Technical Field

This invention relates to a method of supplying power to a lighting plant as disclosed in the preamble of claim 1.

Nowadays, high-pressure gas discharge lamps, also known as metal vapour lamps, eg. halogenide mercury lamps, mercury lamps, White SON lamps, high-pressure sodium lamps and similar lamps, are adapted to be supplied between one phase and zero at 220-230 V.

To start such a lamp requires a voltage difference of approximately 120 V. This means that the lamp does not work continuously, since the voltage between phase and zero varies from -230 V to +230 V at 50 Hz, whereby the lamp is turned off for approximately 2/3 of a sine cycle. As a result, the lamp generates third harmonic currents, said currents collecting in zero, because third harmonic currents are in phase. The lamp forces the network and the power supply connected to said network to generate a third harmonic current, however, in the network's negative sequence system, resulting in a current passing opposite in relation to the first harmonic current for 2/3 of the cycle and parallel to the first harmonic current for 1/3 of the cycle.

This state of the art power supply is explained below with reference to the drawing, Figs. 1A, IB and 1C.

Fig. 1A shows in principle how to supply a high-pressure gas discharge lamp 4, eg. a high-pressure sodium lamp, with power from an alternating current network, said lamp being connected to phase F and zero N of said network via a power

supply circuit 5 provided with two input terminals 6 connected to phase F and zero N, respectively. Depending on which type of lamp 4 is used, the power supply circuit 5 comprises the following components: A capacitor or a capacitor battery 1 for reducing electric noise from the lamp and the supply circuit and preventing said noise from interfering with the network; an inductance coil 3, the main functions of which are to reduce the amount of current drawn from the network and to dampen or equalize the generated third harmonic current; a starter coil or a starter coil circuit 2 generating high frequency voltage pulses in an order of magnitude of 2-4 kV ensuring that the lamp 4 starts each time the voltage across the lamp 4 is sufficiently high for that purpose so that the lamp 4 is lighted during the longest possible part of each half cycle of alternating voltage being applied. Fig. 1A is only one example for a state of the art power supply circuit.

Fig. IB shows a graph of the supply voltage U. during a complete cycle, preferably at a frequency of 50 Hz.

Graph I. represents the current the lamp draws during the two half cycles. The current is substantially zero when the voltage U _]L across the lamp is not sufficiently high to start the lamp. As soon as the lamp can be started, which happens approximately after 1/6 of the cycle, current is drawn and increases, while the voltage increases to a maximum value, whereupon, approximately after 2/6 of the cycle, the current drops again towards zero, because the voltage across the lamp is now too low to maintain a discharge in the lamp. The same procedure is repeated in the next half cycle.

Fig. IB shows clearly that the graph I is not sinusoidal.

Because of this particular mode of operation, a significant

third harmonic current I-, is generated, cf. Fig. IB.

Fig. 1C shows a scaled-down graph of the voltage U-. between phase F and zero N and superimposed a starting voltage U generated by the starter coil or the starting circuit 2, said starting voltage comprising a series of high voltage pulses U _τ of 2-4 kV at high frequency. Thus it is ensured that the lamp 4 starts as soon as the voltage U.. , cf. Fig. IB, is sufficiently high, ie. 120 V, for the lamp to start and a discharge current to pass through the lamp.

In large lighting plants a plurality of lamps is connected to a three-phase network, such that the lamps are connected to zero, which is common to all lamps, and to one of the three phases, usually denoted F.. , F„ and F„ or R, S and T, permitting an even load distribution on the three-phase network. An unfortunate side effect of such a setup is a comparatively large current through the zero wire N, since the third harmonic current generated by the lamps on each phase has to pass through said zero wire, where all third harmonic currents are in phase.

Under field conditions, the amperage through zero is high, usually higher than the amperage through the phase wires. This is highly unsatisfactory, since it requires a robust zero wire with a particularly large cross-section.

It is well-known that the third harmonic current is responsible for a number of disadvantages of the known circuits:

a) The induced large voltage drop in the zero wire reduces the potential difference across the lamps.

b) The large current through the zero wire results in great power losses.

c) Consequently, the efficiency of the power supply is very low.

d) The third harmonic current reduces the lamp life.

e) The transformer supplying three-phase voltage to the lighting plant is exposed to unnessary loads.

f) Great heat development in the lamps causes heat losses to the surroundings.

g) Disconnecting the zero wire results in overload on and destruction of some lamps.

DE-AS no. 1 589 165 discloses a Xenon discharge lamp which is supplied with alternating current by connecting said lamp between two phases of a three-phase network. Lamps of this type are, however, used for special applications and are not based on the use of mercury vapour as starting medium for the electric discharge inside the lamp housing.

Advantages of the Invention

By supplying power to an inventive lighting plant as taught by the characterizing part of claim 1, a number of advantages are obtained while avoiding all disadvantages stemming from conventional phase-to-zero power supply methods as disclosed in the prior art.

Most important of all, the third harmonic current is avoided altogether, since the lamp is lighted continuously and since there is no common zero wire for third harmonic currents to collect in, thus avoiding huge power losses in the supply network, transformers, lamps and cables caused by third harmonic currents. Third harmonic currents result

in increased heat development, thus reducing luminous intensity as well as the life of the lamp and the components involved. Third harmonic currents increase the total amperage through the lamp in the second third of each sinusoidal half cycle of the applied voltage and reduce the amperage to zero during the remainder of the half cycle, resulting in non-uniform heating and unstable operation of the lamp. All these disadvantages are avoided by the invention.

Modern high-pressure gas discharge lamps used for lighting are filled with mercury vapour as starting gas, as mercury permits a suitably low starting voltage of approximately 120 V. Usually, such a lamp also contains a certain amount of sodium (Na) , which is not so easy to light and therefore has to be warmed up by the mercury discharge. Today, mercury sodium discharge lamps are always used as plant lamps in greenhouses etc. and are often used in roadway lighting etc., while pure mercury discharge lamps are used when white light is desired, for example in stadiums etc.

Using a state of the art power supply each lamp must be connected between a phase and zero, because the lamps are made to operate at 230 V. The present invention provides an opportunity to adapt the power supply network to the lamps, for example by connecting a three-phase transformer between the network and the lamps, so that each lamp is supplied with external phase-to-phase voltage in such a way as to prevent the generation of a third harmonic current, since there is no return wire. Lamps of such a lighting plant are lighted continuously, which means increased luminous intensity but reduced power consumption. The latter is due to the fact that there are no power losses caused by third harmonic currents.

An inventive lighting plant as disclosed in the

characterizing part of claim 2 or claim 3 is provided with phase-to-phase voltage being adjustable in a range of 150- 300 V, for example by using an adjustable autotransformer having a primary voltage of 380 V and a secondary three- phase voltage of 0-380 V. Actually, experiments have shown that no detrimental effects are observed when the voltage at each lamp is increased to above 300 V, because the lamp current only increases as a linear function of the voltage and not with a large component of third harmonic current, which, as mentioned above, is avoided altogether. In the absence of a third harmonic current an increase in voltage results in improved luminous intensity without destroying the lamps or reducing their lifes.

The emission spectrum of a lamp depends on the temperature of said lamp and thus the voltage applied. At higher voltages a greater part of the emitted light is in the blue range, ie. the range of visible light or daylight; a great advantage of the present invention. A conventional phase- to-zero power supply cannot be controlled in this manner, ia. because of the third harmonic current component.

It is therefore advantageous, and serves a number of practical purposes, to follow the suggestion of the characterizing part of claim 4, so that luminous intensity and emission spectrum are controlled in accordance with the application of the lighting plant. This is achieved by means of pre-programmed lighting or by means of a pre¬ programmed change in lighting, for example as a function of time or of hour or the like.

The above advantages can be optimized further by the teaching of claim 5, where the control system continuously receives data from the surroundings or from another source, for example data obtained by running measurements of light intensity or emission spectrum etc. in the surroundings,

adjusting the lighting plant to optimum overall illumination. The result is uniform, continuous lighting which can be adapted to changing needs, thereby permitting considerable financial savings.

The invention also relates to a lighting plant as described in the preamble of claim 6, characterized in that the input terminal of the power supply circuit is connected between two phases of a three-phase alternating current network, thus obtaining all the advantages mentioned above without encountering any difficulties. The life of the lamp is considerably prolonged due to the reduction in temperature changes as compared to the changes a lamp is subjected to when using a phase-to-zero power supply. As a result the lamp does not develop leaks, the mercury gas remaining inside the lamp and not being deposited outside the lamp, as is common in connection with a power supply between phase and zero.

An inventive lighting plant corresponding to the teaching of the characterizing part of claim 7 can be controlled as mentioned above, resulting in a number of practical advantages, for example in greenhouses.

An inventive lighting plant as described in the characterizing part of claim 8 is provided with further means to control the emission spectrum by exchanging one lamp with another containing a different substance. When using mixtures of different substances (mixed gasses), the emission spectrum can be altered substantially by changing the voltage across the lamp.

An inventive lighting plant as disclosed in the characterizing part of claim 9 is controlled in a particularly simple way not available when supplying power between a phase and zero. The inventive control method

permits two-stage lighting comprising a stage of maximum intensity and a stage of reduced intensity without any of the lamps being turned on and off.

A preferred embodiment of the inventive method and lighting plant has been developed for growing plants etc. in a greenhouse as disclosed in claim 10, where it is possible to optimize lighting and to adapt light intensity and emission spectrum to the plants cultivated in said greenhouse. Moreover, the lighting can be adapted to the desired speed and form of growth and modulated as a function of the incident sunlight, so that during a period of 24 hours the plants are exposed to a time-flux of light or emission spectrum, which is either constant or changes only according to a pre-defined function, despite of clouds, rain or other etereological phenomena. As a result, running of a greenhouse becomes financially more attractive as the lamps, when modulated down, are less expensive to use.

The method and lighting plant according to the invention also provide considerable practical and financial advantages when used in connection with modulating light at roads etc., as explained in claim 11. The inventive method provides roadway lighting, tunnel lighting, stadium lighting, hall lighting etc. that can be adapted to the surroundings in question, the activities taking place at the site in question.and the natural background lighting.

The above applications can be improved in a novel way by using the inventive method instead of a state of the art phase-to-zero power supply. In a known lighting plant with discharge lamps, light is usually modulated by turning a suitable number of lamps on and off, resulting in uneven modulation and considerably restricted modulation possibilities.

The Drawing

The invention is described in greater detail below and with reference to the accompanying drawing, in which

Figs. 1 A-C show and explain a state of the art method with phase-to-zero power supply of a gas discharge lamp;

Figs. 2 A-B show and explain the method according to the invention;

Fig. 3 shows in principle how lamps are connected to the power supply circuit using the method according to the invention;

Fig. 4 shows several illumination graphs for a gas discharge lamp at different supply voltages;

Fig. 5 shows a lighting plant in a greenhouse according to a field experiment;

Fig. 6 shows a draft method of a lighting plant according to the invention in a greenhouse, and

Figs. 7 A-B shows a control circuit according to the invention.

Best Mode for Carrying Out the Invention

The state of the art method of supplying power to a lighting plant is shown in Fig. 1A-C and explained above. Further explanations are therefore not necessary.

Fig. 2A shows a gas discharge lamp 4, eg. a high-pressure sodium lamp of the Type SON 400 W available from Philips, connected to two phases F of a three-phase alternating current network. The lamp 4 is connected to the network via a supply circuit 5 comprising an inductance coil 3, eg. an inductance coil of the type BSN 400 L33 available from Philips, a starting means 2, eg. a starting means of the type SN 50 or SN 58 available from Philips, and a suitable capacitor 1, eg. a 50 μF capacitor available from Philips. The supply circuit 5 with input terminals 6 may be identical to the one shown in Fig. 1A. Fig. 2A1 shows an alternative supply circuit 5, where the parts identical to the ones of Figs. 1A and 2A are denoted by the same reference numerals.

For clarity reasons only one lamp and one supply circuit are shown, for practical applications a plurality of lamps is used, each lamp having its own supply circuit, said lamps being distributed between the phases as shown in Fig. 3, where three lamps 4 with supply circuits (not shown) are connected between phases, the phase voltages being transformed down to 260 V between two phases.

Fig. 2B shows voltage and current graphs of the circuit of Fig. 2A as a function of the frequency f. A voltage U F„„F is applied to the lamp and the supply circuit and a current I _pF is passing through the lamp, said current being constant when the lamp is lighted. The lighted status is maintained without interruption, since the applied voltage U is constant and higher than the required minimum voltage of approx. 110-120 V for lighting the lamp. The lamp 4 starts as soon as the starting voltage U _τ is applied, whereupon a starting voltage as required by the state of the art technique, cf. Fig. 1C, is no longer necessary. For reasons of clarity the starting voltage U is therefore not shown in Fig. 2B after the lamp has been

lighted .

Since the lamp 4 is lighted continuously and since there is no zero wire, the third harmonic current is not generated either, and accordingly l„ is always zero.

When the applied phase voltage U,r-,Γ--, is increased or decreased, the lamp current I _ increases or decreases correspondingly. Illumination and the lamp's emission spectrum also change in accordance with the above alterations, which is explained below.

Fig. 4 shows the illumination measured in lux as a function of the wave length ■ using one lamp, for example the lamp shown and explained in connection with Figs. 1A and 2A, but with different supply voltages according to the invention.

The graphs are based on the lamp's known properties and specifications and not on direct measurements. The lamp provided with the supply circuit 5 as shown in Figs. 1A and 2A of the drawing is mounted in a holder. Graphs a-f! simply represent expected, relative lighting conditions.

Graph a shows the illumination provided by the lamp when a phase-to-zero voltage of 220 V is applied, ie. the state of the art power supply method. Illumination peaks around 750 nm, ie. the lamp emits red to pink light.

Graph b shows the illumination provided by the same lamp when a voltage of 230 V is applied between two phases, as shown in Fig. 2A. Maximum illumination occurs in the range of around 600 nm, supplying yellow light.

Graph c shows the illumination provided by the lamp of graph b, but the voltage applied being 240 V. Illumination measured in lux is slightly higher and peaks at a somewhat

lower wave length of 550-600 nm, providing slightly more greenish light.

Graph d shows the illumination provided by the lamp of graph b when 260 V are applied. Illumination is increased again, peaking at a wave length of 450-500 nm, ie. in the blue to dark green range.

Graph e shows the illumination provided by the lamp of graph b, this time applying 220 V. The wave length of maximum illumination increases to 600-700 nm, ie. reaching the orange range.

Graph f shows the illumination provided by the lamp of graph b, but this time the voltage is reduced to 160 V. The illumination graph peaks in the range of 700-750 nm, the light has a red to pink tinge. This illumination corresponds to the one on forest soil beneath large trees and is therefore particularly suitable for growing woodland plants such as ivy and the like in a greenhouse. The plants grow extremely well because the emission spectrum corresponds to the spectrum they are exposed to in nature.

Moreover, the power consumption is comparatively low, as the voltage applied is only 160 V. As a result, better plant growth is obtained with less power and thus at lower cost.

In other words, the graphs of Fig. 4 give an indication of the benefits obtainable by controlled growth in a greenhouse, the control not being restricted to controlling the lighting but also encompassing control of the emission spectrum, ie. the colour of light.

The lamp may be filled with other substances or mixtures of substances apart from mercury (Hg) in order to obtain a different emission spectrum which is then adjusted by

changing the phase-to-phase voltage.

By adding, apart from mercury:

lithium (Li) the light becomes red sodium (Na) - yellow potassium (K) - blue-violet calcium (Ca) - violet magnesium (Mg) - blue-green beryllium (Be) - pink

Furthermore, other substances having similar properties and of the same group or sub-group of the periodic table as the above-mentioned substances may be used.

It is well-known that the colour of light greatly influences for example the growth rate of a plant. A person skilled in the art will immediately perceive the numerous possibilities of controlling plant growth etc. not available previously, when a greenhouse was exclusively lighted with sodium discharge lamps with phase-to-zero power supply. Using a phase-to-phase power supply, the voltage across the lamp is adjustable as described above, obtaining previously unattainable spectral distributions of light which can be easily adapted to changing needs.

Fig. 5 shows the setup of a field experiment conducted in the spring of 1994, .growing roses in a greenhouse 7. The greenhouse is positioned geographically with one of the long walls facing north, cf. the arrow indicating north in the drawing. The greenhouse was divided into two areas 9 and 10 by a light-impervious partition 8 made of black, opaque plastics. The two cultivation areas 9 and 10 were identical in size and were adjusted to the same temperature.

In cultivation area 9 nine 400 W lamps were arranged in groups of three between the phases FI, F2 and F3 and connected to the supply voltage via one phase wire and zero, cf. the drawing.

In cultivation area 10 nine lamps 4 of the type used in cultivation area 9 were installed, using the same holders, supply circuits etc. and observing the same distance to the work benches as in cultivation area 9. In cultivation area 10 all lamps 4 were supplied with voltage from two phases, the applied voltage difference being 264 V. The lamps were arranged in groups of three between the phases, cf. the drawing.

After 4 1/2 weeks the roses in cultivation area 10 were ready to be sold, while the roses in cultivation area 9 were first marketable after 6 weeks, the usual growing period for roses. This significant difference occured despite the fact that cultivation area 10 offered slightly worse conditions than cultivation area 9, as cultivation area 10 faced north and received less natural light than cultivation area 9.

Power consumption in both cultivation areas was identical, but in cultivation area 9 part of the power was lost, as it was used to generate third harmonic currents, which do not provide light, but losses in the form of heat.

In a subsequent qualitative evaluation of the plants from the two cultivation areas 9 and 10 the roses of cultivation area 10 got higher marks, because they were more uniform with respect to the number of leaves, buds etc.

The inventive lighting plant did not only result in financial savings and faster growth but also in plants of higher quality, which probably bring higher prices.

Based on the information gained from the experiment explained in connection with Fig. 5 it is for example possible to make and control a power supply for a lighting plant in a greenhouse as shown in Fig. 6. A number of three-phase transformers 16 are supplied with power from a common three-phase transformer 15 controlled by a control means 17, for example a computer or a PLC. The computer or PLC receives data from one or more sensors 18 measuring suitable parameters, such as light intensity on a plant bench, emission spectrum on the plant bench, temperature etc. , and adjusts the voltage across the lamps accordingly. The power supply of the transformer 15 can be a three-phase 15 kV network as shown. When providing a power supply as shown in Fig. 6 and taking into account where north is, cf. Fig. 6, it is preferred to locate the power supply for the lamps connected in parallel on the northern wall of the greenhouse, since there is a certain voltage drop across the supply lines. In this setup a slightly higher voltage is applied to the lamps on the northern wall than to the lamps on the southern wall. However, this is only one example of how such a control can be set up. It is within the scope of the subsequent claims to provide a great variety of methods for supplying power to a lighting plant.

Figs. 7A and 7B show another control method for lamps 4 supplied with power by being connected between two phases, which is suitable for practical application. Fig. 7A shows two lamps 4 connected to phase F2 and phase F3 respectively as well as to phase FI by means of a junction point 20 and a contact breaker 21.

When the contact breaker is open, the two lamps 4 are connected in series, cf. Fig. 7B, between F3 and F2. The lamps are not switched off, but glow with greatly reduced intensity. When more than one set of lamps is used, they

are distributed in the usual way between the phases, so that the load is uniformly distributed in the three-phase network. In this setup the lamps are never switched off, switching-off reducing the lamp life considerably, the power consumption nevertheless being significantly reduced. Such a lighting plant is for example suitable for lighting streets and similar places, where the light is dimmed as soon as nobody is using the road, eg. at night, but where the light must be switched on as soon as a road-user appears. This is achieved by controlling the lighting plant by means of suitable sensors.