Zhihan Xu, Matt Proctor, Ilia Voloh

- GE Digital Energy

Mike Lara - SNC-Lavalin

Presented by: Terrence Smith GE Digital Energy

CT fundamentals – Circuit model, excitation curve, simulation model

CT saturation – AC saturation, DC saturation

Simplified CT saturation analysis

Effects of CT saturation on 87L

Techniques to improve CT saturation tolerance for 87L applications

CT saturation analysis tool for 87L

2

Like any other kind of transformer, consisting of two windings magnetically coupled by the flux in a saturable steel core

Draw an exciting current to keep the core excited

Experience copper losses, core losses, eddy current losses and leakage flux

N1 N2

IP IST

ISIEVs XE RE VB

XS RS

ZB

Ideal CT

3

More practical way to represent the CT steady-state performance

10-5

10-4

10-3

10-2

10-1

100

101

10-2

10-1

100

101

102

103

Exciting Current (A, rms)

Excitin

g V

olta

ge

(V

, rm

s)

Knee point voltage (148V) per IEEE

Saturation voltage

CT Ratio: 2500/5A

Secondary resistance: 0.782 ohm

Knee point voltage (157V)

per IEC

Provided by manufacturers

Verified during field tests

4

IEEE PSRC model

Assume the single-valued saturation curve

Ignore curve in the below-knee-point region

Actual curve

Simplified Curve

10A

Excitation Current (A, rms)

Exci

tati

on V

olt

age

(V, rm

s)

VsSlope=1/S

5

Steady state saturation

Caused by the symmetrical current with no DC component

0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05-1.5

-1

-0.5

0

0.5

1

1.5

Ti me ( s)

Current (pu of fault)

Saturated Secondary

Current (15-65kA)

Ratio Current

CT Ratio: 800/5

Burden: 2 ohms

Vs@10A: 303 V

DC offset: 0%

SSX ZIV

To avoid AC saturation

6

Transient saturation

Caused by DC component in the fault current, unipolar half wave current or remnant flux in the CT

To avoid DC saturation

0.02 0.03 0.04 0.05 0.06 0.07 0.08

-1

-0.5

0

0.5

1

1.5

2

2.5

Ti me ( s)

Current (pu of fault)

Saturated Secondary

Current (15-65kA)

Ratio Current

CT Ratio: 800/5

Burden: 2 ohms

Vs@10A: 303 V

DC offset: 100%

)1(R

XZIV SSX

7

High primary fault current

Excessive secondary burden

Heavy DC offset in current

Large percent remanence

8

Nonlinear exciting current during saturation affects the performance of current-based protection elements

0.02 0.04 0.06 0.08 0.1 0.12 0.14-1.5

-1

-0.5

0

0.5

1

1.5

2

Time (s)

Sa

tura

ted

Cu

rre

nt a

nd

Ma

gn

itu

de

(p

u o

f fa

ult)

Ip=15kA

Ip=65kA

0.02 0.04 0.06 0.08 0.1 0.12 0.140

20

40

60

80

100

120

Time (s)

An

gle

Sh

ift (d

eg

ree, le

ad

ing)

Ip=15kA

Ip=65kA

9

Exact same samples during unsaturation

Saturated current samples are zero during saturation

Saturation is repeated each half cycle with the same pattern

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035-1.5

-1

-0.5

0

0.5

1

1.5

Ti me ( s)

Current (pu)

Ideal Current

Saturated Current

No dc offset

The time to saturation longer than half cycle is not considered since 87 function operates at high speed

10

-0.01 -0.005 0 0.005 0.01 0.015 0.02 0.025 0.030

0.2

0.4

0.6

0.8

1

Time (s)

Ma

gn

itu

de

(p

u)

Ideal Current

Saturated Current

-0.01 -0.005 0 0.005 0.01 0.015 0.02 0.025 0.03-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Time (s)

An

gle

(d

eg

ree

)

Ideal Current

Saturated Current

Time to saturation: 2.86 ms @ 60Hz

Magnitude: 0.3 pu of ideal

Angle: leading ideal by 51.3° 11

Adjust time to saturation from 0 to 0.5 cycle M

ag

nitu

de

(α

, p

u o

f id

ea

l)A

ng

le S

hift fro

m Id

ea

l (β, d

eg

ree

, lea

din

g)

RM

S (

pu

of id

ea

l)

5 10 15 20 25 300

0.2

0.4

0.6

0.8

1

Time to saturation (1/64 cyc)

5 10 15 20 25 300

20

40

60

80

100

12

SATIDEALSATSAT tItI

cyclet

tt

tt

ttt

SAT

SATSAT

SATSAT

SATSATSAT

5.0,0

)12.972sin(01394.0)12.972cos(04234.0

)648.8sin(1621.0)648.8cos(3008.0

)324.4sin(3429.0)324.4cos(6565.0916.0

cyclet

tt

ttt

SAT

SATSAT

SATSATSAT

5.0,0

)466.9sin(4704.0)466.9cos(392.6

)733.4sin(84.49)733.4cos(52.3655.61

13

Consider internal faults in a two-terminal line

Traditional percentage differential plane is analyzed

))(()(

SATPSATR

PL

tKItI

II

γ is an angle difference tolerance factor

K is a magnitude difference tolerance factor Different fault current level at the remote end

Different CT performance between two CTs

Model difference between simplified saturation and real saturation

Other errors caused by DC offset, asymmetrical saturation, etc.

14

87L operates correctly if

15

))(()(1

))(()(

SATSATP

SATPSATPRLDIFF

tKtI

tKItIIII

KtI

KItIIII

SATP

PSATPRLRES

)(1

)(

2

1

)()cos(211

2

SLPK

KK

K

KKDF

1

)()cos(21 2

Define Dependability Factor

16

87L correctly operates on internal faults if DF > 0.7, that is, SLP2 need to be set less than 0.7

γ (degree)

Time to saturation (1/64 cyc)

0

20

5 10 15 20 25 30

0.86

0.88

0.9

0.92

0.94

0.96

0.98

1

De

pe

nd

ab

ility

Fa

cto

r

K=1

γ (degr

ee)

Time to saturation (1/64 cyc)

-20

0

205 10 15 20 25 30

0.7

0.75

0.8

0.85

0.9

0.95

1

De

pe

nd

ab

ility

Fa

cto

r

K=10

17

)180)(()(

SATPSATR

PL

tKItI

II

))(()(1

)180)(()(

SATSATP

SATPSATPRLDIFF

tKtI

tKItIIII

KtI

KItIIII

SATP

PSATPRLRES

)(1

)(

87L misoperates if

1

12

)()cos(21

12

)(1 2

KSLP

KK

KSLP

K

K

KKSF

1

)()cos(21 2

Define Security Factor

18

There exists misoperation if time to saturation is less than 2.6 ms when SLP2 is set to 0.7

5 10 15 20 25 300

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Ti me t o sat ur at i on ( 1/ 64 cyc)

Security Factor

K=0.9

K=1.0

K=1.1Misoperations

SLP2=0.7

19

100% Line

Operating Zone

Restraint Zone

Ip=6pu Ip=8pu

tSAT -> 0

0 2 4 6 8 10 12 14 160

2

4

6

8

10

12

14

16

Restraint Current (pu)

Diffe

ren

tia

l C

urr

en

t (p

u)

tSAT -> 0.5 cyc

Misoperations

PKP=0.3

BP=3

SLP1=30%

SLP2=60%

Add a portion of current distortions such as harmonics, saturated CT signal and noise, into the restraint signal; therefore, the restraint region is adaptively increased.

Dynamically switch the differential settings to more secure values to deal with external faults. Normally, the more secure settings would result in the larger restraint region.

Constantly use the transient bias as the additional restraint signal. A delta signal is mixed into the transient bias to increase the restraint signal.

20

Adaptive restraint dynamically adjusts the operating-restraint boundary.

Adaptive decision process is based on an on-line computation of measurement error.

Squared differences between the actual waveform and an ideal sinusoid is a measure of a “goodness of fit” (a measurement error)

21

Dynamically increase the weight of the adaptive restraint portion in the total restraint quantity, but for external faults only

22

arg(ILOC/IREM)=0

(internal fault)

MULT=1

MULT=1

MULT=abs(arg(ILOC/IREM)×5/180

arg(ILOC/IREM)=180(external fault)

MULT=5

23

Total restraint = Traditional restraint

+ Weight factor * Adaptive restraint

Imaginary (ILOC/IREM)

Real (ILOC/IREM)

OPERATE

REST.

Error factor is high

Error factor is low

It is necessary to develop a type of CT saturation analysis tool because

Utility customers are looking for the relay manufacturer recommendations and warranties for the CT selection at their system with particular relay models.

CT selection recommendations are different from one manufacturer to another and there cannot be any standard giving specific recommendations.

24

A practical CT saturation analysis tool is required to

seamlessly incorporate the CT performance and relay system application

utilize the commonly recognized CT model and CT saturation calculation algorithm

simulate the analog/digital signal processing and data calculations exactly existing in the line current differential relay

25

A practical CT saturation analysis tool is able to Analyze reliability of 87L during CT saturation

Evaluate the differential relay security

Investigate the effect of adjusting 87L settings

Choose the proper size of CT

Examine possibility of reducing CT requirement

26

27

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Based on the simplified saturation analysis, The saturation caused by internal faults will rarely result in the failure to operate

The saturation caused by external faults introduces a spurious differential current that may cause 87L to misoperate.

Adaptive restraint technique used in 87L to tolerate CT errors, reduce CT requirement and improve relay security

Necessity, function requirement, and example of a practical CT saturation analysis tool

29