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, 2005
PROVISIONAL SPECIFICATION (See section 10 and rule 13)
TITLE OF THE INVENTION
A transformer with improved voltage regulation and enhanced fault current limiting capability
APPLICANTS
Name: CROMPTON GREAVES LIMITED
Nationality: Indian Company
Address: Dr Annie Besant Road, Worli, Mumbai 400030, Maharashtra, India
INVENTOR
Name: Raghavan Venkatesh
Nationality: Indian National
Address: Crompton Greaves Limited, Switchgear-6 & Power Quality Business, D2,
MIDC, Waluj, Aurangabad, Maharashtra, India
PREAMBLE TO THE DESCRIPTION
The following specification describes the invention:

FIELD OF INVENTION
This invention relates to a transformer with improved voltage regulation.
This invention also relates to an AC power system comprising of a transformer with improved voltage regulation.
BACKGROUND OF THE INVENTION
With increasing emphasis on power quality, especially voltage quality, it is imperative to improve the voltage regulation of the electric power system.
The voltage regulation of the electric power system is governed by the series impedance (inductive reactance) of the transmission lines itself and also by other series connected impedance, transformer leakage impedance being the most important of this.
Transformers are used in electric power systems to step up or step down the voltages levels and basically comprise of a primary winding, a secondary winding and a magnetic core that provides the magnetic coupling between the primary and secondary winding. Due to the inherent limitation associated with the design & construction of transformers, the magnetic coupling between primary and secondary windings is not 100% and there is a portion of the magnetic flux that does not link the primary and secondary winding, termed as leakage flux. This leakage flux manifests as series leakage impedance that is inductive in nature and affects the voltage regulation. The leakage impedance or
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regulation depends upon the disposition of the primary and secondary windings, construction of the magnetic core and the air gaps in the magnetic core.
All transformers used for transformer of power from one voltage level to another voltage level have their magnetic cores constructed of thin laminations of a magnetic material. Though various designs are available for constructing such cores (interleaved laminations, C & I type etc.) substantially all of them have air gaps in the cores due to the basic construction of the core with laminations, which are stacked. Though various designs are available for disposition of windings (shell type construction, core type construction etc.), all suffer from the disadvantage that the leakage impedance is high.
Typical leakage impedances obtained with conventional prior art designs range from 4% to 7%. While designs with higher leakage impedances and lower leakage impedances are possible using special design and construction, the range is normally from 3% to 15%. While it is possible to extend the leakage impedances on the higher side, it is virtually impossible to design & construct transformer with lower leakage impedance.
While lower leakage impedance offers a better voltage regulation, this increases the fault current in the system as the leakage impedances of the transformers basically limit the fault currents. This puts a constraint of the lower leakage impedance that can be tolerated in the system, if such lower leakage impedances can be achieved.
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Prior art designs have a fixed leakage impedance value, which is same for voltage regulation as well as short circuit conditions and this poses a dilemma in design / selection as to balance between better voltage regulation and higher fault impedance as the requirements are conflicting in nature.
Though the problem is relevant for all transformers irrespective of voltage class or power rating, the problem is more felt in the case of low power / distribution transformers, typically having rated voltages from 0.1 kV to 33kV and power handling capacity from 5 KVA to 500 kVA. Since such low power distribution transformers are normally at load centers located farther away from generating stations, the fault levels are already very low and also suffer from low voltage problems due to long transmission and distribution lines.
Though applicable to transformers of all voltage levels and all power ratings, the present invention is more beneficial when applied to transformers covering voltage class from 0.1 kV to 33kV and power rating from 5 kVA to 500 KVA, covering both single phase and multi-phase configurations.
An object of the invention is to provide a transformer with improved voltage regulation by reducing the leakage impedance to very low values, which could be as low as 1%. The design and construction of the invention offers leakage impedance in the range of 0.8% to 1.5%.
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Another object of the invention is to provide transformer with very low leakage impedance under steady state normal operating conditions thus improving the voltage regulation and high impedance under fault conditions thus effectively limiting the fault currents.
Another object of the invention is to configure a three-phase transformer using three single-phase transformers having low leakage impedance under steady state operating conditions and a high impedance during fault conditions.
DESCRIPTION OF INVENTION
According to the invention there is provided a transformer with a primary winding, a secondary winding and a magnetic core to provide the magnetic coupling between the primary and secondary windings and where the magnetic core is a torroidal core (round core wound with thin ribbons of magnetic steel) would with thin ribbon of magnetic material.
The first winding (primary winding) is wound on the torroidal core with insulation as dictated by the voltage level of this winding and the two ends of the windings are taken out. The second winding (secondary winding) is wound on top of the first winding with adequate insulation between the two windings as dictated by the voltage levels of the two windings.
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According to another embodiment of the invention, an auxiliary winding is provided in series with the main winding and this auxiliary winding comprises of two separate windings would on a torroidal core in antiparallel (winding polarity in opposite direction so that the current flowing in the two windings produce a flux in opposite directions and cancel each other as to create a net zero flux and hence produce a condition of zero inductive reactance.) fashion and the two windings are connected in series with the line and neutral of one of the windings of the main winding. While the auxiliary winding can be connected in series with the primary or the secondary winding of the main winding, it is preferable to connected this auxiliary winding with that winding of the main winding which is feeding the load.
The Functioning of one type of transformer is as follows:
The voltage applied across the primary produces a flux, which is contained to a large extent within the torroidal magnetic core and this flux links the secondary winding and induces a voltage in the secondary winding. The functioning of the transformer is similar to conventional transformers and is governed by all the equations applicable to the conventional electromagnetic transformers except the fact that the flux mutually linking the primary and secondary winding is high compared to conventional transformers. Due to the lower airgaps in the magnetic core and the construction of the core in a torroidal form and the disposition of primary and secondary windings, the permeability of the magnetic core is high and a greater amount of flux is contained in the magnetic core and links both the windings thus reducing the leakage flux.
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It is also possible in this invention to increase the leakage flux and leakage impedance from the natural/typical value of 1% to higher values (typically 3% to 5% as is common with conventional transformers) by providing controlled air gaps in the torroidal magnetic core by building this round core with multiple segments (arc of a circle) of core with controlled air gaps and also by controlling the distance between the primary and secondary winding (termed as Hi-Lo gap).
Functioning of a three phase transformer is as follows :
The functioning of a three phase transformer is similar to that of a single phase transformer and the three phase configuration is obtained by connecting three numbers of single phase transformers externally in star (Wye) or delta configuration.
Functioning of a basic transformer is as follows :
In Basic Transformer, the auxiliary current limiting reactor inserted in series with the secondary winding offers very low impedance during steady state conditions as the current flowing through the two windings are in phase opposition and the flux produced in the common magnetic core cancels to produce a net zero flux and consequently zero inductive impedance. The currents flows from the line end of the secondary winding of the main transformer and through the first winding of the current limiting winding, through the line end of the load, through the neutral end of the load, through the secondary winding of the current limiting reactor and back to the neutral end of the secondary winding of the main transformer.
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In the event of an earth fault, a part of the return current flows through the earth wire and not through the neutral wire in the return path. This creates an imbalance in the currents flowing through the two windings of the auxiliary current limiting winding (between the windings in series with the line side and neutral side). This imbalance in current creates a net flux in the core of the auxiliary current limiting winding and a high inductive
Functioning of the star (wye) or delta configuration type transformer is as follows : The functioning of this transformer is similar to those explained as above, wherein a three-phase transformer is constructed using three numbers of single-phase transformers connected externally in star (wye) or delta configuration.
Functioning of another type transformer is as follows :
The functioning of this transformer is similar to those explained as above , except the difference that the auxiliary current limiting reactor is not made of three numbers of single phase reactors, but a composite three phase reactor, wherein the three phase conductors are wound on a common torroidal magnetic core. Under balanced steady state operating conditions, the three phase currents (being of same magnitude and phase shifted by 120 degrees) add upto zero and thus the net flux produced in the common core is zero and consequently this current limiting reactor offers negligible impedance. In the event of a fault (such as line to ground, line to line etc.) the currents in the three phase windings do not add upto zero and hence sets up a net flux in the common magnetic core and hence offers a significantly higher impedance thus effectively limiting the fault current.
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The invention apart from offering a better voltage regulation also has additional benefits of lower magnetizing current, which improves the no-load power factor. This also reduces the inductive reactive power load on the system. The magnetizing currents typically are in the range of 0.3% to 0.6% of full load currents as against 0.7% to 1.5% in the case of conventional designs.
In accordance with the present invention, a transformer with two windings wound on a torroidal magnetic core with improved magnetic coupling between the two windings with a lower leakage flux and lower leakage impedance and improved voltage regulation.
In accordance with the present invention, a transformer in which the torroidal core is made of any magnetic material such as CRGO, Mu metal (Iron-Nickel alloy), amorphous metal or any magnetic steel.
In accordance with the present invention, a transformer with two winding, is provided with an auxiliary core with two windings wound in a particular manner on a torroidal core as to cancel the flux generated by the currents flowing in the two windings of the auxiliary windings, and wherein the two windings of the auxiliary core are connected in series with the line and neutral windings of one of the windings of the main core so as to offer a improved voltage regulation under normal steady state operating conditions and higher impedance during fault conditions as to limit the fault current.
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In accordance with the present invention, a three-phase transformer constructed out of three single-phase transformers.
In accordance with the present invention, a transformer as mention above, wherein the insulation is either of dry type (resin cast / resin impregnated / gas insulated) or wet type (oil - paper insulation) and transformer is suitable for either indoor or outdoor applications with suitable insulation system.
Dated this 30th day of March 2007


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