Patent Publication Number: US-6700214-B2

Title: Mobile power generation system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of PCT Application Serial No. PCT/US00/03815, filed on Feb. 14, 2000 entitled “Mobile Power Generation System” which claims priority from U.S. Pat. No. 6,157,175 filed on Feb. 26, 1999, which applications are each incorporated herein by reference in its entirety. 
     This application also claims priority from U.S. provisional patent application Ser. No. 60/258,917 filed on Dec. 28, 2000 entitled “Inverter and DC Power Upgrade to Mobile Power Generation System.” 
    
    
     BACKGROUND 
     1. Field of the Invention 
     This invention generally relates to the field of electric power generation and electric power conversion, and in particular to a system for maintaining the voltage level of a power supply bus coupled to a load. 
     2. Description of the Related Art 
     The need for mobile commercial-grade alternating current (AC) power comes in many applications and cuts across many industries. There is an emerging need for a mobile power generation system that can satisfy the demands of power hungry machinery, like heavy construction tools, while still being capable of delivering well-regulated power to sensitive equipment, like computers. Moreover, not only should the mobile system have the capacity to generate such power while operating at a stationary site, it should also provide the same type of power while the system is in transit. For example, a mobile power generation system should be versatile enough to support the power requirements of emergency response vehicles rendering life-saving services while in transit, as well as satisfy the power demands of sensitive intelligence/news-gathering computer and telecommunications equipment in isolated regions of the world. 
     There is also a need to provide this same AC power during occasions when the running of the vehicle&#39;s engine is not desirable. Late night work in residential areas, where engine noise is an issue, is an example of one such occasion. Generation of power without the need to run an engine is also desirable to reduce air pollution caused by engine exhaust. 
     One solution for this need is to derive the power from storage batteries, converting the energy from a direct current (DC) source to AC. A device that performs such a function is called an “inverter”. Inverters are in the prior art and are commercially available. The conventional inverters typically depend on either the charging system of the vehicle to restore charge to the batteries once they are drained, and/or by a plug-in battery charger. It is common for a vehicle with an inverter to be plugged into standard utility power overnight to “top off” the batteries then depend on the vehicle&#39;s alternator to replenish charge level during operation. It is also common for a vehicle to be outfitted with a high output alternator to provide additional charging capacity. 
     The conventional inverters suffer from various disadvantages. For example, in some of the conventional inverters, the power demands from the inverter exceed by a large margin the ability of an alternator to provide charge. In such a case, there comes a point when the batteries are completely drained, and because of the alternator&#39;s inability to keep up alone, inverters shut down and work requiring AC power ceases. 
     Another disadvantage of the conventional inverter system is that a vehicle alternator produces its least amount of charging current at low engine RPM&#39;s. When the batteries are drained, the typical response is to start the vehicle&#39;s engine to recharge the batteries with the alternator. At idle RPM the alternator is limited in capacity and will tend to overheat, reducing alternator life and potentially creating a fire danger. 
     A further disadvantage of the conventional inverter system is that the inverter typically continues to pull power off of the batteries until the batteries are very deeply discharge. Deep discharging of batteries may greatly limit the life of the batteries. 
     Yet another disadvantage of the conventional inverter system is that the typical automotive alternator recharges based on voltage level only, not battery temperature, which in some cases limits the amount and rate of battery charging and reducing battery life. 
     Yet another disadvantage of the conventional inverter system is it typically lacks the capability of providing very high power, short duration demands, such as those that occur when starting certain AC electric motors. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a power generation system is provided for supplying electrical power to a load from at least one of a generator and a battery. The power generation system includes a power bus coupled to the generator and a bi-directional conversion unit coupled between the power bus and the battery. The bi-directional conversion unit is capable of transitioning between a first direction wherein electrical power flows from the power bus to the battery and a second direction wherein electrical power flows from the battery to the power bus. 
     In one aspect of one embodiment, the present invention provides a way to continually supply AC power, even when the generator is no longer producing electrical power, by converting DC power from a battery source into AC power. 
     In another aspect of one embodiment, the present invention provides a system that transitions between a battery power operation to a generator power operation without AC power interruption. 
     In another aspect of one embodiment, the present invention provides a smooth transition from a mode where a battery source is supplying power to AC load to another mode where a generator source is supplying power to the AC load. 
     In another aspect of one embodiment, the present invention is capable of simultaneously supplying DC and AC power without interruption. 
     In yet another aspect of one embodiment, the present invention provides means for supplying power from both a generator source and a battery source during times of high power demand. 
     In yet another aspect of one embodiment, the present invention provides an alarm to an operator that the charge levels of the batteries are getting low, providing time for the operator to start the engine and begin recharging the batteries. 
     In yet another aspect of one embodiment, the present invention monitors battery temperature and adjusts the charging rate accordingly, enhancing battery life and maximizing the amount of useful charge stored in the battery. 
     In yet another aspect of one embodiment, when the electrical power generated by the generator exceeds the AC load demand, the excess power is used to recharge a battery source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, explain the object, advantages, and principles of the invention. 
     FIG. 1A is a block diagram of a power generation system according to one embodiment of the invention. 
     FIG. 1B is a block diagram of a power generation system according to another embodiment of the invention. 
     FIGS. 2A and 2B are schematic diagrams of a bi-directional conversion unit according to one embodiment of the invention. 
     FIG. 3 is a timing diagram of the bi-directional conversion unit of FIGS. 2A and 2B during net energy flow from power supply bus to the battery. 
     FIG. 4 is a timing diagram of the bi-directional conversion unit of FIGS. 2A and 2B during net energy flow out of the battery to the power supply bus. 
     FIG. 5 is a timing diagram of the bi-directional conversion unit of FIGS. 2A and 2B during a transition of energy flow direction, i.e. very small net energy flow. 
     FIG. 6 is an internal block diagram of a DSP based control unit according to one embodiment of the invention. 
     FIG. 7 is an internal block diagram of a PWM generation unit according to one embodiment of the invention. 
     FIG. 8 is a block diagram of a feed forward based control and a current-rise limiter incorporated in the DSP based control unit. 
     FIG. 9 is a block diagram of a feedback tracking correction of FIG.  8 . 
     FIG. 10 is a timing diagram of control signals generated by the PWM generation unit according to one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     In the following description, specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order to avoid obscuring the present invention. 
     FIG. 1A depicts a power generation system according to one embodiment of the present invention. The power generation system includes an electronic control unit (ECU)  104  coupled to a generator  108  to generate power for a power supply bus  116  connected to an AC inverter  118 . In one embodiment, the generator is an induction-type generator, requiring an external source of mechanical power to convert into electrical power. For example, the generator may be coupled to an engine of a motor vehicle so that the required mechanical power is supplied by the rotational energy of the engine. Also included in the power generation system is a bi-directional conversion unit (BCU)  106  coupled between the power supply bus  116  and a battery pack  110 . The battery pack  110  serves to supply power to the system components when demanded and to store energy generated by the generator when the AC load and DC load demand combined does not exceed the generated power. 
     A master control unit (MCU)  102  is coupled between the ECU  104  and the BCU  106  to facilitate communication between the ECU and the BCU and to control the activities thereof. A user interface  112  is coupled to the MCU  102 . The user interface  112  may include a control panel and a display device. The control panel may include, for example, power meters, on-off/control switches, a heavy duty mode switch (i.e., for accommodating machinery that requires high start-up currents), an emergency/fault condition indicator, and other visual indicators, all of which are well known in the art. The display device may be supplied with various system status data, such as battery voltage, battery current charge or draw rate, total power draw, estimated time to battery discharge, etc. to be displayed on the display device. 
     In the illustrated embodiment, the ECU  104  includes a processor  120 , a generator drive  114 , a DC power supply bus  116  and an AC inverter  118 . The generator drive  114  is responsive to the amplitude- and frequency-variant signal produced by the generator  108  and is controlled by the ECU processor  120  to maintain the DC power supply bus  116  at a desired DC voltage level. The AC inverter  118  produces AC output power by converting the DC voltage supplied by the power bus  116  into an AC power signal. It should be noted that the power bus  116  supplying power to the AC inverter  118  derives its electrical power from the generator  108 , the battery pack  110  or both. However, the function of AC inverter  118  is not affected in any way by the source of the bus voltage. Because of this, the AC output will be constant and unaffected by transitions between a generator power source and a battery power source, providing the seamless AC operation required of the present invention between vehicle engine on and vehicle engine off operation. 
     In one embodiment, the power supply bus  116  includes a positive DC bus  126 , a negative DC bus  122  and a neutral rail  124 . The positive DC bus  126  may comprise a plate of a capacitor or a respective plate of each one of several capacitors which are connected in parallel, with the other plate or plates coupled to the neutral rail  124 . Similarly, the negative DC bus  122  may comprise a plate of a capacitor or a respective plate of each one of several capacitors which are connected in parallel, with the other plate or plates coupled to the neutral rail  124 . In one embodiment, the power generation system is configured to maintain 400 Volts across the positive DC bus  126  and the negative DC bus  122 . 
     When the engine is on and the generator  108  is generating power, the generator drive  114  attempts to regulate the electrical power produced by the generator such that a desired voltage level across the power supply bus  116  is achieved. In one embodiment, the generator drive  114 , responsive to control signals received from the ECU processor  120 , attempts to regulate the electrical power produced by the generator  108  such that the DC power bus  116  is maintained at a nominal voltage of positive 200 VDC and negative 200 VDC with respect to the neutral rail  124 . The neutral rail  124  allows the generation of a combination of 120 VAC and 240 VAC signals without requiring an output transformer. In one embodiment, the AC inverter  120  converts the positive 200 VDC and negative 200 VDC into a plurality of AC output power signals, such as for example, two 120 VAC, 60 Hz signals, 180° out of phase. The voltage between the two signals is rated at 240 VAC while the voltage between each signal and a neutral rail  131  is 120 VAC. The AC inverter  120  supplies the plurality of output AC power signals to the AC load. 
     The power generation system may be configured to automatically activate the AC inverter  118  to produce AC power upon engine startup. Alternatively, the system may be configured to enable the AC inverter  118  to be manually activated by an operator. In such a case, if the AC inverter  118  is not started automatically, activation occurs manually through an on/off switch located on the control panel of the user interface  112 . 
     At initial start-up of the system, there exists no voltage on the DC power supply bus  116 . In one embodiment, the generator is an induction-type generator and lacks self-excitation and the lack of DC voltage on the power supply bus  116  precludes the generator drive  114  from developing or applying any current for the generator stator field coils. Thus, shortly after initial start-up, the system switches electrical power from the battery pack  110  to the generator drive  114  via the power supply bus  116  to enable the generator drive to apply sufficient current to drive the generator. Alternatively, the generator drive  114  may receive power from other battery storage source. Once the generator drive  114  has been supplied with adequate power, the generator drive develops a current for the generator stator field coil windings. Using the electrical power generated by the generator  108 , the ECU  104  is able to charge the power supply bus  116 . When the voltage of the power supply bus  116  is sufficient for the generator drive  114  to develop current for the generator stator field coils, the system may disable the flow of electrical power from a battery storage source to the generator drive  114 . 
     According to one aspect of the invention, the power generation system monitors, among other things, the voltage levels of the power supply bus  116  and maintains the power bus at a desired voltage range by regulating the flow of power between the power supply bus and the battery pack via the BCU  106 . The BCU  106  includes a pulse width modulation generation unit (PGU)  137 , a digital signal processor based control unit (DCU)  136  and a bi-directional power supply circuit  138 . The DCU  136  is configured to continuously monitor the various signals including voltage level of the power supply bus  116  and compute required commands values for the PGU  137  to maintain adequate energy flow between two sides of the bi-directional power supply circuit  138  for the desired set parameters of the system in terms of voltages and currents. 
     In one embodiment, the DCU  136  determines the appropriate command values to the PGU  137  by comparing the voltage levels on the power bus  116  and the battery pack  110  or DC load  140 , with reference set-points. These set-points may include charging and over-discharge voltages for battery under different conditions like temperature, reference for power bus voltage generated by the induction generator  108 , reference minimum voltage maintained on the power bus  116  by the battery. Other reference set-points the DCU  136  may use include power level versus RPM for induction generator  108 , current rise limits to and from the battery  110  or DC load  140 , maximum charging rate for the battery  110 . 
     In accordance with one aspect of one embodiment, the bi-directional power supply circuit  138  is capable of transitioning, in a continuous manner, between a first direction wherein electrical power flows from the battery to the power bus and a second direction wherein electrical power flows from the power bus to the battery. The direction and the amount of electrical power, flowing through the bi-directional power supply circuit between the battery and the power bus, are determined based on various parameters. The smooth transition from one direction to the other direction enables the bi-directional power supply circuit to effectively distribute the power provided by the AC and/or DC electrical power sources to AC and/or DC loads without interruption. 
     In one embodiment, pulse width and timing of control signals applied to the switching elements of the bi-directional power supply circuit  138  is adjusted in a continuous manner to maintain the voltage level of the power bus within a desired range. The direction and the amount of the energy flow between the battery and the power bus may be determined by comparing the bus voltage level with upper and lower threshold values. In one implementation, the upper threshold value is set slightly below the power bus voltage level maintained by the ECU  104  when the generator  108  is actively generating power. When the power bus voltage level exceeds the upper threshold value, width and timing of control pulses applied to the switching elements of the bi-directional power supply circuit  138  will be appropriately adjusted such that the power from the power supply bus can be converted to an adequate battery charge voltage level to recharge the battery pack  110 . If the generator  108  is de-activated, for example because the vehicle&#39;s engine is turned off, the ECU  104  will no longer receive the generator power necessary to maintain a desired power bus voltage level, causing the power bus voltage level to fall. When the power bus voltage level drops below a lower threshold value, the width and timing of control pulses applied to the switching elements of the bi-directional power supply circuit will be appropriately adjusted such that the power from the battery pack  100  can be converted into an adequate bus voltage level to charge the power bus to a desired bus voltage level. 
     One advantage obtained by the invention occurs during times of high surge current demand. With the generator  108  activated, the AC load  128  may include a device that requires a large initial current, such as in the case with certain AC motors. When this occurs, the power demand may exceed the capacity of the generator  108 , resulting in a drop of the power bus voltage level. In one embodiment, when the power bus voltage level falls below the lower threshold value, the BCU will smoothly transition the flow of electrical power through the bi-directional power supply circuit  138  to prevent the power bus voltage level from decreasing further. Since the lower threshold voltage value is set above the point where AC load voltage is effected, a brownout condition may be avoided. The AC power supplied to the AC load may be derived from both the generator  108  and the battery pack  110 , resulting in an extremely stable source of power that is not only beneficial but necessary in many applications. Once the generator  108  catches up with the load demand, the generator drive  114  will bring the power bus voltage level above the upper threshold value, which will in turn transition the bi-directional power supply circuit  138  to direct electrical power from the power bus to the battery pack. 
     Another advantage obtained by the invention is the management of the vehicle engine idle speed (e.g., engine RPM). Through an idle control system that is customized for each vehicle, the ECU  104  has the ability to modify the engine idle RPM to maintain minimum generator rotational rates. Increasing idle RPM also puts the engine in a power band that is more suitable for generating the required horsepower necessary for generator operation. 
     In one embodiment, if a sudden, high power demand is placed on the system, the generator drive will not result in a sudden torque applied to the vehicle&#39;s engine. In this regard, the additional power required during a load surge on the power bus  116  may be supplied by the battery pack  110  via the bi-directional conversion unit (BCU)  106 . Advantageously, such arrangement of the invention prevents a sudden torque applied to the engine caused by the generator drive demanding additional power from the generator, thus avoid experiencing the reduction in engine RPM as a result of sudden torque applied to the engine. According to one embodiment of the invention, as the overall system reacts to the increased load, the torque applied by the generator  108  to the engine will occur gradually, allowing the power generation system to change the engine idle slowly and preventing RPM overshoot. This is important for applications that require power while the vehicle is being driven, such as would be the case in recreational vehicles and the like. 
     In one embodiment, a feed-forward control logic is implemented in the BCU. The feed-forward control logic provides near zero delay in restoring power from the battery under conditions due to either surge in AC load  128  or quickly decreasing the amount of power from the generator  108 , for example, when the engine is turned off. Since the BCU  106  does not need to switch modes under these conditions, the battery power is instantaneously available to supply power to the load. This feature of the invention greatly simplifies load transient handling and provides seamless transition for energy flow. In fact, it allows the battery pack  110  to act as a direct storage element in stabilizing the power bus  116 . 
     To compensate for increase in load demand, the generator drive  114  is capable of adjusting the output power of the generator  108  by modifying the applied AC voltage level to the generator stator coils while maintaining optimum slip. The “slip,” expressed as a percentage, is defined as the slip frequency divided by electrical frequency applied to the stators. The slip frequency is the difference between the mechanical frequency—a function of rotor speed—and the electrical frequency. By adjusting the electrical frequency, the slip can be adjusted thereby minimizing the magnitude of the currents in the rotor disk and in the stator coils. 
     In one embodiment, the BCU processor  136  is configured to monitor the rotational speed of the generator  108  and an AC load and a DC load applied to the system as well as monitoring the voltage level of the power supply bus  116 . The generator  108  speed may be determined by a generator rotational speed sensor. The generator speed sensor may be any type of sensor that determines RPMs, such as a hall effect sensor mounted directly onto the generator. Based on this information, the BCU processor  136  coordinates with the MCU  102  and the ECU processor  120  to send appropriate control signals to the generator drive  114  to maintain the power supply bus at a desired voltage level through the power adjustments of the generator  108 . Although illustrated as a separate component for purposes of illustration, it will be appreciated that the function of the ECU processor  120 , the BCU processor  136  and the MCU  102  may be implemented in a single processor. 
     One way the system maintains the voltage level of the power bus at a desired range is by continuously monitoring the voltage levels of the power bus and adjusting the electrical power produced by the generator by virtue of the generator drive. In one implementation, when the voltage level of the power bus drops as a result of power drawn from the system, the stored electrical power from the battery pack is initially used to supply power to the power bus, then the generator drive  114  varies the current within the stator field coils in order to adjust the generator electrical power to produce sufficient power to maintain the power bus within a desired voltage range. According to one aspect of one embodiment, the battery power may be used to compensate for drop in the voltage level of the power bus in various situations, including (1) when load demand increases, (2) when the power bus is experiencing a load surge, i.e., when a sudden, high power demand is placed on the system, or (3) when the engine is running at low RPM in the drive mode. 
     In operation, when AC load  128  and DC load  140  are applied to the system, the power is drawn from the power supply bus  116 . If the power drawn from the system exceeds the electrical power supplied by the generator and the battery pack, the BCU  106 , detecting the drop in the power bus voltage level and measuring the load, will determine if the generator  108  is capable of generating the additional electrical power required by the load at the present generator rotational speed. If the current rotational speed of the generator  108  is sufficient to meet the load demand, the generator drive  114 , responsive to control signals sent by the ECU processor  120 , will cause the generator to produce enough power to compensate for the extra power drawn from the system. This can be achieved by the generator drive  114  increasing the AC voltage level applied to the generator stator coils while maintaining optimum slip. When the applied load is disconnected from the power generation system or is turned off, the power bus voltage level will rise above the desired voltage level and the generator drive  114  will react by modifying current within the stator field coils to reduce the generator electrical signal power production and bring the power bus voltage level back down. If the current rotational speed of the generator  108  is inadequate to meet the load demand, the ECU processor  120  may send appropriate control signals via the MCU  102  to increase the speed the engine by opening up the engine idle or electronic means. 
     To avoid exceeding the overall power capability of the system, the power generation system controls the AC and DC power drawn from the system. For example, if an operator chooses AC priority, the DC power output  140  will be automatically reduced as the AC load  128  demand increases such that the total power does not exceed maximum. The converse is true for a DC priority system. 
     In one embodiment, the power generation system is configured to monitor the voltage level of the battery pack  110  and indicate to the operator when the batteries are nearly discharged, allowing the operator time to start the engine so that generator  108  can directly supply the necessary AC power and DC power for recharging battery pack. In one embodiment, if the battery voltage falls below a pre-programmed discharged voltage, the system will shut down to avoid a possibility of damaging the battery caused by excessive drainage and indicate that the system has shut down due to low battery voltage on the control panel of the user interface  112 . 
     FIG. 1B depicts a power generation system according to another embodiment of the present invention. The major components of the power generation system include a generator  202 , a generator drive  208 , a power supply bus  210 , an AC inverter  220 , a bi-directional power supply circuit  226 , a battery pack  228  and a processor  240 . The generator may be an induction-type generator, which derives its required mechanism power from an engine  246  to convert the rotational energy of the engine into electrical power. In one embodiment, the generator  202  is driven by an engine  246  of a motor vehicle. The power generation system has numerous applications and may be used in any type of vehicles having an engine suitable for driving a generator. Alternatively, the power generation system of the invention may be used as a stand-alone power generation system. The generator drive  208  coupled to the generator  202  is configured to control amount of electrical power generated thereby. In this regard, the generator drive  208 , in response to commands received from the processor  240 , selectively adjusts the power produced by the generator  202  by varying the current within the stator field coils of the generator and optimizing the operating slip frequency. 
     In this embodiment, the power generation system employs a number of sensors to monitor various parameters within the system. The generator  202  is provided with an RPM sensor  204  and a thermal sensor  206 . The battery pack  228  is provided with a voltage sensor  236  and a thermal sensor  238 . A voltage sensor  212  is coupled to the power supply bus  210  to monitor the voltage level thereof. A current sensor  230  is coupled between the bi-directional power supply circuit  226  and the battery pack  228  to monitor the battery charge current and battery discharge current. Additionally, load sensors  224 ,  234  are coupled to the AC load  224  and the DC load  232  to monitor the load demands thereof. The output signals from these sensors are input into the processor  240 . Based on the output signals received from these sensors, the processor  240  is configured to dispatch control signals to the generator drive  208 , the bi-directional power supply circuit  226  and the idle control  244  to maintain the power bus voltage level within a desired voltage range. 
     In operation, when the engine  246  is off and the generator  202  is not generating power, the electrical power demanded by the AC and DC loads  224 ,  232  will be supplied by the battery pack  228 . To supply the power to the AC load  224 , the bi-directional power supply circuit  226 , responsive to control signals received from the processor  240 , enables the electrical power from the battery pack  228  to be used to charge the power supply bus  210 . Once the power bus  210  is sufficiently charged, the electrical energy from the power bus is used to drive the AC inverter  220 , which converts the received DC voltage from the power bus into an AC power signal. 
     To ensure that the system can match the power being drawn from the system, the processor  240  monitors the AC load  222 , the power bus voltage level and the battery voltage level. As long as certain conditions are met, the processor  240  will attempt to maintain the power bus voltage level within a desired voltage range. For example, by monitoring the battery voltage level data from the battery voltage sensor  236 , the processor  240  is able to determine whether the battery pack  228  can continue to support the load demanded by the AC and DC loads. If the battery voltage level falls below a defined voltage level, the processor will activate an alarm to warn that the system is about to shut down. In such a case, if the battery voltage level continues to fall below a certain level, the processor  240  will shut off the system by switching off the bi-directional power supply circuit  226  to avoid damaging the batteries caused by excessive drainage. The switching off the bi-directional power supply circuit  226  will cause the power bus voltage level to drop, which in turn will cause the AC inverter  220  to turn off. 
     Additionally, the processor  240  is also configured to monitor the battery current data from the battery current sensor  230  to ensure that battery charge current and battery discharge current are within an allowable range. If the battery pack  228  is used to supply the AC load and the battery discharge current exceeds a certain level, the processor  240  will instruct the bi-directional supply circuit  226  to shut off for safety reasons and to avoid damaging any internal components. When charging the battery pack  228 , if the battery charge current is exceeds a certain level, the battery charge current will be adjusted down by the bi-directional power supply circuit  226 , in response to control signals received from the processor, to avoid damaging the batteries in the battery pack and other components. In one implementation, the upper limit of the battery charge current is set at 200 amps. 
     When the engine  246  is on and the generator  202  is generating power, the electrical power demanded by the AC and DC loads  224 ,  232  will be supplied by the generator, the battery pack  228  or both. When the AC inverter  220  is turned off, all power produced by the generator  202  will be used to recharge the battery pack  228 . When the AC inverter  220  is turned on, the generator power will supply the AC load  224  and any remaining power that the AC load does not require will be used to recharge the battery pack  228 . If needed, the battery power may be used to supplement the power being drawn by the AC load. 
     To maintain the power supply bus within a desired voltage range, the processor  240  continuously monitors the voltage level of the power supply bus  210 . This voltage detection is performed according to the output data from the power bus voltage sensor  212 . Then, the processor  240  determined whether the detected voltage level of the power bus is within upper and lower threshold voltage values. Based on this information, the processor  240  controls the direction and the amount of electrical power flowing through the bi-directional power supply circuit  226  between the battery pack  228  and the power supply bus  210 . 
     When the detected power bus voltage level exceeds an upper threshold value, the bi-directional power supply circuit  226 , responsive to control signals received from the processor, will use the electrical power from the power bus to recharge the storage batteries in the battery pack  228 . When the detected power bus voltage level drops below a lower threshold value, the processor will convert DC battery voltage from the battery pack  228  to supply the power bus  210  with electrical power via the bi-directional power supply circuit. Accordingly, by controlling the direction and the amount of electrical power flowing through the bi-directional power supply circuit  226  in a continuous manner, the voltage level of the power bus can be maintained within a desired range without changing the rotational speed of the engine so long as the power being drawn from the system does not exceed the power produced by the generator and the battery pack. 
     If the power being drawn from the system exceeds the power produced by the system, this will cause the power bus voltage level to continue to drop. In such a case, the processor  240  examines the rotational speed of the generator to determine if at the current rotational speed, the generator is capable of generating the additional power required the AC and DC loads. This generator rotational speed is detected according to the output data from the generator RPM sensor  204 . If the detected rotation speed of the generator  202  is sufficient to meet the load demand, the generator drive  208 , responsive to commands from the processor  240 , will respond by varying the AC voltage level applied to the stator field coils of the generator unit while maintaining optimum slip to produce enough additional power in the generator electrical signal to bring the power bus back up to a desired voltage level. Conversely, when the load  224 ,  232  is disconnected from the system or turned off, the power bus voltage level will rise above its desired level and the generator drive will react by modifying current within the stator field coils to reduce the generator electrical signal power production and bring the voltage back down. 
     In one embodiment, the processor  240  examines the battery voltage level as well as how much power the generator is capable of generating based on the RPM to determine if the system is capable of matching the power demanded by the AC and DC loads. If at the current RPM, the generator  202  is capable of generating more power than required by the AC load  222 , any access will be used for recharging the battery pack  228 . For example, if the generator is producing 5 kwatts and AC load is 4 kwatts, the processor will control the bi-directional power supply circuit such that only 1 kwatt goes to the battery. On the other hand, if the electrical power demanded by the AC load  222  is greater than what the generator is capable of generating at the current RPM, the battery pack  228  will supply the extra power required by the AC load via the bi-directional power supply circuit  226 . If the AC load  222  is greater than power supplying capability of both the generator  202  and the battery  228  combined, the processor  240  may control the vehicle engine throttle via the idle control  244  to match the power demanded by the AC load  222 . However, if the power drawn from the system exceeds the power supplied by the battery pack  228  and the generator producing power substantially at its maximum capacity, the AC inverter will shut down with a fault, for example, by tripping a circuit breaker. 
     As noted above, the processor  240  is configured to control the vehicle engine RPM via the idle control  244  based on the AC and DC load requirements and the charged voltage level of the battery. For example, if the processor  240  determines that the battery is fully charged and the load requirement is relatively low, the current rotational speed of the engine will not be effected by the processor  240 . However, as the load demand increases, the processor  240  may command the idle control  244  to increase the engine RPM in order to satisfy the DC and AC loads. More specifically, if the detected rotation speed of the generator  202  is insufficient to meet load demand, the processor  240  may instruct the idle control  244  to increase the rotational speed of the engine. The idle control  244 , responsive to control signals received from the processor, operates to adjust the generator rotational speed by manipulating the throttle or electronic idle control means of the vehicle engine  246 . In one implementation, the idle control  244  is operated by the processor  240  only when the vehicle is stationary or parked and is disabled when the vehicle is being driven. The idle control  244  may be any conventional actuator commercially available for throttle positioning. In cases where relatively powerful engines are utilized, such as medium and heavy-duty diesel engines, adjustments of the engine speed by the processor  240  may not be necessary in order to satisfy the loads. 
     In one embodiment, the processor  240  is configured to regulate the amount of AC and DC power applied to the loads  222 ,  232  so that the overall power supplying capability of the system is not exceeded. In AC priority operation, the processor will reduce the amount of DC provided for battery charging if the AC demand increases. For example, if the capacity of the generator is 5,000 watts continuous and 4,000 watts is required to operate AC equipment, after initial surge the DC output would be reduced to 1,000 watts in steady state operation. Similarly, if the processor is set up for DC priority operation, AC power would be limited to supply the DC demand up to it&#39;s maximum. 
     In one embodiment, the power generation system includes a thermal sensor  238  located at the battery pack  228  to provide battery temperature information to the processor  240 . Based on the battery temperature information received from the thermal sensor  238 , the processor regulates the battery charge voltage level such that the battery pack  228  is recharged in an optimal charging condition to maximize battery life and minimize battery recharge time. In response to control signals sent by the processor  240 , the bi-directional power supply circuit  226  is capable of controlling the level of charging current applied to the battery pack  228 . Additionally, a thermal sensor  206  is provided at the generator  202  to enable the processor to monitor the temperature of the generator. If the processor determines that the temperature of the generator  202  exceeds a certain value, the generator is shut off. In one implementation, the generator is turned off by eliminating the excitation voltage thereto. 
     In one aspect of the invention, the power generation system is capable of supplying AC and/or DC power even when the engine is turned off. For example, when the power generation system is used to provide electrical power at a stationary location, there may be occasions when the running of the vehicle&#39;s engine is not desired. In such situations, all of the energy received by the power bus is supplied by the electrical energy stored in the battery pack. In this regard, to supply AC load, the processor  240  will send appropriate control signals to the bi-directional power supply circuit  226  to convert electrical power from the battery  228  to charge the power supply bus  210  to maintain the power bus voltage level within a desired voltage range. In this mode, DC power is supplied directly from the battery pack. 
     FIGS. 2A and 2B illustrate in schematic diagram form a bi-directional conversion unit (BCU) according to one embodiment of the invention. The bi-directional conversion unit according to the invention has various possible applications and may be used with the power generation system shown in FIG.  1 A. In general, the bi-directional power supply circuit can be used to control the direction and the amount of the electrical power flow between two sources, such as for example, a generator and a battery pack. In the illustrated embodiment, the bi-directional power flow path is provided to derive electrical power from the battery to charge the power bus and subsequently share or solely supply power to the load connected to the power bus, or, depending on the available power, derive electrical power from the power bus to charge the battery and supply power to a DC load. 
     In the illustrated embodiment, the bi-directional power converter unit comprises a first subcircuit  450  coupled to a second subcircuit  452  via a transformer  460 , a PWM generator unit (PGU)  422  and a DSP based control unit (DCU)  423 . The first subcircuit  450  is coupled between the power bus terminals  410 ,  412  and the primary winding L 1  of the transformer and serves as an H-bridge converter on the power bus side, wherein net current flow direction depends on voltages in the system and timings of the switching control waveforms. In the illustrated embodiment, the first subcircuit  450  comprises four power transistors, such as insulated gate bipolar transistors (IGBT) designated Q 1 , Q 2 , Q 3  and Q 4 , acting as switches. Each of transistors Q 1 -Q 4  is connected in parallel with a diode D 1 , D 2 , D 3  and D 4 , respectively. The transistors Q 1  and Q 3  are connected in series and coupled to a first terminal  461  of a primary transformer winding L 1 . Similarly, the transistors Q 2  and Q 4  are connected in series and coupled to a second terminal  462  of the primary transformer winding L 1 . 
     The second subcircuit  452  is coupled between the battery terminals  406 ,  408  and the secondary transformer windings L 2 , L 3  and serves as a combination of boost and buck converter on the battery bus side terminals  406  and  408 . In the illustrated embodiment, the second subcircuit  452  comprises two transistors, such as MOSFET Q 5  and Q 6 , an inductor  418  and a capacitor C 2 . The transistors Q 5  and Q 6  are coupled to end taps  424 , 428  of the secondary transformer winding while the inductor  418  is coupled to the center tap  426  of the secondary winding. In the illustrated embodiment, MOSFET transistors Q 5 , Q 6 , each having a built-in zener diode D 5 , D 6 , are utilized. The transistors Q 1 -Q 6  are controlled by control signals generated by a logic block, PWM generator unit (PGU)  422 . The PGU logic block generates synchronized control waveforms for bi-directional operation of the power conversion circuit. 
     FIG. 7 illustrates a block diagram of the PGU  422  according to one embodiment of the invention. The PGU  422  comprises a timer logic block  510 , four identical pulse width modulation (PWM) logic blocks  520 ,  521 ,  522  and  523 , and a deadtime and interlock logic block (DILB)  531 . The timer logic block  510  generates periodic synchronizing signals on its output terminal  511  and determines operating frequency of the PWM control signals VC 1 , VC 2 , VC 5  and VC 6  on PGU output terminals  561 ,  562 ,  563  and  564 , to drive transistors Q 1  to Q 6 . 
     The first pair of PWM logic blocks  520  and  521  generate control signals  571  and  572  with timings intended to control transistors Q 1  and Q 2 . These signals can produce conduction periods for Q 1  and Q 2  ranging from set minimum non-overlapping to a maximum, which has a portion of overlap duration. This allows the second subcircuit  452  to operate in combination of buck and boost conversion any time without any mode switching. 
     The second pair of PWM logic blocks  522  and  523  generates control signals  573  and  574  intended to control transistors Q 1 , Q 2 , Q 3  and Q 4 . There are two signals out of these blocks and each is intended to control two of the four transistors to implement a full bridge converter  450 . These signals can produce a conduction period for each set of transistors from a set minimum non-overlapping to a set maximum non-overlapping pulse every PWM cycle. The starting and ending points in time for these conduction periods are controlled according to an algorithm to maintain bi-directional energy flow path at all times. 
     The deadtime and interlock logic block (DILB)  531  ensures that the control waveforms on terminals  561 ,  562 ,  563  and  564  reaching transistors Q 1  to Q 6  always conform to the switching delay specifications of these transistors. 
     The real-time control of the first and second subcircuits of the BCU shown in FIGS. 2A and 2B is achieved by the functional block DCU  423 , which incorporates control and monitoring algorithms to realize stable bi-directional function of the BCU  106  in one of the embodiments. 
     FIG. 6 shows a block diagram of the DSP based control unit (DCU)  423  according to one embodiment of the invention. The DCU comprises a digital signal processor (DSP)  610  capable of executing numeric and logic computations at speed exceeding several thousand times the underlying sample rate of the control algorithms, compatible speed program memory  621  and data memory  622 , to hold execution code and runtime variable data, non-volatile memory  623  to hold system parameters and a peripheral communication processor  641  for providing access to and from components external to the BCU  106 . A dedicated internal high speed control bus  631  is provided for exchanging data with the PGU  422  and other hardware elements in the system, including voltage and current sensors and protection logic. 
     In one embodiment, the DCU  423  commands PGU  422  with appropriate values for PWM synchronizing signal at terminal  511 , and starting and ending points of the waveforms at terminals  561 ,  562 ,  563  and  564  to be sent to transistors Q 1  to Q 6 . DCU  423  is capable of generating these signals every period of synchronizing signal. In one implementation, the frequency for synchronizing signal is set above 75 KHz, as a result of which significant reduction in size and weight of magnetic components  460  and  418  has been achieved. 
     In one embodiment, the DCU  423  monitors various parameters such as the readings from a battery temperature sensor  414 , the voltage level of the battery  404  and DC load  405  read by a voltage sensor  403 , the amount and direction of current flowing through a shunt resistor R 1  read by a current sensor  416 , voltage level on the power bus read by voltage sensors  477  and  478  and operator commands from the user interface  112  and based on these parameters, generates appropriate signals to properly command waveforms to PGU  422 , to drive the transistors Q 1 -Q 6 . 
     The operations of the bi-directional power converter circuit will now be described with reference to FIGS. 2A to  7 . Since there is no specific charge or invert mode in the system, timing diagrams will be based on net energy flow basis; (1) a moment when net flow is towards the battery  404  and DC load  405 , (2) a moment when net energy flow is towards the power bus terminals  410  and  412  and (3) a moment when net flow is near zero or transitioning between the two. 
     The timing diagram in FIG. 3 depicts operations of the bi-directional power supply circuit during a PWM cycle (Tpwm) when the current I 1  in the inductor  418  continues to flow towards the battery  404  and the DC load  405 . Under this condition, electrical energy flows from the first subcircuit  450  to the second subcircuit  452  and according to the voltage across inductor  418 , the magnitude of current I 1  will only rise when one of the transistor switch pairs in the first subcircuit  450  is conducting. As illustrated in FIG. 3, at instant T 1 , control signal VC 1  is set to turn ON transistors Q 1  and Q 4  thereby allowing a conduction path for energy flow. From T 1  to T 2  this conduction remains in effect and according to bi-directional algorithm, other control signals VC 2 , VC 5  and VC 6  are set such that proper current path exists across the subcircuits  450 ,  452  and transformer  460 . Additional benefit is achieved by keeping transistor Q 5  in conduction state thereby providing lower resistance path for the current I 1 . At T 2  this conduction period ends and negative current I 1  through the inductor  418  is allowed to fall until the beginning of next half cycle of conduction commencing at T 3 . As illustrated in FIG. 3, at instant T 3 , control signal VC 2  is set to turn ON transistors Q 2  and Q 3  thereby allowing a conduction path for energy flow. From T 3  to T 5  this conduction remains in effect and according to bi-directional algorithm, other control signals VC 1 , VC 5  and VC 6  are set such that proper current path exists across the subcircuits  450 ,  452  and transformer  460 . Again a benefit is achieved by keeping transistor Q 6  in conduction state thereby providing lower resistance path for the current I 1 . At T 5  this conduction period ends and negative current I 1  through the inductor  418  is allowed to fall again until the beginning of next full cycle of conduction commencing at T 6 . Waveform during this cycle generated across terminals  461  and  462  is shown in FIG. 3 as V 1  and for terminals  426  and  408  as V 4 . 
     The timing diagram in FIG. 4 depicts operations of the bi-directional power supply circuit during a PWM cycle (Tpwm) when the current I 1  in the inductor  418  continues to flow out of the battery  404  and the DC load  405 . Under this condition, electrical energy flows from the second subcircuit  452  to the first subcircuit  450  and according to the voltage across the inductor  418 , the current I 1  will only rise when only one the transistors Q 5  and Q 6  in the second subcircuit  452  is conducting. As illustrated in FIG. 4, at instant T 1 , control signal VC 6  is set to turn OFF transistor Q 6  while Q 5  remains ON, thereby allowing a conduction path for energy flow. From T 1  to T 2 , this conduction remains in effect and according to bi-directional algorithm of the invention, other control signals VC 1  and VC 2  are set such that proper current path exists across the subcircuits  450 ,  452  and transformer  460 . During this period diodes D 1  and D 4  are conducting to complete the circuit for current in the first subcircuit  450 . At T 2 , this conduction period ends and positive current I 1  through the inductor  418  is allowed to rise by virtue of VC 5  and VC 6  set to turn ON both the transistors Q 5  and Q 6 , this continues until the beginning of next half cycle of energy transfer commencing at T 3 . As illustrated in FIG. 4, at instant T 3 , control signal VC 5  is set to turn OFF transistor Q 5  while Q 6  remains ON, thereby allowing a conduction path for energy flow. From T 3  to T 4 , this conduction remains in effect and according to bi-directional algorithm of the invention, other control signals VC 1  and VC 2  are set such that proper current path exists across the subcircuits  450 ,  452  and transformer  460 . During this period diodes D 2  and D 3  are conducting to complete the circuit for current in the subcircuit  450 . At T 5  this conduction period ends and again, positive current I 1  through the inductor  418  is allowed to rise by virtue of VC 5  and VC 6  set to turn ON both the transistors Q 5  and Q 6 , this continues until the beginning of next full cycle of energy transfer commencing at T 6 . Waveform during this cycle generated across terminals  461  and  462  is shown in FIG. 3 as V 1  and for terminals  426  and  408  as V 4 . 
     The timing diagram in FIG. 5 depicts operations of the bi-directional power supply circuit during a PWM cycle (Tpwm) when the net current I 1  in the inductor  418  is near zero and it may change direction within a PWM cycle. This means that, during a part of the cycle, the current I 1  may flow from the battery  404  to the power bus and during other part of the cycle, the current I 1  may flow from the power bus to the battery  404  and DC load  405 . As illustrated, just before the instant T 1 , there is a small amount of current present in the inductor  418 . At T 1 , VC 6  is set to turn OFF Q 6  while Q 5  is still ON. Therefore a path is opened for energy transfer across the subcircuits  450  and  452 . Since the current I 1  is positive, energy flows towards the first subcircuit  450  and current starts to drop until it reaches zero. Additionally, since at this moment by virtue of VC 1 , Q 1  and Q 3  are in ON state, energy can start flowing back from the first subcircuit  450 . The current I 1  through inductor  418  continues to increase negatively until T 2 , when VC 1  causes transistors Q 1  and Q 3  to turn OFF thereby closing the energy path from the first subcircuit  450 , resulting in fall of negative current I 1  through  418 . This turns into positive current and reaches its peak value at T 3  when next half cycle repeats the process. Also illustrated in FIG. 5 are the waveforms V 4  appearing across terminals  426  and  408  and V 1  shows for terminals  462  and  461 . 
     FIG. 8 shows a block diagram of a feed forward based control and a current-rise limiter incorporated in the DSP based control unit according to one embodiment of the invention. The feed forward component  710  provides a high degree of stability to the power bus voltage across terminals  410  and  412 . As shown, it also includes a correction signal  722  from a slow tracking feedback correction  714  adding precision to the same. 
     The feed forward component  710  is an estimate of pulse width required to maintain desired power bus voltage across  410  and  412  thereby eliminating delays and transient response associated with pure feedback based control loops. A small correction signal  722  is added to remove small errors caused by losses in the circuit which are generally dependent on factors like amount of load shared by battery pack  404 , temperature and component tolerances etc. In the illustrated embodiment shown in FIG. 8, the feed forward component is computed as follows: 
      Ta=Tpwm/2(1+((BusRef− N* Vbatt)/BusRef)  (1) 
     where BusRef represents a desired voltage across power bus terminals  410  and  412 ; 
     N represents a ratio of number of turns of transformer  460  on L 1  side to L 3  side; and 
     Vbatt represents a current voltage level across battery terminals  406  and  408 . 
     FIG. 9 shows a block diagram of the feedback tracking correction of FIG. 8 according to one embodiment of the invention. The feedback correction signal  920  is generated by cumulatively adding small errors computed by comparing the desired power bus voltage (BusRef)  917  with the actual power bus voltage (Vbus)  918 . Only sign of this error is carried on so that feedback is not dominant and also slowing down the accumulation to achieve precise tracking of the bus voltage (Vbus)  918  appearing across the power bus terminals  410  and  412 . Accumulation is implemented by blocks  914 ,  912  and  910  and includes a limiter to limit effect of the feedback based correction signal  920 . 
     Referring back to FIG. 8, the feedback corrected signal  723  passes through a current rise limiter  712 . The purpose of the current rise limiter  712  is to estimate current rise every PWM cycle and limits its output signal  724  to keep current rise to the limits set by the system parameters. The result of the function performed by the current rise limiter  712  is that surge inrush peak current value of I 1  in inductor  418  is automatically controlled to be within the desired limits. The output signal  724  is translated to a suitable format by the block  715  and sent to PGU  422 . 
     The timing diagram in FIG. 10 shows control signals VC 5 , VC 6 , VC 1  and VC 2  generated by the PWM generation unit according to one embodiment of the invention. Each pulse width modulation time period (Tpwm) consists of “overlap section” and “push-pull section.” As seen by referring to FIG. 10, each push-pull section contains a subsection referred to as a “reverse push-pull subsection.” The overlap sections in FIG. 10 are defined as duration of time when control signals VC 5  and VC 6  are both turned ON. The push-pull sections are defined as the remainder of the time within the time period (Tpwm). In other words, a push-pull section may be defined as a time period when only one of control signals VC 5  and VC 6  is turned ON. The reverse push-pull subsection is defined as duration of time within a push-pull section when one of the control signals VC 1  and VC 2  is turned ON. As can be seen in FIG. 10, periods of overlap sections and push-pull sections are exclusive in time. 
     From bi-directional power conversion point of view, the width of the reverse push-pull subsections controls energy transfer from the power bus to the battery, whereas the width of overlap sections plus remaining parts of the push-pull sections controls energy transfer from the battery to the power bus. Hence, by controlling the ratio of these sections, the desired direction and the amount of energy transfer between the battery and the power bus can be achieved. In other words, the desired direction and the amount of electrical power flowing through the bi-directional power supply circuit can be controlled by varying the width of the reverse push-pull subsection and the width of push-pull section within a PWM cycle period (Tpwm). 
     While the foregoing embodiments of the invention have been described and shown, it is understood that variations and modifications, such as those suggested and others within the spirit and scope of the invention, may occur to those skilled in the art to which the invention pertains. The scope of the present invention accordingly is to be defined as set forth in the appended claims.