of Current Transformers Used for Protective Relaying Purposes C37.110-1996

Copyright ATG Consulting 2003

23

CT Classification

CT Accuracy Class ANSI defines accuracy rating classes by a letter and number C100, C800 or T100, etc. Letter designates how the accuracy can be determined Number designates the minimum secondary terminal voltage under a standard burden

Copyright ATG Consulting 2003

24

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 LH 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

Copyright ATG Consulting 2003

25

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

CT Knee Point Voltages45o Tangent A B 300:5 2000:5

VS

IS

Copyright ATG Consulting 2003

26

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

Copyright ATG Consulting 2003

27

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)

Copyright ATG Consulting 2003

28

Example - Equivalent CircuitIP VS = 500 IS = 100 A IE = 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)

Copyright ATG Consulting 2003

29

Examples Determine Accuracy Class Selecting CT Ratings Calculating Time to Saturation

Avoiding CT SaturationVX > IS ZTB (1 + X/R)VX = saturation voltage IS = secondary current ZTB = total ct secondary burden

Copyright ATG Consulting 2003

30

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

Copyright ATG Consulting 2003

31

Transient Response of Current TransformersPower Systems Relaying Committee

Time to Saturate Equation

VK I F R TB

( I - KR )

=

TCT TST CT - T S

{

e

-t TCT

-e

-t TS

}

+1

Copyright ATG Consulting 2003

32

VKVK 2000:5 Tangents Intersect 300:5

VS

IS

Saturation ParametersR TB = RCT + R LEADS +R DEVICES I F = FAULT CURRENT (SEC RMS AMPS) KR =

{

.5 - .75 IRON CORE .1 AIR GAP

= 377L TCT = M R TB

Copyright ATG Consulting 2003

33

VM & I M45o Tangent VM 300:5 2000:5

VS

IM

IS

CT InductanceLM TCT = RTB VM

XM =

IMXM

LM =

Copyright ATG Consulting 2003

34

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

Copyright ATG Consulting 2003

35

Voltage WaveformFIA

0

45

90 4.16

135 180 225 270 345 360 8.33 12.5

Degrees

60 Hz

0

16.67 Time ms

Voltage WaveformFIA

0

45

90 5

135 180 225 270 345 360 10 15

Degrees

50 Hz

0

20 Time ms

Copyright ATG Consulting 2003

36

Power SystemR L

G

Z = R2 + X2 = ARCTAN ( L / R)

Power SystemR L

G

= Characteri