19a_krueger_onsite measure ct and vt.pdf
TRANSCRIPT
On-Site Calibration of Current Transformers
Michael Krüger, Florian Predl, OMICRON AUSTRIA
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
Page 2 © OMICRON
1. Measurement of Residual Magnetism
Page 3 © OMICRON
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
Page 4 © OMICRON
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
Page 5 © OMICRON
Ip
Np Ns
P1
P2
Vcore
RCT
RCTIECTS1
S2
Iexc
Reddy
Lmain
IL
IERH
VRCTVTERMINAL
N
4. Measurement of Eddy Losses
Page 6 © OMICRON
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
Page 7 © OMICRON
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
Page 8 © OMICRON
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
Measurement Accuracy
Page 9 © OMICRON
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)
Page 10 © OMICRON
Residual Magnetism
Page 11 © OMICRON
Ipn
Flux density (B)
Flux intensity (H)
20 x Ipn
Residual Magnetism
Page 12 © OMICRON
20 x Ipn
residual magnetism
Residual Magnetism
Page 13 © OMICRON
Ipn
~10 x Ipn reserve
Residual Magnetism – Transient Fault
Page 14 © OMICRON
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
Page 15 © OMICRON
τp
Residual Magnetism – Transient Fault
Page 16 © OMICRON
Closed core (TPX) with Kr = 80%; Tp = 0.02s; Ts = 1.5s; Tfr = 0.4s
Residual Magnetism: Affectes Distance Protection
Page 17 © OMICRON
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
Page 18 © OMICRON
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?
Page 19 © OMICRON
Demagnetization process
Done automatically by the CTAnalyzer at the very end of the
measurement
Residual Magnetism
Page 20 © OMICRON
CT charged with DC (winding resistance measurement)
CT specs: Class: 5P20 Ratio: 1000:1 Rated Burden: 30VA
