Day 1 Part 2 CTs and VTs
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Transcript of Day 1 Part 2 CTs and VTs
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HV Power Seminar Nov 2009 1
Current and Voltage
Energy Sector© Siemens AG 2008
Voltage TransformersPerformance Requirements
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HV Power Seminar Nov 2009 2
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
Page 2
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
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HV Power Seminar Nov 2009 3
Current transformer, simplified equivalent circuit
jX2
i2 ZBi1
1 : N2
i
R2
1'1
1I
NI ⋅=
Energy SectorEnergy Automation© Siemens AG 2008
Page 3
R2
RB
i2
LW
L2<< LW
im
12
1 IN
I ⋅=
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HV Power Seminar Nov 2009 4
N1
I1
N2
I2
εεεε
I1. N1
N2
Current transformer:Phase displacement ( δδδδ) and current ratio error ( εεεε)
Energy SectorEnergy Automation© Siemens AG 2008
Page 4
I2
Zb
δδδδ
Im
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HV Power Seminar Nov 2009 5
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
Energy SectorEnergy Automation© Siemens AG 2008
Page 5
current (In) In (rated accuracy limit)
5P
10P
±±±± 1 %
±±±± 3%
±±±± 60 minutes 5 %
10 %
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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 atrated accuracy limit
TPX(closed iron core)
± 30 min± 0,5 % %10ˆ ≤ε
Error at rated current Remanence
no limit
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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
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HV Power Seminar Nov 2009 7
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
Energy SectorEnergy 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
rrK
ψψ⋅=100
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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 SectorEnergy Automation© Siemens AG 2008
Page 8
•Max exciting current at rated knee point emf. And/or at a stated percentage thereof
•Max resistance of secondary winding
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HV Power Seminar Nov 2009 9
Current transformer saturation
Steady-state saturation with AC current
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Transient saturation with offset current
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HV Power Seminar Nov 2009 10
CT saturationCurrents and magnetising
saturation flux
t
IP
φφφφ
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Page 10
saturation flux
t
t
IS
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HV Power Seminar Nov 2009 11
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
Energy SectorEnergy Automation© Siemens AG 2008
Page 11
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:
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HV Power Seminar Nov 2009 12
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
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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 constantTN [ms]
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HV Power Seminar Nov 2009 13
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+=
Energy SectorEnergy Automation© Siemens AG 2008
Page 13
5
10
7
TM = 35 ms
TM = 25 ms
50 100 TN [ms]
+= NM
TT
- NTF e-11 TK' ω
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HV Power Seminar Nov 2009 14
Current transformer magnetising curve and point of remanence
BI II III
up to 80%
< 10%
negligible
Energy SectorEnergy 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
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HV Power Seminar Nov 2009 15
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
Energy SectorEnergy Automation© Siemens AG 2008
Page 15
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
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HV Power Seminar Nov 2009 16
Capacitive voltage transformer,Equivalent circuit
ZBLF CF
RF 100 V
3
20 kV
3
400 kV
3C1 = 5 nF
C2 = 95 nF
Energy SectorEnergy Automation© Siemens AG 2008
Page 16
R'BLF CF
RF
LTRTCT
CE = C1 + C2
RE
LO
U*1 = UP ⋅C1
C1 + C2U*2 L'B
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HV Power Seminar Nov 2009 17
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
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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
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HV Power Seminar Nov 2009 18
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:
Energy SectorEnergy Automation© Siemens AG 2008
Page 18
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)
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HV Power Seminar Nov 2009 19
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
Energy SectorEnergy Automation© Siemens AG 2008
Page 19
7SA513
2KTF ≥
5KTF ≥
s50mTif10K NTF ≥≥
Close-in fault
7SA6 / 7SA522
30ms)T if 1( 2K NTF <≥≥
50msTif5K NTF <≥
Fault atbalance point
Close-in fault
Fault atbalance point
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HV Power Seminar Nov 2009 20
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
Energy SectorEnergy Automation© Siemens AG 2008
Page 20
80
60
40
20
0
60 1004020 80 % Z1
K= 3
K= 2
K= 1