FIELD LIGHTING INSTALLATION

Abstract: A field lighting installation includes a plurality of series connected light fittings, supplied from an A.C. mains via a converter unit, which is adapted to convert the substantially constant voltage obtained from the mains to a substantially constant current in departing lines containing the light fittings. A current controlled regulating unit (12) is associated with each light fitting or group of fittings for regulating the current through the associated lamp or lamps (6).

Description: Field lighting installation The present invention relates to a field lighting in¬ stallation including a plurality of series connected light fittings, supplied from an A.C. mains via a converter unit adapted to convert the substantially constant voltage obtai¬ ned form the mains to a substantially constant current in departing current lines containg the fittings. An installation of this kind is described in the US patent specification 4 754 201. The traditional method of controlling and monitoring field lights on an airfield is to supply power to the diffe- rent light configurations via a so-called parallel system or a so-called series system, cf figures 1 and 2. In such a case, the regulating and monitoring unit is centrally placed in a cabinet or the like, and its regulators provide either a constant voltage (parallel system) or a constant current (series system) to the different power supply cables to the different field light configurations. The object of the present invention is to achieve a field lighting installation of the kind mentioned in the introduc¬ tion, wherein individual control of the light fittings, or groups thereof, is possible while cable costs are conside¬ rably reduced at the same time. This object is achieved with an installation having the characteristing features disclosed in claim 1. In the installation according to the invention different light configurations are accordingly supplied by one or more transformers, implemented such that they may be regarded as representing current supply sources. Each light fitting is provided with a local regulating and monitoring unit, which suitably obtains its control information via signals carried by the power cable, a separate control cable or by radio. In the installation in accordance with the invention there is thus used a "current supply" network where the prevailing output voltage will be a function of the prevailing load. The advantages accompanying the use of such a current supply system in a field lighting installation for airfields are as follows: 1) The lamps have a resistance that varies heavily, depending on the filament temperature, a current supplying system then providing a smooth successive voltage increase across the lamp, whereas a voltage supplying system results in severe currrent surges when the lamp is turned on; 2) the lamps are spread over large areas, and if a current supplying system is used, single conductor, high- voltage cables, typically for 5 k , can be used for the supply, which considerably reduces cable costs; and 3) current transformers are cheaper than corresponding voltage transformers. In accordance with an advantageous embodiment of the installation according to the invention the converter unit adapted for converting the voltage obtained form the A.C. mains to a substantially constant current is a so-called Boucherot circuit with a series resonance circuit, tuned substantially to the mains frequencey. This is a simple and advantageous method of obtaining a current source having an indefinite EMF behind an infinite impedance. The Boucherot circuit is described more in detail by E. Arnold, Die echselstromtechnik, Erster Band, Zweite Auflage, Verlag Julius Springer, Berlin, pp 141-4. According to another advantageous embodiment of the installation according to the invention the converter unit includes a further inductance in series with a load connected to the converter unit. If this inductance is of the same magnitude as the one included in the series resonance cir¬ cuit, there will be obtained the advantage that during idling, i.e. shortcircuiting of the current system, the current in the network ideally will be zero. Another advantage in the utilisation of this special Boucherot circuit is that the effect on the network is small and that the sinus wave shape is maintained essentially unaffected, which facilitates signal transmission over the power cables, and is generally advantageous in applications for airfields, where a low interference level is essential. In accordance with a further advantageous embodiment of the installation acording to the invention the regulating unit includes a counter synchronised with the current zero crossings and provided with its own oscillator controlled by a binary number. This binary number can be varied individual¬ ly for each lamp, and is determined preferably from a central control system. In accordance with a still further advantageous embodi- ment of the installation according to the invention the regulating unit includes a triac connected in parallel with the light fitting lamp, for regulating the current through the lamp by controlling the ignition time. The installation in accordance with the invention also preferably includes a safety system, suitably having three levels, since a fault that could lead to an open circuit would cause impermissibly high voltages, The installation according to the invention therefore includes transient protection, primarily in the shape of a component, e.g. a type of two-way Zener diode, which is connected across each lamp and which is short-circuited (not interrupted) when it is driven outside its operating range. As further protection, the triac can be disposed such that in response to over- voltage occurring across the lamp it is forced to a permanent "on" state for short-circuting the transients, and as a third protection means there can be arranged a (mechanical and/or electronic) device for short-circuiting any occurring over- voltages, if these are not short-circuited by the other protective means. In order to explain the invention in more detail, an embodiment of the installation according to the invention; selected as an example, will now be described while referring to figures 3-8. On the drawings, figures 1 and 2 illustrate the prin- ciples of so-called parallel and series supply, respectively, for filed lightings on an airfield according to prior art. Figure 3 illustrates the principle of the installation according to the invention and Figure 4a illustrates the basic implementation of the so-called Boucherot circuit included in the converter unit of the installation according to the invention, and Figure 4b illustrates the electrical properties of the circuit. Figure 5 illustrates a further development of the Boucherot circuit, Figure 6 illustrates the further developed Boucherot circuit of Figure 5 included in the installation according to the invention, Figure 7 schematically illustrates an example of a local regulating and monitoring unit in the installation according to the invention and Figure 8 illustrates the unit of Figure 7 in more detail. In Figure 3 there is schematically illustrated an em¬ bodiment of the installation according to the invention, in which a series system of a plurality of light fittings is supplied from a current generator 10. Each fitting includes a lamp 6 as well as a local regulating and monitoring unit 12. The output voltage is not regulated, and becomes a function of the prevailing load. The regulating and monitoring units 12 are given their control information, suitably from a central control system, by signals carried on the power cable, a separate control cable or by radio. The current source is realised by a converter unit supplied from an A.C. mains having substantially constant voltage. This converter unit converts the voltage obtained from the mains to a substantially constant current in the departing lines which include the light fittings. The converter unit includes a Boucherot circuit, illu¬ strated in its basic implementation in Figure 4a. The circuit contains a series resonance circuit formed of an inductance L N and a capacitor C and is tuned substantially to the mains frequency. The properties of the Boucherot circuit are as follows. When it is supplied with the voltage U N from the mains the voltage seen from the load side is infinitely great when the load impedance goes towards infinity and for a short- circuited load the impedance is formed of the reactance in the inductance L JJ , cf Figure 4b. By applying Thevenin's theorem, the circuit may be represented by an infinitely great EMF behind an infinite impedance, i.e. it constitutes a current source. The magni- tude of the current is: I=U N /X, where X = CO I*, is the reac¬ tance of the inductance, and this current is equal to the short-circuiting current. When the circuit is short-circuited the current in the load line is I N = I and is purely induc- tive. In Figure 5 there is shown a further advantageous deve¬ lopment of the Boucherot circuit, which is used in the installation according to the invention. In this embodiment a second inductance 1^ is connected in series with the load Z^,. If the inductance L j is of the same magnitude as the series resonance circuit inductance 1^, one of the advantages with this embodiment is that the mains current I N is equal to zero, when the system is short-circuited, i.e. in a no-load state, since I_ 2 and C are in parallel resonance. In the description so far of the Boucherot circuit the load has been assumed to be linear, namely a resistance in series with an (ideal) inductance. In the installation according to the invention, the load consists of a resistan¬ ce, i.e. the lamp 6, which is connected in parallel with a triac 8, cf figures 6-8. The effective value of the current through the lamp can then be varied by varying the ignition angle of the triac 8. This combined load is non-linear, but in spite of this the current from the Boucherot circuit is practically sinusoidal, due to the inductance 1^ at the out- put. As previously mentioned, this afffords important advan¬ tages. When the triac 8 is disconnected at the beginning of each half period the Boucherot circuit is resistively loaded, and when the triac 8 is connected for the rest of the half period the Boucherot circuit is short-circuited. The wave form of the voltage across the load is also formed of a part of a sinus form that can be divided into fundamental tone and overtones. The overtones will be (almost) filtered away by the inductances and capacitance of the circuit, while the fundamental tone of the voltage can be divided into an active component, in phase with the current, and a reactive compo¬ nent, phase shifted 90° forwards of the current. In other words, the load acts as a resistive-inductive load. In Figure 6 there is shown an example of a series system of field lights of the kind to which the invention relates, and supplied from a Boucherot circuit via a current trans¬ former 14 on the output side. The series line is loaded by a plurality of current transformers 2, each of which is connec- ted to one or more light fittings on the secondary side. Via a switch 16 the Boucherot circuit is connected between the phases of an ordinary 3-phase mains 18. Several such circuits can be connected distributed between the phases of the mains to balance the 3-phase load. As already mentioned, the installation must be provided with protective means, since very high voltages will occur if a light fitting should form an open circuit, e.g. because of a lamp failure. The triac 8 connected in parallel with the lamp 6 is adapted to be permanently turned-on for short-circuiting the lamp, should the lamp fail. If the circuit for turning on the triac should not enter into fuction, there is a second over- voltage protection in the form of a two-way Zener diode 20 connected across the lamp 6, and it will be short-circuited if an overvoltage occurs across the lamp. The Boucherot circuit is further protected by a short-circuitng means comprising two anti-parallel connected thyristors 22 across the output transformer 14. If the line with the transformers should form an open circuit, e.g. due to a lamp failure, and the voltage across the transformer 14 rises, the short- circuiting means 22 will start to function and short-circuit the Boucherot circuit. If the operation mechanism of the short-circuiting means 22 should not function a break-down will occur in the thyristor as a result of the overvoltage, and a permanent short-circuit will be established. Only a limited overvoltage will appear in the installation for a very short time, and this overvoltage can be used to activate an alarm and for triggering the switch 16, suitably with time a delay of a few periods, so that the current has time to be decay. The installation shown in Figure 6 thus includes a threefold overvoltage protection. As mentioned above in connection with the description of Figure 3, each light fitting includes a local regulator unit 12 (not shown in Figure 6) . An example of such a unit is cursorily illustrated in Figure 7. The regulating and monitoring unit includes a conventio¬ nal current transformer 2, as isolation between the power supply 4 and the lamp 6, as well as a triac 8 connected in parallel with the lamp 6, for regulating the light intensity of the latter. Thyristors can be used instead of the triac 8 for regulating illumination. The current transformer 2 drives a constant current through the secondary side and with the triac 8 not turned on the entire secondary side current flows through the lamp 6. By gradually turning on the triac 8 there is obtained a gradually decreasing current through the lamp 6. The light intensity from the lamp can thus be regulated in this way, which will be explained in greater detail below in connection with Figure 8. The regulating and monitoring unit illustrated in figures 7 and 8 may be essentially divided into: Power supply, detec¬ tor, counter and amplifier. The power supply includes an auxiliary transformer 24, which may be a current transformer having a high transforma¬ tion ratio, the secondary side of which is connected to a rectifier bridge 26. The rectified output voltage from the rectifier bridge 26 is smoothed by a capacitor 28 and stabi¬ lised by a Zener diode 30. The detector is connected to the A.C. terminals of the rectifier bridge 26, where the voltage has a square wave configuration and is in phase with the current in the line containing the light fitt; s. The steepness of the f "* a ks of the square wave are improved with the aid of comparat :s 32, 34 and the square wave is converted into a short pulse PE, which is repeated every half period by transferring the output volt, jes of the comparators 32, 34 to the base of a transistor 36 via their respective capacitors 38, 40. This zero point detector will thus send a pulse PE for each zero crossing of the current in the line containing the light fittings. The counter includes a crystal-controlled oscillator with a binary counter 42, which generates a clock pulse Cl, which in turn clocks a following 8 bit binary count-down counter 44. The count-down counter 44 is activated by the pulse PE, which sets it to the binary number N, to be found at the inputs JO, J1...J7. After N counts the count-down counter 44 delivers a short output pulse CO-. This pulse CO sets an RS- flipflop to zero 46, which is set to the "one" state by the pulse PE. The pulse CO sets the output of the flipflop 46. to 0, in which state it remains for the rest of half period. The output signal P is amplified in the amplifier 48 and forms the control pulse turning on the triac 8, which is turned on for P=0. The pulse trains PE, CO and P are shown in the upper right- hand part of Figure 8. The binary number N is individual for each lamp 6 and is transferred ot the address of the light fitting in question from a computer in the central control system. This transfer is most cheaply achieved by using the power cable, but it can also be effected via separate signal cables or per radio, as already mentioned. It has also been mentioned earlier that there is a means for turning the triac into a permanent on-state if there should be a lamp failure, and suitably there are also means (not shown) for sensing the condition of the lamp 6 and sending information thereon back to the central control system computer, which can thus keep count of which lamps need to be changed.

Claims: Claims 1. Field lighting installation, including a plurality of series connected light fittings supplied from an A.C. mains via a converter unit, adapted to convert the substantially constant voltage obtained from the mains to substantially constant current in departing current lines containing the light fittings, c h a r a c t e r i s e d in that a regulator unit supplied with current is associated with each fitting or group of fittings for individual regulation of the current passing through the associated lamp or lamps. 2. Installation as claimed in claim 1, c h a r a c t e r- i s e d in that the converter unit includes a Boucherot circuit having a series resonance circuit, substantially tuned on the mains frequency. 3. Installation as claimed in claim 2, c h a r a c t e r- i s e d in that the converter unit further includes an induc¬ tance in series with a load connected to the converter unit, said inductance being preferably of equal magnitude as the inductance included in the series resoncance circuit. 4. Installation as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the regulator unit in¬ cludes a counter synchronised to the zero crossings of the current, the counter being intended for current regulation controlled by a set binary number. 5. Installation as claimed in any one of claims 1-4, c h a r a c t e r i s e d in that the regulator unit includes a triac or thyristor connected in parallel with the lamp of the light fitting or regulating the current through the lamp. 6. Installation as claimed in any one of claims 1-5, c h a r a c t e r i s e d in that the regulator unit also in¬ cludes means for monitoring the operational state of the lamp in the light fitting. 7. Installation as claimed in any one of claims 1-6, c h a r a c t e r i s e d in that there is provided overvol- tage protetction in the form of a component, preferably a two-way Zener diode, which is short-circuited when it is driven outside its oepration range, said component being connected across each lamp. 8. Installation as claimed in any one of claims 5-7, c h a r a c t e r i s e d in that the triac connected in parallel with the lamp is adapted to be forced in a permanent on-state in response to the occurrence of overvoltage across the lamp to short-circuit until a resetting signal is given. 9. Installation as claimed in claim 8, c h a r a c t e r- i s e d in that a short-circuting means is arranged across the primary side of a transformer connected to the output of the Boucherot circuit for short-circuiting the transformer if an overvoltage should occur. 10. Installation as claimed in any one of claims 1-9, c h a r a c t e r i s e d in that the regulator units are adapted for being controlled from a central control system.

Inventors: BAECKSTROEM GOERAN, THORBORG KJELD, BAECKSTROEM, GOERAN, THORBORG, KJELD
Applicants: SWEDISH AIRPORT TECHNOLOGY HB
Application year: 1990
Main classification: H05B37
IPC codes: H05B37, H05B39