GB2515912A

GB2515912A - Switching power transformers

Info

Publication number: GB2515912A
Application number: GB1411948.1A
Authority: GB
Inventors: Gary Vizard; Brian Dickinson
Original assignee: POWER EFFICIENT SYSTEMS Ltd
Current assignee: POWER EFFICIENT SYSTEMS Ltd
Priority date: 2014-07-03
Filing date: 2014-07-03
Publication date: 2015-01-07
Anticipated expiration: 2034-07-03
Also published as: GB201411948D0; GB2515912B

Abstract

A transformer 10 comprises a primary winding 12 and secondary winding 14 disposed around a core 16. The primary and secondary windings have a different number of turns and are connected in series between the input 18 and output 20 phase terminals of the transformer. A switch means 44, 46, 48 is associated with the primary winding for selectively connecting or disconnecting it from the phase supply terminal. The core may be a toroidal core and may have a plurality of primary windings, each having a different number of turns. A control circuit is suitably provided for independently switching the or each primary winding into, or out of, the transformer. The control circuit may comprise a first switch 44 interposed between the primary winding and the input phase terminal, a second switch 46 in a shunt around the primary winding, a resistor 42 wired in series between the primary winding and the phase output terminal, and a third switch 48 in a shunt around the resistor.

Description

Title: Switching power transformers

Description:

This invention relates to power transformers, and in particular, to switching power transformers.

Mains supply voltages tend to vary over time in response to power generation output and instantaneous power demand on the grid. In accordance with local regulations, however, mains voltages must be maintained between upper and lower threshold values. For example, in the United Kingdom, mains voltage is nominally 230V ÷10%/-6%, meaning that the actual mains voltage can legally vary between 216V and 253V.

Variations in mains voltages have a direct effect on energy usage (power consumption being theoretically proportional to the square of the voltage); the longevity of connected loads (over-voltages can lead to reductions in duty cycles); and the performance of connected loads (especially electronic loads where under-voltages can lead to noticeable losses in performance).

Variations in mains voltages are accommodated by local regulations, which generally require electrical loads and appliances to be operable at a lower voltage than the minimum rated mains supply voltage. The rated voltage of many appliances, in Europe, is commonly 210V. This means that even at the lower end of the permissible voltage range for the United Kingdom (216V), the instantaneous mains voltage will always be sufficient to power almost all EU-rated appliances. The upshot of this is that UK mains voltage can be safely stepped-down to its lower threshold value without adversely affecting the operation of connected loads and appliances. Similar considerations apply elsewhere, of course: the UK-EU example above being just an example. That being the case, considerable energy savings can be obtained by stepping-down the mains power supply, and this has been the object of much research in recent years.

If mains voltages were constant, then stepping-down the supply voltage by a given percentage would be straightforward. However, because mains voltages vary over time, it is not possible to use fixed wiring to step-down the mains voltage because if the mains voltage were to fall to its lowest permissible value, and a permanent step-down transformer were in place, then the output voltage (as applied to connected loads and appliances) would fall below the appliances' rated minimum voltage, possibly leading to malfunctioning. Thus, systems have been developed that switch-in or switch-out a step-down transformer in response to variations in the instantaneous mains voltage to maintain an output voltage within desired threshold values, i.e. above the rated voltage of connected appliances.

An example of such a system is described in published PCT application No: W02013/079962A1, which describes a transformer with primary and secondary windings. The mains power supply is connected to the primary winding, and an output current is induced in the secondary winding. Due to the difference in the number of turns in the primary and secondary windings, the voltage on the secondary winding is less that the applied mains voltage, leading to a stepped-down voltage, which is connected to a load. The W02013/079962A1 system, and others like it, comprises a processor, which monitors the incoming mains voltage. The processor is configured to actuate bypass switches which either direct the phase of the incoming mains through the secondary winding (leading to a stepped-down voltage at the output), or directly to an output terminal, thereby bypassing the secondary winding of the transformer, thus directly connecting the incoming phase to the output phase terminal. In all cases, the neutral conductor is directly connected between the transformers input neutral and output neutral terminals.

Whilst systems such as that described in W02013/079962A1 function correctly, they do suffer from a number of drawbacks, as follows: Two switches are required to switch-in or switch-out the secondary winding of the transformer. A first switch is used to connect the input phase to the output terminal, and a second switch is used to connect the secondary winding to the output terminal. The operation, and crucially the timing, of these switches, must be perfect if: a short circuit condition is not to be set up; infinite inrush currents in the secondary coil are to be avoided; and unbalanced currents in the primary and secondary windings are to be avoided. To ameliorate the above, W02013/079962A1 uses a microprocessor to monitor the phase input voltage and to control the switches. The microprocessor is configured to delay switching of the first and second switches until the input phase voltage crosses zero. Specifically, the switches must be opened/closed within 0.Sms either side of V=0 to avoid the aforementioned adverse effects. This is very difficult to achieve, except where very high speed (and high-cost) switch gear is used. Since the speed of high power switches is lower than that of low power switches, in a power transformer application there is a practical limit to the switching speed, which makes such systems expensive to install and failure-prone.

A further drawback of the system described in W02013/079962A1 is that in the event of a fault condition, the transformer is not necessarily bypassed. This means that if the switch gear fails when the voltage is stepped down, and the mains supply voltage subsequently falls, then connected loads will be supplied with an under-voltage, the power to the loads can be cut off, which is unacceptable in mission critical applications.

A need therefore exists for a new type of switchable transformer, which addresses one or more of the above problems and/or which provides an alternative solution to the problem of safely, reliably and economically stepping-down mains voltages.

According to a first aspect of the invention, there is provided a transformer comprising a primary winding and secondary winding disposed around a core, the primary and secondary windings having a different number of turns and each being operatively connected, in series, between input phase and output phase terminals of the transformer, wherein the transformer further comprises switch means associated with the primary winding for selectively connecting or disconnecting it from the phase supply terminal.

Suitably, when the primary winding is disconnected from the phase supply terminal, current flows in series, through the secondary winding only. However, when the primary winding is operatively connected to the phase supply terminal, a current in the primary winding induces a magnetic field in the core, which either opposes or cooperates with the current in the secondary winding, which either bucks or boosts the voltage at the output phase terminal.

In other words, the primary coil could effectively be considered to be a passive component of the transformer, which can be switched-in, or switched-out, at will. When it is switched-in, the primary coil may either oppose or cooperate with the secondary coil to buck or boost the output voltage, respectively; however, when the primary coil is switched-out, the current to the may load flow through the secondary coil as if the secondary coil were simply an ordinary conductor.

Because the secondary winding is wired in series between the input phase and output phase terminals, the secondary winding is wired in series with a load. Thus) even if the transformer fails, there is a direct connection between a connected load and the mains power supply: in essence, the secondary winding acts as a conductor between the mains and load in the event of a transformer failure. Such a configuration means that the invention is particularly suited to critical applications.

A further advantage of the invention is that the primary winding circuit does not carry the load: it is merely used to oppose the current in the secondary winding which carries the load. This means that low-power, and hence quicker andjor less expensive, switch gear can be used in the primary winding's circuit, thereby making the invention more economical to install, and less fault-prone.

A second aspect of the invention provides a control circuit for a switching in or out the primary winding of a transformer, the control circuit comprising a first switch interposed between the primary winding and an input phase terminal of the transformer; a second switch in a shunt around the primary winding; a resistor wired in series between the primary winding and a phase output terminal of the transformer; and a third switch in a shunt around the resistor.

Suitably, the control circuit of the second aspect of the invention can be used to switch-in or switch-out the primary winding of a transformer according to a first aspect of the invention.

S

By wiring the secondary winding in series with the load, the power in the primary winding can be greatly reduced, meaning that smaller, faster and/or cheaper switches can be used to control the transformer.

Thus, a third aspect of the invention provides a transformer comprising a primary winding and secondary winding disposed around a core) the primary and secondary windings having a different number of turns and each being operatively connected) in series, between input phase and output phase terminals of the transformer, wherein the transformer further comprises a control circuit for selectively switching-in or switching-out the primary winding of the transformer, the control circuit comprising a first switch interposed between the primary winding and an input phase terminal of the transformer; a second switch in a shunt around the primary winding; a resistor wired in series between the primary winding and a phase output terminal of the transformer; and a third switch in a shunt around the resistor.

A fourth aspect of the invention provides a method of controlling the control circuit of the second or third aspects of the invention, the control method comprising, in the following sequence, the steps of: closing the second switch; closing the first switch, opening the second switch and closing the third switch.

A fifth aspect of the invention provides a method of controlling the control circuit of the second or third aspects of the invention, the control method comprising, in the following sequence, the steps of: opening the third switch; closing the second switch; opening the first switch; and closing the second switch.

Thus) the primary winding can be switched-in or -out of the circuit by the following procedure: Switch-in: Close second switch Close first switch Open second switch Close third switch Switch-out: Open third switch Close second switch Open first switch Open second switch Upon switching-in the control circuit, the aforementioned switching sequence serves to first shunt current around the primary winding and through the resistor between the input phase and output phase terminals before then shunting the current through the primary winding and around the resistor) and vice-versa for switching-out the control circuit.

The first, second and third switches are suitably high-speed, solid state switches, such as thyristors, and are suitably controlled by a microprocessor. The microprocessor is suitably configured to complete the switching-in or switching-out sequence in less than ims. Suitably, the microprocessor is configured to monitor the voltage at the phase input terminal and to affect the switching sequence as the voltage approaches, or passes through its maximum (Vmax) or minimum (Vmin) value. This is the exact opposite of the teaching of W02013/079962A1 (which switches at V=0) and this is possible due to the switching sequence and the resistor, which limits the inrush current in the primary winding and which serves to balance the currents in the primary and secondary windings.

The control circuit may additionally comprise a safety bypass in the form of a fourth switch in parallel with the second switch in a second shunt around the primary winding. The fourth switch can be used during initial start-up of the transformer, and suitably comprises a normally-closed switch, which is held open by a control voltage, which may be the voltage across the primary winding (so that the fourth switch closes automatically in a fault condition or a power outage, thus "resetting" the system), or a separate control voltage, which may be provided by a separate monitoring/control/safety circuit.

Thus, when the transformer is initially powered down, the fourth switch is closed, such that when the transformer is first powered-up the primary winding is bypassed) which simply means that current flows through the secondary winding from the input phase terminal to the output phase terminal with the secondary winding acting as a simple conductor. When it is desired to switch-in the control circuit for the first time, the following sequence is adopted: Switch-in: Close second switch Open fourth switch Close first switch Open second switch Close third switch Switch-out: Open third switch Close second switch Open first switch Close fourth switch Open second switch It will be appreciated that the switching sequence incorporating the fourth switch is substantially the same as without it, except that the switching of the fourth switch is included as a second, or penultimate) step in the sequence for switch-in and switch-out, respectively.

The transformer may comprise two or more independently switchable primary windings, each having a different number of turns) such that each winding) when switched-in, can buck or boost the voltage across the secondary winding by different amounts. Suitably, the first primary winding is configured to reduce or increase the voltage at the phase output terminal by a first certain voltage, say by or 16V. The second primary winding, where provided) may be configured to reduce or increase the voltage at the phase output terminal by a second certain voltage, say 20V or 32V. Thus, by providing two or more primary windings, it is possible to buck or boost the voltage at the phase output terminal by a desired amount in desired increments.

A separate transformer may be provided for each phase of a polyphase supply voltage, for example, three transformers may be provided for a three-phase supply.

Suitably, the core of the transformer comprises a toroidal core. Such a configuration suitably confines the magnetic field within the core, thus improving the intrinsic efficiency of the transformer and ameliorating the losses that arise in association with the use of transformers.

Preferred embodiments of the invention shall now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram for a transformer in accordance with the invention; Figures 2 to 5 are a series of Figure 1 showing the switching sequence for switching-in the primary winding; Figure 6 is a circuit diagram for a transformer in accordance with the invention with a safety bypass fitted; Figures 7 to 12 are a series of Figure 6 showing the switching sequence for switching-in the primary winding; Figure 13 is a circuit diagram for a transformer in accordance with the invention comprising two primary windings; and Figure 14 is a circuit diagram for a three-phase transformer system in accordance with the invention.

In Figure 1, a transformer 10 in accordance with the invention comprises a primary winding 12 and a secondary winding 14 wound around a toroidal core 16 (illustrated schematically as a rectangle in the drawings, but of course the core 16 and windings 12, 14 would be doughnut-shaped, in practice). The transformer 10 has an input phase terminal 18 and an output phase terminal 20 and the secondary winding 14 is connected in series between these terminals 18, 20. The transformer additionally comprises input neutral 22 and output neutral 24 terminals, but this can be omitted because they are directly interconnected 26 and the transformer 10 plays no role in the neutral.

When connected to a mains power supply 28, a load 30 connected to the output terminals 20, 24 is powered directly by currents flowing through the neutral-neutral interconnection 26 and through the secondary winding 14. Whilst the primary winding 12 is inactive, the output voltage 30 is equal to, and in-phase with, the input voltage 28.

The primary winding 12 can be used to buck or boost the voltage in the secondary winding 14 by switching it in or out. The primary 12 and secondary windings 14 have different numbers of turns, and so mains voltage 28 applied to the primary winding 12 induces a magnetic field in the core 16 to oppose or augment the voltage in the secondary winding 14. Thus, the voltage in the secondary winding 14, and hence the output voltage 30 at the output terminals 20, 24 can be bucked or boosted relative to the input voltage 28 at the input terminals 18, 22.

The primary winding 12 is switched-in or switched-out by a control circuit 40, which comprises a resistor 42 and three switches 44, 46, 48. The first switch 44 is wired in series with the primary winding 12 and serves to selectively connect or disconnect the primary winding 12 from the supply voltage 28. It is not possible to simply switch the primary winding 12 into, or out of, the transformer due to the infinite inrush current that would be generated during the infinitesimal switching period, or during the first half-cycle of the applied mains voltage 28. The resistor 42 is therefore wired in series with the primary winding 12 to absorb the inrush current. To enable the resistor 42 to be switched into and out of the control circuit 40 (it is only needed during the initial switch-in or switch-out phase), a first shunt 50 comprising the second switch 46 is wired across the primary winding, and a second shunt 52 comprising the third switch 48 is wired across the resistor 42. The primary winding 12 can thus be switched into the transformer 10 by the following sequence, as illustrated in Figures 2 to 5 of the drawings.

In Figure 2, the second switch 46 is closed. In Figure 3, the first switch 44 is closed and so current flows around the primary winding 12 and through the resistor 42 (as indicated by the dashed line arrow 54). Then, as shown in Figure 4, the second switch 46 is opened such that current now flows through the primary winding 12 and the resistor 42 (as indicated by the dashed arrow 56).

Finally, as shown in Figure 5, the third switch 48 is closed this shorting the resistor 42 and diverting current through the primary winding 12 only (as indicated by the dashed arrow 58).

The reverse procedure can be used to switch-out the primary winding, as described hereinabove.

Figure 6 is an embodiment of the invention in which an additional bypass safety leg is added to the transformer described above. Identical reference sings have been used to identify identical features in the drawings to avoid repetition and to aid intelligibility. In Figure 6 a similar circuit to that described previously further comprises an additional shunt 60 around the primary winding 12, which comprises a normally-closed switch 62 that is held open by a control voltage. Thus, when the transformer is initially powered down, the primary winding 12 is shorted thus diverting current in the control circuit 40 through the resistor 42. Given that the path of least resistance thus becomes the path through the secondary winding 14, current flows directly from the phase input terminal 18 to the phase output terminal 20 via the secondary winding 14, which simply acts as a conductor in the circuit. Thus, in a fault condition, i.e. loss of power to the control circuit 40, the NC switch 62 closes, thereby bypassing the primary winding 12 thus effectively turning the transformer 10 a simple conductor.

The addition of the NC switch 62 and the additional shunt 60 introduces an additional step in the switching sequence, as described with reference to Figures 7 to 11 as follows: In Figure 7, the transformer 10 is powered-down, so the NC switch 62 is closed as a safety precaution. To switch-in the primary winding 12, the second switch 46 is closed, as shown in Figure 8. Then, the NC switch 62 can be opened, as shown in Figure 9 before the closing the first switch 44 as shown in Figure 10, whereupon the current flow path is indicated by dashed arrow 66 around the primary winding 12 and through the resistor 42. As shown in Figure 11, the second switch 46 is then opened to allow current to flow in the primary winding 12 and through the resistor 42 (as shown by dashed arrow 68). Finally, as shown in Figure 12, the third switch 48 is closed thereby shorting the resistor 42 such that current flows through the primary winding 12 only. The switching-out sequence is the reverse of the above, or it can follow the reverse of the sequence of Figures 2 to 5 if the NC switch 62 is held open at all times.

Figure 13 shows a more complicated version of the transformer previously described, this time having a single secondary winding 14 and a first 12 and a second 12' primary winding, each primary winding 12, 12' having a different number of turns to each other and to the secondary winding such that the output voltage at the output terminals 20, 24 can be selected between a number of options by switching in, or out, either or both of the primary windings 12, 12'. For the sake of clarity and for the avoidance of repetition) identical reference signs have been used to describe identical features to those described previously, and it will be appreciated that the features of the second primary control circuit 40' are identified by a prime suffix. Otherwise, the operation of the first 40 and second 40' control circuits is as described previously.

Finally, Figure 14 of the drawings shows how three transformers 10, 10' and 10" can be used in a polyphase wiring arrangement, whereby each transformer 10, 10', 10" is separately connected to one of the phases 18, 18', 18" of the mains supply. Of course, in a balanced three-phase system, a neutral conductor 26 is not necessary, but it may be included for certain applications.

The illustrated embodiments of the invention are merely exemplary of the invention and are not limiting on the scope of the invention, which is defined by the appendent claims. For example, the three-phase arrangement of Figure 14 could comprise any number of primary windings 12 on each of the transformers, the number of primary windings could be greater than the one or two illustrated herein, the safety leg 60 and NC switch 62 could be added or omitted from any of the illustrated embodiments, and transformer configurations other than a toroidal core transformer could be used.

The invention is particularly suited to domestic, commercial or industrial applications whereby it can befitted immediately after the mains head unit and/or before the consumer unit of a premises, where it can regulate the mains voltage within the premises and thus protect loads from over-voltages and optionally step-down the voltage to obtain energy savings.

Claims

Claims: 1. A transformer comprising a primary winding and secondary winding disposed around a core, the primary and secondary windings having a different number of turns and each being operatively connected, in series, between input phase and output phase terminals of the transformer, wherein the transformer further comprises switch means associated with the primary winding for selectively connecting or disconnecting it from the phase supply terminal.
2. The transformer of claim 1, wherein the core comprises a toroidal core.
3. The transformer of claim 1 or claim 2, comprising a plurality of primary windings, each having a different number of turns.
4. The transformer of any preceding claim, further comprising a control circuit for independently switching the or each primary winding into, or out of, the transformer.
5. A polyphase transformer comprising a plurality of transformers according to any preceding claim, wherein a separate transformer is provided for each phase of a polyphase supply voltage.
6. A three phase transformer system comprising three transformers according to any of claims 1 to 4: one transformer for each of the three phases of the three phase supply voltage.
7. The transformer of any of any preceding claim, further comprising neutral input and neutral output terminals interconnected by a conductor.
8. The transformer of any of claims 4 to 7, wherein the control circuit comprises a first switch interposed between the primary winding and the input phase terminal of the transformer; a second switch in a shunt around the primary winding; a resistor wired in series between the primary winding and the phase output terminal of the transformer; and a third switch in a shunt around the resistor.
9. The transformer of claim 8, wherein the first, second and third switches comprise high-speed, solid state switches.
10. The transformer of claim 9, wherein any or each high-speed, solid-state switch comprises a thyristor.
11. The transformer of claims 8 to 10, further comprising a microprocessor operatively connected to, and arranged to effect switching of, each of the switches.
12. The transformer of claim 11, wherein the microprocessor is configured to monitor the voltage at the phase input terminal and to affect the switching sequence as the voltage approaches, or passes through its maximum (Vmax) or minimum (Vmin) value.
13. The transformer of any of claims 8 to 12, wherein the control circuit further comprises a safety bypass comprising a fourth switch wired in parallel with primary winding.
14. The transformer of claim 13, wherein the fourth switch comprises a normally-closed switch, which is held open by a control voltage.
15. A method of controlling the control circuit of any of claims 8 to 14 comprising, in the following sequence, the steps of: closing the second switch; closing the first switch, opening the second switch and closing the third switch.
16. A method of controlling the control circuit of any of claims 8 to 14 comprising, in the following sequence, the steps of: opening the third switch; closing the second switch; opening the first switch; and closing the second switch.
17. A method of controlling the control circuit of claim 13 comprising, in the following sequence, the steps of: closing the second switch, opening the fourth switch; closing the first switch; opening the second switch; and closing the third switch.
18. A method of controlling the control circuit of claim 13 comprising, in the following sequence, the steps of: opening the third switch; closing the second switch; opening the first switch; closing the fourth switch; and opening the second switch.
19. The method of any of claims 15 to 18, when dependent on any of claims 11 01 12, wherein the microprocessor is configured to complete the switching sequence in less than Sms.
20. The method of any of claims 15 to 18, when dependent on any of claims 11 or 12, wherein the microprocessor is configured to complete the switching sequence in less than 2ms.
21. The method of any of claims 15 to 18, when dependent on any of claims 11 or 12, wherein the microprocessor is configured to complete the switching sequence in less than ims.
22. A transformer, control circuit or method substantially as hereinbefore described, with reference to, and as illustrated in, the accompanying drawings.