Abstract:
A multi-unit power generation system comprising a plurality of generators connected in parallel, a switching system for switching between and/or aggregating a generator load produced by the plurality of generators and a utility grid load, and a control system. The control system is in communication with each generator for communicating command signals to each generator. The control system is further in communication with the switching system for commanding the switching system to switch between or aggregate the generator load and the utility grid load. Each generator may comprise, for example, a microturbine generator.

Description:
BACKGROUND OF INVENTION  
       TECHNICAL FIELD  
         [0001]    The present invention relates to distributed power generating systems. More specifically, the present invention relates to parallel operation and control of multiple electrical power generators to provide grid-connected and/or stand-alone power generation.  
           [0002]    Microturbine generators are used as distributed power generation systems to generate on-site power at locations such as businesses, homes, or other facilities. Microturbines can be connected to a power grid or run in stand-alone mode. In addition, multiple microturbines may be connected in parallel in order to combine their outputs. In any such application, multiple microturbines can be controlled and operated to provide total power (load following) or partial power (peak shaving or base loading). However, systems involving multiple microturbines connected in parallel present significant design challenges that have yet to be satisfactorily overcome.  
           [0003]    In particular multiple microturbine systems of the prior art are generally unable to insure safe, reliable and economical operation while also providing the user with a high power quality level. Such prior art systems tend to have low system tolerance to external faults and transients. Such prior art systems are typically not able to match site voltage levels with standard generators, as well as isolate loads and grid utility systems from the possibility of coupling direct current conditions from unbalanced or faulty inverters. In addition, start-up of prior art multiple microturbine systems is costly, typically involving multiple batteries or the need to acquire power from the utility grid.  
           [0004]    To overcome the above-described and other design challenges is a satisfactory and economical manner, a system to function a number of microturbines in a similar fashion that a single microturbine operates is required.  
         SUMMARY OF INVENTION  
         [0005]    A multi-unit power generation system comprising a plurality of generators; connected in parallel, a switching system for switching between and/or aggregating a generator load produced by the plurality of generators and a utility grid load, and a control system. The control system is in communication with each generator for communicating command signals to each generator. The control system is further in communication with the switching system for commanding the switching system to switch between or aggregate the generator load and the utility grid load. Each generator may comprise, for example, a microturbine generator.  
           [0006]    Each generator is connected to a corresponding output isolation transformer to convert a generator output voltage to a site operating voltage. The switching system includes a power meter to monitor the generator load and a feedback signal is provided by the switching system to the control system to inform the control system of the generator load. The switching system may also contain means for measuring load conditions and utility grid conditions. Feedback signals may be provided by the switching system to the control system to inform the control system of the load conditions and utility grid conditions.  
           [0007]    The switching system comprises a utility circuit breaker and a generator circuit breaker. The switching system communicates position information for the utility circuit breaker and the generator circuit breaker to the control system. The control system commands the switching system to open and close the utility circuit breaker and the generator circuit breaker in order to switch between or aggregate the generator load and the utility grid load.  
           [0008]    The control system may further receive input signals from one or more input devices for allowing an operator to control the generator load. The control system may be programmed to controls the generators to provide programmable scheduled start/stop times, load profiles, load following offset and peak shaving power levels. In operation of the system, the control system designates a first generator as a master generator and all other generators as slave generators. The control system commands the master generator to operate at the normal utility frequency, which is in synchronization with the utility frequency when it is present. The master generator communicates a synchronization signal indicating said frequency to the slave generators.  
           [0009]    These and other aspects, features and advantages of the present invention will become apparent upon a reading of the following description of certain exemplary embodiments and with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0010]    [0010]FIG. 1 is a block diagram illustrating certain components of an exemplary microturbine generator, which may be used in certain embodiments of the multi-unit power generation system of the present invention.  
         [0011]    [0011]FIG. 2 is a block diagram illustrating a multi-unit power generation system according to exemplary embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0012]    The present invention provides a multi-unit power generation system. The multi-unit power generation system comprises multiple power generators, such as microturbines, connected and working in parallel. A control system is provided for operating the multiple power generators as if they were a single unit and for maximizing the overall efficiency of the system. A switching system is provided for switching between a stand-alone power generation mode and a grid-connected power generation mode. Exemplary embodiments of the invention will hereinafter be described with reference to the figures, in which like numeral indicate like elements throughout the several drawings.  
         [0013]    [0013]FIG. 1 is a block diagram illustrating certain components of an exemplary power generator  110 . The power generator  110  shown in FIG. 1 is commonly referred to as a microturbine. A microturbine represents one possible power generator that may be used in a multi-unit power generation system according to the present invention. A microturbine  110  useful within an exemplary system of the present invention may be a microturbine sold under the mark “Parallon 75® ” by General Electric Corporation of Schenectady, N.Y. Generally described, the “Parallon 75®” microturbine includes a turbine  114 , a compressor  112 , and a two-pole permanent magnetic generator rotor  134  mounted on a single high-speed shaft  118  via a number of air bearings. The “Parallon 75®” microturbine may generate about 75 kilowatts (75 kW) of electricity. Those skilled in the art will appreciate that other configurations and components for microturbines exist or may be developed. The compressor  112 , the turbine  114  and the generator rotor  134  can be rotated by a single shaft  118  as shown, or can be mounted to separate shafts.  
         [0014]    Air entering an inlet of the compressor  112  is compressed. Compressed air leaving an outlet of the compressor  112  is circulated through cold side passages  120  in a cold side of a recuperator  122 . In the recuperator  122 , the compressed air absorbs heat, which enhances combustion. The heated, compressed air leaving the cold side of the recuperator  122  is supplied to a combustor  24 .  
         [0015]    Fuel is also supplied to the combustor  124 . Both gaseous and liquid fuels can be used. In gaseous fuel mode, any suitable gaseous fuel can be used. Choices of fuel include diesel, flare gas, off gas, gasoline, naphtha, propane, JP-8, methane, natural gas and other man-made gases. The flow of fuel is controlled by a flow control valve  126 . The fuel is injected into the combustor  124  by an injection nozzle  128 .  
         [0016]    Inside the combustor  124  the fuel and compressed air are mixed and ignited by an igniter  127  in an exothermic reaction. The combustor  124  may contain a suitable catalyst capable of combusting the compressed, high temperature, fuel-air mixture at the process conditions. Some known catalysts usable in the combustor  124  include platinum, palladium, as well as metal oxide catalyst with active nickel and cobalt elements.  
         [0017]    After combustion, hot, expanding gases resulting from the combustion are directed to an inlet nozzle  130  of the turbine  114 . The inlet nozzle  130  has a fixed geometry. The hot, expanding gases resulting from the combustion are expanded through the turbine  114 , thereby creating turbine power. The turbine power, in turn, drives the compressor  112  and the electrical generator  116 .  
         [0018]    Turbine exhaust gas is circulated by hot side passages  132  in a hot side of the recuperator  122 . Inside the recuperator  122 , heat from the turbine exhaust gas on the hot side is transferred to the compressed air on the cold side. In this manner, some heat of combustion is recuperated and used to raise the temperature of the compressed air en route to the combustor  124 . After surrendering part of its heat, the gas exits the recuperator  122 . Additional heat recovery stages could be added onto the microturbine  110 .  
         [0019]    The generator  116  can be, for example, a ring-wound, two-pole toothless (TPTL) brushless permanent magnet machine having a permanent magnet rotor  134  and stator windings  136 . The turbine power generated by the rotating turbine  114  is used to rotate the rotor  134 . The rotor  134  is attached to the shaft  118 . When the rotor  134  is rotated by the turbine power, an alternating current is induced in the stator windings  136 . Speed of the turbine can be varied in accordance with external energy demands placed on the microturbine  110 . Variations in the turbine speed will produce a variation in the frequency of the alternating current generated by the electrical generator  116 . Regardless of the frequency of the ac power generated by the electrical generator  116 , the ac power can be rectified to dc power by a rectifier  138 , and then processed by a solid-state electronic inverter  140  to produce ac power having a fixed frequency. Accordingly, when less power is required, the turbine speed can be reduced without affecting the frequency of the ac output.  
         [0020]    Moreover, reducing the turbine speed reduces the airflow because the compressor runs slower. Consequently, the turbine inlet temperature remains essentially constant, thus maintaining a high efficiency at partial load. Use of the rectifier  138  and the inverter  140  allows for wide flexibility in determining the electric utility service to be provided by the microturbine  110  of the present invention. Because any inverter  140  can be selected, frequency of the ac power can be selected by the consumer. If there is a direct use for ac power at wild frequencies, the rectifier  138  and inverter  140  can be eliminated.  
         [0021]    The microturbine  110  also can include one or more batteries  146  for providing additional storage and backup power. When used in combination with the inverter  140 , the combination can provide uninterruptible power for hours after generator failure. Additionally, the controller  142  causes the battery  146  to supply a load when a load increase is demanded. The battery  146  can be sized to handle peak load demand on the microturbine  110 .  
         [0022]    During operation of the microturbine  110 , heat is generated in the electrical generator  116  due to inefficiencies in generator design. In order to extend the life of the electrical generator  116 , as well as to capture useful heat, compressor inlet air flows over the generator  116  and absorbs excess heat from the generator  116 . The rectifier  138  and the inverter  140  also can be placed in the air stream. After the air has absorbed heat from the aforementioned sources, it is compressed in the compressor  112  and further pre-heated in the recuperator  122 .  
         [0023]    The controller  142  controls the turbine speed by controlling the amount of fuel flowing to the combustor  124 . The controller  142  may use sensor signals generated by a sensor group  144  to determine the external demands upon the microturbine  110 . The sensor group  144  could include sensors such as position sensors, turbine speed sensors and various temperature and pressure sensors for measuring operating temperatures and pressures in the microturbine  110 .  
         [0024]    A switch/starter control  148  can be provided off-skid to start the microturbine  110 . Rotation of the compressor  112  can be started by using the generator  116  as a motor. During startup, the switch/starter control  48  supplies an excitation current to the stator windings  136  of the electrical generator  116 . Startup power is supplied by the battery  146 . The controller  142  allows the system to start-up with a single black-start battery  146 , using an algorithm or algorithms based on system efficiency and load requirement to control the number of microturbines  110  running and the power output therefrom. In the alternative, a compressed air device could be used to motor the microturbine  110 .  
         [0025]    [0025]FIG. 2 is a block diagram illustrating a multi-unit power generation system  270  in accordance with certain embodiments of the present invention. As shown, the multi-unit power generation system  270  includes multiple generators  110  connected in parallel, a control system  220  and a switching system  260 . The generators  110  and the switching system  260  are controlled by the control system  220 . All communications to the generators  110  originate from the control system  220 . The control system  220  receives information regarding circuit breaker positions, current and voltage from the switching system  260  for both the utility grid and site load.  
         [0026]    The control system  220  may include a processor or other logic circuitry for executing system control algorithms and operating system software. The control system may also include memory, such as Flash memory, RAM, and non-volatile memory for storing software modules, computational variables, fault information, installation specific information and user programmable information. The control system  220  is configured to generate commands that are received and processed by the controllers  142  of the individual generators  110 .  
         [0027]    The multi-unit power generation system  270  may include two or more generators  110 . The number of generator  110  in the system may be limited by the physical limitations of the switching system  260  and/or cost efficiency. In other words, the cost of supplying a certain number of generators  110  to meet a certain power demand may exceed the cost of obtaining all needed power from the utility grid. The output from each generator  110  is supplied to an output isolation transformer  232 . The output isolation transformers  232  convert generator output voltage to the site operating voltage. The output isolation transformers  232  also isolate and protect the generators  110  from high voltage, maintaining low voltage at microturbine interfaces. Thus, it is preferred that one isolation transformer  232  be provided per each generator  110 . A distribution panel  233  may be used to connect the output of the isolation transformers  232  to the switching system  260 .  
         [0028]    The switching system  260  is connected to the combined output of the multiple generators  110  (e.g., through a distribution panel  233 ) and to the utility grid. The switching system  260  includes a utility circuit breaker  242  and a generator circuit breaker  240 , which are used to transfer the load between the utility grid and the multiple generators  110 . A power meter  262  may be incorporated into the switching system  260  in order to monitor the power output of the multiple generators  110 . Current, power and voltage sensors (e.g., transformers) may also be contained within the switching system  260  to measure load and grid conditions.  
         [0029]    In addition to receiving feedback signals from the switching system  260 , the control system  220  may also be configured to receive input signals from various other devices and interfaces. As an example, a local operator I/O bus  214  may be provided for interfacing with local input/output devices and controls. As another example, a remote operator I/O bus  216  may be provided for interfacing with remote input/output devices and controls located anywhere in the world. Remote input/output devices and controls may communicate with the remote operator I/O bus  216  via dedicated or shared communication links. The control system  220  analyzes input signals coming from local and remote locations and makes decisions concerning the operation of the generators  110  based on how the provided information impacts the operating and fault mode strategies programmed into the controller  142 .  
         [0030]    Other examples of I/O buses that may be provided include a utility manager I/O bus  210 , a site manager I/O bus  212 , and a safety monitor I/O bus  218 . The utility manager I/O bus  210  may be provided, for example, to allow input of an enabling signal for the coordination with utility interfaces. The control system  220  may interpret such an enabling signal such that unless the signal indicates the grid to be in an enabled state, the multi-unit power generation system  270  will not be connected to the grid or if connected will be immediately disconnected from the grid. The site manager I/O bus  212 , for example, may provide for input of a signal that controls the starting and stopping of the multi-unit power generation system  270 . The safety monitor I/O bus  218  may be provided for input of one or more alarm signals. In certain embodiments, the control system  220  may be configured to respond to alarm input signals, such as a fire alarm signal that may be processed by the control system  220  as an emergency power off situation, a gas detection signal that may be processed as a normal shutdown situation, and a site entry signal that may be processed as a warning situation.  
         [0031]    For stand alone operations, the control system  220  designates one of the generators  110  as a master and the other generators  110  as slaves. The control system  220  facilitates start-up of the master generator  110  with the generator circuit breaker  240  being open. Start-up of the slave generators  110  may be staggered so as to reduce total run time on individual units and to reduce the total start-up power requirements and fuel consumption of the multi-unit power generation system  270 . The control system  220  starts sufficient generators  110  to support a preprogrammed load level. The master generator  110  produces a synchronization signal that is transmitted to the slave generators  110  via a synchronization signal bus  252 . The slave generators  110  synchronize to the frequency indicated by the synchronization signal. In this manner, all operational generators  110  share the load equally. Even if a power failures occur in the master generator  110 , the master generator  110  continues to transmit synchronization signals to all operational slave generators  110 . Any failed generators  110  will be logged as a fault to the control system  220  and will be bypassed during the next start sequence. Generator failures will also alert operators that the failed generators  110  require maintenance.  
         [0032]    Once all the generators  110  are running and ready for load, the control system  220  commands the switching system  260  to close the generator circuit breaker  240 . When the generator circuit breaker  240  is closed, the generators  110  assume the load with each machine sharing the load equally. The output of the multi-unit power generation system  270  follows the site demand. However, the site demand cannot require power in excess of the preprogrammed load level, even for startup inrush. If additional generators  110  are available for the site, they may be started-up automatically via sensors in the control system  220  as the total output approaches the capability of the operating generators  110 . This process will implement a “spinning reserve” capability. If the load demand exceeds the preprogrammed load level, the multi-unit power generation system  270  will shut down.  
         [0033]    In a grid-connected mode of operation, the control system  220  will synchronize the generators  110  with the grid frequency and phase before power is delivered from the generators  110 . In grid-connected mode, the control system  220  may control the multi-unit power generation system  270  to provide a programmed load level or peak shaving load levels. In grid-connected mode the generators  110  are loaded in sequence until each reaches its maximum power level. If the desired power level is greater than the total power output capability of the multi-unit power generation system  270 , the system will deliver the maximum power possible.  
         [0034]    The control system  220  may be configured to provide automatic transition between grid-connected mode and stand-alone mode for the multi-unit power generation system  270 . The automatic transition feature may be used to provide back-up power for a facility when the generators  110  are running in grid-connected mode and a loss of grid or grid fault occurs. When the control system  220  senses a grid fault, it commands the switching system  260  to open the utility circuit breaker  242  and the generator circuit beaker  240  and commands all the generators  110  that are running to shift to stand-alone mode and all generators  110  that are not running to start in stand-alone mode. Once all the generators  110  are on and are ready for load, the control system  220  commands the switching system  260  to close the generator circuit breaker  240 . When the grid returns and remains stable for a predetermined time interval, the control system  220  commands the switching system to open the generator circuit breaker  240  and close the utility circuit breaker  242 .  
         [0035]    Based on the foregoing, it will be appreciated that the present invention relates to a multi-unit power generation system that includes multiple generators working in parallel to provide stand-alone and/or grid-connected power for facilities of all sizes. Various methods for implementing such a multi-unit power generation system in accordance with the present invention have been described herein by way of example only. Many other modifications, features, embodiments and operating environments of the present invention were described above by way of example only and are, therefore, not intended as required or essential elements of the invention. It should be understood, therefore, that the foregoing relates only to certain embodiments of the invention, and that numerous changes may be made therein without departing from the spirit and scope of the invention as defined by the following claims.