Patent Publication Number: US-8976526-B2

Title: Providing a cooling system for a medium voltage drive system

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 12/845,406, filed Jul. 28, 2010, now U.S. Pat. No. 8,711,530 which is a continuation-in-part of U.S. patent application Ser. No. 12/495,359, filed Jun. 30, 2009, now U.S. Pat. No. 8,130,501, issued on Mar. 6, 2012, the content of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     Generally, equipment referred to as a power converter, inverter or drive is used to provide power to another piece of equipment such as a motor. Specifically, such a converter (converter is used generally herein to refer to converters, inverters and drives) is coupled to a utility connection to receive incoming input power such as three-phase AC power. The converter conditions the power to provide a conditioned power to the equipment or load. In this way, incoming power to the load may be of improved efficiency, leading to reduced costs to operate the equipment. 
     Multi-level power converters have been gaining popularity mainly due to improved input and output harmonics content, better electromagnetic compatibility, and higher voltage capability. These improvements in power conversion are achieved by using a multiple voltage step strategy. One common multi-level inverter topology is based on H-bridge inverters, in which multiple H-bridge inverters are connected in series. Since this topology consists of series power conversion cells, the voltage and power level may be easily scaled. 
     Multi-level power converters are used to provide power to a load such as a motor. Oftentimes, such multi-level converters are implemented as a large piece of equipment that is housed in a cabinet that receives incoming power, e.g., from a utility connection, conditions the power and provides it to a coupled load. In general, a drive system includes one or more transformers that have secondary windings that are coupled to individual power cells that perform rectification, inversion, and other conditioning tasks. Typically, to perform these functions each power cell includes a rectifier, DC link capacitor and inverter, among other such equipment, and each such power cell is packaged together into a module. 
     However, as voltage and power requirements increase in a system, these modules can be relatively cumbersome, bulky and expensive. For example, in medium voltage (MV) applications, such power cells can easily exceed 50 pounds due to the size of the DC link capacitors and other components present. Another drawback of this conventional design of power cells of a drive system includes a costly and complex customized isolation system as voltage levels increase from a few kilovolts to tens of kilovolts. In general, power cells are provided as a fixed enclosed module customized for a given power and voltage level. For example, power cells for 4160V at 1000 HP applications will radically increase foot print and weight over a power cell designed for 3300V at the same output power. In addition, the overall MV converter package must be individually designed to meet a particular power output, such as 5000 HP, 10000 HP, 20000 HP applications, control, and protection specifications. 
     In current medium voltage drives, a cascaded topology is implemented using a partial modular design. Specifically, the only subcomponent of the system that is common across drive products of a family is the power cell, which as described above contains the power components including inverter, rectifier, and DC-link. Current medium voltage drives have excluded all other system components, including transformers, control, cooling system, communication distribution, packaging, and electrical insulation, from a modular approach. Instead, these major components are optimized for voltage and power rating of a specific design and thus are not easily transferable to other drive ratings. 
     Many drive system are provided with an air-based or water-based cooling system. While suitable for many applications, such systems suffer from high costs, reduced efficiency, and size limitations. 
     SUMMARY OF THE INVENTION 
     According to aspect of the present invention, a medium voltage drive system is provided having a cabinet including multiple slices of the system. In general, the cabinet includes a transformer bay and a power cube bay. The transformer bay is to house at least one transformer to couple between a utility connection and power cubes. In turn, the power cube bay includes multiple portions. A first portion has multiple cube chambers each having a protective enclosure in which to house one of the power cubes, where each cube includes an inlet port to receive a flow of liquid coolant and an outlet port to output a flow of two phase coolant. A second portion is to accommodate at least a portion of a cooling system for the drive system. Depending on the implementation, it is possible for one or both of the power cube and transformer bays to be NEMA-4 enclosures. 
     In general, the cooling system is a modular two phase system and includes an inlet conduit coupled to first connection members each to connect to a corresponding inlet port of one of the power cubes to enable the flow of liquid coolant to the corresponding power cube, an outlet conduit coupled to second connection members each to connect to a corresponding outlet port of one of the power cubes to enable the flow of two phase coolant from the corresponding power cube. The cooling system is configurable to be a single heat exchange system or a multiple heat exchange system based upon a location of the drive system. 
     In an embodiment, each power cube has at least one cold plate configured to receive the flow of liquid coolant and to output the flow of two phase coolant. In many embodiments, each power cube includes multiple cold plates each including a channel adapted to communicate the liquid coolant along a path to enable heat exchange with a switching device coupled to the corresponding cold plate. 
     In certain embodiments, the second portion of the power cube bay includes an accumulator to store the liquid coolant, a pump to provide the flow of liquid coolant to the inlet conduit, and a heat exchanger to receive the flow of two phase coolant from the outlet conduit and to enable heat exchange and to provide the liquid coolant to the accumulator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram view of a drive system adapted in a cabinet in accordance with one embodiment of the present invention. 
         FIG. 2A  is a schematic diagram of components within a power cell chamber in accordance with one embodiment of the present invention. 
         FIG. 2B  is a block diagram illustrating connections available in a power service bus in accordance with an embodiment of the present invention. 
         FIG. 2C  is another view of a power service bus in accordance with one embodiment of the present invention. 
         FIG. 2D  is a schematic diagram of a power cell mobile section in accordance with an embodiment of the present invention. 
         FIG. 3  is a block diagram view of a power cell module in accordance with one embodiment of the present invention. 
         FIG. 4  is a breakaway diagram of a chamber of a power cell cabinet in accordance with one embodiment of the present invention. 
         FIG. 5  is an illustration of an example embodiment of a winding geometry of a modular transformer in accordance with one embodiment of the present invention. 
         FIG. 6  is a block diagram of a system in accordance with an embodiment of the present invention. 
         FIG. 7A  is an illustration of a cabinet in accordance with one embodiment of the present invention. 
         FIG. 7B  is a side view of the cabinet that illustrates air flow for cooling in accordance with an embodiment of the present invention. 
         FIG. 8A  is a block diagram of a modular system configuration in accordance with one embodiment of the present invention. 
         FIG. 8B  is a block diagram of a parallel modular system configuration in accordance with an embodiment of the present invention. 
         FIG. 8C  is a block diagram of a series modular system configuration in accordance with one embodiment of the present invention. 
         FIG. 9A  is a flow diagram for a basic control loop for a master controller in accordance with one embodiment of the present invention. 
         FIG. 9B  is a flow diagram for a basic control loop for a cabinet in accordance with one embodiment of the present invention. 
         FIG. 9C  is a flow diagram for a method for performing control of a power cell in accordance with one embodiment of the present invention. 
         FIG. 10  is a block diagram of a single slice of a drive system that can be cooled in accordance with an embodiment of the present invention 
         FIG. 11  is a block diagram of a single slice of a drive system that can be cooled in accordance with another embodiment of the present invention. 
         FIG. 12  is another example of a drive system having a cooling arrangement in accordance with an embodiment of the present invention. 
         FIG. 13  is yet another implementation of a cooling system in accordance with an embodiment of the present invention. 
         FIG. 14  is an arrangement of another drive system having a different cooling arrangement in accordance with an embodiment of the present invention. 
         FIG. 15  is an isometric view of a power cube in accordance with another embodiment of the present invention. 
         FIG. 16  is an illustration of coupling of a power cube to a manifold assembly in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In various embodiments, a power cell chamber can be separated into multiple portions, namely a stationary or fixed portion and a moveable portion. These different portions may be joined by a power service bus. In this way, a module or other housing or enclosure that forms the moveable portion can be made to be relatively small and lightweight as compared to a conventional power cell. This allows for greater ease of customer access and removal, repair and replacement operations on these moveable portions of a power cell. In turn, the moveable portion may be plugged into a power service bus that couples the moveable portion with the fixed portion within the power cell chamber. The fixed portion of the chamber may include various components that would conventionally be present in a single power cell enclosure including, for example, a DC link by way of one or more capacitors. As such components can be relatively reliable over a number of years, these components present in the fixed portion may be provided on an opposite side of the power service bus from the moveable portion. As used herein, “fixed” or “stationary” means that a component is physically attached directly within a cabinet, not readily customer accessible, and cannot be inserted and removed by simple pluggable/slidable means. Instead, special access and tools are needed to insert or remove the component. In contrast, “moveable” or “pluggable” means a component or group of components that are easily inserted and removed from a cabinet, e.g., via sliding, plugging and so forth without the need for tools, and having ready customer access. 
     Embodiments may provide for a modular cabinet-based drive system that can be used to provide regulated power at a wide range of voltages. For example, some applications may be used for low voltage applications. As used herein, low voltage is used to denote voltages of 600 volts and below. Others may be used for medium voltage applications. As used herein, medium voltage is used to denote voltages between approximately 600 volts and 35000 volts. Still further, owing to the separation between components of a conventional power cell and isolation afforded within a cabinet (as discussed below), embodiments also may be readily used for high voltage (HV) applications. As used herein, high voltage means greater than 35000 volts, e.g., 69000 volts. 
     Referring now to  FIG. 1 , shown is a block diagram view of a power cell cabinet in accordance with one embodiment of the present invention. As shown in  FIG. 1 , cabinet  10  may house at least a portion of a drive system such as a medium voltage drive system that can be a multi-level inverter. The view of  FIG. 1  is with respect to a front portion of the cabinet. In general, the cabinet is arranged such that a plurality of transformers  22   a - 22   c  are present in a transformer bay  20 , which may be configured in a lower portion of cabinet  10 . In turn, a plurality of power cells  32   a - 32   i  may be configured within a power cell bay  30  of cabinet  10 . In the embodiment shown in  FIG. 1 , nine such power cells are present, although the scope of the present invention is not limited in this regard. Also, while shown as single modules in the view of  FIG. 1 , understand that the portions of the power cells shown in  FIG. 1  may correspond to the moveable portions adapted within enclosures. Unseen in  FIG. 1  are the fixed portions of the power cell modules, which may be adapted to a rear portion of a power service bus, also not shown in  FIG. 1 . 
     In some implementations, a single modular power cell cabinet may be sufficient to provide desired power levels for a given system. In such implementations, a cabinet controller  45  present in a controller bay  40  may be used to handle control of the drive system. However, many embodiments may aggregate a plurality of power cell cabinets with a respective cabinet controller  45  along with a single master controller cabinet  40  to control the cabinet arrangement and increase the power capabilities. In these implementations, controller bay  40  including cell controller  45  may in turn be coupled, e.g., via a fiber optic interface to a master controller cabinet (not shown in  FIG. 1 ). Furthermore, in implementations in which a single power cell cabinet is present, a separate cabinet may provide a user interface. That is, to provide user access to information regarding a drive system, a user interface may be present. Such user interface may include a display and an input mechanism such as a key pad or keyboard to enable user input of information and control of various operations including diagnostics and so forth. In other implementations, a master controller cabinet may include such a user interface. 
     With reference to the transformers of  FIG. 1 , each transformer may be configured in a horizontal manner having a core  24  and multiple windings adapted there around, including a primary winding and a set of secondary windings. In addition, some embodiments may further include one or more auxiliary windings to provide power to auxiliary equipment such as fans or the like. Each transformer  22  may be a three-phase transformer that receives three-phase power from a utility connection and provides power to one or more of the power cells (although only a single phase is shown in  FIG. 1 ). More specifically, each transformer may include three sets of secondary windings to provide power to a corresponding power cell. In one implementation transformer  22   a  may provide power to the power cells with which it is generally vertically aligned with, namely power cells  32   a ,  32   d , and  32   g . Similar connections can be configured for transformers  22   b  and  22   c . However, other implementations are possible. Note that the primary windings, secondary windings or both may be phase-shifted in some implementations. 
     By providing transformers in a generally horizontal configuration, improved airflow is realized. For purposes of cooling the transformers and the cabinet in general, a substantially linear and laminar airflow may be provided, e.g., from forward to rear of the cabinet such that the air blows through the windings of the transformers. While ambient air cooling can be used in many implementations, some embodiments may provide some type of liquid cooling for the transformers as well. Further details regarding the horizontal nature of the transformers will be discussed below. 
     As further seen in  FIG. 1 , each power cell chamber  33  (i.e., both the moveable and fixed portions) can be configured as an enclosure (only one representative such chamber shown in  FIG. 1 ). Such cell chamber can be formed using an insulative material such as a polypropylene or other plastic or other such material. Still further, to provide electrical isolation between the power cells and other equipment in cabinet  10 , an isolation barrier  34  can be provided in each chamber (note only one such isolation barrier  34  is shown). As seen, isolation barrier  34  is formed of a plurality of individual members in an interlocking manner or single involving member as shown in  FIG. 7A  to provide efficient isolation. In various embodiments, isolation barrier  34  may be formed of sheets of an insulating material such as FORMEX™ or other such material. While shown with only a few such members for ease of illustration in the embodiment of  FIG. 1 , understand that many such members, e.g., 2 or many more may be provided, as these individual members can be of relatively thin width, e.g., 9.8 to 125.2 mils. In this way, an insulating bubble is formed around each power cell to facilitate voltage insulation. Such insulation may enable a single generic cabinet architecture to be used with a wide variety of voltages, from low voltage applications all the way up to high voltage applications. Generally as the voltage application increases, the number of sheets and thus relative thickness may also increase. Further, while not shown for ease of illustration in  FIG. 1 , understand that spacers may be present to provide separation of the individual power cell chambers from a base over which they are situated. 
     Understand that while shown with this particular implementation in the embodiment of  FIG. 1  with the power cells configured above transformers, the scope of the present invention is not limited in this manner. Furthermore, understand that additional components may be present in a given drive system, and the illustration of  FIG. 1  is at a high level to identify the main components and their general location within a system. 
     Referring now to  FIG. 2A , shown is a schematic diagram of components within a power cell chamber in accordance with one embodiment of the present invention. As shown in  FIG. 2A , chamber  100  is adapted to include both the fixed and moveable portions. A given drive system may include multiple such chambers to form the drive system. In various implementations a power cell module  110  may correspond to the moveable portion of the power cell chamber and may be accessed via a front panel of a cabinet, whereas a fixed portion  115  of the chamber may be accessible via a rear panel of the cabinet or via a front panel when the cell module  110  is not present or is otherwise removed. Thus as shown in  FIG. 2A , from right to left generally corresponds to a direction from a front to a rear of a cabinet. Details of the movable power cell are described further below. In general, the moveable portion may include the rectification and inverter stages of a cell (but not the DC-link power capacitor). 
     Power cell module  110  may couple to a power service bus  120  via a plurality of pluggable terminals located on a back portion thereof. Power service bus  120  may have various connectors to mate with the terminals of power cell module  110 . In some embodiments, power cell module  110  may be installed via blind mating connections with corresponding connectors of power service bus  120 . As will be described below, power cell module  110  may include a rectifier section and an inverter section. The rectifier section may include a plurality of diodes coupled to the inputs from the transformer secondary, along with at least one parallel-connected local capacitor that in turn is coupled to a local capacitor low inductance bus. Similarly, the inverter section may be, for example, an H-bridge inverter including a plurality of switching devices such as IGBTs and further may include a parallel-connected local capacitor. The local capacitor may provide diagnostics and snubber protection capability to the power cell. In different implementations, this capacitor may be sized from a few tens to a few hundreds of nanofarads. In turn, the inverter section is also coupled to the local capacitor low inductance bus. This bus may be constructed in a manner to provide a low inductance path. In various embodiments, the inductance may be between a few nanohenries to less than 20 nanohenries and the bus may be formed from a laminate material and provide connections between these two sections in a low inductance manner. 
     In a given implementation, power cell module  110  may be formed having dual cooling plate heat sinks, e.g., on top and bottom. In various implementations, various controller boards may be present in module  110 . One of the boards provides gate signals to control the H-bridge inverter, while a second board provides input current and DC bus voltage sensing capability for the power cell. A third board may be in communication with these controller boards and may be also implemented within the plug-in section of the cell chamber  110  and which in turn can be coupled via a fiber optic interface to the respective cabinet controller. A power supply also may be present within module  110 , and which may be used to provide a low voltage supply to, e.g., the internal cell controller boards. 
     In the embodiment shown in  FIG. 2A , input, output, DC bus, and control signal connections of a power cell may be provided via a number of blind mating connectors to couple to bus  120 . In the embodiment of  FIG. 2A  the connectors may be cyclindrical-shaped male/female plug connectors that can provide high current density and minimal spacing, although other implementations are possible, as described below. Note that bus  120  may also include a DC low inductance bus. In general, this bus may use at least some of the following to enable a low inductance connection: the bus may be formed of a laminate material, e.g., multiple copper sheets and an insulation sheet to provide electrical isolation to the arrangement. As seen in  FIG. 2A , the AC output of the power cell is connected using plug-in connectors  125   1  and  125   2 . In turn, an input connection to the cell from secondary windings of a transformer may be coupled to power cell  110  via connectors  126   1 - 126   3  (in a three-phase implementation). In turn, a DC bus connector  128  may couple a DC bus link between power cell  110  and a capacitor  130 . In one embodiment, connector  128  may be formed of a single connection having a plurality of concentric fixed connectors to mate with concentric fixed terminals to connect to a low inductance DC bus. The concentric connector  128  may connect to a cell DC-link power capacitor  130  and which is implemented in a fixed portion  115  of the power cell chamber formed on the opposite side of power service bus  120 . The capacitor thus may be a remote capacitor with respect to the moveable power cell. In one such implementation this concentric connector may be a type of coaxial cylindrical cable that reduces the DC bus inductance. In other embodiments, low inductance may be realized by providing a number of connections for the positive and negative portions of the DC bus, e.g., four connectors in parallel, arranged horizontally or in any other pattern, two each for positive and negative portions. Note also a control connector  129  may provide various status and control signals through power service bus  120 . 
     In one embodiment, capacitor  130  may be a single film power capacitor or a plurality of single film capacitors connected in series or parallel or a combination to fit a specific cell design. The capacitor configuration may range from thousands of microfarads to tens of thousands of microfarads and from a few tens to a few hundreds of amperes rms. The capacitor configuration  130  may operate from a DC bus which may be rated from a few hundred volts to several thousands of volts. Other implementations may use electrolytic or other type of capacitors. Each individual capacitor may further be associated with a discharge resistor. As shown in  FIG. 2A , a discharge/ground system may be realized as a resistive divider formed of series resistors R 1  and R 2  having an intermediate node coupled to ground, which may be a floating ground connection to the frame, such that only half of the voltage may be discharged to ground during a fault. More than one such capacitor may be present, and thus the term “capacitor” as used herein refers to a combination of one or more capacitors. Capacitor configuration  130  may also be located on top or bottom of the moveable section of the power cell chamber but within the cell chamber. 
     In addition, fixed portion  115  may further include an input cell protection mechanism  140  such as circuit breakers or other protection mechanisms e.g., fuses to couple between the secondary windings of a transformer and power service bus  120 . The input cell mechanism protection  140  may provide reliable short circuit protection and overload protection within a few 60 Hz current cycles. The control trip scheme to operate mechanism  140  (not shown) may be implemented in moveable portion  110 . Not shown in the embodiment of  FIG. 2A  are various control and switching signals that may couple between a cell and a master or cabinet controller (which may be outside of a given power cell chamber, but present in another part of a cabinet, for example). As further shown in  FIG. 2A , a control transformer, which may be a fuseless transformer, can be coupled to multiple phases of the incoming signals from the power transformer secondary. This transformer provides protection and can withstand short circuit fault currents in a safe manner. Note that T 1  and T 2  are secondary terminals of the fuse-less transformer, and can be used to activate the circuit breaker and bypass shunt trip coils and provide control power to the cell mobile section. 
     In addition, a cell bypass block  150  may be provided in fixed portion  115 , which may provide redundancy power/bypass to a given power cell in a cell failure mode. This mechanism may be implemented using a shunt trip contactor controlled from a cabinet controller via the plug-in section and service bus. For example, during operation when a power switch failure is detected by signals sent to a master controller, the master controller may act to actuate block  150  to thus create a short circuit between the outputs of a power cell having a failure. With reference to  FIG. 2A , by actuating a coil within bypass block  150 , a short circuit between terminals P 1  and P 2  can be realized to thus bypass this power cell in case of a malfunction. For example, if power cell  32   g  (of  FIG. 1 ) is disabled due to a failure in phase A, power cells  32   d  and  32   a  may be also disabled in phases B and C via the master controller to allow the drive system to continue functioning at reduced power and balance voltage output. 
     In one embodiment, block  150  may be an electronic switch coupled via a latching relay to a cabinet controller (not shown). The latching relay thus acts to take the signals from the cabinet controller and automatically control switching of bypass block  150  to open and close to thus provide bypass by responding to a fault command or other adverse condition. Similar connections may be present between a corresponding latching relay and input cell protection mechanism  140  to cause the breaker or other protection mechanism to be enabled to thus prevent input power from being applied to a given power cell. For instance, if a failure occurs in the rectifier section and detected by a current sensor located in the cell input (not shown in  FIG. 2A ), the input protection mechanism  140  may disable the power to the cell via  140 , followed by the actuation of bypass block  150  to isolate the troubled cell. Under this scenario, a master controller may act to disable the entire drive system or bypass this cell to continue operation at reduced power. In some implementations the connection between the cabinet controller and latching relay or shunt trip circuit may be via fiber optic, although the scope of the present invention is not limited in this regard. 
     The structure of power service bus  120  holds concentric DC bus and other terminal mating accesses (plug-in style). Power service bus  120  may integrate bus bars for cell series output, bus bar for cell input, a low inductance laminated DC bus to connect to the DC-link power capacitor(s), and a G-10™ material to provide structural support and insulation between electric buses. In this way, series coupling of a group of power cells can occur to provide a phase output line to a load coupled to the drive system. In different implementations, the service bus can be formed using a division wall (e.g., G-10™ material) or an open structure to provide for air circulation. 
     Referring now to  FIG. 2B , shown is a 3-D mechanical model illustrating connections available in a power service bus in accordance with an embodiment of the present invention. As shown in  FIG. 2B , power service bus  120  may be formed using insulation material, e.g., G-10 and includes contacts for cell input, output, DC link, and control signals (not shown). Specifically as seen in  FIG. 2B , at a top portion of power service bus  120 , the cell outputs P 1  and P 2  may be adapted. Furthermore, the three connections L 1 -L 3  at a bottom of the power service bus may provide the inputs from secondary windings of a transformer, while the DC link connections  128  may be provided via four individual connections, two positive and two negative. In other embodiments only a single positive and negative connection may be provided. 
     Referring now to  FIG. 2C , shown is another view of the power service bus that shows part of a moveable portion of a power cell chamber that provides connections to the cell inputs, outputs, as well as the DC link laminated buses in the moveable portion. Note that on the top portion of moveable module  110 , a plurality of switching devices  114 , which may be IGBTs are seen, while rectification devices  117  are present at the bottom of the cell. Switching devices  114  may be coupled between a laminated DC bus  119  and bus bars  118 . As also seen in  FIG. 2C , the output from a power cell is routed via connectors P 1  and P 2  from bus bars  118 . Connectors P 1  and P 2  may provide connection to another power cell with which power cell  110  is coupled in series, namely another power cell of the same phase output line. Note that in some embodiments, rather than the flat connectors shown, cylindrical connectors such as shown in  FIG. 2A  may be used. 
     While shown with this particular implementation in the embodiment of  FIGS. 2A-C , the scope of the present invention is not limited in this regard. For example, while it is assumed in this embodiment that power cell moveable portion  110  may not include the components of fixed portion  115  described above, in some implementations, a limited amount of local capacitance available in the power cell may be combined with the fixed capacitance present in the fixed portion within the power cell chamber in accordance with an embodiment of the present invention. 
     Referring now to  FIG. 2D , shown is a schematic diagram of a power cell chamber in accordance with an embodiment of the present invention. More specifically, shown in  FIG. 2D  are the components of the moveable portion of the power cell chamber. Specifically, the moveable portion  110  includes a rectifier bridge  111  and an inverter  117  which may be formed as an H-bridge inverter. In addition, a set of local capacitors  113  may be present and coupled between rectifier bridge  111  and inverter  117 . Note however that these local capacitors (which in one embodiment may be film capacitors) do not provide sufficient capacitance to the DC bus of the power cell. Instead, the local capacitors, which may be on the order of between 100 and 200 microfarads, can be used for purposes of bench diagnostics (single cell) and snubbing of transients for the IGBTs of inverter  117  during medium voltage operation. 
     However, these capacitors provide insufficient capacitance to properly operate the inverter drive at a desired medium voltage level. Instead as seen, connectors P and N, which may form a DC bus  128  may receive the DC voltage from the cell power capacitor (e.g., capacitor  130  of  FIG. 2A ) that is present in the stationary section of the cell. Note that connections L 1 -L 3  coupled to rectifier bridge  111  may also be received via a power service bus, e.g., from a power transformer secondary as shown in  FIG. 2A . In turn, the regulated output from moveable portion  110  may be output to the stationary section via connectors P 1  and P 2 , which as shown in  FIG. 2A  may pass through power service bus  120  where the voltages can be provided to a next power cell (for example). Thus in contrast to a conventional power cell, the components shown in  FIG. 2D  provide an appropriate output voltage by receipt of DC bus bulk capacitance from the stationary section. 
     Referring now to  FIG. 3 , shown is a block diagram view of a power cell module in accordance with one embodiment of the present invention. As shown in  FIG. 3 , module  160  may be a basic representation of a removable portion of a power cell. Not shown, an enclosure may be present and which may have various components present therein including, for example a local controller, input current and heat sink sensors, power switch gate drivers and a modular HV power supply. In addition, module  165  includes a split heat sink formed of cooling plate members  162   a  and  162   b , present on opposite sides of housing  165  to provide for improved thermal control and increased power density for module  160 . While not shown in  FIG. 3  for clarity, understand that the heat sink members may have a comb, fin, or fan-like structure to efficiently dissipate heat. The heat sink structure may include a cooling plate with a cooling fluid circulating in pipes through the plate with input/output leaving on the same or opposite sides of the housing. Different components can be coupled to the two heat sinks. In the implementation shown in  FIG. 3  on a top portion, switching devices of an inverter stage may be present. As an example, switching devices  164   a  and  164   b  may be IGBTs. While shown in this front view in  FIG. 3  as including only two such IGBT dual modules, understand that in various implementations an H-bridge of a power cell may include six-pack IGBT modules, and all of which may be adapted on a top portion of power cell enclosure  165 . On the bottom portion, rectification devices  166   a - 166   c  of a rectifier stage may be present. Again, while shown with three such devices in  FIG. 3 , understand that additional rectification devices may be present in different embodiments. In some embodiments, the rectification section may be replaced by an active front end having six IGBT modules to provide regeneration capability in addition to the rectification. Also, by adapting the major components of the moveable portion of the power cell around an exterior of an enclosure  165 , ease of access to the components for heat dissipation as well as increase of power density by maintaining same foot print for a wide range of power and utilizing none or limited DC-link capacitance. This also has a dramatic impact on size and weight reduction, and how diagnostics and removal, repair and replacement operations are realized. 
     To provide a low inductance path between the rectifier and inverter sections, two local film capacitors  168  that provide local ripple and full DC-link current circulation, may be adapted to the outside of enclosure and coupled to the switching devices and rectification devices by way of corresponding sets of terminals (note the terminals are not shown in  FIG. 3 ). This low inductance path has reduced parasitics and provides an adequate path for DC current circulation, as well as for performing snubbing of transients that may arise during normal operation to improve inverter cell performance. In this way, local capacitors  168  may be used to provide a low inductance path directly between rectifier and inverter to enable smoothing of a noisy signal during normal operation, and diagnostics/testing capabilities for a single cell. That is, in contrast to the one or more capacitors provided in a fixed portion of a power cell chamber which are used to provide the bulk capacitance needed to operate the inverter, the local capacitor(s) can act as snubbing circuitry to filter out unwanted voltage transients present at the inverter terminals. This is particularly so during switching events occurring in switching devices of the inverter. While shown with this simplified view in the embodiment of  FIG. 3 , understand the scope of the present invention is not limited in this regard. 
     Referring now to  FIG. 4 , shown is a breakaway diagram of a cell chamber of a cabinet in accordance with one embodiment of the present invention. As seen in  FIG. 4 , chamber  33  includes a power cell insulator portion  32  including rail members  35  at its bottom to allow a movable power cell to be slidably mated into and removed from the chamber. Specifically, rails  31  may enable corresponding rails  35  of the movable power cell to slide. As seen, multiple layers of an isolation barrier  34  may be present to provide insulation to a power cell chamber. In this way, isolation barrier  34  acts as a bubble or cocoon shell in which the power cell chamber is isolated from other equipment of a drive system. The number of layers may depend on a rated voltage of the drive system. For example, depending on the thickness of the individual layers (which can range from less than 1 mm to over 20 mm), operation isolation can exceed 150 kV. In some implementations, 5 or more layers may be provided to enhance isolation. Further, while shown as interlocking members, in other implementations the members can be surrounding or encircling such that each member fully surrounds chamber  33 . 
     In various implementations, the layers of isolation barrier  34  may be present between a perimeter of a chamber and rails  35  of the chamber. As further shown in  FIG. 4 , additional insulation members  37  may be adapted around the power cell insulator portion  32  to enable improved insulation of the power cell. The staggering discussed above may be implemented in part using these additional insulation members  37  of varying sizes. Note that the length of the layers of isolation barrier  34  may extend beyond the length of the power cell module adapted within chamber  33  to improve isolation. Furthermore, this extended length enables a single modular design to be used with power cells of varying rated voltage applications to enable a single modular design to be used with drive systems having widely different voltage rated capabilities. Still further, when adapted in a stack in a modular drive system, the chambers may be in substantial vertical alignment along one axis but not along another. That is, the chambers can be staggered vertically (with respect to the front/back axis of the cabinet) to provide greater isolation protection with minimum vertical separation between chambers. This configuration can reduce or prevent a corona occurrence and do so with a minimal effect on spacing size of the chambers. For example, the staggering may be between 1 and 2 inches, in one embodiment. 
     As described above, a cabinet may include a transformer bay, in which one or more transformers are adapted. Such transformers may have a main primary winding, which may be a three-phase medium voltage winding that receives medium voltage power feed, e.g., from a utility connection. In turn, a set of secondary windings each of which may be a three-phase secondary winding may provide normal operating power to the power cells. These secondary windings may be phase shifted, e.g., by 20 degrees from its neighboring secondary winding, however the scope of the present invention is not limited in this regard. In addition to the main windings for powering the power cells during normal operation, a transformer may further include auxiliary windings to enable pre-charging as well as to handle auxiliary functions, such as for diagnostics, voltage sensing, fan power and so forth. 
     By separating a transformer into modular units, there is more surface area for the core and thus it can dissipate heat more effectively. Further, each modular core volume may reduce in size since the winding window may only need to accommodate one or a small number of secondary windings per output phase. The modular approach allows a single unit transformer to be used across a wide voltage and power range. By increasing the number of modular units, a drive system in accordance with an embodiment of the present invention can be capable of higher voltage and power with lower harmonic distortion. 
     In one embodiment of a drive system having multiple modular transformers, the amount of phase shift of secondary and primary windings can be calculated according to the following equations: 
                     N   s     =       N   dc       N   T               [     EQ   .           ⁢   1     ]                 α   sec     =     360       2   ·     N   ph       ⁢     N   s                 [     EQ   .           ⁢   2     ]                 α   prim     =       α   sec       N   T               [     EQ   .           ⁢   3     ]               
where N T  is the number of transformer modules; N dc  is the number of isolated DC sources; N S  is an integer number of the number of secondary windings in each transformer; N ph  is the number of phases of a supply; α sec  is the secondary windings phase shift in each module; and α prim  is the primary winding phase shift in each module.
 
     Modular transformers may be manufactured using transformer manufacturing techniques and implementing various types of winding designs for both primary and secondary windings. Primary windings may include both extended delta configurations and a standard delta configuration. However, the connection of primary and secondary windings can be freely chosen. In various implementations, a desired phase shift may be realized by changing the geometry of the winding, e.g., by adjusting the number of turns of one or more coils of the transformer or taps with regard to other coils. By controlling the number of turns of coils and connection method of them, a given phase shift can be realized. Secondary windings can include standard delta configurations, as well as polygon configurations, where again by changing the size and/or turns of one or more coils, different phase shifts can be obtained. Of course, other configurations or connections can be used to realize a desired phase shift in different implementations. 
     As described above, modular transformers may be adapted horizontally to aid in cooling the transfer.  FIG. 5  is an illustration of an example embodiment of a winding geometry of a modular transformer. As shown in  FIG. 5  is a geometric illustration of a modular transformer  200 . As seen, transformer  200  may be a single modular transformer having a generally horizontal configuration (i.e., the windings are wrapped around a horizontal axis X) with a core  205 , which may be an iron core having the different windings, both main and auxiliary, wrapped around horizontal columns of the core. Generally, the windings may include a primary coil  210  and a plurality of secondary windings  220 . In addition, some implementations may further provide auxiliary power by way of a LV primary auxiliary winding  230  and a LV secondary auxiliary winding  240 . However, in some implementations, the auxiliary windings may not be present. Further, the scope of invention is not limited to only a horizontal configuration as this method can be applied to a conventional vertical configuration too. 
     While  FIG. 5  shows a three-phase configuration, and thus having three columns for supporting windings, each adapted on a horizontal axis, reference herein will be with regard to a single phase. As shown, a spatial separation exists between the main coils and the primary LV auxiliary coil. This configuration causes a loose coupling with other windings and a high leakage inductance for the LV primary auxiliary windings. However, the scope of invention is not limited in this aspect and other methods can be applied to generate high leakage inductances for the primary LV auxiliary winding. While the scope of the present invention is not limited in this regard, in a medium voltage implementation in which core  205  is approximately 2 to 10 feet high, this separation may be on the order of between approximately 0.5 and 6 inches to provide the desired high leakage inductance between the primary auxiliary winding  230  and the main secondary windings  220  and main primary winding  210 . 
     As seen, the configuration of the main secondary windings  220  is such that these windings are wrapped concentrically around each other, and further that these windings are also concentrically wrapped around the auxiliary secondary winding  240 . Note that in the implementation of  FIG. 5 , auxiliary secondary winding  240  may extend substantially along the entirety of the column length of core  205  and may have the main secondary windings  220  wrapped there around. 
     Thus in the particular implementation, the secondary windings  220  may be next concentrically adapted, e.g., in order of a first phase-shifted secondary winding  220   a , a second phase-shifted secondary winding  220   c , and finally a non-phase shifted secondary winding  220   b . Finally, wrapped concentrically around these windings is the main primary winding, MV winding  210 . The spacing between coils corresponds to cooling method and isolation and voltage level of the coils. Thus while described above as a forced air cooling technique, this method can be applied to natural cooled, and water cooled transformers. Various transformer manufacturing techniques can be used in realizing the coils and insulation. As examples, different wire types (e.g., round, square, or so forth) and different insulation materials (e.g., NOMEX™ felt or paper insulation, fiber, wood, epoxy, or so forth) can be used. 
     The configuration shown in  FIG. 5  thus provides for loose coupling between the primary auxiliary winding  230  and the main windings  210  and  220  (in particular, main secondary windings  220 ). However, different winding geometries or methods can be implemented to provide high leakage inductance for the LV auxiliary primary winding. 
     Note that in  FIG. 5 , direction from left to right may correspond from a front to a rear of a cabinet in which the transformer is adapted. In this way, a horizontal arrangement is realized such that air flow coming from an input mechanism such as a grill at a front panel of the transformer bay may receive air which is pulled through the grill and across the windings (and core) from a front to a rear portion of the cabinet. Then the air may be forced up through a rear of a cabinet and output via fans or other cooling members that act to pull the air across the transformers and up and out through these exhaust fans. In this way, improved cooling can be realized as a large volume of air that travels in a substantially linear and generally laminar flow can occur, without the need for baffles or other air handling or director equipment. Note that  FIG. 5  shows only a single modular transformer and in various implementations three or more such transformers may be present in a transformer bay of a cabinet in accordance with an embodiment of the present invention. When multiple such transformers are present, spacing may be maintained between the transformers such that at least some amount of open space is present between the windings of the two transformers, such that air flow may pass and magnetic and electric isolation is maintained between the transformers. 
     When using auxiliary windings in accordance with an embodiment of the present invention at power-up of a drive, power is supplied through the high leakage inductance LV auxiliary primary winding(s). The high inductance of this set of windings can slow the rate of capacitor charging and limit the in-rush current to the drive. Furthermore, another auxiliary secondary winding can be embedded into a transformer module for providing power to cooling fans or any other auxiliary power usage. Of course, a given system may not implement any auxiliary windings. 
     Referring now to  FIG. 6 , shown is a block diagram of a system in accordance with an embodiment of the present invention. As shown in  FIG. 6 , system  600  may be a medium-voltage drive. Specifically, in the embodiment of  FIG. 6 , a three-phase, medium-voltage drive is shown that includes a plurality of power cell chambers  620   A1 - 620   C3  (referred to in  FIG. 6  as diode front end (DFE) cells). As seen, a local cell controller  626   A1 - 626   C3  is associated with each of the power cell chambers. Understand that while shown as a single enclosure, each of the power cell chambers may be separated into a fixed portion and a moveable portion and that the local cell controller may in some embodiments be adapted within an enclosure of the moveable power cell. Also, understand that while not shown in  FIG. 6  for ease of illustration a plurality of power service buses may be configured between these different portions of the power cells and furthermore may provide for connections between each power cell and a master controller  640 . 
     As seen, each of these local controllers may communicate with a fiber optic interface  660 . In some implementations, a pair of unidirectional fiber optic channels may be coupled between each local controller and fiber optic interface  660 . In turn, fiber optic interface  660  communicates with a master controller  640  that further includes an ADC  645 . 
     Master controller  640  may provide control signals to fiber optic interface  660  for transmission to the different local controllers. In one embodiment, these control signals may be voltage reference signals, which cause the local controllers to perform certain processing to generate the needed switching signals. In other implementations, the switching signals themselves may be sent by master controller  640  for transmission to the local cell controllers. 
     As further seen in  FIG. 6 , a signal conditioning board  650  may be present to sense or perform signal processing with regard to various information, namely voltage and/or current information obtained both from the input power source and the output of the different phase output lines coupled to a load  630  which in one embodiment may be a motor, as well as from an auxiliary secondary winding in accordance with an embodiment of the present invention. 
     In addition to the control information described above, additional information from master controller  640  can be provided to the individual local controllers. In addition, the local controllers can provide information such as status information, both as to normal operation as well as faults, over-temperature situations or so forth, back to master controller  640 . Master controller  640  may further be associated with a user input device  655  such as a keyboard and/or touch screen display and which may be adapted within a user interface bay to enable user input to control various features such as speed, torque, selection of different power cells to be enabled and so forth, as well as to provide status information to the user via a given display or other output means. 
     As shown in  FIG. 6 , input power to transformer modules  610  may include both a medium voltage source, e.g., from an input power supply such as a utility connection, and a low voltage power source, e.g., from an auxiliary low voltage power source, as discussed above. Such sources may be provided to a customer cabinet  605 , which may be at a separate location from a cabinet of drive system  600 . Cabinet  605  may include a control circuit to switch between powering of the drive by either the main power source or the auxiliary power source. For example, at power-up, a switch  606 , which may be a circuit breaker, of the low voltage auxiliary power line is closed such that power is provided to transformer module  610  through a normally closed contact  607  to enable pre-charging of the capacitors of the different power cells  620  via this low voltage source. Accordingly, the capacitors of power cells  620  are charged through LV auxiliary primary winding(s) of transformer module  610  to a predetermined voltage level. Then after passing of a predetermined time, which may be on the order of approximately 50 to 10000 milliseconds (ms), a main power supply power switch  608  (e.g., a MV circuit breaker) is closed. Normally closed contact  607  can be used to disconnect the LV auxiliary power. By closing the main power supply, the capacitors are thus charged to their rated voltage. A timer or programmable logic controller (PLC) or other type of control circuit can be used to control the process and sequence of switching. In another embodiment, the master controller can determine this charging time by monitoring the DC-bus voltage of one or more power cells. After the capacitors of power cells are charged to a predetermined level, the master controller can send an activation signal to MV circuit breaker  608 . However in either implementation method, the sequence of switching for powering-up the drive is first to close the LV auxiliary switch  606  (i.e. circuit breaker). After a predetermined time or receiving a command from a controller, the LV switch  608  is opened and main power switch  608  (i.e., MV circuit breaker) is closed. Note in other implementations, direct connection form utility to drive system  600  may occur (i.e., without customer cabinet  605 ). 
     Referring now to  FIG. 7A , shown is an illustration of a cabinet in accordance with one embodiment of the present invention. As shown as  FIG. 7A , a cabinet that houses a drive system, e.g., a medium voltage drive system, includes a transformer bay  20 , a power cell bay  30 , a fan bay  40 , and a cabinet controller bay  50 . 
     The illustration of  FIG. 7A  further shows the horizontal configuration of three transformers  200   a - 200   c , each of which includes a core having three columns, one for each of three phases, each having a horizontally aligned axis around which primary and secondary coils may be concentrically wrapped. Also seen are the illustrations of individual power cell chambers  32   a , which show the moveable portions that are formed of an enclosure with heat sinks adapted on top and bottom portions. As described above, in some embodiments, each chamber  32  may be located on top of spacers to provide simple and low cost mechanical support and greater air flow. 
     Referring now to  FIG. 7B , shown is a side view of the cabinet that illustrates air flow for cooling in accordance with an embodiment of the present invention. As seen in  FIG. 7B , a grill or other input mechanism  25  may be provided at a forward panel of transformer bay  20  to enable incoming air flow. As seen, the incoming air flow passes horizontally through transformers  200 , as well as passing vertically up to the power cell bay. The laminar air flow through transformers  200  may then be vertically removed through an air duct at a rear of the cabinet by fans within fan bay  40 . 
       FIG. 7B  further shows the configuration of the power cell chambers which are formed from the moveable enclosure  110 , power service bus  120  and capacitor of the fixed portion  115  (note that other components may be present in the fixed portion as discussed above, e.g., with reference to  FIG. 2A ). As seen in  FIG. 7B , the air flow through the power cell bay may further be provided vertically upward via fans within fan bay  40 . While shown with this particular implementation in the embodiment of  FIGS. 7A and 7B , the scope of the present invention is not limited in this regard. 
     As described above, a modular power control system includes building blocks that can be used to configure systems having one or more cabinets. A modular system may include one or more cabinets, each of which may be configured such as described above with regard to  FIG. 1 . Still further, a master control cabinet per system may be provided to enable centralized control for the overall system. Thus in various embodiments, a power control system may include a master control cabinet and one more or more cell cabinets that can be identically configured. The number and interconnect configuration of the cell cabinets determine the current and voltage capacity of the overall system. In different implementations, the cabinets can be configured in parallel for greater current and/or in series for greater voltage applications. 
     The control and diagnostics for the modular system can also be distributed among the major components. The master controller provides control information to each cell cabinet. Each cell cabinet provides local control of the power cells, via a cabinet controller. Status information is provided from the power cells to each cabinet controller. The cabinet controller then provides cabinet status back to the master controller. In various embodiments, diagnostics are run locally for each major component. Each power cell controller, cell controller, and master controller initiates local operations to assess operational readiness. Cell cabinet operational readiness is then communicated back to the master controller, and the master controller then determines the overall readiness of the system. 
     Referring now to  FIG. 8A , shown is a block diagram of a modular system configuration in accordance with one embodiment of the present invention. As shown in  FIG. 8A , system  400  may be a minimal modular system, e.g., for a relatively low medium voltage application in which only a single power cell cabinet  420  is present and which in turn is coupled to a load  430 . As seen, cabinet  420  is coupled to receive input power from, e.g., a set of AC mains. Feedback information, namely the input current and voltage may be provided via a feedback path  405  to a master control cabinet  410 , which may further receive a feedback regarding the output voltage and current from power cell cabinet  420 . 
     In other implementations, a different configuration may be realized. Referring now to  FIG. 8B , shown is a block diagram of a parallel modular system configuration in accordance with an embodiment of the present invention. As shown in  FIG. 8B , system  400 ′ includes a plurality of power cell cabinets  420   a - 420   n  coupled in parallel to increase drive output current capability. Each power cell cabinet is paralleled to the AC mains and provides output current to load  430 . The master control  410  maintains load sharing among cabinets  420   a - 420   n  by processing current feedbacks via  405  and  425 . 
     In yet other implementations a series configuration is possible. Referring now to  FIG. 8C , shown is a block diagram of a series configuration. As seen in  FIG. 8C , system  400 ″ includes a plurality of power cell cabinets  420   a - 420   n  coupled in series, such that the outputs are cascaded to provide a wide range output voltage capability to load  430 . Similar feedback connections to master cabinet control  410  are provided to preserve load voltage sharing among the power cell cabinets. Other implementations may use combinations of parallel and serial configurations to increase load voltage and current capability. 
     Referring to  FIG. 9A , shown is a flow diagram of a basic control loop for a master controller in accordance with one embodiment of the present invention. As shown in  FIG. 9A , method  500  may be performed on a continual basis by the master controller. Specifically, based on input current and output voltage and current information received in the master controller, and various command parameters, including current and voltage ratings, speed, frequency, torque and so forth, a voltage reference may be computed (block  510 ). In one embodiment, the voltage or current reference may be computed using a standard motor vector control algorithm, e.g., processing torque and flux commands, and rotor position. Next, it may be determined based on various information coming from the different cabinets whether the system is operating within its operating parameters (diamond  520 ). If not, an alarm may be triggered (block  525 ) followed by a master control action if required. Otherwise, normal operation may continue and a reference vector may be transmitted to all cabinets (block  530 ). In one embodiment, this reference vector may be serially encoded and sent to the cabinets, e.g., via a fiber optic interface. 
     Referring now to  FIG. 9B , shown is a flow diagram for a basic control loop for a cabinet in accordance with one embodiment of the present invention. As seen in  FIG. 9B , method  550  may begin by receiving the reference vector from the master controller in a cabinet controller. From this reference vector, a switching pattern using a carried based pulse width modulation scheme or any other modulation technique for the cells within the cabinet may be computed (block  560 ). In one embodiment, this PWM calculation may be in accordance with commands received from the master controller and general drive operation status. Then based on various status information, including information from ambient sensors and the various power cells, it may be determined whether the cabinet is operating within its operating parameters (diamond  570 ). If not, an alarm may be triggered (block  575 ). Otherwise, normal operation may continue and the PWM signals may be sent to the power cells of the cabinet (block  580 ). In one embodiment, the PWM signals may be serially encoded and sent to the cells, e.g., via a fiber optic interface. 
     Referring now to  FIG. 9C , shown is a flow diagram of a method for performing control of a power cell via a local controller of the power cell in accordance with one embodiment of the present invention. As shown in  FIG. 9C , method  600  may begin by each local controller receiving encoded PWM signals from the cabinet controller of the cabinet. From this information gate drive signals may be computed (block  610 ). In one embodiment, these signals may be computed by a local FPGA within the moveable cell section, and that these computations are based on cell status and PWM command signals received from the cabinet controller. Then, based on various information including ambient sensors and the status of the switching elements themselves, it may be determined whether the cell is operating within its operating parameters (diamond  620 ). If not, an alarm may be triggered (block  625 ). Otherwise, normal operation may continue and gate drive signals may be sent to the switching elements (block  630 ). While shown with this particular control implementation in the embodiment of  FIGS. 9A-9C , the scope of the present invention is not limited in this regard. Furthermore, understand that the various control operations described may be performed in different orders and may be performed in different controllers such that the different control can be handled at a more local or global basis depending on a desired implementation. 
     Referring now to  FIG. 10 , shown is a block diagram of a single slice of a drive system  1000  that can be cooled in accordance with an embodiment of the present invention. In  FIG. 10 , the portion of drive system  1000  shown may be a single slice that includes a plurality of power cells or cubes  1010   1 - 1010   3  (generically power cube  1010 ). Each of these power cubes can be configured as described above, e.g., with regard to  FIGS. 2A-2D , with each cell configured within a given chamber and including a movable portion and a fixed portion. Or as will be described further below, a unitary power cube design in which the entire power cube is movable may instead be provided. However, understand the scope of the present invention is not limited in this regard. For example, in other embodiments rack mounted power cubes can be provided that can be plugged into a backplane or other supporting mechanism within a cabinet. In any event, the cubes may be inserted in corresponding chambers of an electronics or power cube bay of a cabinet. Given the cooling capabilities of a cooling system in accordance with an embodiment of the present invention, this cabinet may be an enclosed NEMA-4 cabinet (at least for the electronics or power cube bay portion), improving reliability and reducing heat transfer with an ambient environment. 
     Note that the illustration shown in  FIG. 10  is at a high level to show the components involved in a cooling system for a single slice. In various embodiments, the cooling system may be a vaporizable dielectric fluid (VDF)-based cooling system. Specifically, each power cube  1010  may include one or more cold plates or other heat transfer members that acts as an evaporator to receive a flow of cool fluid to thus remove heat from components within the power cube and provide combined fluid and vapor phases to a remainder of the cooling system. 
     In the embodiment shown, the cooling system can include a heat exchanger  1040  which in an embodiment may be an air-based heat exchanger such as a condenser that cools the heated high pressure vapor and fluid with an ambient air source that may be directed via a fan  1044 . Also note the presence of a condensate control valve  1042  to avoid any moisture condensation on high power electronics, cold plates and cooling system components such as tubing, pump tray, etc. Such a condensate control valve may be included in the various cooling loops of a drive system to avoid any moisture accumulation on cold plates, which are in intimate contact with high power electronic components, and on any cooling lines to avoid catastrophic failures. 
     In other embodiments, this heat exchanger can be a water-based heat exchanger or another fluid-based heat exchanger. In general, heat exchanger  1040  may operate by cooling the heated vapor and fluid to thus reduce its temperature and pressure and provide a flow of liquid coolant to an accumulator or reservoir  1020  via an inlet conduit  1045 . Although shown being located externally to the cabinet, understand that in other embodiments the heat exchanger may be located within the cabinet. 
     In an embodiment, inlet conduit  1045  can be a combination of a dielectric hose section (generally shown in  FIG. 10  as dotted lines) such as thermoplastic hoses, copper or other tubing or so forth. There is at least one section of dielectric part from the power cubes to the inlet and outlet manifold or conduits to isolate the floating voltages between the power cubes of a medium voltage drive system. A cooling system in accordance with an embodiment of the present invention can isolate up to or more than 13.8 kV isolation. 
     Still referring to  FIG. 10 , accumulator/two phase fluid separator  1020  (hereafter accumulator) can be coupled via conduit  1045  to a pump  1030  which may be a low pressure pump that provides a regulated flow of liquid coolant via conduit  1045  to each of the power cubes. Note that in the embodiment shown, each power cube  1010  may include a single inlet port  1012   1 - 1012   3 , which as will be described further below may be provided with quick disconnect fitting to enable rapid and independent connection/disconnection of a given power cube to/from the cooling system such that a given power cube can be dynamically inserted/removed during normal system operation without an effect on the remainder of the system. Still further, in some embodiments, these ports may be received via blind mating connections to enable easy manual insertion and disconnection. By using dielectric hoses, electrical isolation (e.g., up to 35 kV) between hose-to-hose (i.e., line-to-line) can be realized, and also between power cube to manifold, which is grounded (i.e., line to neutral). This in turn helps to maintain isolation between adjacent power cubes. 
     After cooling the components within a given power cube, the heated vapor and liquid may exit via a corresponding outlet port  1014   1 - 1014   3  (also realized via quick disconnect and/or blind mating connection, some embodiments). Each port couples to an outlet conduit  1015  which again may be formed of a combination of dielectric section such as thermoplastic hoses, copper or other tubing or so forth, and may be coupled to heat exchanger  1040  (note the terms inlet and outlet may be with respect to the power cubes). In certain embodiments, a manifold assembly may be present to support routing of the conduits and enable connection of ports  1012  and  1014  to corresponding ports or other connection members adapted within this manifold assembly and coupled to conduit  1015 . And in some embodiments, the manifold assembly may also support the accumulator and pump. However, this manifold assembly is not illustrated in  FIG. 10  so as to not obscure other details shown. Note that most joints can be brazed, which improves reliability of the cooling system and shock tolerance. In an embodiment three disconnects are used: between a power cube and thermoplastic hose; blind mate connections; and quick disconnect at the pump tray. 
     As further shown in  FIG. 10 , a modular transformer  1050  may be present and coupled to the power cubes. In various embodiments, this modular transformer may be configured as discussed above. Transformer  1050  may be cooled by a flow of incoming air, e.g., being directed by fans  1055  or other extraction mechanism to enable the transformer to be cooled and allow a heat exchange to occur such that a flow of heated air exits the transformer. In some implementations, multiple fans may be provided for transformer cooling. The fan motors may be controlled responsive to a variable frequency drive (VFD). In this way the fan speed can be maintained proportionally to air flow rate and likewise to the transformer winding temperature. Also, by using VFD in combination with a separate fan for each winding, the temperature of each winding can be controlled separately, allowing greater control on winding performance. Although shown at this high level in the embodiment of  FIG. 10 , understand the scope of the present invention is not limited in this regard. 
     Embodiments may use the heat of vaporization (flow boiling). This VDF-based cooling is isothermal, and can be plumbed in a serial manner to different cold plates in the power cubes, such that all cold plates in that power cube may be maintained close to the same temperature. Each power cube is floating in terms of voltage with respect to others. When fluid passes from manifold to power cube or vice versa, at least one dielectric section of hoses or conduit is provided. All power cubes then can be connected in series or parallel. Evaporative heat transfer in accordance with an embodiment of the present invention can be 2×-4× better than single-phase (e.g., water) convective heat transfer coefficients. In this way, smaller pumps can be used to lower flow rates, which may be of 20% or less flow rate than a single phase system. 
     In different embodiments, any vaporizable dielectric can be used. As examples, R134-a or low pressure refrigerants can be used, which flash to gas at ambient pressures. These various refrigerants are dielectric and will not harm the electronics in case of leakage occurs, and further cause no corrosion or scaling. Note that the dielectric fluid can be interchanged according to the site location and its respective local environmental codes. 
     In a VDF system, a liquid-to-gas phase change of a refrigerant such as R134-a occurs within the evaporator such as a cold plate. As long as there is fluid in the cold plate, the cold plate surface will be held close to the boiling point of the fluid. In an example embodiment, for 40° C. refrigerant, 151J are required to convert 1 g of refrigerant from liquid to gas. Therefore, to dissipate 1 kW of power, a flow rate of 0.43 lit/min is required, in contrast to much higher flow rates for a water-based system. Lower flow rates for a VDF system equate to a smaller pump, power supply, reservoir, and smaller tube diameters for cooling system conduits. And by using lower flow rates of fluid, the parasitic load on the cooling system is reduced, thus improving efficiency of a medium voltage drive system. Also the overall size and weight of the drive system is reduced as compared to water or air cooled systems. 
     Using a dielectric fluid that vaporizes easily at ambient pressures, many positive attributes adhere including safety, operating temperatures, heat of vaporization for optimum performance, minimum volumes, lubrication and component reliability, and materials compatibility. Dielectric fluids in accordance with an embodiment have excellent compatibility with many metals including copper, aluminum, brass, bronze, with set galvanic action from combinations of metals and thus no ionic stripping or transport or galvanic action occurs. In various embodiments self-optimizing system performance for VDF cooling technology may be as follows. In general, sub-cooled fluid enters and flows through pump  1030  in a 100% liquid phase. Liquid flows though conduit  1045  connecting pump  1030  to cold plates within each of power cubes  1010 . Sensible heat transfer occurs in the cold plates, causing fluid flow to boil to a predetermined liquid/vapor percentage. Next multiple phases flow from conduits  1015  from the cold plates to heat exchanger  1040 , which dissipates heat to the cooling fluid medium (air, water, etc.) and condenses to liquid. A mixture of fluid and vapor enters the condenser where heat is released and the mixture condenses to liquid. Sub-cooling occurs via sensible heat transfer, reducing temperature of liquid to ensure 100% liquid enters accumulator  1020  and hereafter to pump  1030 , and the cycle repeats. 
     In an embodiment, for steady-state operation the system design is sized for a maximum of 80% liquid/vapor quality at the outlet of each power cube. As load increases, the system moves further into the vapor dome, but design parameters may be set to size it so cold plates never dry out. As the load decreases, the system boils less. There are no penalties in this in terms of cold plate temperatures because fluid flows are low and pump parasitic power draw is low, as the system reacts quickly to varying power levels and fluid flow is constant. Using a two-phase cooling system, a higher horsepower rating can be achieved, realizing more power density while reducing unit cost of cooling. Other cost benefits may include: more electronics reliability due to less thermal switching; less maintenance as the external air does not enter the electronics section (e.g., when using a NEMA-4 enclosure); less heat load on a HVAC system when a drive system is installed inside a building; the drive can go from zero to full rating without having any thermal issues (i.e., very small time constant for cooling); and can have an option of air and water cooled with the same system. 
     Other embodiments besides the externally located heat exchanger implementation shown in  FIG. 10  are possible. For example, in another embodiment multiple heat exchangers may be provided to enable greater cooling operation. A choice of a single or multiple heat exchange system may be based on a location of the drive system. For example, when it is desirable to limit the amount of heat exchange into an ambient environment such as a room in which a drive system is located, it is possible to include a water-based heat exchanger within the drive system itself that in turn can be coupled to an external heat exchanger, which in various implementations can be a water or air-based heat exchanger. Note that the number of heat exchangers depends also upon customer specifications such as availability of a cold water tower, HVAC limitations for the room, size of installation site, and cost for each additional heat exchanger system and so forth. 
     As with the drive system itself, the cooling system components within the power cubes are also modular and can be replaced quickly in case of failures. For example if, e.g., a pump fails, swapping the pump tray is very easy by disconnecting a quick disconnect mechanism, enabling rapid replacements. Also, the cooling system is modular with each slice. If a customer needs to add more rating in the drive, there is no need to custom design the cooling system for the whole drive system. And as such the slice cooling system can allow mass production of the cooling system independently of the rating of the drive. Cooling systems described herein may be used for regenerative as well as non-regenerative medium voltage drive systems. 
     Referring now to  FIG. 11 , shown is a block diagram of a single slice of a drive system  1100  that can be cooled in accordance with another embodiment of the present invention. As shown in  FIG. 11 , the illustrated drive system  1100  may be configured similarly to that of drive system  1000  of  FIG. 10 . However here note the presence of an internal heat exchanger  1060 . In other aspects, the system shown in  FIG. 11  may be generally arranged similarly to the system shown in  FIG. 10 . Accordingly, a plurality of power cubes  1010   1 - 1010   3  may be present, each of which can be provided with a flow of cooling liquid received via corresponding ports  1012   1 - 1012   3  which receive a flow of cooling liquid from a pump  1030  that is coupled to an accumulator  1020 . Note however that the outlet flow from the power cubes via corresponding ports  1014   1 - 1014   3  may instead be coupled through outlet conduit  1015  to a heat exchanger  1060 . Thus the heated two-phase liquid and vapor may be cooled by a water-based heat exchanger  1060  to provide a flow of cooled liquid to accumulator  1020 . Note that this heat exchanger can handle any type of fluid such as fresh/sea water, oil, ethylene glycol mixture, etc. 
     To achieve this cooling via a water-based heat exchanger, an incoming flow of cool water may be received via a conduit  1070  from an external heat exchanger  1080 , which may be a water or air-based heat exchanger in different implementations. In turn, the heated water from heat exchanger  1060  may be communicated via a conduit  1075  to be provided back to heat exchanger  1080  and cooled using ambient water and/or air at an external location. In a given implementation this may be an external location to a facility having a drive system located internally to the facility. 
     Note that in the implementation of  FIG. 11 , transformer  1050  may still be cooled by a flow of incoming air, e.g., being directed by fans  1055  or other extraction mechanism to enable the transformer to be cooled and allow a heat exchange to occur such that a flow of heated air exits the transformer. Note that although this implementation of  FIG. 11  is different than the implementation of  FIG. 10  in general, both systems can be manufactured in the same manner and having similar location of components, with the simple addition of a water-based heat exchanger within the enclosure of the drive system. In various embodiments, this heat exchanger  1060  (and the accumulator and pump) can be located on a separate shelf or otherwise supported within the structure such as coupled to a portion of the manifold assembly. And as further seen, the outlet conduit  1015  may adapt to heat exchanger  1060  and inlet conduit  1045  may similarly adapt to the heat exchanger. Thus a common design of a drive system may be present and minor changes can be accommodated to enable the internal heat exchanger option of  FIG. 11 . For example, a single cabinet design and arrangement may be provided and minimal changes may be made to the plumbing to accommodate different cooling system implementations. As such, great configurability of a cooling system can be realized to accommodate different cooling requirements of customers. Of course other implementations are possible. 
     Referring now to  FIG. 12 , shown is another example of a drive system having a cooling arrangement in accordance with an embodiment of the present invention. As shown in  FIG. 12 , drive system  1200  may be configured similarly to that of drive system  1100 . However in the embodiment of  FIG. 12 , additional cooling and corresponding reduction of hot air exiting the drive system to an ambient environment can be realized by providing cooling fluid through an outlet air path of transformer  1050 . Thus as seen, inlet conduit  1045  provides a flow of cooling liquid to a heat exchanger  1057 , which may be a finned liquid/air heat exchanger having a compact and lightweight design, along an air exhaust path out of transformer  1050  that further includes fans  1055 . This enables greater heat exchange within the drive system and causes a flow of heated two-phase vapor and liquid to be communicated via outlet conduit  1015  to the internal heat exchanger  1060 . Using such an implementation, greater amounts of heat exchange occur within the drive system, further reducing load on an HVAC system of a facility including the drive system. For example in various implementations, a heat exchange of approximately 90% can occur, with only 10% of heat load being dissipated to the ambient within the facility, thus reducing HVAC load in the facility. In other respects, drive system  1200  may be configured similarly to that of drive system  1100  of  FIG. 11 . Note also that in another implementation, the transformer bay may also be NEMA-4 such that cooled air after heat exchanger  1057  may be directed by fans  1055  back to an inlet side of transformer  1050 . 
     Referring to  FIG. 13 , shown is yet another implementation of a cooling system in accordance with an embodiment of the present invention. In general, cooling system  1300  can be configured similar to the system  1200  shown in  FIG. 12 . However here note that direct connection via conduits  1070  and  1075  are provided to heat exchanger  1057 . Thus in this arrangement, conduits  1015  and  1045  within the power cube bay of the drive system are used only to provide cooling to the power cubes themselves. And as such, heat exchanger  1060  is used only to provide heat exchanging for the power cubes themselves. Instead, for purposes of cooling the outlet air flow from transformer  1050 , external heat exchanger  1080  may be used, by providing a flow of liquid coolant via conduit  1070  and in turn passing two phase vapor and liquid from heat exchanger  1057  to external heat exchanger  1080  via outlet conduit  1075 . In other aspects, drive system  1300  may be configured similar to that of system  1200  of  FIG. 12 . 
     Yet further implementations are possible. For example in another arrangement, no heat exchanger is present within either of the drive system or a facility including the drive system. Instead, cooling liquid and corresponding heated two-phase liquid and vapor may be communicated to an external heat exchanger, e.g., located outside of a facility including a drive system. 
     Referring now to  FIG. 14 , shown is an arrangement of another drive system  1400  having a different cooling arrangement in accordance with an embodiment of the present invention. As seen, an incoming flow of cooling liquid may be received in drive system  1400  via an inlet conduit  1015 . This flow can be received from a pump  1030  that receives the liquid conduit from an accumulator  1020 . Note that in this implementation both the pump and accumulator may be located externally to the cabinet of drive system  1400 . In addition, a heat exchanger  1080  that receives a flow of two phase coolant via outlet conduit  1045  may also be external to the drive system cabinet. Typically, these components may be located within close relation to the drive system, e.g., within a same room of a facility that includes the drive system, although the scope of the present invention is not limited in this aspect. Note that in other implementations one or more of the pump and accumulator may be located externally to the facility. 
     Note that the arrangement in  FIG. 14  may be implemented where a larger, higher power pump is provided to enable the single pump to provide a suitable flow of liquid coolant for multiple slices of a drive system. Although the embodiment shown provides the accumulator and pump external to the drive system, the scope of the present invention is not limited in this regard. In other implementations it may be possible to provide sufficiently small enough sized pumps and accumulators to enable their incorporation within the drive system. 
     Referring now to  FIG. 15 , shown is an isometric view of a power cube in accordance with another embodiment of the present invention. As shown in  FIG. 15 , power cube  1500  may be configured differently from the divided fixed and pluggable-based power cubes, e.g., of  FIGS. 2A-2D . Instead here, a unitary moveable power cube design exists. As seen, the power cube is configured to be blind mate inserted into a backplane or manifold assembly of a cabinet. In the illustration shown, this blind mating may be realized by way of power connections  1510   a  and  1510   b . In addition, to provide communication of data and control signals to and from controllers within the power cube and a master controller, a blind mate electrical connection  1520  may also be provided. In addition, to enable cooling via a cooling system in accordance with an embodiment of the present invention, blind mate quick connect ports  1525   a  and  1525   b  may be provided that can connect with corresponding connection members within a manifold assembly to enable the flow of liquid coolant and return two phase vapor and liquid via inlet and outlet conduits, respectively as described above. 
     For purposes of discussion of the cooling operation of the power cube, note that the inlet conduit via inlet port  1525   a  (for example) may communicate a flow of liquid coolant to a first of a plurality of independent cold plates  1530   a - 1530   d . In general, each of these cold plates may include a channel, where the cold plates are connected together in series to thus enable a series of flow of liquid coolant through the multiple cold plates, and thereafter to enable a flow of heated two phase vapor and liquid coolant to an output conduit (not shown) via outlet port  1525   b . By way of VDF-based cooling, the cold plates of a given power cube may be substantially isothermal, in contrast to a water-based cooling system, in which serial communication of liquid coolant may cause different temperature of the cold plates. 
     In the embodiment shown, cold plates  1530   a - 1530   d  may be configured on a base plate  1540  which in an embodiment may be an aluminum plate to provide heat spreading capability. As further shown, a plurality of resistors  1545   a - 1545   d  may be configured on this plate. Above cold plates  1530   a - 1530   d  may be a corresponding plurality of IGBTs which in the illustration shown are shown as IGBTs  1550   a - 1550   d . Understand that each such IGBT may be associated with a switching controller such as a control board adapted above the corresponding IGBT (these control boards are not shown for ease of illustration in  FIG. 15 ). 
     Sufficient cooling of the power cube may be realized by way of aluminum plate  1540  that acts as a heat spreader along with the cold plates  1530   a - 1530   d . Other components, such as a plurality of capacitors  1560   a - 1560   d  may not have independent cooling mechanisms, since they may not produce great amounts of heat during operation. In other embodiments, to provide additional cooling capabilities, one or more fans or other cooling mechanisms may be included in the power cubes. 
     As further shown in  FIG. 15 , a support bracket  1570  may be configured below ports  1525   a  and  1525   b , to prevent damage to these ports during insertion/removal operations. Although shown at this location, understand that multiple other support brackets may be present in other implementations. Although shown at this high level in the simplified view in  FIG. 15 , understand that additional components may be present within the power cubes, such as diodes (in a non-regeneration-capable power cube), pre-charging resistors, capacitors and other such components. 
       FIG. 16  shows an illustration of coupling of a power cube to a manifold assembly via a dielectric section in accordance with an embodiment of the present invention. As shown in  FIG. 16 , power cube  1500  is shown being inserted into a manifold assembly  1600 . In the high level shown, note the presence of port  1525  and bracket  1570 . As further shown, power connections  1510   a  and  1510   b  are illustrated. All of these connections can be blind mated into corresponding connection members of manifold assembly  1600 . However for ease of illustration only a connection member  1610  to receive port  1525  is shown. Of course similar connection members can be present to enable blind mating of the power connections and electrical connections. In various embodiments, an interface section  1620  of manifold assembly  1600  may be implemented using dielectric components to provide flexibility and insulation. As an example, various members of this dielectric section may be formed of TEFLON or TORLON or another flexible dielectric material such as thermoplastic hoses. Although shown at this high level view in the embodiment of  FIG. 16 , understand the scope of the present invention is not limited in this regard. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.