This invention relates to switched inductor banks, this invention also relates to switching arrangement in switched inductor systems. BACKGROUND OF INVENTION

With increasing reactive loads in power system the current in electrical power systems are increasing to deliver real power. This is leading to increased power losses, overloading and reduced reliability of power equipments, and reduced voltage stability margins. This also gives rise to increased electromagnetic effects in power systems which affect the efficiency of the electrical systems attached thereto.

This necessitated application of reactive power compensation systems to compensate for the capacitive or inductive reactive power consumed by the load. Since the loads are most often not steady, but are time varying in nature, the value of the reactive power required is not constant and the reactive power compensating equipments have to deliver variable reactive power.

Amongst the equipments used for compensating reactive power is a synchronous condenser or synchronous generator. These are synchronous machines operated in over-excited mode and can generate a variable capacitive reactive power. The machine can also be operated as to generate inductive reactive power. Though this type of reactive power generators can generate (or absorb) variable reactive power, they have the disadvantage of being inefficient (consumes significant quantities of real power due to losses in the equipment), being expensive and requiring complex control systems.

Switched shunt inductor banks are also used for reactive power compensation. The total inductor bank is split into multiple steps with a switching device for each step. All steps are connected in parallel and required step is switched ON or OFF depending upon reactive power requirement. In the conventional switched shunt inductor bank each inductor step is provided with a series switching device. A first inductor of inductance Li is provided in series with a first switch Si , and a second inductor of inductance L ₂ in series with a second switch S ₂ . The said inductor bank has two steps each of which can be activated separately by switching respective switches.

For example if Si is closed, then inductor Li gets connected to the circuit and delivers an output reactive power depending upon the system voltage and frequency. The reactive power output will be Q i = V _L ² / 2 n f Li

Where,

Q i is the reactive power delivered,

f is the system frequency

VL is the voltage across inductor Similarly if switch S ₂ is closed, inductor L ₂ gets connected and delivers a reactive power output

2

proportional to the inductor L ₂ and the reactive power output will be Q ₂ = VL / 2 π f L ₂ If both the switches are closed, then both the inductors, L\ and L ₂ are connected to the line and the reactive power delivered is proportional to the equivalent inductance of L] and L ₂ in parallel, i.e (Li

L ₂ ) / (L, + L ₂ )

In this type of configuration, maximum of 3 output combinations are possible, i.e Qi , Q ₂ and (Q i + Q ₂ ).

These conventional switched shunt inductor banks may be replicated in each phase to provide for a three phase delta configuration or a three phase star configuration. Further, there exist various type of reactive power compensating equipments such as synchronous condenser or synchronous generators, thyristor controller shunt inductor, advanced static var- compensator using high speed semiconductor switches such as IGBT. However, each of the above identified equipment for power compensation may be either complex, expensive and/or generate harmonics and increased power losses.

Furthermore, there also exist systems for power compensation having more than one type of power compensating equipment. Various types are combined as to obtain a configuration which may work better than opting for one of the available solutions.

Of all the types of reactive power compensation equipments, switched shunt inductors are the most popular owing to their simplicity and lower cost. In the three phase delta configuration of these banks the switches are generally located outside the delta formatTonT MiTnecessitates the switches to be rated to carry the full line current of the reactor bank/step.

Therefore there is needed an inductor bank providing an efficient and safe means for increased resolution and switching combinations, which may help in providing reactive power compensation to varying reactive loads. OBJECTS OF THE INVENTION

An object of the invention is to provide a switching system for inductors which provides a better resolution and more number of output reactive power combinations for a given number of inductor steps.

Another object of the invention is to provide a switching system for power inductors, which uses switches of lower current rating to switch inductors. Another object of the invention is to provide a switching system for power inductors which enhances the life of switches and inductors.

Yet another object of the invention is to provide an economical, efficient and safe inductive reactive power compensation system.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to embodiments of the invention, examples of which may be illustrated in the accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in the context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.

Fig. 1 illustrates a two step switched inductor bank according to one embodiment of the present invention.

Fig. 2 illustrates a three step switched inductor bank as per one embodiment of the present invention. Fig. 3 illustrates a three phase two step switched inductor bank in delta configuration according to one embodiment of the invention. Fig. 4 illustrates a three phase two step switched inductor bank in star configuration according to one embodiment of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A switched inductor bank operable in multiple switching combinations in an alternating current power supply is described. In one embodiment of the present invention the inductor bank is described having two inductor steps in a single phase power supply. In another embodiment a polyphase switched inductor bank is provided.

Further, a multi-step switched inductor bank allowing numerous step additions to provide for increased switching combinations and better resolution is also provided. Each of the above embodiments are described with reference to a limited number of inductors as referenced in each embodiment, but a person skilled in the art would recognize that each of such inductors can be replaced by multiple inductors.

Also various types of inductors may be used. The switches used may include mechanical or solid state switches. In the following description, for purpose of explanation, specific details are set forth in order to provide an understanding of the invention. It will be apparent, however, to one skilled in the art that the invention may be practiced without these details. One skilled in the art will recognize that embodiments of the present invention, some of which are described below, may be incorporated into a number of different switched inductor banks. It is understood that one skilled in art may modify or change the data used in the examples described in the specification. Reference in the specification to "one embodiment" or "an embodiment" means that a particular feature, characteristic, or function described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment.

As per one embodiment of the present invention a switched inductor bank operable in multiple switching combinations in an alternating current power supply is described with reference to figure 1. The figure 1 shows the version comprising of two inductor steps. The switched inductor bank comprises of a first inductor LI ( 101 ) having one terminal connected to a power line ( 141) and the other terminal connected to a neutral line (143) through a first switch S I (1 1 1). This provides for one inductor step of the switched inductor bank.

Further, a second inductor L2 (102) having one terminal connected to the neutral line (143) and the other terminal connected to the said one terminal of the first inductor LI (l Ol)through a second switch S2 (1 12) is provided thus providing another inductor step of the switched inductor bank.

A third switch S3 (1 13) is provided across the junction of the first inductor LI (101) with the first switch S I (1 1 1 ) and the junction of the second inductor L2 (102) with the second switch S2 (1 12) whereby the number of switching combinations of the inductor bank is increased to correspondingly increase the range of reactive power delivered by the inductor bank. The said switches may be switched manually or automatically to provide for various switching combinations.

The various switching combinations on which the said switched inductor bank may operate comprises of a first switching combination having the third switch S3 (1 13) in closed position while the second switch S2 (1 12) and the first switch S I (1 1 1) are in open position. This switching combination provides for series connection of the said first inductor LI (101 ) and the said second inductor L2 (102). For example the equivalent inductance (Ls) of the series combination of inductance LI (101 ) and inductance L2 (102) would be Ls=Ll+L2. The voltage across each of the inductor will be less than the voltage across the power line (141) and neutral line (143) for which each reactor is rated. In this switching combination the thermal and dielectric stresses are much lower than nominal stresses.

The switched inductor bank also enables a second switching ^' combination having the second switch S2 (1 12)in closed position while the first switch S I (l l l )and the third switch S3 (1 13) are in open position. In such switching combination, only the second inductor L2 (102) is brought in the circuit. Further, the first switch S I (1 1 1 )in closed position while second switch S2 ( 1 12)and the third switch S3 (1 1 3)are in open position provide for a third switching combination having the first inductor LI ( 101 )in the active circuit.

In another switching combination, termed the fourth switching combination, the second switch S2 (1 12) with the first switch S I (1 1 1) in closed position while the third switch S3 (1 13)is open provides for the first inductor LI (101) in parallel to the second inductor L2 (102) for the circuit. The equivalent inductance (Lp) in this case is Lp = (LI x L2)/ (LI + L2).

In one embodiment herein a switched inductor bank is provided having multiple steps allowing multiple switching combinations. Across the switched inductor bank as shown in figure 1 , at least one twig containing an inductor and a switch in series is added as shown in figure 2.

Figure 2 shows an exemplary embodiment herein where a three step version of the said inductor bank is provided. Using four switches namely first switch S I (21 1 ), second switch S2 (212), third switch S3(213) and a fourth switch S4(214) along with the three inductors Ll (201 ), L2(202) and L3(203), nine output combinations are provided, excluding the switching combination having all the four switches open. This configuration facilitates for thirteen switching combinations. More particularly, all the switching configurations available in the two step version of the said inductor bank are provided, in which the fourth switch S4 (214) would be in open position. Further, additional switching configurations would arise by switching combination of switches when the fourth switch S4 (214)-is in closed position.

More particularly, the fourth switch S4 (214) in closed position while the first switch SI (21 1 ), the second switch S2 (212), and the third switch S3 (213) are open provides for a first additional switching combination having the third inductor in the circuit. The fourth switch S4 (214) along with first switch S I (21 1 ) in closed position while the second switch S2 (212) and the third switch S3 (213) are open provides for a second additional switching combination having the first inductor LI (201) in parallel with the third inductor L3 (203) in the circuit. The fourth switch S4 (214) along with second switch S2 (212) in closed position while the third switch S3 (213) and the first switch S I (21 1 ) are open provides for a third additional switching combination having the second inductor L2 (202) in parallel with the third inductor, L3 (203) connected in circuit.

The fourth switch S4 (214) along with the first switch S I (21 1 ) and second switch S2 (212) in closed position while the third switch S3 (213) is open provides for a fourth additional switching combination having the first inductor,Ll (201) the second inductor L2 (202) and the third inductor L3 (203) in parallel in the circuit. The fourth switch S4 (214) along with the third switch S3 (213) in closed position while the second switch S2 (212) and the first switch S I (21 1) are open provides for a fifth additional switching combination having the third inductor L3 (203) in parallel to the series combination of the first inductor LI (201 ) and the second inductor . L2 (202)

In another embodiment of the present invention a polyphase switched inductor bank is provided. The polyphase two step switched inductor bank is explained with reference to an exemplary embodiment having three phases as shown in figure 3 and figure 4. The figure 3 shows the exemplary embodiment of the invention having the delta configuration of the said three phase inductor bank. Between each phase the said bank comprises a first inductor L 1 1 (301 a), L12 (301b), L13 (301c) respectively, having one terminal connected to one phase and the other terminal connected to the other cyclically consecutive phase through a first switch S 1 1 (31 1 a), S 12 (31 1b), S 13 (31 1c) respectively. Further the bank comprises of a second inductor L21 (302a), L22 (302b), L23 (302c) respectively, having one terminal connected to the said cyclically consecutive phase and the other terminal connected to the said one terminal of the first inductor through a second switch S21 (312a),S22 (312b), S23 (312c) respectively, and a third switch S3 1 (313a), S32(313b), S33(313c) respectively, provided across the junction of the first inductor with the first switch and the junction of the second inductor with the second switch, whereby the number of switching combinations of the inductor bank is increased to correspondingly increase the range of reactive power delivered by the inductor bank. The switches present between each cyclically consecutive phase may be gang operated. Such switching of the various switches would lead to various switching combinations. Various switching combinations would be similar to the single phase switched inductor bank as explained above. For example, the third switch between the phases PI and P2, ie. S31 (3 13a), the third switch between phases P2 and P3 i.e. S32 (313b) and the third switch between phases P3 and PI , i.e. S33 (313c) all in closed position with the second switches and the first switches in open position provides for a first switching combination having the first inductor the second inductor of each phase in series in the active circuit.

Further, at least one twig containing a third inductor and a fourth switch in series, connected across the said cyclically consecutive phases may be provided enabling provision of additional switching combinations. A person skilled in the art would realize that multiple such twigs would increase the output combinations and the switching combinations of the switched inductor bank.

In the exemplary embodiment of the polyphase switched inductor bank in star configuration as shown in figure 4, each phase of the said bank comprises of a first inductor LI 1 (401 a), L12(401 b), L13(401 c) respectively, having one terminal connected to one electrical terminal / phase and the other terminal connected to the other electrical terminal or neutral through a first switch S I 1 (41 l a), S 12 (41 1b), S 13 (41 1 c) respectively. Further there is provided a second inductor L21 (402a), L22 (402b), L23(402c) respectively, having one terminal connected to the said cyclically consecutive phase and the other terminal connected to the said one terminal of the first inductor through a second switch S21 (412a), S22(412b), S23(412c) ^' respectively. Further, a third switch S31 (413a), S32(413h), S33(413c) respectively, is provided across the junction of the first inductor with the first switch and the junction of the second inductor with the second switch whereby the number of switching combinations of the inductor bank is increased to correspondingly increase the range of reactive power delivered by the inductor bank.

Further, at least one twig containing a third inductor and a fourth switch in series, connected across the power-line and the neutral-line, may be provided enabling provision of additional switching combinations. A person skilled in the art would realize that multiple such twigs would increase the output combinations and the switching combinations of the switched inductor bank.

The reference to 'a first inductor' in reference to one phase should be understood to be reference to such individual inductors in each phase in case of star configuration and to such inductor between two phases in case of delta configuration. Though the said exemplary embodiment is explained with reference to three phases it is obvious to a person skilled in the art that the said exemplary embodiment may be adapted to be used in four or more phases. In each case, the neutral of all phases would be common or joined. Similarly, polyphase multi step switched inductor bank may also be provided by using the single phase version of the three step switched inductor bank connected as explained above in a star or delta configuration.

Examples that follow are described for illustrating working of the invention during particular values of the inductance used. These examples are illustrative of the invention but not limitative of the scope thereof.

Example 1 : The inductor bank with two equal steps

This example may be understood with reference to figure 1 and corresponding explanation, with a specific limitation of this example having LI =L2.

Total reactive power output = 105 kvar

Reactive power of one of the inductor step = 52.5 kvar

Reactive power of another inductor step = 52.5 kvar

Possible power output combinations when:

All switches off = 0 kvar

Only First switch closed = 52.5 kvar

Only Second switch closed = 52.5 kvar

Only Third switch closed = 26.5 kvar

Only First switch and second switch closed= 105 kvar Neglecting the condition with zero output the number of discrete output combinations is 26.25, 52.5 and 105 kvar, i.e three different combinations.

Example 2: Proposed arrangement with two binary steps

This example may be understood with reference to figure 1 and corresponding explanation, wherein Ll = 2 times L2.

Total reactive power output = 105 kvar

Reactive power of one of the inductor step = 35 kvar

Reactive power of another inductor step = 70 kvar

Possible power output combinations when:

All switches off = 0 kvar

Only First switch is closed = 35 kvar

Only Second switch is closed = 70 kvar

Only Third switch is closed = 23.3 kvar

Only First and second switch is closed = 105 kvar

Neglecting the condition with zero output the number of discrete output combinations possible are 23.3, 35, 70 and 105 kvar, i.e four different combinations.

The foregoing description of the invention has been described for purposes of clarity and understanding. Although embodiments of the present invention have been described relative to a few standards, and associated attributes therein, one skilled in the art will recognize that the present invention is also very much applicable to other such standards. It is not intended to limit the invention to the precise form disclosed. Various modifications may be possible within the scope and equivalence of the appended claims.