Inverter controlled, parallel connected asynchronous generator for distributed generation

A distributed generation system is disclosed that is capable of conditioning power from a utility grid, providing backup power in the event the utility grid fails, and exporting excess power to the utility grid. The system comprises an engine coupled to an asynchronous generator, an energy storage device, an engine controller capable of managing the engine and controlling its torque or speed or power, and an inverter for generating an AC output and also capable of controlling the frequency and voltage of the generator to match the frequency of a coupled utility grid.

DETAILED DESCRIPTION OF THE INVENTION FIG. 4 is a block diagram of a first distributed generation (DG) system 400 that is capable of generating three-phase power. Distributed generation means a system divided up among two or more components. The output of the DG system 400 is preferably connected in parallel to a utility grid to power a load 106 as shown in FIG. 1 . The system 400 comprises an engine 402 coupled to an asynchronous generator 404 , an inverter 406 coupled to an energy storage device 408 such as a battery or ultracapacitor, an engine controller 414 , and an optional filter 412 . The generator 404 is capable of generating a three-phase AC output. The generator AC output may be connected in parallel to the three-phase AC output of the filter 412 . The system is capable of conditioning power from the grid, providing backup power in the event the grid fails, and exporting excess power to the grid. Grid failure may be a complete lack of power (blackout) or insufficient power (brownout). The present invention tightly integrates the energy storage device-backed inverter and an engine-generator, in novel manners. The engine controller 414 may be coupled between the inverter and the engine, and is capable of controlling speed or torque or power level of the engine 402 . The DG system 400 may optionally comprise an inverter controller 416 capable of monitoring the load, and the utility grid in order to perform peak-shaving, net-metering, and time-of-use metering. The system of FIG. 4 is different from that of FIG. 3 in structure and function. The inverter 406 controls the generator 404 output frequency and voltage, which allows connection of the DG system 400 to the grid and export power to it. Since the frequency of the generator 404 can be matched to the grid frequency by the inverter 406 , there is no need for a transfer switch to disconnect the generator 404 when the DG system 400 is connected to the grid. A transfer switch can be very expensive to purchase and install and may be a source of maintenance issues. Another advantage of the present invention, is that since the engine 402 and generator 404 do not need to be electrically disconnected from the grid, they can run permanently and take over the load very rapidly, should the grid fail. As a consequence, and in contrast to the prior art shown in FIG. 3, a smaller battery is needed for load-leveling. It is, therefore, also conceivable to replace the battery with a bank of ultracapacitors; the advantage of ultracapacitors being that they are maintenance free, have a long life cycle, and do not require a sophisticated state of charge management. An ultracapacitor, or “supercapacitor,” stores energy electrostatically by polarizing the electrolytic solution. Though it is an electrochemical device (also known as an electrochemical double-layer capacitor), there are no chemical reactions involved in its energy storage mechanism. This mechanism is highly reversible, allowing the ultracapacitor to be charged and discharged hundreds of thousands of times. An ultracapacitor may be viewed as two non-reactive porous plates suspended within an electrolyte, with a voltage applied across the plates. The applied potential on the positive plate attracts the negative ions in the electrolyte, while the potential on the negative plate attracts the positive ions. This effectively creates two layers of capacitive storage, one where the charges are separated at the positive plate, and another at the negative plate. Ultracapacitors are available from Maxwell Technologies, Inc. The inverter 406 is preferably a PWM—switched design, which may require an output filter 412 to smooth its voltage. The output filter 412 also provides impedance to allow phase adjustment to achieve required power factor when exporting power to the grid. A filter 512 alternatively may be placed at the output of the DG system, as shown in FIG. 5 . When the grid is powering the loads, the generator 404 may need to free-spin synchronously in order not to load the system. A clutch between the engine and generator may then be necessary to keep the engine from having to turn. The generator windings may be in wye or delta configuration, or could be switched from wye for starting to delta for running. Details of the generator winding is disclosed in copending U.S. patent application Ser. No. 09/772,820 entitled “Electromechanically Controlled Changeover Switch” and is incorporated herein by reference in its entirety. The present invention can condition power from the grid, provide backup power in the event the grid power fails, and export excess power to the grid without the costly, inefficient AC-DC-AC process of generating, then rectifying, and then reinverting power, without the added expense of a transfer switch, and with a smaller energy storage device. The present invention is more efficient, more flexible, less expensive, smaller and lighter. The engine controller 414 may be coupled to the inverter 406 or inverter controller 416 via an analog or digital link. For example, the link may be a serial link, such as a Controller Area Network (CAN). The engine controller 414 may be used to manage the start up, cool down, and fault detection of the engine 402 . Further, the engine controller 414 may be used to operate the engine 402 at a specified speed, torque, or power level. The specified operation point may be chosen to maximize efficiency or minimize reaction time depending on the application. The run, stop or operation point value of the engine controller 414 may be sent from the inverter 406 or the inverter controller 416 to the engine controller 414 . Alternatively, the engine controller 414 may determine the optimum speed or torque autonomously. It should be understood that, while the present invention has been described in detail herein, the invention can be embodied otherwise without departing from the principles thereof, and such other embodiments are meant to come within the scope of the present invention as defined in the following claim(s).