Abstract:
An interconnection for connecting a switched mode inverter to a load, the interconnection comprising: a plurality of insulated conductors ( 311 - 313, 321 - 323 ); sleeving means ( 351 ) sleeving the insulated conductors together; and at least one lossy toroidal inductor core ( 352 ) concentric with and partially surrounding the sleeving means to hold the plurality of insulated conductors together; wherein the at least one lossy toroidal inductor core ( 352 ) is arranged to act as a common mode inductor to minimise current flowing through the interconnection to a stray capacitance of the load. Preferably, high frequency eddy current effects are minimised in the interconnection by a suitable choice of diameters of conductive cores of the plurality of insulated conductors and the spacing between the centres of the conductive cores.

Description:
BACKGROUND 
       [0001]    As shown in  FIG. 1 , a switched mode inverter (SMI)  10  for connection to a load  11  by an interconnection  12  is known, in which first and second connectors A, B of the switched mode inverter  10  are connected by the interconnection  12  to first and second connectors A 1 , B 1  respectively of the load  11 . 
         [0002]    Most usually, switched mode inverters use pulse width modulation (PWM) with a waveform  20  substantially as shown in  FIG. 2 . Such PWM waveforms  20  permit very flexible control methods of the load to be employed but the rectangular nature of the pulses means that a fast rate of change of voltage (dV/dt) on the leading edges  22  and trailing edges  21  of the switching pulses  23  can result in high currents flowing into stray capacitances Cpl associated with the switched mode inverter, since I=C*dV/dt where I is the current flowing into a capacitance, C is the stray capacitance and dV/dt is the rate of change of voltage V at the pulse edges Similar current can flow into a stray capacitance Cp 2  of the load. In this case the current I cp2  flows through both conductive elements of the cable connection  12  from the switched mode inverter  10  to the load  11 . To prevent, or at least minimize, the current I cp2  in the stray capacitance Cp 2  of the load, flowing in the same direction in both parts of an interconnection  12 , shown by arrow-headed lines  13 , a common mode choke L 1 , which tends to prevent current flowing in the same direction in both conductive paths of the interconnection, is provided in the interconnection. The choke or inductor L 1  thus provides a high impedance to impede current I cp2  that would otherwise flow in the same direction, shown by arrow headed lines  13 , in both parts of the interconnection joining the first connector A of the SMI to the first connector A 1  of the load and the second connector B of the SMI to the second connector B 1  of the load, while offering minimal impedance to the desired load current I p  flowing in opposed directions of arrow-headed lines  14  to the load via the first connector A of the SMI and first connector A 1  of the load and from the load via second connector B 1  of the load and the second connector B of the SMI. The desired current flows to the load on one cable, and back on another. The core has no effect on this differential signal, because the magnetic flux induced in the core by the outward current is cancelled by that in the return. In the case of stray capacitance, currents are flowing in the same direction on both conductors, the magnetic fluxes add, so that the core acts as a common mode choke. 
         [0003]    The choke L 1 , by minimizing the current flow I cp2  in direction of arrow-headed line  13 , minimizes the voltage that appears across Cp 2 , reducing the voltage across the stray capacitance from V peak  to k*V peak , where, with an appropriate design, the factor k is much less than 1. In prior art arrangements, the common mode choke provided a high impedance to noise, either generated at the SMI or at the load (such as would be generated by a magnetron), but the effect of the impedance was to reflect the noise, so there could have been radiation from the conductors causing EMC problems. 
         [0004]    Voltages that appear across the stray capacitances Cp 1  and Cp 2  stress associated dielectric materials and can lead to premature aging and consequent failure of such dielectric systems, and are therefore preferably avoided or minimised. 
         [0005]    Generally, good design practice minimizes or shields the capacitances Cp 1  and Cp 2  and a choke L 1  is installed in the feed lines connecting the first connector A of the SMI to the first connector A 1  of the load and connecting the second connector B of the SMI to the second connector B 1  of the load, and these lines are, in low power SMI&#39;s (i.e. &lt;1000 watts), quite short and direct. 
         [0006]    However, in high-power systems, EMC issues arise. In such high-power systems, the SMI and load can be physically quite large items, possibly with volumes of many hundreds of litres, and the resultant values of stray capacitances Cp 1  and Cp 2  can be very large, 5 to 30 nF being quite typical. With rates of changes of voltage typically of the order of 1000V/μs, the resultant common mode current I cp2  flowing in direction of arrow-headed lines  13  could be typically 30 A peak. Furthermore, it is not uncommon for the parts of the interconnection connecting the first connector A of the SMI to the first connector A 1  of the load  11  and connecting the second connector B of the SMI to the second connector B 1  of the load to be, perhaps, 5 metres long. Such large pulsed currents in such a long wire represent a source of a very serious EMC problem. 
         [0007]    In known circuits with a motor load, switching the voltage at a high frequency results in a current with a low frequency sine wave oscillation in the range 20 to 100 Hz. With a transformer as a load the current is also of a high frequency form and this requires a different approach to the lead system in the case of a transformer load from that used with a motor load. 
         [0008]    Thus, a further problem in high power systems with a transformer load is that the desired current I p  flowing in the direction  14  in the parts of the interconnection connecting the first connector A of the SMI to the first connector A 1  of the load and the second connector B 1  of the load to the second connector B of the SMI will be of a high frequency nature and also have high rms values. As indicated above, a typical waveform  20  is shown in  FIG. 2 , having a pulse frequency of 2,500 pulses/sec, peak currents of ±150 A, an rms current of 60 A and pulse rise and fall times of the order of 1 μs. 
         [0009]    With high frequency currents, due to eddy current effects, the current flows close to the surface and only the conductive material of thin conductors will be fully utilised. That is, the resultant AC resistance R ac  at high frequencies will be the same as the DC resistance R dc  if a thin conductor is used. So with high frequency currents that are of a high rms value, Ip rms , multiple conductors isolated from each other are required to handle the current without excessive dissipation. As a guide to what is a “high frequency” pulsed current and what is a “thin” conductor, at a pulse rate of 2,500 Hz the skin depth at which the current flow falls to 37% of its value is approximately 1.3 mm in a pure copper conductor. The current penetration of the higher frequency components of the current waveform in  FIG. 2  would be even less than 1.3 mm. 
         [0010]    At high frequencies the inductance of the cable can present a limiting impedance and result in the pulse current flow being restricted or distorted. This could, in principle, be overcome by using a connector such as coaxial cable or other specialised cable that can minimise inductance per unit length. However, such cable tends to be expensive and the copper in the inner conductor usually has a much smaller cross-sectional area than the outer conductor. Coaxial cable is designed for matched impedance transmission at frequencies of the order of 1 MHz and above. Therefore, when, as in the present case, the frequency is only a few kHz, coaxial cable is not an ideal choice for high power/current transmission. 
         [0011]    Moreover, to maximise the transmission of power in high power systems, multiphase power transmission systems are used. The most common of these is a 3-phase connection. The strategies discussed above can also be applied to a 3-phase SMI feeding a 3-phase load. 
         [0012]    The problems described above are well known and numerous solutions to individual aspects of the problems have been proposed in the existing art. 
         [0013]    It is an object of the present invention at least to ameliorate the aforesaid shortcomings in the prior art. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0014]    According to a first aspect of the present invention there is provided an interconnection for connecting a switched mode inverter to a transformer load, the interconnection comprising: a plurality of insulated conductors; sleeving means sleeving the insulated conductors together; and at least one lossy toroidal inductor core concentric with and partially surrounding the sleeving means to hold the plurality of insulated conductors together; wherein the at least one lossy toroidal inductor core is arranged to act as a common mode inductor to minimise current flowing through the interconnection to a stray capacitance of the load and the insulated conductors are arranged to minimize eddy current loss. 
         [0015]    Advantageously, high frequency eddy current effects are minimised by a suitable choice of diameters of conductive cores of the plurality of insulated conductors and of the spacing between the centres of the conductive cores. 
         [0016]    Conveniently, the interconnection further comprises a central insulating member wherein the plurality of insulated conductors are arranged around the central insulating member. 
         [0017]    Advantageously, the plurality of insulated conductors are arranged substantially in a circle around the central insulating member with a first plurality of insulated conductors arranged in a first semicircle for passing electrical current in a first direction through the interconnection and a second plurality of insulated conductors arranged in a second semicircle opposed to the first semicircle for passing electrical current in a second direction opposed to the first direction through the interconnection. 
         [0018]    Alternatively, the plurality of insulated conductors are arranged in a circle with members of a first plurality of insulated conductors alternating with members of a second plurality of insulated conductors and the first plurality of insulated conductors is arranged for passing current in a first direction through the interconnection and the second plurality of insulated conductors is arranged for passing a current in a second direction, opposed to the first direction, through the interconnection. 
         [0019]    Conveniently, the plurality of insulated conductors comprises a plurality of PVC-insulated copper-core cables. 
         [0020]    Advantageously, the interconnection comprises a plurality of lossy toroidal inductor cores spaced along the interconnection and arranged to hold the plurality of insulated conductors together and to act as a common mode inductor to minimise current flowing to a stray capacitance of the load. 
         [0021]    Conveniently, the at least one lossy toroidal inductor core has a quality factor less than 2 at a frequency of 100 kHz. 
         [0022]    Advantageously, the interconnection is arranged for pulse wave modulation of the load. 
         [0023]    Conveniently, the interconnection is arranged to pass a multiphase current between the switched mode inverter and the load. 
         [0024]    Advantageously, the plurality of insulated conductors comprises a go and return pair grouped together in a phase group for each of the phases with at least one lossy toroidal inductor core arranged as a common mode inductor on each phase group. 
         [0025]    Conveniently, the interconnection is arranged to pass a three-phase pulsed current. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    Embodiments of the invention are further described hereinafter, by way of example, with reference to the accompanying drawings, in which: 
           [0027]      FIG. 1  is a block diagram of an interconnection, for which the present invention may be used, for connecting a switched mode inverter to a load; 
           [0028]      FIG. 2  is a waveform typically used for pulse wave modulation in the interconnection of  FIG. 1 ; 
           [0029]      FIG. 3  is a transverse cross-section drawing of an interconnection according to the present invention; 
           [0030]      FIG. 4  is a perspective view of a transverse cross-section of an interconnection according to the present invention; 
           [0031]      FIG. 5  is a illustration of toroidal cores suitable for use in the interconnections of  FIG. 3  or  4 . 
           [0032]      FIG. 6  is a diagram showing magnetic cores spaced along the interconnection of  FIG. 3  or  4 ; and 
           [0033]      FIG. 7  is a schematic diagram of a three-phase interconnection embodiment of the present invention. 
       
    
    
       [0034]    In the Figures like reference numerals denote like parts. 
       DETAILED DESCRIPTION 
       [0035]      FIG. 3  shows a cross-section of a cable interconnection according to an embodiment of the invention that would be suitable for connecting a first connector A of an SMI  10  to a first connector A 1  of a load  11  and connecting a second connector B of the SMI  10  to a second connector B 1  of the load  11  in  FIG. 1 . 
         [0036]    In  FIG. 3 , electrical conductor cross-sections  311 - 313  marked A, with current flowing into the page, are “go” conductors connecting the first connector A of the SMI to the first connector A 1  of the load and the electrical conductor cross-sections  321 - 323  marked B, with current flowing out of the page, are “return” conductors connecting the second connector B 1  of the load to the second connector B of the SMI. 
         [0037]    Minimisation of high frequency eddy current effects, which undesirably make the ratio of the AC resistance R AC  to the DC resistance R DC  much greater than  1 , is dependent on two key parameters: a diameter d of the individual conductors  341  and a spacing Sp between centres of the individual conductors  341 . The calculations required for such minimisation are available in numerous standard texts but only for a relatively simple example, such as, for example, in “Alternating current resistance”, Bell System Technical Journal, Volume 4, April 1925, page 327. The far more complex arrangements of conductors required in this invention can be solved using computer aided design. It is important to retain the mechanical arrangement of the conductors to minimise loss in much the same way as coaxial cable needs to be kept coaxial to perform its function correctly. 
         [0038]    As can be seen in  FIGS. 3 and 4 , the cables  311 - 313 ,  321 - 323  that comprise the conductors are arranged transversely in two opposed semicircular halves respectively of a circle around an insulating central member  33 . Arranging the conductors substantially in a circle causes the high frequency current to flow at the outer surface of the cores of the interconnection. A conducting central member would do little to increase the current flow so that using, for example, copper for the central member instead of a less expensive insulating member would increase the cost of the interconnection without improving electrical conductivity. 
         [0039]    Individual cables such as Tri-rated BS6231 single core PVC insulated flexible cables with a single core copper conductor  341  insulated by a PVC insulating outer layer  342  are suitable for uses as the cables  311 - 313  and  321 - 323 . To keep the interconnection loosely in its required pattern, the group of cables  311 - 313 ,  321 - 323  and insulating centre member  33  are sheathed in expandable braided insulated sleeving  351 , such as RS 408-205. As shown in  FIGS. 3 ,  4  and  6 , to keep the cables in their grouping, torroidal cores  352  of a suitable magnetic material, to form the inductance L 1  of  FIG. 1 , also act as clamps to keep or hold the cables grouped together to form the interconnection. Although it is convenient for the toroidal cores to be used to hold the insulated conductors together as well as acting as common mode inductors, embodiments of the invention are envisaged in which the toroidal cores act solely as a common mode inductor and other clamping or holding means are used to clamp or hold the insulated conductors of the interconnection together. 
         [0040]    Any magnetic material normally currently used in inductor design is suitable for use in the toroidal cores. Appropriate laminar iron dust cores, or ferrites can be used. An important feature is that the magnetic material particle size is much greater or the laminations of the core are much thicker than would be used in a normal or typical inductor. This is to increase eddy current loss and thus increase resistance. For a 100 kHz inductor, a particle size or lamination thickness in a typical inductor is approximately 25 μm. Using a particle size or lamination thickness of 300 μm or even more in the present invention, eddy current loss becomes sufficiently high to produce a lossy inductor at 100 kHz. 
         [0041]    A quality factor Q, which is a ratio of the reactive component to the resistive component of the common mode choke, is intentionally very low, so causing resistive dissipation of the common mode switching edge transitions rather than reflection. A value of Q below 2 is ideal, compared with a typical inductor which would have a value of the quality factor greater than 50. As shown in  FIG. 6 , the magnetic cores are spaced at intervals along the interconnection suitable for the magnetic cores to act both as inductors and cable clamps. A wide variety of suitable cores from Micrometals Inc., 5615 E. La Palma Avenue, Anaheim, Calif. 92807 USA or Fair-Rite Products Corp. PO Box 288, 1 Commercial Row, Wallkill, N.Y. 12589 can be employed for the toroidal inductor cores. A photograph of a typical interconnect arrangement, including two toroidal cores, is shown in  FIG. 4 . 
         [0042]    In the invention, the lossy choke dissipates as heat the noise generated at the SMI or at the load, thereby reducing or eliminating the EMC problem of the prior art. 
         [0043]    The cable grouping shown in  FIGS. 3 and 4  is only one example of possible groupings of the insulated conductors. Other groupings which can be usefully used include a grouping with alternate cables located around a circle being used as “go” and “return” conductors. Also a random assembly, with or without the central insulating core of the conductors, will under many circumstances prove adequate. The total number of cables to be used in the interconnection is determined by a predetermined required current rating. It is found that, by correct calculation and appropriate design, the total amount of copper used in an interconnection of the invention is no greater than that required for an equivalent direct current interconnection. However, the overall diameter of the interconnection of the invention may be larger than required for an equivalent DC interconnection, because of the required insulation and spacing between individual conductors. 
         [0044]    For a three-phase application, a suitable arrangement of cables is shown in  FIG. 7 . This arrangement uses a pair of cables per lead and each go and return pair for each of the phases is grouped together and the common mode inductors L A , L B  and L c  are arranged on each phase grouping of leads. The inductance formed by the loops between the three-phase SMI having phased sources U n , V n  and W n  and the load having terminals A 1 , A 2 , B 1 , B 2 , C 1  and C 2  should be minimised as shown in  FIG. 7 . It will be understood that the lines connecting A 1  and C 2 ; A 2  and B 1  and B 2  and C 1  do not represent leads but imply interconnects. The arrangement shown is typical for a 2,500 Hz PWM waveform with 50 A rms rating per phase from a source voltage of 690V rms. This has each individual lead formed of a pair of parallel 4 mm 2  1.1 kV rated SIWO-KUL™ cables with four cables closely grouped in a bundle and sleeved together. Ten suppression cores of type RS 239-062 are fitted over the sleeved bundle of four cables to clamp the cables together and provide the common mode inductor or choke. It will be seen that separate inductors L A , L B , L C  are used for each group of cables with the same phase. 
         [0045]    Thus this invention when applied to poly-phase systems uses a simple method that overcomes at least some of the problems in the prior art, uses standard electrical single core wires in a suitable arrangement, instead of specialised and more expensive coaxial cable, and provides the required inductance L 1  using multiple magnetic toroidal cores that double as cable clamps to keep the cables in a required arrangement. 
         [0046]    Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not necessarily limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
         [0047]    Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.