19a_KRUEGER_Onsite measure CT and VT.pdf
Transcript of 19a_KRUEGER_Onsite measure CT and VT.pdf
Measurement Principle of the CT Analyzer
1. Measurement of Residual Magnetism
2. Measurement of Winding Resistance
3. Measurement of Excitation Losses
4. Measurement of Eddy Losses
5. Measurement of Turns Ratio N (core ratio)
6. Calculation of current ratio error and phase displacement using excitation curve, core ratio N, Rct and desired burden values
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1. Measurement of Residual Magnetism
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Ip
Np Ns
P1
P2
Vcore
RCT
RCTIECTS1
S2
Iexc
Reddy
Lmain
IL
IERH
VRCTVTERMINAL
N
t
Ψ
resΨ
1Ψ
2Ψ
3Ψ
01t
2t
3t
nn ΨΨ − ,2
nn Ψ−=Ψ −1
2´ −nt1−nt
nt
n∆Ψ
1tΨ∆⋅−∆Ψ≈Ψ 5.01res
2. Measurement of Winding Resistance
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Ip
Np Ns
P1
P2
Vcore
RCT
RCT
S1
S2
Reddy
Lmain
IL
IERH
VRCTVTERMINAL
N IDC
U
t
DC
DCCT I
UR =
3. Measurement of Excitation Losses
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Ip
Np Ns
P1
P2
Vcore
RCT
RCTIECTS1
S2
Iexc
Reddy
Lmain
IL
IERH
VRCTVTERMINAL
N
4. Measurement of Eddy Losses
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Ip
Np Ns
P1
P2
Vcore
RCT
RCTIEXCS1
S2
Reddy
Lmain
IL
IERH
VRCTVTERMINAL
NIEXC
Measurement at two different frequencies with the same resulting magnetic flux in the core
222
211
2
**
**
*
*
2
1
fbfaP
fbfaP
fbP
faPPPP
f
f
CurrentEddy
Hysteris
CurrentEddyHysterisIron
+=
+=
=
=
+=
−
−
)(
)(
1221
2112
1221
212
221
fffffPfPb
fffffPfPa
−−
=⇒
−−
=⇒
5. Measurement of Turns Ratio N
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Ip
Np Ns
P1
P2
RCT
RCT
S1
S2
Reddy
Lmain
IL
IERH
VCore
VRCT=Iexc*RCT
VTerminal
IExc
VPRIM
IExc
PRIM
CORE
VVN =
6. Calculation of Current Ratio Error and Phase Displacement
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Ip
Np Ns
P1
P2
R CT
S1
S2
R eddy
L main
I L
I E
R H
N
Z B
IS IEXC
IP/N = IEXC+IS
VCore
VRCT=IS*RCT IP = N*(IEXC+IS)
( ) )* 22BBCTSCore XRRIV ++=
IS
IEXC
IP/N
Residual Magnetism
> Typical Residual Magnetism effects in cores occur:
... after being confronted with very high transient fault currents during fault conditions
... Circuit Breakers: electric arc extinguishes at zero crossing of the current signal
... DC currents due to wrong handling on site (winding resistance measurement)
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Residual Magnetism – Transient Fault
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2/max πφα =−= whenIDC
[ ])sin(*)/(*2 φαω −+= tZVI PP [ ]τφα /*)sin(*)/(*2 tP eZV −−
PPPPPP RLRLTanLjRZ //)( 1 ==+= − τφω
=PI ACI
+
DCI+
Residual Magnetism – Transient Fault
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Closed core (TPX) with Kr = 80%; Tp = 0.02s; Ts = 1.5s; Tfr = 0.4s
Residual Magnetism: Affectes Distance Protection
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Netz 1F2F1
Schutzgerät 1 Schutzgerät 2
Schutzabschnitt des Schutzgerätes 1operating section of protection unit 1
protection unit 1 protection unit 2
Main 1
Distance protection relay sees higher fault impedance -> it trips with a time delay
Residual Magnetism: Affects Differential Protection
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CT1Netz 1
F2F1CT2
Schutzgerät 1 Schutzgerät 2protection unit 1 protection unit 2
Main 1
Unwanted tripping due to pre-saturation of one CT -> not the same current measured at both protection units
Residual Magnetism – How to get rid of?
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Demagnetization process
Done automatically by the CTAnalyzer at the very end of the
measurement
Residual Magnetism
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CT charged with DC (winding resistance measurement)
CT specs: Class: 5P20 Ratio: 1000:1 Rated Burden: 30VA