WO2016020432A1

WO2016020432A1 - Constant current regulator with third harmonic power injection

Info

Publication number: WO2016020432A1
Application number: PCT/EP2015/068044
Authority: WO
Inventors: Lorenzo Dellacorna; Federico CANETTA
Original assignee: Adb Bvba
Priority date: 2014-08-08
Filing date: 2015-08-05
Publication date: 2016-02-11

Abstract

Constant current regulator (10) for supplying and controlling an AC series circuit (9) current of an airfield lighting system, comprising an input port (11) arranged for being coupled to an AC power supply, an output port (12) arranged for outputting the series circuit current at an output voltage, an interface port (13) arranged for receiving a command signal representative of a set value of the series circuit current, and a transformer (15) coupled between the input port and the output port, for providing galvanic insulation between the input and output ports. A control unit (171, 172) is arranged for determining a root mean square voltage level value required to generate the set value of the series circuit current, and for determining an output voltage signal comprising a fundamental component of sinusoidal shape and of a fundamental frequency, and a third harmonic component of the fundamental component having a frequency three times the fundamental frequency, wherein the third harmonic component has a peak value of at least 1% of a peak value of the fundamental component, wherein the output voltage signal has a root mean square value equal to the root mean square voltage level value. Electric circuitry (14, 16) is arranged for generating the output voltage signal and for applying it to the output port as the output voltage.

Description

CONSTANT CURRENT REGULATOR WITH THIRD HARMONIC POWER

INJECTION

[0001] The present invention is related to constant current regulators, in particular regulators used for controlling series circuit currents of airfield lighting systems.

[0002] Lighting installations on an airfield emit light signals for the orientation and guidance of aircraft which are on the approach to an airport or are moving on the take-off or landing runways, or taxiways. Lighting installations comprise all the lighting aids which are intended to guarantee safe flight operations and safe taxiing of aircraft in the area of an airport, even when it is dark or when visibility conditions are poor. A distinction is drawn, inter alia, between approach lighting, glide- angle lighting, threshold lighting, side and centre lighting, take-off and landing runway lighting, taxiway lighting, identification lights, hazard lights, obstruction lights and rotating lights.

[0003] In such airfield lighting installations it is demanded that all lights within a same circuit (e.g. runway centreline lights) have a same brightness, so that the distance of the airplane to each light is the only parameter that influences how the light appears to the pilot. E.g., a runway centreline light that is less bright than another one is thus further away from the airplane than the other one. Constant current regulators (i.e., current controlled regulators) are used for this purpose, and provide a constant rated output current to the light circuits under varying conditions, for example fluctuations in the mains input voltage or frequency, the ambient temperature, the height above sea level of their location, relative air humidity and the applied load.

[0004] Voltage controlled regulators are not suitable for this purpose, since most airfield lighting circuits extend over very long distances (up to 15 km). There will hence be a voltage drop along the cable resulting in a different power and a different light output in function of the place of the light in the circuit. As such, a light closer to the airplane could appear less bright than a light further away from it. This could lead to the pilot making erroneous manoeuvres and would jeopardize air traffic safety. To avoid the voltage drop, one would have to use unacceptably thick (and expensive) cables. Thus, current control is used in order to guarantee a same current and power for each light along the circuit. [0005] Constant current regulators (CCRs) of the above kind are known from e.g. US 2010/283400 and WO 2014/001402. A CCR's goal is to regulate a current output level around a working point which can e.g. be set by an operator or an airport control tower. The current output level is maintained as constant as possible near the pre-set working point in order to ensure that the airfield lights emit at constant luminous intensity (brightness). Since it is desirable to use different intensities depending on the circumstances (e.g. day or night, good or low visibility), the working point can change and the CCR must be able to adapt the current output level accordingly. Once the working point of the current has been set, the CCR is arranged to maintain an RMS (root mean square, also 'rms') value of the current, typically an AC current, as constant as possible at the value of the working point.

[0006] The frequency of the current output by a CCR is typically identical to the frequency of the supply voltage, e.g. 50 Hz or 60 Hz. The voltage level output by the CCR is a (dynamic) consequence of the load present on the series circuit, and the pre-set current level. The CCR power rating and the current level determines the maximum voltage possible for this regulator. For example, a CCR with a rated current output of 6.6 Arms and power rating of 5kVA will have a maximum output voltage level of: 5000 / 6.6 = 757 Vrms. However, if the actual load on the series circuit (cables and light fixtures) is only 2000 VA (e.g. 1000 VA loss in the cable and 1000 VA = 50 x 20 VA light fixtures), then the actual voltage level output by the CCR will also be lower than the maximum: 2000 / 6.6 = 303 Vrms. The voltage level is dynamic in the sense that, if the load changes, the voltage level across the series circuit will also change (e.g. if one light goes defect: (2000 - 20) / 6.6 = 300 Vrms). Clearly, the voltage level will also be dynamic for maintaining a constant current.

[0007] CCRs for use in airfield lighting systems must have an output which is galvanically insulated from the CCR input, as prescribed by FAA (Federal Aviation Administration) or IEC (International Electrotechnical Commission) standards. To this end, CCRs comprise a transformer coupled between an input stage circuit and an output stage circuit. Power electronic components, such as thyristors or transistors (e.g. MOSFET or IGBT) are used in either the input stage, the output stage, or both for creating the desired voltage and current output levels.

[0008] Reliability is a key aspect of the above CCRs since they supply electric power to lighting installations which ensure safety of the air traffic on the airfield. In WO 2014/001402, reliability is obtained by a modular arrangement of the CCR, which is composed of different modules which can communicate with one another and can cope with a situation when one module would fail. [0009] It is an aim of the present invention to provide aspects which enhance the reliability of airfield lighting systems and particularly of constant current regulators for controlling series circuit currents of such airfield lighting systems. It is also an aim of the present invention to reduce the operational (maintenance) cost of such constant current regulators.

[0010] According to first aspects of the invention, there is therefore provided a constant current regulator, and an airfield lighting system comprising such constant current regulator, as set out in the appended claims. Constant current regulators according to first aspects of the invention comprise an input port arranged for being coupled to an AC power supply, and output port arranged for outputting a series circuit current at an output voltage, an interface port arranged for receiving a command signal representative of a set value of the series circuit current, and a transformer coupled between the input port and the output port, for providing galvanic insulation between the input and output ports. The constant current regulator further comprises a control unit configured for determining a root mean square voltage level value required to generate the set value of the series circuit current. In an aspect of the invention, the control unit is configured for generating a pulse width modulated signal. The constant current regulator further comprises electric circuitry comprising input stage circuitry coupled between the input port and the transformer, and output stage circuitry coupled between the transformer and the output port.

[0011] In an aspect of the invention, the output stage circuitry comprises inverter circuitry arranged for generating an output voltage signal having a root mean square value equal to the root mean square voltage level value. The electric circuitry is arranged for applying the output voltage signal to the output port.

[0012] In an aspect of the invention, the control unit is coupled to the inverter circuitry for applying the pulse width modulated signal to it. The pulse width modulated signal is configured for determining the output voltage signal when applied to the inverter circuitry.

[0013] In an aspect of the invention, the control unit is configured for generating the pulse width modulated signal allowing for the output voltage signal to comprise a fundamental component of sinusoidal shape and of a fundamental frequency, and a predetermined amount of a third harmonic component of the fundamental component having a frequency three times the fundamental frequency. The third harmonic component has a peak value of at least 1 % of a peak value of the fundamental component, and advantageously more than 1 %, such as at least 3%, at least 5%, or even at least 10%. The control unit is advantageously implemented to generate the pulse width modulated signal based on a reference signal, which can be representative of a waveform of the output voltage signal. This reference signal advantageously comprises the pre-determined amount of the third harmonic component. The reference signal can have a waveform defined by the formula:

sin(oot) + A ^* sin(3oot), wherein A represents the relative amount of third harmonic component (ranging between 0 and 1 ), t is time and ω = 2ττΐ with f the fundamental frequency (e.g. 50 or 60 Hz).

[0014] According to second aspects of the invention, there is provided a method of supplying an electrical AC current to a series circuit of airfield lights, as set out in the appended claims.

[0015] Aspects of the present invention enable reducing peak voltage levels which are applied to the power electronic components of the CCR, and which are applied to the series circuits of the airfield lighting system, without affecting the RMS output power levels (Wrms). By so doing, the power electronic components (e.g. MOSFET or IGBT, capacitors and other components) are subjected to a lower peak voltage level, and can operate at a higher voltage margin (i.e. the difference between the voltage applied to the component compared to the maximal voltage which the component can manage). This increases the lifetime of such components, which is dependent on the voltage level applied to them, resulting in reduced failure rate and longer intervals between maintenance. Reduced peak voltage levels are also beneficial for field transformers which are coupled in the series airfield lighting circuit. These transformers will experience lower peak voltage levels for a same RMS power level, such that transformer insulation will be able to stand longer, resulting in increased reliability. Reduced peak voltage levels are also beneficial for field cables and connectors which are provided in the series airfield lighting circuit. These cables and connectors will experience lower peak voltage levels for a same RMS power level, such that cables' and connectors' insulation will be able to stand longer, resulting in increased reliability. The increase of the voltage margin also reduces power losses in the electronic components and increases efficiency.

[0016] According to aspects of the present invention, both the peak voltage levels inside the CCR and those output from the CCR can be reduced by superposition of a third harmonic component on the AC voltage signal (and consequently on the AC current signal). A suitable superposition of a third harmonic component can reduce the peak amplitude of the AC signal without affecting the RMS level of the signal. Since the power transmitted is determined by the RMS value of voltage and current, the peak amplitude can be reduced without affecting the transmitted RMS power level (Wrms). Also, since the airfield lights are controlled by adjusting the light intensity (brightness) of the lights based on the RMS current level flowing through the series circuit, and since the RMS (current) level is unaffected by the invention, no additional measures are required, and a CCR according to the invention can advantageously be used for replacing existing CCRs without requiring additional measures.

[0017] The use of third harmonic superposition is known in electric motor drive control for increasing a motor torque for a same peak current level. US 2003/0085627 describes the use of pulse width modulated (PWM) current controlled inverters for providing power to two three-phase windings of an electric induction motor. The PWM control signals for the inverters include a third harmonic component. PWM control signals including a third harmonic component for motor drive control are also described in EP 1480329 and EP 2393200.

[0018] It will be convenient to note that in the present invention, the reason for using a third harmonic component is different than in motor drive control. A CCR for airfield lighting systems works at fixed frequency, typically equal to the input line frequency, and at a pre-set RMS value of the current of the lighting series circuit. These values therefore should remain unaltered. The third harmonic component is hence used to reduce the voltage peak amplitude to which power electronic components within the CCR and the series circuit at the CCR output are subjected. By reducing these voltage peak amplitudes, the reliability of the CCR is increased.

[0019] A light emitting diode (LED) driver with superposition of a third harmonic component is described in WO 2009/109888. A resonant tank is employed in an input stage of the driver for superposing the third harmonic component. The driver allows the LED to generate more light for the same peak current as compared to driving the LED directly from the power supply. Conversely, the present invention aims at reducing the peak current so as to reduce failure rates and increase reliability of the system.

[0020] According to aspects of the invention, the requirement of galvanic insulation is exploited for generating an output signal with third harmonic component in the output stage of the CCR, while a power reserve for superposing the third harmonic component can be built up in the input stage without requiring large capacitors, which are expensive and have limited lifetime. A compact, yet reliable CCR is thus obtained.

[0021] Aspects of the invention will now be described in more detail with reference to the appended drawings, which are illustrative only and wherein same reference numbers indicate same features, wherein: [0022] Figure 1 represents a scheme of an example embodiment of a constant current regulator according to aspects of the invention;

[0023] Figure 2 represents an enlarged scheme of an example input stage voltage rectifier circuitry as used in the scheme of Fig. 1 ;

[0024] Figure 3 represents a combined signal with superposition of a third harmonic component on a fundamental sine wave;

[0025] Figure 4 represents a flow chart of a process of modifying the output voltage signal of the CCR by superposition of a third harmonic component according to aspects of the invention.

[0026] The term 'third harmonic' as used in the present description refers to a signal component being a harmonic of a fundamental signal component and having a frequency of three times the frequency of the fundamental component. That is, if the fundamental component is a 50 Hz signal, the third harmonic component has a frequency of 150 Hz.

[0027] The term 'rated power', 'rated current', or 'rated voltage', as used in the present description, refers to the maximum operating power, current, or voltage respectively.

[0028] The term 'rms' in 'Arms' or 'Vrms' or 'Wrms' refers to the root mean square value of current (A), voltage (V), or power (W). The term 'pk' in 'Vpk' refers to the peak value (amplitude) of the voltage.

[0029] Figure 1 depicts a schematic diagram of a CCR 10 according to the invention, with power line input port 1 1 for connection to an electrical AC power supply line and power output port 12 for connection to a series circuit 9 of airfield lights. Series circuit 9 typically serially connects field transformers 92, the primary coils of which are connected to the series circuit 9 and the secondary coils of which are connected to the airfield lights 91.

[0030] CCR 10 is operable to provide an output electrical power at the output port 12 of the CCR 10 and hence to the series circuit 9. The CCR 10 is configured to generate and output a single phase AC current at output port 12 in accordance with pre-set levels which may be read from interface port 13. The (single phase) output voltage at port 12 is related to the desired output current and to the resistance of the load (e.g., the series circuit 9) and can change between zero and a maximum (rated) voltage.

[0031] In order to comply with national and international standards, such as FAA (Federal Aviation Administration) or IEC (International Electrotechnical Commission), the output port 12 is galvanically insulated from the input port 1 1 through a transformer 15. Transformer 15 may for example provide galvanic insulation, which, according to international standard requirements, can be higher than or equal to 23 kV (e.g. for CCRs having power rating of 30kVA). The transformer 15 is advantageously a 1 :1 transformer. The CCR can hence comprise input stage circuitry 14 coupled between input port 1 1 and transformer 15, and output stage circuitry 16 coupled between transformer 15 and output port 12.

[0032] Typical CCRs for controlling airfield lighting series circuits have power ratings ranging between 2.5 kVA and 30 kVA. With typical current ratings of 6.6 Arms, and in some cases 12 Arms or 20 Arms, of sine wave shape, this means that typical voltage ratings at the output of the CCR are between 757 Vrms and 4545 Vrms. Since the output signal is of sinusoidal shape, the voltage peak amplitudes are between 1071 Vpk and 6428 Vpk. The power electronic components of e.g. the output stage circuitry 16, such as MOSFET (metal oxide semiconductor field effect transistor) or IGBT (insulated gate bipolar transistor) or capacitors (in particular electrolytic capacitors) or other electronic components must withstand such large peak values. Needless to say, economical considerations impose that the power electronic components are designed with minimal voltage margin. Since the lifetime of the power electronic components is inversely proportional to the voltage margin, it becomes evident that regular preventive maintenance (substitution) of the power electronic components is required in order to avoid unexpected CCR failure.

[0033] The inventors have found that by superposition of a third harmonic component on the fundamental wave of the output signal at port 12, it is possible to reduce the voltage peak amplitude of the output signal, without affecting the RMS value of the voltage signal, i.e. without affecting the real power (Wrms) that is output by the CCR to the series circuit 9. If the output signal at port 12 is a 50 Hz sine wave, the third harmonic component, being a 150 Hz sine wave which is in phase with the fundamental wave is superposed. If the output signal is a 60 Hz sine wave, the third harmonic component, being a 180 Hz sine wave which is in phase with the fundamental wave is superposed. By way of example, superposing a third harmonic component having a peak amplitude of 10% of the peak amplitude of the fundamental wave, allows for obtaining an output signal with peak amplitude of 1000 Vpk (in case of a 5 kVA at 6.6 Arms) and 6000 Vpk (in case of 30 kVA at 6.6 Arms) without affecting the RMS values of 757 Vrms and 4545 Vrms. This means a reduction of the voltage peak amplitude by 71 V for 5 kVA to 426 V for 30 kVA and hence a voltage margin of the power transistors can be significantly increased, which reduces losses in the transistors and is beneficial for the lifetime of transistors, capacitors (in particular electrolytic capacitors) and other electronic components. As a result, less maintenance is required for the CCR and reliability of the CCR is increased.

[0034] The superposition of a third harmonic component has furthermore no effect on the control of the airfield lighting series circuit, since both the resistive losses in the cables, and the control of the brightness of the lights are determined by the RMS value of the output signal, which remains unaltered. An additional benefit is that the field transformers 92 are subjected to lower voltage peak amplitudes, which spares the insulation and increases lifetime and hence reliability. Yet an additional advantage is that the voltage signal comprising the third harmonic has a shape that better resembles the shape of a square wave as compared to a pure sine wave. With such shapes, transformer efficiency can be increased. Hence the efficiency of the CCR as well as the lighting system as a whole can be increased due to reduced transistor losses and increased transformer efficiency.

[0035] Advantageously, the third harmonic component which is superposed has an amplitude (i.e., peak value), relative to the amplitude (peak value) of the fundamental wave, of at least 1 %, advantageously at least 2%, advantageously at least 3%, advantageously at least 4%, advantageously at least 5%, advantageously at least 6%, and advantageously 20% or less, advantageously 18% or less, advantageously 16% or less, advantageously 15 % or less, advantageously 14% or less. An example is a third harmonic component of about 10% (in amplitude) of the fundamental wave.

[0036] Advantageously, the third harmonic component that is superposed is in phase (i.e. has no phase shift) with the fundamental wave.

[0037] It will be convenient to note that higher order harmonic components can be superposed in addition to the third harmonic component, or in the alternative thereto.

[0038] A third harmonic superposition can be obtained with an example

CCR 10 as depicted in Fig. 1 and being configured as follows.

[0039] Input stage circuitry 14, which is coupled between input port 1 1 and transformer 15, may comprise a low pass filter 141 for filtering the line input signal applied at port 1 1 of the CCR 10. Input stage circuitry 14 comprises an input voltage conversion circuit, schematically represented by blocks 142-143 in Fig. 1 . Input voltage conversion circuit 142-143 is advantageously configured for generating a regulated (AC) voltage widely independent of the input voltage level and of the frequency of the input voltage (e.g. mains frequency 50 or 60 Hz). [0040] Voltage conversion circuit 142-143 advantageously comprises input rectifier circuitry 142 followed by input stage inverter circuitry 143. Referring to Fig. 2, input rectifier circuitry 142 can comprise a rectifier diodes bridge circuit 144, which is operable to transform an AC (alternating voltage/current) signal (such as an advantageously filtered line input, at grid frequency), received at input terminals 1421 of circuitry 142, to a DC (direct current) signal at terminals 1425.

[0041] In addition to the diodes bridge 144, the circuit 142 advantageously comprises a boost converter circuit 145 connected between the rectifier bridge 144 (terminals 1425) and the output terminals 1422 of rectifier circuitry 142. The boost converter circuit 145 is configured to generate an output voltage (at output 1422) higher than the voltage which the rectifier bridge 144 can provide at terminals 1425, i.e. higher than the rectified supply voltage. As will be described, the boost converter circuit 145 hence is configured to provide the supplementary power needed for generating the third harmonic component in the output stage circuit 16.

[0042] Boost converter circuit 145 can form part of, or consist of, a power factor corrector circuit in rectifier circuit 142, or can provide for power factor correction if required.

[0043] Input stage inverter circuitry 143 is operable to transform the DC signal from the input rectifier 142 at output terminals 1422 to an AC signal applied to transformer 15.

[0044] The regulated voltage output by voltage conversion circuit 142-143 is advantageously, though not necessarily, a high frequency AC voltage, such as of at least 1 kHz, advantageously at least 10 kHz, advantageously at least 20 kHz, advantageously at least 25 kHz, advantageously at least 30 kHz. The (high) frequency of the AC voltage is in principle limited only by the transistor switching speed defined by the available technology, and can be lower than or equal to 500 kHz, possibly lower than or equal to 200 kHz. A suitable frequency is 40 kHz. In order to obtain such high frequency signals (voltage, current), input stage inverter circuitry 143 advantageously comprises a quasi resonant converter circuit, advantageously a LLC quasi resonant converter power circuit.

[0045] An example of a suitable LLC quasi resonant converter power circuit is described in an internet paper titled "Phase Shifted Full Bridge LLC Resonant Converter" by Martin Zhang and Sober Hu, and downloadable at the following web page, the contents of which being incorporated herein by reference: http://blog.dianyuan.eom/blog/u/42/1 150776421.pdf. [0046] An advantage of a high frequency voltage output in the input stage is that size and weight, and hence cost of transformer 15 can be significantly reduced. By way of example, a high frequency transformer working at about 40 kHz is 10 to 20 times smaller in weight and size compared to its 50 Hz counterpart.

[0047] Additionally, at the indicated high working frequency, the power losses in the transformer due to the high frequency remain acceptable, such that an optimal balance between size reduction and power loss is achieved.

[0048] The input inverter circuitry 143 is connected to the primary side

151 of transformer 15. The secondary side 152 of transformer 15 is connected to the output stage circuitry 16, which can comprise: rectifier circuitry 161 and inverter circuitry 162, to provide a regulated output (voltage) at output port 12. Output stage inverter circuitry 162, which can comprise a pulse width modulated transistor H-bridge inverter circuit is advantageously configured for working at a high frequency, such as the ones indicated hereinabove in relation to the input voltage conversion circuit 142- 143 (e.g., 40kHz). The output of inverter circuitry 162 can be fed to a low pass filter 163 prior to being applied at CCR output port 12.

[0049] The output low pass filter 163 connected between the output stage inverter circuitry 162 and the output 12 is advantageously operable to filter out any signal components of the above indicated high frequency (i.e. frequency of operation of the rectifier and transistor H-bridge). The output filter 163 is advantageously arranged for having a cutoff frequency of at least 100Hz, advantageously at least 200Hz, advantageously at least 500Hz, advantageously at least 1 kHz. Operating at such high frequencies is advantageous, since it allows for making the output low pass filter 163 very small and economical.

[0050] The regulated output at port 12 is an AC (voltage) signal advantageously having a same frequency as the power line supply (voltage) signal at input port 1 1 , e.g. a frequency of 50 or 60 Hz, and advantageously having a same phase as the power line supply signal at input port 1 1. Maintaining the AC signal at output port 12 in phase with the power line signal at input port 1 1 is advantageous, as it avoids the requirement of installing large capacitor banks, since the maximum power is transmitted to the load at the same time as the maximum power entering the CCR. Frequency correspondence, or phase correspondence, or both, between the signals at input 1 1 and at output 12 can be obtained through synchronisation. To this end, CCR 10 advantageously comprises a (micro)controller 171 which is operable to gather frequency and/or phase information of the signal at input 1 1 . This information can be provided to control input stage inverter circuitry 142, or output stage inverter circuitry 162, or both.

[0051] Advantageously, the frequency and/or phase information of the power line input, and advantageously collected by controller 171 , is used for generating a pulse width modulated control signal with appropriate duty cycle that is fed to output stage inverter circuitry 162. The PWM control signal is such that, when applied to inverter circuitry 162, it allows for outputting a (voltage) signal having a same frequency and phase as the input signal at port 1 1 . To this end, the CCR can comprise a second (micro)controller 172 operable to control the output stage circuits 16. In this case, controller 171 may be operable to control the input stage circuits 14. The output stage controller 172 may be connected to the input stage controller 171 through a data communication link 173. Galvanic insulation between the two controllers 171 and 172 can be obtained through opto-isolators 174 and/or optical transmission of data (optical fibre).

[0052] The input stage controller 171 and the output stage controller 172 may be integrated in a single (micro)controller. Either one or both controllers 171 and 172, or the single controller, can be a microprocessor.

[0053] It will be convenient to note that the electronic circuits in input/output stage blocks 141 -143 and 161-163 advantageously allow digital communication with the controllers 171 and 172. Alternatively, digital to analog converters (DAC) may be provided for such communication.

[0054] The frequency and/or phase information of the power line signal at input 1 1 can be collected by input stage controller 171 and can be transmitted to output stage controller 172 over communication link 173. Output stage controller 172 can be programmed for generating the PWM control signal applied to inverter circuitry 162.

[0055] The CCR 10 advantageously comprises a current feedback loop, as known in the art, for controlling the (RMS value of the) electric current output at port 12. This current feedback loop comprises sensors for e.g. measuring the electric current and possibly additional signal parameters at the CCR output. Part of the feedback loop may be integrated in the controller(s) 171 , 172. The current feedback loop, together with the input and output stage controllers 171 , 172 forms a control unit of the CCR 10.

[0056] The third harmonic component can be superposed on the fundamental signal of the output by appropriate modification of the PWM control signal that is supplied to the inverter circuitry 162. To this end, the controller 172 can be programmed/implemented to be operable to generate a pulse width modulation control signal with appropriate duty cycle, which, when applied to the inverter circuitry 162, generates a voltage signal comprising a third harmonic superposition as indicated above.

[0057] The PWM signal (also referred to as PWM control signal) can be generated by an algorithm running on the controller 172. A look-up table with several values related to PWM signal generation can be provided in computer memory linked to controller 172. The algorithm is run in the controller to process values stored in the look-up table in order to determine a duty cycle of the PWM control signal based on a desired CCR output voltage RMS value and predetermined amount of the third harmonic component. The desired CCR output voltage rms value can be determined based on a pre-set current value and current control feedback. The predetermined amount of the third harmonic component that is to be superposed on the fundamental voltage signal can be a fixed value, or can change, e.g. depending on the desired voltage RMS level. It can be set by hardware or software (in controllers 171 , 172), or provided through interface port 13.

[0058] Advantageously, the (duty cycle of the) PWM signal generated and/or determined by the controller 171/172 is based on a reference signal which comprises, or consists of, the fundamental wave component 31 and a predetermined amount of the third harmonic component 32 as shown in Fig. 3. The reference signal can be generated with the following formula:

Reference signal wave = sin(oot) + A ^* sin(3oot),

wherein A represents the relative amount of third harmonic component (ranging between 0 and 1 ), t is time and ω = 2nfwith f the fundamental frequency (e.g. 50 or 60 Hz). The PWM signals used in the present invention therefore incorporate information representative of a predetermined amount of the third harmonic component 32 and therefore differ as compared to a PWM signal determined based on a pure sinusoidal (fundamental) wave signal.

[0059] The PWM control signal controls the switching times of the transistors of the inverter circuitry 162 which are operable to generate the output signal. As a result, a voltage signal is output from inverter circuitry 162 comprising, or consisting essentially of, a fundamental wave component, e.g. a sine wave at the frequency of the power supply, 50 Hz or 60 Hz, and a third harmonic component of the fundamental wave component. The low pass filter 163 can be arranged for filtering out higher order harmonic components, such as those introduced by PWM switching. It will be convenient to note that, if the load is resistive, the shape of the current signal at the output of the inverter circuitry 162, and hence at the output port 12, will follow the shape of the voltage signal and therefore also comprise, or consist essentially of, a superposition of a fundamental current component and a third harmonic component of the fundamental component. However, if the load is inductive or capacitive, the current signal will have an altered shape, which is determined by the electrical characteristics of the load of the series circuit.

[0060] It will be convenient to note that the amount or level of the third harmonic component in the output voltage signal is predetermined by the PWM signal, and advantageously independent of the load at the output port 12 (full or less than full load).

[0061] Since the phase of the output (voltage) signal at output port 12 is advantageously the same as the phase of the input signal at input port 1 1 , an instantaneous power reserve is required for being able to superpose the third harmonic component on the fundamental signal. Referring to Fig. 3, one can see that the signal 30 obtained by superposition of the fundamental wave 31 and the third harmonic wave 32 has intervals Τι, T2 in which it has a higher value than the fundamental wave 31. An overpower relative to the power provided by the line input is hence required during these intervals in order to generate the signal 30. According to an aspect of the invention, the overpower is created in the input stage circuitry 14, in particular by the boost converter circuit 145 located in input stage rectifier circuitry 142.

[0062] Referring again to Fig. 2, boost converter circuit 145 is configured for boosting the DC voltage provided by rectifier bridge 144 when the AC supply voltage at input 1 1 is not at peak value. To do so, boost converter circuit is connected to controller 171 which controls switching times of the boost converter circuit 145 to alter voltage levels at output terminals 1422.

[0063] As represented in Fig. 2, boost converter circuit 145 can be formed of two or more boost converter cells L1 , D1 , Q1 and L2, D2, Q2 which are interleaved (two interleaved cells are represented in Fig. 2). Semiconductor switch Q1 controls energy storage in inductor L1. Semiconductor switch Q2 controls energy storage in inductor L2. A capacitor C is connected across the output 1422. A diode D1 , D2 is placed between each inductor and the capacitor. Switches Q1 and Q2 are coupled to controller 171 through ports 1423 and 1424 respectively. Controller 171 is advantageously programmed to generate control signals, such as PWM control signals, for operating switches Q1 , Q2. Control signals for switches Q1 , Q2 are applied to the boost converter circuit 145 through ports 1423 and 1424 connecting the boost converter circuit to the controller 171 . [0064] In order to generate the overpower, e.g. at instants when the third harmonic component is at peak value, the controller 171 is advantageously programmed for altering the duty cycle of the PWM control signal applied to the switches Q1 and Q2. This PWM control signal can suitably be sampled each 25 s for a line input frequency of 50 Hz.

[0065] Advantageously, the switches Q1 , Q2 of the cells of the interleaved boost converter circuit are controlled in a time-shifted manner. That is, the trigger edges of the PWM signals for switches Q1 and Q2 are time-shifted by an amount smaller than the base period of the PWM signals. By way of example, let us consider a PWM control with switching frequency at 40 kHz, which corresponds to a (base) period of 25 s. This means that the duty cycles of both switches Q1 and Q2 are controlled/sampled with 25 s intervals. In the above time-shifted control, however, the triggers for switches Q1 and Q2 occur with a time shift of e.g. 12.5 s (half the period) between the switches. The result is that a boost converter circuit apparently working at double frequency (e.g. 80 kHz) can be obtained with each separate boost converter cell (Q1 , Q2) operating at 40 kHz.

[0066] Controller 171 can be programmed to determine instants of time on which the overpower is required and possibly an amount of overpower, based on line input voltage information and information on the third harmonic component that is added in the output stage. The information regarding the third harmonic component can be provided by controller 172 to controller 171 , e.g. at the instant the component is added in the output stage. Based on the above information, controller 171 adapts the control signal for boost converter switches Q1 and Q2.

[0067] The PWM signals for the boost converter circuit 145 and for the inverter circuitry 162 can be synchronized by synchronizing each one with a fundamental sinewave of the line input.

[0068] As a result, an instantaneous power reserve is made available without requiring large capacitors, which further increases reliability of the CCR and saves cost.

[0069] It will be convenient to note that the output stage advantageously does not comprise any boost converter circuit or PFC.

[0070] The CCR 10 can be operated as follows. A message with pre-set current level information is received at interface port 13, which can be a CAN bus interface, and fed to the control unit (e.g. controller 171 or 172), which introduces the pre-set current level in the current feedback loop controlling the current RMS value at output port 12. The current feedback loop adapts/sets the RMS voltage level for the output corresponding to the pre-set current level.

[0071] Fig. 4 represents a flow chart of the operation of aspects the CCR, with the current feedback indicated at 41 by comparing the pre-set current level /_s with the actual (measured) current RMS value k at the output port. The controller(s) 171/172 are implemented, e.g. with program code, to determine the RMS voltage level VRMS for the output at step 42.

[0072] An amount of the third harmonic component, either fixed or variable, can be pre-programmed in either controllers 171 , 172, e.g. in memory, or can alternatively be read from interface port 13.

[0073] Controller 171 or 172 can be programmed to generate a PWM control signal, which, when applied to the output stage inverter 162, is configured to generate a voltage output signal comprising a third harmonic superposition and having a RMS value which corresponds to the voltage level RMS set by the current feedback loop. The controller can be programmed to generate a pulse width modulated signal at a predetermined switching frequency, e.g. 40 kHz. The pulse width modulated signal is supplied by the controller to the output stage inverter circuitry 162 for controlling the switching of the transistors to obtain an AC output voltage signal with the desired amount of third harmonic component and desired RMS value.

[0074] To generate the PWM signal for inverter 162, a look-up table in controller 171 or 172 can store duty cycle information of the PWM signal in function of the RMS voltage level.

[0075] Controllers 171 and 172 are arranged to co-operate, such that the required (over)power for superposing the third harmonic component is available in the output stage 16 when needed. By way of example, input stage controller 171 is configured to generate appropriate (PWM) control signals for operating switches Q1 , Q2 of the boost converter circuit 145 in conjunction with controller 172 determining the PWM signal for inverter circuitry 162. When the PWM signal for inverter circuitry 162 is changed by controller 172, input stage controller 171 can be configured to react and change control signals for the boost converter switches, hence increasing or decreasing the electric power transmitted to the output stage 16 (there can be a small time delay of about 1 ms in practice) in view of generating the AC output voltage signal, which is supplied to the load 9.

[0076] This is shown in Fig. 4 at step 43, where the PWM control signals, both for the inverter circuit 162 and the boost converter circuit 145 are generated based on VRMS and a predetermined amount A of the third harmonic component. [0077] It will be convenient to note that the CCRs according to aspects of the invention can be used in a modular arrangement, such as modules as described in WO 2014/001402, when provided with suitable features.

Claims

1. Constant current regulator (10) for supplying and controlling an AC series circuit (9) current of an airfield lighting system, comprising:

an input port (1 1 ) arranged for being coupled to an AC power supply, an output port (12) arranged for outputting the series circuit current at an output voltage,

an interface port (13) arranged for receiving a command signal representative of a set value of the series circuit current, and

a transformer (15) coupled between the input port and the output port, for providing galvanic insulation between the input and output ports,

a control unit (171 , 172) configured for determining a root mean square voltage level value required to generate the set value of the series circuit current and configured for generating a pulse width modulated signal,

electric circuitry (14, 16) comprising input stage circuitry (14) coupled between the input port (1 1 ) and the transformer (15), and output stage circuitry (16) coupled between the transformer (15) and the output port (12),

wherein the output stage circuitry (16) comprises inverter circuitry (162) arranged for generating an output voltage signal having a root mean square value equal to the root mean square voltage level value, wherein the control unit (171 , 172) is coupled to the inverter circuitry (162) for applying the pulse width modulated signal thereto, the pulse width modulated signal being configured for determining the output voltage signal when applied to the inverter circuitry, and wherein the electric circuitry is arranged for applying the output voltage signal to the output port (12),

characterised in that the control unit (171 , 172) allows for generating the pulse width modulated signal such that the output voltage signal comprises a fundamental component (31 ) of sinusoidal shape and of a fundamental frequency, and a predetermined amount of a third harmonic component (32) of the fundamental component having a frequency three times the fundamental frequency, wherein the third harmonic component has a peak value of at least 1 % of a peak value of the fundamental component.

2. Constant current regulator of claim 1 , wherein the control unit is configured for generating the pulse width modulated signal having a duty cycle which is determined based on the predetermined amount of the third harmonic component and based on the root mean square voltage level value.

3. Constant current regulator of claim 1 or 2, wherein the third harmonic component has a peak value of at least 3% of a peak value of the fundamental component.

4. Constant current regulator of claim 1 or 2, wherein the third harmonic component has a peak value of at least 5% of a peak value of the fundamental component.

5. Constant current regulator of claim 1 or 2, wherein the third harmonic component has a peak value of at least 10% of a peak value of the fundamental component.

6. Constant current regulator of any one of the preceding claims, wherein the control unit (171 , 172) is arranged for generating the pulse width modulated signal, such that the fundamental frequency corresponds to a frequency of an input signal applied at the input port (1 1 ).

7. Constant current regulator of any one of the preceding claims, wherein the control unit (171 , 172) is configured for generating the pulse width modulated signal, such that a phase of the output voltage signal corresponds to a phase of an input signal applied at the input port (1 1 ).

8. Constant current regulator of any one of the preceding claims, wherein the output voltage signal consists essentially of the fundamental component and the third harmonic component.

9. Constant current regulator of any one of the preceding claims, wherein the control unit (171 , 172) is implemented such that the generated pulse width modulated signal is based on a reference signal comprising the predetermined amount of the third harmonic component (32).

10. Constant current regulator of any one of the preceding claims, wherein the input stage circuitry comprises a boost converter circuitry (145) coupled between the input port (1 1 ) and the transformer.

11. Constant current regulator of claim 10, wherein the boost converter circuitry (145) is configured for providing instantaneous additional power in addition to instantaneous power available at the input port (1 1 ) for generating the voltage output signal in the inverter circuitry (162).

12. Constant current regulator of claim 10 or 1 1 , wherein the control unit (171 ) is coupled to the boost converter circuitry (145) and is configured for controlling the boost converter circuitry to provide instantaneous additional power at an instant of time in which the output voltage signal creates an instantaneous output power at the output port (12) which exceeds an instantaneous power available at the input port (1 1 ).

13. Constant current regulator of claim 12, wherein the control unit (171 ) is configured for controlling the boost converter circuitry (145) through a pulse width modulated control signal.

14. Constant current regulator of claim 13, wherein the control unit (171 ) is programmed to determine an instantaneous duty cycle of the pulse width modulated control signal.

15. Constant current regulator of claim 13 or 14, wherein the control unit (171 , 172) is configured for changing the predetermined amount of the third harmonic component in the output voltage signal by modifying a duty cycle of the pulse width modulated signal, and wherein the control unit is configured to modify the pulse width modulated control signal in response to a change of the pulse width modulated signal.

16. Constant current regulator of any one of the preceding claims, wherein the input stage circuitry (14) comprises rectifier circuitry (142) for rectifying a voltage signal applied to the input port to obtain a primary DC voltage signal, and inverter circuitry (143) coupled between the rectifier circuitry and the transformer for converting the primary DC voltage signal into a primary AC voltage signal applied to a primary side (151 ) of the transformer (15), the primary AC voltage signal having a frequency of at least 10 kHz.

17. Constant current regulator of any one of the preceding claims, wherein the output stage circuitry (16) comprises rectifier circuitry (161 ) for rectifying a voltage signal available at a secondary side (152) of the transformer (15) to obtain a secondary DC voltage signal, and wherein the inverter circuitry (162) is coupled between the rectifier circuitry (161 ) of the output stage and the output port (12) for converting the secondary DC voltage signal into a secondary AC voltage signal representative of the output voltage signal.

18. Constant current regulator of any one of the preceding claims, wherein the control unit (171 , 172) allows for generating the pulse width modulated signal based on a reference signal having a waveform defined by sin(oot) + A ^* sin(3oot), wherein A represents the predetermined amount of the third harmonic component, t is time and ω = 2ττΐ with f the fundamental frequency.

19. Constant current regulator of claim 18, wherein the control unit (171 , 172) is configured for generating the pulse width modulated signal with different values for the predetermined amount of the third harmonic component (A).

20. An airfield lighting system comprising a series circuit (9) of airfield lights and a constant current regulator (10) of any one of the preceding claims, wherein the power output of the constant current regulator is connected to the series circuit for supplying and controlling an electrical current flowing in the series circuit.

21. Method of supplying an electrical AC current to a series circuit (9) of airfield lights, comprising:

receiving a pre-set current level at an interface port (13) and introducing the pre-set current level in an electric current feedback loop to control an electric current root mean square value at an output port (12),

generating a pulse width modulated signal and applying the pulse width modulated signal to inverter circuitry (162) of output stage circuitry (16) of a constant current regulator (10), which output stage circuitry is coupled between an insulating transformer (15) and the output port (12), wherein the constant current regulator comprises input stage circuitry (14) coupled between an input port (1 1 ) and the insulating transformer (15),

wherein the pulse width modulated signal, applied to the inverter circuitry (162), generates a voltage output signal having a root mean square value corresponding to a root mean square voltage level set by the electric current feedback loop, the voltage output signal comprising a fundamental component (31 ) of sinusoidal shape and of a fundamental frequency, and a predetermined amount of a third harmonic component (32) of the fundamental component having a frequency three times the fundamental frequency, wherein the third harmonic component has a peak value of at least 1 % of a peak value of the fundamental component.

22. Method of claim 21 , comprising generating a pulse width modulated control signal for operating a boost converter circuit (145) of the input stage circuitry (14), wherein the pulse width modulated control signal, applied to the boost converter circuitry (145), provides an overpower compared to an electric power supplied at the input port (1 1 ), wherein the overpower is transmitted to the output stage circuitry for generating the output voltage signal.

23. Method of claim 22, comprising changing the pulse width modulated control signal in response to a change in the pulse width modulated signal.