On-Site Calibration of Current Transformers Michael Krüger, Krüger, Florian Predl, OMICRON AUSTRIA
Mea Measurement su rement Princ Pri ncip iple le of the th e CT CT Anal Anal yze yzer 1. Measu Measureme rement nt of Resi Residua duall Magneti Magnetism sm 2. Measu Measureme rement nt of Windin Winding g Resist Resistan ance ce 3. Measu Measureme rement nt of Exci Excita tatio tion n Losses Losses 4. Measu Measureme rement nt of Eddy Eddy Loss Losses es 5. Measure Measurement ment of of Turns Turns Ratio Ratio N (core (core ratio) ratio) 6. Calcula Calculation tion of current current ratio error error and and phase displa displacemen cementt using excitation curve, core ratio N, Rct and desired burden values
1. Me Measurement su rement of o f Residu si dua al Magn Magne etism ti sm
N
Ip
RCT
IECT
P1
S1
RCT
Vcore
Iexc
IL
Lmain
IE
RH
Np
VTERMINAL
VRCT
Reddy
Ns
P2
S2
Ψ Ψ1 Ψ3 Ψn − 2 , Ψn
∆Ψn
Ψres
t 2
0 t
1
Ψ2 Ψ
n −1
= −Ψ
n
t 1
t 3
t n −1 t ´n
−
2
t n
t
Ψres ≈ ∆Ψ1 − 0.5 ⋅ ∆ Ψ
2. Me Measurement su rement of Windi in ding ng Resist si sta ance nc e
IDC
N
Ip P1
RCT
Vcore
IL
Lmain RH
Np
IE
VRCT
R CT S1
VTERMINAL
Reddy
Ns
P2
S2
U RCT
=
U DC I DC
t
3. Mea Measurement su rement of Exc Excit ita ation ti on Los L osses ses
N
Ip
IECT
P1
RCT
Vcore
Np P2
S1
Iexc
IL
Lmain RH
RCT
IE
VRCT
VTERMINAL
Reddy
Ns S2
4. Me Measurement su rement of Eddy Losses Los ses
N
Ip
IEXC
P1
S1
RCT
Vcore
IEXC
IL
Lmain RH
Np
RCT
VRCT
VTERMINAL
IE
Reddy
Ns
P2
S2
Measurement at two different frequencies with the same resulting magnetic flux in the core P Iron
=
P Hysteris
P Hysteris + P Eddy −Current =
2
a * f
⇒
a=
2
P Eddy −Current = b * f
2
P f 1
=
a * f 1 + b * f 1
P f 2
=
a * f 2
⇒b= 2
+ b * f 2
P1 f 2
2
− P2 f 1
f 1 f 2 ( f 2 − f 1 ) P2 f 1 − P1 f 2 f 1 f 2 ( f 2 − f 1 )
5. Measur Measure ement of Turns ur ns Ratio N
VRCT=Iexc*RCT RCT
Ip P1
VCore
Lmain RH
VPRIM
Np
S1
RCT
IExc
IL
IExc
IE
VTerminal
Reddy
Ns
P2
S2
N
=
V CORE V PRIM
6. Calcul lc ula ation ti on of o f Curre Curr ent Ratio ti o Error Erro r and and Phase Displacement
IP = N*(IEXC+IS) N Ip
VRCT=IS*RCT
IP/N = IEXC+IS
P1
S1
RCT
VCore
L main RH
Np
IS
IEXC
IL
IS
IE
R eddy
IEXC IP/N
Ns
S2
P2
V Core = I S *
ZB
( R
CT
+ R B
2
)
2
+ X B )
Mea Measurement su rement Accuracy Accu racy
Residual si dual Magn Magne etism ti sm
> Typical ypical Residual Residual Magneti Magnetism sm effects effects in in cores occur: 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)
Residual si dual Magn Magne etism ti sm Flux density (B)
20 x Ipn
Ipn Flux intensity (H)
Residual si dual Magn Magne etism ti sm
20 x Ipn
residual magnetism
Residual si dual Magn Magne etism ti sm
~10 x Ipn reserve Ipn
Residual si dual Magn Magne etism ti sm – Transi Transie ent Fault ul t
Z = ( RP I P I P
=
+ jω LP )
2 * (V P / Z ) * sin(ω t + α − φ ) I AC
=
I DC
φ = Tan −1 LP / RP
= max
when α − φ = π / 2
+ +
τ = LP / RP 2 * (V P / Z ) * sin(α − φ ) * e − t / τ I DC
Residual si dual Magn Magne etism ti sm – Transi Transie ent Fault ul t
p
Residual si dual Magn Magne etism ti sm – Transi Transie ent Fault ul t
Closed core (TPX) with Kr = 80%; Tp = 0.02s; Ts Ts = 1.5s; 1. 5s; Tfr = 0.4s
Residual si dual Magn Magne etism ti sm:: Affecte Affect es Distance is tance Protection
Main 1
Netz 1
operating section des of protection unit 1 1 Schutzabschnitt Schutzgerätes
F1
Schutzgeunit rä t 1 protection
F2
Schutzge rät22 protection unit
Distance protection relay sees higher fault impedance -> it trips with a time delay
Residual si dual Magnetism Magnetism:: Affects Differe if ferenti ntia al Protection
Main11 Netz
CT1 CT 1
Schutzgerät protection unit 11
F1
CT2
F2
Schutzgerät 2 protection unit 2
Unwanted tripping due to pre-saturation of one CT -> not the same current measured at both protection units
Residual si dual Magnetism Magnetism – How How to get rid ri d of? of ?
Demagnetization process
Done automatically by the CTAnalyzer CTAnalyzer at the th e very end of the measurement
Residual si dual Magn Magne etism ti sm CT charged with DC (winding resistance measurement) CT specs: Class: 5P20 Ratio: 1000:1 Rated Burden: 30VA 30VA
[email protected]
Questions uesti ons and Re Remarks? marks ?
