Abstract:
A constant-current regulator for high-powered airport lighting loops combined a ferroresonant transformer and a digital programmable logic device to provide a versatile, software-modifiable current control for power transfer throughout a range of about 30 kW to 50 kW with uniformly good power factor and low harmonics without switching winding taps, and without requiring oil cooling.

Description:
This application is a continuation of application Ser. No. 09/932,758 filed Aug. 16, 2001, now U.S. Pat. No. 6,570,345 the disclosure of each of which is incorporated in its entirety herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to constant current regulators, and more specifically to a high-power regulator controlled by a programmable logic for airport lighting applications. 
     BACKGROUND OF THE INVENTION 
     Approach lights for airport runways typically include sets of high-wattage lamps connected in series in a lighting loop. In order to maintain a uniform intensity throughout the loop regardless of supply voltage variations, and to allow selected changes of intensity to cope with various weather and natural light conditions, the lighting loop has to be supplied with an adjustable constant current that is unaffected by supply voltage variations or other electrical disturbances. 
     In addition, airport lighting is subject to strict FAA regulations which require, for example, minimization of switching harmonics and minimization of inductive loading of the power supply. Switching harmonics are undesirable both as a reflection into the power supply, and in the lighting loop. In the latter, the skin effect from high power harmonics can require the use of heavier copper cables (which can be quite long in airport applications), and the radiation of harmonics from the lighting loop can interfere with sensitive communication systems such as instrument landing systems. 
     The individual lamps of the loop are typically fed from the loop through isolation transformers. If a lamp burns out, the isolation transformer primary winding acts as an inductor and puts a substantial inductive load on the circuit. For that reason, a shorting device is mounted across the secondary of the isolation transformer. When the lamp fails (i.e. opens up), the shorting device is activated to keep the integrity of the loop intact. 
     With conventional analog control circuitry, control of an airport lighting constant current regulator is feasible but is not very flexible or efficient. It is therefore desirable to give the regulator a maximum of flexibility, e.g. automatic power reduction on power-up, power-down and in error conditions. 
     Another problem of the prior art is that regulators exceeding about 30 kW power capacity usually required oil cooling, which was expensive and environmentally undesirable. Also, a regulator adaptable to a wide range of power outputs generally required switchable taps on the main transformer windings that required resetting the transformer when changing the load. 
     SUMMARY OF THE INVENTION 
     The present invention solves the problems of the prior art by providing a constant current regulator using an air-cooled ferroresonant transformer that maintains a good power factor and efficiency over the entire 30 kW to 50 kW output power range without requiring any tap switching, in conjunction with a control system using a programmable logic to track circuit conditions and user interfaces, and to take appropriate control actions in response thereto. 
     Each and every feature described herein, and each and every combination of two or more of such features, is included within the scope of the present invention provided that the features included in such a combination are not mutually inconsistent. 
     These and other aspects of the present invention are set forth in the following detailed description and accompanying claims, particularly when considered in conjunction with the accompanying drawings in which like parts bear like reference numerals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the regulator system of this invention; 
     FIG. 2 is an equivalent circuit diagram of the ferroresonant transformer used in this invention; 
     FIG. 3 is a block diagram of the transformer control circuit; and 
     FIG. 4 is a block diagram showing relevant data inputs and outputs of the programmable logic used in the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates the basic circuitry of the inventive regulator. Power (typically 480 VAC) is applied to the power input  10 . Input voltage sensor  12  and input current sensor  14  monitor the input power and supply error signals  16 ,  18  to the current control  20  if input voltage or input current are outside a predetermined safe range. The input power is applied through an electromagnetic interference (EMI) filter  22  to the primary winding  24  of ferroresonant transformer  26 , the details of which are discussed below. 
     A first secondary winding  28  powers the lighting loop  30 . The loop  30  is equipped with a loop voltage sensor  32  which, together with the loop current sensor  36 , provides an indication of the power input to the lighting loop  30 . The loop current sensor  36  senses the actual loop current and provides a signal  34  representative thereof to the current control  20  for control purposes. 
     The primary windings  38   a  through  38   n  of a plurality of isolation transformers  40   a - 40   n  are connected in series in the loop  30 . The secondaries of the isolation transformers  40   a - 40   n  drive the individual lamps  42   a - 42   n , respectively, of the lighting array. Because the isolation transformer primaries  38   a - 38   n  are connected in series, the same current flows through all of them, and the brightnesses of the lamps  42   a - 42   n  are therefore identical at the level selected by brightness selector  43 . 
     A pair of parallel-connected second secondary windings  44   a ,  44   b  (shown separately for conformance with is FIG. 2) drive a resonant capacitor  46 . As described below, the capacitor  46  is so dimensioned as to resonate at the 60 Hz line frequency with the leakage inductance of the transformer  26 , for a reason discussed below. 
     A third secondary winding  48  on transformer  26  is switched into and out of the circuit at a 60 or 120 Hz rate by an SCR switch  50  for a purpose described below. A snubber  52  is connected between the control line  54  and the switch  50 , more specifically across the gate/cathode connections of the dual SCRs that make up the switch  50 . This provides an interface between the gate trigger circuitry of the current control  20  and the gates/cathodes of the switch SCRs, as well as transient protection for the switch  50 . 
     FIG. 2 shows in more detail the equivalent circuit of the ferroresonant transformer  26 . 
     The principle of the ferroresonant transformer  26  is that the leakage inductances  56   a  and  56   b  resonate with the capacitors  46   a ,  46   b , respectively, at 60 Hz. At resonance, the transformer  26  transfers power to the lighting loop  30  with maximum efficiency through windings  24   e  and  28  (in the equivalent circuit of FIG. 2, the windings  24   a ,  24   b ,  24   c ,  24   d  and  24   e  are all functional sections of a primary winding that drives the secondary windings  28 ,  44   a ,  44   b  and  48 ). 
     The inductances  56   a ,  56   b ,  58  and  60  cooperate to keep high frequency harmonics out of the AC input and field wiring to reduce the need for heavy wiring and to prevent interference with airport avionics. The resonant filtering action of the ferroresonant transformer  26  provides a very low harmonic distortion, so that the output waveform is almost purely sinusoidal. The resonant filtering also provides maximum noise immunity and complete isolation between the input and output circuitry. 
     When inductance is added to the circuit by lamp inductance  58  and control inductance  60 , the transformer  26  becomes less resonant, and the gain of the transformer network, i.e. its ability to transfer power into the lighting loop  30 , is reduced. By closing switch  50  in FIG. 2 during a selected portion of each cycle or half-cycle of the oscillation of the resonant circuit, the gain of the transformer  26  can thus be adjusted. 
     The manner in which this adjustment is made is shown in FIG.  3 . In that figure, the inputs to the current control  20  are the voltage across capacitor  46 , the actual loop current signal voltage  34 , and the desired loop current signal voltage generated by the brightness selector  43 . 
     The voltage across capacitor  46 , which is a 60 Hz sine wave, is transmitted through isolation transformer  62  to a filter  64  which removes any spurious frequency components and puts out a clean 60 Hz sine wave  66 . This sine wave is applied to a zero crossing detector  68  which puts out a positive-crossing pulse  70  on rail  72 , and a negative-crossing pulse  74  on rail  76 . The pulses  70  and  74  provide a timing reference to a programmable logic  78  which controls the SCR switch  50  in synchronism with the oscillations of the resonant circuit of transformer  26 . 
     The actual loop current signal  34  is applied through summing resistor  80  to the subtractive input of an integrator  82 , and the loop current reference signal  83  generated by the brightness selector  43  is applied to the same input through summing resistor  84 . As long as the actual loop current is equal to the selected loop current reference, the current signals  34  and  83  (which are of opposite polarity) cancel each other out, and the continuously variable output of integrator  82  stays at a steady error voltage  87  which is applied to the negative input of comparator  88 . The positive input of comparator  88  is a sawtooth generator  90  that resets at each zero crossing of the 60 Hz signal  66 . When the level of the sawtooth wave equals the error signal  87 , the comparator  88  puts out a logic “1” signal that causes the programmable logic device  78  to trigger the switch  50  closed until the next zero crossing of the signal  66 . The zero crossing of signal  66  opens the switch  50  and resets the sawtooth generator  90 . 
     When the brightness selector setting is changed; an extra inductive load due to lamp burnout changes the resonance of transformer  26 ; or some other imbalance between signals  34  and  83  occurs, the integrator  82  translates that imbalance into an increase or decrease in the error signal  86  so as to change the portion of the sawtooth wave during which the SCR switch  50  is closed. The purpose of diode  91  is to prevent overdriving the comparator  88  by limiting the negative change of voltage  86  to a single diode drop. 
     FIG. 5 diagrammatically illustrates the programmable logic device  78 . Programming software within the device  78  is designed by conventional methods to scan the inputs  92  and, based thereon, carry out, i.e., the following functions: 1) following a power-up or shut-down command, the device  78 , by forcing the switch closed, reduces brightness to a selectable low level for a short, selectable time before carrying out the command, so as to prevent large switching transients; 2) selectable out-of-range levels of circuit parameters such as AC input power voltage and lighting loop current are made to cause selectable responses such as alarms, power-downs or shut-downs; and 3) data for front-panel indicators and monitors is generated to show real-time status and operating parameters of the equipment. Because the inventive regulator maintains a constant output current even if the loop shorts out, the logic device reacts only to no-loop-current (i.e. open-loop) and overcurrent conditions. The latter may occur momentarily, for example, when the AC supply is toggled between commercial power and generator power. 
     The efficient, low heat-generating construction of the ferroresonant transformer  26  makes it possible to smoothly power any substantially resistive load from 30 to 50 kW or more without the use of switchable taps or oil cooling. That ability, coupled with the wide-ranging ability of the digital programmable logic device  78  to adjust the regulator&#39;s performance characteristics by software changes, results in a versatile and economic airport lighting control. 
     It should be understood that the exemplary constant current regulator for airport lighting described herein and shown in the drawings represents only a presently preferred embodiment of the invention. Indeed, various modifications and additions may be made to such embodiment without departing from the spirit and scope of the invention. Thus, other modifications and additions may be obvious to those skilled in the art and may be implemented to adapt the present invention for use in a variety of different applications.