Emergency mode voltage-controlled power supply

Abstract

This invention relates to a method and system for providing a trade-off between operating range and grid load / stability. This is achieved by increasing the operation range by allowing a driver (e.g., an LED driver) to operate at lower voltages but at lower power levels. The driver may then use the DC input voltage level as an indication of the maximum output power. In a lighting system example, a lighting controller may communicate via a wired (e.g., DALI) or wireless (e.g., Interact) communication link to luminaires. Both, the lighting controller and the luminaires may be powered from an industrial DC power system. The lighting controller may be configured to measure the DC supply voltage. Users of the lighting system can adjust various lighting control parameters with a user interface, e.g., a touch display or an app on a portable device. To support the energy saving in an emergency power operation, users can adjust one or more parameters of the lighting control system via the user interface.

Description

[0001] Emergency mode voltage-controlled power supply

[0002] FIELD OF THE INVENTION

[0003] The invention relates to the field of direct current (DC) power supply systems which may be used lighting systems or other systems.

[0004] BACKGROUND OF THE INVENTION

[0005] CO2-neutral industry is a central basis for transformation to a climate-neutral society. An important contribution to this is the supply of factories and plants with DC power. The so-called DC-INDUSTRIE system concept describes an open system for efficient integration of renewable energies with reduced energy requirements for sustainable production. To this end, a so-called Open Direct Current Alliance (ODCA) including 56 companies from industry, academia and research has been founded by ZVEI e. V. with support of the Federal German Ministry for Economic Affairs and Climate Action. The goal is to build a worldwide direct current ecosystem and establish direct current technology across applications.

[0006] Further details can be gathered e.g. from System concept DC-INDUSTRIE2, ZVEI & consortium DC-INDUSTRIE2, 29 April 2022 (grant number 03EI6002A-Q).

[0007] This system concept describes properties of industrial low-voltage DC (LVDC) installations based on industrial DC grids that are characterized by many electrical drives that already use DC internally even in existing alternate current (AC) installations. DC-INDUSTRIE grids connect DC links with each other as well as with storage (e.g., batteries), renewable energy supply such as solar power, and a connection to the AC supply grid. This enables direct use of recuperation energy within the DC grid rather than dissipating braking energy in resistors. Storage within the DC installations provides ride-through capacity for power outages thus reducing downtime significantly. The stored energy also provides an efficient way to reduce infeed power from the supply grid. 80 % reduction of infeed power was reached in one of the model applications - 50 kVA for DC rather than 450 kVA in the AC case. The operating voltage of LVDC grids is 620 V to 750 V when the connection to the AC grid is made with a bi-directional active infeed converter (AIC). Such AICs are capable of supplying energy in both directions, i.e., they can also feed energy back into the AC grid and thus support supply grid stability. For an uncontrolled rectifier (e.g., a B6- bridge), the voltage range may be from 485 V to 750 V.

[0008] A driver for a network load (e.g., an outdoor luminaire driver connected to an AC mains voltage) can be configured to turn off its output (shutdown) in case of an excessive low mains voltage. Depending on the driver type, a shutdown could occur between 150 and 180 V AC. This shutdown functionality may be intended to prevent overcurrent conditions in the mains grid. Without shutdown, the driver input current may increase (up to 2.5 times) with decreasing mains voltage since the driver is designed to maintain full light output of the luminaire and thus output power. This would further exacerbate the undervoltage condition further. Consequently, mains cables and relays may be overloaded and melting fuses and / or miniature circuit brakers (MCBs) may trip.

[0009] However, a disadvantage of such a shutdown functionality is a resulting total loss of lighting.

[0010] Therefore, a more flexible control option for power supply grids is desired for mixed AC and DC power supply grids.

[0011] SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to provide a controllable emergency mode for a power supply grid.

[0013] This object is achieved by an apparatus as claimed in claim 1, by a lighting controller as claimed in claim 8, by a lighting system as claimed in claim 9, by a method as claimed on claim 10, and by a computer program product as claimed in claim 15.

[0014] According to a first aspect, an apparatus for controlling an emergency mode of a power supply system that supplies power to one or more load devices is provided, wherein the apparatus is configured to allow an input of one or more power control parameters via a user interface and to control an energy saving characteristic in the emergency mode of the power supply system based on the one or more power control parameters.

[0015] According to a second aspect, a lighting controller of a lighting system is provided, the lighting controller comprising an apparatus of the first aspect.

[0016] According to a third aspect, a lighting system comprising a DC power supply system and a lighting controller of the second aspect is provided. According to a fourth aspect, a method of controlling an emergency mode of a power supply system that supplies power to one or more load devices is provided, wherein the method comprises: inputting one or more power control parameters via a user interface; and controlling an energy saving characteristic in the emergency mode of the power supply system based on the one or more power control parameters.

[0017] According to a fifth aspect, a computer program product is provided, which comprises program code for producing the steps of the above method of the fourth aspect when run on a computer device.

[0018] Accordingly, a trade-off between operating range and grid load / stability can be provided by allowing a load driver to operate at lower voltages but at lower power levels during the emergency mode. The actual supply voltage level may be used as an indication of the maximum output power. To support the energy saving in the emergency mode, users can adjust one or more power control parameters via the user interface.

[0019] In a lighting system example, a lighting controller may communicate via a wired or wireless communication link to luminaires. Both, the lighting controller and the luminaires may be powered from an industrial DC power system.

[0020] According to a first option of any of the first to fifth aspects, a voltage level of the supply voltage of the power supply system may be determined and the emergency mode may be triggered when the determined voltage level has fallen below a predetermined trigger voltage level defined by one of the one or more power control parameters. A wired or wireless communication link to luminaires may not work anymore because the supply voltage is too low below this trigger voltage level.

[0021] According to a second option of any of the first to fifth aspects, which may be combined with the first option, a power level supplied to the one or more load devices may be controlled as a function of the determined voltage level during the emergency mode.

[0022] According to a third option of any of the first to fifth aspects, which may be combined with the first or second option, the power supply system may be a DC power supply system for a lighting system, wherein the one or more load devices may comprise one or more luminaires of the lighting system, and wherein the energy saving characteristic may be a dimming characteristic of the one or more luminaires of the lighting system during the emergency mode.

[0023] According to a fourth option of any of the first to fifth aspects, which may be combined with any of the first to third options, the one or more power control parameters may comprise a maximum luminous flux and a lower luminous flux of a dimming range of the dimming characteristic.

[0024] According to a fifth option of any of the first to fifth aspects, which may be combined with any of the first to fourth options, the dimming characteristic may be defined by subtracting from the maximum luminous flux a value determined as a predetermined function of a determined actual voltage level of the supply voltage of the power supply system.

[0025] It is noted that the above apparatus may be implemented based on discrete hardware circuitries with discrete hardware components, integrated chips, or arrangements of chip modules, or based on signal processing devices or chips controlled by software routines or programs stored in memories, written on a computer readable media, or downloaded from a network, such as the Internet. More specifically, the software may run on a virtual device such as a Docker Container, or virtual machine, which may be implemented on a physical server on a respective premise or as part of a cloud infrastructure.

[0026] It shall be understood that the apparatus of claim 1, the lighting controller of claim 8, the lighting system of claim 9, the method of claim 10, and the computer program product of claim 15 may have similar and / or identical preferred embodiments, in particular, as defined in the dependent claims.

[0027] It shall be understood that a preferred embodiment of the invention can also be any combination of the dependent claims or above embodiments with the respective independent claim.

[0028] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0029] BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In the following drawings:

[0031] Fig. 1 shows schematically a block diagram of a power control system with emergency mode according to various embodiments;

[0032] Fig. 2 shows schematically a diagram of a controlled emergency operating mode according to an embodiment; and

[0033] Fig. 3 shows a flow diagram of control process in an emergency operating mode according to an embodiment. DETAILED DESCRIPTION OF EMBODIMENTS

[0034] Various embodiments of the present invention are now described based on a lighting system with DC power supply, e.g., as defined in the initially described DC- INDUSTRIE concept. The lighting system may be a solid-state lighting system comprising luminaires which can be any type of lighting unit or lighting fixture that comprises one or more light sources (e.g., visible or non-visible (infrared (IR) or ultraviolet (UV)) light sources) for illumination and / or communication purposes and optionally other internal and / or external parts necessary for proper operation of the lighting, e.g., to distribute the light, to position and protect the light sources and ballast (where applicable), and to connect the luminaires to a power supply. More specifically, luminaires may comprise semiconductor light emitting diodes (LEDs), semiconductor lasers, vertical -cavity surface emitting lasers (VCSELs), organic light-emitting diodes (OLED), or polymer light-emitting diodes (PLED) as sources of illumination or light sources in visible or non-visible light spectra.

[0035] The following embodiments may be implemented in connection with any type of luminaire module or board and may be applicable to various kinds of luminaire drivers.

[0036] Solid-state lighting (SSL) is a type of lighting that uses semiconductor lightemitting diodes (LEDs), organic light-emitting diodes (OLED), or polymer light-emitting diodes (PLED) as sources of illumination rather than electrical filaments, plasma (used in arc lamps such as fluorescent lamps), or gas. Solid state electroluminescence is used in SSL, as opposed to incandescent bulbs (which use thermal radiation) or fluorescent tubes. Compared to incandescent lighting, SSL creates visible light with reduced heat generation and less energy dissipation.

[0037] A luminaire driver may be required to deliver a highly stable constant current to the luminaire(s) irrespective of variations in the luminaire characteristics or the supply voltage while complying with increasingly stringent regulations covering power factor and harmonic distortion. Furthermore, the driver may be used to store luminaire-specific information (such as occupancy, activity patterns, changes in temperature or humidity, daylight levels, etc.).

[0038] A mains guard function or functionality is understood as a control function or functionality that is used to control a load device when an AC mains voltage is becoming too low (e.g., lower than 180 V in case of a light emitting diode (LED) driver for 230 V AC mains voltage). Thereby, the load device (e.g., a luminaire) and mains grid can be protected against the initially described overcurrent conditions without loss of lighting. In case of drivers operating at a DC voltage, a minimum input voltage may be defined at which the luminaire driver may operate. A voltage below e.g. 404 V may cause the driver to shut down. Below this voltage level, the DC grid may not be stable enough and the loads (e.g., luminaires) may need to be disconnected from the grid.

[0039] An example of a mains guard function or functionality may be to decrease input load current as a function of a decreasing mains voltage. This decrease may be continued down to a lower mains voltage before an eventual shut-down of the driver output. As a further option, a small hysteresis of e.g. 5 to 10V against on / off nuisance cycling may be implemented before the driver output becomes automatically active again once the mains voltage starts to recover.

[0040] Benefits of such a mains guard function or functionality include that load(s) and grid are protected against undervoltage and overcurrent, that MCBs, fuses and / or relays are protected against current overloading, and / or that power supply (e.g., light) can remain on even at excessive low mains voltages.

[0041] It may however be desirable to maintain the power supply switched on as long as possible for safety reasons (e.g., keeping the lights on).

[0042] In embodiments, it is therefore proposed to provide an enhanced emergency mode with a trade-off between operating range and grid load / stability. This can be achieved by allowing the load driver (e.g., a luminaire driver) to operate at lower voltages but at lower power levels to thereby increase the operation range.

[0043] Fig. 1 shows schematically a block diagram of a power control system with emergency mode according to various embodiments.

[0044] It is noted that - throughout the present disclosure - the structure and / or function of blocks with identical reference numbers that have been described before are not described again, unless an additional specific functionality is involved. Moreover, only those structural elements and functions are shown, which are useful to understand the embodiments. Other structural elements and functions are omitted for brevity reasons.

[0045] According to Fig. 1, a DC power grid with a supply voltage VDC comprises a first line L+ for a positive polarity, a second line L- for a negative polarity, and a third line PE for ground potential or another reference potential.

[0046] The grounding concept via the third line PE may be implemented as "AC-side grounding" where the ground reference may be realized via a star point grounding at a transformer of an AC grid (mains grid) connected to the DC power grid. Further low- impedance groundings as connections from L+ or L- to PE may not be permitted in the DC grid or in the connected devices. The operational grounding may take place centrally at the transformer. If there are several feeding transformers, the star points may need to be connected to each other with low impedance so that no significant potential differences occur between the grounding points (e.g., to prevent corrosion via DC stray currents).

[0047] The DC power grid may be set up in a bus or ring structure with at least two DC sectors connected via a DC breaker (not shown in Fig. 1) which is a circuit breaker used in DC power distribution systems to prevent overcurrent and potential hazards. It may be configured to interrupt the flow of current if it exceeds a predefined threshold, and / or to protect electrical components from damage caused by excessive current, and / or to prevent electrical fires and other safety hazards that can result from overcurrent conditions. In general, the DC breaker may include one or more of a DC miniature circuit breaker (MCB), a DC molded case circuit breaker (MCCB), and a type B residual current device (RCD). Compared to a bus structure, a ring structure offers the advantage that in case of a disconnection of one connection (for instance, in a substation), all DC sectors can still be supplied with power.

[0048] Behind each DC breaker a subordinate hierarchy level may optionally be built up, which can be designed as a bus or ring. Theoretically, this methodology can be broken down "downwards" as far as desired.

[0049] In embodiments, the lighting system comprises one or more lighting controllers (LC) 10 configured for wired (W), e.g., Digital Addressable Lighting Interface (DALI), or wireless (e.g., a portfolio of connected lighting software applications for the Internet of Things (loT) that delivers data-driven insights and services to users and managers of illuminated spaces) communication to luminaires (LUM) 20.

[0050] DALI is a digital communication protocol commonly used in lighting systems. Using DALI, it is possible to send dimming commands (1-254 levels), set fade rates and fade times, query driver or LED status, etc. Luminaire drivers (which may be included in the module or board of the luminaire 20 may also respond to public luminaire-specific DALI commands (e.g., query if the luminaire module is short- circuit or open-circuit). More information on DALI can be gathered from www.digitalilluminationinterface.org.

[0051] In embodiments, a user interface (UI) 30 (which may be implemented e.g. as a touch display or an app on a portable device) is provided, by which users of the lighting system can adjust various lighting control parameters of the lighting controller 10 with a user interface. Thus, to support energy saving in an emergency power operation of an emergency mode, users can adjust one or more parameters of the lighting control system via the user interface 30.

[0052] Both, the lighting controller 10 and luminaires 20 (with their luminaire drivers) are powered from the DC power grid. The lighting controller 10 may be configured to measure the DC supply voltage VDC.

[0053] In a lighting system example, the lighting controller 10 may communicate via the wired or wireless communication links to the luminaire(s) 20. The driver of the luminaire 20 may then use a measured and communicated DC input voltage level as an indication of the maximum output power.

[0054] Fig. 2 shows schematically a diagram of an example of a controlled emergency operating mode according to an embodiment for a proposed LED lighting system with a dimming area and programmed luminaire flux reduction in emergency operating mode.

[0055] In the two-dimensional diagram of Fig. 2, the horizontal axis (X-axis) indicates the DC voltage VDC of the DC power grid and the vertical axis (Y-axis) indicates the output power P of the luminaire driver.

[0056] The shape of the hysteresis-type of operating curve of Fig. 2 depends on several different parameters which may at least partly be controllable (e.g., settable or adjustable) by a user via the user interface 30 of the lighting controller 10. To support the energy saving in an emergency power operation, users can thus adjust different parameters of the lighting control system via the user interface 30.

[0057] The settable or parameters include a first parameter which is a trigger value (threshold value) Voim for the DC supply voltage VDC, below which the output power of the luminaire driver and thus the lighting level of the luminaire 20 will be reduced. That is, when the DC supply voltage VDC falls below Voim, the lighting level will be reduced.

[0058] Furthermore, the controllable parameters include a second parameter which is a maximum luminous flux (power consumption) Loimi of the luminaire(s) 20 in the emergency mode, i.e., when the DC supply voltage VDC is below the trigger value Voim. That is, Ldimi defines the maximum luminous flux and power consumption of the luminaire(s) 20 in the emergency mode when VDC is just below Voim.

[0059] Additionally, the controllable parameters include a third parameter which is a lower luminous flux (power consumption) Loim2 of the luminaire(s) 20 in the emergency mode. In combination with the maximum luminous flux Loimi, the lower luminous flux Loim2 indicates the dimming range of the luminaire(s) 20 in the emergency mode. That is, Loim2 in combination with Loim i defines a (linear) dimming function to reduce luminous flux and power consumption of the luminaire(s) 20 as a function of the measured VDC in the emergency mode.

[0060] The lower luminous flux (power consumption) Loim2 can be adjusted in a range LMin (minimum luminous flux) < Loim2 < Loimi. In the special case of Loim2 = Loimi, the luminous flux is constant in the emergency mode. LMin may be selected as 10% of the maximum luminous flux.

[0061] According to the exemplary hysteresis-type of operating curve of Fig. 2, the output power of the luminaire driver(s) stays (no light) at zero until a minimum DC voltage Von = 525 V has been reached. Then, the luminaire driver is switched on to provide 100% luminous flux (LF) (maximum luminous flux) of the luminaire 20, which may correspond to a nominal output power PNom= 63W in the exemplary embodiment. This on-state power output is kept up to a maximum DC power grid voltage Vmax = 750V.

[0062] Now, when the DC power grid voltage VDC decrease down to the trigger value Voim = 500V, the emergency mode is entered, and the luminaire driver is controlled to reduce its output power to 75% luminous flux. Below the trigger value of 500V, the output power of the luminaire driver is further decreased (dimmed), e.g., in a linear relation to the voltage change, until the lower luminous flux Loim2 has been reached at a minimum DC power grid voltage Voff = 404V. The lowest dimmed luminous flux (i.e., LMin) may be 10% of the maximum luminous flux, which may correspond to a minimum output power PMin = 10W of the luminaire driver(s). Thus, a hysteresis of 25V (525V-500V) is provided to prevent continuous switching between the normal operating mode and the emergency mode at the trigger value.

[0063] The luminaire driver(s) may be configured to dim the output to the luminaire(s) 20 e.g. by means of continuous amplitude modulation (AM) dimming of a DC output current or by pulse width modulation (PWM) dimming of the output current. AM dimming guarantees a smooth and flicker-free operation over the entire dimming range.

[0064] Fig. 3 shows a flow diagram of a control process with activated emergency mode according to an embodiment, which may be implemented as a software routine that controls the lighting controller 10 of Fig. 1.

[0065] The flow diagram of Fig. 3 is started when an emergency mode (EM) operation option is activated or allowed.

[0066] In step S301 (L=Lm), the luminous flux L of the luminaire(s) is set to the maximum value Lm (e.g., 100% which may correspond to a driver output power of 63 W). Then, in step S302 (RD PAR), emergency mode parameters (e.g., Voim, Loimi, Loim2) are read from the user interface or a memory in which they have been stored after entering via the user interface.

[0067] In subsequent step S303 (CNK DC), the actual level of the DC power grid voltage VDC is checked (e.g., measured or read from a sensing input) and compared with the emergency mode trigger value Voim.

[0068] In step S304, it is determined whether the actual level of the DC power grid voltage VDC is smaller than the trigger value (VDc>VDim?). If not (branch “N”), the procedure jumps back to step S301 and starts again.

[0069] On the other hand, if the actual voltage level VDC has fallen below (or reached) the trigger value Voim (branch “Y”), the procedure continues with step S305 where the luminous flux is set as a function of the measured actual level VDC. This may be the maximum luminous flux Loimi minus an output value of a predetermined dimming function f(Voc), which may be a linear function or other continuous (e.g., monotonically decreasing) function down to the lower luminous flux Loim2.

[0070] Then, the procedure jumps back to step S303 where the actual level of the DC power grid voltage VDC is measured again, and the procedure continues based on the new measuring result.

[0071] To summarize, a method and system method and system for providing a tradeoff between operating range and grid load / stability have been described. This can be achieved by increasing the operation range by allowing a driver (e.g., an LED driver) to operate at lower voltages but at lower power levels. The driver may then use the DC input voltage level as an indication of the maximum output power. In a lighting system example, a lighting controller may communicate via a wired (e.g., DALI) or wireless (e.g., Interact) communication link to luminaires. Both, the lighting controller and the luminaires may be powered from an industrial DC power system. The lighting controller may be configured to measure the DC supply voltage. Users of the lighting system can adjust various lighting control parameters with a user interface, e.g., a touch display or an app on a portable device. To support the energy saving in an emergency power operation, users can adjust one or more parameters of the lighting control system via the user interface.

[0072] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed LED lighting system. It may be applied to all kinds of industrial lighting systems (e.g., with 650 V DC power systems) according to the “DC-INDUSTRIE” concept or to other types of load control systems with voltage-controlled emergency operation mode in other industrial production processes (e.g., motor drives or robots).

[0073] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in the text, the invention may be practiced in many ways, and is therefore not limited to the embodiments disclosed. It should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being re-defined herein to be restricted to include any specific characteristics of the features or aspects of the invention with which that terminology is associated.

[0074] A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0075] The described operations like those indicated in Fig. 3 can be implemented as program code means of a computer program and / or as dedicated hardware of the transmitter devices, receiver devices or transceiver devices, respectively. The computer program may be stored and / or distributed on a suitable medium, such as an optical storage medium or a solid- state medium, supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Claims

CLAIMS:

1. An apparatus (10) for controlling an emergency mode of a power supply system that supplies power to one or more load devices (20), wherein the apparatus (10) is configured to receive an input of one or more power control parameters via a user interface (30) and to control an energy saving characteristic in the emergency mode of the power supply system based on the one or more power control parameters, wherein the apparatus is configured to determine a voltage level of a supply voltage of the power supply system and to trigger the emergency mode when the determined voltage level has fallen below a predetermined trigger voltage level (Voim) defined by one of the one or more power control parameters.

2. The apparatus of claim 1, wherein the apparatus is configured to control a power level supplied to the one or more load devices (20) as a function of the determined voltage level during the emergency mode.

3. The apparatus of any one of the preceding claims, wherein the power supply system is a direct current, DC, power supply system for a lighting system, wherein the one or more load devices comprise one or more luminaires (20) of the lighting system, and wherein the energy saving characteristic is a dimming characteristic of the one or more luminaires (20) of the lighting system during the emergency mode.

4. The apparatus of claim 3, wherein the one or more power control parameters comprise a maximum luminous flux (Loimi) and a lower luminous flux (Liim2 of a dimming range of the dimming characteristic.

5. The apparatus of claim 4, wherein the dimming characteristic is defined by subtracting from the maximum luminous flux (Loimi) a value determined as a predetermined function of a determined actual voltage level of the supply voltage of the DC power supply system.

6. The apparatus of claim 5, wherein the predetermined function is a linear function.

7. A lighting controller (10) of a lighting system, the lighting controller (10) comprising an apparatus of any one of claims 1 to 6.

8. A lighting system comprising a DC power supply system and a lighting controller of claim 7.

9. A method of controlling an emergency mode of a power supply system that supplies power to one or more load devices (20), wherein the method comprises: inputting one or more power control parameters via a user interface (30); controlling an energy saving characteristic in the emergency mode of the power supply system based on the one or more power control parameters; and determining a voltage level of a supply voltage of the power supply system and triggering the emergency mode when the determined voltage level has fallen below a predetermined trigger voltage level (Voim) defined by one of the one or more power control parameters.

10. The method of claim 9, wherein a power level supplied to the one or more load devices (20) is controlled as a function of the determined voltage level during the emergency mode.

11. The method of claim 9 or 10, wherein the one or more power control parameters comprise a maximum luminous flux (Loimi) and a lower luminous flux (Liim2) of a dimming range of a dimming characteristic of one or more luminaires (20) of a lighting system during the emergency mode.

12. The method of claim 11, wherein the dimming characteristic is defined by subtracting from the maximum luminous flux (Loimi) a value determined as a predetermined function of a determined actual voltage level of the supply voltage of the power supply system.

13. A computer program product comprising program code for performing the steps of any one of claims 9 to 12 when run on a computer device.

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