CICED2008
Technical Session 1
81-47
Distribution network equipment
CP1510
Self-excited measurment of CVT and its error analysis
LI Tao, DU Xiao-ping,
zhu jian-sheng, LIU Huang-guang, LI Jing-hua, NIU Qing-song
Linyi Power Supply Company of, Linyi, Linyi 276003, China
E-MAIL:lita04541@126.com
Abstract: Measuring partial-voltage capacitance and tan 8
As a result of it's high performance, the Capactive
value of superposition capactive voltage transformer (CVT)
Voltage Transformer (CVT) have been applied extensively.
by CVT self-excited method is discussed. This paper
From the principle figure I, we can know that the
introduces
the
middle-transformer T in the electromagnetic unit (the B in
frequency-conversion bridge, and what we should pay
figure) have two major types: having derivation line and
attention to in the test . The feasibility and necessity of
having
self-excitation measurement for superposition CVT are
capacitance unit in the oiltank under the low pressure
how
to
perform
test
on
CVT
with
no
derivation
line.
The
basic
analyzed by experiment. Also, the main factors that affect
partial-voltage
self-excitation measurementare discussed. On this basis, some
middle-transformer T and compensating reactor Land
problems worth attention are indicated concerning CVT
impedance Z . The CI is a high voltage capacitance.
capacitance
C2
,is
partial-voltage
make
up
of
operation, maintenance, and test.Test data make it clear that
improving self-excited method adapts to on-site test.
:---- -----------0-----
Keywords: Capactive Voltage Transformer (CVT);
self-excited method
;
error analysis
;
1
partial-voltage
1n
Tid
l__T~-~------Ltzj
Foreword
d
Capactive Voltage Transformer (CVT) is a kind of
electric power equipment which is used more extensive in
Shandong power system. Because of it's preferable preventing
resonance
capability,
CVT
also
can
be
used
as
a
communicating carrier capacitance at the same time. The
CVT is gradually substituting original Electromagnetic
Voltage Transformer in systems of II0kV and above. So it
become a kind of essential equipment in the
power
system.At the present time, the bulk of CVT is superposition
structurel'ISince it's capaitor divider can't be
separated
from the electromagnetic unit, there are much problem when
measuring partial-voltage capacitance and tan 8 value of
1a
1
1
I
capacitance ; dielectric loss (tan 8 )
1
B
I
:
1 I"
d:
Fig. 1 Circuit diagram ofCVT
2.2 The working principle
conversion bridge
The frequency-conversion bridge
of
frequency-
is base on the
electronics and the microprocessor technique" frequencyconversion
anti-jamming
technique
and
digital
filter
technique. Exerting the voltage of certain frequency on the
tested equipment and the standard capacitor, we can get
capacitance and tan 8 value through comparing the values
of their current and phase.
superposition CVT on-site test.
x.I .
Therefore, it's necessary for us to study applicability and
accuracy of the self-excited method, and look for a
R
practical
1 XCt )
improving method in this paper.
2
The basic principle of CVT and frequencyconversion bridge
2.1 The basic structure and working principle of
CVT
Fig. 2
Equivalent circuit of measuring CX
In Diagram 2:
Rt-----the sampling resistance of the standard capacitance
circuit
CICED2008
Technical Session 1
Distribution network equipment
R2 --the tested capacitance circuit
When we test by the self-excited method, CVT's
CN--the Equivalent capacitance of standard capacitor
middle-transformer is acted as a experiment transformer. The
Rx--the equivalent resistance
exerted voltage on its secondary coil can arouse magnetizing
Cx--the equivalent capacitanceof the tested equipment.
Through carrying on an analysis and calculation With
the u Rl(t) and the u R2(f) , we can figure out the phase
difference Sx between the
time,
we can
and the
iN(t)
iX(f).
iX(f)
and capactive current
on primary winding which can be used as power supply to
measure the value of capacitance C2 and C I and tan 0 .
At the same
work out the dielectric loss value tan Ox
resistance current
current .At the same time , the high voltage can be induced
iX(f)C
3.1
~
of the
Measuring the value of capacitance C. and
tant5.
3.1.1
tested equipment.
As known from Diagram 2:
Measuring the value of C. and tand, by self-excited
method
Since the experiment voltage can't be exerted on B
terminal directly, the testing voltage can be rised by
(1)
middle-transformer of the electromagnetic unit. It's very easy
·2 fil"
U R2(t)
•
lX(t)=--~-J 1i '--'xus
R2
From the formula
CX
(1)~
()
us(t).
•
(2)
t +--=lx(t)C+lX(f)R
Rx
to measure C I of CVT by self-excited method with the
frequency-conversion bridge .The A point connects signal
(2) .we can know that:
= iX(t)C
. crN
rrr
terminal. The experiment voltage of C I can be exerted
indirectly on the <i terminal and the high-voltage terminal of
(3)
=m~N
standard capacitor (C N) .As known from the figure 3, we can
IN(t)
iX(t)C
m=--
see the equivalent circuit of measuring C I and tand1 value by
(4)
self-excited method.
iN(f)
In the formula:
j---plural factor. It means that the phase difference
between current and voltage
is plus 90 0;
f---the output frequency of the frequency-conversion
bridge;
m---the ratio of the capactive current in tested equipment
to the current in the standard capacitance circuit
Fig.3
From Diagram 2 , we can know the dielectric loss value
ofCX.
Equivalent circuit of measuring Cl
3.1.2 The error analysis of measuring Cl and tanol value
by self-excited method
(5)
From figure 3 .we can know that the value of
2.3
capacitance C2 and the insulation resistance
Measuring the CVT by self-excited method
The
frequency-conversion
bridge
has
been
terminal will directly have an influence on the measure
used
results.
extensively , because it has a good ability to limit primary
voltage
( Ro ) of <i
(1) In the vectorgraph 4(a),
the dielectric loss angle of
and secondary voltage of the CVT. When measuring
C1 is equivalent to the phase difference between the current
the CVT by the frequency-conversion bridg with self-excited
IN and leI [3] without taking the influence of the capacitance
method,
C2 and the <i terminal into account.
it can effectively avoid
resonance. Because the
capacitance C2 and C I in the superposition CVT are
connected for a integral,
(2) In the vectorgraph 4(b) , without calculating the
measuring the capacitance and
influence of the capacitance C2 and the <i terminal,
tan 0 value of capacitance C2 and C I with self-excited
see the formula
we can
( 6) and (7):
method is recommended in the specification'f .
(6)
Through correcting the error of test result according to
the practice, the testing result can reflect their actual value of
(7)
capacitance and tan 0 more accurately.Therefore .we also
can give an accurate judgment to the CVT's quality.
In the formula:
2
CICED2008
Technical Session 1
Distribution network equipment
The subscript
p---the actual value
testing precision of measuring instrument through distributed
The subscript
m---the measured value
capacitance. It will make the testing value more bigger than
Because of the influence of the C2,
C1p<C 1m,
the actual value.
iiClp>iiClm. When C2»CN, the influence can.be ignored.
l
(3) When we calculate the ii terminal influences and
ignore the taniic2' we can see the formula (8) and (9) from
the vectorgraph 4( c):
CI P ~
tan
0CIP
Cl m • C2
(8)
(C 2 + CNtan a)
~ tan
0Clm -
1
(
m R g(C2 + CN )
Fig. 5 Equivalent circuit of measuring C2
(10)
m R g(C2 + CN )
The tanu is
9)
3.3 Other error analysis and attentions of
self-excited method
the ratio of the resistance current flowing
through the ii terminal to the capacitive current of standard
capacitor CN'
(1) The voltage of electromagnetic unit
When the ii terminal has fault insulation, the
interference source will disturb testing accuracy of measuring
measured value of C 1 will be bigger more than the actual
instrument through ii terminal and distributed capacitance. It
value. Because of the leakage resistance Rii, the voltage phase
will make the measured value bigger more than actual
of ii terminal will lead the testing power supplies phase.
value[4l.
As a result,
the current phase of standard capacitor (CN)
will be lead. The measured dielectric loss value of C 1 will be
(2) According to the formula ZL=l/ro(C1+C2), ZL
includes compensating-reactor's
bigger more than the actual value. From the formula 10, we
transformer's drain
can see the increased value of the dielectric loss value.
3.2
middle-transformer when measuring C2.
Therefore, the measuring voltage can't be exerted too
high. Moreover, the capacity of secondary winding in CVT is
not high, generally not more than 200VA. Owing to the high
magnetizing voltageof secondary winding, it causes testing
(
c ) calculating R 0
instrument protection to be acted or testing precision to
Vectorgraph consideing C2 and R 8
bedisturbed.
For example , the frequency-conversion bridge that we
Measuring the value of C2 and tano2 by
self-excited method
use commonly is applied to that the testing high voltage is
less than 4 kV . At the same time ,low-voltage magnetizing
From figure 5 .we can see that experiment voltage is
current is not allowed exceeding 30A. It is worry about that
exerted on B terminal indirectly through middle-transformer
the high voltage
of electromagnetic unit. Since C1is in series with CN, it will
(3) Measuring superposition capactive voltage transformer
to its small influence, we can ignore it.The testing accuracy
( CVT)
should be influenced by electromagnetic unit in a similar way
500-1000 V in 220 kV equipment area, As a result, the testing
connected with the ii terminal on electromagnetic unit . The
precision should be disturbed.
connecting wire is so nearly with adjustive winding of
capacitance. The
voltage
as
to
bring
of adjustive
some
4
distributed
winding
by self-excited method on site,We usually adopt the
way of part blackout. The induced electricity can reach
of positive connection. The low-voltage terminal of C2 is
reactor
and magnetizing current will bring
experiment instrument to fault.
influence the dielectric loss of branch circuit with CN. Owing
compensating
measuring C2, C2 is
voltage is smaller than the voltage directly exerted on
"
Fig. 4
reactancell.When
close to inductance ZL. Therefore the exerted secondary
11""
( b) calculating C 2
reactance and middle-
in series with ZL .At the same time , the capacitance ZC2 is
,,
,,,
,,
,,,
,,
,, ,,
(a) ideal state
as an
within
Conclusions
We can arrive at a conclusion that test results of
compensating reactor as a interference source will influence
3
CICED2008
Technical Session 1
Distribution network equipment
partial-voltage capacitance and tan 8 value of the CVT by
[3]
Huang Tao, Liu Van, Li Guowei. "The Use of the
self-excited method should be proofread. On this basis, the
Frequency-Conversion Bridge in Test of the Capacitor
test results can reflect the true device status. Test data make it
Voltage Transformer without Disconnecting the Wire".
clear that improving self-excited method adapts to on-site
High Voltage Engineering, Vol 56, No.7, pp. 32-36,
test.
July. 2004.
[4]
Liang Zi-meng,Tan Van-mingo "tanbTest and Analysis
References
for CVT's EM Unit".High Voltage Engineering, Vol 72,
[1]
No.8, pp. 36-41, August. 2006.
Jin Xu-dong, Jing Ke-yu. "Self-excited Method for
Superposition CVT". Jiangsu Electrical Engineering,
[2]
[5]
Zhang Ren-yu, Chen Chang-yu, Wang Li-chang. The
Vol 26, No. 11, pp. 19-22, November. 2005.
high electric voltage experiments technique, Qing Hua
Chen Huan-gang. The electric power equipments
university publishing company, Bei jing,2003.
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experiments
technique,
Chinese science technique publishing
method
and
diagnosis
company, Beijing,2001.
4