Abstract:
By controlling a power source in response to a load power demand, more efficient operation is obtained. Faster response is also realized by incorporating a compensation for inertia in the path between the power source and the load. In one exemplary embodiment, a further-driven system for electric power generation is controlled according to increased values of a load power demand and a turbine output shaft speed.

Description:
RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 09,645,567, filed Aug. 25, 2000, now U.S. Pat. No. 6,380,639 B1 on Jun. 12, 2002 the entirety of which is incorporated herein by reference. 
     This application claims the benefit of U.S. Provisional Application No. 60,203,588, filed May 11, 2000 , and entitled “SYSTEM, METHOD AND APPARATUS FOR POWER REGULATION”. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to power regulation. More specifically, this invention relates to control of a power source based at least in part on a power demand by a load. 
     2. Description of Related Art 
     FIGS. 1 and 2 show two conventional systems for power generation. In FIG. 1, an engine produces mechanical power that is inputted to a generator. For example, the engine may rotate a shaft that is coupled to the generator (possibly through a gearbox or transmission). The generator converts the mechanical power to electrical power that supplies a load. In response to an error between a predetermined engine speed output, and an actual engine output speed as measured by a speed sensor, a governor provides a command to a fuel supply controller, which controls the supply of fuel to the engine. Because the control of the engine is based upon the engine output speed, this type of control system may be called a speed reactive system. 
     A generator as shown in the system of FIG. 1 may produce electricity in either an alternating current (a.c.) or direct current (d.c.) form, depending on the requirements of the load. If a.c. is required, then the frequency of the current supplied by the generator must typically be regulated to within a small margin in order to avoid damage to the load. In the United States and Canada, e.g., a.c. current is typically maintained at 60 Hz. Because the rotational speed of the generator rotor determines the frequency of the electrical power produced, it is essential that the speed of the engine be regulated to a constant value. 
     Even if the load requires d.c. from the generator (possibly supplied via a rectifier or inverter), it may also be important to regulate the speed of the engine. In this case, the voltage of the d.c. output by the generator depends upon the speed of the engine. Although small variations in output voltage (i.e., ripples) maybe filtered out, avoiding damage to the load will typically require that an average voltage output of the generator (and hence an average speed of the engine) be kept constant. That is, even though the predetermined engine speed for a system according to FIG. 1 may yield good results over a broad range of conditions with respect to some criterion (e.g., minimal fuel consumption), one single value will typically be sub-optimal with respect to changes in such variables as pressure, temperature, and load power demand. Because fuel costs may account for 80% of the operating costs of such a system, increasing efficiency by even a few percent may result in a considerable cost savings. 
     Another shortcoming of speed reactive systems is that they operate without any knowledge of the power actually demanded by the load. Because the system reacts only to the engine output speed, its response is slowed by the inertia of the moving components in the power generation path, as no command with respect to speed may be met until the components are accumulated or decelerated as necessary. 
     FIG. 2 shows another system for engine speed regulation that is used, for example, in propeller-driven aircraft. In this system, it is also desirable to maintain engine output at a constant speed. An operator varies the power supplied by the engine to the load by commanding the final supply controller to adjust the supply of fuel to the engine. In order to maintain a constant engine output speed another controller varies the characteristics of the load. In response to a speed increase (possibly due to a command to increase power), for example, the controller may vary the pitch of the propeller, thereby increasing torque and maintaining a constant engine speed. For a given output speed, however, an engine will typically perform optimally (for example, with respect to fuel consumption) at only one particular power output value. Therefore, a control system as shown in FIG. 2 may also perform sub-optimally during much of its operation. 
     SUMMARY 
     In a system, method, and apparatus for power regulation as disclosed herein, a load power demand value determines a power supply behavior that is optimal with respect to some criterion (e.g., fuel consumption). Such a system, method and apparatus also complies with load demand requirements more quickly and efficiently than existing approaches. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a speed reactive system; 
     FIG. 2 shows a conventional power generation system; 
     FIG. 3 shows a block diagram for a system according to an embodiment of the invention; 
     FIG. 4 shows a block diagram for a system according to an exemplary embodiment of the invention; 
     FIG. 5 shows a system according to an exemplary embodiment of the invention; 
     FIG. 6 shows a system according to an embodiment of the invention; 
     FIG. 7 shows a block diagram for a system according to an embodiment of the invention; 
     FIG. 8 shows a block diagram for a system according to an embodiment of the invention; 
     FIG. 9 shows a block diagram for a system according to an embodiment of the invention; 
     FIG. 10 shows a block diagram for a system according to an embodiment of the invention; 
     FIG. 11 shows a block diagram for an apparatus according to an embodiment of the invention; and 
     FIG. 12 shows a block diagram for an apparatus according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     As shown in FIG. 3, system  100  according to an embodiment of the invention includes a power source  110  which supplies power through power converter  120  to drive a load  800 . Demand sensor  180  senses the power consumed by the load. Value converter  130  receives the observed power consumption value and projects the power output that power source  110  must produce in order to meet the load demand. This power supply requirement value is outputted to lookup tables  140  and  170 . 
     Lookup table  140  maps the power supply requirement value to a power demand value. A value based at least on part on this power demand value is inputted to fuel supply controller  160 , which controls the supply of fuel to power source  110  accordingly. Lookup table  170  maps the power supply requirement value to a rate target value. After an observed rate value as indicated by supply rate sensor  190  is subtracted from the rate target value, the resulting rate error value is inputted to stabilizer  150 , which produces an inertia compensation value. The power demand value is modified by this inertia compensation value, and the modified power demand value is inputted to fuel supply controller  160 . 
     Power source  110  receives fuel via fuel supply controller  160  and produces power. This power, which is inputted to power converter  120 , may be produced as mechanical energy, as thermal energy or as some other form of energy. Power source  110  may be an engine  210 , as shown in FIG. 4, such as a diesel engine. In an exemplary implementation, as shown in FIG. 5, power source  110  is a gas turbine  212  which consumes a fossil fuel (e.g., diesel fuel) received via fuel supply controller  160  and which produces mechanical power by turning a drive shaft. In a particular implementation, the operation of gas turbine  212  is characterized by a speed of a compressor and a speed of a turbine output shaft, which need not be linked except by thermodynamic pressure. Turbine  212  may have several compressors and/or output shafts, and turbine  212  may also be of another type such as a vapor or steam turbine. 
     Power converter  120  converts the power received from power source  110  into a form as required by load  400 . For example, this conversion may be from thermal to electrical power, or from electrical to mechanical power, or from thermal to mechanical power. In one implementation of the invention, power converter  120  comprises a generator that receives mechanical power outputted by power source  110  (e.g., by turning a drive shaft) and converts it into electrical power. In an exemplary implementation, power converter  120  comprises two or more generators and a gearbox that receives the mechanical power and distributes it among the generators in predetermined proportion. Power converter  120  may also include rectifiers, inverters, and/or filters to produce an output in the form required by load  800 . In the exemplary implementations of FIGS. 4 and 5, electrical load  810  may comprise one or more motors which propel a fuel-powered locomotive that incorporates a system as described herein. 
     Demand sensor  180  outputs an observed power consumption value that relates to the power demand of load  800 . This demand value may be expressed, for example, as watts or Joules. In the exemplary implementations of FIGS. 4 and 5, demand sensor  180  outputs a signal that relates to a level of electrical power demanded by electrical load  810 . For example, demand sensor  180  may include one or more voltage sensors and one or more current sensors, wherein a power demand value corresponding to a particular component of electrical load  810  is obtained by multiplying together corresponding voltage and current values. Values may be sensed or outputted by demand sensor  180  according to a given digital sampling rate, for example, 50 Hz. 
     In a case where load  800  consumes thermal power (i.e., for heating and/or for cooling), demand sensor  180  may produce a value relating to a difference between a temperature of a component of load  800  and an ambient or a desired temperature. As above, the sensing and/or the output of values by the demand sensor  180  may be either analog or digital. 
     Value converter  130  receives the observed power consumption value from demand sensor  180  and produces a power supply requirement value in response. The power supply requirement value represents a projection of the power that power source  110  must supply in order to meet the load demand. In one implementation, value converter  130  produces the power supply requirement value by applying an efficiency reciprocal function to the observed power consumption value, wherein the efficiency reciprocal function represents the inverse of the efficiency of conversion of power converter  120 . In this way, an estimate of the level of power output by power source  110  that will produce the load power demand at the output of energy converter  120  may be obtained. In a case where the efficiency reciprocal function may be expressed as a constant factor (i.e., the relationship between power inputted to and power outputted by power converter  120  is linear), value converter  130  may be constructed as a constant multiplier. In another case (e.g., for a more complex relationship), value converter  130  may comprise one or more lookup tables whereby a particular demand level may be interpolated to a corresponding power supply requirement. In the exemplary implementations of FIGS. 4 and 5, value converter  130  may compensate for the efficiency of one or more generators, gear boxes, rectifiers, inverters, and/or other components that may appear in the power path between the output of power source  110  and the input of electrical load  810 . 
     Lookup table  170  receives the power supply requirement value produced by value converter  130  and outputs a corresponding rate target value. The map between the input power value and the output rate value that characterizes lookup table  170  may be optimized according to a particular criterion such as fuel consumption, emissions production, operating temperature, etc. In an exemplary implementation, for example, lookup table  170  applies a map that is optimized for minimum fuel consumption. 
     The relationship mapped by lookup table  170  may be a function of one or more other values as well. For example, a performance of a turbine  212  may also depend on a temperature of the air at the compressor intake, as the density of air may change dramatically with large changes in temperature. Similarly, large changes in altitude may also result in air changes in the ambient air pressure that will affect air density as well. Therefore, lookup table  170  may be implemented as a matrix of tables. Each such table may indicate a different mapping of power to rate, and the choice between the tables may be determined by sensed operating parameters such as air pressure and temperature, fuel characteristics, etc. 
     Supply rate sensor  190  outputs an observed rate value that relates to the rate of energy supply by power source  110 . In the exemplary implementations of FIGS. 4 and 5, supply rate sensor  190  is implemented as a speed sensor  290  and outputs a value obtained by sampling (at a rate of 50 Hz) the rotational position or speed of a drive shaft driven by power source  110 . In this case, the value outputted by speed sensor  290  relates to a speed of the drive shaft in revolutions per minute (rpm). 
     In adder  80 , the observed rate value produced by supply rate sensor  190  is subtracted from the rate target value outputted by lookup table  170  to obtain a rate error value. Stabilizer  150  receives the rate error value and outputs an inertia compensation value according to one or more predetermined stabilization profiles. In an exemplary implementation, stabilizer  150  is a governor  250  that is implemented as a proportional-integral controller. The operation of such a controller is characterized by a proportional gain Kp and an integral gain Ki. Proportional-integral controllers tend to decrease rise time and steady state error while increasing overshoot. Other forms of controllers (such as proportional, derivative, or proportional-integral derivative) may also be used for stabilizer  150  if different performance characteristics are desired. Although incorporating a derivative gain allows a system to reach the steady state sooner, such controllers also tend to add noise to the system. In exemplary implementations of FIGS. 4 and 5, the extra response provided by incorporating a derivative gain has not been necessary in practice. 
     Lookup table  140  receives the power supply requirement value produced by value converter  130  and produces the power demand value. In the exemplary implementation of FIGS. 4 and 5, the power demand value relates to a speed of a compressor of a turbine  212 , which speed is strongly correlated with turbine output power. 
     As with lookup table  170 , the relationship between power and speed, for example, that is mapped by lookup table  140  may depend on other operating parameters such as pressure and temperature as well. Therefore, lookup table  140  may be similarly implemented as a matrix of (e.g., 64) tables, wherein sensed values of such other operating parameters are used to select the desired mapping. 
     Fuel supply controller  160  receives the modified power demand value outputted by adder  70  and adjusts the supply of fuel to power source  110 , accordingly. In the exemplary implementations of FIGS. 4 and 5, fuel supply controller  160  monitors the speed of a compressor of turbine  212 , and adjusts the fuel supply such that the compressor speed complies with the modified power demand value. In another implementation, fuel supply controller  160  may control the fuel supply based upon other monitored values in order to maintain a desired correspondence between the modified power demand value and the operation of power source  110 . 
     Because of the quick feedback path provided through lookup table  140 , system  100  may adapt quickly to meet changes in the load demand level, thereby reducing both response time and fuel consumption. In the exemplary implementation of FIGS. 4 and 5, because the power demand value is modified by the inertia compensation value before it is inputted to fuel supply controller  160 , the inertia encountered by the generated power in transfer from power source  110  to load  800  may be counteracted by increasing or decreasing the power demand value as appropriate. In these exemplary implementations, inertia is encountered in attempting to change the speed of the turbine output shaft, the components of the gear box, and the components of the generator, for example. 
     When adder  80  returns a positive value (i.e., the speed of the output shaft of turbine  212  is insufficient to support the power demand of electrical load  810 ), an augmented power demand value is inputted to fuel supply controller  160  in order to accelerate the inertial components more quickly to the desired speed. When the value outputted by adder  80  is negative (i.e., the turbine output speed is higher than the power demanded by electrical load  810  requires), then a reduced power demand value is inputted to fuel supply controller  160  such that the load may pull part of the required power out of the inertial components and thereby decelerate them to the appropriate speed more quickly. 
     FIG. 6 is a block diagram of a system according to an alternative embodiment of the invention. In this embodiment, override  320  inputs a minimum rate requirement B to maximum function block  310 . Maximum function block  310  outputs the maximum between the override rate B and the rate target value outputted by lookup table  170 . Such an embodiment may be used in a case where a minimum rate is desired or required, regardless of the power demand of load  800 . In the exemplary implementations of FIGS. 4 and 5, for example, it may be necessary to operate a generator at a minimum speed, regardless of the power drawn by electrical load  810 . It is possible that the additional mode of operation described in FIG. 6 may only be initiated in the case of a malfunction of one or more components of load  800 . 
     Because the relationship between power and rate which is mapped in lookup table  140  may be nonlinear, it may not be desirable to use fixed values for K p  and K i  in stabilizer  150 . K p  and K i  values which are appropriate at low values of low power demand, for example, may not be appropriate at high levels of demand, due in part to this nonlinearity. In the alternative implementation shown in FIG. 7, such a difficulty may be avoided by moving the conversion from power to rate performed in lookup table  140  to a point subsequent to the summation in adder  70 . In this case, stabilizer  450  converts the rate error value into an inertia compensation value that relates to power rather than rate. As a consequence, fixed values of K p  and K i  may be used in stabilizer  450 . Lookup table  440  may be adjusted as necessary to perform the new mapping or mappings. Alternatively, as shown in the embodiment of FIG. 8, the rate values may be squared in squaring function blocks  460  and  470  before differentiation in adder  80 . In this case, stabilizer  452  performs a similar conversion with possibly different fixed values of Kp and Ki. Because of the squaring of the rate values, the value outputted by stabilizer  452  bears. a closer resemblance to power; and, therefore, it is possible in the implementation to use the same mappings in lookup table  140 . 
     As shown in FIGS. 9 and 10, the modification described in FIG. 6 may be combined with the modifications shown in FIGS. 7 and 8. 
     FIG. 11 shows an apparatus  105  according to an embodiment of the invention. This apparatus may be constructed and supplied separately from power source  110  and power converter  120  for use with possibly different power generation systems. Similarly, the apparatus shown in FIG. 12 may be used to perform a function similar to that of the system shown in FIG.  8 . 
     The foregoing presentation of the described embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments are possible, and the generic principles presented herein may be applied to other embodiments as well. For example, the invention may be implemented in part or in whole (as appropriate to the particular embodiment) as a hard-wired circuit or as a circuit configuration fabricated into an application-specific integrated circuit or field programmable gate array. Likewise, the invention may be implemented in part or in whole as a firmware program loaded or fabricated into non-volatile storage (such as read-only memory or flash memory) as machine-readable code, such code being instructions executable by an array of logic elements such as a microprocessor or other digital signal processing unit. Further, the invention may be implemented in part or in whole as a software program loaded as machine-readable code from or into a data storage medium such as a magnetic, optical, magnetooptical, or phase-change disk or disk drive; a semiconductor memory; or a printed bar code. Thus, the present invention is not intended to be limited to the embodiments shown above but rather is to be accorded the widest scope consistent with the principles and novel features disclosed in any fashion herein.