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CURRENT-TRANSFORMER TESTING WITH CPC 100
David Topolánek
Doctoral Degree Programme (1), FEEC BUT E-mail: xtopol02@stud.feec.vutbr.cz
Supervised by: Petr Toman
E-mail: toman@feec.vutbr.cz
ABSTRACT
This paper describes main problems of current-transformers measuring. There is descrip-tion of parameters, which specifies an accuracy class of CT and also the method for deter-mination of those parameters according to standard IEC 600044-1 (
Č
SN EN 600044-1) by a measuring system CPC 100. In the end, measured results are compared with CT nominal parameters.
1.
INTRODUCTION
The instrument transformers or the current transformers (CT) are devices, whose primary current corresponds to secondary current and phases shifting of secondary current are in-considerable in comparison with shifting of primary current. Its mechanical design brings about distortion of secondary current and its bad function. Reason of this incorrect function is its ferromagnetic circuit. To be able to use CT properly with respect of error in mea-surement, we have to know its rated transformation ratio, accuracy class and parameters of transformer magnetization curve (knee point, instrument security number).
2.
THE MEASUREMENT OF ACCURACY CLASS
One of the transformer characteristic is that its primary current I
p
is independent on burden value Z
b
which makes it different from secondary current I
s
. Relationship between primary and secondary currents is defined by nominal ratio K
n
. This nominal ratio is always de-fined as rate of rated primary current I
pn
and rated secondary current I
sn
. /
sn pnn
I I K
=
e.g. K
n
= 100/5 A (1) Figure 1. shows the equivalent circuit of the transformer. From this circuit is evident that CT is weight down by error which is made by magnetic current i
e
. Value of error is speci-fied as difference between rated transformation ratio and actual transformation ratio. The current error (ratio error) [1] is expressed in percentage by equation (2).
( )
p psni
I I I K
100
⋅−⋅=
ε
(2)
Figure 1:
Equivalent circuit of current transformer
. The standard IEC 60044-1 (
Č
SN EN 60044-1) [1] defines the phase displacement
i
δ
in a similar way as current error. This is difference in vectors of primary and secondary cur-rents and is supposed to be positive if the vector of secondary current is overtaking the vec-tor of primary current
1
. Orientation of those vectors is defined as for an ideal transformer where is zero phase displacement. The standard defines accuracy class of measuring and protective current transformer, which specifies maximum errors. The errors correspond to rated current in percentage. The maximum current and phase errors are shown at the Table 1 and Table 2 [1].
Table 1:
The accuracy class of the measuring current transformer.
Table 2:
The accuracy class of the protective current transformer.
The errors are proportional to a burden of current transformer. The burden is impedance of secondary circuit express as an absorbed apparent power (VA) with rated power factor. The errors dependence on the shunt impedance
Z
e
(
mm
L R
,) and the burden
Z
b
is defined by equation (3) [2].
1
This definition is true only for harmonic voltage.
beb
zzz
+=+
rad ii
j
δ ε
(3) The secondary winding has to be short-circuited (has not to be no-loaded) when the CT is measured. It is very important for safety reasons. If this precondition is not realized, there are generated high-voltage peaks. These peaks can damage or destroy the transformer, but an accident hazard is more dangerous. The next reason, why secondary winding has to be short-circuited, is influence of remanent flux. If the CT is working with no-load secondary winding its iron core will be magnetized. The measured CT has two different burdens, the first is used for a CT as the measuring current transformer (5VA) its accuracy class is 0,5. The second burden is used for the pro-tective current transformer (10VA) its accuracy class is 5P. Both secondary windings (1S1-1S2 and 2S1-2S2) have to correspond to choice of accuracy classes, because it is double-core current transformer. Therefore the measuring has to be done for both CT cores with rated burdens 5VA and 10VA. The standard [1] defines power factor 1cos
=
ϕ
for the bur-den into 5VA and 8,0cos
=
ϕ
for the burden over 5VA. For measuring of accuracy class are used burdens with power factor 1cos
=
ϕ
(burden 5VA is replaced by resistance 0,2
Ω
and burden 10VA is replaced by resistance 0,4
Ω
). The CT has been demagnetized before the measuring starts. The measures errors of current and phase are shown on the Figure 2. and Figure 3. There are limits which matched the accuracy class 1 and 0,5 (Table 1).
-1,5-1-0,500,511,500,250,50,7511,251,51,752
I
p
/I
pn
[-]
ε εε ε
i
[ % ]
Class 0,51S1-1S2 Burden 5VA2S1-2S2 Burden 5VA1S1-1S2 Burden 10VA2S1-2S2 Burden 10VAClass 1
Figure 2:
The current error of the double-core CT
.
010203040506070809000,250,50,7511,251,51,752
I
p
/I
pn
[-]
δ δδ δ
i
[ % ]
Class 0,51S1-1S2 Burden 5VA2S1-2S2 Burden 5VA1S1-1S2 Burden 10VA2S1-2S2 Burden 10VAClass 1
Figure 3:
The phase displacement of the double-core CT
.
2.1.
T
HE
M
EASUREMENT OF
M
AGNETIZATION
C
URVE
The information about the current transformers iron-core is given by a magnetization curve. The ideal current transformer has no linear dependence between exciting voltage E
s
and exciting current I
m
,
which is necessary for excitation secondary current. This is the rea-son why the exciting voltage E
s
has to be increased if bigger burden is connected. This vol-tage can be increased into limited (maximum) voltage E
lim
. If the exciting voltage E
S
ex-ceeds limited voltage E
lim
, the CT will be saturated and its function will be incorrect.
IEC/BSANSI 45°
ANSI 30°ANSI 45°ANSI 30°1,0V10,0V100,0V0,001A0,01A0,1A1,0A10,0A
I
m
[A]
E
s
[ V ]
2S1-2S21S1-1S2
Figure 4:
Magnetization curve of current transformer
.

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