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
A method for determining pre-arcing time corresponding to a SF6 gas pressure value in a high voltage circuit breaker.
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road. Worli, Miimbai 400 030, Maharashtra, India, an Indian Company
INVENTOR
Gandhi .lay Rasik of Crompton Greaves Ltd, S3 Division. Switchgear Complex. A-3 Ml DC. Ambad. Nashik 422 010. Maharashtra, India, an Indian National
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
This invention relates to a method for determining pre-arcing time corresponding to a SF6 gas pressure value in a high voltage circuit breaker. BACKGROUND OF THE INVENTION
Typicaily: the opening and closing of a high voltage circuit breaker is controlled through a controller to open and close at a pre-determined instance. The pre-determined instance is such that it coincides with a particular point on input line voltage/current wave, typically a zero or peak voltage/current point. Such is necessary to ensure longevity of the life of the circuit breaker contacts which otherwise are prone to damage due to the arc created at the time of opening and closing of the contacts of the circuit breaker. Opening or closing at the pre-determined instance ensures that a minimum intensity or no arc is generated at the time of opening or closing. Since the command to OPEN/CLOSE the circuit breaker is issued randomly by the user to the controller, the controller first calculates and then implements the delay in executing the open or close command. Since there are a number of operational parameters within the circuit breaker which can affect the opening or closing of the circuit breaker, the controller is pre-programmed to take into account such parameters and compensate for them in calculating the final delay in issuing the OPEN/CLOSE command. Such parameters being control voltage, air pressure, line frequency, SF6 gas pressure, temperature etc within the circuit breaker. The manner of calculating the delay and compensating the operational parameters in calculation of the delay is well known which includes determining the closing time of the circuit breaker at different values of a particular operational parameters and inputting such values into the controller. For example. to compensate, say SF6 gas pressure within the circuit breaker, its value is first determined within the circuit breaker by means of a pressure sensor. The controller then determines the

closing time corresponding to the detected SF6 gas pressure Value stored therein and processes it further (through well known method(s)) in calculation of the total closing delay time. Similarly. other parameters mentioned above are compensated. After calculating the total closing delay time (compensating the above parameters), pre-arcing time is usually subtracted therefrom to arrive at the final closing delay time. Pre-arcing time is the interval of time between instances at which there is a physical connection between the contacts and electrical connection. Pre-arcing lime is just interval between electrical connection to mechanical contact touch. Usually, the pre-arcing time is assumed to be a constant value. However, varying SF6 gas pressure within the circuit breaker may vary the value of the pre-arcing time, But since the pre-arcing time is assumed to be a .constant value,, the calculation of final closing delay time may not be accurate. Thus, there is a room to improve the accuracy in calculating the final closing time delay. For that, there is a need to determine the values of pre-arcing time at different SF6 gas pressure values within the circuit breaker which can be inputted into the controller. The controller can thereafter be configured to compensate the pre-arcing time as well in calculation of the final closing delay time based on the values of the pre-arcing time corresponding to different SF6 gas pressure values within the circuit breaker. DETAILED DESCRIPTION OF THE INVENTION
According to the invention, there is provided a method for determining pre-arcing time corresponding to a SF6 gas pressure value in a high voltage circuit breaker, the method comprising the steps of determining the values of g associated with a plurality of known values oft and s from the formula g=195-s*t; wherein g being the ga,p value between the stationary and moving arcing contacts of the circuit breaker, s being the speed of the moving arcing contact of the circuit breaker, t being a time value in a pre-defined time range and 195 being the gap

between the contacts in full open condition in mm; determining a breakdown voltage (BDV) value for each gap value through the formula, BDV=E critical/ E max * V , wherein, V being a preset input line voltage (V), E critical being electric field value within the circuit breaker corresponding to said SF6 gas pressure value within the circuit breaker and E max being a maximum electrical stress value (E max) amongst the electrical stress value measured on the stationary and moving main & arcing contacts at each gap value between the stationary and moving arcing contacts, thus each BDV value corresponding to a time value; plotting a graph of a breakdown voltages (Y-axis) versus their corresponding time values (X-axis); plotting a modulus sine wave of the input line voltage on the said graph: determining a lust time value which being the time value on the X-axis of said graph corresponding to the point of intersection of the breakdown voltage curve and the modulus sine wave; determining a second time value which being the time value on the X-axis of said graph corresponding to the point of intersection of the breakdown voltage curve and the X-axis; and subtracting the first time value from the second time value, the result of which being the pre-arcing time corresponding to said SF6 gas pressure value within the circuit breaker.
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 I is a flowchart showing steps involved in a method for determining pre-arcing lime corresponding to a SF6 gas pressure value in a high voltage circuit breaker.
Fig 2 is BDV Vs its corresponding Time graph.
Fig 3 is graph showing input sine wave voltage plotted on the graph of Fig 2.

Fig [ is a flowchart showing the steps involved in the method for determining pre-arcing time corresponding to a SF6 gas pressure value in a high voltage circuit breaker. Pre-arcing time is the interval of time between instances at which there is a physical connection between the contacts and electrical connection but no physical connection between the contacts. For a given value of SF6 gas pressure, values of g is determined through the formula g=195-s*t for a plurality values oft and s wherein g is the gap value between the stationary and moving arcing contacts of the circuit breaker, s is the speed of the moving arcing contact of the circuit breaker, t is the time taken by the moving contact to travel and 195 is the contact gap in full open condition in mm. Plurality of time values is used from a pre-defined time range. Subsequently, a breakdown voltage (BDV) value for each gap value is determined through the formula BDV=E critical/ E max * V . wherein. V is a preset input line voltage (V). E critical is electric field value within the circuit breaker corresponding to the SF6 gas pressure within the circuit breaker and E max is the maximum electrical stress value (E max) amongst the electrical stress value measured on the stationary and moving main & arcing contacts at each gap value between the stationary and moving arcing contacts. However, there may be other methods/formulas to determine BDV. Since each gap value corresponds to a time value, each BDV value corresponding to a gap value also corresponds to a time value. Based on this a BDV curve is plotted with values of breakdown voltages on Y-axis and their corresponding time values on X-axis. Thereafter, a modulus sine wave of the input line voltage is plotted on the same graph. Next, the following values are determined from the graph:
o first value on the X-axis corresponding to the intersection point of the BDV curve and the sine wave

o second value on the X-axis corresponding to the intersection point of the BDV curve and the X-axis Finally, the first value is subtracted from the second value to have the pre-arcing time corresponding to the given value of SF6 gas pressure. Similarly, pre-arcing times for different values of SF6 gas pressure values within the circuit breaker is determined. EXAMPLE
Assume, SF6 gas pressure value = 5 kg/cm2 in a high voltage circuit breaker - 420 kV sp - pn Speed of moving contacts s = 4 m/s
time values 11=38.75, t2=39.75, t3=40.75, t4=41.75, t5=42.75. t6=43.75. t7=44.75, 18=45.75, t9=46.75,110=47.75 (all in milli seconds) Values of gl, g2, g3, g4.g5, g6, g7, g8, g9. g10 with respect to tl. t2. t3, t4. t5. t6. t7. t8. t9. UO are determined through the formula g=l95- s*t Therefore, g 1 = l95-s*tl = 195-4* 38.75=40 mm
g2= 195-s*t2 =195- 4* 39.75=36 mm
g3= l95-s*t3 = 195-4* 40.75=32 mm
g4= J95-s*t4= 195-4* 41.75=28 mm
g5= l95-s*t4 = 195-4* 42.75=24 mm
g6= 195-s*t4 = 195-4* 43.75=20 mm
g7= 195-s*t4 = 195-4* 44.75=16 mm
g8= 195-s*t4 = 195-4* 45.75=12 mm
g9= !95-s*t4 = 195-4* 46.75=8 mm
gl0= 195-s*t4= 195-4*47.75=4 mm

Subsequently, values of breakdown voltage (BDV)1 10 are determined through the formula
(BDV)|... ,0 =E critical/ (E max),... 10 * V, wherein. V is a preset input line voltage (V) being 100 V
E critical is electric field value within the circuit breaker being 24 kV/mm at 5 kg/cm2 (different value at different SF6 gas pressure) and E max is the maximum electrical stress value amongst the electrical stress value measured on the stationary and moving main & arcing contacts at a gap values between the stationary and moving arcing contacts.
Values of (E max) 1.....10 being 5972.9, 6309.7, 6812.6, 7596.6, 8729.0, 10350.9, 12587.9, 16104.3, 23532.8, 47279.5 in V/m corresponding to the above mentioned gap values gl...gl0 between the stationary and moving arcing contacts.
Values of (BDV)1.....10so determined are 402, 380, 352, 316, 275, 232, 188.59, 149, 102, 5 I in kV corresponding to the gap values 40, 36, 32, 28, 24, 20, 16, 12. 8, 4 in mm. Different E critical values at different SF6 gas pressure are obtained analytically. Since each gap value gl..gl0 corresponds to the respective time values tl,..tl0 and each (BDV)1....10 value corresponds to the gap values gl...g10, each (BDV)1....10 value also corresponds to the time value tl..t10 respectively. Next, a graph of BDV Vs time curve is plotted, as shown Fig 2. On the same graph, modulus sine wave of input voltage V is plotted, as shown in Fig 3. The following values are then derived from the graph:
o first value, 44.75 ms, which is value on the X-axis corresponding to the point of intersection between the BDV curve and sine curve
o second value, 48.75 ms, which is the value on the X-axis where the BDV curve intersects the X-axis.

Finally, the pre-arcing time 4 ins, for the SF6 gas pressure value 5 kg/cm2 is derived by subtracting first value from the second value, i.e. 48.75 - 44.75 ms
Similarly, pre-arcing time values (4. 3.5. 3, 2.8) are derived for SF6 gas pressure values (5 kg/cm2. 6 kg/cm2, 7 kg/cm2, 8 kg/cm2)
The pre-arcing time values determined above are fed into a controller device of a circuit breaker which uses it in the calculation of the total delay time to be implemented in issuing a control signal for closing the circuit breaker.
For example, whenever the controller device of a circuit breaker receives a command signal for closing the circuit breaker from a user, the controller does not instantly implements the command. It is pre-programmed to issue a control signal for closing the circuit breaker such that the closing of the circuit breaker takes place at a pre-determined instance on the input voltage sine wave (for example, zero or peak voltage/current point). To do this, invariably a delay has to be implemented in issuing the control signal for closing the circuit breaker such that the closing of the circuit breaker coincides at the pre-determined instance on the input voltage sine wave. This helps in minimum generation of arc thereby inducing minimum stress on the contacts. Since there are a number of operational parameters within the circuit breaker which can affect the opening or closing of the circuit breaker, the controller is pre-programmed to take into account such parameters and compensate for them in calculating the final delay in issuing the OPEN/CLOSE command. Such parameters being control voltage, air pressure, line frequency. SF6 gas pressure, temperature etc within the circuit breaker. The method of calculating the delay and compensating the operational parameters in calculation of the delay is well known which includes determining the closing time of the circuit breaker at different values of a particular operational parameters and inputting such values into the controller. For example, to

compensate, say SF6 gas pressure within the circuit breaker, its value is first determined within the circuit breaker by means of a pressure sensor. The controller then determines the closing time corresponding to the detected SF6 gas pressure value stored therein and processes it further ( through well known method(s)) in calculation of the total closing delay time. Similarly, other parameters mentioned above are compensated. After calculating the total closing delay time (compensating the above parameters), the value of the pre-arcing time corresponding to the SF6 gas pressure value detected as determined through the method described above and stored within the controller is fetched and subtracted from the total closing delay time to arrive at the final closing delay time.
According to the invention, determining a pre-arcing corresponding to each SF6 gas pressure value within the circuit breaker improves the accuracy of calculating the delay time in closing the circuit breaker.
The steps of the above method can be performed either manually or by means of an algorithm or a combination of both.
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 method for determining pre-arcing time corresponding to a SF6 gas pressure value in a high voltage circuit breaker, the method comprising the steps of:
o determining the values of g associated with a plurality of known values of t and s from the formula g=195-s*t; wherein g being the gap value between the stationary and moving arcing contacts of the circuit breaker, s being the speed of the moving arcing contact of the circuit breaker, t being a time value in a pre-defined time range and 195 being the gap between the contacts in full open condition in mm. o determining a breakdown voltage (BDV) value for each gap value through the formula, BDV=E critical/ E max * V , wherein. V being a preset input line voltage (V). E critical being electric field value within the circuit breaker corresponding to said SF6 gas pressure value within the circuit breaker and E max being a maximum electrical stress value (E max) amongst the electrical stress value measured on the stationary and moving main & arcing contacts at each gap value between the stationary and moving arcing contacts, thus each BDV value corresponding to a time value; o plotting a graph of a breakdown voltages (Y-axis) versus their corresponding
time values (X-axis); o plotting a modulus sine wave of the input fine voltage on the said graph: o determining a first time value which being the time value on the X-axis of said graph corresponding to the point of intersection of the breakdown voltage curve and the modulus sine wave;

o determining a second time value which being the time value on the X-axis of
said graph corresponding to the point of intersection of the breakdown voltage
curve and the X-axis; and o subtracting the first time value from the second time value, the result of which
being the pre-arcing time corresponding to said SF6 gas pressure value within
the circuit breaker.
2. The method as claimed in claim 1 is performed manually.
3. The method as claimed in claim 1 is performed by means of an algorithm.
4. The method as claimed in claim I is performed by a combination of manual and algorithm means.
5. The method as claimed in claim I. wherein the critical electric field values (H critical) corresponding to different gas pressure values within the circuit breaker are determined analytically.
6. The method as claimed in claim 1, wherein the electrical stress values on the stationary and moving main & arcing contacts at different air gaps between the stationary and moving contacts at a preset input line voltage (V) are determined by means of a FEMM (Finite Element Method Magnetics) software.