Patent Publication Number: US-2006017328-A1

Title: Control system for distributed power generation, conversion, and storage system

Description:
RELATED APPLICATION DATA  
      This is a continuation-in-part of co-pending patent application Ser. No. 10/361,400, for DISTRIBUTED POWER GENERATION, CONVERSION, AND STORAGE SYSTEM, filed Feb. 10, 2003. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention pertains to the generation of electrical power. In particular, this invention relates to a control system for distributed power generation systems used close to where electricity is used (e.g., a home or business) to provide an alternative to or an enhancement of the traditional electric power system.  
      2. Description of Related Art  
      Centralized electric power generating plants provide the primary source of electric power supply for most commercial, agricultural and residential customers throughout the world. These centralized power-generating plants typically utilize an electrical generator to produce electrical power. The generator has an armature that is driven by conversion of an energy source to kinetic energy, such as a water wheel in a hydroelectric dam, a diesel engine or a gas turbine. In most cases, steam is used to turn the armature, and the steam is created either by burning fossil fuels (e.g., oil, coal, natural gas, etc.) or through nuclear reaction. The generated electrical power is then delivered over a grid to customers that may be located great distances from the power generating plants. Due to the high cost of building and operating electric power generating plants and their associated power grid, most electrical power is produced by large electric utilities that control distribution for defined geographical areas.  
      In recent years, however, there has been a trend away from the centralized model of electric power generation toward a distributed power generation model. One reason for this trend is the inadequacy of the existing electric power infrastructure to keep pace with soaring demand for high-quality, reliable power. Electric power distributed in the traditional, centralized manner tends to experience undesirable frequency variations, voltage transients, surges, dips or other disruptions due to changing load conditions, faulty or aging equipment, and other environmental factors. This electric power is inadequate for many customers that require a premium source of power (high quality) due to the sensitivity of their equipment (e.g., computing or telecommunications providers) or that require high reliability without disruption (e.g., hospitals). The utilities that traditionally operate centralized power generating plants are increasingly reluctant to make the large investments in modernized facilities and distribution equipment needed to improve the quality and reliability of their electric power due to regulatory, environmental, and political considerations.  
      More recently, technological advancements in small-scale power generating equipment has led to greater efficiencies, environmental advantages, and lower costs for distributed power generation. Various technologies are available for distributed power generation, including turbine generators, internal combustion engine/generators, microturbines, photovoltaic/solar panels, wind turbines, and fuel cells. Distributed power generating systems can complement centralized power generation by providing incremental capacity to the utility grid or to an end user. By installing a distributed power generating system at or near the end user, the electric utility can also benefit by avoiding or reducing the cost of transmission and distribution system upgrades. For the end user, the potential lower cost, higher service reliability, high power quality, increased energy efficiency, and energy independence are all reasons for interest in distributed power generating systems.  
      There are numerous applications for distributed power generating systems. A primary application is to produce premium electric power having reduced frequency variations, voltage transients, surges, dips or other disruptions. Another application is to provide standby power (also known as an uninterruptible power supply or UPS) used in the event of a power outage from the electric grid. Distributed power generating systems can also provide peak shaving, i.e., the use of distributed power during times when electric use and demand charges are high. In such cases, distributed power can be used as baseload or primary power when it is less expensive to produce locally than to purchase from the electric utility. By using the waste heat for existing thermal processes, known as co-generation, the end user can further increase the efficiency of distributed power generation.  
      Not withstanding these and other advantages of distributed power generation, there are other disadvantages that must be overcome to achieve wider acceptance of the technology. Conventional distributed power generating systems require further improvements in reliability and efficiency in order to compete effectively with centralized power generation. Distributed power generating systems that utilize an engine to drive a generator tend to be slow to achieve an operational speed from start up, and consequently are slow to provide a source of back-up power. During the time necessary to bring the engine and generator up to operational speed, the distributed power generating system must rely on stored power (i.e., batteries) to supply the back-up source. Battery storage systems are large, expensive, heavy, and have relatively short life expectancy. It is therefore desirable to minimize reliance of the distributed power generating system on batteries.  
      Accordingly, it would be desirable to provide a distributed power generating system to serve as an alternative to or enhancement of centralized power generation that overcomes these and other drawbacks of conventional distributed power generation. More particularly, it would be desirable to provide a control system for a distributed power generating system that brings the power generating system to an operational state very rapidly so as to reduce the reliance on stored power.  
     SUMMARY OF THE INVENTION  
      The present invention is directed to a distributed power generating system that enables very rapid and reliable start-up of the engine used to generate back-up power, thereby substantially reducing the need for stored power. The distributed power generating system does not include many of the mechanical components of conventional power generating systems, such as the mechanical switchgear, starter motor and associated linkage, which represent significant failure points of the conventional systems. As a result, the present invention provides a highly reliable and cost effective distributed power generating system.  
      More particularly, the distributed power generating system comprises a power bus electrically coupled to commercial power and to a load, an engine comprising a rotatable shaft, a starter/generator operatively coupled to the shaft of the engine and electrically coupled to the power bus, and a temporary storage device electrically coupled to the power bus. The starter/generator is adapted to start the engine from a standstill condition and rapidly brings the engine to an operational speed sustainable by the engine alone. To accomplish this, the starter/generator has a short time torque capability higher than the rated torque of the engine and starter/generator. When the engine reaches the operational speed, the starter/generator delivers electrical power to the power bus. Upon a fault of the commercial power, the temporary storage device supplies electrical power to the power bus for delivery to the load and for powering the starter/generator until the engine reaches the operational speed, whereupon the starter/generator takes over supply of electrical power to the power bus for delivery to the load.  
      In an embodiment of the invention, the distributed power generating system further comprises a control system adapted to detect a failure of the commercial power and cause the starter/generator to start the engine from a standstill condition. The control system provides the starter/generator with an initial voltage vector selected to rapidly bring the engine to an operational speed sustainable by the engine alone. The temporary storage device supplies electrical power to the power bus for delivery to the load and for powering the starter/generator until the engine reaches the operational speed, whereupon the control system causes the starter/generator to take over supply of electrical power to the power bus for delivery to the load. The starter/generator further comprises a rotor and a stator, with the stator including a plurality of phase windings. The control system starts the engine upon detection of a voltage on the power bus below a predetermined lower limit. After the engine has started, the control system monitors speed of the engine to determine whether the operational speed is reached. The control system terminates operation of the engine upon detection of a voltage on the power bus above a predetermined upper limit.  
      More particularly, the control system identifies an initial position of the rotor relative to the stator and selects the voltage vector based on the initial position to provide maximum torque to the rotor. The control system first measures the self-inductance of said phase winding of the stator. Then, the control system estimates an angle of self-inductance of the stator based on the self-inductance of each phase winding in accordance with the following equation:  
         2   ⁢   θ     =     -       tan     -   1       (             3     2     ⁢   Δ   ⁢           ⁢     t   b       -         3     2     ⁢   Δ   ⁢           ⁢     t   c             Δ   ⁢           ⁢     t   a       -       1   2     ⁢   Δ   ⁢           ⁢     t   b       -       1   2     ⁢   Δ   ⁢           ⁢     t   c           )           
 
 wherein, θ is the estimated angle of self-inductance of the stator, Δt a  is the time for current in phase A of the stator to fall from a positive selected level to a negative selected level, Δt b  is the time for current in phase B of the stator to fall from the positive selected level to the negative selected level, and Δt c  is the time for current in phase C of the stator to fall from the positive selected level to the negative selected level. Thereafter, the control system tests the estimated angle of self-inductance of the stator to determine if it is accurate or off by 180°. 
 
      A more complete understanding of the control system for a distributed power generating system will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings, which will first be described briefly.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of a conventional distributed power generating system;  
       FIG. 2  is a block diagram of a distributed power generating system in accordance with an embodiment of the invention;  
       FIG. 3   a  is a block diagram showing the flow of power in the distributed power generating system prior to start up;  
       FIG. 3   b  is a block diagram showing the flow of power in the distributed power generating system during a first interval following start up;  
       FIG. 3   c  is a block diagram showing the flow of power in the distributed power generating system during a second interval following start up;  
       FIG. 4  is a block diagram of an exemplary control system for the distributed power generating system;  
       FIG. 5  is a flow diagram depicting operation of the distributed power generating system;  
       FIG. 6  is an electrical schematic diagram showing a rotor of a generator of the distributed power generating system; and  
       FIG. 7  is a flow diagram depicting an algorithm for identifying initial position of the rotor of the starter/generator. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      The present invention satisfies the need for a distributed power generating system to serve as an alternative to or enhancement of centralized power generation. Specifically, the present invention provides a distributed power generating system that achieves an operational state very rapidly so as to reduce the reliance on stored power. In the detailed description that follows, like element numerals are used to describe like elements illustrated in one or more of the figures.  
       FIG. 1  illustrates a block diagram of a conventional distributed power generating system  10 . The distributed power generating system  10  includes switchgear  22  that enables the coupling of AC power to a load  24  from a variety of sources. Under normal conditions, AC power is delivered to the load  24  through the switchgear  22  from the AC power mains connected to the commercial power grid. In the event of a fault of the AC mains, the switchgear  22  cuts off the AC mains and delivers AC power to the load from either a generator  14  or a battery bank  28 . The switchgear  22  can also supply the AC output of the generator  14  back to the power grid. The switchgear  22  may comprise a mechanical switch that is manually actuated by an operator or may be adapted to automatically actuate the switch upon detection of a fault.  
      The power generating system  10  further includes an engine  12  that drives the generator  14 . The engine  12  may comprise a reciprocating engine using a combustible fuel such as propane, diesel or gasoline. The generator  14  converts the rotational energy of a rotor shaft driven by the engine  12  into AC power. The generator  14  is electrically connected to a rectifier  16  that converts the AC power into DC. The rectifier  16  is further electrically coupled to an inverter  18  that converts the DC power back into an AC output, such as a high voltage, three-phase AC output (e.g., 400/480 volts AC), that is delivered to the load  24  through the switchgear  22 . Alternatively, the generator  14  may deliver AC power directly to the switchgear  22  without the intervening rectifier  16  and inverter  18 , but it is advantageous to include the rectifier  16  and inverter  18  in order to regulate the frequency, phase and/or amplitude of the AC power delivered to the load  24 .  
      A starter motor  32  connected to the engine  12  by an associated mechanical linkage  34  is used to start the engine  12  from a cold condition. The mechanical linkage  34  enables the starter motor  32  to be disengaged from the engine  12  once the engine has started. A battery  36  provides DC power to the starter motor  32 . The battery bank  28  comprises a plurality of batteries (e.g., lead-acid batteries) that are coupled together in parallel to provide a source of DC power. The DC power is converted to AC power by inverter  26 , which is in turn delivered to the switchgear  22  for delivery to the load  24 . Rectified AC passing through the switchgear  22  from either the generator  14  or the AC mains may be used to charge the battery bank  28 .  
      Upon the detection of a fault with the AC mains, the distributed power generating system  10  goes into the back up mode. The switchgear  22  first connects the battery bank  28  to the load  24  as discussed above to continue to supply AC power to the load. Meanwhile, the engine  12  is started by operation of the starter motor  32 . Particularly, the starter motor  32  turns the shaft of the engine  12  at a minimal rate sufficient to begin a reciprocating cycle of the engine  12  (e.g., 500 rpm). When fuel within the cylinders of the engine  12  begins to ignite and the shaft of the engine is able to turn on its own, the starter motor  32  disengages from the engine  12 . Eventually, the engine  12  reaches an operational speed (e.g., 3,000 rpm) and the generator  14  begins producing reliable AC power. The switchgear  22  then disconnects the battery bank  28  from the load  24  and connects the generator  14  to the load  24 .  
      As discussed above, there are a number of significant drawbacks with the conventional distributed power generating system  10 . First, there are a high number of components, including various mechanical components that are subject to failure. The mechanical switchgear  22  represents a particularly critical component, the failure of which can totally disable the power generating system  10  and further cause the failure of other system components. The mechanical linkage  34  also represents a critical failure point, since the engine  12  cannot be started if there is a failure of the linkage. Second, the engine  12  has a relatively long start-up time due to the use of a small capacity starter motor  32 . Since the starter motor  32  is only used to turn over the engine  12  at a minimal rate sufficient to initiate internal combustion, it is known to use a low torque starter motor. If the engine  12  has been sitting idle for a while, it may take several seconds for the engine  12  to start. The battery bank  26  must therefore have sufficient capacity (and hence size) to supply the load  24  during the relatively long start-up time of the engine  12 . Batteries have relatively limited life expectancies (e.g., approximately five years) and require routine maintenance to keep them in serviceable condition. Moreover, the battery bank  26  is used only for supplying the load  24  and not for powering the starter motor  32 . The separate battery  36  used to power the starter motor  32  is susceptible to discharge, representing yet another critical failure point of the system  10 .  
      The present invention overcomes these and other drawbacks of conventional distributed power generating systems. Particularly, the present invention enables very rapid and reliable start-up of the engine used to generate back-up power, thereby eliminating altogether the need for a battery bank. Moreover, the present invention does not include many of the mechanical components of conventional power generating systems, such as the mechanical switchgear, starter motor and associated linkage, which represent significant failure points of the conventional systems. As a result, the present invention provides a highly reliable and cost effective distributed power generating system.  
      Referring now to  FIG. 2 , a power generating system  100  is illustrated in accordance with an embodiment of the invention. The power generating system  100  includes an engine  112  and a starter/generator  114 . The engine  112  may be provided by a reciprocating internal combustion engine using a fuel such as propane, diesel or gasoline, although other types of engines such as turbines could also be advantageously utilized. The engine  112  drives a rotatable shaft  113  that is operatively coupled to the starter/generator  114 . Unlike the conventional systems, the starter/generator  114  provides the dual functions of starting the engine  112  from a standstill condition and producing electrical power after the engine  112  reaches an optimum operational speed, thereby eliminating the need for a separate starter motor, linkage or battery.  
      Further, the present power generating system  100  avoids the use of mechanical switchgear by including a common DC power bus  120 . DC power is supplied to the DC power bus  120  by the AC mains, the starter/generator  114 , and a temporary storage  130 . Rectifier  122  is electrically connected to the AC mains and delivers rectified DC power onto the common DC power bus  120 . The starter/generator  114  is electrically connected to rectifier  118  that converts AC power produced by the starter/generator  114  into DC power that is provided to the common DC power bus  120 . The temporary storage  130  provides short term or transient power. In an embodiment of the invention, the temporary storage  130  comprises one or more electrolytic capacitors that are charged by the DC power on the common DC power bus  120  and deliver DC power to the bus during transient load conditions. The temporary storage  130  also provides power to the starter/generator  114  through the DC power bus  120  and rectifier  118  to power the starter/generator  114  during start-up of the engine  112 . Alternatively, the temporary storage  130  may be provided by other known sources, such as flywheels, batteries, fuel cells, and the like.  
      The DC power of the common power bus  120  is delivered to a load through the DC-to-DC converter  124  and the inverter  126 . The DC-to-DC converter  124  converts the DC power from the common power bus  120  into a different voltage DC output (e.g., 48 volts DC) used to supply a DC load  132 . The inverter  126  converts the DC power from the common power bus  120  into an AC output, such as a reliable high voltage, three-phase AC output (e.g., 400/480 volts AC), used to supply an AC load  134 . It should be understood that the AC output of the inverter  126  and the DC output of the converter  124  represent premium electric power that is substantially free of undesirable frequency variations, voltage transients, surges, dips or other disruptions.  
       FIG. 3   a  illustrates normal operation of the distributed power generating system  100  with the AC mains supplying the common DC power bus  120  through rectifier  122 . The temporary storage  130  is charged by the rectified DC power on the power bus  120 . The DC power of the common power bus  120  is delivered to a load through the DC-to-DC converter  124  and inverter  126  as discussed above. The engine  112  and starter/generator  114  are not operating at this time.  
       FIG. 3   b  illustrates a condition of the distributed power generating system  100  in a first interval following failure of the AC mains. The temporary storage  130  provides DC power to the starter/generator  114 , which commences rotating the rotor shaft of the engine  112 . The temporary storage  130  also supplies power to the common DC power bus  120  for delivery to a load through the DC-to-DC converter  124  and inverter  126  as discussed above.  FIG. 3   c  illustrates a condition of the distributed generating system  100  in a second interval following failure of the AC mains. The engine  112  has started and reached an operational speed. The direction of current in the starter/generator  114  reverses, and the starter/generator now supplies power to the common DC power bus  120  for delivery to a load through the DC-to-DC converter  124  and inverter  126  and to recharge the temporary storage  130 . This condition will continue until such time as the AC mains have recovered from the fault.  
      It should be appreciated that the distributed power generating system must strike a balance between the size/capacity of the temporary storage  130 , the power drawn by the starter/generator  114 , and the start-up time of the engine  112 . It is desirable to limit the size of the temporary storage  130  to the minimum necessary to supply the load and the starter/generator  114  for the time needed to bring the engine  112  up to operational speed. If the engine  112  were brought up to speed too slowly, the temporary storage  130  would have to supply the load for a longer period of time and would hence require greater size and capacity. At the same time, if the power rating of the starter/generator  114  is not properly matched to the engine  112 , the starter/generator would draw excessive power from the temporary storage  130  without appreciably decreasing the time for the engine  112  to be brought to operational speed.  
      In the present invention, an optimal balance between these parameters is met with the starter/generator  114  selected to have a short time torque capability higher than the rated torque of the engine  112  and starter/generator  114 , so that the starter/generator  114  can bring the engine  112  quickly to full operation with respect to ignition, speed and torque. The fraction of the short time torque capability of the starter/generator  114  compared to the moment of inertia of the rotating part of the engine  112  can be optimized to achieve an acceleration time from zero to rated speed within less than a second, and more particularly within less than 0.2 second. In an exemplary embodiment of the invention, the starter/generator  114  has a short time torque capability at least two times higher than the rated torque of the engine  112  and starter/generator  114 . In yet another exemplary embodiment of the invention, the starter/generator  114  has a short time torque capability at least four times higher than the rated torque of the engine  112  and starter/generator  114 . Due to a typically lower short time torque capability (roughly 1/10 of the rated torque of the engine  112  and starter/generator  114 ) and higher moment of inertia, conventional systems result in substantially longer start-up times.  
      Referring now to  FIG. 4 , an exemplary control system for the distributed power generating system is shown. The control system includes a power control unit  202  that provides central control and monitoring of various functions of the distributed power generating system. As understood in the art, the power control unit  202  may comprise general purpose or specialized circuitry such as a microprocessor, digital signal processor (DSP), application specific integrated circuit (ASIC), field programmable gate array (FPGA), discrete logic circuits, and the like, along with suitable memory for storing programming instructions and data. The power control unit  202  may be accessed by one or more personal computers  204 ,  206  coupled to the power control unit through conventional network system interface, such as RS232 and Ethernet. It should be further appreciated that a long distance network connection with the power control unit  202 , such as via the Internet, could also be established. Using the personal computers  204 ,  206 , a user can monitor the operation of the distributed power generating system, execute tests and measurements, be alerted to fault conditions, check fuel levels and pressures, set operating parameters, and the like.  
      In an embodiment of the invention, the power control unit  202  may provide an output signal constructed as a set of hypertext markup language (HTML) pages with an associated set of executable components, such as Java applets. These applets may be used, for example, to perform functions such as generating grids, charts, and tables, which appear within an HTML page when displayed by a web browser. Accordingly, the user would be able to monitor and control operation of the distributed power generation system using the web browser executing on a personal computer connected to the power control unit  202  through an associated network.  
      The power control unit  202  communicates with a plurality of subsystem controllers through a suitable communication bus. The communication bus may include a Controller Area Network (CAN) bus, which is a simple two-wire differential serial bus system suitable for operating in noisy electrical environments with a high level of data integrity. The CAN bus has an open architecture and a user-definable transmission medium that make the bus extremely flexible. Capable of high-speed (e.g., 1 Mbits/s) data transmission over short distances (e.g., 40 m) and low-speed (e.g., 5 kbits/s) transmissions at lengths of up to 10,000 m, the CAN bus is highly fault tolerant, with powerful error detection and handling capability. Alternatively, the communication bus may include an RS485 bus, another known standard adapted to support thirty-two drivers and thirty-two receivers bi-directionally over a single or dual twisted pair cable. An RS485 network can be connected in a two or four wire mode. Maximum cable length can be as much as 4000 feet because of the differential voltage transmission system used. It should be appreciated that the communication bus may further comprise a hybrid of these interface types, with a portion of the subsystem controllers communicating over a CAN bus and another portion communicating over an RS485 bus. Other communication bus configurations could also be advantageously utilized in the present invention.  
      The exemplary subsystem controllers include a DC/AC control module  212 , a starter/generator control module  214 , a fuel control module  216 , a DC/DC control module  218 , an AC/DC control module  222 , and a storage control module  224 . The DC/AC control module  212  is associated with the inverter  126  used to convert the DC power from the common power bus  120  into an AC output. The DC/AC control module  212  manages the operation of the inverter  126  and communicates status data to the power control unit  202 , such as AC phase voltage and current, DC bus voltage measurement, operating temperature, cooling fan speed, frequency, operation time, status and errors. The power control unit  202  also communicates instructions to the DC/AC control module  212 , such as to change operating parameters of the inverter  126 .  
      The motor/generator control module  214  is associated with the starter/generator  114  used to start the engine  112  and generate power after the engine reaches operational speed. The motor/generator control module  214  manages the operation of the starter/generator  114  and communicates status data to the power control unit  202 , such as DC bus voltage measurement, starter/generator speed, cooling fan speed, temperature, frequency, operation time, status and errors. The power control unit  202  also communicates instructions to the motor/generator control module  214 , such as to change operating parameters of the motor/generator  114 .  
      The fuel control module  216  is associated with the engine  112  and manages the operation of the engine  112  and the delivery of fuel to the engine. The fuel control module  216  receives as inputs various measurements from the engine, including fuel tank weight, fuel line pressure, oil level, oil pressure, oil temperature, etc., and communicates this measurement data to the power control unit  202 . The power control unit  202  also communicates instructions to the fuel control module  214 , such as to change throttle level, switch fuel tanks, change check valve conditions, turn on/off cooling fan, and the like.  
      The DC/DC control module  218  is associated with the converter  124  used to convert the DC power from the common power bus  120  into another DC level output. The DC/DC control module  218  manages the operation of the converter  124  and communicates status data to the power control unit  202 , such as the DC voltage and current, operating temperature, cooling fan speed, switching frequency, operation time, status and errors. The power control unit  202  also communicates instructions to the DC/DC control module  218 , such as to change operating parameters of the converter  124 .  
      The AC/DC control module  222  is associated with the rectifier  118  used to convert the AC power from the starter/generator  114  into DC while in power generation mode, and to convert the DC voltage from the intermediate bus to AC while in engine startup mode. The AC/DC control module  222  manages the operation of the rectifier  118  and communicates status data to the power control unit  202 , such as the DC voltage and current, operating temperature, switching frequency, operation time, status and errors. The power control unit  202  also communicates instructions to the AC/DC control module  222 , such as to change operating parameters of the rectifier  118 .  
      The storage control module  224  is associated with the temporary storage  130  used to supply DC power to the intermediate bus after a failure of the AC mains and before power is supplied from the starter/generator  114 . The storage control module  224  manages the operation of the temporary storage  130  and communicates status data to the power control unit  202 , such as the voltage of each capacitor within the temporary storage  130  and temperature.  
       FIG. 5  illustrates a flow diagram depicting operation of the distributed power generating system under the control of the power control unit  202 . The operation occurs in a continuous cycle that may be interrupted by alarms received from the various control modules indicating fault conditions of the distributed power generating system. As will be further described below, the power control unit  202  uses a measurement of the voltage on the intermediate bus as a trigger to determine when distributed power generation is needed.  
      In particular, at step  302 , the DC voltage on the intermediate bus is compared to a desired level (e.g., 300 volts). When the AC power mains are operating properly, the DC voltage on the intermediate bus will remain at this desired level and the distributed power generation system can remain in a standby mode. But, when there is a fault of the AC power mains, the DC voltage on the intermediate bus will drop, thereby signaling the distributed power generation system to activate. Thus, if the intermediate bus voltage is equal to or greater than the desired level, the operation flow remains on step  302 . Alternatively, if the intermediate bus voltage drops, the operational flow passes to step  304 .  
      In step  304 , the power control unit  202  identifies the initial position of the rotor of the starter/generator  114 . As discussed above, the starter/generator  114  is used to start the engine  112  rapidly from a standstill condition. In order to achieve rapid start of the starter/generator  114 , and hence the engine  112 , it is desirable to know the precise position of the rotor of the starter/generator  114  relative to the corresponding stator. This way, a voltage vector can be applied to the rotor having a phase angle that will produce maximum torque on the rotor, and thereby enable the starter/generator  114  to bring the engine  112  to an operational speed as quickly as possible. An exemplary algorithm for identifying the initial position of the rotor will be described below with respect to  FIG. 7 .  
      Next, in step  306 , the power control unit  202  starts the engine  112 . To accomplish this, the power control unit  202  may first command the opening of check valves in the fuel delivery system to enable the delivery of fuel to the engine. An exemplary fuel delivery system for a distributed power generation system is disclosed in co-pending patent application Ser. No. ______, which is incorporated herein by reference. The power control unit  202  also provides a voltage vector to the starter/generator  114  having a phase angle corresponding to the identified initial position of the rotor. At step  308 , the power control unit  202  determines whether the operational speed of the engine  112  has been reached, which is detected by signals provided by the starter/generator control module  214 . As long as the operational speed is not yet reached, the power control unit  202  will continue to execute step  308 . But, when the engine  112  reaches the desired operational speed, the operational flow passes to step  310 .  
      In step  310 , the power control unit  202  changes the operation of the starter/generator  114  from startup mode to power generation mode. The engine  112  is able to continue operating on its own without being driven by the starter/generator  114 . The starter/generator  114  delivers AC power to the rectifier  118 , which in turn provides DC power to the intermediate bus. At step  312 , the power control unit  202  monitors the operation of the engine  112  to ensure that the operational speed is maintained. If the engine speed drops below a predetermined limit, possibly indicating a problem with the engine  112 , the operational flow returns to step  306  and the startup sequence is repeated. Conversely, if the engine speed remains at or above the predetermined limit, the operational flow continues to step  314  in which the power control unit  202  checks the voltage of the intermediate bus. If the voltage of the intermediate bus is at or below the desired level, then the AC mains are still in a fault condition and the distributed power generation system must continue to supply back up power. The operational flow cycles through steps  312  and  314  again. Conversely, if the voltage of the intermediate bus is above the desired level, then the fault condition of the AC mains has cleared and it is no longer necessary for the distributed power generation system to supply back up power. The operational flow then passes to step  316  in which the engine  112  is shut down. This step may also include the closing of check valves in the fuel delivery system to cut off the delivery of fuel to the engine  112 . The operational flow then returns to step  302 , and the entire process repeats.  
      Referring now to  FIGS. 6 and 7 , the identification of the initial rotor position will now be described. The starter/generator  114  comprises a magnetic rotor  404  having a plurality of permanent magnets (depicted by magnetic polepiece  406 ) and a stator  402  having three-phase windings  402   a ,  402   b , and  402   c  arranged radially separated at equal intervals by 120°. It should be understood that the rotor  404  would rotate around a common axis shared by the stator  402 . In the startup mode, the rotor is caused to rotate by applying a three-phase AC voltage from the rectifier  118  to the stator windings to produce a rotating magnetic field. Conversely, in the generator mode, the rotor is caused to rotate by operation of the engine  112 , thereby inducing a three-phase AC voltage on the stator windings. The AC voltage is full-wave rectified to a direct current by the rectifier  118  to supply a DC voltage to the intermediate bus.  
      More particularly, the rectifier includes a driving circuit  400  shown in  FIG. 6  as comprising a plurality of semiconductor rectifying devices connected in a bridge form. The driving circuit  400  includes three serially-coupled pairs of transistors connected in parallel between respective input terminals. More particularly, stator winding  402   a  is connected to the junction between the emitter terminal of transistor  412  and the collector terminal of transistor  416 , stator winding  402   b  is connected to the junction between the emitter terminal of transistor  422  and the collector terminal of transistor  426 , and stator winding  402   c  is connected to the junction between the emitter terminal of transistor  432  and the collector terminal of transistor  436 . Diodes  414 ,  425 ,  434 ,  418 ,  428 ,  438  are coupled between the emitter and collector of respective transistors  412 ,  422 ,  432 ,  416 ,  426 ,  436 . A capacitor  440  provides smoothing of a DC driving voltage applied (V D ) from the intermediate bus to the input terminals coupled across the transistor pairs. Driving signals applied to the base terminals of the transistors selectively activate the transistors to provide a three-phase AC voltage to the stator windings to thereby produce the rotating magnetic field.  
      As discussed above, if the initial angular position of the rotor  404  relative to the stator  402  is known, then initial driving signals can be applied to the driving circuit  400  that matches the angular position and thereby applies maximum torque on the rotor. In a permanent magnet synchronous in which the magnets are mounted inside the rotor, the variation in the self-inductance is sinusoidal and the frequency of the variation is twice the motor frequency. Since the self-inductance varies with the rotor angular position, knowledge of the inductance can therefore be used to determine the rotor angular position. And, since the variation in inductance from motor to motor can be significant, it is preferred to measure the inductance in all three motor phases and derive the average inductance from the measurement. Ignoring the effect of the stator resistance (r s ) (which is small), and assuming that the time it takes to perform the inductance measurement is much shorter than the mechanical time constant given by the moment of inertia of the rotor, the voltage (V s ) across the stator winding as a function of time (t) and current (I) is defined by the following expression:  
         V   s     =     L   ⁢       ⅆ   I       ⅆ   t             
 
 The magnitude of the voltage vector that is applied to the stator windings is equal to the driving voltage (V D ). Accordingly, the self-inductance can be determined by the following expression:  
       L   =           V   D       Δ   ⁢           ⁢   I       ·   Δ     ⁢           ⁢   t         
 
 wherein ΔI is the change in current over the time Δt. The driving signals applied to the driving circuit  400  can define a phase angle of the voltage vector as 0°, 60°, 120°, 180°, 240°, 300°, or 360°, depending upon which transistor of the driving circuit is activated. 
 
       FIG. 7  illustrates a flow diagram depicting an algorithm  350  for identifying the initial angular position of the rotor of the starter/generator. Starting at step  352 , the self-inductance of phase A (winding  402   a ) is measured. This step is performed by first activating transistors  412 ,  426 , and  436 . When the current in phase A reaches a positive selected level, transistors  412 ,  426 , and  436  are deactivated and transistors  416 ,  422 ,  432  are activated. In a preferred embodiment of the invention, the selected current level (positive or negative) corresponds to three times the nominal current through the winding (or per units (pu)). The time (Δt a ) is measured for the current in phase A to fall from the positive selected level (e.g., 3 pu) to a negative selected level (e.g., −3 pu). Since the self-inductance variation is very small, the present invention uses a higher than nominal current to measure the self-inductance in order to achieve a higher signal-to-noise ratio. It should be appreciated that the selected current level (positive or negative) is limited by the maximum allowable current limit of the transistors of the driving circuit  400 .  
      Next, at step  354 , the self-inductance of phase B (winding  402   b ) is measured. This step is performed by first activating transistors  416 ,  422 , and  436 . When the current in phase B reaches a positive selected level, transistors  416 ,  422 , and  436  are deactivated and transistors  412 ,  426 ,  432  are activated. The time (Δt b ) is measured for the current in phase B to fall from the positive selected level (e.g., 3 pu) to a negative selected level (e.g., −3 pu). Then, at step  356 , the self-inductance of phase C (winding  402   c ) is measured. This step is performed by first activating transistors  416 ,  426 , and  432 . When the current in phase C reaches a positive selected level, transistors  416 ,  426 , and  432  are deactivated and transistors  412 ,  422 ,  436  are activated. The time (Δt c ) is measured for the current in phase C to fall from the positive selected level (e.g., 3 pu) to a negative selected level (e.g., −3 pu).  
      At step  358 , an initial estimate of the phase angle of the self-inductance (2θ) is calculated, using the following expression:  
         2   ⁢   θ     =     -       tan     -   1       (             3     2     ⁢   Δ   ⁢           ⁢     t   b       -         3     2     ⁢   Δ   ⁢           ⁢     t   c             Δ   ⁢           ⁢     t   a       -       1   2     ⁢   Δ   ⁢           ⁢     t   b       -       1   2     ⁢   Δ   ⁢           ⁢     t   c           )           
 
 Since the frequency of the variation in the self-inductance is two times the motor frequency, the initial estimate of the rotor angle is θ. This initial estimate may be correct or it may be incorrect (i.e., out of phase) by 180°. 
 
      Accordingly, at step  360 , the initial estimate of the self-inductance is tested by calculating the phase angle of the next voltage vector in order to determine whether the initial estimate is correct. In this step, the phase angle of the next voltage vector is used to find the position of the rotor&#39;s d-axis. In an induction motor, the direct, or d-axis, current component flows through the parallel inductor, and the quadrature, or q-axis, current component flows through the parallel resistor (see  FIG. 6 ). The d-axis component produces rotor flux; the q-axis component produces torque. A positive current vector in the same direction as the d-axis will increase the flux density in the stator, resulting in higher saturation and a lower inductance as compared to a negative current vector.  
      More particularly, this test step  360  is similar to the measurements of self-inductance performed in the preceding steps. A voltage vector is applied to the stator having the estimated phase angle calculated in step  358 , i.e., by activating/deactivating appropriate ones of the transistors of the driving circuit  300 . Since it is not practical to apply the exact phase angle of the voltage vector (such as 57°), the closest approximation of the phase angle (such as 60°) is applied. First, the time is measured for the current to fall from a positive selected level (e.g., 3.5 pu) to zero. Then, the activated transistors are deactivated and the deactivated transistors are activated, and the time is measured for the current to rise from a negative selected level (e.g., −3.5 pu) to zero. The rate of change of the current reflects whether the estimation of the phase angle is correct or off by 180°. Specifically, if the current falls more quickly from the positive selected level to zero than it rises from the negative selected level to zero, then the estimated phase angle was correct. Conversely, if the current rises from the negative selected level to zero more quickly than it falls from the positive selected level to zero, then the estimated phase angle was not correct and should be shifted by 180°. Following confirmation of the estimated phase angle, the algorithm ends at step  362 .  
      Having thus described a preferred embodiment of the control system for a distributed power generating system, it should be apparent to those skilled in the art that certain advantages of the within system have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is further defined by the following claims.