Abstract:
A power generating system having a variable speed genset is provided. The variable speed genset includes an engine and a variable speed generator. The variable speed generator is mechanically coupled to the engine and is configured to generate electrical power. The power generating system further includes an energy storage device, which is charged or discharged during transient load conditions of a power grid. The power generating system includes a controller to generate a speed control signal to select a speed for the genset. The speed control signal is selected based upon stored energy in the energy storage device and power generating system conditions, power grid conditions or combinations thereof.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to an engine-generator set (genset), and, more particularly, to an operation of the genset during transient conditions. 
         [0002]    Gensets are used to supply electrical power in places where utility (central station) power is not available, or where power is needed only temporarily. Currently, gensets typically include engines connected directly to generators to produce electricity. In some cases the generators are permanently installed and kept ready to supply power to critical loads during temporary interruptions of the utility power supply. Typically the gensets operate at a fixed speed to produce electricity at a grid frequency. The fixed speed may typically be 1500 rpm for a 50 Hz grid frequency or 1800 (or 1200) rpm for a 60 Hz grid frequency. 
         [0003]    For low power engines, such as engines operating below about 25 KW, higher speeds are typically possible as are higher output power and higher efficiency. However, operation of the engine at very high speeds is not practical when the genset needs to generate electricity at a fixed grid frequency. In some cases, the output power required from the genset is below a rated value while the engine is still running at a fixed speed. This results in reduction in the engine efficiency. In islanded grids, the engine efficiency may be improved by reducing the engine speed and hence reducing the fuel consumption. However, the genset frequency may then drop below an acceptable value of the grid frequency. 
         [0004]    A variable speed genset may be used to improve the efficiency of the engine. Other advantages of variable speed gensets are reduced fuel consumption, reduced noise, prolonged engine life, and reduced emissions. One challenge for variable speed gensets as well as for fixed speed gensets is that, when there is a step change or a step increase in a load on the genset, the engine may take time to accelerate to its required speed. The time delay may result in poor transient performance or poor transient response of the engine. A variable geometry turbocharger is sometimes used to improve the transient response of the engine. However, such turbochargers are expensive and are not easily obtainable. 
         [0005]    Therefore, it would be desirable to have a system and a method that will address the foregoing issues. 
       BRIEF DESCRIPTION 
       [0006]    In accordance with one exemplary embodiment of the present invention, a power generating system is provided. The system includes a variable speed genset, having an engine and a variable speed generator. The variable speed generator is mechanically coupled to the engine and is configured to generate electrical power. The system also includes an energy storage device. The energy storage device is charged or discharged during transient load conditions of a power grid. The system further includes a controller to generate a speed control signal to select a speed for the genset based upon stored energy in the energy storage device and power generating system conditions, power grid conditions or combinations thereof. 
         [0007]    In accordance with another exemplary embodiment of the present invention, a power generating system is provided. The power generating system includes a genset, wherein an engine and a generator mechanically coupled to the engine and configured to generate electrical power is provided. The system also includes an energy storage device, which is charged or discharged during transient load conditions of a power grid. The system further includes an auxiliary machine mechanically coupled to the engine and a converter configured to couple the energy storage device to the auxiliary machine. A controller generates a converter control signal to control supply of power from the energy storage device to the auxiliary machine based upon stored energy in the energy storage device and power generating system conditions, power grid conditions, or combinations thereof. 
         [0008]    In accordance with yet another embodiment of the present invention, a method for use in a power generation system for supplying stored energy from an energy storage device to a genset is provided. The genset includes an engine and a generator coupled to it. The method includes obtaining energy signals indicative of stored energy in the energy storage device. The method also includes obtaining condition signals indicative of power generation system conditions, power grid conditions, or combinations thereof. The method further includes controlling engine control signals to select a speed for the engine and storage control signals to charge or discharge the energy storage device based upon the storage and condition signals. 
     
    
     
       DRAWINGS 
         [0009]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0010]      FIG. 1  is a diagrammatical representation of a genset system with an energy storage device connected to the power grid; 
           [0011]      FIG. 2  is a diagrammatical representation of a variable speed genset system with an energy storage device connected to the power grid in accordance with an embodiment of the present invention; 
           [0012]      FIG. 3  is a diagrammatical representation of a variable speed genset system with a Doubly Fed Asynchronous Generator (DFAG) and an energy storage device in accordance with an embodiment of the present invention; 
           [0013]      FIG. 4  is a diagrammatical representation of a variable speed genset system with an energy source in accordance with an embodiment of the present invention; 
           [0014]      FIG. 5  is a diagrammatical representation of a genset system with an auxiliary machine for transient response in accordance with an embodiment of the present invention; 
           [0015]      FIG. 6  is another diagrammatical representation of a genset system with an auxiliary machine for transient response in accordance with an embodiment of the present invention; 
           [0016]      FIG. 7  is yet another diagrammatical representation of a variable speed genset system with an auxiliary machine for transient response in accordance with an embodiment of the present invention; and 
           [0017]      FIG. 8  is a diagrammatical representation of a variable speed genset system with an auxiliary machine for transient response in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    As discussed in detail below, embodiments of the present invention function to provide a system to control a variable speed genset connected power generating system. The system includes a genset with an energy storage device and a power electronic converter interface to a power grid or a power generating system. Although the present discussion focuses on a genset system, the present invention is applicable to any power generating system with a controllable or uncontrollable input energy source and a power electronic converter interface. 
         [0019]      FIG. 1  shows a genset system  10  with an energy storage device connected to the power grid. A generator  12  is mechanically coupled to an internal combustion (IC) engine  14 . The generator  12  and the engine  14  together form a genset  16 . In one embodiment, the generator comprises a synchronous generator. In another embodiment, the internal combustion engine comprises a natural gas engine or a diesel engine. The generator  12  is electrically connected to the power grid  18  to which electrical loads (not shown) such as residential loads and industrial loads are connected. As described earlier, the rated rotational speed of the IC engine and the generator is typically 1500 rpm for 50 Hz grid applications or 1800 (or 1200) rpm for 60 Hz applications. In one embodiment, the system  10  may be used in an islanded grid. When the system  10  is used in the islanded grid, fluctuations in electrical loads connected to the grid cause a fluctuation of engine speed, which translates into a fluctuation of the grid frequency. In another embodiment, where the system  10  is connected to the grid, the engine  14  may be commanded to track a reference power demand signal with high bandwidth. The engine may not be able to track such a reference signal due to limitations of the engine dynamic response. As the electrical loads are rated for a fixed frequency, fluctuations in the grid frequency are harmful to the performance of electrical loads connected to the grid. Hence, an energy storage device  20  connected to an energy storage (ES) converter  22  may also be coupled to the power grid. In one embodiment, the energy storage device comprises a supercapacitor. As will be appreciated by those skilled in the art, supercapacitors offer very high capacitance in a small package. In another embodiment, the energy storage device may be a battery storage or a direct current (DC) flywheel. The energy storage device  20  is charged or discharged during times of transient load change to reduce the impact of load changes on the genset. Thus, the IC engine speed deviations remain within defined limits and the harmful consequences of frequency variations on the grid-connected loads are avoided. In one embodiment, the energy storage converter  22  includes power electronic devices such as Insulated Gate Bipolar Transistors (IGBTs). A controller  24  provides control signals  26  to the ES converter  22  to control its output voltage amplitude, output voltage frequency and phase and thus to control the output current of the ES converter  22 . The controller  24  generates the control signals  26  based on IC engine parameters  28 , grid voltage  30  and the energy storage device parameters  32 . In one embodiment, the IC engine parameter includes IC engine speed. In another embodiment, the energy storage device parameter  32  includes status of charge and current from the energy storage device. In another embodiment, transformers  34 ,  36  are used to step up the output voltage of the generator  12  and the ES converter  22  to match it to the grid voltage. This embodiment responds more slowly to transient events than is desired for certain applications. 
         [0020]      FIG. 2  shows a variable speed genset system  50  with an energy storage device connected to the power grid for transient response in accordance with an embodiment of the present invention. The genset  16  includes a synchronous generator  12  mechanically coupled to the IC engine  14 , and the system  50  further includes the energy storage device  20  connected to the power grid  18  through the energy storage converter  22 . In the embodiment of  FIG. 2 , a variable speed generator (VSG) converter  52  is included in the genset system. In one embodiment, the VSG converter comprises power electronic components such as IGBTs and includes a rectifier stage  54  and an inverter stage  56 . The rectifier stage  54  converts the alternating current (AC) power from the generator  12  to direct current (DC) power. The inverter stage  56  converts the DC power back to AC power and feeds it to the power grid  18  at an appropriate voltage and frequency. 
         [0021]    The VSG converter  52  enables the IC engine  14  to run at a variable speed such that, at times of low power demand on the power grid, the IC engine can run at low speeds rather than rated speed thus saving on fuel consumption. As described earlier, it may also reduce emissions, noise production, and wear and tear of the genset. In this embodiment, the controller  24  commands the speed response of the engine depending on the power output of the VSG converter  52 . This embodiment represents an improvement but a continuing challenge is that the optimal engine speed for fuel efficiency is often a speed where the output power of the engine is limited and close to that of the loads. If the load demand spikes much above normal levels, then there is little additional torque available to accelerate the engine to the required new speed for the higher power level. In one embodiment, the additional energy storage device  20  may be used to accommodate the transient response scenario. As described earlier, the energy storage device  20  is charged or discharged through the ES converter  22  during times of transient load change to reduce the impact of load changes on the genset. The controller  24  provides control signals  26  to the ES converter  22  to control its output voltage. Transformers  34  and  36  may be used to match the output voltage of the VSG converter  52  and the ES converter  22  to the grid voltage. In one embodiment (not shown), the output of the ES converter  22  may be directly connected to the output of the VSG converter  52  instead of to the grid  18  through the transformer  36 . This avoids the expense for an additional transformer connection between the ES converter and the grid. 
         [0022]      FIG. 3  shows a variable speed genset system  70  with a Doubly Fed Asynchronous Generator (DFAG) and an energy storage device for transient response in accordance with an embodiment of the present invention. In the embodiment of  FIG. 3 , the genset  16  includes a DFAG machine  72  with a DFAG converter  74  connected between the DFAG machine stator and the rotor. The DFAG machine may also be referred to as a Doubly Fed Induction Generator (DFIG). As will be appreciated by those skilled in the art, the DFAG machine or the DFIG machine is an asynchronous machine with multiphase windings on the stator and the rotor. The stator and rotor windings participate actively in the electrical energy conversion process. The rotor winding is connected to the grid via a DFAG converter and the stator winding is connected directly to the grid. The advantage of a DFAG machine is that the rating of the DFAG converter  74  is typically one third of the full power rating of the DFAG machine  72 . The DFAG converter  74  enables the output frequency of the DFAG machine to remain constant despite variations of approximately +/−30% in the DFAG machine speed. The advantage of using the DFAG machine and the DFAG converter is that, due to the reduction of power rating of the converter, the costs associated with the genset system are considerably lower than for a fully rated converter genset system such as might be used in the embodiment of  FIG. 2 . As described earlier, in one embodiment, the transformer  36  may be eliminated by directly connecting the ES converter output to the input of transformer  34 . 
         [0023]      FIG. 4  is a diagrammatical representation  80  of the variable speed genset system with an energy source in accordance with an embodiment of the present invention. In the embodiment of  FIG. 4 , the energy storage device is directly connected to a DC bus (not shown) of the VSG converter  52 . The advantage of this configuration is that it helps in eliminating an extra DC/AC conversion stage represented by converter  22  of  FIG. 2 . In one embodiment, the synchronous generator  12  and the VSG converter  52  may be replaced by the asynchronous generator  72  and a DFAG converter  74  of  FIG. 3 . 
         [0024]      FIG. 5  shows a genset system  90  with an auxiliary machine for transient response in accordance with another embodiment of the present invention. In the embodiment of  FIG. 5 , an electrical motor-generator (M/G) set  92  is mechanically coupled to the crankshaft  94  of the IC engine  14 , and the energy storage converter  22  is electrically connected to the M/G set instead of the power grid  18 . The M/G set is controlled by the controller  24  such that if the load on the power grid  18  increases resulting in a decrease in the genset speed, the M/G set  92  is driven in a motoring mode to supply additional torque to the genset  16  and thus maintain the genset speed. In the motoring mode, the energy to drive the M/G set  92  is obtained by discharging the energy storage device  20 . On the other hand if the load on the power grid  18  decreases and results in engine overspeed, the controller  24  controls the M/G set  92  to operate in a generating mode providing a breaking torque on the crankshaft  94  of the genset. In the generating mode, the energy generated by the M/G set  92  is utilized to charge the energy storage device  20 . An advantage of the system  90  is that no additional connection of the energy storage device  20  to the grid  18  is required and the ES converter voltages can be adapted to the rating of the M/G set  92 , which could be potentially lower than the grid voltage and therefore less expensive. In one embodiment, the M/G set  92  is rated at a fraction of the size of the asynchronous generator  72 . In one embodiment, the M/G set includes an AC motor and the ES converter includes a AC to DC converter to convert DC power from the energy storage device to AC power and feed it to the AC motor of the M/G set. In another embodiment, the M/G set includes a DC motor and the ES converter includes a DC to DC converter to feed the DC power to the DC motor of the M/G set. 
         [0025]      FIG. 6  shows a genset system  110  with an auxiliary machine for transient response in accordance with an embodiment of the present invention. In the embodiment of  FIG. 6 , as compared to  FIG. 5 , the M/G set is located at a different position on the crankshaft  94  of the genset 
         [0026]    Similarly,  FIG. 7  shows a genset system  120  with an auxiliary machine for transient response in accordance with an embodiment of the present invention. In the embodiment of  FIG. 7 , as compared to  FIG. 5 , the M/G set is located in between the IC engine and the generator on the crankshaft of the genset. 
         [0027]      FIG. 8  shows a variable speed genset system  130  with an auxiliary machine for transient response in accordance with another embodiment of the present invention. The system  130  is similar to the system  90  of  FIG. 5 . However, a DFAG machine  72  and a DFAG converter  74  are used in system  130 . Thus, compared to genset system  90 , there is also a flexibility of controlling speed of the engine  14 . In one embodiment, the DFAG machine and DFAG converter may be replaced by a synchronous machine and a VSG converter as described earlier. 
         [0028]    In embodiments wherein existing gensets use an external electric starter motor (not shown) for black start capability, the M/G set  92 , the ES converter  22  and the energy source device  20  could replace such electric starters with a potential for slightly uprated capability in exchange for the modest additional cost of these components as compared to starter motors. As with earlier configurations, the transformer  34  may be used in the case that the output voltage of the generator  72  does not match the grid voltage. 
         [0029]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.