day 1 part 2 cts and vts

HV Power Seminar Nov 2009 1

Current and Voltage

Energy Sector© Siemens AG 2008

Voltage TransformersPerformance Requirements


2

1

12 NN

⋅=′ IIR1jX1 jX2 R2I1 I2

Zb

S2P2Ideal CT

S1P1 Im

N1 N2 ZmU2

Equivalent current transformer circuit

Energy SectorEnergy Automation© Siemens AG 2008

X1 = Primary leakage reactanceR1 = Primary winding resistanceX2 = Secondary leakage reactanceZm = Magnetizing impedanceR2 = Secondary winding resistanceZb = Secondary load

Note: Normally the leakage fluxes X 1 and X2 can be neglected


Current transformer, simplified equivalent circuit

jX2

i2 ZBi1

1 : N2

i

R2

1'1

1I

NI ⋅=


R2

RB

i2

LW

L2<< LW

im

12

1 IN

I ⋅=


N1

I1

N2

I2

εεεε

I1. N1

N2

Current transformer:Phase displacement ( δδδδ) and current ratio error ( εεεε)


I2

Zb

δδδδ

Im


Accuracy classCurrent errorat nominal current (In)

Angle error δδδδat rated current

In

Total error at n x In(rated accuracy limit)

IEC 60044-1 specifies the following classes:

Current transformer, Standard for steady-state performance


current (In) In (rated accuracy limit)

5P

10P

±±±± 1 %

±±±± 3%

±±±± 60 minutes 5 %

10 %


Current transformers, Standard for transient performance

IEC 60044-6 specifies four classes (1992/3):

Class

Ratio error Angle error

Maximum error atrated accuracy limit

TPX(closed iron core)

± 30 min± 0,5 % %10ˆ ≤ε

Error at rated current Remanence

no limit


TPYwith anti-remanence

air gap

TPZlinear core

± 60 min

± 180 ± 18 min± 1,0 %

± 1,0 %

only)current (a.c.

%10ˆ ≤ε

%10ˆ ≤ε

TPSclosed iron core

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

< 10 %

negligible

No limit


Accuracy classCurrent errorat nominal current (In)

Angle error δδδδat rated current

In

Total error at n x In(rated accuracy limit)

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

Current transformer, Standard for steady-state performance


current (In) In (rated accuracy limit)

5PR

10PR

±±±± 1 %

±±±± 3%

±±±± 60 minutes 5 %

10 %

The remanence factor Kr shall not exceed 10%s

rrK

ψψ⋅=100


IEC 60044-1 (Edition 1.2 2003) specifies the Class PX:

Current transformer, Class PX

•Rated turns ratio: turns ratio error < 0.25%

•Rated knee point emf.

•Max exciting current at rated knee point emf. And/or at a stated percentage thereof


•Max exciting current at rated knee point emf. And/or at a stated percentage thereof

•Max resistance of secondary winding


Current transformer saturation

Steady-state saturation with AC current


Transient saturation with offset current


CT saturationCurrents and magnetising

saturation flux

t

IP

φφφφ


saturation flux

t

t

IS


ΙP primary current

TN

B

total flux

t

d.c. component )(ω

1 TSt

TNt

SN

SN

~

−−−

−⋅⋅+= eeTTTT

B

Bˆ

TNTSTS

S

NS

Maxω1

~

−

⋅⋅+=TT

TBB

TTT

Course of CT-flux during off-set short-circuit curr ent


a.c. flux

transient d.c. flux

BMax

B~

tB~

N

S

NS

SN

Max B nTT

lTTTT

t ⋅−

= ⋅

δtanω

1Bi

W

S ⋅=

+=

RRL

T

[ ]

[ ] ms10900

min

S δ=TFor 50 Hz:


CT transient over-dimensioning factor K TF

KTF

TS [ms]

30

40

50

60

500

1000

5000

∞∞∞∞ (KTF ≈≈≈≈1+ωωωωTN)

Closed iron core

NS

S

TTT

S

NSTF ω1

−

+=TTTK


50 100 150 200

10

20

30

100

250

Linear core

TN = network time constant(short-circuit time constant)

TS = CT secondary time constantTN [ms]


CT with closed iron core,Over-dimensioning factor for a specified time to sa turation (T M)

15

20

K'TF

TM → ∞

16

N

NNTF 11

RX

TK' +=ω+=


5

10

7

TM = 35 ms

TM = 25 ms

50 100 TN [ms]

+= NM

TT

- NTF e-11 TK' ω


Current transformer magnetising curve and point of remanence

BI II III

up to 80%

< 10%

negligible


H = im ⋅ w

I: closed iron core (TPX)

II: core with anti-remanence air-gaps (TPY)

III: Linearised core (TPZ)

negligible


Standards of voltage transformers

Classdesignation

5P

3P

Voltage error FU Angle error δPermissible error at 0.05 ·UN and 1.0 · UN

± 3.0 %

± 6.0 %

120 minutes

240 minutes

VT classes to IEC 60044-3

VT classes for measurement and protection VDE 0141, Teil 3


Classdesignation at 1.0 · UN at 0.05 ·UN at 1.0·UN at 0.05 ·UN

0.10.2

0.5

13

0.1

0.2

0.5

1

3

1.01.0

1.0

2.0

6.0

10

5

2030

40

40

40

40

80

120

Permissible voltage error in % Permissible angle error in minutes


Capacitive voltage transformer,Equivalent circuit

ZBLF CF

RF 100 V

3

20 kV

3

400 kV

3C1 = 5 nF

C2 = 95 nF


R'BLF CF

RF

LTRTCT

CE = C1 + C2

RE

LO

U*1 = UP ⋅C1

C1 + C2U*2 L'B


Time Ts

in ms

Ratio

Classes

3PT16PT1

3PT26PT2

3PT36PT3

10 --- ≤ 25 ≤ 4

20 ≤ 10 ≤ 10 ≤ 2

40 ≤ 10 ≤ 2 ≤ 2

%100U2

)t(U

s

s ⋅⋅

t

Measured voltage transients after fault at voltage maximum and zero-crossing (Example)

Transient performance of CVTs, Recommendations acc. to IEC 60044-5


Legend:

UP(t) Primary voltage

US(t) Secondary voltage

(1) Fault inception

(2) Aperiodic damping of US(t)

(3) Periodic damping of US(t)

40 ≤ 10 ≤ 2 ≤ 2

60 ≤ 10 ≤ 0.6 ≤ 2

90 ≤ 10 ≤ 0.2 ≤ 2

t

Recommendations to IEC 60044-5


BCT

BNCT

B

BNiALF

i RRRR

PPPP

ALFALF'++⋅=

++⋅=

rated CT burden: PBNinternal burden of the CT: Pi=Ri ⋅ I2N

2

Actual connected burden : PB= RB ⋅ I2N2

RB=Rl+RR= burden resistanceRl= resistance of connecting cablesRR= burden resistance of the relay

BNCT

BCT

BNi

BALF'i

RRRR

PPPP

ALF'ALF++⋅=

++⋅=

KIK' ⋅≥ALF No saturation XBwith

CT dimensioning

Theory:


N

KOD I

IK' ⋅≥ALF No saturation

for the totalshort-circuit duration:

N

NN

~

MaxTF 11

RX

TB

BK' +=+== ω

ˆ

−

−⋅⋅+=

−−S

M

N

M

SN

SN Tt

Tt

(ω

1 eeTTTT

TFK ''

No saturation for the specified time tM:mTFOD KKK Re⋅≥

100Remanence %

1

1Re

−=mK

with

Practice:

Theory:

by vendors drecommende values-K of Use TF

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


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

2KTF ≥Close-in fault

50msTif2K NTF <≥

100msT50msif3K NTF <≤≥

10KTF ≥

7SA510 / 511Close-in fault

Fault atbalance point


7SA513

2KTF ≥

5KTF ≥

s50mTif10K NTF ≥≥

Close-in fault

7SA6 / 7SA522

30ms)T if 1( 2K NTF <≥≥

50msTif5K NTF <≥


Close-in fault



Operating timems

140

120

100

80

TN = 100 ms K=10

K= 5

K= 4

7SA6 / 7SA522: Operating time with CT saturationInfluence of CT dimensioning


80

60

40

20

0

60 1004020 80 % Z1

K= 3

K= 2

K= 1

day 1 part 2 cts and vts

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