alternating current electricity supply

The present invention relates to an apparatus and method for controlling the voltage in an alternating current electricity supply and relates particularly, but not exclusively, to a voltage optimisation system for a domestic and commercial power supply.

It is well known that the electricity supply presented to residential and commercial premises is not constant. Due to the varying loads presented by the customers, and the resistances in the distribution network, the electricity supply companies struggle to keep the supplied voltage within recommended limits of 209V - 253V. Even when the supplied voltage is within the intended limits many appliances waste energy or fail to operate optimally due to the wrong supply voltage.

The use of transformers contained in voltage optimisation systems located in or near the premises receiving the supply are known and are used to ensure that a more nearly correct voltage is provided.

Examples of the prior art include the use of a simple autotransformer. Although reliable, this is of limited use as it only provides a fixed reduction or increase in supply voltage. Also the transformer must be rated to withstand the maximum load of the supply continuously making the apparatus large and expensive.

Another example of the prior art is the use of an autotransformer with automatic bypass relay. In this instance when the supply is insufficient, the load too great for the transformer or the voltage sufficiently close to the optimum, a changeover relay routes the incoming supply directly to the load, "bypassing" the transformer. A smaller transformer, compared to the previous example, may be used but the operation of the relay momentarily interrupts the supply causing surges and spikes in the power delivered to the premises.

A further example of the prior art is the use of a series-connected transformer, supplied by a digital pulse width modulation (PWM) pulses, or phase-angle adjusted waveform, to produce an adjustable average output voltage. This solution provides good regulation without relay switching spikes, but generates interference which is difficult and costly to filter, and is complicated and expensive to mass-produce.

Specific examples of the prior art include CN20062110134U that refers to an invention, which uses one or more autotransformers to select a voltage reduction, and a bypass switch, with voltage sensing and microprocessor control. WO2007/017618 discloses an invention which uses a single transformer, exited by a variable supply voltage, connected in series with the supply, to generate a variable voltage which compensates for the variation in the incoming supply. This example uses digital electronics to generate either PWM or phase angle variation to adjust the supply voltage. The transformer is deliberately under-rated, and the invention includes an automatic mechanism for reducing the effective regulation when the transformer becomes hot. EP2244353 is an example of an autotransformer with automatic bypass relay.

Preferred embodiments of the present invention seek to overcome the above described disadvantages of the prior art.

According to an aspect of the present invention there is provided an apparatus for controlling output voltage of an alternating current electricity supply, the apparatus comprising:- input terminals for receiving a power supply voltage; output terminals for supplying a first output voltage; transformer means for receiving a first input voltage dependent on said power supply voltage and providing a second output voltage, different from said first input voltage; switching means having a first mode in which said second output voltage is supplied to said output terminals and a second mode in which said power supply voltage is supplied to said output terminals; voltage determining means for providing a first control signal dependent on said power supply voltage; and processing means for receiving said first control signal and providing a second control signal for selecting said first and second mode such that said switching between said first and second modes occurs when the amplitude of said supply voltage is within a predetermined range relative to zero.

By switching between a mode in which the transformer adjusts voltage (voltage optimisation mode) and a mode in which no such adjustment takes place (bypass mode) at or around the point of zero voltage in an alternating current, the advantage is provided that the likelihood of arcing in the switches and the production of spikes in the voltage is significantly reduced or eliminated. As a result, apparatus downstream of the voltage controlling apparatus is protected without the use of further significant voltage damping apparatus. As a result, the apparatus of the present invention can be used economically in a domestic or commercial premises.

In a preferred embodiment, the transformer means comprises at least one autotransformerj[GJAi].

By using an auto transformer the advantage is provided that the overall size of the apparatus is minimised.

In another preferred embodiment, the transformer means comprises at least one toroidal transformer.

By using a toroidal transformer this offers reduced size of transformer along with efficiency gains.

In a further preferred embodiment the transformer means comprises at least one transformer including a plurality of taps. By providing the transformer with a plurality of taps, the advantage is provided that the output voltage can be controlled with a reasonable degree of accuracy to maintain a voltage close to the optimum voltage.

In a preferred embodiment, the voltage detection means measures the voltage across said input terminals.

In another preferred embodiment, the voltage detection means measures the voltage across said output terminals.

In a further preferred embodiment, the switching means comprises a plurality of switching devices.

In a preferred embodiment, a plurality of said switching devices act to operate respective said taps on said transformer.

In another preferred embodiment, at least one said switching device comprises a latching relay.

In a further preferred embodiment the switching devices each has an open position and a closed position and said predetermined range is such that at least one switching device changes between one of said open and closed positions before the amplitude of said supply voltage is zero and at least one other switching device changes between the other of said open and closed positions after the amplitude of said supply voltage is zero.

The use of latching relays enables the control of large currents with limited power draw and such relays can be controlled with accurate timing. By timing the opening and closing of the pair of the switches such that one switch changes its state just before the zero voltage point in the alternating current wave and the other alters its state just after the zero point, this provides the advantage that the gap between the opening and closing ensures maximum protection of downstream circuits. Furthermore, this reduces contact wear within the latching relays and limits negative effects to wave form. ln another preferred embodiment, the said processing means delays providing said second control signal dependent upon characteristics of said apparatus which are compared to a look up table or by calculation.

By using a look-up table to determine whether a switching operation should take place immediately or should be delayed dependent upon certain measured characteristics, provides the advantage that switching only takes place when it is most suitable and provides the optimum efficiency and prevents repeated switching on and off between voltage optimisation and bypass modes.

In a further preferred embodiment, the characteristics include at least one of input voltage, output voltage, current load, transformer temperature, time since last change of mode and proximity to main circuit.

According to another aspect of the present invention, there is provided an apparatus for controlling output voltage of an alternating current electricity supply, the apparatus comprising:- input terminals for receiving a power supply voltage; output terminals for supplying a first output voltage; transformer means for receiving a first input voltage dependent on said power supply voltage and providing a second output voltage, different from said first input voltage; switching means having a first mode in which said second output voltage is supplied to said output terminals and a second mode in which said power supply voltage is supplied to said output terminals; voltage determining means for providing a first control signal dependent on said power supply voltage; and processing means for receiving said first control signal and providing a second control signal for selecting said first and second mode such that said switching between said first and second modes is delayed dependent upon characteristics of said apparatus which are compared to a look up table or by calculation.

By using a look-up table to determine whether a switching operation should take place immediately or should be delayed dependent upon certain measured characteristics, provides the advantage that switching only takes place when it is most suitable and provides the optimum efficiency and prevents repeated switching on and off between voltage optimisation and bypass modes.

In a preferred embodiment, the characteristics include at least one of input voltage, output voltage, current load, transformer temperature, time since last change of mode and proximity to main circuit.

Preferred embodiments of the present invention will now be described by way of example only, and not in any limitative sense, with reference to the accompanying drawings in which: Figure 1 is a schematic representation of an electricity supply incorporated in the present invention;

Figure 2 is a circuit diagram showing an embodiment of the present invention; Figure 3 is a diagram representing the timing of operations used in the present invention;

Figure 4 is a circuit diagram showing the use of multiple relays in an embodiment of the present invention;

Figure 5 is a schematic representation showing the use of multiple relays in the present invention;

Figure 6 is a portion of an embodiment of the present invention; Figure 7 is a circuit diagram of a further embodiment of the present invention;

Figure 8 is a circuit diagram of another embodiment of the present invention;

Figure 9 is a circuit diagram showing a further embodiment of the present invention for use in three phase supply; and

Figure 10 is a circuit diagram showing a further embodiment of the present invention.

Referring to Figure 1 , an apparatus, hereinafter referred to as a voltage optimisation unit 10, receives a supply voltage at input terminals 12 and 14 and supplies an output voltage at output terminals 16 and 18. The voltage optimisation unit is typically for use in domestic and commercial premises and the supply of voltage comes from an external electricity supply via electricity meter 20. Also typically downstream of the voltage optimisation unit 10, the output voltage is provided to a consumer unit fuse panel 22 through which electricity is supplied to various circuits. The electricity meter 20 and consumer unit 22 do not form part of the present invention and are only included as typical examples of an arrangement within which the voltage optimisation unit 10 of the present invention may be used.

Referring additionally to Figure 2, the voltage optimisation unit 10 also includes a transformer 24 that receives a first input voltage that is dependent on the power supply voltage (and is most typically the same as the power supply voltage) and is able to provide a second output voltage (typically identical to the output voltage of the voltage optimisation unit) that is different from the input voltage received by the transformer 24. The transformer 24 may be any suitable type of transformer and in particular auto transformers and toroidal transformers are preferred for reasons of size. The transformer 24 has a primary coil 26 and a secondary coil 28.

The voltage optimisation unit 10 also has switching means 30 which has a first mode in which the output voltage from the transformer 24 is supplied to the output terminals 16 and 18 and has a second mode in which the input voltage to the voltage optimisation unit via input terminals 12 and 14 and is provided to the output terminals 16 and 18. These first and second modes are therefore a voltage optimisation mode and a bypass mode respectively since in the first mode the input voltage is optimised using transformer 24 and the second mode bypasses the transformer therefore leaving the voltage unaffected.

The switching means 30 comprises at least two switching devices 32 and 34 which operate as a co-operating pair such that when the voltage optimisation unit is running in either the voltage optimisation mode or bypass mode, such that one of the switches is open whilst the other is closed and vice versa (although this is not the case very briefly during the switching as explained below). Therefore if the voltage optimisation switch 32 is closed and the bypass switch 34 is open, the device is operating in voltage optimisation mode. If the bypass switch 34 is closed and the voltage optimisation switch is open, then the voltage optimisation unit 10 is operating in bypass mode. The switching devices 32 and 34 are preferably latching relays which are large fast acting switches capable of comfortably handling the currents involved in the device of the present invention.

The voltage optimisation unit 10 also includes voltage determining means for providing a first control signal dependent upon the power supply voltage. In the embodiment shown in Figure 2, there are two voltage measuring devices 36 and 38. Voltage measuring device 36 measures the voltage across the input terminals 12 and 14 and therefore measures the power supply voltage. The second voltage measuring device 38 measures the voltage across the output terminals 16 and 18 and therefore measures the output voltage which is dependent on the power supply voltage. It is preferable to measure the voltage at both the input and output terminals and it is also possible to measure the voltage at other points within the voltage optimisation unit. However, the present invention will work with a single voltage measurement either at the input terminals, at the output terminals or elsewhere within the voltage optimisation unit.

The voltage optimisation unit 10 further includes a processor 40 that receives the first control signal from the or each voltage measuring device (in this instance 36 and 38) and provides a second control signal for selecting the first mode (voltage optimisation mode) and second mode (bypass mode) in the switching means 30. This is achieved by sending signals to the latching switches 32 and 34 via signal connections 42 and 44. These signal connections along with the signal connections 46 and 48 that respectively carry the first control signals from the voltage measuring devices 36 and 38 to the processor 40, are shown as dotted lines in Figure 2 to indicate that they carry control signals and do not form part of the power supply circuit.

With additional reference to Figure 3, the operation of the circuit, in particular the control of the voltage optimisation switch 32 and bypass switch 34 will now be described. The upper portion of Figure 3 shows a plot of 50hz mains voltage with voltage on the Y axis and time in milliseconds on the X axis. The point on the X axis at which time equals zero coincides with the point at which the voltage equals zero.

The bars in the middle portion of Figure 3 show the status of the voltage optimisation and bypass switches 32 and 34. Bar 50 indicates the status of voltage optimisation switch 32 and bar 52 indicates the status of bypass switch 34. Below these bars the further bar 54 indicates the status of the voltage optimisation unit as a whole. The status bars 50, 52 and 54 are drawn so as to be coincident with the X axis time of the graph in the upper portion of Figure 3. Figure 3 illustrates the switchover from voltage optimisation mode to bypass mode. This occurs when the processor has determined that the voltage optimisation mode is no longer required. This will typically be when the input voltage is sufficiently close to the optimum voltage required to make the use of a transformer to reduce or increase the voltage inefficient. In this embodiment, the latching relays used for switches 32 and 34 take approximately 7 milliseconds to change from open to closed and vice versa. As indicated in bars 50 and 52, the voltage optimisation relay 32 is closed at time T = -9 milliseconds and, as indicated by bar 52, bypass switch 34 is open. As a result, and as indicated by bar 54, the voltage optimisation unit is in voltage optimisation mode.

At time T = -7.5 milliseconds, processor 40 sends a signal to voltage optimisation switch 32 to open. This signal, or open relay pulse, takes 7 millseconds to cause the voltage optimisation relay to open, causing the relay to open at time = -0.5 milliseconds. Turning to bar 52 at time = -6.5 milliseconds the processor 40 sends a signal pulse to the bypass switch 34 which likewise takes 0.7 milliseconds to alter its state to a closed position. This therefore occurs at time = 0.5 milliseconds. As can be seen from Figure 3, there is a period of 1 millisecond split either side of the point of zero milliseconds (that is from -0.5 milliseconds to 0.5 milliseconds) where the voltage optimisation relay 32 has opened and the bypass switch 34 has not yet closed. However, because the voltage at this point is very close to zero, this does not interfere with the supply of power downstream of the voltage optimisation unit and any arcing, resulting in sparks, is minimised due to the minimal voltage passing through the system at that point. Similarly, any voltage spikes from the transformer or elsewhere are also dampened due to the minimal voltage at the time of switch over.

In the reversed situation where the circuit is changing from bypass to voltage optimisation mode, a similar timed switching is applied. However, in this instance the pulse signal to the bypass switch 34 is sent first in order to open the switch and then one millisecond later the signal to the voltage optimisation switch 32 is sent closing it to close. As a result, it is again the case that there is a brief moment of one millisecond on either side of the time zero where both the switches 32 and 34 are open.

It is also the case that the timing of the control pulse signal to the switches 32 and 34 can be fine-tuned to ensure correct switching takes place. For example, relay contact wear, change in the temperature around the relay and changes in control board holding the processor reducing feed voltage to the relays can all alter the time it takes for the relay to open or close. To overcome this processor 40 has the ability to adjust the timing of the pulse signals being sent to the relay switches 32 and 34 by measuring a number of parameters and by comparing them to the design limits adjustments to the timing can be made. These parameters include the timing of the transfer from voltage optimisation mode to bypass mode and vice versa by analysing data from the voltage measuring devices 36 and 38 or by the inclusion of further voltage measuring devices. This therefore ensures minimum voltage changes received by downstream appliances and maintains life span of the unit.

The embodiment shown in Figure 2 indicates a transformer that is either operational or non-operational and therefore provides only a single voltage change. It is therefore preferable to include a transformer that has a plurality of taps into the secondary coil 28 so as to be able to produce a variety of output voltages. A schematic representation of this is shown in Figure 4. Like reference numerals have been used to label the parts of the circuit diagram and the additional voltage optimisation relays have been labelled 32a, 32b, 32c and 32d, each being attached to respective taps 56a, 56b, 56c and 56d. The same principle of timings of the opening and closing of the voltage optimisation tap relays 32a to 32d and the bypass relay 34 apply as described above. The difference in this instance is that the processor determines which of the relays should close to provide the correct output voltage. Furthermore, the same principle can be applied when the processor determines that it is necessary to switch from one tap to another so that. For example, in switching from the first tap 56a to the second tap 56b, the first tap relay 32a opens 0.5 milliseconds before time = zero and the second tap relay 32b closes 0.5 milliseconds after. Referring to Figure 5, a portion of a further embodiment, similar to that shown in Figure 4, includes three taps 56a to 56c and their respective relays 32a to 32c. The bypass relay 34 is connected to the supply feed via the input terminal 12 and an additional relay, referred to as the transformer relay 58 is also included. This relay isolates the transformer and acts as a safety relay in the event of the failure of any of the other relays which could result in circulating current within the transformer This relay, under the control of processor 40 is timed to open and close shortly before (approximates 0.5ms) the opening and closing sequence described above for the voltage optimisation or tap and bypass relays 32 and 34.

It is sometimes the case that the supply voltage may be very close to the voltage where the voltage optimisation unit is used and may vary around this point causing the voltage optimisation unit to switch between bypass and voltage optimisation modes regularly. Although the invention set out above reduces any danger of flicker or variations in output voltage, it is preferable to reduce the number of switching operations by using the processor to determine whether switching is required, advisable or optimal at the present time. As an example, the following rules can be applied to determining whether to change mode.

The change from voltage optimisation to bypass mode occurs immediately if: Input voltage too low

Output voltage too low

Load Current >60A

Transformer too hot

The change from bypass mode to voltage optimisation mode occurs, subject to satisfactory parameters of V _in (the voltage across in the input terminals) and V _ou t (the voltage across in the output terminals) and load and current etc. and is subject to a wait, calculated according to:

How long since last change to VO mode.

How much 'headroom' in input voltage.

Transformer temperature.

How close to main circuit.

Where:

TVO = time in s since last selection of VO mode

DVIN =volts over minimum needed for selection of VO mode.

STEMP - degrees under maximum temperature measured at transformer core.

SIMAX = amps less than max current rating in VO mode.

WAIT(s) = (1800 - TVO) x (SVIN + STEMP + SIMAX)

Further embodiments of the present invention are now described with reference to Figures 6 to 10.

Figure 6 shows two simple single-pole relays connected in series, to effectively produce a change-over relay. If a conventional change-over relay were used, there would be an interval, during switching, when both sets of contacts were open-circuit. Power relays typically take 50mS to 500mS to operate.

Thus in a conventional arrangement, using a change-over relay to switch between "supply" and "optimized" voltage there is a 50mS to 500mS interruption in the supply. In this embodiment the change-over switching time is reduced to <1 mS by energizing both relays several milliseconds earlier than required, according to their operating time.

Figure 7 shows the change-over relay configuration applied to a simple autotransformer voltage optimizer. Having established a technique for reducing the open-circuit time to <1 mS, in this embodiment the relays are operated in anticipation of the zero voltage instant in the AC waveform, resulting in a near-perfect transition from one supply to the other.

The operating time for each relay can be measured. This measurement is used to set the timing of the next drive pulse. If the relay takes 5mS to operate, it is next driven 5mS early. If the relay takes 6mS to operate, it is subsequently driven 6mS early and so on.

Some relays require a different length of time to close than to open. Again, this requirement is anticipated and the drive pulse sent accordingly in advance.

Figure 8 shows the power switch, relay3. Relay 3 turns the autotransformer supply on and off. The microcontroller system measures the incoming supply voltage and chooses the optimum instant to switch on relay 3 for minimum inrush current.

The microcontroller measurement and timing circuit is used to control relayl and relay2, with minimum interval between their resulting switch operations, and synchronized with the zero-voltage point on the incoming AC waveform.

In Figure 9, each relay represents a pair of relays as depicted in Figure 2. In Figure 5 the <1 mS change-over relay pairs can be used, one per phase of a 3-phase system, and synchronized to the AC waveform zero switching point.

The switch instant is delayed, as per the AC mains frequency, according to the phase of Live2 and Live3 by 5.5mS or 6.6mS (depending on 50Hz or 60Hz mains). In this way, each phase is switched at the zero-voltage point, and with <1 mS switch interval, so no surges or dips are produced. Figure 10 shows a simple single-phase voltage optimizer, including multiple relays as follows. RelaylA and Relayl B together control the power to the autotransformer. They are driven in parallel, and because the switching instant is selected to be at the zero voltage phase of the AC waveform, the contacts are closed before any current begins to flow, therefore the contacts share the current appropriately. A pair of relays or a plurality of relays can be connected thus to develop a greater current carrying rating.

Relay2 and relay3 form the change-over switch, used to select optimized supply or incoming supply. Each relay may be a single relay, a pair of relays (relay2A and relay2B) or a plurality of relays, to achieve the required unbroken supply during switching, at the zero voltage. Switching at the zero voltage instant ensures that the relay contacts may share the load equally, avoiding the typical problem where the first relay to close suffering the greatest contact burden. Similarly, relay 3A and Relay 3B represent a single or a plurality of relays.

It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only and not in any limitative sense, and that the various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims.

For example, it should be noted that the voltage to the transformer may not be identical to that of the supply voltage reaching the input terminals 2 and 14 as a result of additional components included. Similarly, the output from the transformer may not be identical to the output from the output terminal 16 and 18. It will also be apparent that the timing delay in the latching relays may not be 7ms and as a result the time at which the closing of the relays is started may vary so that the opening and closing actions of the relays remains around the Ov point in the alternating current cycle. Similarly the gap between opening of one relay and closing of another is not limited to 0.5ms either side of the Ov point and can be any suitable time that minimised the voltage at the point the relays open and close.