ARC SUPPRESSION TECHNIQUE

Abstract: Apparatus and a corresponding method for the suppression of switching arcs in direct-current (dc) circuitry. Resistive loads, such as lighting loads (20), supplied with dc through a rectifier circuit (14) and a filter circuit (16) are subject to arcing conditions when the loads are switched. The invention includes an arc detection circuit (18) having a transformer (50) with two primary windings (52 and 54) and one secondary winding (56). The primary windings are oppositely wound to eliminate the effects of externally induced alternating signals but to detect high-frequency signals caused by arcing conditions. Detected arcing signals are compared with a threshold in a comparator (60), and the result is used to trigger a one-shot pulse generator (66), the output of which is applied to a disabling circuit (70) connected to disable gate control signals applied to the rectifier circuit. The rectifier circuit is thereby temporarily disabled and the arcing condition is automatically suppressed.

Description: ARC SUPPRESSION TECHNIQUE CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of U.S. Application Serial No. 06/713,017, filed March 18, 1985 and entitled "Light Control Circuit and Related Method." BACKGROUND OF THE INVENTION This invention relates generally to circuits for the control of direct-current (dc) power supplied to electrical loads such as incandescent lights. More particularly, the invention relates to light dimmers of the type used to control lighting in live theatri¬ cal performances, or in the production of motion pictures or programs for television. An important requirement for such stage lighting is for convenient and precise control of the brightness of a number of incandescent lamps, and the ability to switch select¬ ed lamps on and off without significantly affecting the others. A well known approach to the control of lighting loads uses selective switching of alterna¬ ting-current (ac) power signals in a controlled rectifier. Each time the alternating signal reaches a selected phase angle, a controlled rectifier compo¬ nent is switched on, and when the signal next reaches a zero-crossing point the controlled rectifier is automatically switched off. The resulting "chopped" ac signal is filtered to provide a direct current (dc) signal whose voltage level is dependent on the phase angle selected for switching. A significant problem with the use of dc power to supply large lighting loads is that switches used to turn the lights on and off are subject to serious arcing problems. The present invention is directed to a technique for detecting and suppressing arcing in dc switching circuits. SUMMARY OF THE INVENTION The present invention resides in means for automatically suppressing arcing conditions, which are inherent in the switching of dc loads. The means for suppressing arcing includes a transformer having two oppositely wound primary windings connected in the power supply lines to the dc loads, such that high-frequency signals induced equally in both lines will cancel in the transformer, and a secondary winding for detecting high-frequency signals carried by the lines when an arc is formed. In the control system in which the invention is used, the average voltage level of the dc power signal is controlled by dc control means. The inven¬ tion further includes means for comparing the signal sensed by the transformer with a preselected thres¬ hold level, and means responsive to the means for comparing, for generating a disabling pulse for application to the dc control means, to disconnect the dc power temporarily from the load, and thereby suppress the arc. In the preferred embodiment of the inven- tion, the dc control means includes a three-phase rectifier having silicon controlled rectifier (SCR) components. Application of the disabling ' pulse in this context has the effect of disabling gate signals applied to the SCR's, and thereby reducing the output of the rectifier to zero. The technique of the inven- tion would, of course, be equally well suited for application to other types of controlled rectifiers, whether full-wave or half- wave. In addition, the dc control means may take other forms, such as circuitry for pulse-width modulating a dc signal, as disclosed in the cross-referenced application Serial No. 06/713,017. The method of the invention includes the steps of detecting an arcing condition in the load circuit, and temporarily disconnecting the dc power supply from the load to eliminate the arcing con¬ dition. More specifically, the method includes the steps of detecting the presence of high-frequency signals in the load lines, caused by an arcing condition, comparing the magnitude of the detected high-frequency signals with a preselected threshold value, and, when the detected signals exceed the threshold value, applying a disabling pulse to temporarily reduce the dc power level to zero, and thereby suppress the arcing condition. It will be appreciated from the foregoing that the present invention represents a significant advance in the field of control systems for resistive loads, such as lighting loads. In particular, the invention provides a novel technique for the auto¬ matic suppression of switching arcs in dc circuitry. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the acco - panying drawings . BRIEF DESCRIPTION OF THE DRAWINGS FI GURE 1 i s s chemat i c di agram o f the relevant portions of a controlled dc power supply incorporating the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the drawings for purposes of illustration, the present invention is concerned with control circuitry for direct-current loads, such as lighting loads. Although the use of dc power in lighting controllers has significant advantages, such as elimination of power-frequency noise in lighting equipment, and a high output voltage that eliminates the need for booster transformers in large lighting systems, the switching of large direct currents can result in serious arcing problems. Arcing not only affects the lighting loads still connected to the power supply, but causes serious and cumulative damage to switch contacts. In accordance with the invention, arcing conditions are detected and automatically suppressed by the temporary disconnection of power from all of the loads, without significant effect on the lighting conditions, and before significant damage can occur to the switch contacts at which arcing is occurring. As shown in FIG. 1, power is supplied to the control circuitry in the form of three-phase alterna¬ ting current (ac) , the power supply lines being indicated by reference numeral 10. In addition to the three-phase "hot" lines, there is a neutral line 12. The circuitry includes a rectifier circuit 14, a filter circuit 16, an arc detection circuit 18, and a load circuit 20. The three phase lines 10 are connected to a three-phase circuit breaker 22A and in parallel to a second three-phase circuit breaker 22B. As will be observed from the figure, the power conversion cir- cuitry comprises two symmetrical sections, which will be referred to as the A section and the B section. Suffixes A and B are used on reference numerals to indicate the section to which a component belongs. The first circuit breaker 22A is connected, in turn, to the anode terminals of three silicon controlled rectifiers (SCRs) 24A. The cathode terminals of the SCRs 24A are connected in common to a single output line 26A. Similarly, the other circuit breaker 22B has its three terminals connected to the cathodes of three SCRs 24B, and the anodes of these SCRs are connected in common to a single output line 26B. A diode 28A has its anode connected to the neutral line 12 and its cathode connected to the output line 26A. Another diode 28B has its cathode connected to the neutral line 12 and its anode connected to the output line 26B. Output line 26A is connected to one terminal of a series inductor 30A, the other terminal of which 'provides one output from the filter section 16, on line 26A 1 . The filter circuit 16 also includes a shunt capacitor 32A connected between line 26A' and the neutral line 12. A symmetrical arrangement is provided on the other output line 26B, which is connected to a series inductor 3OB to output line 26B 1 , with a shunt capacitor 32B connected between line 26B' and the neutral line 12. Output line 26A" and the neutral line 12 are connected through the arc detection circuit 18 to a first lamp load 20A and output line 26B 1 and the neutral line 12 are connected through the arc detec¬ tion circuit 18 to a second lamp load 2OB. The three A-section SCRs 24A function as a three-phase half- wave rectifier to supply direct current to the lamp load 20A, and the other three SCRs 24B similarly act as a three-phase half-wave rectifier to supply direct current to the other lamp load 2OB. Each of the SCRs 24A has a gate electrode 34A and each of the SCRs 24B has a gate electrode 34B. The gate electrodes are controllable in a conventional manner to vary the output voltage on output lines 26A* and 26B-. Each SCR must have a bias voltage applied to its gate electrode for conduction to take place. Basically, each SCR is in a conductive state from the time that a signal is applied to its gate electrode until the time that the voltage between its anode and cathode reaches a zero point. Control of the signal applied to the gate electrode allows the production of a "chopped" ac current waveform through the SCR, and this effects control of the output voltage of the rectifier circuit 14. These "chopped" ac waveforms, of which there are three overlapping versions, one for each phase, are filtered to an. approximately flat dc waveform by the action of the filter circuit 16. The B-section SCRs 2 B operate in the same manner as the SCRs 24A, except that they conduct only in the negative-going halves of the input power signals. Control of the gate signals to achieve a desired output voltage is effected by any suitable technique, the details of which are not pertinent to the present invention. This is shown in the figure by the presence of a controller 36, which receives as inputs the levels of the output voltages on lines 26A 1 and 26B 1 and control signals on line 38 indica- tive of the desired output voltage levels. Output signals from the controller 38 are transmitted over lines 40A and 40B to the two sets of gate electrodes 34A and 34B. The loads 20A and 2OB are typically balanced, but the desired output voltages may not necessarily be identical. The lighting loads 20A and 2OB are switched on and off by switches 42A and 42B, it being under¬ stood that there may be multiple lamps in the loads and corresponding multiple switches. During these switching operations, arcing can and usually does occur in the switches 42, and it is the function of the arc detection circuitry to sense when an arcing condition occurs. The phenomenon surrounding the formation of a switching arc is not completely under- stood but is known to produce high frequency current components in the load lines while the arcing is occurring. If the switch 42A is arcing, a high-fre¬ quency component will be present in both the line 26A and the neutral line 12. Unfortunately, high-frequen- cy signals can also be induced in power lines by external sources, and some means must be provided to distinguish between the two types of high-frequency signals. The arc detection circuitry 18 also per¬ forms this function. The arc detection circuitry 18 includes a transformer 50A connected in the A-section load lines and a transformer 50B connected in the B-section load lines. The transformer 50A has two primary windings 52A and 54A, one of which (52A) is connected in series in the output line 26A' and the other of which is connected in a branch 12A of the neutral line 12. The primary windings 52A and 54A are wound in oppo¬ site directions, so that an externally induced high- frequency signal would affect the two windings in opposite senses, resulting in cancellation. When arcing occurs in the load circuit 20, however, the effect on the two primary windings 52A and 54A will be cumulative, and there will be a corresponding high-frequency component in the magnetic flux asso- ciated with the transformer 50A. A secondary winding 56A detects this high-frequency magnetic flux and generates a corresponding output signal on lines 58A. A similar structure is provided for the B-section load lines, including a transformer 50B, two primary windings 52B and 54B connected in lines 26b 1 and 12B, repsectively, a secondary winding 56B, and secondary output lines 58B. The output signals from the A-section arc detector, on lines 58A, are connected as one input to a comparator circuit 60A, which compares the peak or the average value of the detected high-frequency signals with a selected thresheold value input on lines 62A. An output signal is generated on line 64A whenever the detected high-frequency signals exceed the selected threshold value. This output signal on line 64A is used to trigger a one-shot circuit 66A, which generates a pulse output on line 68A. The pulse output is sustained for a preselected period of time, such as one tenth- of a second, chosen 'as the time required to ensure the extinction of an switching arc. The output of the one-shot circuit 66A is connected by line 68A to a disabling circuit 70A in the line 40A to the gate electrodes of the SCRs 24A. The disabling circuit 70A may take any convenient form, but is shown diagrammatically as an AND gate having line 68A as one inverted input. Thus, when the one-shot circuit 66A is normally quiescent, there is no signal on line 68A and the AND gate is enabled, to pass the desired control signals to the gate electrodes of the SCRs 24A. However, during the short period of output from the one-shot circuit 66A, line 68A provides a disabling input to the AND gate and the control signals applied to the gate elec¬ trodes of the SCRs 24A are temporarily reduced to zero. This renders the SCRs 24A non-conductive for this short period of time, and the arcing condition is automatically suppressed. When the disabling signal is removed from the AND gate 70A, the normal control signals are applied to the gate electrodes of the SCRs 24A and normal operation of the rectifier circuit resumes. Similar circuitry is provided for the B-section arc suppression circuitry, including a comparator 60B with additional input lines 62B and output line 64B, a one-shot circuit 66B with output line 68B, and a disabling circuit 70B connected in line 40B. From the foregoing, it will be appreciated that the present invention represents a significant advance in the field of dc controllers for loads such as lighting loads. In particular, the invention provides a method and means for automatically detect¬ ing and suppressing switching arcs, thereby avoiding t one of the most serious disadvantages , of dc power supply systems used to control lighting loads. It will also be appreciated that, although an embodiment of the invention has been described in detail for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.

Claims: CLAIMS I claim: 1. For use in a direct-current (dc) power supply having control means for varying the output of the power supply to a dc load circuit, apparatus for detecting and suppressing an arcing condition in the dc load circuit, the apparatus comprising: means for detecting high-frequency signal components in the load circuit and distinguishing such signal components from externally induced high- frequency signals; means for comparing the detected high-fre- quency signal components with a preselected threshold level; and means, responsive to the means for compar¬ ing, for disabling the control means of the dc power supply for a preselected short period of time, to disconnect power from the load circuit temporarily, and to thereby suppress the arcing condition. 2. Apparatus as defined in claim 1, wherein the means for detecting high-frequency signal compo- nents includes: a transformer having two oppositely wound primary windings connected in the power supply lines to the dc loads, such that high-frequency signals induced equally in both lines will cancel in the transformer, and a secondary winding for detecting high-frequency signals carried by the lines when an arcing condition occurs. Apparatus as defined in claim 2, where¬ in: the means for disabling includes means for generating a disabling pulse for transmission to the control means when the means for comparing detects high-frequency signal components over the threshold level. 4. A controllable direct-current (dc) power supply system capable of automatically suppressing an arcing condition in a load circuit, the system com¬ prising: a controlled rectifier circuit for converting alternating-current power to dc power for supply to the load circuit; means for detecting high-frequency signal components indicative of an arcing condition in the load circuit and distinguishing such signal compo¬ nents from externally induced high-frequency signals; means for comparing the detected high-fre¬ quency signal components with a preselected threshold level; and means, responsive to the means for com¬ paring, for disabling the controlled rectifier circuit for a preselected short period of time, to disconnect power from the load circuit temporarily, and to thereby suppress the arcing condition. 5. A power supply system as defined in claim 4, wherein the means for detecting high-frequen¬ cy signal components includes: a transformer having two oppositely wound primary windings connected in the power supply lines to the dc loads, such that high-frequency signals induced equally in both lines will cancel in the transformer, and a secondary winding for detecting high-frequency signals carried by the lines when an arcing condition occurs. 6. A power supply system as defined in claim 5, wherein: the means for disabling includes means for generating a disabling pulse for transmission to the control means when the means for comparing detects high-frequency signal components over the threshold level. 7. A power supply system as defined in claim 6, wherein: the controlled rectifier circuit includes multiple silicon controlled rectifiers (SCRs) having gate electrodes for the control of conduction of the SCRs; the means for disabling the controlled rectifier circuit further includes means responsive to the disabling pulse, for temporarily disconnecting control signals from the gate electrodes of the SCRs, to disable output from the rectifier circuit until the arcing condition is suppressed. 8. A power supply system as defined in claim 7, wherein: the controlled rectifier circuit includes two three-phase half-wave rectifiers connected to supply dc power to two load circuits. 9. For use in a direct-current (dc) power supply system having control means for varying the output of the power supply to a dc load circuit, a method for detecting and suppressing an arcing condition in the dc load circuit, the method com¬ prising the steps of: detecting an arcing condition in the load circuit; and temporarily disconnecting the dc power supply from the load to eliminate the arcing condi¬ tion. 10. A method as defined in claim 9, wherein the detecting step includes: detecting the presence of high-frequency signals in load lines connecting the power supply to a dc load, the high-frequency signals being caused by the arcing condition; distinguishing these high-frequency signals from other high-frequency signals that may be exter¬ nally induced in the load lines; and comparing the magnitude of the detected high-frequency signals with a preselected threshold value. 11. A method as defined in claim 10, where¬ in the step of temporarily disconnecting includes: applying a disabling pulse to the dc control means when the comparing step indicates that the detected high-frequency signals are above the prese¬ lected threshold, to temporarily reduce the dc power level to zero, and thereby suppress the arcing condi¬ tion.

Inventors: LA BELLE JOHN T, LA BELLE, JOHN, T.
Applicants: STRAND CENTURY INC, STRAND CENTURY, INC.
Application year: 1986
Main classification: H05B37
IPC codes: H02H1, H02M7