ELECTRICAL POWER DISTRIBUTION CONTROL SYSTEM

Abstract: An electrical power distribution control system is capable, by means of a pair of transformers (T1, T2), of supplying a number of loads with a reduced voltage during continuous operation. When a new load is applied to the power distribution network, the transformer (T1) is reconfigured by a relay (A1) to increase the voltage on the distribution network to enable ignition of fluorescent lamps etc.. The control system also includes signalling means comprising a timer (17), a timer control circuit (18) and a further relay (TM). Signalling is effected by further reducing the voltage on the distribution network for predetermined short periods. In an alternative embodiment, the reconfiguring of the transformer (T2) is carried out by means of four triacs (29, 30, 31, 32) (not shown) connected in a bridge circuit.

Description: ELECTRICAL POWER DISTRIBUTION CONTROL SYSTEM DESCRIPTION This invention relates to an electrical power distribution control system which permits remote control of electrical loads such as electric lamps connected to a power distribution conductor system. The invention has particular but not exclusive application to the remote control of loads in buildings such as offices or domestic premises. Remote control systems for electrical lighting have been previously proposed in which a central control unit applies bursts of an A.M. carrier to supply conductors of a domestic ac supply. Remote switching units connected to the supply conductors respond to the bursts of carrier to switch selectively electrical load circuits e.g. electric lamps. The bursts of A.M. carrier may be of different lengths or different frequencies so that the load circuits can be switched individually by the remote switching units. A disadvantage of these proposals is that they are expensive to manufacture and also requires special inductances connected in the main supply to prevent signals produced in one dwelling from feeding back along the main supply to another dwelling. It is an object of the present invention to provide a much simpler remote switching system. In accordance with a first aspect of the present invention, an electrical power distribution control system is provided wherein signalling to remote switching means is effected by means of signalling means that reduces the voltage of the electrical power supply temporarily to provide a signal for operating the switching means. The invention has particular but not exclusive application to the control system described in our co- pending International Patent Application PCT/GB87/00756 which discloses a control system for electrical lighting e.g. flourescent lamps, wherein the supply voltage is reduced from normal mains voltage after start up of the lamps. A first transformer provides a reduced voltage (216 v) as compared with mains supply voltage (240 v) . A second transformer provides a supplementary voltage (24 v). Upon start up of the circuit, a control circuit connects both the reduced voltage from the first transformer and the supplementary voltage from the second transformer to the output so as to provide a voltage which is sufficient to strike the flourescent lamps. The control circuit after a predetermined delay e.g. 15 seconds, disconnects the supplementary voltage. The lamps then continue to operate on the reduced voltage (216 v) thereby reducing the power consumed by the lamps. Preferably, the reduced voltage for signalling operation of the switching means is produced by means of the second transformer. The transformer is connected in such a manner as to produce an impedance which reduces the nominal supply voltage produced by the first transformer i.e. reduces the output below 216 volts to e.g. 204 volts. According to a second aspect of the present invention there is provided a source of variable voltage ac electric power comprising a first transformer, a second transformer including a primary winding having first and second tappings and controlled solid state switching means for varying the current flowing in said primary winding of the second transformer relative to the current output from the first transformer so as to vary the output voltage or for creating a temporary low impedance path between the said tappings. . Preferably, the source of variable voltage ac power is employed in an electrical power distribution system having a remote switching capability. In order that the invention may be more fully understood, an embodiment thereof will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram in block form of an electrical power distribution control system in accordance with the invention; Figure 2 is a schematic circuit diagram of a signalling circuit according to a first aspect of the invention; Figure 3 is a schematic circuit diagram of a switch and control circuit shown in Figure 1; Figure 4 is a schematic circuit diagram in partial block form of a signalling circuit according to a second aspect of the invention; and Figure 5 is a schematic circuit diagram of part of the supply voltage monitoring circuit. Referring firstly to Figure 1, domestic ac supply is connected to live and neutral input terminals 10, 11 of a signalling circuit 12 having live and neutral output terminals 20, 21 that are connected to supply conductors 13, 14 of a conventional ac power supply distribution system, as found conventionally in offices dwellings etc. A first electric lamp circuit 15 is connected to the supply conductors 13, 14 under the control of an electric switch and control circuit 16. The lamp circuit may comprise a number of flourescent tubes and the switch and control circuit 16 comprises a switch operable manually in the vicinity of the lamp circuit 15 together with a control circuit responsive to signals from the signalling circuit 12 to permit remote switching of the lamp circuit 15. The circuit 16 is connected in the live line 13 and may be embodied as a replacement unit for a conventional wall switch for an ac supply. A second electric lamp circuit 15' and a corresponding second switch and control circuit 16' are connected to the main supply conductors 13, 14. As will be explained in more detail hereinafter with reference to Figures 2 and 3, the signalling circuit 12 produces remote switching of the lamp circuits by temporarily reducing the supply voltage on conductors 13, 14. The switch and control circuit 16, 16' are individually responsive to different durations of reduction of the supply voltage. Thus, the circuit 16 may be responsive to a reduction for a period of five seconds whereas the circuit 16' may be responsive to a voltage reduction for 10 seconds. Thus, if the signalling circuit 12 produces a temporary reduction in the supply voltage for five seconds, circuit 16 operates to switch the lamp circuit 15 whereas if the signalling circuit 12 produces a voltage reduction for 10 seconds the switch and control circuit 16' operates to switch lamp circuit 15' . It will be appreciated that further switch and control circuits 16 could be provided responsive to yet further different temporary voltage reduction periods. The reduced voltage that occurs during signalling is selected so that any lamps which are lit remain operational. Conveniently, the signalling circuit may be based on the control circuit described in our aforementioned PCT Patent Application. Accordingly, the live and neutral terminals 10, 11 are configured to receive a supply voltage of 240 volts and the output terminals 20, 21 are arranged to deliver a nominal supply voltage of 216 volts during normal operation of the lamps. However, at start up the nominal supply voltage is boosted to 240 volts to enable the flourescent lamps to strike. During signalling, the nominal supply voltage of 216 volts is temporarily reduced to 204 volts for either five or 10 seconds in order to operate the switch and control circuits 16, 16' selectively. The signalling circuit 12 will now be described in more detail with reference to Figure 2 which can be considered to be a modification of Figure 1 of the PCT Application. The first transformer Tl in the form of an auto-transformer has winding tapping Wl, W2 connected across the main supply voltage terminals 10, 11. The transformer Tl also has an output tapping W3 which provides the nominal supply voltage of 216 volts that is supplied to output terminals 20, 21. The second, step-down transformer T2 has primary winding tappings W4, W5 connectible to the main supply terminals 10, 11 through a changeover switch contact Al. Transformer T2 has secondary winding tappings W6, W7 connected in series with output terminals 20 and the tapping 3 of the first transformer Tl. A control circuit CC shown schematically in Figure 2, controls operation of the changeover contact Al. In a first position of Al shown in solid outline in Figure 2, the transformer T2 is connected to provide a voltage in its secondary which increments a reduced voltage of 216 volts produced by the transformer Tl so as to provide an output voltage at the terminals 20, 21 which closely approximates full mains voltage (i.e. 240 volts). When the control circuit operates changeover switch Al to the position shown in dotted outline in Figure 2, the terminals W4, W5 of the primary of T2 are effectively short circuited such that T2 no longer produces the supplementary voltage and also does not impede current flow from Tl to the output terminals 20, 21. The control circuit CC is so arranged that upon start of the lamps, switch Al is in the position shown in solid outline in Figure 2 so that a voltage approximating to mains voltage is produced at the terminals 20, 21 to enable switch on of the lamps. A short period thereafter e.g. 15 seconds, the control circuit CC switches Al to the dotted line position so as to disable operation of transformer T2 and thereby reduce the voltage supplied to the lamps by 24 volts i.e. approximately 10% of normal mains voltage. This circuit operation is described in more detail in our aforementioned PCT Application. Also, a more detailed description of two embodiments of the control circuit CC are described in the PCT Application and the contents thereof are incorporated herein by reference. It has been found that by operating the lamps at the reduced voltage of 216 volts, no substantial loss in light output occurs but a substantial saving in power is achieved. The remote switching according to the invention is achieved by means of a timer 17 which switches current from the mains supply through the coil of a relay TM having switching contacts TM1 so arranged to switch the primary winding of the second transformer T2 between an open and a closed circuit condition. The timer 17 is controlled by a control circuit 18 which includes a manually operable switch (not shown) . In use, the control circuit 18 can be used to set the timer for a period appropriate for switching lamp circuit 15 or 15' i.e. 5 or 10 seconds. Thus, after start up of the lights, the contactor CC operates contact Al to the position shown in dotted outline so as to apply the nominal supply voltage of 216 v to the output terminals 20, 21 and hence to the supply conductors 13, 14. When the contact TM1 is in the position shown in Figure 2, the terminals W4, W5 of the primary of transformer T2 are short circuited which results in the transformer T2 presenting a low impedance such that a negligable voltage drop occurs between the secondary winding tappings W6, W7. As a result, the voltage produced at winding tapping W3 is substantially the same as that produced across terminals 20, 21. When it is desired to produce remote switching of lamp circuit 15 or 15' , the timer control circuit is operated to operate timer 17 for a period of either 5 or 10 seconds. This produces energisation of relay coil TM for a commenserate period which opens relay contacts TM1 so as to produce an open circuit between terminal W4, W5 of the primary of transformer T2. As a result, the impedance presented by the transformer T2 increases such that a voltage drop of approximately 20 volts occurs between the winding terminals W6, W7 of its secondary. Consequently, the voltage developed across terminals 20, 21 drops to approximately 20 volts during the period of operation of timer 17. This voltage drop is used to operate switching circuits 16, 16' as will now be explained with reference to Figure 3 which shows the circuit 16 in more detail. The live conductor 13 is connected to live input terminal 19 and the output of the circuit is applied to switched live terminal 22. A rechargeable battery 23 is connected to a power supply unit comprising a full wave rectifier bridge 24 through a step down transformer T3. The output of the rectifier bridge 24 is stabilised by a zener diode ZD1 and a capacitor Cl. The rechargeable battery 23 acts as a power supply for the circuit and drives a bistable relay B having contactor Bl arranged to switch power from the live terminal 19 to the switched live terminal 22. The voltage on the live terminal 19 is detected by means of a voltage sensor circuit which operates a switch Cl for triggering the relay B. The switch Cl is conveniently a transistor switch. In addition, a manually operable push button switch SI is provided for latching the relay B. A manual push button switch S2 is provided for unlatching the relay B. On operation of push button SI, the relay B latches on and contact Bl changes over from the position shown in Figure 3 so as to connect the live terminal 19 to the switched live terminal 22. Thereby disconnecting the transformer T3 and energising the lamp circuit 15 (Figure 1). In the event that the signalling circuit 12 (Figure 2) produces a switching signal to circuit 16, the voltage on the live terminal 19 is reduced below 204 volts for a period of five seconds. The voltage sensor C is arranged to detect this condition by comparing the voltage at the live terminal 19 with a reference voltage V * which is conveniently earth. When the five second voltage reduction is detected, voltage sensor C closes switch Cl, which provides a release signal to latching relay B and thereby causes contact Bl to change over into the position shown in Figure 3 so as to switch off the lights and reconnect transformer T3 thereby to charge the battery 23. The transformer T3 has a high input impedance so that when the contactor Bl is in the position shown in Figure 3 insufficient current flows through the transformer to illuminate the light circuit 15. It will be appreciated that with this type of circuit, the lights are off more than they are on and accordingly, the transformer T3 and rectifier 24 are connected to the battery 23 for sufficient time to maintain it fully charged, thereby ensuring reliable operation of the switching circuit. The voltage sensor C is so arranged that in the event of a power failure, and the subsequent • restoration of power, the relay B remains in the mode in which it was set immediately before the power failure. To this end, the voltage sensor C is arranged to operate switch Cl in response to an increase in voltage at the end of the signalling period i.e. five second. If the voltage restoration does not occur within this period, the voltage sensor assumes that the reduction in voltage is due to a power failure and is configured to maintain the relay B in the same mode upon restoration of the power at the end of the power failure. The voltage sensor C is programmable to detect recovery of the voltage from below the reduced voltage e.g. 204 volts after five seconds, 10 second, 15 seconds etc. so that a corresponding circuit 16 can be used for control circuit 16' in Figure 1. Clearly, further lamp circuits 15 and associated control circuits 16 can be connected to the supply conductors 13, 14 and can be switched selectively by appropriate control of the timer 17 shown in Figure 2 and appropriate settings of the voltage sensor C shown in Figure 3. An alternative embodiment of the signalling circuit 12 will now be described in detail with reference to Figure 4. Again the first transformer Tl in the form of an autotransformer has winding tappings Wl, W2 connected across the mains supply voltage terminals 10, 11. The transformer Tl also has an output tapping W3 which provides the nominal supply voltage of 216 volts that is supplied to the output terminals 20, 21. The second, step-down transformer T2 has a secondary winding connected in series with the output tapping W3 of the first transformer. The primary winding tappings W4, W5 of the second transformer T2 are connected to opposite nodes of a bridge circuit 24. The remaining two nodes of the bridge are connected to the mains supply. The arms of the bridge circuit 24 are identical to each other and each comprises a triac 25, 26, 27, 28, connected by its main terminals to consecutive nodes, in parallel with a varistor 29, 30, 31, 32. The triacs 25, 26, 27, 28 are triggered by pulses produced by four pulse forming circuits 33, 34, 35, 36. A monitor unit 37 monitors the mains input current and voltage, and the output voltage. Signals indicative of the various monitored conditions are passed to a control unit 38. The control unit 38 also receives signalling time signals from a timer. The control unit 38 produces four outputs which are .fed to the pulse forming circuits 33, 34, 35, 36 respectively, in order to control the switching of the triacs 25, 26, 27, 28. In operation, the monitoring unit 37 monitors the input current and voltage, and should the current increase, indicating that an additional light has been switched-on, or the input voltage fall, it sends a BOOST command to the control unit 38. Since the remote switching is dependant on causing periods of reduced supply voltage, it is confusing for the system if there is an inadvertant but brief drop in the supply voltage between the terminals 20, 21. In order to overcome this, the monitor unit 37 monitors the output voltage between terminals 20, 21 and on detection of an unwanted drop in output voltage sends a BOOST signal to the control unit 38. When the control unit 38 receives a BOOST signal it outputs signals which cause the triac 27 between the tapping W5 of the primary winding of the second transformer T2 and the live line, and the triac 26 between tapping W4 of the primary winding of the second transformer T2 and the neutral line to conduct. This causes a current to flow in the primary winding of the second transformer T2 which is in phase with that flowing in its secondary winding due to the first transformer Tl. The current flowing in the primary winding of the second transformer T2 induces a voltage in the secondary winding thereof which is superimposed on the voltage output by the first transformer Tl. Since the voltages are in phase the result is an increase in peak voltage between the output terminals 20, 21. Under normal operating conditions, i.e. in economy mode, it is necessary for the primary winding of the second transformer T2 to be shorted out. This is achieved by the control unit 38 producing signals to trigger the two triacs 26, 28 which are connected to the neutral line. The two triacs 26, 28 then conduct and provide low impedance paths between the tappings W4, W5 of the primary winding of the second transformer T2 and the neutral line. This means that the secondary winding of the second transformer T2 presents a low impedance to the current flowing in the lighting circuit. When it is desired to produce remote switching of lamp circuits 15 or 15' , the signalling timer 39 is operated to pass a signal having a duration of either 5 or 10 seconds to the control unit 38. During signalling it is necessary to reduce the voltage between the output terminals 20, 21. When the system is operating in economy mode, this achieved by inducing a voltage in the secondary winding of the second transformer T2 in anti-phase to the voltage output from the first transformer Tl. To this end the triac 25 between the tapping W4 of the primary winding of the second transformer T2 and the live line, and the triac 28 between the tapping W5 of the primary winding of then second transformer T2 and the neutral line are triggered by the control unit 38. This causes a current to flow in the primary winding of the second transformer T2 in anti-phase to that flowing in the primary thereof as a result of the first transformer Tl. If, however, the system is operating in boost mode, the voltage drop produced, by inducing a voltage in the secondary winding of the second transformer T2 in anti-phase to the voltage output from the first transformer Tl, would be in the order of twice the required value. Therefore, when signalling is carried out during boost mode, the control unit 38 causes the the primary winding of the second transformer T2 to be shorted for the duration of the signalling pulse. The shorting of the primary winding of the secondary transformer T2 is carried out in the same manner as during economy mode. For effective signalling the signalling pulses should be applied to a steady supply voltage. To this end the monitor unit includes a supply voltage steadiness determining circuit, part of which is shown in Figure 5. Referring to Figure 5, the circuit comprises a pair of voltage comparators IC1,IC2, a pair of diodes D1,D2 having their anodes connected to the outputs of respective comparators IC1,IC2 and their cathodes connected together, first and second potential dividers formed by resistors R2 and R4, and R3 and R5 respectively, and a low pass filter R1,C1,C2. The cathodes of the diodes D1,D2 are also connected to the output connection 39 of the circuit. The low pass filter R1,C1,C2 is in the form of a pi-network, the input of which is connected to the circuit input connection 40. The output of the low pass filter R1,C1, C2 is coupled to the non-inverting input of IC1 and to the inverting input of IC2. The first potential divider R2,R4 is connected between the circuit .input 40 and the positive supply rail 41 with its mid-point coupled to the inverting input of IC1. The second potential divider R3,R5 is connected between the circuit input 40 and the 0V rail 42 with its mid-point coupled to the non-inverting input of IC2. In operation the circuit input 40 is supplied with a dc voltage representative of the supply voltage amplitude, peak-to-peak or r.m.s., for instance 3.7V. This signal is passed to the low pass filter R1,C1,C2 which has a relatively long time constant. The output from the low-pass filter essentially tracks the long term trends in the supply voltage and provides a reference voltage for the comparators IC1,IC2, also 3.7V in this case. The potential dividers are arranged such that under steady conditions, the voltage on the inverting input of IC1 will be slightly higher than the reference voltage ,e.g. 3.75V, and the voltage on the non-inverting input of IC2 slightly lower, e.g. 3.65V. Under these conditions the outputs of the comparators IC1,IC2 are low. If, however, there is a drop in supply voltage for a duration significantly below the time constant of the low-pass filter R1,C1,C2, the reference voltage will remain substantially constant due to the action of the filter R1,C1,C2, while the voltages at the mid-points of the potential dividers will fall. Should the voltage on the non-inverting input of the comparator IC1 fall below the reference voltage the output of the comparator will go high and consequently the voltage at the output 39 of the circuit will go high. Similarly, if there is a short increase in the supply voltage, the voltage on the inverting input of IC2 will rise. If it becomes greater than the reference voltage, the output of this comparator will go high and consequently the voltage at the output 39 of the circuit will go high. It can, therefore, be seen that the output 39 going high indicates unsteadiness in the supply voltage. It is apparent that this circuit is capable of producing an ac voltage at one of three different levels; a low level for signalling, an intermediate level for economy lighting and a high level for igniting flourescent lamps. It is envisaged, however, that this circuit will find application in other systems where two or three ac voltage levels are required.

Claims: CLAIMS 1. An electrical power distribution control system comprising: a power distribution conductor system; an electrical power supply operative to feed electrical power to the conductor system at a predetermined nominal supply voltage; at least one electrical load to receive power from the conductor system; switching means for controlling the supply of electrical power from the conductor system to the load; signalling means associated with the electrical power supply to change temporarily the supply voltage thereof for a predetermined time so as to signal operation of the switching means; and control means responsive to the change in supply voltage produced by the signalling means for operating said switching means. 2. A system according to claim 1 wherein said electrical power supply is operative in such a manner that said nominal supply voltage comprises a reduced voltage below normal mains voltage for the load, and said power supply includes means for providing a supplementary voltage when initially operating the lighting to increment the reduced voltage to a value approximating to normal mains voltage, and means for thereafter removing the supplementary voltage. 3. A system according to claim 1 or 2 wherein the signalling means is operative to reduce temporarily the nominal supply voltage. 4. A system according to any preceding claim, wherein signalling is inhibited for a predetermined period after a supply voltage excursion. 5. A system according to any preceding claim including more than one load and control means, and wherein said -signalling means is operative to produce said temporary change in the supply voltage for different predetermined times selectively so as to operate different ones of said control means. 6. A system according to claim 2 wherein said reduced voltage is produced by a first transformer and the supplementary voltage is provided by a second transformer having a primary winding connected to receive the mains voltage and a secondary winding that produces the supplementary voltage. 7. A system as claimed in claim 6 wherein the secondary winding of the second transformer is connected so as to provide a series connection with the first transformer. 8. A system according to claim 6 or 7 including timed control means for connecting the primary winding of the second transformer in an open circuit disconnected from the mains supply, for producing a temporary reduction in the nominal supply voltage so as to signal operation of the switching means. 9. A system according to claim 6 or 7 including timed control means for causing a voltage generated in the secondary winding of the second transformer to be anti-phase with respect to the voltage produced by the first transformer for producing a temporary reduction in the nominal supply voltage so as to signal operation of the switching means. 10. A system according to claim 8 or 9 wherein said time controlled means includes means for selectively controlling the time period during which the swpply voltage is reduced. 11. A system according to any preceding claim wherein the control means includes voltage sensor means responsive to said change in the supply voltage for said predetermined time and a relay responsive to said voltage sensor for switching the supply of electrical power from the conductor system to the load. 12. A system according to claim 11 including a rechargeable power supply for the relay, arranged to be charged from the supply conductors. 13. A system according to claim 11 or 12 including manually operable switches for operating said relay. 14. A source of variable voltage ac electric power comprising a first transformer, a second transformer including a primary winding having first and second tappings and controlled solid state switching means for varying the current flowing in said primary winding of the second transformer relative to the current output from the first transformer so as to vary the output voltage or for selectively creating a low impedance path between the said tappings. . 15. A source of variable voltage ac electric power according to claim 14 wherein the solid state switching means are triacs. 16. A source of variable voltage ac . electric power according to claim 14 or 15 wherein the first transformer is an auto transformer. 17. A source of variable voltage ac electric power according to claim 14, 15 or 16 wherein the secondary winding of the second transformer is connected so as to provide a series connection with the first transformer. 18. A source of variable voltage ac electric power according to any claim 14 to 17 wherein the switching means are connected so as to form a bridge circuit. 19. A source of variable voltage ac power according to any claim 14 to 18 wherein said first transformer is connected to a supply of electric power at conventional mains voltage and said variable voltage source normally outputs a voltage less than the input mains voltage. 20. A source of variable voltage ac power according to claim 19 including means for reducing the output voltage from said normal value thereof for signalling. 21. A source of variable voltage ac power according to claim 19 or 20 including means for increasing the output voltage from said normal value thereof for facilitating the ignition of a gas discharge lamp. 22. A control system including an electrical load rated to operate at conventional mains supply voltage and normally operated at a reduced voltage below conventional mains voltage including a source of variable ac electric power according to any claim 14 to 21. 23. A control system for electrical load according to claim 22 wherein the electrical load is a lamp. 24. A control system for electrical loads according to claim 23 wherein the lamp is a flourescent lamp.

Inventors: FINCH DAVID ALLEN, LAWRENCE JOHN ARTHUR, FINCH, DAVID, ALLEN, LAWRENCE, JOHN, ARTHUR
Applicants: ECONOLIGHT LTD, ECONOLIGHT LIMITED
Application year: 1990
Main classification: H05B37
IPC codes: H02J13