Day 1 Part 2 CTs and VTs

Current and Transformers Performance Requirements © Siemens AG 2008 Energy Sector

Equivalent Equivalent current transformer circuit

I 1

jX1

R1

I 2′ = I 1 ⋅

N1 N2

jX2 I m

P1 N1

N2 U

I 2

R2

S1

Zm

Zb

2

P2

S2

Ideal CT

X1 = Primary leakage leakage reactance R1 = Primary winding resistance X2 = Secondary leakage reactance Zm = Magnetizing impedance R2 = Secondary winding resistance Zb = Secondary load Note:

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Normally the leakage fluxes X1 and X2 can be neglected

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© Siemens AG 2008 Energy Sector

Current transformer, transformer, simplified equivalent circuit

1 : N2

'

1 N 2

⋅ I

L2<< LW

LW

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R2

i2

i1

I =

jX2

im

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ZB

R2

i2

RB


Current transformer: Phase displacement (δ (δ) and current ratio error (ε (ε) I 1

I 1

. N1 N2 ε

N1 I 2

N2 I 2

δ Zb

I m

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Current transformer, transformer, Standard for steady-state steady-state performance

IEC 60044-1 specifies the following classes: classes:

Accuracy class

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Current error at nominal

5P

±1%

10P

± 3%

Angle error δ at rated current n

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± 60 minutes

Total error at n x In rate rated d accu accura rac c limi limitt

5% 10 %


Current transformers, Standard for transient performance IEC 60044-6 specifies four classes (1992/3):

Class

Error at rated current Ratio error

TPX

Maximum error at rated accuracy limit

Angle error

± 0,5 %

± 30 min

ε ˆ ≤ 10%

with anti-remanence anti-remanence air gap

± 1,0 %

± 60 min

ε ˆ ≤ 10%

TPZ

± 1,0 %

± 180 ± 18 min

(closed iron core)

TPY

linear core

TPS closed iron core

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Remanence

ε ˆ ≤ 10% (a.c. current only)

Special version for high impedance im pedance protection (Knee point voltage, internal secondary resistance)

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no limit < 10 %

negligible

No limit


Current transformer, transformer, Standard for steady-state steady-state performance

IEC 60044-1 (Edition 1.2 2003) specifies the following classes for low remanence:

Accuracy class

Current error at nominal

5PR

±1%

10PR

± 3%

Angle error δ at rated current n ± 60 minutes

The remanence factor K r shall not exceed 10%

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Total error at n x In rate rated d accu accura rac c limi limitt

5% 10 %

K r = 100 ⋅

r

ψ s


Current transformer, Class PX

IEC 60044-1 (Edition 1.2 2003) specifies the Class PX: •Rated turns ratio: turns ratio error < 0.25% •Rated knee point emf. •Max exciting current at rated knee point emf. And/or And/or at a stated percentage thereof •Max resistance of secondary winding

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Current transformer saturation

Steady-state saturation with AC current

Transient saturation with offset current

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CT saturation Currents and magnetising I P

t

φ

t I S

t

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Course of CT-flux during off-set short-circuit current Ι P primary current d.c. component

B

ˆ B ~ t TN

= 1+

B Max B ~

total flux

ω ⋅T N ⋅T S

T N − T S

−

(e

t TN

− e

 T N    T S 

= 1 + ω ⋅T S ⋅  

−

t TS

)

TS TS −TN

B

t B Max =

BMax

transient d.c. flux

B~

T S =

a.c. flux t B~

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N

S

⋅

T S − T N LW

R i + R B

For 50 Hz:

=

⋅ l n

S

T N

1 ω ⋅ tanδ

T S =

10900 [ms] δ [min ]


CT transient over-dimensioning factor KTF KTF TS

60

 T N  T K TF = 1+ ωT S  T S   

∞ (KTF ≈ ≈1+ω 1+ωTN)

S − TN

5000

Closed iron core

50 1000

40

TS [ms] 500 250

20 100

Linear core

10

TN = networ network k time const constant ant (short-circuit time constant) 50

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10 0

1 50

200

TN [ms]

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TS = CT secondary time constant


CT with closed iron core, Over-dimensioning Over-dimensioning factor for a specified time to saturation (TM) K'TF TM → ∞

20 16

K' TF = 1+ ωT N = 1 +

X N R N

15

10

TM = 35 ms TM = 25 ms

7

 

TM 



K' TF = 1 + ω T N 1-e TN  

-



5

50

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100 TN

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[ms]


Current transformer magnetising curve and point of remanence I

II

III

B up to 80%

< 10% negligible

H = im ⋅ w I: closed iron core (TPX) II: core with anti-remanence anti-remanen ce air-gaps (TPY) III: Linearised core (TPZ)

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Standards of voltage transformers VT classes to IEC 60044-3 Class designation

Permissible error at 0.05 ·U N and 1.0 · U N Voltage error FU

Angle error δ

3P

± 3.0 %

120 minutes

5P

± 6.0 %

240 minutes

VT classes for measurement and protection VDE 0141, Teil 3 Class designation

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Permissible voltage error in % at 1.0 · UN

at 0.05 ·UN

0.1

0.1 0.2

0.2

1.0 1.0

0.5

0.5

1.0

1 3

1 3

Permissible angle error in minutes at 1.0·UN

at 0.05 ·UN

5

40 40

2.0

10 20 30

6.0

40

120

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40 80


Capacitive voltage transformer, Equivalent circuit 400 kV C1 = 5 nF

C2 = 95 nF

3

RF

20 kV LF

3

C E = C 1 + C 2 C 1 U *1 = U P ⋅ C 1 + C 2

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100 V

CF

3

ZB

LO RE

CT

RF RT

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LT

LF

CF

U*2

R'B L'B


Transient performance of CVTs, Recommendations acc. to IEC 60044-5 Measured voltage transients after fault at voltage maximum and zero-crossing (Example)

t

Legend: UP(t) Primary voltage

Ratio Time Ts in ms

U s (t ) 2 ⋅ Us

⋅ 100%

Classes

10

3PT1 6PT1 ---

3PT2 6PT2 ≤ 25

3PT3 6PT3 ≤4

20

≤ 10

≤ 10

≤2

40

≤ 10

≤ 2

≤2

60

≤ 10

≤ 0.6

≤2

90

≤ 10

≤ 0.2

≤2

Recommendations to IEC 60044-5

US(t) Secondary voltage

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(1)

Fault in incepti ption

(2) (2)

Aper periodi iodicc dam damping ping of US(t)

(3) (3)

Peri eriodic odic dam damping ping of US(t)

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CT dimensioning ALF' = ALF ⋅

ALF = ALF' ⋅

with

P i + P BN P i + P B

=

P i + P B = P i + P BN

ALF ' ≥ K OD ⋅

ALF ⋅

ALF'⋅

IN

K OD ≥ K TF ⋅ K Re m K Re m =

1−

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1 % Remanence

R CT + R BN R CT + R B

R CT + R B

P BN

Actu Actual al conn connec ecte ted d burd burden en :

P B= R B ⋅ I 2N2

P i=R i ⋅ I 2N2

RB=Rl+RR= burd burden en resis resista tanc nce e Rl= resi resist stan ance ce of conn connec ecti ting ng cabl cables es RR= burd burden en resi resist sta ance nce of the the rela relay y

R CT + R BN

Theory:

rate rated d CT burd burden en:: inte intern rnal al burd burden en of the the CT: CT:

No saturation X N Max = 1 + ω T N = 1 + K' TF = for the total ˆ R N B ~ short-circuit duration:

  − − No saturation for ω ⋅ T N ⋅ T S TN − e TS   ' ' = + 1 ( e K the specified time tM: TF tM

 

T N − T S

tM

 

Note: this equation is not fully valid for very small tM (less than 1 cycle)

Practice: Use of K - values recommended by vendors TF

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Distance relays, practical CT requirements Transient over-dimensioning over-dimensioning factors KTF (ARC not considered)

Close-in fault

7SA510 / 511 Fault at balance point

Fault at balance point

Close-in fault

7SA6 / 7SA522

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Fault at balance point

if T N < 50ms

K TF ≥ 3

if 50ms ≤ T N < 100ms

K TF ≥ 10

Close-in fault

7SA513

K TF ≥ 2

TF

K TF ≥ 5

K TF ≥ 10 K TF ≥ 2

if T N < 50ms

if T N ≥ 50ms ( ≥ 1 if T N < 30ms)

K TF ≥ 5

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7SA6 / 7SA522: Operating time with CT saturation Influence of CT dimensioning

Operating time ms

140 120 K=10

TN = 100 ms 100

K= 5

80

K= 4 K= 3

60

K= 2 40

K= 1

20 0 20

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40

60

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80

100

% Z1


Day 1 Part 2 CTs and VTs

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