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A3-106
CIGRE 2012
Critical Design Aspects for Reliability of 1200 kV UHV AC Capacitor Voltage
Transformer under Severe System Conditions.
R.K. TIWARI
M. KURRE S. MAHENDRA
K.M. NAIK A. DHAMIJA
BHARAT HEAVY ELECTRICALS LIMITED, BHOPAL
INDIA
SUMMARY
Indian Power system is expanding at an accelerated pace due to rapid industrialization. The Planning
Commission projections indicated 3 times increase in peak demand by 2021-22. Power generating
units are being installed more and more at remote places due to unevenly distributed energy sources in
the country. Right of Way, High Transmission losses, economical transmission of bulk power over
long distances etc are the main factors for considering the up-gradation of existing transmission
system voltage in a country like India having dense population. 1200 kV is considered as the next
level transmission voltage to overcome the above challenges. [1]
Capacitor Voltage Transformer (CVT) is used in the power system for measurement of voltage,
protection of Bus Bar against over voltage and PLCC (Power Line Carrier Communication)
application. It is one of the most complicated equipment in the switchyard because of its sensitivity to
the system transients. This paper discusses the critical design aspects of a reliable CVT considering the
system severity and all system transient conditions like ferroresonance, Electrostatic field distribution,
Air saturation at high switching transient voltages, Corona Suppression etc. This paper also illustrates
the anticipated problems for reliable operation of the equipment and recommendations to overcome
these problems. Some of the unique features of 1200 kV CVT discussed in this paper are internal and
external design of Capacitor voltage divider, various methods of ferroresonance damping and the
method adopted in 1200 kV CVT, electrostatic field distribution and design of corona shields.
Apart from the above unique features, the 1200 kV CVT has to withstand many system transients in
the system which can affect its performance. As the solid state relays are used, the CVT has to
faithfully reproduce the changes on the primary voltage but this is also the time at which transients are
produced in the systems. The CVT needs to be designed to take care of these transients, so that much
superior transient response is guaranteed.
Finally the approach for successful development and testing of 1200 kV Capacitor Voltage
Transformer is illustrated in this paper.
KEYWORDS
Capacitor Voltage Transformer, Capacitor Voltage Divider, Intermediate Voltage Transformer,
Electrostatic Field Distribution, Ferroresonance, Damping, Air Saturation.
rktiwari@bhelbpl.co.in
1.0
INTRODUCTION:
Capacitor Voltage Transformer (CVT) is one of the most important equipment in the Power System.
The measurement of the system voltage and its protection depends on the performance of CVT. The
CVT comprises of Capacitor Voltage Divider (CVD) and Electromagnetic unit. The Capacitor Voltage
Divider consists of high voltage capacitance (C1) and intermediate capacitance (C2). The value of
high voltage and intermediate capacitance is such that it transforms the high voltage to a medium
voltage. The Electromagnetic unit further steps down this medium voltage to the measuring voltage.
The CVT is represented as following figure
A
Lr
a
C1
H. V.
C A P A C IT O R
C2
IN T E R M E D IA T E
C A P A C IT O R
IV T
D A M PIN G
D E V IC E Z 0
n
V OLTAG E
A D JU S T M E N T
W IN D I N G S
Fig: 1 Schematic Diagram of CVT
Depending upon the design philosophy, Lr is provided either between Intermediate Voltage tapping
points and Intermediate Voltage Transformer (IVT) primary HV end or between earth end of IVT
primary and earth. Also the voltage adjustment windings are connected either at the HV end of IVT
primary or at its low voltage end.
Considering the requirement of 1200 kV transmission voltage in India, it was decided to develop 1200
kV CVT.
2.0
FINALIZATION OF CAPACITOR VOLTAGE TRANSFORMER (CVT)
RATING:
The technical Specification of 1200 kV CVT has been evolved through Technical Committee formed
by International and National experts of the Instrument Transformer along with the system experts for
1200 kV National test station. Based on the system envisaged and extensive simulation study of the
proposed UHV line configuration, the following specification was arrived at for the 1200 kV CVT.
Table: I Technical Specification of 1200 kV CVT
Applicable Standards
Highest system Voltage
BIL
SIL
Voltage Ratio
Voltage Factor
Capacitance
Simultaneous Burden
IEC 60044 – 5
1200 kV
2400 kVp
1800 kVp
1150 kV/√3 : 110 V/√3
1.2 Cont. / 1.5 for 30 sec.
2000 pF +10%, -5%
50 VA
1
3.0
CAPACITOR VOLTAGE DIVIDER (CVD):
The number of capacitor units to form the Capacitor Voltage Divider (CVD) depends on many
parameters such as Impulse Voltage Withstand level of individual capacitor unit, manufacturing
technology, ease of handling at manufacturing unit and at site etc. The number of capacitor units to be
selected after critical analysis of all the above factors.
3.1
Higher Dielectric Stress:
The design of CVD shall be such that it should withstand the Dielectric stresses throughout its service
life. The CVD has to withstand stresses due to lightning & switching impulses in addition to the
stresses due to continuous voltages and temporary over voltages.
3.2
Internal Dielectric Design:
The capacitor elements of the developed CVD comprised of tissue paper having dielectric constant of
1.2 and polypropylene film having dielectric constant of 2.25, impregnated with synthetic liquid. The
capacitor elements are mounted inside porcelain housings and are connected in series. The number of
capacitor elements depends on the rated voltage and Lightning Impulse Voltage Withstand level. The
temperature co-efficient of the tissue paper is positive and that of polypropylene film is negative. The
overall temperature co-efficient of the CVD shall be almost near to zero to ensure stability of accuracy
over the entire temperature range. To keep the overall temperature co-efficient of the CVD near to
zero, the ratio of tissue paper and polypropylene film thickness has been judiciously decided.
The utmost precaution shall be taken during winding and drying & impregnation process of capacitor
elements. These processes are the backbone of the reliability of CVT. The capacitor housings shall be
sealed immediately after processing to avoid ingress of any moisture.
4.0
AIR SATURATION AT 1200 kV AND SWITCHING IMPULSE VOLTAGE
LEVEL OF 1800 kVp:
For 1200 kV CVT having Switching Impulse Withstand Level of 1800 kVp, the flashover
characteristics of air insulation is very critical. As the Switching Impulse voltage increases beyond
1000 kVp, the external air starts saturating. The saturation results into reduction of flashover withstand
level of the porcelain. Thus much higher flashover clearance is required as compared to 765 kV CVT.
The flashover characteristic of air insulation is a very important factor while designing 1200 kV CVT.
The minimum air clearance required and the final product dimension taking suitable margins by
judicious design approach is an important design aspect for the reliability of CVT.
For determining the air spark over clearance reference is made of [2]. The minimum air clearance for
switching impulse level of 1800 kVp was kept more than 8 m. This was further validated by various
high voltage tests at in-house Ultra High Voltage Laboratory.
5.0
FERRORESONANCE STABILITY:
Due to the presence of iron cored Intermediate Voltage Transformer and the capacitance of the CVD,
the 1200 kV CVT can enter into Ferroresonance oscillations. The phenomenon of the Ferroresonance
can lead to high permanent over-voltages, over-currents, distorted voltage and current wave forms,
heating of intermediate transformer, undesirable tripping of protection devices etc. For 1200 kV
system voltage, the higher value of capacitance (C1 + C2) where C1 is Primary Capacitance and C2 is
Secondary Capacitance ultimately requires increased damping.
2
5.1
Calculation for Ferroresonance Damping Device:
L
C
RL
Z
1
L:
Z 2 Z0
Fig: 2 General Equivalent circuit of CVT
For calculating the parameters of ferroresonance damping device, following method [3] has been used,
except the derivation involving quadratic equations for arriving at the final solution.
When third sub harmonic oscillations are considered which is most difficult to damp, the magnetizing
current ie of the non linear inductance may be represented by:
ie = a1ψ + a 3ψ 3
---------- (1)
Where ψ is flux linkage, a1 and a3 are coefficient and ω is the angular frequency that equals 2πf , f
being the power frequency
 ωt 

 3 
ψ = ψ 1 cos(ωt + φ ) + ψ 3 cos
---------- (2)
From 1 & 2
 ωt 
 ωt

ie = F1 cos(ωt + φ ) + F2 cos(ωt ) + F3 cos  + F4 cos + φ 
3
3
 


Where F1, F2, F3 & F4 are function of ψ 1 &ψ 3
----------- (3)
Primary voltage V = Vm sin (ωt + α )
When reactor Lr is tuned, the voltage drop across series elements is negligible, hence:
ψ1 =
Vm
ω
; and α = φ + π
 ωt 
 ωt

 + F4 cos + φ 
 3 
 3

Sub harmonic Current i3 = F3 cos
---------- (4)
Where
3
3


F3 =  a1 + a 3ψ 32 + a3ψ 12  ×ψ 3 ;
4
2


F4 =
3
a3ψ 1ψ 32
4
The third sub harmonic equivalent circuit can be expressed as:
3
L
C
RL
^
1
I
^
0
Z1
^
I3
I
I
^
2
Z2
Z0
Fig.: 3
Complex variables are used further as the above circuit is for a single frequency
I1 + I2 + I0 = -I3
----------- (5)
or
ω 
− j  ψ 3Y = F3 + F4 e jϕ
3
----------- (6)
Where Y = Complex admittance of 3 parallel branches Z0, Z2, /Z1+L+R1+C/, at third sub harmonic
frequency ( ω / 3 )
Separating real and imaginary parts for equation 6, the value of ψ 3 is given by:
 4ω

4
ψ 32 = 
Im(Y ) − a1 − 1.5ψ 12  ±
9
 9a 3

4
± − 1.75ψ +
3a 3
4
1

ω
 2  4ω
 3 Im(Y ) − a1 ψ 1 −  9a Re(Y )


 3

2
------------ (7)
The sub harmonic oscillations cannot be sustained if ψ 3 is not real positive. Therefore, if the
expression under square root is always negative whatever be the value of fundamental flux ψ 1 (or
Fundamental voltage), that is CVT will not have sustained ferroresonance at any voltage.
For deriving this condition the theory of quadratic equations ax2 + bx + c = 0 is used, from which it is
well known that the sign of (ax2 + bx + c) is same as that of 'a' when
b2- 4ac ≤ 0 or b2 ≤ 4ac.
Therefore, the expression under the square root will always be negative when
2
 4 ω
 4ω


×  Im(Y ) − a1  ≤ 4 × 1.75 × 
× Re(Y )


 3a3  3
 9a3

4 ω

×  Im(Y ) − a1  ≤
3a3  3

7 x
4ω
× Re(Y )
9a3
2
----------- (8)
----------- (9)
Where Im(Y) = Imaginary complex admittance and
Re(Y) = Real complex admittance
4
From equation -- (9)
ω
3
× Im(Y ) − a1 ≤
Im(Y ) −
3a1
ω
7
× ω × Re(Y )
3
---------- (10)
− 7 Re(Y) ≤0
---------- (11)
Equation – (11) has been used for designing the damping circuits for the condition of no sustained
Ferroresonance at all voltage levels.
5.2
Ferroresonance Damping Device:
The suppression of Ferroresonance needs extensive and careful study of various methods of damping
and adopting the best possible technique. The damping device consists of R-L-R high impedance
circuit under normal conditions. The damping is increased manifold on the initiation of sub harmonic
oscillation and the device reverts back to the normal state of high impedance when the Ferroresonance
oscillations subside. The design is made such that the 1200 kV CVT will not enter in the
Ferroresonance zone of permanent oscillations at any voltage upto 150% rated voltage and even
beyond. A few of the possible damping devices are given below:
L
C
Series impedance Ferroresonance damping circuit
R
L
R
L
R
R
L
C
R
R
Parallel impedance
Ferroresonance
damping circuits
Fig: 4 Various Damping Devices
After evaluating the merits and demerits of the above possible damping circuits, the non electronic RL-R damping circuit shown at the end of the above figure has been adopted. This gives better
performance and reliability as compared to electronic damping devices. The damping device used in
the developed CVT has given the satisfactory performance during type testing for Ferroresonance test
at 80%, 100%, 120% and 150% rated voltages according to IEC: 60044-5.
6.0
ELECTROSTATIC FIELD DISTRIBUTION:
The electrostatic field distribution in a 1200 kV CVT is very critical to withstand transient voltages.
Design of the Corona Shield suitable for suppression of external corona during transient as well as
power frequency over voltages depends on this field distribution. Also the total height of CVT,
internal and external stresses, height of the mounting structure etc, are the factors affecting the corona
5
suppression. The electrostatic field distribution study shall be carried out to calculate the stresses at the
surface and at the nearest electrode.
After detailed study, it was decided to provide 1 No. top corona shield having two rings of 400 mm
dia. and overall dia. of 1500 mm. 2 nos. Corona shields having ring dia of 250 mm and overall dia of
1000 mm have been provided on 4th & 5th porcelain housings. The CVT has successfully withstood the
lightning and switching impulse level of 1800 kVp and 2400 kVp at the test lab.
Fig: 5 Corona Shields provided on the 1200 kV CVT
7.0
PRODUCT REALISATION:
1200 kV, 2000 pF Capacitor Voltage Transformer has been successfully designed, manufactured and
routine and type tested at in-house Ultra High Voltage Laboratory. The CVT has been installed at
1200 kV National Test Station.
7. 1
Performance Evaluation Tests:
The developed CVT has successfully withstood all type tests in addition to the routine electrical tests.
Some of the type tests are as below:
•
•
•
•
•
Verification of Accuracy on Equivalent circuit of CVT.
Ferroresonance test on CVT.
High Frequency Capacitance and Equivalent Series Resistance Measurement on CVD.
Dry Lightning Impulse & Chopped Impulse Voltage withstand test.
Dry Switching Impulse Voltage withstand test.
6
A photograph of the developed 1200 kV CVT under test is shown below:
Fig. 6 1200 kV CVT under test
8.0
CONCLUSION:
As the system voltage is increased higher and higher, the system severity increases exponentially. The
successful development of 1200 kV CVT is a result of detailed study of each and every critical aspect
for UHV class equipment for its reliable operation in a severe system conditions. The performance of
1200 kV CVT during stringent routine and type tests at in-house validates the design concepts.
9.0
ACKNOWLEDGEMENT:
The authors would like to thank the management of BHEL for giving permission to publish this paper.
We also thank our esteemed customer M/s POWERGRID for giving us the opportunity to develop
1200 kV CVT for their 1200 kV experimental line at BINA which is the first of its kind in the country.
7
BIBLIOGRAPHY
[1]
[2]
[3]
1200 kV National Test Station: Key Issues (GRIDTECH 2011, 19-21 April 2011, Pages 3-11)
High Voltage laboratory planning by Mr. Nils Hylten Cavallius.
Influence of Ferroresonance suppression circuits upon the transient of capacitive voltage
transformers (International Conference on Developments in Power System Protection. Power
and Control and Automation Divisions of the IEE London, 11-13 March 1975, Pages 181188)
8