WO1994001915A1

WO1994001915A1 - Method and apparatus for an asynchronous static switch

Info

Publication number: WO1994001915A1
Application number: PCT/AU1993/000329
Authority: WO
Inventors: Richard Colin
Original assignee: Powerline Systems Pty. Limited
Priority date: 1992-07-03
Filing date: 1993-07-02
Publication date: 1994-01-20
Also published as: CN1084327A

Abstract

This invention concerns a method for switching a load between two or more asynchronous electrical supplies. The method switches on the second supply when the instantaneous magnetic flux within the load is equal to the instantaneous magnetic flux that would be produced by the second supply. By ensuring that the load flux continues unchanged as the second supply is switched on, heavy in-rush currents due to direct current flux bias are prevented. In a second aspect the invention provides an apparatus for switching a load between two or more asynchronous electrical supplies.

Description

"Method and Apparatus for an Asynchronous Static Switch"

TECHNICAL FIELD

This invention relates to a method and apparatus for switching an electrical load between two sources of asynchronous alternating current electrical power. The invention finds application in switching between a primary power supply and an alternative power supply. In some applications it might be necessary to switch between two power supplies for maintenance purposes, in other situations the alternative supply may be a backup supply used in the event of a failure in the primary supply.

BACKGROUND OF THE INVENTION

Turning power supplies off and on creates transients which must be managed during switching. The problems are particularly acute where there are loads containing magnetic circuits, such as those presented by motors and transformers where the transients may give rise to the phenomena of current in-rush. Conventional static switches overcome this problem by deliberately lengthening the supply break time in order to allow the transient effects to decay before connecting the alternative supply. This has the effect of mitigating current in-rush phenomena, but is unsatisfactory for switching the supply to sensitive equipment such as electronic computers. This type of equipment can tolerate only very short supply break times while operating continuously.

SUMMARY OF THE INVENTION

According to a first aspect, the invention provides a method for switching a load between a first and second asynchronous electrical supply. Each of the supplies has at least one active conductor connected to the load by two switching devices arranged to conduct, when enabled, in respective half cycles of the active signal. The method comprises the steps of: disabling the switching devices of the first supply; monitoring the switched states of the switching devices of the first and second supplies; monitoring the load voltage; monitoring the voltage of the second supply; simulating the instantaneous magnetic flux within the load; simulating the magnetic flux that would be produced by the second supply; and then enabling one of the switching devices of the second supply when the instantaneous magnetic flux within the load is equal to the instantaneous magnetic flux that would be produced by the second supply, the voltage of the second supply and the output voltage are such that the enabled switching device will conduct, and no short circuit will exist between the enabled switching device and the switching devices of the first supply.

Regardless of the reactive behaviour of the load, which may vary considerably, the basic relationship between flux and voltage is preserved; flux is proportional to the time integral of voltage. This is relied upon to simulate a magnetic flux in the simulation steps . By ensuring that the load flux continues unchanged as the second supply is switched on, heavy in- rush currents due to direct current flux bias are prevented.

Since the flux in an alternating supply achieves the same instantaneous value twice in every cycle of the supply, there are two opportunities for switching in each cycle, and it is possible to guarantee switching within a single cycle of the supply. For conventional 50 Hz supplies this is switching in 20 milliseconds and it is achievable even when the load is highly inductive. The time scale is also sufficiently fast to switch supplies for most computer equipment having switch mode power supplies, with a changeover time operating within a 100% to 200% safety margin. In a second aspect the invention provides an apparatus for switching a load between a first and second asynchronous electrical supply. Each of the supplies having at least one active conductor connected to the load via two switching devices arranged to conduct, when enabled, in respective half cycles of the active signal. The apparatus comprising: means to disable the switching devices of the first supply; means for monitoring the switched states of the switching devices of the first and second supplies,- means for monitoring the load voltage; means for monitoring the voltage of the second supply; means for simulating the instantaneous magnetic flux within the load; means for simulating the magnetic flux that would be produced by the second supply; and logic means to enable one of the switching devices of the second supply when the instantaneous magnetic flux within the load is equal to the instantaneous magnetic flux that would be produced by the second supply, no short circuit will exist between the enabled device and the switching devices of the first supply, and the voltage of the second supply and the output are such that the enable switching device will conduct .

The switching devices are preferably power semiconductors. Currently thyristors are a first choice for switching electrical current of any significant size. However, other devices such as TRIACS, Gate Turn Off thyristors (GTO) , Insulated Gate bipolar transistors (IGBTs) or any other family may be used. The devices may be arranged in back to back pairs with one pair for each conductor. The two devices in any pair conduct during the positive and negative half cycles respectively.

These devices may be enabled by powering their third electrodes, or disabled so that they will not turn on with a rising input voltage, by not driving their third terminals.

Since magnetic flux is proportional to the time integral of voltage, the means for simulating the instantaneous magnetic fluxes may be embodied by electronic integrators .

The logic means may be constructed from analog or digital circuitry, alternatively it may be supplied by a microprocessor system. It should be appreciated that the invention may be configured to switch from a first to a second supply and back from the second to the first .

The invention can be applied to switching between single, two, three or polyphased supplies, and the invention will switch all power conductors including the neutral conductors. When used with polyphase conductors a pair of switching devices must be provided for the conductor of each phase as well as for the neutral conductor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only with reference to the accompanying drawing, which shows a functional block diagram of an embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiment shown is for switching between two single phase power supplies with common neutral conductors .

The first power supply has an active conductor Al and a neutral conductor Nl . The second power supply has an active conductor A2 and a neutral conductor N2. The output, to which the load is connected, also has an active conductor A¹ and a neutral conductor N' .

Thyristors 1, 2, 3 and 4 are able to conduct when they are enabled by their respective latches 5, 6, 7 and The truth table for a latch is as follows:

When the D input goes low the output goes low immediately, and when the D input goes high the output goes high after the arrival of the next clock pulse.

The D inputs of the latches 5 and 6 are connected to input RQ1, and the D inputs of latches 7 and 8 are connected to input RQ1 via an inverter 34. In this way it is not possible for the pair of thyristors 1 and 2 and the pair of thyristors 3 and 4 to be enabled at the same time, and consequently they are not able to conduct at the same time.

When thyristors 1 and 2 are enabled they conduct respective half cycles of the first supply signal on active conductor Al . Thyristor 1 conducts positive half cycles to produce an output Al⁺. During positive half cycles thyristor 2 does not conduct, but as the negative half cycles commence it conducts once thyristor 1 stops conducting. During the negative half cycles thyristor 2 passes negative half cycles Al^". In this way both half cycles of the supply signal on active conductor Al are presented to the output or load terminal A' .

If the user decides to switch from the first supply to the second supply for any reason; for instance, if the first supply falls outside acceptable levels. The signal on input RQ1 changes state, so that the D input and Q output of latches 5 and 6 goes LOW. This disables thyristors 1 and 2 since the Q output of latches 5 and 6 goes LOW. If this happens during a positive half cycle of the first supply then the current flowing through thyristor 1 will decay until that thyristor eventually turns OFF. Thyristor 2 will not be able to conduce when the negative half cycle commences and thyristor 1 will not conduct the next positive half cycle. The precise turn off time of the conducting thyristor is unpredictable, but is not important for the operation of a circuit.

At the point in time when input RQ1 changes state, the D input of latches 7 and 8 goes HIGH. This enables thyristors 2 and 4 since their Q outputs will go HIGH once the next clock pulse arrives. The clock pulses to the two latches are separately derived, and one of the two thyristors 3 and 4 will be triggered ON by the arrival of a clock pulse to its respective latch when three conditions are simultaneously satisfied.

The clock input of each latch 5, 6, 7 and 8 is connected to the output of a corresponding AND gate 9, 10, 11 and 12. Each AND gate has three inputs and each input signal must go HIGH in order to supply a HIGH output to the clock input of its respective latch.

The first requirement for one of the AND gates to trigger a thyristor is for the thyristors of the other supply to be confirmed as OFF and not conducting, or if conducting, conducting in such a direction that will cause the current to be picked up by the newly triggered thyristor. Each of the AND gates 9, 10, 11 and 12 is associated with a corresponding OR gate 21, 25, 29 and 33, and their respective comparators 19 and 20, 23 and 24, 27 and 28, and 31 and 32 to test for this condition. Each of the comparators has two inputs and provides the comparisons as set out in the following table:

The second requirement for one of the AND gates to trigger a thyristor is for that thyristor to be in a position to pick the load. In other words for a thyristor arranged to pass positive half cycles, the supply voltage must be greater than the output; and for a thyristor arranged to pass negative half cycles, the supply voltage must be less than the output . The second input to each of the AND gates 9, 10, 11 and 12 is connected to the respective comparator 18, 22, 26 and 30 to test for this condition.

The third requirement for one of the AND gates to trigger a thyristor is for magnetic flux equivalence on each side of that device. In other words for devices 3 and 4 the magnetic flux within the load must be equivalent to the magnetic flux which would be produced by the second supply A2.

Three integrators 13, 14 and 15 respectively integrate the first supply voltage, the second supply voltage and the output supply voltage to the load. Since the time integral of voltage is proportional to the magnetic flux, the outputs II, 12 and I' of the integrators 13, 14 and 15 represent the simulated fluxes of the first supply, the second supply and the load respectively. Comparator 16 compares the outputs of integrators 13 and 15, that is the flux II of the first supply and I' of the load, and when they are equal produces a first flux equivalence signal Φ = 1.

Similarly comparator 17 compares the outputs 12 and I ' of the integrators 14 and 15, and when they are equal produces a second flux equivalence signal Φ = 2.

The signals Φ = 1 and Φ = 2 are narrow pulses which occur every time there is instantaneous flux equivalence between the respective supply and the output . The narrow width of the pulses is not significant with respect to the alternating current mains frequency. The pulses Φ = 1 and Φ = 2 occur twice in every cycle of the respective supplies. They are each supplied to both AND gates associated with their respective supplies, so pulse Φl is supplied to AND gates 9 and 10 of the first supply and pulse Φ2 is supplied to AND gates 11 and 12 of the second supply.

The AND gates ensure that both latches enable their respective thyristors at the correct time. In greater detail, for thyristor 3 to turn ON the three requirements for AND gate 11 are as follows:

First supply voltage must be greater than the load voltage (Al > A') or the first supply voltage must be less than the load voltage less the voltage across a thyristor (X) , (Al < (A'-X)) . If either (or both) condition is satisfied, the corresponding comparator 27 and 28 produces a HIGH output and OR gate 29 also produces a HIGH output . This indicates that the Al thyristors 1 and 2 are either both OFF, or at least thyristor 2 is OFF. If thyristor 3 conducts when thyristor 2 is ON then a catastrophic short circuit would result. This logic arrangement will, however, validly allow a triggering ON of thyristor 3 if thyristor 1 is still ON since the active signal on the second supply will pick up the active signal on the first supply.

Second, the second supply voltage must be greater than the load voltage (A2 > A' ) , this is tested by comparator 26 which prevents triggering of thyristor 3 unless it is capable of picking up the load.

And third, the second flux equivalence signal Φ = 2 from comparator 17, indicating that the flux in the load is equal to the flux that would be produced by the second supply, must come along when the first two requirements are both satisfied.

When all three conditions are satisfied the incoming positive half cycle A2⁺ of the second supply 2 will appear at the output A¹ at the instant the second flux equivalence pulse occurs.

In the succeeding half cycle, when the flux equivalence pulse Φ = 2 again occurs, AND gate 12 will clock latch 8 to trigger thyristor 4 into. conduction to pass the negative half cycle A2^" of the second supply. In succeeding cycles thyristors 1 and 2 cannot conduct as long as the input at RQl is low and the inputs of latches 5 and 6 are low. The thyristors 3 and 4 will conduct as the second supply voltage enters the appropriate half cycle. The selection signal input terminal RQl could be driven by a number of means depending upon the application and functionality required from the invention. For example input RQl could be driven directly from a manual switch, or by a computer, or through sensing circuits such as voltage failure sensing circuits which detect for supply failure. For this reason the invention may well be combined with power protection features such as lightning and surge protection, isolation and noise suppression, sine wave stability detection, or overload protection circuitry. Intelligence features may also be included, such as checking that the normal supply is within an acceptable limit, that the alternative supplies are within acceptable limits.

It should be appreciated that although the invention has been described with reference to a particular embodiment it could be embodied in many other forms. For instance there are a number of permutations for the logic components that could be used to obtain the same functionality, for similar or different types of switching devices.

Claims

THE CLAIMS :

1. A method for switching a load between a first and second asynchronous electrical supply, each of which has at least one active conductor connected to the load by two switching devices arranged to conduct, when enabled, in respective half cycles of the active signal; the method comprising the steps of : disabling the switching devices of a first supply; monitoring the switched states of the switching devices of the first and second supplies,- monitoring the load voltage; monitoring the voltage of the second supply; simulating the instantaneous magnetic flux within the load; . simulating the magnetic flux that would be produced by the second supply; and then enabling one of the switching devices of the second supply when the instantaneous magnetic flux within the load is equal to the instantaneous magnetic flux that would be produced by the second supply, the voltage of the second supply and the output voltage are such that the enabled switching device will conduct, and no short circuit will exist between the enabled switching device and the switching devices of the first supply.

2. A method for switching according to claim 1, wherein the magnetic flux is simulated by integrating the voltage with respect to time.

3. An apparatus for switching a load between a first and second asynchronous electrical supply, each of the supplies having at least one active conductor connected to the load via two switching devices arranged to conduct, when enabled, in respective half cycles of the active signal; the apparatus comprising: means to disable the switching devices of a first supply; means for monitoring the switched states of the switching devices of the first and second supplies; means for monitoring the load voltage; means for monitoring the voltage of the second supply; means for simulating the instantaneous magnetic flux within the load; means for simulating the magnetic flux that would be produced by the second supply; and logic means to enable one of the switching devices of the second supply when the instantaneous magnetic flux within the load is equal to the instantaneous magnetic flux that would be produced by the second supply, no short circuit will exist between the enabled device and the switching devices of the first supply, and the voltage of the second supply and the output are such that the enable switching device will conduct .

4. An apparatus according to claim 3, wherein the switching devices are power semiconductors.

5. An apparatus according to claim 4, wherein the switching devices are thyristors.

6. An apparatus according to any one of claims 3 to 5, wherein the means for simulating the instantaneous magnetic fluxes are electronic integrators.

7. An apparatus according to any one of claims 3 to 6, wherein the logic means is constructed from analog or digital circuitry.

8. An apparatus according to any one of claims 3 to 6, wherein the logic means is supplied by a microprocessor system.

9. A method substantially as herein described with reference to the accompanying drawing.

10. An apparatus substantially as herein described with reference to the accompanying drawing.