Ct Dimensioning

How to select CT’s for modern protection relays?

The Siemens Answer: CT Dimensioning Innovation for Generations

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CT – Measuring vs Protection Measuring CT

Protection CT

• Under over current, the CT should limit the secondary current to avoid thermal overload of the connected equipment

• In case of over current, the CT should maintain the secondary current within the rated limit to enable proper protection operation

• At rated burden (cosφ = 0.8) and rated security factor, the composite error should be > 10%

• At rated burden (cosφ = 0.8) and rated accuracy limit factor, the composite error should be less than 5% or 10%

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CT – Standards • IEC/AS 60044-1: Instrument transformers - Part 1 Current transformers (ed. 1.2 – 2003) • IEC/AS 60044-6: Instrument transformers - Part 6 – Requirements for protective current transformers for transient performance • IEC/AS 60044-8: Instrument transformers -Part 8 Electronic current transformers • AS 1675: Current Transformers – Measurement & Protection (now superseded) • BS7626 (BS 3938): Specification for current transformers (now superseded)

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CT – AS 60044 - 1 – Class P Rated Composite Error at ALF (turn ratio error, iron / copper losses, saturation)

AS 60044:

5P20 5VA

AS 1765:

5P 100 F20

Rated Secondary Reference Voltage Vref (the RMS secondary voltage upon which the performance is based)

Conversion: Vref ⋅ I sec = VA ⋅ ALF Copyright ©

IEC/AS 60044-1 / BS3938: multiple of rated current at which, with rated burden, will maintain the accuracy IEC/AS 60044-1 / BS3938 specify Rated Burden

Accuracy Limit Factor (ALF) (the factor which, when applied to at rated primary current, gives the accuracy limit primary current)

Siemens. Innovation for generations.

CT – AS 60044 - 1 – Class PX (similar to BS 3938 Class X AS 1675 Class PL)

0.05PX 450 R3 10VA Kx Rb Max. exciting current (Iexc) at rated knee point voltage

Dimensioning Rated resistive factor (Kx) burden (Rb)

Rated knee point Max. secondary winding o voltage (Ek or Vk) resistance (Rct) at 75 C

Turn ratio error within ± 0.25% AS 1675: or ±50/Ns whichever the greater, where Ns = number of secondary turns Copyright ©

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CT Dimensioning Operational Accuracy Limit Factor – ALF’ ALF' = ALF ⋅

Pi + PN Ri + RN = n⋅ Pi + PB Ri + RB

P + PB Ri + RB ALF = ALF' ⋅ i = ALF'⋅ Pi + PN Ri + RN

Rated CT burden: Internal CT burden:

PN Pi=Ri ⋅ I2N2

Actually connected burden :

PB= RB ⋅ I2N2

with: RB=RL+RR= total burden resistance RL= resistance of connecting cable RR= relay burden resistance Copyright ©

Siemens. Innovation for generations.

CT Dimensioning Operational Accuracy Limit Factor – ALF’ I ALF ' ≥ K OD ⋅ SC IN Where KOD is the total over-dimensioning factor:

K OD ≥ K TF ⋅ K Rem K Rem

% remanence = 1+ 100

Theory: No saturation during BMax XN K' TF = = 1 + ω T N = 1 + total fault duration: ˆ

B~

No saturation for the specified time tM:

RN

tM  tM  − − ⋅ ω T N ⋅ TS K ' 'TF = 1+ (e TN − e TS  TN − TS    

Practice:

K OD = KTF Remanence only considered in extra high voltage systems (EHV) KTF-values acc. to relay manufacturers‘ guides Copyright ©

Siemens. Innovation for generations.

CT Dimensioning Choice of CT rated power • Rated burden > total connected burden • For GIS and MV switchboard with modern relays/measuring devices, class P 1A CTs with small rated burden are sufficient, e.g. 5VA, 10VA Choice of Accuracy Limit Factor • Relay manufacturer’s recommendation should be followed + engineering secuirty factor • CT performance depends on operational ALF

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CT Dimensioning Choice of CT Ratio • CT ratio should be based on the protected object, however, it may need to be go higher: • limitation of difference in CT ratio for low impedance differential protection as recommended by the relay manufacturer • Avoid excessive over-dimensioning: • Imax, switchgear / Ipri-rated > 100 ! • Such CT can damage the relay because there is no CT saturation

If a more precise estimation of the CT performance is necessary, then RCT and the magnetising curve is required.

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CT Dimensioning Choice of Measuring CT • For classes 0.1 – 1, the accuracy is based on a total connected burden between 25% to 100% of the rated burden • For classes 0.1, 0.2 and 0.2S with rated burden < 15VA, an extended range of burden can be specified (from 1VA to 100% of the rated burden) • For classes 3 and 5, the accuracy is based on a total connected burden between 50% to 100% of rated burden • Instrument security factor (FS) defines the maximum secondary current that will flow under short circuit of primary circuit • Instrument security factor (FS) is based on the rated burden • Under-burdened measuring CT can damage connected instrument during short circuit or overload of the primary circuit

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CT Dimensioning Overcurrent Protection CT saturation is allowable provided the remaining fundamental current is higher than I>> setting

Isec, fundamental > I >

Isec,funfamental > I >> Pn + PCT I >> ALF' = ALF ⋅ > P'+ PCT In

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CT Dimensioning Differential Protection Differential protection should be stable on external faults High sensitivity for internal fault Protected Object

Relay Type

Requirement for Internal Faults

Requirements for External Faults

min. saturation free time (ms)

min. over dimensioning KTF

min. saturation free time (ms)

min. over dimensioning KTF

Generator

7UM6

4

0.75

5

1.20

Transformer

7UT6

4

0.75

5

1.20

O/H Line, Cable with digital communication

7SD52

3

0.50

5

1.20

7SD61

3

0.50

5

1.20

O/H Line, Cable with pilot wires

7SD600

5

1.20

1

1.00

Busbar

7SS52

3

0.50

3

0.50

7SS600

4

0.75

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CT symmetry

0.75

4 0.75 Siemens. Innovation for generations.