4: Study of transient performance of capacitive voltage transformer

International Journal of Conceptions on Electrical and Electronics Engineering
Vol. 2, Issue. 1, April 2014; ISSN: 2345 - 9603
Study of transient performance of capacitive voltage
transformer
Ashish S Paramane, Avinash N Sarwade and Pradeep K Katti
Jayant G Ghodekar
Electrical Engineering Department,
Dr. Babasaheb Ambedkar Technological University,
Lonere, Maharashtra, India.
[email protected], [email protected],
[email protected]
Govt. College of Engineering,
Karad,
Maharashtra, India.
Abstract— Capacitive Voltage Transformers (CVTs) are widely
used as source of voltage signals for monitoring, protection relays
and control application at transmission and sub transmission
voltage level. The energy storing elements like inductors &
capacitors cause CVTs to exhibit transient behaviour. Further
the performance of protection devices is also affected by the ratio
errors of CVTs.
This paper addresses the issue of CVT transients using
PSCAD software package while analysing CVT ratio error
using experimental case study.
II.
CAPACITIVE VOLTAGE TRANSFORMER
The circuit diagram of CVT can be shown as in Fig.1
The present paper addresses the issue of CVT transients
using PSCAD software along with the results of ratio error
& thus proposes a solution of adopting rogowski coil in
place of CVT.
Keywords- Capacitive Voltage Transformer, Ratio error, PSCAD,
Rogowski Coil etc.
I. INTRODUCTION
Capacitor voltage transformer is used for protective
relaying purpose and monitoring purpose. The monitoring
equipment is required low voltage to operate. In purpose to
achieve power supply requirement for monitoring equipment,
capacitor voltage transformer is used to step down the high
voltage to low voltage. [1]
Theoretically, the output waveform of a CVT should be an
exact replica of the input waveform under all operating system
conditions. This requirement can easily be satisfied under
steady-state condition. However, electric power systems are
subjected to many types of disturbances that results in electric
transients due to lightning, system fault, line energization and
deenergisation, switching of inductive or capacitive load.
Under such transient conditions, the CVT output waveform
may not follow closely to its input waveform due to internal
storage elements such as capacitive, inductive and non-linear
components (saturable magnetic core) of the CVT. They take
time to dissipate their stored energy. Electromechanical relays
can cope with unfavourable CVT transients due to their
natural mechanical inertia at the expense of slower operation.
Digital relays are designed for high-speed tripping and
therefore they face certain CVT related transient problem.
[1][4]
Fig.1.Equivalent circuit diagram of Capacitive voltage transformer
Basically, the CVT model is composed of Capacitive Voltage
Divider (C1 and C2), Step-down Transformer (SDT),Compensating Series Reactor (SR), Ferro resonance Suppression
Circuit (Cf, Rf, Lf), and Overvoltage Protection devices
(Vgap, Rgap).[2][3]
Ferro-resonance:
In theoretically, the ideal CVT voltage output should be same
with the initial. Since CVT contains non-linear component
such as capacitance and inductance components, transient
voltage and current output will be produced. The suddenly
saturated current phenomenon is known as Ferro resonance.
Usually to reduce Ferro resonance, Ferro resonance
Suppression Circuit (FSC) is located in CVT. The purposes of
FSC are to prevent dangerous and destructive overvoltage.
FSC is installed at secondary transformer.
Two types of FSC model can be used in CVT as follows,
i. Active Ferro resonance Suppression Circuit (AFSC)
ii. Passive Active Ferro resonance Suppression Circuit
(PFSC) [2]
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International Journal of Conceptions on Electrical and Electronics Engineering
Vol. 2, Issue. 1, April 2014; ISSN: 2345 - 9603
III.
SIMULATION OF CVT TRANSIENTS IN PSCAD
Fig.2 shows the simulation of 400 kV, 50Hz system model
to show the effect of CVT transients whereas Fig.3 shows the
results obtained.
A. The system model for four fault positions is shown in
Fig.4 along with the transducer scheme employed as
shown in Fig.5. The breaker-1 & breaker-2 plots are
shown in Fig.6 & 7 respectively.
Fig.4 CVT transients for four different fault positions
Fig.2 PSCAD simulation of 400 kV system
Details of CVT used in CVT Simulation:
Upper Capacitor- 0.00484 µF
Lower Capacitor- 0.0880 µF
Compensationg Series Reactor- 1.42 H
Fig.5 Scheme used for sensing voltage of system & controlling fault positions
Fig.3 Results of Simulated System
Fig.6. Plots at Breaker B1
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International Journal of Conceptions on Electrical and Electronics Engineering
Vol. 2, Issue. 1, April 2014; ISSN: 2345 - 9603
V. ROGOWSKI COIL
The Rogowski coil is a conceptually simple device. Its
theory of operation illustrates some basic principles of electromagnetism applied in a practical device. The coil itself
provides an elegant demonstration of Ampère's Law and,
because of its inherent linearity, the response of a coil under
extreme measuring situations is much easier to treat
theoretically than iron-cored measuring instruments such as
CTs & CVTs.[5][6]
Integrator circuits are required due to differential inputoutput relationship of rogowski coil. The rogowski coil is
wound around the current carrying conductor whose current is
to be sensed as shown in Fig.8.[7][8]
Fig.8 Basic Diagram of Rogowski coil with integrator
A.
Performance of Rogowski Coil at 50 Hz:
1)
Table.1 – Results at the site installed for induction heating
application
Fig.7 Plots at Breaker B2
IV.
Sr
No.
CASE STUDY OF CVT RATIO ERROR
According to the calculation of CVT ratio,
= 3636.36
For the purpose of testing 1kV input to the CVT was
applied which must result an output of 275 mV. If it deviates
from the standard value it may affect the performance of
protective devices too.
B.
CURRENT
ROGOWSKI
INTEGRATOR
VALUE
OUTPUT
OUTPUT
1
10 KA
10V
20 mA
2
7.5 KA
7.5V
16 mA
3
5 KA
5V
12 mA
4
2.5 KA
2.5V
8 mA
5
0 KA
0V
4 mA
Linear Characteristics of Rogowski Coil
The results obtained as per the ratio error formula for the
line under consideration have been shown in ANNEXURE-A.
The results show that the ratio error of CVT exceeded the
limits. Such kind of error may occur due to stack capacitor
element failure & internal short circuit due to internal flash
over may be due to ageing effects.
Therefore for the calculation of ratio error, it can be
tabulated as in ANNEXURE-A
Where,
Vprim= Primary voltage
Vssc = Secondary Voltage
Fig.9 Linear V-I characteristics of Rogowski coil
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International Journal of Conceptions on Electrical and Electronics Engineering
Vol. 2, Issue. 1, April 2014; ISSN: 2345 - 9603
rogowski coils are found to give effective devices for sensing
the signals to relays.
REFERENCES
[1]
[2]
[3]
[4]
[5]
Fig.10 Application of rogowski coil for induction heating
From the results in Table-1, a linear characteristic is
obtained as in Fig.9. It can be seen that rogowski coil do not
posses any saturation or transient behaviour because of absence
of magnetising branch. The installed application prototype is
shown in Fig.10.
[6]
[7]
VI. CONCLUSION
From the results in Table-1 & 2, it is apparent that rogowski
coils overcome most of the limitations of the existing devices
such as CTs & CVTs because of absence of magnetising
branch. Further rogowski coils being air core the problem of
saturation doesn’t arise. Also problem like ratio error does not
arise because of absence of magnetising branch. Thus,
[8]
Dr Saad Mekhilef, Mr. Cheng Hock Lim and Dr Ab. Halim Abu Bakar,
“Investigation of Transient Performance of Capacitor Voltage
Transformer (Cvt)”, Journal - The Institution of Engineers, Malaysia
(Vol. 71, No.2, June 2009), PP-44-50,
Ahmad Zakuan Bin Ahmad Dahalan, “Analysis Of Lightning-Caused
Ferroresonance In Capacitor Voltage Transformer (Cvt) Using
Harmonic Method”. Thesis report
General Electric, “IEC Capacitive & Coupling Capacitor Voltage
Transformers (CVT & CCVT) guide” 2013.
Roger A. Hedding, “Where did those transients come from and how are
we going to handle them? A fresh look at CCVT transient’s
phenomena”
D. A. Ward and J. La T. Exon , “Using Rogowski coils for transient
current Measurements”, Engineering Science And Education Journal
June 1993
Special report Sponsored by the Power System Relaying Committee of
the IEEE Power Engineering Society, “Practical Aspects of Rogowski
Coil Applications to Relaying”, September 2010
Ashish S. Paramane, Dr. P.K.Katti , “Improved Performance in Current
Measurement of Rogowski Coil in Power System” International
Conference at Asia Pacific Institute of Technology (ICACCT-2013)
ISBN:978-93-83083-38-1,PP:183-188.
Ashish S. Paramane, Avinash N. Sarwade, Pradeep K. Katti, Jayant G.
Ghodekar,, “Rogowski Coil- A Novel Transducer for Current
Measurement” in press, 6th International Conference on Power System
Automation & Protection, by Central Board of Irrigation & Power, India
with CIGRE.
ANNEXURE-A
Table -2 Case study results of CVT ratio error
400
110
3636.36
5
Specified
of CVT
for 1 kV
(mV)
275
400
110
3636.36
5
400
110
3636.36
400
110
3636.36
at
CVT (kV)
at VT
(V)
Ratio
Specified
Ratio Error
(%)
Actual
of CVT
for 1 kV
Actual ratio
error at the
field (%)
293
6.55
275
291
5.81
5
275
292
6.18
5
275
295
7.27
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