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 INVENTION
An automatic vacuum capacitor switching device
INVENTORS
Dr Raghavan Venkatesh and Kulkarni Laxmikant Narharrao, both Indian nationals and
both of Crompton Greaves Ltd, Switchgear-6 & Power Quality Business, D2, MIDC,
Waluj, Aurangabad 431136, Maharashtra, India
APPLICANTS
Name : CROMPTON GREAVES LIMITED
Nationality : Indian Company
Address : CG House, Dr Annie Besant Road, Worli, Mumbai 400025, Maharashtra, India
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 INVENTION
This invention relates to an automatic vacuum capacitor switching device.
BACKGROUND OF INVENTION
In a power distribution system it is essential to maintain the power factor of the distribution system at the desired level for efficient power utilization. When the power factor of the distribution system falls below the desired level, the various capacitors provided in the power distribution system are switched on so as to increase the power factor to the desired level. The switching on and off of the capacitors is carried out by switching devices comprising vacuum interrupters, oil filled circuit breakers, sulphur hexafluoride gas (SF6) filled circuit breakers or air circuit breakers.
In one vacuum capacitor switching device, three vacuum interrupters are connected in parallel with the three phases of the AC mains and in series with the capacitors in the power distribution system. The switching device further comprises a solenoid connected to a controller which energizes or de-energizes the solenoid to control the operation of the vacuum interrupters ie to close and open the contacts of the vacuum interrupters. When the power factor of the power distribution system comprising the switching device drops, the drop in the power factor is sensed at the power station and the operator has to manually go from the power station to the power distribution system which may be remote to the power station so as to operate the controller. The controller energizes the solenoid and closes the contacts of the vacuum interrupter to switch on the capacitors and increase the power factor of the power distribution system to the desired level. Manual
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operation of the switching device is inconvenient and cumbersome. The switching device as such does not provide over/under voltage or current protection. If over/under voltage or current protection is required for the power distribution system, separate over/under voltage or current protection devices are to be used. Such protection devices add to the cost of the power distribution system besides increasing the size of the switching device.
In another vacuum capacitor switching device, three vacuum interrupters are connected in parallel with the three phases of the AC mains and in series with the capacitors in the power distribution system. The primary winding of a current transformer is connected in series with one of the phases of the AC mains and the secondary winding of the transformer is connected to a controller. The controller is also connected to the contacts of the vacuum interrupters through a solenoid and to an auxiliary power source through a potential transformer. The controller is programmed to open and close the contacts of the vacuum interrupter at preset intervals of time irrespective of whether or not the power factor level of the power distribution system has dropped below the desired level. This may reduce the life of the capacitors as the capacitors may be switched on when they are not required to be switched on ie when the power factor of the power distribution system is at the desired level Once the contacts of the vacuum interrupter are closed, the current transformer continuously senses the current flowing through the respective phase of the AC mains. The current sensed by the current transformer is fed to the controller which compares the sensed current with a reference current set therein. If the sensed current is less than the reference current, the controller keeps the solenoid energized so that the
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contacts of the vacuum interrupter remain closed and the capacitors remain switched on. If the sensed current is more than the reference current the controller deenergizes the solenoid and the contacts of the vacuum interrupters are opened and the capacitors are switched off. The potential transformer and the controller derive their operating power from the auxiliary power source. In this switching device also as in the case of the above switching device over voltage and under voltage protection is not provided. If over voltage and under voltage protection is required for the power distribution system, separate over voltage and under voltage protection devices are to be used. Such protection devices will add to the cost of the power distribution system and increase the space occupied by the switching device. The auxiliary power source will also increase the cost of the switching device. The current being sensed by the current transformer is not the source current but the current flowing through the capacitors after the contacts of the vacuum interrupter are closed. Therefore, the power factor of the distribution system is not continuously monitored, and there are chances the capacitors are not switched on when the power factor of the distribution system is below the desired level. This results in the inefficient usage of the capacitors and malfunctioning of the switching device.
OBJECTS OF INVENTION
An object of the invention is to provide an automatic vacuum capacitor switching device, which switching device continuously senses the current and voltage in all the three phases of the AC mains and performs the dual function of switching the capacitors for
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maintaining the desired power factor level of the power distribution system and providing protection to the power distribution system against over/under voltage and current.
Another of the invention is to provide an automatic vacuum capacitor switching device, which switching device eliminates the auxiliary power source for powering the various components thereof.
Another object of the invention is to provide an automatic vacuum capacitor switching device, which switching device is compact and economical.
DETAILED DESCRIPTION OF INVENTION
According to the invention there is provided an automatic vacuum capacitor switching device comprising three vacuum interrupters connected in parallel with the three phases of an AC mains and in series with the capacitors of a power distribution system, a step down voltage transformer whose primary winding is connected across the two phases of the AC mains, a step down current transformer whose primary winding is connected in series with the third phase of the AC mains and a controller connected to the ends of the secondary winding of the voltage transformer and to the ends of the secondary winding of the current transformer, the controller being further connected to the contacts of the vacuum interrupters through a solenoid, tappings being provided in the secondary winding of the voltage transformer and connected to the controller for powering the controller.
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The following is a detailed description of the invention with reference to the accompanying drawings, in which:
Fig 1 is an electrical layout of the automatic vacuum capacitor switching device of the invention according to an embodiment of the invention; and
Fig 2 is a block diagram of the controller of the switching device of Fig 1.
The switching device 1 as illustrated in the accompanying drawings comprises three vacuum interrupters represented as contacts 2 connected in parallel with the three phases R, Y and B of an AC mains 3 and in series with a capacitor bank 4 comprising capacitors 4a, 4b and 4c. 5 is a step down voltage transformer whose primary winding 5a is connected across R and B phases of the AC mains and 6 is a step down current transformer whose primary winding is connected in series with the Y phase of the AC mains (Fig 1). 7 is a controller comprising a microprocessor 8 connected to a rotary switch 9 through a logic controller 10. 11 is a thyristor connected to the microprocessor and a solenoid 12 which in turn is connected to the contacts 2 (Figs 1 and 2). The secondary winding ends marked T3 & T4 of the current transformer are connected to the logic controller. The secondary winding 5b ends marked T1 and T2 of the voltage transformer are also connected to the logic controller. Tappings (not marked) in the secondary winding of the voltage transformer are connected to the microprocessor, logic controller and thyristor for powering them.
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During operation of the switching device, the voltage and current transformers step down the voltage and current and continuously sense the voltage and the current in the three phases of the AC mains, respectively. The sensed voltage and current are fed to the logic controller which outputs a digital signal to the microprocessor corresponding to the sensed voltage and current. The microprocessor compares this input signal corresponding to the sensed voltage and current with the reference voltage and current preset therein. If the sensed voltage and current are less than the reference voltage and current, the microprocessor gives an output to the thyristor which in turn deenergizes the solenoid so as to open the contacts of the vacuum interrupters thereby not allowing the current to flow into the vacuum interrupters and protecting the power distribution system 13 against over/under voltage and current conditions. This operation occurs irrespective of whether the power factor of the power distribution system is above or below the desired level. If the sensed current is more than the reference current, the thyristor energizes the solenoid so as to close the contacts of the vacuum interrupters and switch on the capacitors of the capacitor bank thereby increasing the power factor of the power distribution system to the desired level.
The switching device can be used for different ratings of the current transformer by changing the position of the rotary switch. The rotary switch is optional if the switching device is to be operated only for one current rating. The number of tappings in the secondary winding of the voltage transformer can vary.
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According to the invention, the voltage and current in the AC mains are automatically continuously sensed by the voltage and current transformers and the capacitors are switched on only when the power factor of the power distribution system falls below the desired level. As the voltage and current in the phases are continuously sensed, fault conditions like over/under voltage or current are promptly detected and the faulty voltage and/or current is/are not allowed into the vacuum interrupters thereby protecting the power distribution system against over/under voltage and current conditions. The over/under voltage or current protection is inbuilt into the switching device of the invention thereby rendering it compact and economical besides reducing the space requirement of the switching device. The switching device of the invention eliminates the auxiliary power source and is self powered through the tappings in the secondary winding of the voltage transformer. This further reduces the cost of the switching device.
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WE CLAIM
1. An automatic vacuum capacitor switching device comprising three vacuum interrupters connected in parallel with the three phases of an AC mains and in series with the capacitors of a power distribution system, a step down voltage transformer whose primary winding is connected across the two phases of the AC mains, a step down current transformer whose primary winding is connected in series with the third phase of the AC mains and a controller connected to the ends of the secondary winding of the voltage transformer and to the ends of the secondary winding of the current transformer, the controller being further connected to the contacts of the vacuum interrupters through a solenoid, tappings being provided in the secondary winding of the voltage transformer and connected to the controller for powering the controller.
2. A switching device as claim in claim 1, wherein the controller comprises a microprocessor connected to a rotary switch through a logic controller and a thyristor connected to the microprocessor and the solenoid, the logic controller being connected to the ends of the secondary winding of the voltage transformer and to the ends of the secondary winding of the current transformer, the tappings in the secondary winding of the voltage transformer being connected to the microprocessor, logic controller and thyristor for powering them.
Dated this 2nd day of February 2007.
(Prita Madan)
of Khaitan & Co
Agent for the Applicants
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Abstract
An automatic vacuum capacitor switching device (1) comprising three vacuum interrupters (2) connected in parallel with the three phases of an AC mains (3) and in series with the capacitors (4a, 4b, 4c) of a power distribution system. 5 is a step down voltage transformer whose primary winding 5a is connected across the two phases (R, B) of the AC mains. 6 is a step down current transformer whose primary winding is connected in series with the third phase (Y) of the AC mains. 7 is a controller connected to the ends of the secondary winding (5b) of the voltage transformer and to the ends of the secondary winding of the current transformer. The controller is further connected to the contacts of the vacuum interrupters through a solenoid (12). The secondary winding of the voltage transformer is provided with tappings (not shown) which are connected to the controller for powering the controller (Fig 1).