FORM 2
THE PATENTS ACT 1970
As amended by the Patents (Amendment) Act, 2002
COMPLETE SPECIFICATION
(See Section 10; Rule 13)
TITLE
A switching current limiting reactor for three-phase balanced ac power systems
APPLICANTS
Crompton Greaves Ltd., CG House, Dr Annie Besant Road, Prabhadevi, Mumbai - 400025, Maharashtra, India, an Indian Company
INVENTOR
Raghavan Venkatesh, Corporate Research and Development Centre, Crompton Greaves Ltd, Kanjur Marg (East), Mumbai -400042, Maharashtra, India, an Indian National
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 a switching current limiting reactor for balanced three-phase ac power systems.
PRIOR ART
In balanced three-phase ac power systems comprising grounded star connected capacitor banks, there will be heavy switching inrush currents, which are to be limited to reduce power losses and to improve reliability/life of the switches and capacitor banks. Several current limiting reactors are known for use in such power systems to limit the switching inrush currents. A first type of switching current limiting reactors comprises three inductor coils connected in series with the capacitor banks. The net impedance of such reactors under normal operating conditions is high resulting in significant power losses and increased voltage across the capacitor banks. A second type of switching current limiting reactors comprises preclosing resistors and switches. They are very expensive and are not reliable. A third type of current limiting reactors comprises solid state switching devices like thyristors, Gate Turn Off Thyristor (GTO), Insulated

Gate Bipolar Transistor (1GBT) and point on wave triggering circuits. Such devices are complicated in design and operation besides being very expensive and associated with large power losses. A fourth type of current limiting reactors comprises single phase (pole) switches with complex timing and control circuits for point on wave switching. This type of devices are very expensive.
OBJECTS OF INVENTION
An object of the invention is to provide a switching current limiting reactor for three-phase balanced ac power systems comprising neutral grounded star connected capacitor banks, which reduces power losses.
Another object of the invention is to provide a switching current limiting reactor for three-phase balanced ac power systems comprising neutral grounded star connected capacitor banks, which improves reliability/life of the switches and capacitor banks.

Another object of the invention is to provide a switching current limiting reactor for three-phase balanced ac power systems comprising neutral grounded star connected capacitor banks, which improves voltage regulation.
Another object of the invention is to provide a switching current limiting reactor for three-phase balanced ac power systems comprising neutral grounded star connected capacitor banks, which ensures continued operation of the power system in case one or more voltage phases fail,
DESCRIPTION OF INVENTION
According to the invention there is provided a switching current limiting reactor for three-phase balanced ac power systems comprising neutral grounded star comiected capacitor banks, wherein the reactor comprises three inductor coils of equal number of turns wound unidirectionally in torroidal configuration and magnetically coupled, one ends of the inductor coils being connected in series with the three phases of an ac voltage source through circuit breakers and the other ends of the

inductor coils being comiected in series with the neutral grounded star connected capacitor banks.
The following is a detailed description of the invention with reference to the accompanying drawings, in which:
Fig 1 is plan of a switching current limiting reactor according to an embodiment of the invention;
Fig 2 is crosssection at A-A in Fig 1;
Fig 3 is circuit diagram of a balanced three-phase ac power system comprising the switching current limiting reactor of Figs 1 and 2;
Fig 4 is circuit diagram of a balanced three-phase ac power system comprising a switching current limiting reactor according to another embodiment of the invention;

Fig 5 is computer simulated graphical plot showing the waveforms representing the currents flowing through the individual phases and sum current at one instant of closing of the switches of a balanced three-phase ac power system comprising a typical switching current limiting reactor of Figs 1 and 2 during the transient period;
Fig 6 is computer simulated graphical plot showing the waveforms representing the currents flowing through the individual phases and sum current at the one instant of closing of the switches of a balanced three-phase ac power system comprising a typical switching current limiting reactor of Figs 1 and 2 after switching transients have died down;
Fig 7 is computer simulated graphical plot representing flux in the torroidal core of a typical switching current limiting reactor of Figs 1 and 2 with unbalanced currents; and
Fig 8 is computer simulated graphical plot representing flux in the torroidal core of a typical switching current limiting reactor of Figs 1 and 2 with balanced currents.

Referring to Figs 1, 2 and 3 of the accompanying drawings, the switching current limiting reactor 1A comprises three inductor coils Lr, Ly and Lb of equal number of turns wound unidirectionally in torroidal configuration on a torroidal core 2 and magnetically coupled. One ends of the inductor coils are connected in series with the three phases Vr , Vy and Vb of an ac voltage source (not shown) through circuit breakers Br, By and Bt>. The other ends of the inductor coils are connected in series with three neutral grounded star connected capacitor banks Cr, Cy and Cb.
Due to the inductor coils being wound on the torroidal core and magnetically coupled, under normal operating conditions of the balanced power system ie under steady state conditions, the vector sum of currents add upto zero that is net current and hence net flux are zero as shown below;


As the three inductors are wound on a torroidal magnetically coupled core and linking the same flux, inductances of the inductor coils are Lr, Ly and Lb = Nx (doo/dI), where N denotes number of turns of the inductor coils and doo/dI is rate of change of flux with respect to phase or line current lr, lb or ly. As the net flux is zero, the impedance (inductance) offered by the inductors will be nearly zero ie very small and negligible. Under switching conditions (during transient period), due to non-simultaneity of closing of the three poles of the ON-OFF switches of the ac power system, the currents in the three phases would be distinctly different and the net current and hence net flux would not add upto zero. Consequently net impedance offered by the reactor would be substantially high thereby limiting the inrush currents.
The circuit diagram of Fig 4 of the accompanying drawings comprises a switching current limiting reactor 1B, wherein the inductor coils include bypass switches Sr, Sy and Sb connected across them. In case one or more of the voltage phases fail, the inductor coils and hence the current limiting reactor may be bypassed by closing the bypass switches in order to facilitate continued operation of the ac power system without affecting the

losses and the regulation. The bypasss switches may be manually operated or automatic.
Fig 5 of the accompanying drawings shows the development of a net current in the inductor coils of the reactor during the switching transient period. The waveforms representing the currents flowing through the phases R, Y and B are marked I'r, I'y and I'b, respectively. The waveform representing the net current is marked I's. Due to the net current, a net flux is generated. This gives rise to a high impedance which limits the switching inrush currents.
Fig 6 of the accompanying drawings shows development of net current in. the inductor coils after the switching transients have died down. Since the net current is zero the net flux is also zero and hence the impedance offered by the switching current limiting reactor under steady state operation is very negligible.
Fig 7 of the accompanying drawings shows net flux in the torroidal core with unbalanced currents and fluxes in the three phase

windings, which is not equal to zero, Due to non simultaneous pole closure at the switching instance the current through the reactor is not balanced and the zero flux condition is destroyed. Therefore, the impedance offered by the current limiting reactor increases substantially thereby effectively limiting the switching inrush current.
Fig 8 of the accompanying drawings shows the net flux in the torroidal core with balanced currents and fluxes in the three phase windings, wliich is equal to zero. Under this zero flux condition the net impedance offered by the current limiting reactor is nearly zero or negligible.
A three-phase switching current limiting reactor was designed with 10 turns of the inductor coils per phase. The winding was made up of 35 sq.mm PVC insulated copper cable. The core was constructed out of cold rolled grain oriented steel (CRGO) in torroidal fashion having an inner diameter of 100 mm, outer diameter of 200 mm and a width (thickness) of 50 mm. The ac impedance of the reactor was measured under balanced current conditions by passing

balanced currents from a three-phase ac supply and measuring the voltage drops across each phase winding. The ac impedance was found to be 4 milli ohm for each phase winding under balanced conditions. The inductance measured for each phase independently was 140 micro henries, winch corresponds to an ac impedance of 43.9 milli ohms. This also corresponds to the ac impedance measured by passing current through only one winding. The reactor was connected in a circuit configuration of Fig 3. The switching inrush current was measured with and without the current limiting reactor a number of times. Switching inrush current without the current limiting reactor was = 732 A which corresponds to a source impedance of 92.9 milli ohms. Switching inrush current with reactor was only 577 A which corresponds to a total circuit impedance of 117.85 milli ohms. The derived impedance of current limiting reactor under fault condition was 24.95 milli ohms as against its steady state impedance of 4 milh ohms. This indicates that the reactor offers a low impedance under normal / balanced/ steady state conditions and a high impedance under transient / unbalanced conditions, thereby effectively limiting the switching inrush current.

Under normal balanced operating conditions of the power system, impedance offered by the reactor of the invention is very small and negligible. Therefore, power losses associated with the switching current limiting reactor are very small. Since the switching current limiting reactor of the invention offers negligible impedance during normal operating conditions, it's effect on the system voltage regulation is negligible. The reactor of the invention effectively limits the switching inrush currents. Therefore, the reliability/life of the switches and capacitor banks is improved. It also ensures continued operation of the power system in case one or more voltage phases fail.
The torroidal core is made of magnetic material such as electrical steel comprising cold rolled grain oriented steel (CRGO), amorphous metal or ferrite. Alternatively, the inductor coils may be wound in torroidal configuration with air as the core instead of a torroidal core of magnetic material. Such inductor coils configuration is to be construed and understood to be within the scope of the invention.

We Claim;
1) A switching current limiting reactor for three-phase balanced ac power systems comprising neutral grounded star connected capacitor banks, wherein the reactor comprises three inductor coils of equal number of turns wound umdirectionally in torroidal configuration and magnetically coupled, one ends of the inductor coils being connected in series with the three phases of an ac voltage source through circuit breakers and the other ends of the inductor coils being connected in series with the neutral grounded star connected capacitor banks.
2} A switching current limiting reactor as claimed in claim 1, wherein the inductor coils are wound in torroidal configuration with air as the core.
3) A switching current limiting reactor as claimed in claim 1, wherein the inductor coils are wound in torroidal configuration with magnetic material as core.
4} A switching current limiting reactor as claimed in claim 3, wherein the magnetic material is electrical steel.

5) A switching current limiting reactor as claimed in claim 4, wherein the electrical steel comprises cold rolled grain oriented steel (CRGO).
6) A switching current limiting reactor as claimed in claim 4, wherein the electrical steel comprises amorphous metal.
7) A switching current limiting reactor as claimed in claim 3, wherein the core comprises ferrite.
8) A switching current limiting reactor as claimed in any one of claims 1 to 7, wherein each inductor coil includes a bypass switch connected across it i.e parallel to it.
9) A switching current limiting reactor as claimed in claim 8, wherein the bypass switch is manually operated.
10) A switching current limiting reactor as claimed in claim 8, wherein the bypass switch operation is automatic.

11) A switching current limiting reactor for balanced three-phase ac power systems substantially as herein described particularly with reference to Figs 1,2 and 3 or Figs 1,2 and 4 of the accompanying drawings.
Dated this 25th day of November 2002