Background Embodiments of the invention relate to an apparatus and method for controlling a voltage of an AC electrical supply. In particular, embodiments of the invention relate to an apparatus and method for selectively reducing or increasing the voltage of the AC electrical supply. WO 2010/067104 describes a voltage reduction system for energy efficiency and is herein incorporated by reference for all purposes. As shown in Figure 1, the system comprises a transformer 10 comprising primary 11 and secondary 12 windings in circuit between an electrical supply 21 and a load 30. A power convertor 20 is connected to the electrical supply 21 to provide voltage regulation to the primary 11 winding. The primary 11 and secondary 12 windings are arranged such that a voltage developed in the secondary winding is in anti-phase to the supply voltage and is therefore subtracted therefrom. The voltage applied to the primary 11 winding is varied by the power convertor 20. The electrical load receives a voltage V _L which is equal to the supply voltage V _IN less the magnitude of a secondary voltage Vs developed across the secondary 12 winding, ~~ ^IN ~ ^s. The secondary voltage Vs is proportional to a primary voltage V _P across the primary 11 winding, as controlled by the power convertor 20, wherein the proportion is determined by the turns ratio of the transformer, for example 10: 1. Thus the system serves to controllably reduce the voltage V _L applied to the load 30.

The system of WO 2010/067104 is provided for the purpose of energy efficiency to reduce the voltage applied to the load 30, such as in a domestic or small commercial application, since it is recognised that the power drawn by the load is also reduced. However it has been recognised that in some situations it would be desirable to increase a voltage provided to a load. For example in certain countries the electrical supply voltage is unreliable and may periodically drop. It is an object of embodiments of the invention to at least mitigate one or more of the problems of the prior art.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example only, with reference to the accompanying figures, in which:

Figure 1 shows a voltage reduction system;

Figure 2 shows an apparatus according to an embodiment of the present invention; and

Figure 3 shows a method according to an embodiment of the present invention.

Detailed Description of Embodiments of the Invention

Figure 2 illustrates an apparatus 200 according to an embodiment of the invention. The apparatus 200 is arranged to control a load voltage by selectively reducing or increasing an electrical voltage provided to a load 230. The apparatus 200 may be used to increase the electrical voltage provided to the load 230 in response to a voltage of an electrical supply falling, for example. The apparatus 200 may further reduce the voltage provided to the load to increase energy efficiency. The apparatus may be used in domestic or light commercial applications to control the voltage provided to the load 230.

The apparatus 200 comprises a transformer 210 having primary 211 and secondary 212 windings, a power converter 220 and a switching device 240 for controlling the transformer to selectively decrease or increase an output voltage. In one embodiment the switching device 240 is arranged to control an orientation of connections of the primary 211 winding of the transformer 210, as will be explained.

The transformer 210 has a predetermined winding ratio between the primary 211 and secondary 212 windings. The winding ratio may be, for example 10: 1 although it will be realised that this is merely exemplary. Thus a 250V supply applied to the primary winding 211 produces an output of 25V on the secondary winding 212. The power converter 220 is arranged to vary the voltage applied to the primary winding 211, such as from 0 to a supply voltage applied to electrical input terminals 221, which may be, for example, 250V. As discussed in WO 2010/067 104 advantageously this allows a power rating of the power convertor 220 and transformer 210 to be reduced depending upon the ratio of the transformer i.e. to 10%. The power converter 220 may continuously vary the voltage applied to the primary winding within an operating voltage range of the power converter. The power converter 220 may operate according to an input signal to output a desired output voltage.

The secondary winding 212 is arranged to be wound in anti-phase to the primary winding 211. Due to the anti -phase winding, a positive voltage applied to the primary winding 211 causes a negative voltage to be induced in the secondary winding 212 which is proportional to the positive voltage applied to the primary winding 211. Similarly, a negative voltage applied to the primary winding 211 causes a positive voltage to be induced in the secondary winding 212 which is proportional to the applied negative voltage. The switching device 240 is arranged to selectively apply the voltage output from the power convertor 220 to the primary winding 211 in first and second directions. In one embodiment the switching device 240 is a double pole double throw (DPDT) relay, although it will be realised that other types of switching device may be used, such as comprising solid-state switching devices, for example a MOSFET -based switching device.

The switching device 240 is arranged to apply the voltage output by the power convertor to the primary winding 211 in the first direction such that a negative voltage is output from the secondary winding 212 which is subtracted from the voltage applied to the terminals 221 i.e. as described in WO 2010/067104.

In the second direction the voltage output by the power convertor 220 is applied in an opposite configuration to the terminals of the primary winding 211 , such that a positive voltage is output by the secondary winding 212 which is added to the voltage applied to the terminals 221. Thus in this configuration the apparatus is arranged to boost the voltage of an electrical supply provided to the load, which may be responsive to the electrical supply voltage falling. Thus in a first configuration of the switching device 240 the apparatus 200 is arranged to reduce the voltage applied to the load 230, whilst in the second configuration the apparatus increases the voltage applied to the load 230. It will be realised that the terms reduces and increases are determined with respect to the voltage at the terminals 221. Thus if 250V is applied to the terminals 221 and the power converter 220 is configured to apply the 250V to the primary winding 211, -25V is output by the secondary winding 212 with the switching device 240 in the first configuration such that 225V is applied to the load, whereas in the second configuration the secondary winding outputs +25V such that 275V is applied to the load 230. Therefore the voltage applied to the load 230 may be selectively increased or decreased with respect to the terminal 221 voltage.

The usefulness of embodiments of the invention is particularly apparent with respect to supply voltages which fluctuate, particularly which drop intermittently. When the supply voltage drops, the apparatus 200 may selectively apply the voltage output from the power converter 220 to the primary winding in the second direction such that the voltage applied to the load 230 is increased with respect to the supply voltage 231. The apparatus 200 may thus selectively compensate for the drop in the supply voltage. The voltage output by the power converter 220 may be controlled to maintain the voltage applied to the load 230 at an intended supply voltage, such as a voltage regulated to 220V When the supply voltage remains relatively constant the apparatus 200 may be used to selectively reduce the voltage applied to the load 230, as described in WO 2010/067104 for energy efficiency. In this way embodiments of the present invention may provide efficiency gains whilst compensating for variable supply voltages.

Further details of the exemplary embodiment of the apparatus 200 will now be provided. The apparatus further comprises a bypass switch 243. A temperature sensor 242 is arranged to continuously or intermittently measure the temperature of the transformer 210. A signal is fed from the sensor 242 to an electronic control module 250 which, when a certain temperature is achieved, outputs a signal to close the switch 243, thus connecting the load 230 directly to the power supply 221 via a first fuse 263. This enables the power converter 220 and transformer 210 to be rated for the average load expected of the apparatus 200. When the bypass switch 243 is operated the transformer 210 is allowed to cool. A thermal cut-out (TCO) 260 is provided in an output from the secondary winding 212 to protect the transformer 210. The TCO 260 is arranged to operate in an instance that the switch 243 fails for some reason. Should switch 243 fail the transformer will operate for a period at a higher temperature before the TCO 260 operates. The TCO has a maximum sustainable current rating above which a second fuse 262 operates to protect the transformer 210. The second fuse 262 is set to operate below a maximum current rating of the TCO 260 to prevent nuisance operation of fuse 262 before the transformer 210 has reached its thermal trip level. A current sensor 241 is arranged to measure the current of the supply and is connected to provide a signal indicative of the current to the control module 250. The control module 250 operates to close switch 243 at a current level below the rating of the fuse 262.

The second fuse 262 is arranged in series with the supply to the transformer 210. The second fuse 262 may have a lower current rating than the first fuse 263. The second fuse may be selected with a trip rating equal to the transformer 210 continuous rating and is arranged in series with the secondary winding 212 in order to interrupt current flow to the load 230 through the transformer 210.

In another embodiment, although not shown, the transformer 210 may comprise a thermally operated switch wherein an output of the switch is connected to a latching relay. The latching relay is arranged to disconnect the secondary winding 211. In this way, when the transformer 210 exceeds a predetermined temperature the secondary winding 211 is disconnected by the latching relay. Figure 3 illustrates a method 300 according to an embodiment of the invention. The method 300 may be performed by the apparatus shown in Figure 2. In particular, the method 300 may be performed by the control module 250. The method comprises a step 310 of determining whether a supply voltage, that is the voltage applied to terminals 221 in Figure 2, is less than a predetermined threshold. The threshold may be, for example, 220V where the input voltage is expected to be 250V although it will be realised that this is merely exemplary. The predetermined threshold may be defined with respect to the expected input voltage, for example as a predetermined percentage of the expected voltage.

In step 320, if the voltage is less than the threshold in step 310, the load voltage is increased with respect to the input voltage. As previously described the load voltage is increased in the apparatus of Figure 2 by the switching device 240 operatively connecting the primary winding 211 in the second direction such that the output of the secondary winding 212 is additive to the input supply voltage.

In step 330, if the supply voltage is not less than the threshold, the voltage to the load 230 may be reduced for energy efficiency. As previously described the load voltage is increased in the apparatus of Figure 2 by the switching device 240 operatively connecting the primary winding 211 in the first direction such that the output of the secondary winding 212 is subtractive to the input supply voltage.

In step 340 it is determined whether a temperature of the transformer 210 is above a predetermined threshold. If the transformer is above the threshold then operation moves to step 350 where the transformer 210 is bypassed, such as by switch 243. The transformer is allowed to cool whereupon the switch 243 may be reconnected to allow operation of transformer 210 to resume and method returns to step 310. It will be realised that fuses 262, 263 may be operable to interrupt current flow if the current exceeds the rating of either fuse.

Embodiments of the invention provide a method and apparatus for selectively increasing or decreasing a voltage provided to a load. The voltage provided to the load may be selectively decreased for energy efficiency and selectively increased in case of a voltage supply dropping in voltage.

It will be appreciated that embodiments of the present invention can be realised in the form of hardware, software or a combination of hardware and software. Any such software may be stored in the form of volatile or non-volatile storage such as, for example, a storage device like a ROM, whether erasable or rewritable or not, or in the form of memory such as, for example, RAM, memory chips, device or integrated circuits or on an optically or magnetically readable medium such as, for example, a CD, DVD, magnetic disk or magnetic tape. It will be appreciated that the storage devices and storage media are embodiments of machine-readable storage that are suitable for storing a program or programs that, when executed, implement embodiments of the present invention. Accordingly, embodiments provide a program comprising code for implementing a system or method as claimed in any preceding claim and a machine readable storage storing such a program. Still further, embodiments of the present invention may be conveyed electronically via any medium such as a communication signal carried over a wired or wireless connection and embodiments suitably encompass the same. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.