WO2011024167A1

WO2011024167A1 - System and method for controlling voltage, useful for controlling the voltage at the user site

Info

Publication number: WO2011024167A1
Application number: PCT/IL2010/000693
Authority: WO
Inventors: Shimon Limor
Original assignee: Powersines Ltd.
Priority date: 2009-08-26
Filing date: 2010-08-25
Publication date: 2011-03-03

Abstract

A method and apparatus for reducing the energy used by a home or business having a mixed load and designed to operate from a given nominal mains voltage and a given tolerance, the method including lowering the voltage supplied to a home or business having a mixed load, to a new nominal voltage, within the tolerance, that is substantially below the given nominal; controlling the supplied voltage to a second tolerance that is tighter than the given tolerance with changes of the mains voltage within the tolerance, wherein lowering the voltage supplied comprises inserting a voltage is series with the line which reduces the voltage fed to the load.

Description

SYSTEM AND METHOD FOR CONTROLLING VOLTAGE, USEFUL FOR CONTROLLING THE VOLTAGE AT THE USER SITE

FIELD OF THE INVENTION

The present invention is concerned with providing a variable voltage, for example a variable voltage system suitable for stabilizing the voltage supply especially to a home or other small consumer, such as a small workshop or office.

BACKGROUND OF THE INVENTION

The mains voltage fed to a consumer's premises is invariably variable to a certain extent. This variability can be caused by fluctuations at the power stations or sub-stations and also by the variation in line losses dependent on the current drawn by other subscribers supplied by the same power lines or power grid.

Thus, electrical power consuming devices invariably are designed to operate over a range of voltages. As might be expected, the efficiency of these devices and the KVA, KVAR, Current, power factors.... drawn by the devices varies with voltage supplied.

Devices for varying the voltage on the line are well known. In general, these devices suffer from various deficiencies, such as commutation problems, cost, size and weight.

Israel patent 133307 filed December 5, 1999 the disclosure of which is incorporated here by reference, describes a system for lighting control in which a transformer primary is placed across the input (between "line" and "return" connections) and the secondary is in series between the load and the line. The secondary is wound and attached to oppose (and thus reduce) the line voltage supplied to the load. This provides a reduced voltage at the load. When the full voltage is needed, the transformer input is disconnected from the return and short circuited, forcing the voltage on the secondary to zero. The primary can then be short circuited. Shorting the primary does not require switching high currents through the short since the current in the primary is much lower than that in the secondary, which carries the full line current. Thus, such systems are especially suited for controlling the voltage for relatively large areas with multiple users. Multiple transformer stages can be utilized in series to provide a greater variation in load voltages. For three phase, the configuration is repeated for each phase.

US Patent 7,330,000 B2, the disclosure of which is incorporated herein by reference, describes a system in which an output transformer has its secondary placed in series between the source and the load. The primary of the output transformer is fed by a controller system providing a variable voltage. This voltage is provided by feeding a series of control units from the line with a secondary of a transformer in each control unit being in a series configuration between the line and the primary of the load transformer with each unit's primary being connected between line and neutral or ground or disconnected from the line. When the primary of a particular unit is connected to the line, the secondary is configured to reduce the voltage fed to the primary of the output transformer. If the primary of a particular unit is disconnected from the line, the secondary of the unit is shorted and the voltage to the output transformer is not reduced. Generally, the secondary of the output transformer is configured so that the load voltage is reduced by the voltage on the secondary transformer. Since the current in the controller system is much lower than the line current, shorting the secondaries of the control unit's transformers does not require a high current short.

The devices described in the previous two references have been used for controlling the voltage supplied to large numbers of light fixtures (for example in offices, factories and gas stations) where supplying reduced voltages can result in substantial savings.

PCT Publication WO 2008/010213, the disclosure of which is incorporated herein by reference describes a three phase variable voltage control suitable for providing a variable voltage for start up for a motor in system in which secondaries of three transformers are placed in series with each of the lines and the primaries can be switched between phase to phase and phase to neutral. This allows for the injection of additive, subtractive and out of phase voltages to the line voltages. As in Israel Patent 13307, when the line voltage is desired, the primary is disconnected and shorted to force a zero voltage on the secondary, which can also be shorted. Since the voltage control is originally designed for motor starting, the line current is not excessive and in any event the short on the primary is imposed at full voltage after high starting currents have fallen to steady state values.

SUMMARY OF THE INVENTION

An aspect of some embodiments of the invention is the provision of a system and method for providing a variable voltage, for example a variable voltage system suitable for stabilizing the voltage supply especially to a home or other consumer.

Some embodiments of the invention utilize a novel circuit design for varying the voltage, which design combines a minimum of commutation effects and ease of operation.

There is thus provided, in accordance with an embodiment of the invention a method of reducing the energy used by a home or business having a mixed load and designed to operate from a given nominal mains voltage and a given tolerance, the method comprising:

lowering the voltage supplied to a home or business having a mixed load, to a new nominal voltage, within the tolerance, that is substantially below the given nominal;

controlling the supplied voltage to a second tolerance that is tighter than the given tolerance with changes of the mains voltage within the tolerance,

wherein lowering the voltage supplied comprises inseπing a voltage is series with the line which reduces the voltage fed to the load.

In some embodiments of the invention, the supplied voltage is substantially below the mains voltage at least for values of mains voltage at the nominal voltage or above.

In some embodiments of the invention, for all mains voltages values above the nominal voltage the supplied voltage is at least 5% or 3% below the mains voltage.

In some embodiments of the invention, wherein the range of supplied voltage is within ±3% of a target voltage over a range of variation of 11% of the mains voltage.

In some embodiments of the invention, the target voltage is at least 4% or at least 5.5% below the nominal voltage. In some embodiments of the invention, the voltage supplied is below the nominal voltage for at least 60% of the range of the allowed variation of the mains voltage, when the allowed variation of the mains voltage is ±10%.

In some embodiments of the invention, the voltage supplied is below the nominal voltage for at least 75% or 80% of the range of the allowed variation of the mains voltage.

In some embodiments of the invention the inserted voltage is derived from a same phase as the line voltage.

In some embodiments of the invention, the line voltage is a three phase voltage and wherein the voltage inserted in any given phase is derived from the voltage of a different phase.

There is further provided, in accordance with an embodiment of the invention apparatus for controlling the voltage supplied to a load comprising:

at least one transformer having a primary and a secondary winding, the secondary winding being in series with a line transmitting power from an input line to the load and the primary being situated between the line transmitting power and either a neutral or another line,

wherein the secondary is placed closer to the input than the primary, such that the voltage on the primary is effected by the voltage on the secondary.

Optionally, the apparatus includes a switching arrangement that switches the primary from a first connection in which it is connected between the transmitting line and the neutral or other line and a second configuration in which it is not connected to the any live line.

Optionally, the switching arrangement provides a short circuit on at least one of the primary and the secondary in the second configuration.

Optionally, the switching arrangement provides for switching the primary from being connected from the primary to one other line of a three phase system to a second line of the three phase system

Unless otherwise defined, all technical and/or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the invention, exemplary methods and/or materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and are not intended to be necessarily limiting. BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced. For illustrative clarity certain elements in some of the drawings are illustrated not-to-scale. The drawings are not to be considered as blueprint specification.

In the drawings:

FIG. 1 is a schematic block diagram of a system for controlling the voltage input to a home, business or small factory, in accordance with some embodiments of the invention;

FIG. 2 is a simplified circuit diagram of a voltage control module suitable for use in the system of FIG. 1;

FIG. 3 is a flow diagram of the operation of a voltage control module according to an embodiment of the invention;

FIG. 4 shows the output voltage as a function of input voltage for a particular choice of transformer ratio;

FIGs. 5A and 5B show simplified circuit diagrams of variations of the circuit of FIG. 2; and

FIGs. 6A and 6B are simplified circuit diagrams of voltage control modules suitable for use in a three phase system.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

The present invention is concerned with providing a variable voltage, for example a variable voltage system especially suitable for stabilizing the voltage supply especially to a home or other small consumer, such as a small workshop or office. While the prior art described above shows methodologies for controlling selected types of loads, mainly to provide stepped increases in voltage for start-up or specified voltages for specific loads, the present inventors have found that placing a voltage controller at each dwelling, business, etc., is a cost effective way to cut overall electrical usage by many percent. As an important economic by-product, the power savings are so great that construction of power plants costing many times the cost of installation of the devices is deferred.

An aspect of some embodiments of the invention is concerned with a system for controlling the voltage supplied to individual dwellings or small workshops. Generally such users receive one or three phase voltage with a current of up to 30 amps per phase. The present inventors have surprisingly found that large savings in power and large reductions in KVA can be achieved by controlling the voltages of individual users. These savings are large enough to justify installation of voltage control devices at individual users.

As is well known, line voltage at the user may vary over a considerable range.

Usually, the allowable variation is up to ±10%. Appliances and lighting fixtures are designed to operate satisfactorily over this range of voltages. However, the efficiency of almost all devices and the power factor or many devices varies depending on the voltage. While prior art voltage controllers have been mooted for control of individual devices or for controlling the voltage supplied by power stations or substations or to large users, until the experiments described herein, the art has apparently not considered the possibility of controlling the voltage at each user, such as a home, workshop or small business, perhaps considering it to be uneconomic.

In some embodiments of the present invention, a voltage regulator is added at an individual user level (i.e., less than about 30-40 amp per line service for mixed loads) to provide a smaller band of voltage variation, than that allowed by the standard of ±10%. In experiments such reduction has been shown to reduce energy consumption by an average of 15% and to reduce KVA by an average of 20% under certain load conditions and by more than 10% under mixed load conditions. This is a very significant reduction and easily justifies the capital cost of retrofitting homes, small businesses and workshops with voltage controllers. This saving is marked when the voltage supplied to the consumer is reduced by several percent from nominal. In one aspect of some embodiments of the invention, the voltage to a user is set at a voltage which is less than the nominal voltage. The present inventors have found that many large users of electricity within a home operate more efficiently and at lower KVA than when operated at nominal voltage. One reason that these devices are designed in this way is that their ability to function at voltages much lower than the optimal voltages is limited. Thus, such systems are designed to be most efficient at a lower voltage (say 215 volts for a 230 volt system) but not at the nominal voltage itself, since they would not operate well at a voltage that is 10% less than their optimal voltage. It is to understood that reducing the voltage by any significant amount from nominal will result in savings, which may be significant. Thus, in some embodiments of the invention the voltage reduction from nominal may be 5 or 10.

In the case of many devices, trade-offs in design have to be made to ensure that the devices operate well over the entire range of expected line voltages. In addition to the optimum being at voltages other than the nominal voltage, the requirement for operation over a larger range does not allow for the highest possible efficiency at the optimal voltage itself. In some embodiments of the invention a tighter control over the voltage at the user allows for the design of more efficient devices.

In an aspect of some embodiments of the system, devices which have a higher efficiency at lower voltages are assigned to specific load circuits. The voltage to these load circuits is reduced and controlled to a higher tolerance than the line voltage, while the other load circuits are not controlled. Optionally, the other circuits may be controlled to a voltage other than the line voltage.

An aspect of some embodiments of the invention is concerned with an apparatus and method for controlling line voltage. In this apparatus, one or more voltage varying units are placed between the line and the load. Each of these units comprises a transformer whose secondary is in series with the load and a primary which is in parallel with the line at the input or the output. Each of the units preferably further includes a switch which substantially simultaneously is capable of disconnecting the primary from the line and shorting the secondary. Shorting of the primary is not necessary. For each unit the transformer is configured for either increasing or decreasing the voltage to the load. A series of such units allows for reducing the variations of voltage on the load under varying line voltage conditions. A controller is preferably provided which measures the incoming line voltage and determines which of the units should be operated to change the voltage delivered to the load to provide a narrower range of voltages to the load.

For three phase control, a similar controller can be placed in each line. Since the line to neutral voltages for different phases are sometimes different controlling each of the phases separately may be advantageous to equalize the voltages on the phases. When three phase equipment is part of the load, this equalization can result in even greater power savings and reduced stress to the equipment. Three separate transformers for each three phase unit could also be replaced by a single three phase transformer.

Before explaining various embodiments of the invention in some detail, it is to be understood that the invention is not necessarily limited in its application to the details of construction and the arrangement of the components and/or methods set forth in the following description and/or illustrated in the drawings and/or the Examples. The invention is capable of other embodiments or of being practiced or carried out in various ways. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. In particular, some features that may be applied to more than one or all of the embodiments are described with respect to only one of the embodiments.

Fig. 1 is a schematic block diagram of a system 10 for controlling the voltage input to a home, business or small factory, in accordance with some embodiments of the invention. A single phase circuit is shown. However, a separate or combined circuit can be provided for three phase systems, as described below.

A main line 12 supplies nominal voltage to a home or small business. A voltage control module 14 controls the voltage as described below. Load lines 16, optionally fused by circuit breakers or fuses 18, feed power from control module 14 to the individual load circuits.

In a first embodiment of the invention, the voltage to all of the load lines is reduced to a voltage below nominal at which, for example, most motors work more efficiently most of the time. This embodiment has the advantage of simplicity. However, the system has the disadvantage of higher current and power requirement and potentially from somewhat lower efficiency since the voltage is lowered for devices which do not gain from such lowering. In some embodiments when the voltage is above nominal it may be reduces, but to a voltage between the target low voltage and nominal or slightly above or below nominal.

In a second embodiment of the invention the voltage is kept at the nominal voltage, but is controlled to reduce the spread of voltages. Alternatively, control is applied only when the voltage is above nominal. At present, this would result in only a limited saving, based mainly on the reduction of energy use when the voltage is higher than nominal. However, as devices that are less voltage tolerant but which have a higher efficiency become available, such systems would be worthwhile.

Fig. 2 shows a first embodiment of a voltage control module 14. This system is applicable for controlling a single phase system or one phase of a three phase system. A line voltage is applied between terminals 20 and 22. In general, the line voltage is applied to terminal 20 and terminal 22 is neutral. A controller 24 receives an indication of the line voltage via a connection 26 and optionally compares it to a reference. Controller 24 controls two double pole single throw switches A and B, each of which comprises two switches al and a2 or bl and b2. When al is open, a2 is closed and vice versa. Similarly for bl and b2.

Voltage control module 14 also includes a first transformer 28, associated with switch 'A' and a second transformer 30 associated with switch 'B'. The primary of each transformer is designated by the reference number prime (28', 30') and the secondary designated by the reference number double prime (28", 30").

Before going on to the actual operation of the system, consider a situation where the line voltage is nominally 230 volts. Under normal circumstances, this voltage may vary ±10% or between 207 and 253 volts. It is desired to have the load voltage close to 215 volts using the circuit of Fig. 2. The reasons for providing a reduced voltage will be explained later.

Consider the case where transformer A has a 230:5 primary.secondary ratio and transformer B has a 230:10 primary:secondary ratio. Thus, is switch al is closed and switch a2 is open, the voltage at the load side of the secondary of A will be (for nominal input voltage) 230-5= 225 volts. Similarly, if al is open and a2 is closed (shorting the secondary of A) and bl is closed and b2 is open, the load voltage (for nominal input) will be 230-10=220 volts. If al is closed, bl is closed and a2 and b2 are open the voltage fed to the load will be 230-10-5=215 volts. It is clear that for all voltages greater than 212.5 and less than 232.5 the load voltage can be controlled to within 215±2.5 volts according to the graph of Fig. 3. For lower input voltages the load and line voltages are the same and for higher voltages the load voltage is at least 15 volts less than the line voltage.

It should be understood that it is possible to control the voltage to different tolerances over different ranges, for example by utilizing other than two transformers (one gives poorer control and three or more, better control) and different turn ratios which can extend the tolerance/range of control. However, the present inventor has found that the choice of 5 and 10 volt feedback gives a good balance between energy savings on the one hand and size, losses and cost of the control system on the other.

The operation of system 10 is explained with the aid of Fig. 4, which is a simplified flow chart 100 of the operation of a voltage control system according to some embodiments of the invention.

At 102 the input line voltage is measured. At 104, the input voltage is compared to a number of thresholds. As seen on Fig. 3, there is a transition between which sets of switches are open and closed to provide the desired tolerance and center voltage. Based on the voltage measured the switches that should be opened and closed are determined at 106. To avoid excessive switching some hysteresis may be built into the switching thresholds. At 108, the switch configuration is compared to the current configuration, which is changed (at 110) as necessary.

The system continues to check the voltage and adjust the switches as necessary.

It should be understood that there are other circuits that can be used to control the voltage. Some of these are shown in Figs. 5A-C.

Fig. 5A shows a system similar to that of Fig. 2, except that the primaries are on the load side of secondaries. Although the voltages are slightly different from the system of Fig. 2, the operation is generally the same.

Fig. 5B shows yet another system similar to that of Fig. 2 except that each stage of transformers are in separate modules 50, 52 and the short circuits are across the primaries instead of the secondaries. The operation of this system is similar to that of those described above.

It should be understood that the various positions of the short circuits and the order of the primaries can be combined. The circuits shown in Figs. 2 and 5A have a switch which short circuits the secondary when no voltage is to be injected into the line. In some embodiments of the invention, the secondary is not shorted and the primary is shorted instead as in Fig. 5B. This configuration requires a shorting switch with a lower current capacity. This type of circuit was used in the prior art to control large discharge lighting circuits since the current capacity of the short circuiting switches in Figs 2 and 5 A would be high and the switches large. Moreover, shorting the primary has somewhat greater commutation problems.

All of the circuits shown above are essentially single phase circuits. For three phases, these circuits can be placed in each line optionally with a single controller to control all of the switches.

Fig. 6A shows a three phase control circuit 200. A three phase transformer 202 having primary windings A, B and C and corresponding secondary winding a, b and c. Two three pole, single throw switches 204 and 206 switch the primaries between neutral N and one or another phase.

Switch 204 has two positions Kl and K2. Similarly, switch 206 has two positions K2 and K3. There are four modes of switching controlled by controller 26.

For the purposes of explanation, the primary secondary ratio is 400:20 (or

230:11.5) and the line voltages are nominal (400 V line to line and 230 V line to neutral). In a first mode, switch 204 is in position K2 and switch 206 is in position K4.

In this case all of the primaries are shorted and the load voltage is the same as the line voltage.

In a second mode switch 204 is in position Kl and switch 206 is in position K4. In this case the primary of each line is connected from line to neutral and 5 volts, negative phase are fed back into each of the lines, so that each load to neutral is 225 volts,

In a third mode, switch 204 is in position Kl and switch 206 is in position K3. 15 volts is induced on each of the secondaries. In this mode, phase 2 leads phase 1 so that the sum of the load and the sum of the input and secondary voltages is 215 volts.

In a fourth mode switch 204 is in position K2 and switch 206 is in position K3,

This feeds 10 volts in leading phase in each line. The total load voltage is 220 volts. Using the same methodology for determining switching points for switches Kl and K2, if the line voltage is between 215v and 23Ov, the load voltage will be 215v ± 2% and

if the line voltage is between 230 and 253, the load voltage will be 215v - 238v.

Fig. 6B shows a similar circuit to that of Fig. 6A, except that the primaries follow the secondaries. This operates in a manner similar to that of Fig. 6A, except that the since the secondary is always in series, there are fewer commutation problems.

In general, it is desirable to control the voltage supplied so that it is around the desired voltage (215 volts in the above examples). However, doing so would require that a rather large transformer system be used and/or that more stages of control be used. Bother of these increase the costs and/or losses of the system. However, the present inventor believes that a good compromise is that described above, which results in the voltage being below nominal for at least 60, 70, 80 or 85% of the range of allowed variation of the mains voltage. In particular, the above designs generally provide for the voltage supplied to the householder or business to be below the nominal mains voltage over 85% of the allowed range of the mains voltage.

As used herein, the terms substantially below the nominal means more than 2.5% below nominal.

EXPERIMENTS

Homes of two of the officers of the assignee of this application were fitted with a three phase system as in Fig.όA. The Power, KVAR and KVA were measured at 235 V (the actual line voltage) and at 215 V. The load includes lighting, an electric clothes dryer, water heater, several air-conditioners, dishwasher, washing machine and (electric) oven. The on times for each of the loads was varied to simulate the normal mix that might be expected over a day. The reduction in power was between 10 and 16%; the reduction in KVAR, between 17 and 26% and the reduction in KVA between 11 and 16%.

Similar simulation experiments were performed and gave similar results.

Economics

There are two sets of economic issues involved in the utility of the present invention. One is the savings to the user (householder or businessman). The other is the savings for the electric company. As to the consumer, a conservative estimate is a 10% savings in average energy use. If the consumer uses 3000 KWH/month then the annual savings is 3600 KWH per year. Since the cost for installation of a 30 Amp, 230 V 3 phase system is estimated at under $500, the payback period for a cost per KWH of 10 cents, is about 1.5 years.

However, the major saving is to the electric company. A major cost for electric companies is the cost of generation plants and infrastructure. Studies have shown that this cost is generally between $2,000 and $6,000 per KW of capacity. Thus, for each $500 of investment and assuming a peak power usage of 20 KW, the peak power would be reduced by 2 KW or more, resulting in a capital saving in power plant construction of at least $4,000. In addition, since the peak current would be reduced, the line losses, which are proportional to the square of the peak current, would also be reduced.

While a single system connected to a single home is described above, it is understood that such systems can be supplied to many homes or businesses to provide increased savings. Preferably, a different device is supplied to each home or business.

Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. In particular, while the disclosed embodiments define various values of voltage change and tolerance, these should not be considered to be limiting. Reference is made to additional tolerances and voltage changes defined in the summary of the invention and defining variations on the disclosed embodiments that are included within the scope of the invention. Accordingly, the claims are intended to embrace all such alternatives, modifications and variations that fall within scope of the appended claims.

All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. To the extent that section headings are used, they should not be construed as necessarily limiting.

Claims

1. A method of reducing the energy used by a home or business having a mixed load and designed to operate from a given nominal mains voltage and a given tolerance, the method comprising:

wherein lowering the voltage supplied comprises inserting a voltage is series with the line which reduces the voltage fed to the load.

2. A method according to claim 1 wherein the supplied voltage is substantially below the mains voltage at least for values of mains voltage at the nominal voltage or above.

3. A method according to claim 1 or claim 2 wherein for all mains voltages values above the nominal voltage the supplied voltage is at least 5% below the mains voltage.

4. A method according to any of the preceding claims wherein for mains voltages between a target voltage and the nominal voltage the supplied voltage is at the target voltage ±3%.

5. A method according to claim 4 wherein the range of supplied voltage is within ±3% of a target voltage over a range of variation of 11% of the mains voltage.

6. A method according to claim 4 or claim 5 wherein the target voltage is at least 4% below the nominal voltage.

7. A method according to claim 6 wherein the target voltage is at least 5.5% below the nominal voltage.

8. A method according to any of preceding claims wherein the voltage supplied is below the nominal voltage for at least 60% of the range of the allowed variation of the mains voltage, when the allowed variation of the mains voltage is ±10%.

9. A method according to any of the preceding claims wherein the voltage supplied is below the nominal voltage for at least 75% of the range of the allowed variation of the mains voltage.

10. A method according to claim 9 wherein the voltage supplied is below the nominal voltage for at least 80% of the range of the allowed variation of the mains voltage.

11. A method according to any of the preceding claims wherein the inserted voltage is derived from a same phase as the line voltage.

12. A method according to any of the preceding claims wherein the line voltage is a three phase voltage and wherein the voltage inserted in any given phase is derived from the voltage of a different phase.

13. Apparatus for controlling the voltage supplied to a load comprising:

14. Apparatus according to claim 13 and including a switching arrangement that switches the primary from a first connection in which it is connected between the transmitting line and the neutral or other line and a second configuration in which it is not connected to the any live line.

15. Apparatus according to claim 14 wherein the switching arrangement provides a short circuit on at least one of the primary and the secondary in the second configuration.

16. Apparatus according to claim 14 or claim 15 wherein the switching arrangement provides for switching the primary from being connected from the primary to one other line of a three phase system to a second line of the three phase system.