Abstract:
A high voltage, high power multi-level drive structure includes a plurality of neutral-point-piloted (NPP) converter cells stacked together. At least one clamping diode is connected to one or many NPP converter cell to provide a neutral-point-pilot-clamped (NPPC) converter structure. Flying capacitors connected to the NPPC converter structure yield a neutral-point-clamped-flying-capacitor converter cell structure.

Description:
BACKGROUND 
       [0001]    This subject matter of this disclosure relates generally to high voltage, high power multi-level drive structures, with many input and output phases, and more particularly to a multi-level drive structure that combines the architectures of classical multi-level neutral-point-clamped (NPC) and multi-level neutral-point-piloted (NPP) converter topologies. 
         [0002]    A multiplicity of multi-level drive structures have been proposed. One known multi-level drive structure is the classical three-level neutral-point-clamped converter  10  such as shown in  FIG. 1 . Another known multi-level drive structure is the three-level neutral-point-piloted converter  20  such as shown in  FIG. 2 . Although known NPC and NPP converter structures have proven to be advantageous in numerous applications, these known converter structures leave room for improvements in both operational efficiency and waveform quality. 
         [0003]    Yet, another known multi-level drive structure  30  is illustrated in  FIG. 3 . Although the multi-level converter  30  structure shown in  FIG. 3  offers improvements over the NPC and NPP converter structures, it still leaves room for improvements in operational efficiency as well as reliability. 
         [0004]    Still another known multi-level drive structure is depicted in  FIG. 4  that illustrates a five-level neutral-point-piloted converter structure  40 . The converter structure  40  shown in  FIG. 4  employs flying capacitors  42 ,  44  to provide a flying capacitor (FC) based multi-level converter. Such converter topologies have proven to be advantageous in certain high voltage, high power drive applications, but continue to leave room for improvements in operational efficiency. 
         [0005]      FIGS. 5-8  illustrates further classical converter topologies known in the art including a cascaded bridge converter structure  50 , a modular multi-level converter structure  60 , and active neutral point converter structure  70  and a hybrid converter structure  80 . Similar to known NPC and NPP converter structures, these classical converter structures continue to leave room for improvements in operational efficiency and/or waveform quality and/or reliability. 
         [0006]    In view of the foregoing, there is a need to provide a high voltage, high power multi-level drive topology that provides further improvements in operational efficiency and/or waveform quality and/or reliability beyond that afforded by known multi-level converter structures. 
       BRIEF DESCRIPTION 
       [0007]    An exemplary embodiment of the disclosure is directed to a high voltage, high power (HVHP) multi-level drive structure. The exemplary HVHP embodiment comprises a plurality of neutral-point-piloted converter cells stacked together and configured with clamping diodes to achieve a zero output voltage level. 
         [0008]    Another embodiment is directed to a high voltage, high power (HVHP) multi-level drive structure comprising a plurality of neutral-point-piloted (NPP) converter cells stacked together, and at least one clamping diode connected to each NPP converter cell to provide a neutral-point-pilot-clamped (NPPC) converter structure. 
     
    
     
       DRAWINGS 
         [0009]    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: 
           [0010]      FIG. 1  is a simplified diagram illustrating a 3-level NPC converter structure known in the art; 
           [0011]      FIG. 2  is a simplified diagram illustrating another 3-level NPP converter structure known in the art; 
           [0012]      FIG. 3  illustrates a high voltage, high power multi-level converter structure known in the art; 
           [0013]      FIG. 4  illustrates another high voltage, high power multi-level converter structure known in the art; 
           [0014]      FIG. 5  illustrates is a simplified diagram illustrating a cascaded bridge converter structure known in the art; 
           [0015]      FIG. 6  illustrates a modular multi-level converter structure known in the art; 
           [0016]      FIG. 7  illustrates an active neutral point converter structure known in the art; 
           [0017]      FIG. 8  illustrates a hybrid converter structure known in the art; 
           [0018]      FIG. 9  illustrates a five-level, neutral-point-pilot-clamped converter structure, according to one embodiment; 
           [0019]      FIG. 10  illustrates a seven-level, neutral-point-clamped-flying-capacitor (NPCFC) converter structure, according to one embodiment; and 
           [0020]      FIG. 11  illustrates an n-level, NPCFC converter structure, according to one embodiment. 
       
    
    
       [0021]    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 
       [0022]      FIG. 9  illustrates a five-level, neutral-point-pilot-clamped (NPPC) converter structure  90 , according to one embodiment. The NPPC converter  90  is particularly useful for multi-phase variable frequency drive applications, among others. NPPC converter  90  can be seen to combine the architectures of classical three-level neutral-point-clamped (NPC) and three-level neutral-point-piloted (NPP) multi-level converter topologies such as those depicted in  FIGS. 1 and 2 . The present inventors discovered the NPPC converter  90  to have almost 40% reduced semiconductor switching losses and substantially better electrical waveform quality when compared to classical NPC and NPP converters. 
         [0023]    According to one aspect, NPPC converter  90  advantageously provides a high voltage output/power by stacking two medium voltage NPP converter cells  92 ,  94 . The NPP converter cells  92 ,  94  are each clamped with one or more respective diodes  96 ,  98  to achieve a zero output voltage level by connecting the central node  95  to the output node  93 . Output voltage levels +1 and −1 are achieved by using intermediate arms  97 ,  99 . Only two devices are advantageously switched to achieve an output voltage in the range 1&lt;|V 0 |&lt;2, resulting in low switching losses. 
         [0024]    The high voltage NPPC converter structure  90  uses medium voltage NPP cells  92 ,  94  resulting in a fewer number of semiconductor devices in each arm  97 ,  99  when compare to classical high voltage NPP cell structures. Further, high voltage NPPC converter  90  advantageously achieves output voltage levels of +1 and −1 volts, which are not possible to achieve with classical NPP structures. Classical high voltage NPP converter cells undesirably requires four devices be switched to achieve an output voltage in the range 1&lt;|V 0 |&lt;2, resulting in high switching losses. NPPC converter  90  advantageously requires only two devices be switched to achieve an output voltage in the range 1&lt;|V 0 |&lt;2, as stated herein. 
         [0025]    Although the multi-level converter cell structure  30  illustrated in  FIG. 3  appears similar to the high voltage NPPC converter structure  90 , there are significant differences between the two structures  30 ,  90 . A zero voltage is output to the AC output terminal  32  of multi-level converter  30  when transistors Q 2 , Q 5  or Q 3 , Q 6  are switched on and Q 1 , Q 4  and SW 1 , SW 2  are switched off, resulting in electric current passing through two semiconductor switches Q 2 , Q 5  or Q 3 , Q 6 . In contradistinction, the NPPC converter cell structure  90  achieves a zero output voltage without switching of corresponding semiconductor switches since switching off of SW 1   97  is sufficient. Instead, the clamping diodes  96 ,  98  automatically conduct, resulting in a zero output voltage at output terminal  93 . The NPPC converter cell structure  90  thus results in decreasing losses and increased reliability beyond that achievable with the multi-level converter  30 . 
         [0026]      FIG. 10  illustrates a seven-level, neutral-point-clamped-flying-capacitor (NPCFC) converter structure  100 , according to one embodiment. Converter structure  100  combines the NPPC topology shown in  FIG. 9  with a flying capacitor (FC) based multi-level converter topology to provide the seven-level NPCFC converter  100 . NPCFC converter cell  100  advantageously generates seven levels in the output phase voltage waveform using the same number of flying capacitors as a five-level NPP converter cell such as shown in  FIG. 4 , resulting in a higher waveform quality, reduced filter requirements and higher efficiency. 
         [0027]      FIG. 11  illustrates an n-level, NPCFC converter cell structure  110 , according to one embodiment. It can be appreciated that the five-level NPPC converter  90  shown in  FIG. 9  is a special case of the n-level, NPCFC converter cell structure  110 , without any flying capacitor. 
         [0028]    In summary explanation, a high voltage high power (HVHP) multi-level drive topology for multi-phase variable frequency drive, dc/ac, ac/ac, ac/dc or ac/dc/ac power conversion applications is described herein. The HVHP multi-level drive topology combines the architectures of classical neutral-point-clamped and neutral-point-piloted multi-level converter topologies to provide increased operating efficiency and/or higher waveform quality and/or increased reliability. The HVHP multi-level drive topology can be further combined with a flying capacitor based multi-level converter topology to provide a novel n-level neutral-point-clamped-flying-capacitor (NPCFC) converter topology with additional advantages over known multi-level converter cell topologies. 
         [0029]    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.