Abstract:
An electronics power system includes a plurality of substantially identical power electronic modules. Each power electronic module includes a single-phase DC/AC inverter having an output side. Each power electronic module further includes a medium/high-frequency-isolated DC/DC current-to-voltage converter having an input side. The medium/high-frequency-isolated DC/DC current-to-voltage converter drives the single-phase DC/AC inverter. Each DC/DC converter and its corresponding DC/AC inverter are connected back-to-back sharing a common DC-link. The plurality of power electronics modules is stacked together in series at the input side and in parallel or series/parallel at the output side.

Description:
BACKGROUND 
       [0001]    The subject matter of this disclosure relates generally to power electronic systems, and more particularly to a scalable-voltage current-link power electronic system suitable for use in high-voltage mega-watt drives located at the offshore platform for oil and gas, current-link based high voltage DC (HVDC) taps, mega-watt drives for subsea oil and gas, and HVDC transmission and distribution (HVTD). 
         [0002]    The distance between the source (three-phase 60 Hz grid) and the load (e.g. many compressor drives, each P&gt;10 MW) may be more than 100 km for an exemplary current-link system. Three-phase grid voltage at the source side is actively rectified and converted to a constant current source. Current source inverters (CSI) at the load side may be used to generate three-phase voltage at the load terminals. Hence, the power is supplied through a current-link based DC transmission system which is similar to the HVDC-classic. The value of the current source is limited by two factors: 1) transmission line rated current capability and 2) transmission line losses. A typical value for multi mega-watt transmission and distribution system is 400 A. 
         [0003]    One example of a three-phase compressor drive  10  using state-of-the-art technology for the current-fed system described above is illustrated in  FIG. 1 . The DC current source  12  is a converted into a constant DC voltage source using a three-level DC-DC current-to-voltage converter  14 . A three-level DC/AC inverter  16  connected back-to-back with the converter  14  then generates three-phase voltage of desired magnitude and frequency at the machine terminals. 
         [0004]    Due to the limitation on the blocking voltage of the Si devices (e.g. IGCTs up to 6.6 kV) the DC-link voltage is limited to 5.4 kV. To supply 12 MW power to the compressor, the reflected DC voltage at the input of the drive system (assuming 400 A current source) is required to be at least 30 kV. Hence, six 5.4 kV drive modules as shown in  FIG. 1  are required. They are connected in series at the input terminals (current source side). The outputs of the modules are connected in series/parallel with the help of low-frequency transformers  18 . The transformers are required to combine the output voltages of each 5.4 kV modules, and to maintain the machine isolation voltage at a low value. 
         [0005]    The state-of-the-art system depicted in  FIG. 1  is disadvantageous in that the switching frequency (typically 400-600 Hz) of 5.5 kV devices is limited due to thermal management requirements. Hence, it causes the following: a) low band-width of the control loops, b) application of selective harmonic elimination (SHM); due to low PWM frequency, space vector PWM is not possible, and c) poor input-output waveforms. 
         [0006]    Further, six low frequency transformers  18  are required to provide isolation and to combine the output voltages from each 5.4 kV drive module. Due to the presence of transformers  18 , there are significant challenges in generating very low frequency three-phase output voltage. The DC output generation is not possible which is often required to start a three-phase PMAC. 
         [0007]    Scalability of the state-of-the-art technology is possible to drive a machine with a higher voltage rating. However, at the cost of the increase in the number of low-frequency transformers described above, this may not be feasible if power density is the premium requirement e.g. for the subsea oil and gas applications. 
         [0008]    Therefore, what is needed is a scalable-voltage current-fed power electronic system for multi-phase AC or DC loads that avoids the drawbacks of state-of-the-art technology for current-fed power electronics systems. 
       BRIEF DESCRIPTION 
       [0009]    One aspect of the present disclosure is directed to an electronics power system comprising a plurality of substantially identical power electronic modules. Each power electronic module comprises a medium/high-frequency-isolated DC/DC current-to-voltage converter driving a single-phase DC/AC inverter. Each DC/DC converter and its corresponding DC/AC inverter are connected back-to-back sharing a common DC-link. The plurality of power electronics modules is stacked together in series at the input side and in parallel or series/parallel at the output side. 
         [0010]    Another aspect of the present disclosure is directed to an electronics power system comprising a plurality of substantially identical power electronic modules. Each power electronics module comprises a medium/high-frequency-transformer isolated current-to-voltage converter driving a single-phase DC/AC inverter. The plurality of substantially identical power electronic modules is stacked together in series at the input side and in parallel or series/parallel at the output side to provide a scalable output voltage. 
         [0011]    According to yet another aspect of the present disclosure, an electronics power system comprises a plurality of substantially identical power electronic modules. Each power electronics module comprises a medium/high-frequency-isolated soft switching resonant based DC/DC current-to-voltage converter driving a DC/AC inverter. Each DC/DC converter and its corresponding DC/AC inverter are connected back-to-back sharing a common DC-link. The plurality of power electronic modules is stacked together in series at the input side and in parallel or series/parallel at the output side. 
         [0012]    According to one more aspect of the present disclosure, an electronics power system comprises a plurality of substantially identical power electronic modules. Each power electronics module comprises a medium/high-frequency-isolated soft switching resonant based DC/DC current-to-voltage folder-converter driving a DC/AC un-folder inverter. The DC/DC current-to-voltage folder-converter converts a constant DC current to a two-pulse or multi-pulse DC voltage which is unfolded to a sine wave ac voltage by the DC/AC un-folder inverter. Each DC/DC folder-converter and its corresponding DC/AC un-folder inverter are connected back-to-back sharing a common pulsating DC-link. The plurality of power electronic modules is stacked together in series at the input side and in parallel or series/parallel at the output side. 
         [0013]    According to one more aspect of the present disclosure, an electronics power system comprises a plurality of substantially identical power electronic modules. Each power electronics module comprises plurality of a medium/high-frequency-isolated soft switching resonant based DC/DC current-to-voltage folder-converter driving a DC/AC un-folder inverter. A plurality of DC/DC current-to-voltage folder-converters, controlled in interleaved fashion, converts a constant DC current to a fixed DC voltage (requiring a very small snubber capacitor in the dc-link), driving a DC/AC inverter. A plurality of power electronics modules comprising a plurality of DC/DC converters and corresponding DC/AC inverters are connected back-to-back sharing a common DC-link (requiring very small snubber capacitor). The plurality of power electronic modules is stacked together in series at the input side and in parallel or series/parallel at the output side. 
         [0014]    These and other features, aspects and advantages of the present embodiments will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and con-stitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
     
    
     
       DRAWINGS 
         [0015]    The foregoing and other features, aspects and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0016]      FIG. 1  illustrates an exemplary multi mega-watt drive using state-of-the-art technology; 
           [0017]      FIG. 2  illustrates a modular three-phase drive according to one embodiment; 
           [0018]      FIG. 3  illustrates a modular 6.6 kV, 12 MW drive according to one embodiment; 
           [0019]      FIG. 4  is a simplified schematic illustrating a power electronic module according to one embodiment; 
           [0020]      FIG. 5  illustrates a modular power electronic module with a resonant tank circuit according to one embodiment; 
           [0021]      FIG. 6  illustrates a modular power electronic module with a resonant tank circuit according to another embodiment; 
           [0022]      FIG. 7  illustrates a modular power electronic module with a resonant tank circuit according to yet another embodiment; 
           [0023]      FIG. 8  illustrates a 1 MW, 3-cell stack power electronic system according to one embodiment where a plurality of DC/DC converters are interleaved to form a DC voltage link with a very small snubber capacitor; 
           [0024]      FIG. 9  illustrates a plurality of modular power electronic modules configured to distribute multi-phase AC/DC loads according to one embodiment; 
           [0025]      FIG. 10  illustrates a scalable-voltage power electronic system using a plurality of modular power electronic modules according to one embodiment; and 
           [0026]      FIG. 11  illustrates a current-link based HVDC power transmission and distribution system using a plurality of modular power electronic modules according to one embodiment; 
           [0027]      FIG. 12  illustrates a current-link based HVDC power transmission and distribution system, for bidirectional power flow, using a plurality of modular power electronic modules according to one embodiment; and 
           [0028]      FIG. 13  illustrates a current-link based drive system using a plurality of power electronics modules containing a DC/DC folder-converter followed by DC/AC un-folder inverter according to one embodiment. 
       
    
    
       [0029]    While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
       DETAILED DESCRIPTION 
       [0030]    Referring to  FIG. 2 , an exemplary multi mega-watt modular three-phase drive system  20  is illustrated using state-of-the-art technology. Identical power electronic modules  22  are used to generate AC voltage at the machine terminals  24 . However, as described herein, n-phase DC or AC output can be generated using plurality of modules  22 . A module  22  comprises a medium/high-frequency-isolated DC/DC current-to-voltage converter  26  and a single-phase DC/AC converter  28 . The DC/DC and DC/AC converters  26 ,  28  are connected back-to-back sharing the same dc-link  29 . A more detailed description of DC/DC converter  26  and DC/AC converter  28  are presented herein with reference to  FIGS. 4-11 . 
         [0031]    Those skilled in the transformer art will appreciate that a higher excitation frequency of a transformer will allow a reduction in its size and weight for a particular application. Hence, each module  22  is expected to have high power density. With continued reference to  FIG. 2 , one module  22  per output phase is used. However, as stated herein, many modules per-phase can be used which is suitable for a mega-watt drive where multi-level voltage at the machine terminals is desirable. 
         [0032]      FIG. 3  illustrates a modular 6.6 kV, 12 MW drive system  30  for a 400 A DC current source. Drive system  30  uses four modules  22  per phase. The output phase voltage  32  has 9 levels. The modular nature of drive system  30  allows the use of many modules per phase to advantageously provide for a scalable output voltage. Further, the modules  22  can advantageously be interleaved (both at the input and output) to generate high quality input-output waveforms. 
         [0033]      FIG. 4  is a schematic illustrating a more detailed view of a power electronic module  40  suitable for use with drive system  20  according to one embodiment. Power electronic module  40  comprises a dc/dc converter stage  42  followed by a single phase dc/ac inverter stage  44 . The module  40  shown in  FIG. 4  is simplified for purposes of discussion by depicting the dc/ac inverter stage  44  as a resistor load R L . The current-to-voltage conversion is achieved by a soft switching resonant based dc/dc converter  42 , according to one embodiment. The current fed parallel resonant converter  42  shown in  FIG. 4  can be considered as the dual of the conventional voltage fed series resonant converter. This resonant converter  42  provides a relatively flat efficiency curve versus load; and with proper tuning of the switching frequency, it can provide soft switching for the bridge devices  46 . Further, more control flexibility can be provided through the use of multiple control variables (pulse width and frequency). 
         [0034]    With continued reference to  FIG. 4 , a programmable controller  48  is employed to control without limitation, switching frequencies, pulse widths, and frequency modulations i.e. timing and interleaving. More specifically, programmable controller  48  may control switching frequencies associated with the bridge devices  46 . Pulse widths generated by the bridge devices  46  may also be controlled via programmable controller  48 . Further, a plurality of modules  22 ,  42  can advantageously be interleaved (both at the input and output) to generate high quality input-output waveforms, as stated herein. 
         [0035]    The use of a combination of pulse width and frequency modulations to regulate the output voltage for different load values helps reduce the range of variation of both variables, thus avoiding the application of very narrow pulse widths at light load conditions, which can help maintain the soft switching operation over a wider load range as compared to using a fixed frequency approach. The range of frequency variation is also narrow (1-1.5 times the resonant frequency), which does not complicate filter designs. 
         [0036]    Numerous resonant topology variants such as, but not limited to, those shown in  FIGS. 5-7  can also be used in accordance with the principles described herein to provide different dynamic characteristics and voltage/current regulation capabilities.  FIG. 5  illustrates another modular power electronic module  80  with a resonant tank circuit  82  according to one embodiment.  FIG. 6  illustrates a modular power electronic module  90  with a resonant tank circuit  92  according to another embodiment.  FIG. 7  illustrates a modular power electronic module  100  with a resonant tank circuit  102  according to yet another embodiment 
         [0037]    A flexible modular approach can be used to stack the converters such that the outputs of the rectifier stage  112  are connected in series for high voltage applications, such as illustrated in  FIG. 8 . Furthermore, applying a phase shift between the currents of each converter provides a lower output ripple and thus smaller dc link filter requirements.  FIG. 8  shows an exemplary 1 MW, 3-cell stack power electronic system  110  according to one embodiment. The resistor load R L  is now replaced by a dc/ac inverter (H-bridge) stage  114 . 
         [0038]      FIG. 9  illustrates a plurality of modular power electronic modules  22  configured to distribute multi-phase AC/DC loads  120  according to one embodiment. The distribution system  120  may comprise of n-phase AC loads  122 ,  124 ,  128  and DC loads  126  operating at various voltage levels. Each power electronic module  22  can generate single-phase ac/dc voltage waveforms. Hence, by connecting a plurality of modules in series at the input side, as shown in  FIG. 9 , n-phase output waveforms can be generated. It can be observed from  FIG. 9  that a variety of single-phase, n-phase ac or dc loads can be driven by simply connecting many modules  22  in series at the input 
         [0039]    The principles described herein can be extended to per-phase applications. If it can be assumed for example, the magnitude of output voltage from each module is 1 per-unit (p.u.), and since the output terminals are isolated (provided by the medium/high frequency transformer used in the resonant circuit topology depicted in  FIG. 4 , the output of n modules  40  can be connected in series to generate n per-unit voltage per output phase as shown in  FIG. 10 .  FIG. 10  illustrates a scalable-voltage power electronic system  130  using a plurality of modular power electronic modules  22  according to one embodiment. 
         [0040]    With continued reference now to  FIG. 2 , the input to the embodied system  20  is a dc current source  21 . The outputs are n-phase voltage waveforms of adjustable magnitude and frequency. However, following the principle of duality, the input to the system  20  can be an n-phase voltage source and the output can be a constant dc-current load. A dual power electronic topology is used at the grid side (sending end), as shown in  FIG. 11 , to convert the three-phase 60 Hz grid voltage to a constant dc-current. Once conversion to dc-current is achieved, the principles described herein are applied to drive multi-phase ad dc loads at the receiving end of a high voltage DC (HVDC) power transmission and distribution (T/D) system.  FIG. 11  illustrates a current-link based HVDC power transmission and distribution system  140  using a plurality of modular power electronic modules  22  according to one embodiment. 
         [0041]    The series connected modular structure of the power electronic modules provides the capability of bypassing any faulted module with a fast bypass switch  150 , as shown in  FIG. 12  while the remaining modules stay operational, hence increasing the system reliability and availability according to one embodiment. 
         [0042]    In a HVDC transmission application where pluralities of modules are connected in series as shown in  FIG. 12 , the overall DC transmission voltage can be controlled by engaging or bypassing modules while each module operating at a fixed loading condition. 
         [0043]    In another embodiment, as illustrated in  FIG. 13 , the plurality of power electronic modules, each containing a DC/DC current-to-voltage folder/un-folder converter connected back-to-back to a AC/DC or DC/AC folder/un-folder converter, are configured to realize a high voltage AC/DC or DC/AC power conversion system  160 . The rectifier/inverter  162  advantageously requires only a small snubber capacitor  164  such that the dc-link voltage  166  is a rectified sinusoidal waveform. It should be noted that a snubber capacitor is not used to account for unbalance energy such as generally associated with a dc-link capacitor that typically stores instantaneous unbalance energy between a DC/DC converter and a DC/AC converter. A snubber capacitor is small compared to a dc-link capacitor since it is used to protect devices from switching overvoltage instead of unbalance energy. 
         [0044]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.