Energy and Power Engineering, 2013, 5, 1240-1242
doi:10.4236/epe.2013.54B235 Published Online July 2013 (
Analysis on the Influence Factors of Capacitor Voltage
Transformer Dielectric Loss Measurement
Yongdong Li, Qing-da Meng, Po, Yang Zheyuan Zhao, Wei Zhang, Zhuo Pan
Jibei Electric Power Maintenance Company, Beijing, China
Received April, 2013
Capacitor voltage transformer (CVT), which is with simple structure, convenient maintenance, functional diversity and
high impact pressure strength, is widely used. And its capacitance and dielectric loss Angle measurement is an impor-
tant test on testing the insulation of the equipment. This paper is mainly to introduce and discuss one of the CVT test
methods——the “self excitation method”, combined with the actual situation encountered in the work, sums up and
expounds the maintenance of CVT and preventive test of influence factors.
Keywords: Capacitive Voltage Transformer; Dielectric Loss; the Influencing Factors
1. Introduction
In recent years, the capacitive voltage transformer is widely
used, because of insulation structure reasonable, high
dielectric strength, can make full use of the carrier com-
munication required for the coupling capacitor, in more
than 110kV voltage grade [1].
2. The CVT Transmission Structure
Capacitive voltage transformer (CVT) is combined with
the capacitive voltage divider, electromagnetic unit (in-
cluding medium voltage transformer, reactor) and the
terminal box. And the wiring principles are shown in
Figure 1.
Capacitive voltage transformer, which is set as a con-
verting device between the high tension line and ground,
is to drop the system voltage into medium first, and after
converting secondary voltage through intermediate
transformer, the voltage will be output to the measuring
instrument and relay protection device [2].
3. The Significance and Principle of CVT
Dielectric Loss Measurement
For CVT in operation, its running situation overall is
good, however, how to discover the CVT defects, such as
the manufacturing quality, water be affected with damp
be affected with damp, oil shortage, insulation aging
factors and determine the operation state is of great sig-
nificance to its safe and reliable operation. At present,
each capacitance differential pressure unit and the di-
electric loss value of CVT was tested by the power out-
age method as one of the preventive test project is also
the principal means of judging its running status.
According to the structure it is divided into separate
loading CVT and Superposition CVT, and the Superposi-
tion CVT which has no intermediate extraction voltage
terminal is in widely used in China. When this kind of
CVT is in field operation, we take the Self-excitation
method to measure the partial pressure capacitor of ca-
pacitance and dielectric loss instead of the common test-
ing methods [3]. Self-excitation method is based on the
middle of the transformer as a testing transformer, ex-
cited voltage from the secondary side, Induction of high
pressure as power supply for measurement. In recent
C11, C12 - coupling capacitance; C13,C2—partial-voltage capacitance ;
L—compensation reactor; T—medium voltage transformer
Figure 1. The principle diagram of the capacitive voltage
Copyright © 2013 SciRes. EPE
Y. D. LI ET AL. 1241
Figure 2. Self-excitation measurement schematic diagram.
the excitation method, which can achieve accurate meas-
urement of the partial pressure of capacitance values and
dielectric loss, has been used widely in the CVT routine
test at the scene of the substation. Its measuring principle
is shown in Figure 2.
In theory any secondary terminals can be used as a
self-excited terminal test. But in fact, considering the
capacitor voltage effect in the unit test process, we
should choose the secondary terminal of large capacity as
self-excited terminals to make the higher applied voltage
and the test values are more accurate [4]. The capacity of
da-dn in secondary terminal is the largest in most of the
CVT, and with damping resistance winding, da-dn for
the remainder of the experiment can make the process
safer, so the self-excitation capacitance measuring pres-
sure method generally choose da-dn terminal is pressur-
Considering the insulation safety factors in low pres-
sure end, the test voltage should not exceed the end of C2
maximum allowable voltage, 2 kV is selected as a gen-
eral method of self-excited test voltage in field experi-
4. Analysis of Influence Factors of
In CVT dielectric loss measurement process, errors
sometimes influence the accuracy of the measurement
due to some of the factors. The followings analyzed
some common influence factors.
4.1. The Influence of the Temperature
Temperature has a big influence on the measurement of
dielectric loss Angle tanδ, the extent of the impact varies
from the different materials, structures [5]. In general,
tanδ increases with temperature rise. Some insulation
material when temperature is below a certain threshold,
the tanδ may be increased with the decreasing of tem-
perature; And wet material under 0 when water
freezes, tanδ will be lower. So, measured the insulation
of the dielectric loss value in both too high and too low
temperature cannot reflect the real situation, but easy to
lead the wrong judgment. As a result, the measurement
of dielectric loss should be not less than 5.
4.2. The Influence of the Test Voltage
In general, at its rated voltage range, tan delta value is
almost unchanged. The tanδ of good insulation is not
significantly increased with the rising of over voltage.
But things are different if bubbles, delaminating, shell
happens in the insulation [6]. When the test voltage is not
enough to make the air bubbles or air gap of the insula-
tion free, the tanδ and normal has no obvious difference;
When test voltage makes the air in insulation free and
produces corona or partial discharge, the tanδ will be
increased as the test voltage increases. Several kinds of
typical testing curve is shown in Figure 3.
4.3. Standard Capacitor
Capacitance and dielectric loss of the standard capacitor
plays an important role in the measurement of CVT by
applying self-excitation method. The accuracy of C2 has
certain relationship with these parameters since Cn is in
series connection with C1. For example, when a CVT
(Model TYD110/—0.01 H, Nominal voltage divider
C1 = 12500 pF, C2 = 50000 pF, dielectric loss tanδ =
0.1%) is tested with Cn = 100 pF, C1 equivalent to series
connection of a resister and a capacitor [7].
1- good insulation; 2 - when the insulation aging; 3 - there is air gap in
insulation; 4 - insulation be affected with damp be affected with damp
Figure 3. Tan delta and the curve of voltage.
Copyright © 2013 SciRes. EPE
Copyright © 2013 SciRes. EPE
The error of the standard capacitor becomes:
And the equivalent dielectric loss is:
As is shown above, the measurement error is negligi-
ble as long as the standard capacitor is properly opted, i.e.
capacitance high and dielectric loss low, or the accuracy
will be affected.
4.4. Insulation of the Low Voltage End
As a result of insulation degradation of the terminal
block on the low voltage end of the voltage divider, the
measured dielectric loss of C1 may exceed its true value
by applying self-excitation method. Since terminal N has
a relatively high potential about 2 kV in testing, leakage
current upon the minor bushing and terminal block will
lower down the accuracy due to the degradation of insu-
lation, which is commonly caused by damp conditions of
the secondary terminal box. In order to get the true value,
measurements can be put out after drying process.
In the case of C2 testing, the main influencing factor
lies in the method of measurement, since terminal N has
a relatively low potential and directly connects to the
testing bridge. The error is negligible and the testing re-
sult makes approximately its true value.
Many factors will have influence on the testing result
by applying self-excitation method, yet the measurement
value of C2 well indicates the real situation of the voltage
divider, as it shares the same insulator and the corre-
sponding dielectric loss are of the same, while the higher
value of C1 represents the damp conditions of secondary
terminal and minor insulator.
4.5. Other Factors
The voltage of PT components interfere the voltage phase
and amplitude of terminal N through the distributed ca-
pacitance coupling, thus affect the measurement accuracy
Electric filed in the vicinity caused by other operating
equipments also influence the testing result. To eliminate
the interference, frequency conversion method is often
applied in field testing.
5. Conclusions
As is discussed, the accuracy in the measurement of CVT
dielectric loss is influenced by various factors, i.e. ambi-
ent temperature, testing voltage, standard capacitor and
the insulation status of low voltage end, the last two of
which have greater impact on the testing result and are
commonly emerged in field. When the measurement
value exceeds its true value, further analysis should be
carried out before judgment so as to ensure the stable
operation of power grid.
[1] T. X. Chen, Y. Z. Wang and S. J. Hai, “Electrical Test,”
China Electric Power Press, Beijing, 2008.
[2] W. He, “Measurement of Tangent Loss in Capacitor
Voltage Transformer,” Northwest China Electric Power,
Vol. 31, No. 5, 2003.
[3] W. L. Yang, H. Q. He and H. S. Wang, “Impacts of CVT
Damped Secondary Outlet Block on Dielectric Loss
Measurement by Self-Excitation Method,” East China
Electric Power, Vol. 39, No. 9, 2011.
[4] T. Li, X. P. Du and H. G. Liu, “Discussion about
Self-excited Method Error on Capacitive Voltage Trans-
former,” Power System Protection and Control, Vol. 37,
No. 5, 2009.
[5] Q. Rao, “Discussion about Measurement on 110-220 kV
Capacitive Potential Transformer,” Guangxi Electrical
Engineering, 2006.
[6] Z. M. Liang and Y. M. Tan, “Tanδ Test and Analysis for
CVT’s EM Unit,” High Voltage Engineering, 2006.
[7] X.D. Jin and K. Y. Jing , “Self-excited Method for Su-
perposition CVT,” Jiangsu Electrical Engineering, 2005,
[8] Y. G. Yue, J. B. Yin and Y. P. Wang, “Analysis on 500
kV Capacitor Voltage Transformer Site Testing By
Self-excitation Method,” Power Capacitor & Reactive
Power Compensation, Vol. 31, No. 4, 2010.