Patent Publication Number: US-2007100506-A1

Title: System and method for controlling power flow of electric power generation system

Description:
BACKGROUND  
      The invention relates generally to a system for controlling power flow of an electric power generation system, and particularly to a system and method for controlling power flow of a power generation system.  
      Power generation systems comprising a power converter constitute a higher share of the overall power generation equipment. Power generation systems comprising a power converter include wind turbines, gas turbines, solar generation systems, hydro-power systems or fuel cells. Power generation systems typically complement conventional power generation equipment such as diesel generators or large turbo generators directly coupled to the grid without a solid-state power conversion stage.  
      Power converters coupled to the power generation equipment typically have integrated dissipative elements, which serve protective functions. These dissipative elements dissipate energy out of the electrical system, typically by a conversion into thermal energy. For example, dissipative loads connected to the power converter in wind turbines protect the power conversion stage and the generator during grid failures. During normal operation these dissipative loads remain unused.  
      A power imbalance in an alternating current (AC) utility system results in a frequency and/or voltage deviation from the nominal values or frequencies and voltages outside a prescribed tolerance band. If voltages and/or frequencies of the utility system are outside the prescribed tolerance band, load equipments and generation equipments may be damaged. For example, tolerance bands for voltages may be in the range of +/−10% of a nominal voltage value, although higher values may be permitted depending on the utility system. Similarly, for example tolerance band for frequencies may be in the range of +/−5% of a nominal frequency value.  
      Specifically in smaller grids, which are not coupled to a large utility system, (also referred as “islanded grids”), power demand and power production need to be matched to provide stability to the grid. In the islanded grids with power generation equipment comprising a power converter often presenting a larger share of the total generation system, sudden load changes, such as load shedding, may result in a transient voltage and frequency that is outside the tolerance band. This is due to the fact that both conventional power generation equipment (for example, diesel generators) or alternative power generation equipment such as wind turbines, fuel cells, or the like are too slow in adjusting the power generation instantaneously. Furthermore, sudden load variations put additional stress on all rotating power generation units in the grid leading to pre-mature failure of generators, bearings and gears.  
      Accordingly, there is a need for a technique that enables a faster control of the electric power balance of an electric power generation system. In addition, a system that enables control of the electric output power of a power generation system is also desirable.  
     BRIEF DESCRIPTION  
      In accordance with one aspect of the present embodiment, a method for controlling power flow of an electric power generation system is provided. The method includes generating or dissipating electric power in power generation equipment comprising a converter to maintain a predetermined grid voltage and frequency. The electric power is transferred to or received from a grid; and the current and voltage of the electric power thus transmitted are sensed. The frequency of the grid is determined based on the sensed current or voltage. A grid-side converter is then controlled to regulate the voltage and frequency of the electric grid via scheduling the power flow to a compensating circuit when the sensed voltage falls outside a predetermined voltage range or the determined frequency falls outside a predetermined frequency range.  
      In accordance with another aspect of the present embodiment, a method for controlling power flow of an electric power generation system is provided. The method includes generating or dissipating electric power to maintain a predetermined grid voltage and frequency. The electric power is transmitted to or received from a grid; and the current and voltage of the electric power thus transmitted are sensed. The frequency of electric power transmitted to the grid is determined based on the sensed current or voltage. A grid-side converter is then controlled to regulate voltage and frequency of the electric grid by reverting power flow in a power generator, when the sensed voltage falls outside a predetermined voltage range or the determined frequency falls outside a predetermined frequency range.  
      In accordance with another aspect of the present embodiment; a system for controlling power flow of an electric power generation system is provided. The system includes a grid-side converter configured to inject or receive electric power at predetermined voltage and frequency to a grid. A current sensor is communicatively coupled to the grid and configured to detect the current at a pre-determined location in the grid. A voltage sensor is communicatively coupled to the grid and configured to detect voltage at a pre-determined location in the grid. A control circuit is configured to determine frequency of electric power transmitted to the grid based on detected current or voltage in the grid. The control circuit is also configured to control the grid-side converter to regulate the voltage and frequency of the grid via scheduling a power flow to the compensating circuit, when the sensed voltage falls outside a predetermined voltage range or the determined frequency falls outside a predetermined frequency range.  
      In accordance with another aspect of the present embodiment; a system for controlling power flow of an electric power generation system is provided. The system includes a grid-side converter configured to inject or receive electric power at predetermined voltage and frequency and transmit the electric power to a grid. A current sensor is communicatively coupled to the grid and configured to detect the current at a predetermined location in the grid. A voltage sensor is communicatively coupled to the grid and configured to detect voltage at a predetermined location in the grid. A control circuit is configured to determine frequency of electric power transmitted to the grid based on detected current or voltage in the grid. The control circuit is also configured to control the grid-side converter to regulate the voltage and frequency of the grid by reverting power flow in a power generator when the sensed voltage falls outside a predetermined voltage range or the determined frequency falls outside a predetermined frequency range. 
    
    
     DRAWINGS  
      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:  
       FIG. 1  is a diagrammatical view of a power generation system in accordance with an exemplary aspect of the present embodiment;  
       FIG. 2  is a diagrammatical view of a wind power generation system having a plurality of wind turbines within a wind farm in accordance with an exemplary aspect of the present embodiment;  
       FIG. 3  is a diagrammatical view of a grid stability control system in accordance with an exemplary aspect of the present embodiment;  
       FIG. 4  is a further diagrammatical view of a grid stability control system in accordance with aspects of  FIG. 2 ; and  
       FIG. 5  is a flow chart illustrating exemplary steps involved in controlling grid stability of a power generation system in accordance with an exemplary aspect of the present embodiment.  
    
    
     DETAILED DESCRIPTION  
      As discussed in detail below, aspects of the present embodiment provide a system and method for regulating voltage and frequency of power transmitted to a grid during load fluctuations, so as to control the net output power of a power generation system. In the embodiments illustrated, the power generation system includes a compensating circuit provided within the power generation system. Specific embodiments of the present technique are discussed below referring generally to  FIGS. 1-5 .  
      Referring to  FIG. 1 , a power generation system is illustrated, and represented generally by reference numeral  10 . In the illustrated embodiment, the power generation system  10  includes a power generator  11  having a wind turbine generation system  13  or a hydro power system  15  or gas turbine system  17  or a fuel cell system  19 , or a solar power system  21  or a combination thereof adapted to collectively supply electrical power to a grid  20 . The power generator  11  produces an electrical output  23 .  
      In the illustrated embodiment, a plurality of auxiliary power sources such as a diesel generator  38 , a fuel cell  40 , a gas turbine  41 , a hydro power generator  45 , or the like are provided to supply electric power to the grid  20 . Prescribed power output levels to the grid  20  may be based on power ramp-up/ramp-down capabilities of auxiliary power sources conjointly supplying power to the grid  20 .  
      In the illustrated embodiment, the system  10  includes a grid-side power converter  42  coupled to the power generator  11 . The converter  42  is configured to convert the power transmitted from the power generator  11  and transmit the power to the grid  20 . As appreciated by those skilled in the art, the converter  42  may include a single-phase inverter, a multi-phase inverter, or a multi-level inverter, or a parallel configuration or a combination thereof. In the illustrated embodiment, although one grid  20  is illustrated, the system  10  may supply power to a plurality of grids, or more generally, to various loads. Similarly, in certain other embodiments, a plurality of power converters may be used to convert DC power signals to AC power signals and transmit the signals to the grid  20 .  
      The system  10  includes a grid stability control system  43  adapted to control voltage and/or frequency of the electric power grid by the power injected into or received from the grid  20 . The grid stability control system  43  includes a sensing circuitry  44  having a current sensor  46  and a voltage sensor  48  communicatively coupled to the grid  20 . A control circuit  50  is configured to receive current and voltage signals from the current sensor  46  and the voltage sensor  48 , and to determine frequency and power flows of the grid  20  based on the detected current and/or voltage detected at the grid  20  in any suitable manner generally known to those skilled in the art.  
      The control circuit  50  may include a processor having hardware circuitry and/or software that facilitate the processing of signals from the sensing circuitry  44  and calculation of frequency of the grid  20 . As will be appreciated by those skilled in the art, the processor  36  includes a range of circuitry types, such as a microprocessor, a programmable logic controller, a logic module, as well as supporting circuitry, such as memory devices, signal interfaces, input/output modules, and so forth.  
      In an exemplary embodiment, a compensating circuit  52  having a dump load resistor  54  and a dump load capacitor  56  is integrated into the power generator  11 . Compensating circuit  52  is adapted to dissipate electric power. When the detected frequency of the grid  20  is outside a predetermined frequency range, the control circuit  50  actuates the power converter  42  to generate a reverse power flow from the grid  20  to the power generators. The excess power is dissipated via the dump load resistor  54 . The excess power may be temporarily stored in the dump load capacitor  56 . Thereby, the instantaneous difference between the power demand and power generated is balanced. For example, during short-term load fluctuating conditions, the compensating circuit  52  dissipates the excess electric power to stabilize the voltage and frequency of electric power at the grid  20  without adjusting the power generation or the generation of the auxiliary power generation system. Especially during low wind conditions, the full capacity of the power converter  42  is available for load regulation purposes. In the illustrated embodiment, there is an added advantage that the presence of the compensating circuit  52  is also required to stop the generator in case of an emergency for example, in permanent magnet generators.  
      Referring now to  FIG. 2 , a wind power generation system  13  is illustrated. In the illustrated embodiment, the wind power generation system  13  includes a wind farm  12  having a plurality of wind turbine generators  14 ,  16 ,  18  adapted to collectively supply electrical power to a grid  20 . The wind turbine generators  14 ,  16 ,  18  include bladed rotors  22 ,  24  and  26  respectively that transform the energy of wind into a rotational motion which is utilized to drive electrical generators drivingly coupled to the rotors  22 ,  24 ,  26  to produce electrical outputs  28 ,  30  and  32 .  
      In the illustrated embodiment, power outputs of individual wind turbine generators are coupled to a low or medium voltage ac or dc distribution network  34  to produce a collective wind farm power output  36 . As appreciated by those skilled in the art, the distribution network  34  is preferably a dc network. The power output may be stepped up in voltage by a transformer (not shown) before being supplied to the grid  20 . The collective power output  36  may vary significantly based on wind conditions experienced by individual wind turbine generators. Embodiments of the present technique function to control the net power output transmitted to the grid  20  to a level acceptable by the grid  20 , without necessarily curtailing the total power output  36  of the wind farm  12 .  
      In the illustrated embodiment, the system  10  includes the grid-side power converter  42  coupled to the network  34 . The converter  42  is configured to convert the power transmitted from the network  34  and transmit the power to the grid  20 . If the network  34  is an ac network, an ac-to-ac converter is required. The system  10  includes the grid stability control system  43  adapted to control voltage and/or frequency of the grid via the electric power injected into or received from the grid  20 . The grid stability control system  43  includes the compensating circuit  52  having the dump load resistor  54  and the dump load capacitor  56  integrated into at least one of the wind turbine generators  14 ,  16 ,  18 , or located centrally closer to the power converter  42 . The function of the grid stability control system  43  is similar to as described above.  
      Referring to  FIG. 3 , this figure illustrates the grid stability control system  43 . Referring generally to  FIG. 3 , the wind turbine system includes a turbine portion  58  that is adapted to convert the mechanical energy of the wind into a rotational torque (TAero) and a generator portion  60  that is adapted to convert the rotational torque produced by the turbine portion  58  into electrical power. A drive train  62  is provided to couple the turbine portion  32  to the generator portion  34 .  
      The turbine portion  58  includes the rotor  22  and a turbine rotor shaft  64  coupled to the rotor  22 . Rotational torque is transmitted from the rotor shaft  64  to a generator shaft  66  via the drive train  62 . In certain embodiments, such as the embodiment illustrated in  FIG. 3 , the drive train  62  includes a gear box  68  configured to transmit torque from a low speed shaft  70  coupled to the rotor shaft  64  to a high speed shaft  72  coupled to the generator shaft  66 . The generator shaft  66  is coupled to the rotor of an electrical generator  74 . As the speed of the turbine rotor  22  fluctuates, the frequency of the output power of the generator  74  also varies. The generator  74  produces an air gap torque, also referred to as generator torque (TGen), which opposes the aerodynamic torque (TAero) of the turbine rotor  22 .  
      As discussed above, the grid stability control system  43  is adapted to control voltage and frequency of the grid via the electric power transmitted to the grid  20 . The sensing circuitry  44  is configured to detect current and voltage transmitted to the grid  20 . The control circuit  50  is configured to receive current and voltage signals from the sensing circuitry  44  and to determine frequency of electric power transmitted to the grid  20  based on the detected current and/or voltage detected at the grid  20 .  
      The compensating circuit  52  is integrated into the converter  42  and adapted to dissipate electric power. In one example, when the detected voltage exceeds a predetermined voltage and/or the detected frequency of electric power at the grid  20  exceeds a predetermined frequency, the control circuit  50  actuates the power converter  42  to generate a reverse power flow from the grid  20  to the wind generators. The predetermined frequency may be a threshold frequency or a nominal frequency as appreciated by those skilled in the art. The excess power is dissipated via the compensating circuit  52 .  
      Referring to  FIG. 4 , a grid stability control system  43  in accordance with aspects of  FIG. 3  is illustrated. In the illustrated embodiment, the converter  42  is configured to convert the AC power signal transmitted from the power source to another AC power signal, and to transmit the resulting AC signal to the grid  20 . The control circuit  50  is configured to receive current and voltage signals from the sensing circuitry  44 , and to determine frequency of electric power transmitted to the grid  20  based on the detected current and/or voltage.  
      The control circuit  50  may further include a database  76 , an algorithm  78 , and a processor  80 . The database  76  may be configured to store predefined information about the power generation system. For example, the database  76  may store information relating to the number of wind power generators, power output of each wind power generator, number of auxiliary power sources, power output of each auxiliary power source, power demand, power generated, wind speed, or the like. Furthermore, the database  76  may be configured to store actual sensed/detected information from the above-mentioned current and voltage sensors, as well as frequency data. The algorithm  78 , which will typically be stored as an executable program in appropriate memory, facilitates the processing of signals from the above-mentioned current and voltage sensors (e.g., for the calculation of frequency).  
      The processor  80  may include a range of circuitry types, such as a microprocessor, a programmable logic controller, a logic module, or the like. The processor  80  in combination with the algorithm  78  may be used to perform the various computational operations relating to determination of the voltage, current and frequency of electric power transmitted to the grid  20 . In certain embodiments, the control circuit  50  may output data to a user interface (not shown). The user interface facilitates inputs from a user to the control circuit  50  and provides a mechanism through which a user can manipulate data and sensed properties from the control circuit  50 . As will be appreciated by those skilled in the art, the user interface may include a command line interface, menu driven interface, and graphical user interface.  
      In the illustrated embodiment, when the detected frequency of electric power at the grid  20  is outside a predetermined frequency range, the control circuit  50  actuates the converter  42  to generate a reverse a power flow from the grid  20  to the wind generators. In an exemplary implementation, a dump load control circuit  82  of the compensating circuit is triggered, facilitating dissipation of the excess power via the dump load resistor  54 . In another embodiment, when the detected frequency of electric power at the grid  20  exceeds a predetermined frequency, the control circuit  50  actuates the converter  42  to generate a reverse power flow from the grid  20  to the wind generators, and the wind generators are effectively operated as a load to dissipate energy. Thereby, excess power is dissipated, and the power and frequency of electric power of the grid is regulated. In yet another embodiment, when the detected frequency is below the predetermined frequency, larger amount of power is supplied to the grid  20 .  
      Referring to  FIG. 5 , a flow chart illustrating exemplary steps involved in controlling grid stability of a wind power generation system is illustrated. The method includes collectively supplying electrical power to a grid via a plurality of wind generators, as represented by step  84 . The wind turbine generators transform the energy of wind into a rotational motion, which is utilized to drive electrical generators. Electric power is also supplied to the grid via plurality of auxiliary power sources. As will be appreciated by those skilled in the art, such “auxiliary power sources” may, in fact, be the primary power supply resources of the grid, and may include fossil fuel-based power plants, nuclear power plants, hydroelectric power plants, geothermal power plants, and so forth.  
      Voltage and frequency of electric power transmitted to the grid or at a pre-determined location in the grid are detected, as represented by step  86 . In particular, in the presently contemplated embodiment, a separate current sensor detects current transmitted to the grid, and a voltage sensor detects voltage transmitted to the grid. The control circuit receives current and voltage signals from the current sensor and the voltage sensor, and determines frequency of electric power transmitted to the grid based on the detected current and/or voltage. The detected voltage is then compared with a predetermined voltage, and the detected frequency of electric power is compared with a predetermined frequency, as represented by step  88 . When the detected voltage falls outside a predetermined voltage range and/or the detected frequency of electric power at the grid  20  falls outside a predetermined frequency range, the control circuit  50  actuates the power converter  42  to generate a reverse power flow from the grid  20  to the wind generators. In the illustrated exemplary embodiment, when the detected voltage exceeds the predetermined voltage (or exceeds the predetermined voltage by a certain amount and/or for a certain period of time), and/or detected frequency of electric power at the grid exceeds the predetermined frequency (or more generally, when a difference between the frequencies exceeds a tolerance), the control circuit actuates the power converter to generate a reverse power flow from the grid to the wind generators, as represented by step  90 . The excess power is dissipated via the dump load resistor  54 , as represented by step  92 . Thereby, the instantaneous difference between the power demand and power generated is balanced. The power and frequency of electric power transmitted to the grid is regulated by dissipating excess power as represented by step  94 . As noted above, in certain embodiments, the instantaneous difference between the power demand and power generated may be balanced by generating a reverse power flow from the grid to the wind generators, effectively operating the wind generators as motors to drive other utility devices.  
      When the detected voltage and/or detected frequency are within the desired ranges, the cycle is repeated as described above. That is, normal production and supply of power from the wind turbine may be resumed. The above mentioned steps are also equally applicable to wind power generation systems having a plurality of wind generators supplying electric power to separate grids. Depending on the load conditions, some wind turbines may be required to supply or to consume electric power while the remaining wind generators may not be required to supply or consume electric power. Thus, as will be appreciated by those skilled in the art, the compensating circuits of the wind generators not required to supply electric power may be operated as load sinks to dissipate excess power while the remaining wind generators are operated at optimum operating conditions. The resulting control scheme facilitates stabilization of the voltage and frequency of electric power at the grid. Although in the illustrated embodiment, the control scheme is described with respect to wind turbine, in certain other embodiments, aspects of the present embodiment may be equally applicable to other power generators.  
      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.