FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
Transient and ferro-resonance damping circuit for Coupling Capacitor Voltage Transformer
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTORS
Dr Vinay Kumar Jaiswal & Dr Narayan Kashirao Deshmukh of Crompton Greaves Ltd. Analytics Centre, Global R&D, Kanjurmarg (East), Mumbai 400042, Maharashtra, India and Mr Shailesh Chintaman Mahajan & Mr John Yesuraj D of Crompton Greaves Ltd, Switchgear (SI) Division, A-3, M.l.D.C. Ambad, Nashik 422010, Maharashtra, India, all Indian Nationals
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
The invention relates to a transient and ferro-resonance damping circuit for a Coupling Capacitor Voltage Transformer (CCVT). More specifically, the present invention relates to a damping circuit which satisfies the 1EC standards for the transient response classes 3PT3 and 6PT3 and ferro resonance requirements.
BACKGROUND OF THE INVENTION
A Coupling Capacitor Voltage Transformer (CCVT) is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal for measurement and instrumentation. A typical CCVT comprises a capacitive voltage divider for reducing the supply voltage to an intermediate voltage, a nonlinear compensating reactor and a voltage transformer (VT). The intermediate voltage is supplied to the primary winding of VT and a secondary winding of VT is used for measurement and relaying purposes.
When a fault occurs in a transmission line, input voltage to the CCVT may drop to a relatively very low voltage. Similarly, under switching conditions, instead of following precisely the change in input voltage, the output of the CCVT may include a temporary transient, which remains for a certain amount of time following the change in input voltage. The transient occurs because the capacitive voltage divider and the compensating reactor are energy storage elements which cannot instantaneously change their charge or flux, it is desirable to minimize the magnitude and time duration of the transient so as to make the CCVT output in conformity with the international standards.
Also, under switching conditions, non-linearities in the magnetization characteristics of the capacitive elements and compensating reactor may cause undesirable ferro-resonant electrical

oscillations in the CCVT and can cause undesirable effects to CCVT and measurement and relaying equipments, if not suppressed.
To suppress transients and ferro-resonance, a transient and ferro-resonance damping circuit needs to be connected in parallel with all or part of voltage transformer's secondary windings, thereby damping undesirable electrical oscillations in the CCVT. However, such damping circuit has to satisfy International Electrotechnical Commission (IEC 60044-5) standards for the transient response classes 3PT3 and 6PT3, and ferro-resonance requirements.
The existing damping circuits which are made up of components such as resistor. inductor and capacitor do not satisfy the latest IEC standards, and the electronic damping circuits are quite complicated and expensive. Therefore, there is a need for a damping circuit, which includes basic components like resistor, capacitor and inductor and also satisfies the IEC standards for the transient response classes 3PT3 and 6PT3, and ferro resonance requirements.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a transient damping circuit for a CCVT which complies the IEC standards for the transient response classes 3PT3 and 6PT3.
Yet another object of the present invention is to provide a ferro-resonance damping circuit for a CCVT which complies the IEC standards for ferro-resonance requirements.
Yet another object of the present invention is to provide a simple, low cost and low power loss transient and ferro-resonance damping circuit for the CCVT.
DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION
According to the invention, there is provided a transient and ferro-resonance damping circuit for a coupling capacitor voltage transformer, which comprises a first damping resistor

connected across a secondary winding of the transformer; and a second damping resistor and a resonant circuit in series, connected across another secondary winding of the transformer. The resonant circuit comprises a first capacitor connected in parallel with a series combination of a first inductor, second inductor and second capacitor, such that the second inductor and capacitor forms a series resonant circuit at power frequency of the transformer, enabling the first inductor and first capacitor to form a parallel resonant circuit thereof.
Preferably, the resonant circuit is a combination of series and parallel resonance parameters.
Preferably, the series and parallel resonance parameters are set so as to cause critical damping of the residual voltage across the secondary windings of the coupling capacitor voltage transformer.
Preferably, the parameters of the series and parallel resonant circuits are set for a resonant frequency of 50Hz.
Preferably, at power frequency, the series resonant circuit offers zero impedance and the parallel resonant circuit offers high impedance, thereby allowing very low current to pass through the second damping resistor.
Preferably, at frequencies higher than the power frequency, the series resonant circuit offers very high inductive reactance as compared to capacitive reactance of the first capacitor, resulting in low equivalent impedance of the resonant circuit and allowing very high current to pass through the second damping resistor.
According to the invention, there is provided a coupling capacitor voltage transformer comprising a transient and ferro-resonance damping circuit, which comprises a first damping resistor

connected across a secondary winding of the transformer: and a second damping resistor and a resonant circuit in series, connected across another secondary winding of the transformer, the resonant circuit comprising a first capacitor connected in parallel with a series combination of a first inductor, second inductor and second capacitor, such that the second inductor and capacitor forms a series resonant circuit at power frequency, enabling the first inductor and first capacitor to form a parallel resonant circuit thereof.
These and other aspects, features and advantages of the invention will be better understood with reference to the following detailed description, accompanying drawings and appended claims, in which,
Fig. 1 illustrates source side circuit of a CCVT;
Fig. 2 illustrates a transient and ferro-resonance damping circuit of the CCVT;
Fig. 3 illustrates the current in a second damping resistor at various time intervals;
Fig. 4 illustrates the transient response of the CCVT when the input voltage becomes zero at
t=0.1 seconds; and
Fig. 5 illustrates the ferro-resonance response of the CCVT using the damping circuit of FIG.2.
Fig. 1 illustrates source side circuit of a CCVT. The source side of the CCVT includes an input voltage supply 101, source resistance 102, a first circuit breaker 103, a second circuit breaker 104. capacitive voltage divider 105, compensating reactor 106, stray capacitor 107, input voltage measurement system 108, and a step-down transformer 100.
The capacitive voltage divider 105 comprises capacitors 109 and 110 in series. The capacitors 109 and 110 may each comprise a number of individual capacitors, but in all cases, form a voltage divider between the input voltage and the ground. The compensating reactor 106 is

connected from a point between the capacitors 109 and 1 10 and a primary winding of the transformer 100 and typically comprises resistor 111. nonlinear inductor 112, and stray capacitor 1 !3. Compensating reactor 106 compensates the reactance of the capacitors 109 and 110 at the fundamental frequency of the power system, and prevents a phase shift between the primary and secondary voltages at that frequency. The stray capacitor 107 represents stray capacitance of primary winding of the transformer.
Fig. 2 shows a transient and ferro-resonance damping circuit 200 connected across secondary windings of the CCVT. The damping circuit 200 comprises a first damping resistor 201 connected across a secondary winding 202. and a second damping resistor 203 in series with a resonant circuit 204 connected across the secondary winding 205 of the transformer. In addition to the damping circuit 200, an output voltage measurement system 206 is connected to the secondary winding 207 for measuring the output voltage thereof.
The resonant circuit 204 includes a first capacitor C1 connected in parallel with series combination of a first inductor L1 second capacitor C2 and second inductor L2. In an embodiment of the present invention, the second capacitor C2 and second inductor L2 form a series resonant circuit, whose parameters are set in such a manner that it resonates at the fundamental (power) frequency of the power system signal. Further, the values of the first capacitor C1 and first inductor Li are set such that they form a parallel resonant circuit, resonating at power frequency. Thus, the resonant circuit 204 comprises both series and parallel resonance parameters. The damping circuit 200 is designed so as to cause critical damping of the residual voltage across the secondary windings of the transformer. The damping circuit 200 may be designed for a power frequency of either 50Hz or 60Hz.

Operationally, at power frequency, the series resonant circuit offers zero impedance and the parallel resonant circuit offers very high impedance thereby allowing very low current to pass through the second damping resistor 203. A very low current through the second damping resistor 203 results in negligible or very low power loss through heating.
At frequencies higher than the resonant frequency, the series resonant circuit 208 offers very high inductive reactance as compared to capacitive reactance of the first capacitor C|. This makes the equivalent impedance of the resonant circuit 204 very low, thereby allowing very high current to pass through the second damping resistor 203, resulting in high power dissipation across the resistor 203.
During steady state condition, the voltage signals appearing across the secondary windings 202, 205 and 207 include power frequency signals. Therefore, under steady state, very low power loss takes place across the second damping resistor 203.
During transient conditions such as fault/switching conditions, the voltages appearing across the secondary windings 202 and 205 and 207 do not instantaneously fall to zero and include high frequency signals of short time duration. The resonant circuit 204 offers very low impedance to such high frequency signals at 205, resulting in high current flow in the second damping resistor 203, thus dissipating high power and damping transient oscillations.
The first damping resistor 201 primarily damps the ferro-resonance effects in the secondary windings, by dissipating high energy in the system during ferro-resonance.
Thus, in this manner, the damping circuit 200 suppresses the transient and ferro-resonance effects in the secondary windings of the CCVT.

It may be noted that the characteristic of transient response is the ratio of secondary voltage Us(t) at a specified time t after application of a primary short circuit, to the peak value of
the secondary voltage calculated before the application of the primary short circuit (IEC

In an exemplary embodiment of the present invention, the rating of CCVT is 400k V and the fundamental (power) frequency of the system is 50Hz. The values of the first and second damping resistors 201 and 203 are set as 138Ω and 20.5Ω respectively. The values of the first inductor L, and capacitor C, for the parallel LC circuit are set as 0.025356H and 400μ.F. Further. the values of the second inductor L2 and capacitor C2 for series LC circuit are set as 1.0142399H and 10μF respectively. The first damping resistor 201 is connected across a secondary winding tapping of value 1 17.47 V. Whereas, the second damping resistor 203 in series with the resonant circuit 204 is connected across another secondary winding tapping of value 201.37 V.
FIG. 3 illustrates the current across the second damping resistor 203 i.e. 20.5 ohms resistor at various time intervals, it may be noted that from the figure, that from 0 to 0.1 seconds, the CCVT is in steady state. The peak value of current in the second damping resistor 203 under the steady state is 1.42 amperes. Hence, the steady state power loss in 20.5 ohms resistor= 1.42* 1.42*20.5/2= 20.67 watts. Further, power loss in the first damping resistor 201 i.e. 138 ohms resistor=100 watts. Therefore, total power loss across the secondary windings under steady state is 120.67 watts which is quite a low value.

At t=0.1 seconds, the input voltage becomes zero due to a fault/switching condition and transients are generated across the secondary windings of the CCVT. As a result, from time t= 0.1 s, high current flows through the second damping resistor 203 to suppress the transient oscillations. As apparent in the figure, the magnitude of the current flowing in the second damping resistor 203 decreases with time till it reaches a steady state.
Fig. 4 illustrates the transient response of the CCVT when the input voltage becomes zero at t=0.1 seconds. When the input voltage becomes zero, the voltage at the tap winding 207 is measured at different time intervals and is illustrated in the table below along with corresponding characteristic of the transient response


Table 1: The transient voltages and ratios

100%) at various instances when Us=63.37V

Time(ms) Us(t) (V) Ratio
10 3.56 3.97
20 0.93 1.04
30 1.78 1.99
40 1.38 1.54
50 1.70 1.90
60 1.44 1.61
70 1.79 2.00
80 1.24 1.38
90 1.76 1.96
100 0.86 0.96
It can be observed from the above facts that the transient response of the CCVT complies with the I EC standards for the transient response classes 3PT3 and 6PT3.
Fig. 5 illustrates the ferro-resonance response of the CCVT using the damping circuit 200, when the secondary winding 207 has been short-circuited at t=700milliseconds, and opened at t=800 milliseconds.

Although the invention has been described with reference to a specific embodiment, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the scope of the invention as defined in the appended claims.

We claim:
1. A transient and ferro-resonance damping circuit for a coupling capacitor voltage transformer comprising:
a first damping resistor connected across a secondary winding of the transformer; and
a second damping resistor and a resonant circuit in series, connected across another secondary winding of the transformer, the resonant circuit comprising a first capacitor connected in parallel with a series combination of a first inductor, second inductor and second capacitor, such that the second inductor and capacitor forms a series resonant circuit at power frequency of the transformer, enabling the first inductor and first capacitor to form a parallel resonant circuit thereof.
2. The transient and ferro-resonance damping circuit as claimed in claim 1, wherein the resonant circuit is a combination of series and parallel resonance parameters.
3. The transient and ferro-resonance damping circuit as claimed in claim 1, wherein the series and parallel resonance parameters are set so as to cause critical damping of the residual voltage across the secondary windings of the coupling capacitor voltage transformer.
4. The transient and ferro-resonance damping circuit as claimed in claim 1, wherein the parameters of the series and parallel resonant circuits are set for a resonant frequency of 50Hz.
5. The transient and ferro-resonance damping circuit as claimed in claim 1. wherein at the power frequency, the series resonant circuit offers zero impedance and the parallel

resonant circuit offers high impedance, thereby allowing very low current to pass through the second damping resistor.
6. The transient and ferro-resonance damping circuit as claimed in claim 1. wherein at frequencies higher than the power frequency, the series resonant circuit offers very high inductive reactance as compared to capacitive reactance of the first capacitor, resulting in low equivalent impedance of the resonant circuit and allowing very high current to pass through the second damping resistor.
7. A coupling Capacitor voltage transformer comprising a transient and ferro-resonance damping circuit, which comprises:
a first damping resistor connected across a secondary winding of the transformer; and
a second damping resistor and a resonant circuit in series, connected across another secondary winding of the transformer, the resonant circuit comprising a first capacitor connected in parallel with a series combination of a first inductor, second inductor and second capacitor, such that the second inductor and capacitor forms a series resonant circuit at power frequency, enabling the first inductor and first capacitor to form a parallel resonant circuit thereof.