HEATING SYSTEM COMPRISING A RESISTIVE HEAT ELEMENT, CONTROLLER FOR SUCH HEATING SYSTEM, AND METHOD OF CONTROLLING A LOAD CURRENT THROUGH SUCH RESISTIVE HEAT ELEMENT

Abstract

A heating system having: i) at least one resistive heat element; ii) at least two terminals for receiving a grid voltage from a power grid, and iii) a controller for being connected to the terminals for receiving the grid voltage, the controller connected to the at least one resistive heat element and being configured for controlling a load current through the at least one resistive heat element, wherein the controller is configured controlling the load current though the at least one resistive heat element. The controller comprises an FCFO-bidirectional power switch connected in series with the at least one resistive element for controlling the load current that is received from the power grid. A method is for controlling the load current through the at least one resistive element, which applies a certain algorithm to avoid inrush current, shortens the length of a cold start and solves problems such as EML.

Claims

1.-15. (canceled)

16. A heating system comprising: at least one resistive heat element; at least two terminals for receiving a grid voltage from a power grid, and a controller for being connected to the terminals for receiving the grid voltage, the controller connected to the at least one resistive heat element and being configured for controlling a load current through the at least one resistive heat element, wherein the controller is configured controlling the load current though the at least one resistive heat element, wherein the controller comprises a forced-closure forced-opening bidirectional power switch connected in series with the at least one resistive element for controlling the load current that is received from the power grid.

17. The heating system according to claim 16, wherein the bidirectional power switch is for controlling of the load current in a binary way, includes a closed state, wherein the bidirectional power switch allows load current to flow in both directions and an open state, wherein the bidirectional power switch blocks the load current.

18. The heating system according to claim 17, wherein the bidirectional power switch is a Solid-State Relay comprising power MOSFETs.

19. The heating system according to claim 18, wherein the controller further comprises a bidirectional power switch driver connected to the bidirectional power switch for driving the power MOSFETs with a driving signal.

20. The heating system according to claim 16, wherein the controller further comprises a digital processing unit for controlling the bidirectional power switch by a control signal.

21. The heating system according to claim 20, wherein the controller further comprises a power monitoring module connected to the at least one resistive heat element for measuring the load current and for providing this information to the digital processing unit including detection of zero-crossings.

22. The heating system according to claim 20, wherein the digital processing unit is configured for controlling the load current in accordance with at least two operational modes.

23. The heating system according to claim 22, wherein a first mode of the at least two operational modes is a cold-start mode.

24. The heating system according to claim 23, wherein a second mode of the at least two operational modes is a synchronous mode or a non-regulated mode.

25. The heating system according to claim 24, wherein, in the cold-start mode, the controller causes the bidirectional power switch to block the load current during a phase angle interval wherein an absolute value of the load current would be equal to or higher than a predefined current threshold unless a start of the phase angle interval comes later than a predefined phase angle threshold.

26. The heating system according to claim 25, wherein the controller switches to the second mode when the start of the phase angle interval is larger than the predefined phase angle threshold or when the phase angle threshold is reached before the current threshold is reached.

27. A method of controlling a load current in a resistive load with a bidirectional power switch in a heating system that is connected to an alternating grid voltage, wherein the bidirectional power switch is controlled in a binary way, which includes a closed state, wherein the bidirectional power switch allows load current to flow in both directions and an open state, wherein the bidirectional power switch blocks the load current, the method comprising steps of: a) starting a cold-start mode; b) setting a current threshold and a phase angle threshold; c) detecting a zero-crossing of the grid voltage and setting an actual phase angle to zero at this point; d) if not already switched on then switching on the bidirectional power switch for allowing load current to flow through the resistive load; e) measuring an actual load current through the resistive load; f) determining an actual phase angle of the grid voltage; g) comparing an absolute value of the actual load current with the current threshold and if the actual load current is larger than or equal to the current threshold then going to step h), otherwise going to step j); h) switching off the bidirectional power switch and storing the actual phase angle as a phase interval start value; i) comparing the phase interval start value with the phase angle threshold and if the phase interval start value is larger than the phase angle threshold then going to step o), otherwise going to step k); j) comparing the actual phase angle with the phase angle threshold and if the actual phase angle is larger than or equal to the phase angle threshold then going to step o), otherwise going to step f); k) determining the actual phase angle; l) comparing the actual phase angle with a value equalling pi minus the phase interval start value and if this value is reached going to step m), otherwise going to step k); m) switching on the bidirectional power switch for allowing load current to flow through the resistive load; n) repeating from step c); o) stopping the cold-start mode and optionally switching to a second mode.

28. A non-transitory computer-readable medium encoded with instructions that, when executed by a control unit, cause the control unit to execute a method of controlling a load current in a resistive load with a bidirectional power switch in a heating system that is connected to an alternating grid voltage, wherein the bidirectional power switch is controlled in a binary way, which includes a closed state, wherein the bidirectional power switch allows load current to flow in both directions and an open state, wherein the bidirectional power switch blocks the load current, the method comprising steps of: a) starting a cold-start mode; b) setting a current threshold and a phase angle threshold; c) detecting a zero-crossing of the grid voltage and setting an actual phase angle to zero at this point; d) if not already switched on then switching on the bidirectional power switch for allowing load current to flow through the resistive load; e) measuring an actual load current through the resistive load; f) determining an actual phase angle of the grid voltage; g) comparing an absolute value of the actual load current with the current threshold and if the actual load current is larger than or equal to the current threshold then going to step h), otherwise going to step j); h) switching off the bidirectional power switch and storing the actual phase angle as a phase interval start value; i) comparing the phase interval start value with the phase angle threshold and if the phase interval start value is larger than the phase angle threshold then going to step o), otherwise going to step k); j) comparing the actual phase angle with the phase angle threshold and if the actual phase angle is larger than or equal to the phase angle threshold then going to step o), otherwise going to step f); k) determining the actual phase angle; l) comparing the actual phase angle with a value equalling pi minus the phase interval start value and if this value is reached going to step m), otherwise going to step k); m) switching on the bidirectional power switch for allowing load current to flow through the resistive load; n) repeating from step c); o) stopping the cold-start mode and optionally switching to a second mode.

Description

BRIEF INTRODUCTION OF THE FIGURES

[0055] In the following is described examples of embodiments illustrated in the accompanying figures, wherein:

[0056] FIG. 1 discloses a heat module arrangement wherein the current invention may be used;

[0057] FIG. 2 discloses an embodiment of the heat module in FIG. 1 in an exploded view;

[0058] FIG. 3 shows a high-level schematic view of a first embodiment of the heating system according to the invention;

[0059] FIG. 4 shows a high-level schematic view of a second embodiment of the heating system according to the invention;

[0060] FIG. 5a shows a first embodiment of a FCFO-bidirectional power switch, which can be used in the controller of the invention;

[0061] FIG. 5b shows a second embodiment of a FCFO-bidirectional power switch, which can be used in the controller of the invention;

[0062] FIG. 6 shows an example of a FCFO-bidirectional power switch driver, which can be used in the controller of the invention;

[0063] FIG. 7 shows an example of a power monitoring module, which can be used in the heating system of the invention;

[0064] FIG. 8 illustrates very important aspects of the current invention, in particular the cold-start mode of the heating system of the invention;

[0065] FIG. 9 shows a cold-start algorithm in accordance with an embodiment of the method of the invention, and

[0066] FIG. 10 shows a further embodiment of the method of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0067] Various illustrative embodiments of the present subject matter are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

[0068] The present subject matter will now be described with reference to the attached figures. Various systems, structures and devices are schematically depicted in the figures for purposes of explanation only and to not obscure the present disclosure with details that are well known to those skilled in the art. Nevertheless, the attached figures are included to describe and explain illustrative examples of the present disclosure. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase.

[0069] The purpose of this description is to provide more detailed input on circuitry and an algorithm for an optimized way to control a resistive load like a heater element, during its coldstart and transitioning to normal operation. The focus is on delivering criteria such as algorithm efficiency, that is to shorten the cold-start period, limit the EMI, and limit the drawn current. A further purpose is to provide power efficient circuitry to shorten the cold-start and limit the power loss, current consumption limitation and low electromagnetic interferences.

[0070] The focus in the given embodiments is on both TN- and IT-grids. IT grids typically have Live1, Live2 and Earth/Ground terminals. TN-grids typically have Live, Neutral, and Earth/Ground terminals. In the case of a TN-grid a safety switch on the Neutral line may be dispensed with as this line does not carry a voltage.

[0071] IT-grids represent roughly 90% of the grids found on ships and offshore platforms. However, the invention is not limited to these two grid types and is equally applicable to other grid types, as long as they provide AC grid voltage.

[0072] The invention will be discussed in more detail with reference to the figures. The figures will be mainly discussed in as far as they differ from previous figures.

[0073] FIG. 1 discloses a heat module arrangement 1 wherein the current invention may be used. The heat module arrangement 1 is configured to be arranged on or in a flooring 3. The heat module arrangement 1 comprises a heat module device comprising two or more heat modules 5, each comprising at least one heat element 10 (visible in FIG. 2), and an electrical cabinet 15 (referred to as electric power source in non-prepublished patent application NO20191312) connected to the at least one heat element 10. The heat module arrangement 1 further comprises a junction box 18 connecting the heat elements 10 of the heat modules 5. In FIG. 1 two heat modules 5 are shown. However, it shall be understood that further heat modules 5 may be connected correspondingly.

[0074] In FIG. 2 an embodiment of one of the heat modules 5 from FIG. 1 is disclosed in an exploded view. The heat module 5 comprises a stepping plate 20 and a base plate 22. The stepping plate 20 comprises a first side S1 and second side S2. The at least one heat element 10 is attached to the first side S1 of the stepping plate 20 so that the emitted heat is directly conducted to the stepping plate 20. The second side S2 of the stepping plate 20 is configured to be stepped on by a person.

[0075] The heat modules 5 further comprises a connection member 30 configured to connect the stepping plate 20 and the base plate 22 and hold them separated from each other, thereby forming a spacing. The connection member 30 further has the function of isolating the spacing from the surrounding environment. The connection member 30 may comprise a first portion 32a at the stepping plate 20 and a second portion 32b at the base plate 22. The first portion 32a and the second portion 32b are configured to jointly connect the stepping plate 20 and the base plate 22.

[0076] The stepping plate 20 may comprise an anti-slip layer at the second side S2.

[0077] The heat modules 5 in FIG. 1 further comprises a water impermeable insulator 40 arranged so that the at least one heat element 10 is isolated from the surrounding environment. The insulator 40 is for example an epoxy-foam or a polyurethane-foam. The insulator 40 may be arranged filling said spacing to more than 99.5%. In addition to isolating the heat element 10 from the surrounding environment, and directing the heat to the stepping plate 20, in certain implementations where the insulator is epoxy-foam or polyurethane-foam, also adds significant structural strength to the heat modules 5.

[0078] The stepping plate 20 comprises for example aluminium and has a wall thickness in an interwall between 0,5 and 3 mm. Preferably, the stepping plate 20 has been processed by a rolling rib. By preparing the stepping plate 20 by means of such a rolling rib, it becomes possible to provide the stepping plate 20 with a very low thickness, considerably reducing the weight of the heat module 5.

[0079] The at least one heat element 10 is preferably attached to the stepping plate 20 by means of one of an adhesive tape, such as aluminium tape, a glue connection, such as heat transferring glue, and a bolt connection. Alternatively, the heat modules 5 comprises at least one further connection member 42 for connecting the at least one heat element 10 to the stepping plate 20. The at least one further connection member 42 is attached to the stepping plate 20 or coextruded with the stepping plate 20. See FIG. 3e.

[0080] The base plate 22 mainly comprises a rigid structural material, such as a metal plate or extruded epoxy.

[0081] The at least one heat element 10 preferably comprises an electric self-regulating heating cable. The electric self-regulating heating cable enables the temperature to be regulated to a predetermined temperature or a predetermined temperature interval. Alternatively, the at least one heat element 10 comprises an electric heating cable, a heat mat, or a heating paint.

[0082] The heat module 5 comprises connection means, such as a bolt connection or a magnet connection, for connecting the heat module 5 to the flooring 3. Alternatively, the heat module 5 may comprise one or more legs for holding the heat module 5 elevated from the flooring 3.

[0083] More information about the heat module arrangement 1 can be found in non-prepublished patent application Ser. No. 20/191,312. The current invention deals with the aspect of steering (controlling current through) the heat module arrangement 1. The at least one heat element 10 of the heat module arrangement 1, just like many other types of heating modules, forms together with the junction box 18 at least form a resistive load for the electrical cabinet 15.

[0084] When such heating modules 5 are switched on different kinds of problems may occur. And that is where the current invention provides a solution.

[0085] As discussed in the introduction, the primary feature of the invention is about the FCFO-bidirectional power switch, opening up a load of new possibilities. The embodiments discussed hereinafter are focusing in more detail on these possibilities.

[0086] FIG. 3 shows a high-level schematic view of a first embodiment of the heating system 100 according to the invention. The figure shows a power grid, which is a TN-grid, i.e., it as a phase line (or live line) L carrying an AC-voltage signal having the full amplitude and a neutral line N carrying no signal. This results in a grid voltage Vg having the full swing. The main blocks of the heating system 100 are a controller 50 connected with the resistive load, which comprises at least one heat element 10, but also a junction box 18 as previously discussed. It is not so important how this impedance is built up as long as it has a resistive component. The controller 50 has a first terminal T1 and a second terminal T2 that are connected with the phase line L and the neutral line N of the power grid, respectively. The resistive heat element 10, 18 is connected between an output of the controller 50 and the neutral line N as illustrated. The controller 50 controls the load current I_RT through the resistive heat element, which obviously is drawn from the power grid. It is assumed that the controller itself will hardly consume any power. Expressed differently, its contribution to the total power consumption can be ignored compared to the power consumption in the heating elements 10). Hence, the actual current drawn from the power grid is substantially equal to the load current drawn by the electrical load 10. In the current embodiment the controller 50 comprises of multiple blocks. The controller 50 comprises a power monitoring module 52, which measures and monitors the current I_RT delivered to the resistive load 10. The power monitoring module 52 delivers this information digitally to a digital processing unit 54 via a communication bus CB. The digital processing unit 54 is coupled with an output to an input of a FCFO-bidirectional power switch driver 56, the output carrying a control signal CS as illustrated. The FCFO-bidirectional power switch driver 56 delivers a driving signal DS to a FCFO-bidirectional power switch 58. In order for the heating system 100 to work properly galvanic isolation 57 is required, particularly in the FCFO-bidirectional power switch driver 56. This means that a galvanically isolated DC-supply voltage Vddi is required at an output side of the FCFO-bidirectional power switch driver 56. Also, the isolation requires an isolated ground GNDi. This isolated voltages GNDi, Vddi are created in separate voltage generators/separators (not shown) such as isolated DC-DC converters. The remaining (parts of) the blocks may be provided with a normal DC-supply voltage V.sub.CC and ground GND as illustrated. The power monitoring module 52 also preferably comprises reinforced isolation 53 for electrically isolating/shielding. It must be noted that everything related to isolation in the current embodiment is purely done from a safety perspective in a practical implementation using existing electronic components. However, the invention is by no means limited to such solutions.

[0087] FIG. 4 shows a high-level schematic view of a second embodiment of the heating system 100-2 according to the invention. The figure shows a power grid, which is an IT-grid, having a first phase line L1 carrying an AC-voltage signal and a second phase line L2 carrying an AC-voltage signal. The resulting swing of the grid voltage Vg is effectively the same as in FIG. 3. This embodiment of the heating system 100-2 will only be discussed in as far as it deviates from FIG. 3. The main difference is that this heating system 100-2 comprises an extra safety measure in the form of an electro-mechanically controlled switch 59 that is connected with one terminal to the second phase line L2 in series with the resistive load 10, 18. The electro-mechanically controlled switch 59 (such as a relay) is controlled by the digital processing unit 54, which generates a controlled switch control signal CSCS for the electro-mechanically controlled switch 59. A main function of this mechanically controlled switch 59 is to be able to completely deactivate the heating system 100-2 when not being used, that is that no voltage is present over the resistive heat elements 10, 18. Without the electro-mechanically controlled switch 59 the voltage signal on the neutral line N may be transferred to the resistive heat elements 10, 18, which forms a safety hazard for people working on the heating system. The relay could be replaced with an SSR or SCR combination or a TRIAC.

[0088] FIG. 5a shows a first embodiment of a FCFO-bidirectional power switch 58, which can be used in the controller 50 of the invention. This figure shows a Solid-State Relay circuit having two N-channel type power MOSFETs NM connected with their respective sources Src1, Src2 together, i.e., back-to-back. The inputs IN1, IN2 of the two MOSFETs NM are to be driven with the same gate signal, wherein a high voltage causes the switch 58 to be closed and a low voltage causes the switch 58 to be open. The outputs OUT1, OUT2 of the switch 58 are defined by respective drains Dr1, Dr2 as illustrated. When closed the switch 58 will allow current to flow in both directions through the MOSFETs NM. This is the reason the switch is called a bidirectional power switch.

[0089] FIG. 5b shows a second embodiment of a FCFO-bidirectional power switch 58-2, which can be used in the controller 50 of the invention. This figure shows a Solid-State Relay circuit having two P-channel type power MOSFETs PM connected with their respective sources Src1, Src2 together, i.e., back-to-back. The inputs IN1, IN2 of the two MOSFETs PM are to be driven with the same gate signal, wherein a low voltage causes the switch 58-2 to be closed and a high voltage causes the switch 58-2 to be open. The outputs OUT1, OUT2 of the switch 58-2 are defined by respective drains Dr1, Dr2 as illustrated. When closed the switch 58-2 will allow current to flow in both directions through the MOSFETs PM. The inversion of the response to gate signals will have to be accounted for in the control signal CS generated by the digital processing unit.

[0090] The FCFO-bidirectional power switches of FIGS. 5a and 5b are available components from Texas Instruments and other semiconductor manufacturers. More information about these components can be found in: [0091] SLVA948-December 2017 Achieve Bidirectional Control and Protection Through Back-to-Back Connected eFuse Devices pp: 2-3.

[0092] FIG. 6 shows an example of a bidirectional power switch driver 56, which can be used in the controller 50 of the invention. There are many components available on the market, which may be used. This figure shows just one example, which is available from Infineon. It concerns the 1EDI EiceDriver? Compact 1EDI20N12AF. It features the earlier discussed galvanic isolation 57 and is capable of driving the large gate capacitances on the inputs IN1, IN2 of the power MOSFETs of the FCFO-bidirectional power switch 50. As it is just a gate driver, its function is not discussed in detail here.

[0093] For the digital processing unit 54 in FIGS. 3 and 4 there are many alternative available, such as a microcontroller, a microprocessor with its peripheral circuitry, a digital signal processor (DSP) or a field-programmable gate array (FPGA). As long as the chosen digital processing unit 54 can be programmed it will be capable of carrying out methods (algorithms) for controlling the FCFO-bidirectional power switch 58 of the invention in accordance with a program or set of instructions. Alternatively, the method (algorithm) may be implemented on an application-specific integrated circuit (ASIC) as well. This is a choice of a designer when implementing the invention.

[0094] FIG. 7 shows an example of a power-monitoring module 52, which can be used in the heating system 100, 100-2 of the invention. There are many components available on the market, which may be used. This figure shows just one example which may be used, which is available from Allegro Microsystems. It concerns the ACS71020 Single Phase Isolated, AC Power Monitoring IC with Voltage Zero Crossing and Overcurrent Detection. It features the earlier discussed reinforced isolation 73. The figure shows how the chipset is16urrcted to the phase line L and neutral line N (polarity not important). A major function of the chipset that is used is its zero-crossing detection capability, which may be conveniently used in the algorithm of the invention as will be discussed later. As an alternative to monitoring power (where the current is obtained by dividing the power by the voltage) one might monitor the current directly.

[0095] FIG. 8 illustrates very important aspects of the current invention, in particular the cold-start mode M1 of the heating system 100, 100-2 of the invention. This figure will be first briefly explained and subsequently in more detail when discussing FIG. 9. The figure illustrates the current in a transition from an OFF-state to a cold-start mode M1, and subsequently from the cold-start mode M1 into a second mode M2, which is a synchronous mode in this example. The figure shows how an actual load current I_act varies over time during these consecutive different modes. It must be stressed that the drawing is purely for illustration purposes and may in practise deviate a lot from what is shown, in particular as regards the number cycles and half cycles in cold-start mode M1. In the current example there are slightly more than three half cycles hc1, hc2, hc3 in the cold-start mode M1 and about two half-cycles hc4, hc5 in the synchronous mode M2. The transition between the cold-start mode M1 and the synchronous mode M2 does not exactly coincide with a cycle transition (from hc3 to hc4) for reasons that will be explained with reference to FIG. 9.

[0096] FIG. 8 shows three sinusoidal current curves 11, 12, 13 which each represent what the actual load current I_act would be based upon the grid voltage Vg and the actual value of the resistance of the resistive load 10, 18, when we consider that there would be no limitations on the current carrying capacity of the resistive load 10, 18 when considered in a steady-state situation. The first curve 11 with the highest amplitude represents the current signal with the lowest resistance value of the resistive load 10, 18, which is typical when the resistive heat element 10 is cold, i.e., lower temperature generally results in a lower resistance. The second curve 12 with a lower amplitude than the first represents the current signal with a medium resistance value of the resistive load 10, 18, which may occur when the temperature of the heat element 10 has already increased. The third curve 13 with the lowest amplitude represents the current signal with the highest temperature of the heat element 10.

[0097] The essence of the algorithm in accordance with some embodiments of the invention is that it needs to ensure that the controller 50 (FIG. 3) causes the FCFO-bidirectional power switch 58, BPS (FIG. 3) to block the load current I_RT, I_act (FIGS. 3+8) during a phase angle interval ai1, ai2, ai3 (FIG. 3) where an absolute value of the load current 11, 12, 13 (FIG. 8) would be equal to or higher than a predefined current threshold I_th (FIG. 8) unless a start of the phase angle interval a0 (FIG. 8) comes later than a predefined phase angle threshold a_th (FIG. 8). The resulting phase angle intervals ai1, ai2, ai3 and their respective lengths extending between the start a0 of the phase interval ai1, ai2, ai3 and the end ?-a0 are clearly illustrated in FIG. 8. Here it must be stressed that the actual phase angle a_act is set on zero at each zero-crossing of the current signal, as illustrated in FIG. 8.

[0098] In an embodiment of the method the controller 50 switches to the second mode M2 when the start of the phase angle interval a0 is larger than the predefined phase angle threshold a_th or when the phase angle threshold a_th is reached before the current threshold I_th is reached.

[0099] It will be understood that there are many ways of implementing such methods, i.e., different algorithms may be developed, which have the same or similar result. In FIGS. 9 and 10 two different implementations are discussed. However, the invention is not limited to these specific implementations.

[0100] FIG. 9 shows a cold-start algorithm in accordance with an embodiment of the method 200 of the invention. The method concerns a method of controlling a load current I_act in a resistive load 10, 18 with a FCFO-bidirectional power switch 58, 58-2, BPS in a heating system 100, 100-2 that is connected to an alternating grid voltage Vg, wherein the FCFO-bidirectional power switch 58, 58-2, BPS is controlled in a binary way, which includes an closed state, wherein the FCFO-bidirectional power switch 58, 58-2, BPS allows load current I_RT to flow in both directions and an open state, wherein the FCFO-bidirectional power switch 58, 58-2, BPS blocks the load current I_RT. Even though the focus on heating systems, the method may be used in different application areas.

[0101] The method represents an algorithm which comprises the following steps: In a first step 201 (step a) the cold-start mode M1 is started. This generally happens when a system is switched on when for the first time or after some idle time. The cold-start mode M1 represents a so-called asynchronous mode as already discussed.

[0102] In a second step 210 (step b) the current threshold I_th and phase angle threshold a_th are set or determined. These threshold values may obviously have been set before the method is carried out. What is important, however, is that these values are available when the method is carried out.

[0103] In a third step 230, 240 (step c) a zero-crossing of the grid voltage Vg is detected and an actual phase angle a_act is set to zero at this point. Here it must be noted that the actual current I_act may not necessarily make a zero-crossing at start-up as the current is most likely zero before start-up. Hence, it is the grid voltage Vg that needs to be monitored for zero-crossings.

[0104] In a fourth step 250 (step d) the FCFO-bidirectional power switch (58, BPS) is switched on, if not already switched on, for allowing load current I_act to flow through the resistive load (10, 18). From this moment the actual load current I_act will follow a respective current curve 11, 12, 13 depending on the temperature and associated resistance of the resistive load (10, 18).

[0105] In a fifth step 260 (step e) the actual load current I_act through the resistive load 10, 18 is measured. This feature is key to making the algorithm adaptive and self-regulating. The prior art solutions do not show such feature.

[0106] In a sixth step 270 (step f) the actual phase angle a_act of the grid voltage Vg is determined. There are different ways of doing this, one of them is discussed in view of FIG. 10.

[0107] In a seventh step 280 (step g) an absolute value of the actual load current I_act is compared with the current threshold I_th. If the actual load current I_act is larger than or equal to the current threshold I_th then the next step will be step h), otherwise the next step will be step j).

[0108] In an eight step 282, 284 (step h) the FCFO-bidirectional power switch 58, BPS is switched off and the actual phase angle a_act is stored as a phase interval start value a0.

[0109] In a ninth step 286 (step i) the phase interval start value a0 is compared with the phase angle threshold a_th. If the phase interval start value a0 is larger than the phase angle threshold a_th then the next step is step o), otherwise the next step is step k).

[0110] In a tenth step 290 (step j) the actual phase angle a_act is compared with the phase angle threshold a_th. If the actual phase angle a_act is larger than or equal to the phase angle threshold a_th then the next step is step o), other the next step is step f).

[0111] In an eleventh step 300-1 (step k) the actual phase angle a_act is determined.

[0112] In a twelfth step 300-2 (step I) the actual phase angle a_act is compared with a value equalling PI (?) minus the phase interval start value a0. If this value is reached the next step is step m), otherwise the next step is step k).

[0113] The eleventh and the twelfth step together effectively form a detection of a predefined phase angle, that is a detection when the actual phase angle equal ?-a0. In other words, the two steps might be combined into a single phase-angle detection step.

[0114] In a thirteenth step 310 (step m) the FCFO-bidirectional power switch 58, BPS is switched on for allowing load current I_act to flow through the resistive load 10, 18.

[0115] In a fourteenth step 320 (step n) the method is repeated from step c).

[0116] In a fifteenth step 400 (step o) the cold-start mode M1 is stopped and optionally it is switched to a second mode M2.

[0117] It must be noted that, even though the steps are numbered here above, that this has nothing to do with the total number of steps being carried out in the method. The numbering only serves for referencing purposes.

[0118] Considering this algorithm of FIG. 9 following may be noted about FIG. 8. In the first half cycle hc1 the temperature is low, and the actual load current I_act follows the first current curve 11. It then rapidly hits the current threshold I_th as illustrated, where step g) will trigger step h) and switch off the FCFO-bidirectional power switch 58, BPS at step h). The actual load current I_act becomes zero at the phase interval start value a0 (which is determined to be lower than the phase angle threshold a_th in step i) and remains zero up to a phase angle of ?-a0 in the first half cycle hc1 in steps k), l), where the FCFO-bidirectional power switch 58, BPS is switched on in step m). At this point the actual load current I_act will become substantially equal to the first current curve 11 and follow it this signal towards the zero-crossing. Then step n) makes the method go back to step c). At the zero-crossing defined by the transition from the first half cycle hc1 to the second half cycle hc2 the actual phase angle a_act is set to zero in step c).

[0119] Now the first half cycle hc1 has caused the temperature of the resistive heat element to rise somewhat, which makes the actual load current I_act follow the second current curve 12 in the second half cycle hc2, as illustrated. It must be noted that this temperature rise may be a gradual effect (and the transition from the first current curve 11 to the second current curve 12 may be smooth) and does not need to occur exactly on the transition from the first half cycle hc1 to the second half cycle hc2. However, for illustration purposes it is shown that way.

[0120] The actual load current I_act follows the second current curve 12 in the beginning of the second half cycle hc2 and becomes negative. When an absolute value of the actual load current I_act reaches the current threshold I_th the FCFO-bidirectional power switch 58, BPS is switched off again. The actual load current I_act becomes zero at the phase interval start value a0 (which is larger than the previous half cycle hc1, but still determined to be lower than the phase angle threshold a_th in step i) and remains zero up to a phase angle of ?-a0 in the second half cycle hc2 in steps k), l), where the FCFO-bidirectional power switch 58, BPS is switched on in step m). At this point the actual load current I_act will become substantially equal to the second current curve 12 and follow it this signal towards the zero-crossing. Then step n) makes the method go back to step c). At the zero-crossing defined by the transition from the second half cycle hc2 to the third half cycle hc3 the actual phase angle a_act is set to zero in step c).

[0121] Now the second half cycle hc2 has caused the temperature of the resistive heat element to rise somewhat, which makes the actual load current I_act follow the third current curve 13 in the third half cycle hc3, as illustrated. It must be noted that this temperature rise may be a gradual effect (and the transition from the second current curve 12 to the third current curve 13 may be smooth) and does not need to occur exactly on the transition from the second half cycle hc2 to the third half cycle hc3. However, for illustration purposes it is shown that way.

[0122] The actual load 21urrentt I_act follows the third current curve 13 in the beginning of the third half cycle hc3 and becomes positive. When an absolute value of the actual load current I_act reaches the current threshold I_th the FCFO-bidirectional power switch 58, BPS is switched off again. The actual load current I_act becomes zero at the phase interval start value a0 (which is larger than the previous half cycle hc2, but still determined to be lower than the phase angle threshold a_th in step i) and remains zero up to a phase angle of ?-a0 in the third half cycle hc3 in steps k), l), where the FCFO-bidirectional power switch 58, BPS is switched on in step m). At this point the actual load current I_act will become substantially equal to the third current curve 13 and follow it this signal towards the zero-crossing. Then step n) makes the method go back to step c). At the zero-crossing defined by the transition from the third half cycle hc3 to the fourth half cycle hc4 the actual phase angle a_act is set to zero in step c).

[0123] Now the third half cycle hc3 in this example has caused the temperature of the resistive heat element reach its steady-state value, which makes the actual load current I_act follow the third current curve 13 in the fourth half cycle hc3, as illustrated. It must be noted that this temperature may in fact be a little bit larger causing the actual load current I_act to actually follow a fourth current curve (not shown) having even lower current values. The consequence of this is that something else happens in the fourth half cycle hc4, namely that the phase angle threshold a_th is reached before the load current threshold I_th. This is illustrated in FIG. 8 and will actually cause the method to go from step j) to step o) causing the FCFO-bidirectional power switch 58, BPS to remain ON and the cold-start mode M1 to transition to the second mode M2. When the load current threshold I_th has been chosen to be the nominal value of the circuit breaker and the phase angle threshold a_th is set around 45 degrees, this will prevent the actual load current I_act to rise above problematic levels causing the circuit breaker (fuse) to interrupt the current.

[0124] It must be noted that FIG. 8 pure illustrative. The temperature rising effect may in practise be much slower, causing the actual load current I_act to follow certain current curves many more cycles than illustrated in FIG. 8.

[0125] The inventor has discovered that it in a practical example it may take from a few up to a couple of hundred cycles for the cold-start to finish. This number depends on the thermal characteristics of the heater modules 5 and the environmental conditions such as temperature, wind, snow, water, sun exposure and location.

[0126] FIG. 10 shows a further embodiment of the method 200-2 of the invention. This embodiment will only be discussed in as far as it differs from FIG. 9. It may not be very easy to directly measure the actual phase angle a_act in the method. Instead, indirect measuring may be preferred, that is that a counter is used, which starts at a zero crossing and as time passes by the counter will increase, thereby giving an indication of the actual phase angle a_act.

[0127] As the grid frequency fg tends to be constant, all what is required is to calibrate the counter with the period (=1/fg) of the alternating voltage. The embodiment of FIG. 10 provides some flexibility in that incorporates the determination of the grid frequency fg together with calibration of the counter.

[0128] Between step b) and c) the following substeps are inserted:

[0129] In a first substep 220 it is determined whether the grid frequency fg is known or set. If not known or set the next step will be substep 222, otherwise the next step is step c) as earlier discussed.

[0130] In a second substep 222 a zero-crossing is detected, and the timer is started.

[0131] In a third substep 224 the timer is stopped after a predefined number of zero-crossings, for example 20 zero-crossings.

[0132] In a further substep 226 the grid frequency is calculated from the timer and the predefined number of zero-crossings.

[0133] It must be noted that the above-mentioned method of determining the grid frequency may also be carried out before the cold-start or at any other moment. Once done, the grid frequency may be stored in the controller 50.

[0134] Now that the grid frequency is known and the relation between the counter and the predefined number of zero-crossings, the counter may be calibrated such that is indirectly indicates the actual phase angle a_act.

[0135] In FIG. 9 this implies that step f) and step k) are implemented by determining a value of the counter. And that all phase angle comparisons in step i) and step j) are comparing counter values instead.

[0136] With reference to FIG. 8 it is further noted that the grid frequency fg typically lies between 50 and 60 Hertz, which results in a period between 0,0167s and 0,02s, and a half cycle between 0,00833s and 0,01s.

[0137] The algorithms of FIGS. 9 and 10 may be run on a state-machine. For the algorithms of FIGS. 9 and 10 the inventor designed the controller such that the sampling frequency is 1 kHz. However, other sampling frequencies may be used as well.

[0138] The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, instead of the gate driver presented in FIG. 6 a Photovoltaic Coupler for MOSFET driver as provided by Toshiba may be chosen. And there are many other circuits that are suitable.

[0139] The person skilled in the art may easily find alternative solutions for the power monitoring module, which may be replaced by a current measurement module as previously discussed. Also, in the embodiments having a relay as a mechanically controlled switch (FIG. 4) the relay may be replaced by a TRIAC or other SCR.

[0140] The invention covers all these variants as long as they are covered by the independent claims. No limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope of the invention. Accordingly, the protection sought herein is as set forth in the claims below.

[0141] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use 5 of the verb comprise and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article a or an preceding an element does not exclude the presence of a plurality of such elements. 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 invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claims enumerating several means, several of these means may be embodied by one and the same item of hardware.