Abstract:
A reconfigurable power system that includes a gas turbine, flywheel, a first electric machine coupled to the gas turbine, a second electric machine coupled to the flywheel, the first and second electric machines being substantially similar in configuration, a first power device for coupling power from the first electric machine to a power grid, a second power device coupled to the second electric machine for driving the flywheel and coupling power from the second electric machine to the power grid, and a switch for coupling either the power generated by the first electric machine or the second electric machine to the grid.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   A reconfigurable power system that comprises multiple loads and prime movers and electric machines connected to an AC bus via power electronic devices. 
   2. Description of the Prior Art 
   Efforts have been underway to develop high-speed generators and power converters used to transfer power between a high speed turbine, a high speed energy storage flywheel and a 450 Vrms, 3-phase, 60 Hz distribution system. The system would incorporate high speed generators which convert rotational energy to electrical energy, rectifiers that convert high frequency AC power to DC power and inverters which convert DC power to AC power. The system also includes a high frequency drive motor to allow charging of the flywheel energy store directly from the 450 Vrms 3-phase Hz distribution grid. During discharge of the flywheel energy store, the power flow can be directed to the 450 Vrms distribution grid or be rectified and routed through the inverters. 
   Although the system noted hereinabove when implemented, will meet the system requirements, it would be desirable if the system had the capability of being reconfigured such that the flywheel portion is essentially capable of operating as a full back-up to the turbine portion of the system. In addition, it would be desirable if the high speed generators were multiple phase-set electric machines. 
   SUMMARY OF THE INVENTION 
   The present invention provides a reconfigurable power system that comprises multiple loads and prime movers and electric machines connected to an AC bus utilizing multiple phase-set electrical machines in conjunction with suitable power electronic devices. 
   The advantages of this system is that common power electronic devices can be used for both the flywheel and turbine (both potential loads and prime movers), and common electric machines can be used for coupling with both flywheel and turbine. The nature of the power requirements of a flywheel are well suited for a multiple phase set electric machine. In particular, when providing power to the flywheel, the power demand is low and when power is extracted from the flywheel a much higher power capacity electric machine is needed. A multiple phase set electric machine can be configured to run on one phase set (or any number of phase sets corresponding to the number of power electronic devices dedicated for a variable frequency drive), when motoring the flywheel and all of the phase sets when providing power to the grid through the same power electronic devices normally used to provide power to the grid from the multiple phase set electric machine coupled to the turbine. Some built-in system redundancy can be provided, but if common electric machines are used and common electric power electronic devices are used, then what would otherwise be a special purpose variable frequency drive for motoring the flywheel can be eliminated in favor of one of the common power electronic devices. If a common power electronic device normally feeding generating power to the grid fails, then the system user has the option of re-configuring the system using the common power electronic device normally serving as a variable frequency device and vice versa. This system re-configuration could be performed on a real-time as needed basis; for example the flywheel could be powered periodically rather than continuously as the needs and priority of the system change. 
   The present invention thus provides an efficient power system comprising a number of electric machines with multiple phase set stators and power electronic devices (which may or may not include switch gear and filters) configured to provide bi-directional power flow through at least one of the electric machines. In particular, a first electric machine is coupled to a turbine engine and a second electric machine is coupled to a flywheel. The first electric machine is used as a motor to start the turbine and as a generator when the turbine is producing power. The second machine is used as a motor to “spin up” the flywheel and as a generator when the flywheel is providing power. 

   
     DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention as well as other objects and further features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawing wherein: 
       FIG. 1  illustrates a preferred embodiment of the system of the present invention; 
       FIG. 2  illustrates an operating mode of the preferred embodiment shown in  FIG. 1  wherein the electric machine acts as a motor to start the turbine and the flywheel portion of the system is charging, both sub-systems being independent; 
       FIG. 3  illustrates an operating mode of the preferred embodiment shown in  FIG. 1  wherein the turbine operates to run the elective machine as a generator and the flywheel is charging, each sub-system acting independently; 
       FIG. 4  illustrates an operating mode of the preferred embodiment shown in  FIG. 1  wherein the gas turbine is off-line and the flywheel is discharging, both sub-systems acting cooperatively; 
       FIG. 5  illustrates an alternative embodiment wherein a DC connection is provided; and 
       FIG. 6  illustrates an operating mode of system shown in  FIG. 5  wherein the gas turbine is on-line and the flywheel is discharging, both sub-system acting cooperatively. 
   

   DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , the reconfigurable power system  10  using multiple phase-set electric machines in accordance with the teachings of the present invention is illustrated. Electric machines  12  and  14  (although only two machines are illustrated, more than that number can be utilized) are illustrated as being coupled to turbine  16  and flywheel  18 , respectively (although other prime movers can be utilized). System  10  is configured to provide bi-directional power flow in at least one of the electric machines  12  and  14 . Electric machine  12 , coupled via shaft  20  to turbine  16 , can be used as a motor to start turbine  16  and, alternately, as a generator when turbine  16  is producing electric power. Electric machine  14 , coupled to flywheel  18  via shaft  22 , is used as a motor to “spin up” flywheel  18  and as a generator when the flywheel is generating electric power. As is well known, flywheels store kinetic energy to be used in driving a machine for a short time period and functions essentially as a back-up in case of a system power failure. 
   The preferred electric machine for use in system  10  is disclosed in copending application Ser. No. 11/751,450, filed May 21, 2007 and assigned to the assignee of the present invention. The advantages of using such a machine is described in that application and the teachings thereof necessary for an understanding of the present invention is incorporated herein by reference. The multi-phase winding sets used in the machine can be independent, space shifted, three phase winding sets. Each set is supplied by a dc-ac power electronics building block (“PEBB”), such as block  156  discussed hereinafter. Permanent-magnet machines are the preferred machine topology. 
   Sets of three phase windings  30 ,  32 ,  34  and  36  from machine  12  is coupled to switches  40 ,  42 ,  44  and  46  respectively. The output from switches  40 ,  42 ,  44  and  46  are coupled to input/output filters  50 ,  52 ,  54  and  56 , respectively (the system can operate without the filters if necessary). The output from the filters are coupled to block  70  (first power device) comprising a series of AC/DC and DC/AC converters, the output therefrom being coupled to input/output filters  80 ,  82 ,  84  and  86 , the outputs of which are coupled to three-phase AC bus  100 , bus  100  operating at a frequency range between 50 and 60 hz and at a voltage range between 450V and 1000 VAC. The AC/DC converters comprise blocks  71 ,  72 ,  73  and  74  and the DC/AC converters comprise blocks  75 ,  76 ,  77  and  78 . It should be noted that the converter blocks are bi-directional i.e. they can be used as either AC/DC or DC/AC converters. 
   Referring to that portion of system  10  involving flywheel  18 , the output from four sets of three phase winding  140 ,  142 ,  144  and  146  are, in one version, coupled to switches  40 ,  42 ,  44  and  46 , the system then operating in the manner described hereinabove with reference to machine  12 . In some modes of operation, three phase winding  146  is connected to power device  150  comprising switch  152 , input/output filter  154 , AC/DC converter  156  DC/AC converter  158  and input/output filter  160 . The output of power device  150  is connected to three phase AC bus  101 . Blocks  41 ,  43  and  45  are part of the dual-pole double system throw switches that either connect the high speed turbine  16  and motor/generator  12  to grid  100  or motor/generator  14  and flywheel  18  to grid  101 . 
   The flywheel generates power while the gas turbine  16  is generating power through the high speed generator  12  or when gas turbine  16  is disconnected from the system  10 . 
   The system  10  described hereinabove has three modes of operation. In the first mode, blocks  150 , powered by bus  101 , causes machine  14  to operate as a motor to spin-up flywheel  18  (switch  152  closes the connection between machine  14  and PEBB  150 ) and switches  40 ,  42 ,  44  and  46  connect machine  12 , operating as a generator, to grid  100  through the filters and block  70 . In the second mode ( FIG. 3 ), blocks  150 , powered by bus  101 , causes machine  14  to operate as a motor to maintain power on the flywheel  18  (switch  152  is open) and switches  40 ,  42 ,  44  and  46  connect machine  12 , operating as a generator, to grid  100  through the filters and block  70 . When there is a failure ( FIG. 4 ) in gas turbine  16  or generator  12  (or if there is a requirement for power from the flywheel), switches  40 ,  42 ,  44  and  46  disconnect generator  12  and turbine  16  and instead connect to machine  14  which is running as a generator as flywheel  18  feeds power back to grid  100  through the filters  50 ,  52 ,  54  and  56  and block  70 . 
   In an alternate mode of operation, when the system requests power simultaneously from gas turbine  16  and flywheel  18 , flywheel  18  is sized to handle the peak load (turbine power, base load and any pulsed load or overload) so when flywheel  18  is on line it provides sufficient power for the total peak load. This eliminates the need for gas turbine  16  to supply power to the load thereby providing a system with improved efficiency over the prior art since gas turbine  16  is optimized for the base load only and would be unloaded when the load increases beyond the base load. 
   Under normal operation, the switch blocks are connecting the gas turbine  16  to PEBB block, or converter,  70  and then to AC grid  100 ; at this time, switch  152  is connecting the motor  14  to flywheel  18  to keep the flywheel spinning, i.e. storing energy and ready for use. When the generator  12  and turbine  16  is disconnected from the system by switches  40 ,  42  . . .  46  the flywheel generator  14  is connected to feed the base load and the additional pulsed or peaking load. When a pulsed or peaking load is completed, switches  40 ,  42  . . .  46  reconnect gas turbine  16  which is still operational even through having been disconnected from the system. The switch  152  connecting flywheel  18  is the same as switches  40 ,  42  . . .  46  and comprise dual pole transfer switches, either for connecting the generator turbine/generator to the AC grid  100  or for connecting the flywheel/generator  14 ,  18  to the grid  101 . 
   The PEBB is used to control the electric machine coupled to the turbine such that it switches between functioning as a motor or a generator “on the fly” i.e. the direction of power flow determines if the electric machine is a motor or generator (the PEBB corresponds to the AC/DC blocks forming converter  70 ). Alternatively, a separate active module could be used for motoring and a separate possible module used for generating. In this case, a contactor can be used to toggle which PEBB is active. In the case of starting the turbine  16  and then using the turbine as a prime mover, the contactor would only be switched after the rotation of the turbine is self-sustained. Otherwise, any active PEBB would be used without a contactor. 
   Each PEBB preferably comprises a three-phase diode bridge or active rectifiers (diode blocks are not used as dc-ac blocks; if a 2-level insulated-gate bipolar transistor three phase bridge is used as a AC/DC converter then the same bridges can be used as DC/AC converter). 
   The time-dominant mode of operation for the electric machine coupled to the flywheel is low power motoring (only providing make-up and initial spin-up power). 
   The key differences between both power paths are the time involved and the disparate power levels for motoring and generating for the turbine and flywheel. 
   Since the charge/discharge (motoring/generating) cycles of the flywheel are significantly disproportionate in power requirements, the ability to have a variable frequency device  150  essentially χ/N (wherein χ is preferably 1 and N the number of phase sets) allows the system to use the same PEBB&#39;s for both turbine generating and flywheel generating. 
   In summary, system  10  provides a power generation system that consists of a gas turbine/generator (or multiples thereof) and a motor/generator that is spinning a flywheel. System  10  can be a stand alone network or can be used to support an existing AC network handling peak loads. For example, the AC network might be able to handle  5  MW continuously, but there can be loads that can come in and out intermittently that are approximately 10 MW. In that case, system  10  can be used to support the extra load. A unique feature of the system  10  is that the same machine, configured as a space shifted split stator as disclosed in the copending &#39;450 application can be used to be the generator rotated by gas turbine  16  and also the motor/generator  14  spinning flywheel  18  (as a motor) and rotated by the flywheel acting as a generator. The PEBB&#39;s used in the system can also be identical on the AC/DC side and DC/AC side. One block that is AC/DC can be used to spin, or rotate, the motor that spins up flywheel  18 . Multiples (N) of the same blocks can be utilized to convert the energy from the flywheel/generator  18  to feed back to the bus, or grid,  100 . The same blocks are used to convert the energy from the gas turbine generator  16  to the common AC bus, or grid  100 . The reconfiguration enables switching between the flywheel/generator  14 ,  18  and gas turbine/generator  12 ,  16  without having to bring in new PEBB&#39;s assuming that the gas turbine generator and flywheel do not have to be on at the same time. 
   System  10  can be adapted to the following configurations: (1) using multiple PEBB&#39;s that are switched from the flywheel subsystem to the gas turbine subsystem; (2) the flywheel subsystem contains at least one conventionally wound three-phase machine; (3) the flywheel sub-system contains multiple flywheels, motor/generators, PEBB modules, not necessarily in a 1:1:1 relationship. 
     FIG. 2  illustrates a variation of the system shown in  FIG. 1 . In particular, system  100 ′ comprises subsystems  102  and  104 , sub-system  102  functioning to start gas turbine (generator)  106  via motor  108 . Sub-system  104  functions to charge (rotate) flywheel  110  utilizing motor  112 . The power flow of sub-systems  102  and  104  is in the direction illustrated by arrow  114 . Subsystems  102  and  104  function independently of each other. 
     FIG. 3  illustrates the system of  FIG. 1  wherein (system  200  comprising sub-systems  202  and  204 ) gas turbine  206  in sub-system  202  operates in a manner such that generator  208  generates AC power in the direction of arrows  210 . Sub-system  204  utilizes motor  212  to charge (rotate) flywheel  214 . Power flows in the direction of arrow  216 . Sub-system  202  and  204  act independently of each other. In this mode of operation, motor  212  is used to provide make-up and initial spin-up power for flywheel  214 . 
     FIG. 4  illustrates the system of  FIG. 1  wherein the gas turbine  16  is off-line and flywheel  18  is discharging (rotating) and causing generator  12  to generate power. Since sub-systems  300  and  302  act cooperatively, the power from generator  14  flows to the switches in sub-system  302  and then to the grid  100  as illustrated by arrows  306 . 
     FIG. 5  illustrates system  400  comprising sub-systems  402  and  404 . The system provides a DC connection wherein prime movers  406  and  408  operate simultaneously. In particular, sub-systems  402  and  404  share a DC connection whereas in  FIG. 4  an AC connection is shared. 
     FIG. 6  illustrates system  500  comprising sub-systems  502  and  504 , an operating mode of system  400 . System  500  is used to meet temporary peak power demand. In particular, flywheel  504  has the capacity to work simultaneously with gas turbine  508  to meet peak power demand. In this system the gas turbine DC/AC modules are rated for peak power and can use passive rectification and flywheel AC/DC modules are selected for active rectification and are rated for flywheel charging. The negative DC connection can be always active; the positive DC connection requires a contactor for safety and/or margin reasons. 
   While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings.