CT Residual Magnetism

8/19/2019 CT Residual Magnetism

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23 January 2012

Residual Magnetism

Will Knapek


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AGENDA

> What is Residual Magnetism

> Ways to Reduce Remanence

> Determining the residual magnetism in a field test

> Summary

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Significance of Residual Magnetism

It has been said that one really knows very little

about a problem until it can be reduced to figures.

One may or may not need to demagnetize, but until

one actually measures residual levels of

magnetism, one really doesn’t know where he or

she is.

One has not reduced the problem to figures.

R. B. Annis Instruments, Notes on Demagnetizing

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Physical Interpretation ofResidual Magnetism

res

t

C d V t 0

)()(

• When excitation is removed from the CT, some of the magnetic

domains retain a degree of orientation relative to the magnetic

field that was applied to the core. This phenomenon is known as

residual magnetism.

• Residual magnetism in CTs can be quantitatively described by

amount of flux stored in the core.


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Significance of Residual Magnetism

WHY DO I CARE????

Bottom Line: If the CT has excessive Residual

Magnetism, it will saturate sooner than expected.

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Magnetization Process and Hysteresis

*Picture is reproduced from K. Demirchyan et.al.,Theoretical Foundations of Electrotechnics


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Remanence Flux (Residual

Magnetism)

© OMICRON Page 7

*Source: IEEE C37.100-2007

Remanence is dissipated verylittle under service conditions.

Demagnetization is required to

remove the remanence.

When excitation stops,Flux does not go to zero


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Residual Remanence and Remanence Factor

%100*S

resr M

• Saturation flux (Ψs)that peak value of the flux which would exist in a

core in the transition from the non-saturated to the

fully saturated condition and deemed to be that

point on the B-H characteristic for the core

concerned at which a 10 % increase in B causes H

to be increased by 50 % (IEC 60044-1, 2.3.6)

• Remanent flux (Ψr)that value of flux which would remain in the core 3

min after the interruption of an exciting current of

sufficient magnitude to induce the saturation flux

(Ψs) (IEC 60044-1, 2.3.7)• Remanence factor (Kr)the ratio Kr = 100 × Ψr / Ψs, expressed as a

percentage

(IEC 60044-1, 2.3.8).

• Residual remanence (Mr)the ratio Mr = 100 × Ψres / Ψs, expressed as a

percentage.

* Picture from CT-Analyzer User Manual


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Residual Magnetism• Maximal Remanence Flux Ψr-max (physically)

- the flux which remains after magnetization of the core to total saturation

and removal of this magnetizing current

• When is the “total saturation” achieved? How defined? or Specified?

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Residual Magnetism• Remanence Flux Ψr (IEC 60044-1)

that value of flux which would remain

in the core after the interruptionof an exciting current of sufficient magnitude

to induce the saturation flux Ψs

Page 10

• Saturation flux Ψs (IEC 60044-1)

that peak value of the flux which would exist

in a core in the transition from the non-saturatedto the fully saturated condition ( Knee point)


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Residual Magnetism

Page 11

Ipn

Flux density (B)

Flux intensity (H)

20 x Ipn


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Residual Magnetism

Page 12

20 x Ipn

residual magnetism


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Residual Magnetism

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Ipn

~10 x Ipn reserve


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Residual Magnetism1 Ip`Current of an ideal current transformer

2 Is

Current of a saturated current transformer

The difference I= Ip‘-Is is the current floating through the saturated inductance.

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Residual Magnetism• Impact due to residual remanence with “Over Current Protection” and

“Distance Protection”

• failure to operate

• unwanted operation

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Netz 1F2F1

Schutzgerät 1 Schutzgerät 2

Schutzabschnitt des Schutzgerätes 1operating section of protection unit 1

protection unit 1 protection unit 2

Main 1


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Residual Magnetism• Impact due to residual magnetism with “Differential Protection“

• no impact on inner failure

• unwanted operation in case of outer failure

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CT1

Netz 1F2F1

CT2

Schutzgerät 1 Schutzgerät 2protection unit 1 protection unit 2

Main 1


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Effect of Remanence (0%)

© OMICRON Page 17

Time to Saturate = 1.5 cyc

1200:5A

C800 CT

24,000A Ifault

X/R = 19


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© OMICRON Page 18


1200:5A

C800 CT

24,000A Ifault

X/R = 19


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© OMICRON Page 19

1200:5A

C800 CT

24,000A Ifault

X/R = 19



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Effect of Remanence

• Remanence as much as 80% of saturationflux can be expected

• Can significantly reduce the burden capability

of the CT

*Source: IEEE C37.100-2007

© OMICRON

Page 20


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Ways to Reduce Remanence

• Use different grade of steel for core (hot-rolled instead of cold-rolled steel reduces up

to half the max. remanence)

• Use Gapped Core CT (TYP Class)

© OMICRON Page 21


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Reduction of Residual Magnetism

> Hot-rolled steel canreduce the residualmagnetism to 40-50%of saturation flux

> Use of air-gappedcore: higher excitingcurrent and lowersaturation levels;

> Drawbacks: larger

and more expensivecores, loweraccuracy

> Used when stablerelay operation iscritical for systemsecurity

* Picture from Electric Power Transformer Engineering,

ed. by James H. Harlow


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Residual Flux Measurement: Cumulative Method

CT U - terminal voltage CT I - terminal current - secondary winding resistanceCT R

- core voltageC U - interlinked (core) flux - residual fluxres

nnn

nnn

res

1

121

323

212

11

ni

i

t

t

C res

n

i

t

t

C res

n

i

iresn d U d U 01

)()(11

n

n

n

n

nn

n

n

n

t

t

CT CT

t

t

CT

t

t

CT CT

t

t

CT

t

t

C

t

t

C res

d I Rd U d I Rd U

d U d U

11

1

0

1

0

1

1

0

)(*)(*2

1)(*)(

)(*2

1)(

res

1

2

3

1t

2t

3t

nn

,2

nn 1

2´ nt

1nt

nt

n

1t

Magnetic flux variation under

rectangular magnetization


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Residual Flux Measurement: Averaging Method

111

000

1 )(*)()(

t

CT CT

t

CT

t

C d I Rd U d U

*5.01res

n

i

t

t

CT CT

n

i

t

t

CT

n

i

t

t

CT CT

t

t

CT

n

i

i

i

i

i

i

i

i

i

i

d I n

Rd U n

d I Rd U n

n

22

2

2

11

11

)(*1

1*)(

1

1

)(*)(1

1

1

1

CT U - terminal voltage CT I - terminal current - secondary winding resistanceCT R

- core voltageC U - interlinked (core) flux - residual fluxres

res

1

2

3

1t

2t

3t

nn

,2

nn

1

2´ nt

1nt

nt

n

1t

Magnetic flux variation under rectangular magnetization


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Hysteresis Loop Symmetry Condition

nn 1

1

21

)()(n

n

n

n

t

t

CT

t

t

CT d I d I

1

21

)()(

n

n

n

n

t

t

CT

t

t

CT d U d U

CT U - terminal voltage CT I - terminal current - interlinked (core) flux

res

1

2

3

1t

2t

3t

nn

,2

nn

1

2´ nt

1nt

nt

n

1t

Magnetic flux variation under rectangular magnetization


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Residual Remanence and Remanence Factor (2)

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* Picture from Wikipedia

Family of hysteresis loops for grain-oriented electrical steel


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Residual Flux Measurement: Implementation Issues

• To determine residual flux it is essential to calculate

voltage and current time integrals taken over

measurement duration.

• If calculation of these integrals can be made real-time (i.e.

simultaneously with input sampling), there is no need to

store input data of current and voltage channels.

• Thus, even if saturation process is very long, it will still be

possible to calculate residual flux, which allows applying

this method to residual remanence measurement for both

CTs and transformers.

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CTA Residual Magnetism Card

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Residual Magnetism• Demagnetization process

• by applying minimum the same electrical force as the force caused the

magnetization effect.

• recommendation:

• starting with similar force as the force which drove the core into saturation

than reducing step by step to demagnetize the core

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Determining the residual

magnetism

• Determining the residual magnetism in a field test

• Analysis of the measured values

• Determining the residual magnetism with the CT Analyzer

30

D t i i th id l


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magnetism• Determining the residual magnetismin a field test

– The residual magnetism can be determined relatively preciselyusing simple test apparatus.

P1

P2

s1

s2

+

-

Transformer Battery

I0U0

CMC 256 as recorder

31

Determining the resid al


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magnetism• Determining the residual magnetismin a field test

– The test is performed in three steps• Load is applied until I0 and V0 are constant.

P1

P2

s1

s2

+

-

Transformer Battery

CMC 256 as recorder

I0V0

RCT

RH LHnP nS

Main inductance DC internal resistance

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magnetism• Determining the residual magnetismin a field test – The test is performed in three steps

• Load is applied until I0 and V0 are constant.• This is then repeated with opposite polarity.

P1

P2

s1

s2

-

+

Transformer Battery

I0V0

RCT

RH LHnP nS


CMC 256 as recorder

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magnetism• Determining the residual magnetism

in a field test

P1

P2

s1

s2

+

-

Transformer Battery

I0V0

RCT

RH LHnP nS


– The test is performed in three steps

• Load is applied until I0 and V0 are constant.• This is then repeated with opposite polarity.

• This is then repeated once more with opposite polarity.

CMC 256 as recorder

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magnetism• Analysis of the measured values – With voltage V0 applied, the current I0 increases.

The internal load of the transformer Z0 drops until V0, I0 and Z0

are constant.

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magnetism• Analysis of the measured values

– As the flux in the core increases, the main inductance LH of the

transformer changes. At maximum flux, the unsaturated

inductance LS becomes the saturated inductance LS.

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magnetism• Analysis of the measured values – The reactance XLS of the saturated main inductance LS is several

times lower than the DC internal resistance RCT of the

transformer. As such, Z0 = RCT at constant current flow.

const.Iwhen88,3 000

0 CT R I V

Z

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magnetism• Analysis of the measured values – If RCT is known, the voltage can be calculated via the main

inductance LH.

CT R L R I V V V V CT H 000

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magnetism• Analysis of the measured values – The area below the voltage VLH is the magnetic flux

[Φ in Vs] in the current transformer's core.

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magnetism• Analysis of the measured values – Approximate calculation of the areas (of the flux in the core).

VssV t V 16.64.14.41

VssV t V 5.105.22.42

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magnetism• Analysis of the measured values – Calculation of the flux via the integral of the voltage VLH.

VsdxV f s

s

LH 24.6

0,2

0,0

1

Vs72.102

Vs65.103

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magnetism• Analysis of the measured values

– Conclusions regarding the flux of the transformer at the start of

the measurement. The difference between Φ3 and Φ1 is the flux

level prior to starting the measurement.

VsVsVs 41.424.665.1013magnetismResidual

magnetismResidual31

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magnetism• Analysis of the measured values – To determine the residual magnetism, the core is fully

magnetized in the positive direction right up to saturation

(I0 = constant) prior to starting

measurements.

VsdxV f s

s

L 25.2

0.2

0.0

1

VsVsVs 4.825.265.1013magnetismResidual

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magnetism• Analysis of the measured values – The residual magnetism determined [Kr] for this core is around78.9%.

%9.7865.10

4.8%100

Vs

VsKr

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magnetism• Analysis of the measurement results – For the sake of completeness it should be mentioned, that the

saturation flux acc. IEC is reached when a 10% increase of B

causes a 50% increase of H.

saturationremanence

saturation

remanenceremanence 100

K

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Summary

> Residual Magnetism will effect how a CT performsduring a fault.

> Demag after all tests.

> May want to consider Demag after faults on criticalcircuits.

> Questions??

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Thanks for your attention.

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CT Residual Magnetism

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