Patent Publication Number: US-2017359011-A1

Title: Reconfigurable multi-permanent magnet generator based power generating system

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a reconfigurable permanent magnet generator based power generating system, and more specifically to a power generating system including dual voltage regulated permanent magnet generators. 
     BACKGROUND 
     Certain vehicles, such as ground based vehicles, favor hybrid electric technology using high voltage electrical power generating as a primary power source for vehicle components. Many such applications utilize one or more permanent magnet generators (PMG) to generate power due to the high power density of permanent magnet generators. Permanent magnet generators convert rotational motion into electrical output power via the interaction between a magnetic field of one or more permanent magnets and an armature including multiple power windings. In typical permanent magnet generator arrangements, the excitation of rotor flux cannot be excited due to the nature of permanent magnets, and the permanent magnet generator may operate in an undesirable manner during short circuit conditions. 
     In some example vehicles, output voltage regulation is achieved utilizing two permanent magnet generators connected in series. By adjusting the position of the stator windings of the first permanent magnet generator, relative to the position of the stator windings of the second permanent magnet generator, the combined output stator voltage can be controlled over a wide range of speed and load configurations. This type of voltage regulation is achieved by physically actuating one of the stator windings. 
     Voltage regulated permanent magnet generators that operate based on the known principle of saturation can regulate the output voltage over a wide speed and load range. However, saturation based permanent magnet generators are similarly limited in controlling the output voltage to zero volts. 
     SUMMARY OF THE INVENTION 
     In one exemplary embodiment a high voltage power generating system includes a first poly-phase permanent magnet generator including at least one control winding and a plurality of power windings, a second poly-phase permanent magnet generator including at least one control winding and a plurality of power windings, the first and second poly-phase permanent magnet generators being arranged in series in at least one operational mode, and the first poly-phase permanent magnet generator being phase shifted relative to the second poly-phase permanent magnet generator such that residual voltage output of the first poly-phase permanent magnet generator is offset by the second poly-phase permanent magnet generator during a first operational mode. 
     In another exemplary embodiment of the above described high voltage power generating system the first operational mode is a short circuit condition at a power generating bus. 
     In another exemplary embodiment of any of the above described high voltage power generating systems the first poly-phase permanent magnet generator is connected to an input of the second poly-phase permanent magnet generator via a first set of switches and connected to a first power generating bus via a second set of switches, the first set of switches and the second set of switches having an inverted state relative to each other. 
     In another exemplary embodiment of any of the above described high voltage power generating systems an input of the second poly-phase permanent magnet generator is connected to a second power generating bus via a third set of switches, and an output of the second poly-phase permanent magnet generator is connected to the first power generating bus via a fourth set of switches, the third set of switches and the fourth set of switches having an inverted state relative to each other. 
     In another exemplary embodiment of any of the above described high voltage power generating systems the state of each set of switches is controlled by a generator control unit, and wherein the first set of switches and the third set of switches are closed during said first operational mode. 
     In another exemplary embodiment of any of the above described high voltage power generating systems the first power generating bus and the second power generating bus are AC busses. 
     In another exemplary embodiment of any of the above described high voltage power generating systems the first power generating bus and the second power generating bus are DC busses. 
     Another exemplary embodiment of any of the above described high voltage power generating systems further includes a selectively removable neutral node connected to an output of said second poly-phase permanent magnet generator via a DC rectifier. 
     In another exemplary embodiment of any of the above described high voltage power generating systems each of said poly-phase permanent magnet generators has an identical number of phases. 
     In another exemplary embodiment of any of the above described high voltage power generating systems each of said poly-phase permanent magnet generators has three phases. 
     An exemplary method for achieving null output voltage of a power generating system including multiple permanent magnet generators includes reducing a control current of a first permanent magnet generator to zero, detecting a residual output of the first permanent magnet generator, and controlling a control current to a second permanent magnet generator, such that the second permanent magnet generator generates an offsetting voltage, thereby reducing the residual voltage to 0 volts. 
     In another example of the above described exemplary method for achieving null output voltage of a power generating system including multiple permanent magnet generators reducing the control current of the first permanent magnet generator to zero, detecting the residual output of the first permanent magnet generator, and controlling a control current to a second permanent magnet generator, such that the second permanent magnet generator generates an offsetting voltage, are performed in response to detecting a short circuit event at a power generating bus. 
     Another example of any of the above described exemplary methods for achieving null output voltage of a power generating system including multiple permanent magnet generators further includes reconfiguring a power generating system such that the first permanent magnet generator and the second permanent magnet generator are arranged in series in response to detecting the short circuit event. 
     In another example of any of the above described exemplary methods for achieving null output voltage of a power generating system including multiple permanent magnet generators reconfiguring the power generating system comprises inverting at least a first, second, third and fourth set of switches. 
     Another example of any of the above described exemplary methods for achieving null output voltage of a power generating system including multiple permanent magnet generators further includes removing a selectively removable neutral node from said second permanent magnet generator. 
     In another example of any of the above described exemplary methods for achieving null output voltage of a power generating system including multiple permanent magnet generators removing the selectively removable neutral node comprises opening a switch. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a high voltage power generating system including two voltage regulated three-phase permanent magnet generators. 
         FIG. 2  schematically illustrates a second example high voltage power generating system including two voltage regulated three-phase permanent magnet generators. 
         FIG. 3  illustrates a method to achieve null output voltage regulation of a power generating system including multiple voltage regulated permanent magnet generators. 
     
    
    
     DETAILED DESCRIPTION OF AN EMBODIMENT 
       FIG. 1  schematically illustrates a high voltage power generating system  100 . The power generating system  100  includes a first voltage regulated permanent magnet generator  110  and a second voltage regulated permanent magnet generator  120 . The permanent magnet generators  110 ,  120  are connected to a shaft  102  and receive a rotational input from the shaft  102 . 
     Each permanent magnet generator  110 ,  120 , includes a three-phase armature windings  112 ,  122 . Each phase on the armature windings  112 ,  122  is alternatively referred to as a power winding. Also included within each of the permanent magnet generators  110 ,  120  is a control winding  114 ,  124 . The control windings  114 ,  124  are connected to a DC voltage source  130  via a switched current regulator  132 . The switched current regulator  132  controls current in each control winding  114 ,  124  separately, thereby allowing for independent power output control of each of the permanent magnet generators  110 ,  120 . A generator control unit  140  is connected to, and controls, the switched current regulator  132 , thereby controlling the output voltage of the system. The generator control unit  140  provides further generator controls as needed, according to known generator control unit operations. 
     An output of the second permanent magnet generator  120  is provided to a DC rectifier  150  that converts the three phase output to a DC power, and provides the DC power to a load  152 , such as a DC power bus. 
     The respective phases of the armature windings  112 ,  122  of each of the permanent magnet generators  110 ,  120  are connected in series. The armature windings  122  of the second permanent magnet generator  120  are phase shifted, relative to the armature windings  112  of the first permanent magnet generator  110 , such that a zero voltage combined output condition is created when the control current of the first control winding  114  is set to zero, and the control current of the second control winding  124  is set to match a residual output voltage of the first permanent magnet generator  110 , and is above zero volts. 
     During normal operation (e.g. when no short circuit is present on the bus  150 ), the generator control unit  140  receives signals from the output DC voltage of the first permanent magnet generator  110  and regulates the output voltage of the system  100  by controlling the current through the first current winding  114  using the switched current regulator  132 . During this mode of operations, the current through the second control winding  124  is maintained at zero amps. 
     When a short circuit condition is present, the generator control unit  140  sets the current through the first control winding  114  to zero amps, and the control current through the second control winding  124  is set to a level that reduces the residual output voltage of the first permanent magnet generator  110  to zero volts. Thus, the high voltage DC power generating system of  FIG. 2 , utilizes the two independently controlled control windings  114 ,  124  and the phase shifted armature windings  112 ,  114  to prevent a power output during a short circuit event. 
     While illustrated in  FIG. 1  as a DC power generating system  100 , one of skill in the art will recognize that some power generating systems utilize poly-phase AC power generating. With continued reference to  FIG. 2 , and with like numerals indicating like elements,  FIG. 2  schematically illustrates a high voltage power generating system  200  configured to provide AC power to a load, such as a high voltage AC bus  252  and a low voltage AC bus  254 . As with the DC power generating system  100  of  FIG. 1 , the AC power generating system  200  includes two voltage regulated permanent magnet generators  210 ,  220 , each of which includes a three-phase armature windings  212 ,  222  and at least one control winding  214 ,  224 . The control windings  214 ,  224  are independently controlled by a switched current regulator  232 , which is, in turn, controlled by a generator control unit  240 . A DC power source  230  is connected to the switched current regulator  232 . 
     An output of each phase of the first voltage regulated permanent magnet generator  210  is connected to an input of the corresponding phase in the second voltage regulated permanent magnet generator  220  by a first set of switches  216 . The output of each phase is also connected directly to the AC bus  252  via a second switches  218 . The first set of switches  216  and the second set of switches  218  are controlled by the generator control unit  240  such that when the first set of switches  216  is closed (conducting), the second set of switches  218  is open (non-conducting). 
     Similarly, an input of each phase of the armature  224  of the second permanent magnet generator  220  is connected to a low voltage AC generating bus  254  by a third set of switches  226 , and an output of each phase is connected to the high voltage AC bus  252  by a fourth set of switches  228 . The third set of switches  226  and the fourth set of switches  228  are controlled by the generator control unit  240  so that while the third set of switches  226  is closed (conducting), the fourth set of switches  228  is open (non-conducting), and vice-versa. 
     Also connected to each phase of the second permanent magnet generator  220  is a DC rectifier  260  and a selectively removable neutral node  262 . The selectively removable neutral node  262  is a switch that, when closed, provides a neutral connection (node) between all phases of the second permanent magnet generator  220  armature  224 . By way of example, the neutral node  262  is, in some embodiments, a transistor. When the switch is opened, no neutral exists between the phases, and power is not output. The state of the selectively removable neutral node  262  is controlled by the generator control unit  240 . During practical operations, the selectively removable neutral node  262  is normally maintained in a closed position, and is opened when a short circuit condition is detected. 
     Under standard (non-short circuit) conditions, the first permanent magnet generator  210  outputs power to the high voltage AC power generating bus  252  through the second set of switches  218 , and the second permanent magnet generator  220  outputs power to the low voltage AC power generating bus  260  through the fourth set of switches  226 . The neutral point of the second permanent magnet generator  220  is the selectively removable neutral node  262 , and is connected to the neutral node  262  by a DC rectifier  260 . In some examples the DC rectifier  260  is a three-phase diode bridge, and the selectively removable neutral node  262  is connected across the negative rails of the diode bridge. While closed (conducting), the selectively removable neutral node  262  effectively creates the neutral point for the second permanent magnet generator  220 . At the same time, the first and third sets of switches  216 ,  228  are maintained open. 
     In the illustrated example, the control current through each of the control windings  214 ,  224  is controlled by an asymmetric H-bridge within a switched current regulator  232 . This control maintains the respective RMS voltage at specified value by controlling current in the control coil of respective voltage regulated PMG. Control is achieved by the generator control unit  240  which includes functions for power input conditioning, voltage regulation of each of the permanent magnet generators  210 ,  220 , current limiting, gate drives, and any other known function(s). 
     When a short circuit occurs on the high voltage AC bus  252 , the first and third sets of switches  216 ,  228  close and the second and fourth sets of switches  218 ,  226  open. This effectively alters the configuration of the permanent magnet generators  210 ,  220 . After inverting the state of all the switches  216 ,  218 ,  226 ,  228 , during a short circuit event, the first permanent magnet generator  210  and the second permanent magnet generator  220  are arranged in a series configuration, and the selectively removable neutral node  262  is opened, removing the neutral from the second permanent magnet generator  220 . Due to the series connection between the permanent magnet generators,  210 ,  220  the neutral node of the first permanent magnet generator  210  functions as the neutral node  262  for both permanent magnet generators  210 ,  220 . Once in the series configuration with the selectively removable neutral being open, the permanent magnet generators  210 ,  220  are electrically similar to the system  100  of  FIG. 1 , and is controlled in the manner described above with regards to  FIG. 1 . 
     While described and illustrated in  FIGS. 1 and 2  as three phase systems, one of skill in the art will understand that the illustrated embodiments can be adapted to suit any poly-phase system, and are not limited to the illustrated three phase configuration. Furthermore, one of skill in the art, having the benefit of this disclosure, will understand that the power generating system  200  illustrated in  FIG. 2  can be adapted to provide DC power to a high voltage DC power generating bus and a low voltage DC power generating bus, in the same manner as the three phase power generating system  200  provides power to the high voltage AC bus  252  and low voltage AC bus  262 . 
     With continued reference to  FIGS. 1 and 2 ,  FIG. 3  illustrates a method  300  for achieving a null output voltage in a power generating system having multiple permanent magnet generators. Initially, a generator controller  140 ,  240  detects a short circuit at the power generating bus  150 ,  252 ,  254  using any known short circuit detection technique in a “Detect Short Circuit” step  310 . 
     Once a short circuit is detected, the controller reduces a control current in a first control winding  114 ,  214  to zero in a “Reduce Control Current in First PMG to 0” step  320 . Reduction of the control current has the effect of reducing the output voltage of the first permanent magnet generator  110 ,  210  to residual levels. 
     The generator controller  140 ,  240  then detects the residual voltage levels in the “Detect Residual Output” step  330 . Based on the detected residual output, the generator control unit  140 ,  240  determines a magnitude of control current to be applied to the phase shifted second permanent magnet generator.  120 ,  220  in order to generate an opposite output power and drive the net output voltage to 0. The generator controller then increases the control current in the second control winding  124 ,  224  accordingly in an “Increase Control Current in Second PMG to Offset” step  340 . 
     In some examples, such as the system of  FIG. 2 , where the first permanent magnet generator  210  drives a high power bus  252  and the second permanent magnet generator  220  drives a low power bus  254 , the generator controller  240  further includes a “Reconfigure Power Generating” step  312  immediately after the detection of the short circuit. The reconfiguring is described above with regards to the examples of  FIG. 2 , and places the two permanent magnet generators  210 ,  220  in a series allowing the following steps  320 ,  33 ,  340  to function properly. 
     It is further understood that any of the above described concepts can be used alone or in combination with any or all of the other above described concepts. Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.