POWER SUPPLY FOR NON-LINEAR LOADS

Abstract: The invention relates to a power supply for non-linear loads (2), such as high pressure sodium lamps in greenhouses or road lighting. Non-linear loads connected between phase (R, S, T) and neutral (N) generate third harmonic currents causing losses and temperature rise in for example conductors and transformers. To reduce the third harmonic current a zero-inductor (6) is inserted either with one winding on a common core in series with each of the phase conductors, or just one winding in series with the neutral conductor. In a preferred version the conductors (R, S, T or N) are threaded through the core forming single-turn windings. In a further development of the invention the core is fitted with an air gap (9) and a multi-turn winding (8) connected to a capacitor (7), which together with the zero-inductor constitute a parallel resonant filter tuned to a frequency in the vicinity of that of the third harmonic.

Description: Power supply for non-linear loads. The present invention relates to three-phase power supplies for non¬ linear loads, especially to such with lamp fittings for gas discharge tubes, like high pressure sodium lamps. Electric plants with a great number of fittings in parallel are used for example in green house cultivation and for road lighting. A discharge tube constitutes from an electrical point of view a non- linear element, which requires a current-limiting inductor in series, and which generates an approximately square wave load voltage, causing harmonic currents in the supply network. The inductor makes the current from the supply highly inductive, and for phase compensation a capacitor is usually connected in parallel to the inductor plus lamp. If this is not done, the fundamental compo¬ nent of the supply current can be more than twice as high, as when compensated, and the power losses in the network increase con¬ siderably. Inserting capacitors in a network may cause problems because of re ¬ sonance phenomena between them and the inductance of the net-work, mainly originating from the supply transformer of the plant. Resonance implies amplification of harmonic currents and gives rise to increased losses in bars, cables and transformers. A typical supply voltage for a lamp fitting is 230 V, corresponding to the phase voltage of a 400 V system. The fittings are therefore usually connected between phase and neutral in a three-phase system. The dominating harmonic is the third, which has the property of hav¬ ing the same phase position in all phases. In other words, it appears as a zero-sequence current. It adds up in the neutral conductor, which therefore carries a three times as big third harmonic current as the phase conductors, and it appears as a circulating current in the primary delta winding of the supply transformer, causing losses and heating. In new plants the problem can be solved by havin each of the ca- pacitors series-connected with an inductor (see Swedish patent ap¬ plication 9500380-2). In existing plants with many lamps it is expensive to alter all lamp fittings, and a solution is needed for larger groups of fittings. The invention aims at a solution which in a simple, inexpensive and flexible way reduces the third harmonic current to a harmless level. What characterises a power supply according to the invention is disclosed in the attached patent claims. The invention is in the following described referring to the attached Figures 1 - 6. Figure 1 shows a three-phase plant of current design, Figure 2 an equivalent diagram, Figure 3 an equivalent diagram of a plant according to one alternative of the invention and Figure 4 to another alternative. Figure 5 shows the zero- inductor in a preferred version and Figure 6 a further developcment of the invention in a preferred version. Figure 1 shows a three-phase plan t wilh a Δ/Y-0 connected trans¬ former 1 having the loads 2 connected between phases and neutral. Figure 2 shows a simplified diagram for a group of lamp fittings, drawn as one fitting 2 per phase, with lamp 3, series inductor 4 and capacitor 5. The third harmonic component of the current 1(3) has the same phase position in all three phases and adds up in the neutral conductor to Iθ(3 = 3 ' 1(3)- Figure 3 shows one version of the zero-inductor 6. It has three essentially equal windings each of them connected in series with a phase coductor all magnetizing the core in the same direction. It is so dimensioned, that its impedance for the third harmonic, generated by the lamps, is high. In consequence the main part of this current finds its way through the phase-compensating capacitors and only a minor part flows through the network. The zero-inductor effectively impedes a third harmonic amplification caused by a resonance between capacitors and network impedance. The third harmonic voltage across a winding of the inductor is in the order of 10 V when the phase voltage is 230 V. Figure 4 shows a power system with the neutral conductor (N) threaded through the zero-inductor 6 forming a single-turn winding. It is dimensioned in the same way as above. The zero-inductor encircles the inductor without breaking it, which is desirable from a safety point of view. Figure 5 shows a similar system wi th the three phase conductors threaded through the zero-inductor. It is dimensioned using the same principles as outlined above and it functions in the same way. An advantage is, that it can be placed anywhere in the network independ¬ ently of if it is a 4- or 5-conductor system. In a symmetrical three-phase power system with a symmetrical load, no fundamental current flows through the neutral conductor, and the sum of the three phase currents is zero. In praxis no plant is completely symmetrical, and with no zero-inductor the fundamental zero current Iθ( l ) > 0- Its magnitude depends on the degree of asymmetry. For Iθ( l ). too, the zero- inductor acts as an impedance, and under normal service conditions a fundamental voltage Uθ( l ) applies itself across it, causing an (insignificant) phase voltage asymmetry . In a preferred version of the invention the iron core of the zero-inductor has no air gap and can be a ring core made of oriented transformer iron. It is designed for a few volts of fundamental voltage. For higher voltages it will saturate, and then the zero-inductor only represents a low impedance. This design keeps down the size of the zero-inductor to a minimum and also ensures that no large voltage asymmetries appear, if the load current becomes asymmetrical, for example when a fuse breaks. Figure 6 shows a further development of the invention. The zero- inductor 6 is provided with one or more multi-turn windings 8 connected to circuits 7 comprising capacitors. By appropriate choice of zero-inductor air gap 9, number of turns of the windings and capacitor data one can achieve, that the device to the network appears as a parallel resonant circuit tuned to a frequency in the vicinity of that of the third harmonic. Because of its high impedance to the third harmonic, the third hamonic currents will be reduced to a harmless level. For the resulting fundamental current, deriving from system asymmetry, the impedance can, however, be kept low. This tolerance towards asymmetry also implies, that the power system can be suit¬ able for supplying other types of non -linear loads, such as computers and copying machines, where larger asymmetries may be expected than in pure lighting systems.

Claims: Claims: 1. Power supply (I) for loads (2). or groups of loads, comprising non¬ linear loads connected between phase and neutral, ch arac ter¬ ized in, that in order to reduce third harmonic currents generated by said loads, an inductive element, a zero-inductor (6), is connected either in series with the phase conductors (R.S.T), and/or in series with the neutral conductor (N) in such a way that its continuity is unbroken. 2. Power supply according to claim 1, c haracterized in, that the non-linear loads (2) consist of lamp fittings with gas discharge tubes, such as high pressure sodium lamps. 3. Power supply according to claims l or2, characterized in, that the zero-inductor (6) has three windings with essential the same number of turns, each of said windings connected in series with a power supply phase (R,S,T). 4. Power supply according Lo claims 1 or 2, characterized in, that the neutral conductor (N) is threaded through the core of the zero-conductor (6), thereby forming a single- turn winding. 5. Power supply according to claim 3, c aracterized in, that the three phase conductors (R.S.T) arc threaded through the core of the zero-inductor (6), thereby forming three single-turn windings magnetising the core in the same direction. 6. Power supply according to claim 4 or 5, characterized in, that the zero-inductor has a ring core of orientated transformer iron, which saturates if the voltage across it reaches a certain level, preferably corresponding to a few volts of fundamental frequency. 7. Power supply according to any of the claims 4-6, charac ter- i z e d in, that the zero-inductor (6) has an air gap (9) and also is provided with one or more multi-turn windings (8) connected to circuits (7) comprising capacitors, which together with the zero- inductor (6) constitu te a parallel resonance circuit tuned to a frequency in the vicinity of that of the third harmonic.

Inventors: THORBORG KJELD, THORBORG, KJELD
Applicants: SWEDEPONIC INTERNATIONAL N V, THORBORG KJELD, SWEDEPONIC INTERNATIONAL N.V., THORBORG, KJELD
Application year: 1996
Main classification: H02J3
IPC codes: H05B41