Current & Voltage Transformers Performance Requirements

1. HV Power Seminar Nov 2009 1
Current and
Voltage
Energy Sector
© Siemens AG 2008
Voltage
Transformers
Performance
Requirements

2. HV Power Seminar Nov 2009 2
2
1
1
2
N
N
⋅
=
′ I
I
R1
jX1 jX2 R2
I1 I2
Zb
S2
P2
Ideal CT
S1
P1 Im
N1 N2 Zm
U
2
Equivalent current transformer circuit
Energy Sector
Energy Automation
© Siemens AG 2008
Page 2
X1 = Primary leakage reactance
R1 = Primary winding resistance
X2 = Secondary leakage reactance
Zm = Magnetizing impedance
R2 = Secondary winding resistance
Zb = Secondary load
Note: Normally the leakage fluxes X1 and X2 can be neglected

3. HV Power Seminar Nov 2009 3
Current transformer, simplified equivalent circuit
jX2
i2
ZB
i1
1 : N2
i
R2
1
'
1
1
I
N
I ⋅
=
Energy Sector
Energy Automation
© Siemens AG 2008
Page 3
R2
RB
i2
LW
L2<< LW
im
1
2
1 I
N
I ⋅
=

4. HV Power Seminar Nov 2009 4
N1
I1
N2
I2
ε
ε
ε
ε
I1
. N1
N2
Current transformer:
Phase displacement (δ
δ
δ
δ) and current ratio error (ε
ε
ε
ε)
Energy Sector
Energy Automation
© Siemens AG 2008
Page 4
I2
Zb
δ
δ
δ
δ
Im

5. HV Power Seminar Nov 2009 5
Accuracy class
Current error
at nominal
current (In)
Angle error δ
δ
δ
δ
at 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
Energy Sector
Energy Automation
© Siemens AG 2008
Page 5
current (In) In
(rated accuracy limit)
5P
10P
±
±
±
± 1 %
±
±
±
± 3%
±
±
±
± 60 minutes 5 %
10 %

6. HV Power Seminar Nov 2009 6
Current transformers, Standard for transient performance
IEC 60044-6 specifies four classes (1992/3):
Class
Ratio error Angle error
Maximum error at
rated accuracy limit
TPX
(closed iron core)
± 30 min
± 0,5 % %
10
ˆ ≤
ε
Error at rated current Remanence
no limit
Energy Sector
Energy Automation
© Siemens AG 2008
Page 6
TPY
with anti-remanence
air gap
TPZ
linear core
± 60 min
± 180 ± 18 min
± 1,0 %
± 1,0 %
only)
current
(a.c.
%
10
ˆ ≤
ε
%
10
ˆ ≤
ε
TPS
closed iron core
Special version for high impedance protection
(Knee point voltage, internal secondary resistance)
< 10 %
negligible
No limit

7. HV Power Seminar Nov 2009 7
Accuracy class
Current error
at nominal
current (In)
Angle error δ
δ
δ
δ
at 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
Energy Sector
Energy Automation
© Siemens AG 2008
Page 7
current (In) In
(rated accuracy limit)
5PR
10PR
±
±
±
± 1 %
±
±
±
± 3%
±
±
±
± 60 minutes 5 %
10 %
The remanence factor Kr shall not exceed 10%
s
r
r
K
ψ
ψ
⋅
=100

8. HV Power Seminar Nov 2009 8
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
Energy Sector
Energy Automation
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Page 8
•Max exciting current at rated knee point emf. And/or at a stated percentage thereof
•Max resistance of secondary winding

9. HV Power Seminar Nov 2009 9
Current transformer saturation
Steady-state saturation with AC current
Energy Sector
Energy Automation
© Siemens AG 2008
Page 9
Transient saturation with offset current

10. HV Power Seminar Nov 2009 10
CT saturation
Currents and magnetising
saturation flux
t
IP
φ
φ
φ
φ
Energy Sector
Energy Automation
© Siemens AG 2008
Page 10
saturation flux
t
t
IS

11. HV Power Seminar Nov 2009 11
ΙP primary current
TN
B
total flux
t
d.c. component
)
(
ω
1 TS
t
TN
t
S
N
S
N
~
−
−
−
−
⋅
⋅
+
= e
e
T
T
T
T
B
B
ˆ
TN
TS
TS
S
N
S
Max
ω
1
~
−







⋅
⋅
+
=
T
T
T
B
B
T
T
T
Course of CT-flux during off-set short-circuit current
Energy Sector
Energy Automation
© Siemens AG 2008
Page 11
a.c. flux
transient
d.c. flux
BMax
B~
t
B~
N
S
N
S
S
N
Max
B
n
T
T
l
T
T
T
T
t ⋅
−
=
⋅
δ
tan
ω
1
B
i
W
S
⋅
=
+
=
R
R
L
T
[ ]
[ ]
ms
10900
min
S
δ
=
T
For 50 Hz:

12. HV Power Seminar Nov 2009 12
CT transient over-dimensioning factor KTF
KTF
TS [ms]
30
40
50
60
500
1000
5000
∞
∞
∞
∞ (KTF ≈
≈
≈
≈1+ω
ω
ω
ωTN)
Closed iron core
N
S
S
T
T
T
S
N
S
TF ω
1
−








+
=
T
T
T
K
Energy Sector
Energy Automation
© Siemens AG 2008
Page 12
50 100 150 200
10
20
30
100
250
Linear core
TN = network time constant
(short-circuit time constant)
TS = CT secondary time constant
TN [ms]

13. HV Power Seminar Nov 2009 13
CT with closed iron core,
Over-dimensioning factor for a specified time to saturation (TM)
15
20
K'TF
TM → ∞
16
N
N
N
TF 1
1
R
X
T
K' +
=
ω
+
=
Energy Sector
Energy Automation
© Siemens AG 2008
Page 13
5
10
7
TM = 35 ms
TM = 25 ms
50 100 TN [ms]










+
= N
M
T
T
-
N
TF e
-
1
1 T
K' ω

14. HV Power Seminar Nov 2009 14
Current transformer
magnetising curve and point of remanence
B
I II III
up to 80%
< 10%
negligible
Energy Sector
Energy Automation
© Siemens AG 2008
Page 14
H = im ⋅ w
I: closed iron core (TPX)
II: core with anti-remanence air-gaps (TPY)
III:Linearised core (TPZ)
negligible

15. HV Power Seminar Nov 2009 15
Standards of voltage transformers
Class
designation
5P
3P
Voltage error FU Angle error δ
Permissible 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
Energy Sector
Energy Automation
© Siemens AG 2008
Page 15
Class
designation
at 1.0 · UN at 0.05 ·UN at 1.0·UN at 0.05 ·UN
0.1
0.2
0.5
1
3
0.1
0.2
0.5
1
3
1.0
1.0
1.0
2.0
6.0
10
5
20
30
40
40
40
40
80
120
Permissible voltage error in % Permissible angle error in minutes

16. HV Power Seminar Nov 2009 16
Capacitive voltage transformer,
Equivalent circuit
ZB
LF CF
RF 100 V
3
20 kV
3
400 kV
3
C1 = 5 nF
C2 = 95 nF
Energy Sector
Energy Automation
© Siemens AG 2008
Page 16
R'B
LF CF
RF
LT
RT
CT
CE = C1 + C2
RE
LO
U*1 = UP ⋅
C1
C1 + C2
U*2
L'B

17. HV Power Seminar Nov 2009 17
Time
Ts
in ms
Ratio
Classes
3PT1
6PT1
3PT2
6PT2
3PT3
6PT3
10 --- ≤ 25 ≤ 4
20 ≤ 10 ≤ 10 ≤ 2
40 ≤ 10 ≤ 2 ≤ 2
%
100
U
2
)
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
Energy Sector
Energy Automation
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Page 17
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

18. HV Power Seminar Nov 2009 18
B
CT
BN
CT
B
BN
i
ALF
i R
R
R
R
P
P
P
P
ALF
ALF'
+
+
⋅
=
+
+
⋅
=
rated CT burden: PBN
internal burden of the CT: Pi=Ri ⋅ I2N
2
Actual connected burden : PB= RB ⋅ I2N
2
RB=Rl+RR= burden resistance
Rl= resistance of connecting cables
RR= burden resistance of the relay
BN
CT
B
CT
BN
i
B
ALF'
i
R
R
R
R
P
P
P
P
ALF'
ALF
+
+
⋅
=
+
+
⋅
=
K
I
K
' ⋅
≥
ALF No saturation X
B
with
CT dimensioning
Theory:
Energy Sector
Energy Automation
© Siemens AG 2008
Page 18
N
K
OD
I
I
K
' ⋅
≥
ALF No saturation
for the total
short-circuit duration:
N
N
N
~
Max
TF 1
1
R
X
T
B
B
K' +
=
+
=
= ω
ˆ








−
−
⋅
⋅
+
=
−
−
S
M
N
M
S
N
S
N
T
t
T
t
(
ω
1 e
e
T
T
T
T
TF
K '
'
No saturation for
the specified time tM:
m
TF
OD
K
K
K Re
⋅
≥
100
Remanence
%
1
1
Re
−
=
m
K
with
Practice:
Theory:
by vendors
d
recommende
values
-
K
of
Use TF
Note: this equation is not fully valid for very small tM (less than 1 cycle)

19. HV Power Seminar Nov 2009 19
Distance relays, practical CT requirements
Transient over-dimensioning factors KTF (ARC not considered)
2
KTF ≥
Close-in fault
50ms
T
if
2
K N
TF <
≥
100ms
T
50ms
if
3
K N
TF <
≤
≥
10
KTF ≥
7SA510 / 511
Close-in fault
Fault at
balance point
Energy Sector
Energy Automation
© Siemens AG 2008
Page 19
7SA513
2
KTF ≥
5
KTF ≥
s
50m
T
if
10
K N
TF ≥
≥
Close-in fault
7SA6 /
7SA522
30ms)
T
if
1
(
2
K N
TF <
≥
≥
50ms
T
if
5
K N
TF <
≥
Fault at
balance point
Close-in fault
Fault at
balance point

20. HV Power Seminar Nov 2009 20
Operating
time
ms
140
120
100
80
TN = 100 ms K=10
K= 5
K= 4
7SA6 / 7SA522: Operating time with CT saturation
Influence of CT dimensioning
Energy Sector
Energy Automation
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Page 20
80
60
40
20
0
60 100
40
20 80 % Z1
K= 3
K= 2
K= 1