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Current TransformerDimensioning
Siemens AG, PTD SE PT 5Postfach 3220, 91050 Erlangen
Freyeslebenstraße 1
Tel +49 9131 7- 34324Fax +49 9131 7- 35017
Power Transmission and DistributionCopyright SIEMENS AG. PTD SE PT 2005. All rights r eserv ed.
Power TechnologiesCurrent Transformer Dimensioning, Page 1
s
Current Transformer Dimensioning
Power Transmission and DistributionCopyright SIEMENS AG. PTD SE PT 2005. All rights r eserv ed.
Power TechnologiesCurrent Transformer Dimensioning, Page 2
s
Current Voltage
Inductive
Rogowski-Coil
Ohmic
Inductive/Optical
Optical
Inductive
Ohmic
Capacitive
Different Measuring Methods I
2
Current Transformer Dimensioning
Power Transmission and DistributionCopyright SIEMENS AG. PTD SE PT 2005. All rights r eserv ed.
Power TechnologiesCurrent Transformer Dimensioning, Page 3
s
Different Measuring Methods II
Current Transformer Dimensioning
Power Transmission and DistributionCopyright SIEMENS AG. PTD SE PT 2005. All rights r eserv ed.
Power TechnologiesCurrent Transformer Dimensioning, Page 4
s
Dimensioning Criteria
Requirements of modern relays Installation points Relay functions Relay burden, cable burden and CT burden Short-circuit current and system time constant Technical criteria
Customer criteria Requirements of relays in use Habits Tender invitation National standards
CT-Ratio, Accuracy Limiting Factor, Nominal Burden,Class, Knee- Point Voltage
3
Current Transformer Dimensioning
Power Transmission and DistributionCopyright SIEMENS AG. PTD SE PT 2005. All rights r eserv ed.
Power TechnologiesCurrent Transformer Dimensioning, Page 5
s
Important Standards
AdditionalCEI (Italy) SEN (Sweden) CSA (Canada) SEU (Suisse) NF (France) ABNT (Brasil)
International Standards
IEC 60044-1 1996/12 Current Transformers IEC 60044-1 2000/07 Current Transformers (amendment 1)
IEC 60044-6 1992/03 Current Transformers (transient performance TP)
IEC 60044-2 1997/02 Voltage TransformersBS EN 60044-1 CurrentTransformers
ANSI C57.13/1993 Current TransformersAS 1675-1986 Current Transformers
IEC 185/1978BS 7626/1993BS 3938/1973
VDE 0414 Specifications for Instrument Transformers
obsolete !obsolete !obsolete !
Current Transformer Dimensioning
Power Transmission and DistributionCopyright SIEMENS AG. PTD SE PT 2005. All rights r eserv ed.
Power TechnologiesCurrent Transformer Dimensioning, Page 6
s
CT: Equivalent Circuit
CT
4
Current Transformer Dimensioning
Power Transmission and DistributionCopyright SIEMENS AG. PTD SE PT 2005. All rights r eserv ed.
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s
Saturation Effect
20 40 60 80 100 120 140 160 180 t/ms0
ipis
5 P10 60 VAU/V
900
90
0,001 0,010 0,100 1,000
Ritz
Pfiffner
IM /A
iP iM
Lh
L σσσσ RCTLZ RZ
Lb
Rb
is
Current Transformer Dimensioning
Power Transmission and DistributionCopyright SIEMENS AG. PTD SE PT 2005. All rights r eserv ed.
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s
Definitions of Accuracy According to IEC 60044-1
Definition of current error:
Definition of angle error:
Definition of composite error:
prim
primnseci I
InI100F
−=
)I(angle)I(angle primseci −=ϕ∆
( )
rms.prim
T
0
2)t(primsecnT
1
i I
dti)t(in
100F
∫ −
=
5
Current Transformer Dimensioning
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s
Qualifying Symbols According to IEC
10 P 10, 15 VA
Nominal burden
Accuracy Limiting Factor (ALF)
Core Type P = Protection
Accuracy limit in % at ALF * I n
Measuring cores are not suitable for protection!
Current Transformer Dimensioning
Power Transmission and DistributionCopyright SIEMENS AG. PTD SE PT 2005. All rights r eserv ed.
Power TechnologiesCurrent Transformer
Dimensioning, Page 10
s
IEC-60044 Accuracy Classification for Protection CT in Steady State
2 standard accuracy classes: 5P and 10PLimits of error
Example: 5 P 10 , 50 VA
accuracy limit in % at I=ALF × I n
accuracy limiting factor (ALF)nominal burden
Accuracyclass
5P10P
At nominal primary current
Current error Phase angledisplacement
Phase angledisplacement
% minutes centirad
± 1± 3
± 60-
± 1.8-
At nominal accuracy limitprimary current
Composite error
%
510
6
Current Transformer Dimensioning
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However:
Definition for type P according toIEC 60044-1, only AC componentis taken into account
Only symmetrical saturation isconsidered
Short-circuit current consists of ACand decaying DC components
For CT-dimensioning DC componentshould also be considered
10P10, 15 VA
Isecondary/In
10
5
0 5 10 15 Iprimary /In
burden < 15 VA
burden = 15 VA
burden > 15 VA
F i = 10%
Definition of Protection CT Performance in Steady-S tate According to IEC
Current Transformer Dimensioning
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IEC: Not high nominal VA or high nominal ALF fac tor leads toa better CT performance, but higher actual ALF’ fact or
Accuracy Limiting Factor ALF’
The CT’s performance under steady-state conditions is described by
A ccuracy L imiting F actorBetriebsüberstromziffer
ALF’ = Actual Accuracy Limiting FactorALFn = Nominal Accuracy Limiting FactorPn = Nominal VA Output (Nominal Burden)PCT = Internal CT BurdenP’ = Actual Connected Burden
CT
CTnn PP`
PPALFALF`
++⋅=
7
Current Transformer Dimensioning
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s
Definition of Knee-Point Voltage According to BS Cl ass X and IEC Class PX
Usecondary/V
200
0 Isecondary/mA
∆∆∆∆U = 10%
50 100 150
400
∆∆∆∆I = 50%
Uknee
RCT (internal burden)
R‘(external burden)
Knee-point voltage is the voltage at which a 10% increase in rms voltage results in a 50% increase in rms magnetizing current
Current Transformer Dimensioning
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BS Performance Classification for Relaying CTs
Class P: 5P and 10P similar to IEC 60044
Class X: Defined by Rated primary current Turns ratio (the error shall not exceed ± 0.25%) Rated knee-point voltage Magnetizing current at rated knee-point voltage Resistance of secondary winding corrected to 75°C
Class X CT will be applied if accuracy limits of cl ass 5P or 10P are notappropriate
Class X quantities can also be measured and given f or class 5P and 10 P
8
Current Transformer Dimensioning
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Relation Between Accuracy Limiting Factor ALF and K nee-Point Voltage
I1N I2N
RCT(internal burden)
Rn(nominalexternalburden)
Uknee
( )1.3
ALFIRRknee
n2nCTnU ⋅⋅+=( )
n2
nCTnI3.1
ALFPP
⋅⋅+
=
( )146V
1VA1.3104VA15VA
U knee =⋅
⋅+=
Example:IEC Class 5P: 600/1A, 5P10, 15 VA, RCT= 4ΩtoIEC Class PX: 600/1A:
Usecondary/V
200
0 Isecondary/mA
∆∆∆∆U = 10%
50 100 150
400
∆∆∆∆I = 50%
Current Transformer Dimensioning
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Relation Between Knee-Point Voltage and Accuracy Li miting Factor ALF
Example : An IEC PX CTI2N = 1A Rated secondary currentUknee = 600V Knee-Point VoltageRCT = 5Ω Internal burdenR‘ = 1Ω Wire and relay burden
100A1Ω5Ω
600VRR
UI
CT
kneemax =
+=
+=
100A1
A100I
I'ALF
n
max ===
245VA20VA
5VA1VA100
PPPP'
ALF'ALFCTn
CTn =
++=
++=
405VA10VA
5VA1VA100
PPPP'
ALF'ALFCTn
CTn =
++=
++=
CT with 20VA:
CT with 10VA:
For small connected burdens, like for most GIS, wit h short lines to the relay and for modern relays with a burden of ca. 0.1 VA only P-cores with small Nominal Accuracy Limiting Factors are required
Maximum transmittable AC currentwithout DC components (secondary):
I1N I2N
RCT(internal burden)
Rn(nominalexternalburden)
Uknee
9
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Conversion of ANSI to IEC CT Data for 1A CTs
RCT
R’Uterm
Example C200, 1A
C100 10P20, 5 VAC200 10P20, 10 VAC400 10P20, 20 VAC800 10P20, 40 VA
Nominal Accuracy Limiting Factor ALF n
Uterm is defined as terminal voltage at 20*I nand connected external standard burden.
ALFn = 20 for all class C cores
Nominal ANSI-Standard burden
Class P Nominal Burden
Ω=⋅
=⋅
= 10A120V200
I20U
'Rn
term
VA1010)A1('RIP 22nn =Ω⋅=⋅=
Current Transformer Dimensioning
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Conversion of ANSI to IEC CT Data for 5A CTs
RCT
R’Uterm
Example C200typical 5A
C100 10P20, 25 VAC200 10P20, 50 VAC400 10P20, 100 VAC800 10P20, 200 VA
Nominal Accuracy Limiting Factor ALF n
Uterm is defined as terminal voltage at 20*I nand connected external standard burden.
ALFn = 20 for all class C cores
Nominal ANSI-Standard burden
Class P Nominal Burden
Ω=⋅
=⋅
= 2A520V200
I20U
'Rn
term
VA502)A5('RIP 22nn =Ω⋅=⋅=
10
Current Transformer Dimensioning
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s
Class C: indicates that the transformer ratio can be calculated (bushing type CTs)Class T: indicates that the transformer ratio must b e determined by test
800
700
600
500
400
300
200
100
0
0 10 20 30 40 50 60 70 80 90 100
8 Ω
4 Ω
2 Ω
1 Ω
C800
C400
C200
C100
wound-type CTs limits of error
secondary amperes
secondary
volts
terminal
Error will not exceed 10% for secondary voltage equivalent to or less than value described by curve
Class C: Ratio error will not exceed 10% between 1 t o 20 times nominalsecondary current.
ANSI C57.13 Performance Classification for Relaying CTs
Current Transformer Dimensioning
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Effective Burden
Ri R1
RRelay
RRelay
RRelay
Ri R1
Ri R1
R2
R2
R2
3 CT Earth-Returns
RBurden = R1 + RRelay + R2PBurden = In2 (R1 + R2) + PRelay
Ri R1
Ri R1
Ri R1
R2
RRelay
RRelay
RRelay
1 CT Earth-Return
3-phase and 2-phase faults:RBurden = R1 + RRelayPBurden = In2 *R1 + PRelaynon-symmetrical earth faults:RBurden = R1 + RRelays + R2PBurden = In2 (R1 + R2) + PRelays
11
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Effective Secondary Cable Burden with Nominal Curre nt 1A, 5A
0 100 200 3000
10
20
30
40
50
60VA
length/m
2,5 mm2
0,70 ΩΩΩΩ / 100 m
4,0 mm2
0,44 ΩΩΩΩ / 100 m
6,0 mm2
0,29 ΩΩΩΩ / 100 m
10,0 mm2
0,17 ΩΩΩΩ / 100 m
16,0 mm2
0,11 ΩΩΩΩ / 100 m
I n = 5 A
2,5 mm2
0,70 ΩΩΩΩ / 100 m
4,0 mm2
0,44 ΩΩΩΩ / 100 m
6,0 mm2
0,29 ΩΩΩΩ / 100 m
10,0 mm2
0,17 ΩΩΩΩ / 100 m
16,0 mm2
0,11 ΩΩΩΩ / 100 m
I n = 1 A
0 100 200 300length/m
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
2,2VA
(I n)2 = 25 A2 ⇒⇒⇒⇒ high burden (In)2 = 1,0 A2 ⇒⇒⇒⇒ low burden
better
Current Transformer Dimensioning
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Steady State Dimensioning - Transmission Capability
RCT RI
……Ω ……Ω
RI
Relay
RRelay
……Ω
P’ = …… VAn = …… / 1A
CT-Transmission Capability with nominal burden
I’’ k = …… x In = …… kA
Accuracy Limiting Factor
CT-Transmission Capabilitywith actual burden
I’’ k = ALF’ x I n = …… kA
=++⋅=
CT
CTnn PP'
PPALFALF'
..................
.................. =
++
⋅=
Current Transformer CT
…… / 1A, …… P ……, …… VA
1000
2 1
0,1
1000
12
Current Transformer Dimensioning
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Steady State Dimensioning
RCT RI
……Ω ……Ω
RI
Relay
RRelay
……Ω
P’ = …… VAn = …… / 1A
Current Transformer CT
…… / 1A, …… P ……, …… VA
Short-Circuit Current
I’’ k = …… kA = …… x In
Necessary Accuracy Limiting Factor
Necessary Nominal Burden
Necessary Nominal AccurancyLimiting Factor
......I''I
'ALFn
k ==
VA......PVA......'PP nn =⇒=≥
=++⋅=
CTn
CTn PP
PP'ALF'ALF
..............................
...... =++⋅=
1000
2 1
0,1
2,1
1000 5? ?
Current Transformer Dimensioning
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Short Circuit Current
Constant AC component 50/60 Hz Decaying DC component Time constants:
Medium voltage 10ms .. 50msHigh voltage 50ms .. 100msExtra high voltage 80ms .. 150ms
Decaying AC component 50/60 Hz Decaying DC component Time constants:
Medium voltage 10ms .. 100msHigh voltage 50ms .. 250msExtra high voltage 80ms .. 250msGenerator level up to 500ms
Fault far from generator Fault near to generator
A
i p
'' 22 kI
'' 22 22 kk II =
Bottom envelope
Decaying component iDC
Top envelope
Time
Current
Bottom envelope
Decaying component iDC
Top envelope
Time
Current
A
i p'' 22 kIk 22 I
IK'' = Subtransient short-circuit current
ip = Peak value of short-circuit value
IK = Steady short-circuit current
iDC = Decaying DC component
A = Initial value of DC component
13
Current Transformer Dimensioning
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s
Meaning of the accuracy limiting factor ALF‘
The multiplication of the accuracy limiting factor and nominal CT-current specifies the symmetrical short-circuit current which can be transmitted without saturation.
Relation between magnetic flux density and secondary nominal CT-current.
dti*)R(RdtuB s(t)Bcts(t) ∫∫ +=≈
-1,5
-1
-0,5
0
0,5
1
1,5
-1,5
-1
-0,5
0
0,5
1
1,5
+Bmax
-Bmax
In* ALF´ x 1.41
maxBB
kmax
s
Ii
Current Transformer Dimensioning
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Transient Dimensioning Factor I
Asymmetrical short circuit current with a superposing DC-component
The DC-component of an asymmetrical short circuit is causing an additional magnetic flux.
Transient dimensioning factor
TN : System time constant
-0,5
0
0,5
1
1,5
2
2,5
0
2
4
6
8
10
12
kmax
s
Ii
maxBB
tfrei
10ms
=
=ACmax
DC/ACmax
B
B
∞=+≈ freeNtd for t T1K ω
14
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Bsat Magnetic Flux
secondary current
primary current
The DC-component is producing a steep rising of the magnetic flux density
A CT can be saturated by an asymmetrical current DC-component nevertheless it will not be saturated by the pure symmetrical current
t free
Transient Dimensioning Factor II
Current Transformer Dimensioning
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CT - Dimensioning according to saturation-free trans mission time
Overdimensioning factor for saturation-freetransmission time t free:
)e(1T1K NTfreet
Ntd
−−+= ω(iron-cored CT with T S >> TN)
tfree = 10 ms
0 0,05 0,10 0,15 0,20
tfree = 20 ms
tfree = 30 ms
tfree = ∞Ktd
0
5
10
15
TN
15
Current Transformer Dimensioning
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CT-Dimensioning for Transformer Differential Protec tion
A transformer differential protection should be sta ble on external short-circuit faults.The CT saturation detection of 7UT612 relays needs an unsaturated transmission ofmaximum short-circuit current in
For high CT time constants, the maximum transmittab le current must not exceed
tfree = 10 ms
0 0,05 0,10 0,15 0,20
tfree = 20 ms
tfree = 30 ms
tfree = ∞Ktd
0
5
10
15
mit ALF'tdK
nSC
⋅= II
TN
Current Transformer Dimensioning
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CT-Dimensioning for Line Differential Protection
A current differential protection should be stable on external short-circuit faults.The CT saturation detection of 7SD51 relays need an unsaturated transmission ofmaximum short-circuit current in
For high CT time constants, the maximum transmittab le current must not exceed
tfree = 10 ms
0 0,05 0,10 0,15 0,20
tfree = 20 ms
tfree = 30 ms
tfree = ∞Ktd
0
5
10
15
mit ALF'tdK
nSC
⋅= II
TN
16
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CT-Dimensioning for Distance Relay
General:
The CT may be saturated, but any short circuit faul ts in zone 1 mustbe cleared undelayed, short circuit faults outside zone 1 must be cleared delayed.
CT may be saturated, but the remainingmagnitude of fundamental current and its phase angle must lead toimpedance within zone 1.
CT should keep to transmit short circuitcurrents without saturation in a longer time than relay operating time.
Current Transformer Dimensioning
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CT-Dimensioning for Overcurrent Relays
General:
The CT may be saturated, but the remaining magnitud e of fundamentalcurrent has to be higher than the relay settings I >>.
IS > I> or I S > I>>
nCT
CTnn I
IPP'PP
ALF >>>++⋅
nII
ALF >>>
In systems with high time constants the tripping co mmand for therelay can be delayed for faults at the zone boundar ies.
17
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......kA......kA............
IK
ALF'I n
tdk =×=⋅=
Transient Dimensioning - Transmission Capability
RCT RI
……Ω ……Ω
RI
Relay
RRelay
……Ω
P’ = …… VAn = …… / 1A
Current Transformer CT
…… / 1A, …… P ……, …… VA
Accuracy Limiting Factor
......PP'PP
ALFALF'CT
CTnn =
++⋅=
Tsys = 0 ms, t free = ∞, steady stateIk = ALF’ * I n = …… kA
Tsys = 50 ms, t free = ∞, transient
Tsys = 50 ms, t free = 10 ms, transient
1000
2 1
0,1
2,1
1000 5 10 10
.......kA......kA............
IK
ALF'I n
tdk =×=⋅=
Current Transformer Dimensioning
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Transient Dimensioning
RCT RI
……Ω ……Ω
RI
Relay
RRelay
……Ω
P’ = …… VAn = …… / 1A
Current Transformer CT
…… / 1A, …… P ……, …… VA
Short-Circuit Current
I’’ k = …… kA = …… x InTsys = …… ms
Necessary Transient Accuracy Limiting Factor
Necessary Nominal Burden
Necessary Nominal AccuracyLimiting Factor
VA......PVA......'PP nn =⇒=≥
.................................... =⋅=
++
1000
2 1
0,1
2,1
1000 5? ?
7UT612
............KI'I'
ALF' ...................
tdn
k =×=⋅=
=++⋅=
CTn
CTn PP
PP'ALF'ALF
18
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CT - Demagnetizing
circuit breaker switched-off at current zero crossing
Maximum of induction
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TP - current transformer classes acc. IEC 60044 - 6
- Iron - cored CT without limited remanence- Mechanical construction corresponds to class P (IE C 60044 - 1)- transient behavior is specified additionally
- same as TPX- remanence limited to < 10% (anti - remanence air ga p)
- remanence neglectable- error limit is only formulated for AC component- DC - component will be shortened considerably
TPX
TPY
TPZ
19
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Exemplary magnetizing characteristics of Class TP - CTs:
Current Transformer Dimensioning
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CT - Induction curve during AR
−
−⋅⋅++⋅
−
−⋅+=
== −−−−−
∝
+
)e(eTTTTω
1e)e(eTTTωT
1B
BK ST
F2t
NTF2t
STF2tSpt
STF1t
NTF1t
SN
S1
SN
SNmaxtd ˆ
tF1 tSp tF2 t
A
B
BR
BR
A – Iron cored CT TPX
B – Iron cored CT with anti - remanenceair gap TPY
20
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• Advantages by AR• Cost intensive partitioned core • very short sec. Time constant (<100ms): sec. DC component is no longer transformed correctly• high demagnetizing current
CT - Induction curve during AR
Linear core CTTPZ
−
−⋅+=
== −−
∝
)e(eTTTT
1B
BK ST
F2t
NTF2t
SN
SNmaxtd
ωˆ
Ip
Is
Im
B
Current Transformer Dimensioning
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Error limits of TP - current transformer classes acc. to IEC 60044 - 6
class Error at rated currentratio angle
Peak error at ratedovercurrent
TPX
TPY
TPZ
± 0,5% ± 30 min
± 1,0% ± 60 min
± 1,0% ± 180 ± 18 min
ε ≤ 10%
ε ≤ 10%
ε ≤ 10% (only I ~)
21
Current Transformer Dimensioning
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CTDim 3.2 - Current Transformer Dimensioning Program
Technical Optimization
Dynamic Check of Requirements
Automatic Report Generation
Standard Conversion
Dynamic Simulation