Abstract:
A generator set includes an internal combustion engine and a generator driven by the engine. The generator provides electricity to a plurality of loads, each of which is selectively connected to the generator by a switch. The generator set is controlled by a method that includes receiving a command to connect a given load to the generator. In response to that command, a throttle of the engine is changed to a transition throttle position and an transition excitation voltage is applied to the generator. Then the switch is operated to connect the given load to the generator. Thereafter, in response to a defined event occurring, the throttle is changed to a normal position and a normal excitation voltage is applied to the generator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the control of an electric generator set that includes an engine and a generator. In particular, the present invention relates to the control of an electric generator set that has a variable load at the output of the generator. 
     2. Description of the Related Art 
     Electrical generators driven by an internal combustion engine are used to provide electrical power in situations in which power is unavailable from an electric utility company. The engine-generator combination is often referred to as a “generator set” or simply a “genset.” The generator produces alternating electric current and thus often is referred to as an “alternator”. The output voltage of a genset is proportional to both the magnetic flux density within the generator, and the speed of the engine. The magnetic flux density is typically determined by controlling an armature voltage or field current of the generator, while the speed of the engine is usually determined by an engine governor. 
     When an electrical load is attached to the output terminals of the generator or when an attached load increases in magnitude, the speed of the engine tends to drop unless the engine governor appropriately adjusts the position of the engine throttle. In practice, adding or increasing the load does not adversely affect the performance of a genset, if the load only changes gradually or if the load is very small, at which times the engine governor is capable of effectively responding to the increased load. However, if a generator load changes too quickly, particularly if the load is large, an excessive drop in the speed of the engine can occur. In this situation, the engine governor is unable to open the throttle fast enough to maintain the engine&#39;s speed. Because the speed of the engine decreases excessively, the output voltage and frequency of the generator also decreases excessively. Such sizeable variation in the output voltage and frequency can adversely affect operation of other loads connected to the generator. 
     Although it would be desirable if the engine governor could respond to a speed decrease in order to maintain an engine&#39;s speed, mechanical time constraints inherent in conventional engine systems limit the rate at which a throttle can be opened. 
     Furthermore, while certain prior art systems exist that maintain engine speed despite sudden increases in the electrical load on the generator, none of those prior art systems both (a) maintains the voltage output level of the generator at the desired level and (b) applies to gensets in which the AC power output of the generator is directly supplied to outside power lines or other loads without rectification or inversion. 
     That is, one type of previous control systems maintain engine speed approximately constant by momentarily relaxing the armature voltage or field current when the load on the generator suddenly increases. By relaxing the armature voltage or field current, the effective load on the engine is decreased, and consequently the speed of the engine does not decrease as much as it otherwise would. While an excessive drop in the engine speed is prevented by these systems, the output voltage of the generator cannot remain at the desired level, but rather must decrease because of the reduction in the armature voltage or field current. 
     Other previous systems prevented an excessive drop in the engine speed by further opening the throttle of the engine rather than by relaxing the armature voltage or field current of the generator. The necessity to open the engine throttle further was determined by measuring DC power output of the genset. That is, these systems applied only to gensets in which the AC power output from the generators was rectified into DC power. Such gensets include rectifiers to convert the AC power into DC power, and must further include inverters to reconvert the DC power into AC power suitable for output to power lines and other AC loads. Thus, these control systems are inapplicable to gensets in which the AC power output of the generators is to be directly connected to power lines and other AC loads. 
     It would therefore be advantageous if another method and apparatus were developed for minimizing the transient effects on the engine speed and output voltage that occur when the genset load changes suddenly. 
     SUMMARY OF THE INVENTION 
     A generator set includes an internal combustion engine and a generator that is driven by the internal combustion engine. The generator provides electricity to a plurality of loads, each of which is selectively connected to the generator by a switch. The engine has a throttle for controlling speed and an excitation voltage is applied to the generator to produce a magnetic field. 
     The generator set is controlled by a method that comprises receiving a command to connect a given load to the generator. In response to receiving the command, a position of the throttle is altered and the excitation voltage applied to the generator is altered in ways that compensate for an effect that connecting the given load to the generator has on the electricity provided by the generator. For example, altering the position of the throttle involves placing the throttle into a predefined transition throttle setting, and alerting the excitation voltage involves applying a predefined transition excitation voltage to the generator. Thereafter, the switch is operated to connect the given load to the generator. 
     An aspect of the present method includes, after operating the switch, sensing an operational characteristic of the generator set; and in response to the operational characteristic, redefining at least one of the predefined transition throttle settings and the predefined transition excitation voltage. 
     Another aspect of the present method includes, in response to a defined event occurring after operating the switch, changing the position of the throttle to a normal throttle setting, and changing the excitation voltage applied to the generator to a normal excitation voltage level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an electrical generator set; 
         FIG. 2  is a block diagram of the engine control subsystem in  FIG. 1 ; 
         FIG. 3  is a block diagram showing the generator set connected to a plurality of electrical loads; and 
         FIG. 4  is a flowchart of a software routine for performing adaptive control of an electrical generator set based on load magnitude. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , a generator set (genset)  10  comprises an engine  12  coupled by a shaft  14  to an electrical generator  16 . A control panel  18  enables a human operator to start the genset and control its operation. Control signals are exchanged between the control panel  18  and a genset controller  22 , which in turn exchanges other control signals with an engine control subsystem  24  via a communication bus  20 . The communication bus  20  may conform to the Computer Area Network (CAN) J-1939 standard promulgated by SAE International, however another communication bus protocol may be used. The genset controller  22  and the engine control subsystem  24  respectively control the operation of the electrical generator  16  and the internal combustion engine  12 . 
     The genset controller  22  is a microcomputer based subsystem that executes a control program which governs the operation of the generator  16  to ensure that a constant output voltage level is produced. An example of one type of genset controller is described in U.S. Pat. No. 6,555,929, which description is incorporated by reference herein. In addition to receiving input signals from the control panel  18 , the genset controller  22  also receives signals from output sensors  26  that sense the voltage and current levels of the electricity produced by the generator  16 . In response to the sensed voltage and current levels, the genset controller employs a conventional voltage regulation technique to control an exciter  28  that applies an excitation voltage to the field coils of the generator. Application of the excitation voltage to the field coils produces a magnetic field within the generator. By selectively varying the excitation voltage, the output voltage produced by the generator  16  is regulated to a substantially constant nominal level (e.g. 240 volts) in a known manner. 
     With reference to  FIG. 2 , the engine control subsystem  24  may incorporate another microcomputer  30  which includes a memory that stores a control program and data for operating of the engine  12 . The engine control subsystem  24  also has internal interface circuits for receiving signals from components on the engine and for producing output signals to control other devices that govern engine operation. For example, the engine control subsystem  24  receives signals from several sensors on the engine  12 , such as an oil pressure sensor  32  and an engine temperature sensor  33 . 
     A speed sensor  35  also provides an input signal indicting the speed of the engine to the engine control subsystem  24 . Although the speed sensor  35  is connected to the engine  12  and thus senses the engine speed, alternatively the speed of the generator can be sensed. In the exemplary genset  10 , the engine  12  is directly connected the generator  16  so that their speeds are the same, thus the speed of either component may be sensed. It should be appreciated that the engine  12  can be connected to the generator  16  by a transmission so that the engine and generator operate at different speeds. In this case, knowing the speed conversion ratio provided by the transmission enables a measurement of the speed of either the engine or generator to be used to determine the speed of the other one. 
     The engine control subsystem  24  produces output signals to control the engine starter  34 , the ignition system  36 , and a throttle control  37  that varies the amount of fuel flow. The microcomputer  30  executes software that implements a conventional governor function which via the throttle control  37  operates a throttle of the engine to maintain the engine at a predefined nominal speed as the mechanical load on the engine varies. 
       FIG. 3  shows the electrical outputs  15  of the genset  10  connected to three loads  51 ,  52 , and  53  by three electrically operated switches  54 ,  55 , and  56 , respectively. Each switch  54 - 56  may be a conventional contactor, relay, or similar device. For example, the first load  51  may include lighting circuits in a home, the second load  52  may be a refrigerator and other kitchen appliances, and the third load  52  may comprise an air conditioning system. It should be understood that any plurality of loads can be supplied with electrical power from the genset  10  according to the present technique. The genset  10  may produce single or three-phase voltages which are selectively and independently connected by the switches  54 - 56  to each of the loads  51 - 53 . Alternatively, one of the loads may be connected directly to the output of the genset  10  without an intervening switch. As shown in  FIG. 1 , the genset  10  includes switch drivers  25  that receive signals from the genset controller  22  to produce output signals for independently operating the three switches  54 - 56 . 
     When an additional load is initially connected to the outputs  15  of the genset  10 , the increase in the total electrical load causes a momentary reduction in the speed of the engine  12  and in the voltage and frequency of the electricity produced at the genset outputs  15 . To avoid this adverse effect, genset controller  22  in  FIG. 1  receives a command, such as produced by a human operator activating an input device on the control panel  18  or at a predetermined time, which indicates that a new load is to be connected to the output of the genset. In response, the genset controller  22  and the engine control subsystem  24  adjusts the engine throttle and excitation of the generator  16  in anticipation of the additional load. After the engine and generator have responded to those parameter changes, the genset controller  22  operates the switch  54 ,  55  or  56  that is associated with the new load  51 ,  52  or  53  that is to be connected. Thereafter, the genset controller  22  monitors operating parameters of the engine  12  and the generator  16  to observe the operational response to the application of the new load. This observation enables the genset to learn new settings for the engine throttle and the generator excitation that may be used when that same load is to be activated at another time. Those new parameter settings are then stored in the memory of the genset controller  22  for subsequent use. 
     With reference to  FIG. 4 , this load control technique is implemented by a software routine  100  executed by the microcomputer within the genset controller  22 . That load control routine  100  commences at step  102  whenever the genset controller  22  receives a load connection change request. The load change request designates which of the loads  51 ,  52 , or  53  is desired to be activated at this time. In response to the request, a determination is made at step  104  whether this is the first load to be applied to the outputs  15  of the genset  10 . If that is the case, the execution of the load control routine branches to step  106  at which a default throttle setting for the engine governor is retrieved from the memory in the genset controller  22 . Also retrieved from the memory at step  108  is a default regulator setting specifying an excitation voltage to be applied to the field windings of the generator  16 . The routine then advances to step  116  at which the governor and regulator settings are applied to operate the genset  10 . Specifically, the governor setting, defining the desired throttle setting for the engine  12 , is sent via the communication bus  20  in  FIG. 1  to the engine control subsystem  24 . That latter component utilizes the throttle setting to operate the throttle control  37  that controls the flow of fuel to the engine  12 . At the same time, the regulator setting is used by the genset controller  22  to control operation of the exciter  28  so that the designated excitation voltage is applied to the field windings of the generator  16  so that the generator will produce the nominal output voltage upon connection of the new electrical load. 
     If, however, at step  104 , the load change request designates an additional load to a load or loads already applied to the genset  10 , the execution of the control routine branches to step  110 . At this time, the operation of the engine  12  and generator  16  are changed based on the transient conditions which occurred during a previous time that this same additional load was initially connected to the genset output. Those transient conditions previously produced a set of values for the operation of the engine governor and voltage regulator which are now to be used in controlling those functions. Specifically, at step  110 , the historical data regarding the particular new load and the genset operating parameters are read from the memory of the genset controller  22 . There is a similar set of data for each of the three loads  51 ,  52  and  53 . 
     Many electrical loads, such as those that have large motors, e.g., a building air conditioning system, have a significantly greater power consumption during startup as compared to the power level required to maintain operation of the load after startup, i.e., a steady state condition. Compensation for the effects of this larger startup power demand is achieved by increasing throttle setting and field winding excitation voltage for an initial period of time during which the load startup produces transient conditions on the genset output. In other words, the operation of the genset  10  is altered to counteract the anticipated transient effects experienced on a previous occasion when this load was initially connected to the genset output. This alteration of the genset operation involves changing the engine throttle control and the excitation of the generator field winding. 
     At step  112 , the adapted governor setting for the specific new load  51 ,  52  or  53  is obtained from memory. This adapted governor setting designates an increased transition throttle setting at which the engine is to operate while transient conditions occur at the generator output. Although the transition throttle setting ordinarily would increase the engine speed, it counteracts the speed decrease that adding the new electrical load otherwise produces. In other words, the fuel flow to the engine  12  is increased to compensate for the added mechanical load on the engine due to the addition of the new electrical load to the generator. 
     At step  114 , an adapted regulator setting for the specific new load  51 ,  52  or  53  is obtained from memory. The adapted regulator setting designates a transition excitation voltage for the exciter  28  to apply to the field coil of the generator  16 . In the absence of adding the new electrical load, this adapted regulator setting would increase the output voltage produced by the generator  16 . In this case, however, since the new electrical load will result in the output voltage decreasing if the excitation voltage remained unchanged, the adapted regulator setting provides compensation and keeps the output voltage substantially at the nominal voltage level as the new load is activated. 
     The load control routine  100  then advances to step  116  at which the transition throttle setting and the transition excitation voltage are applied to operate the engine  12  and generator  16 , respectively. At step  118 , the control process waits for a period of time to allow the new settings to take effect. This period of time is relatively brief so that any change in output voltage does not adversely affect other loads that are already connected to the genset output. After this period of time, the new load is connected to the genset output at step  120  by the genset controller  22  sending a signal to the switch drivers  25  in  FIG. 1  to operate the corresponding load switch  54 ,  55  or  56  for the designated load  51 ,  52  or  53  being added. This signal causes the associated load switch to close, thereby coupling the new load to the outputs  15  of the genset  10 . 
     Thereafter, the engine control subsystem  24  implements a closed speed control loop by monitoring the signal from the speed sensor  35  and adjusting the throttle control  37  as needed to maintain the engine at the nominal speed. In a similar manner, the genset controller  22  monitors the signals from output sensors  26  and issues signals that cause the exciter  28  to adjust the excitation voltage so as to maintain the nominal output voltage from the generator. Both the speed control and voltage regulation at this time use similar techniques to those employed in prior gensets. 
     Although the transition throttle setting and the transition excitation voltage ideally mitigate transients that occur at the output of the generator set due to the application of the new load, some transients may still occur due to changes in load operating characteristics with time. As a consequence, the genset controller  22  enters a learning mode immediately following connection of the new load. In the learning mode, any significant transients of the genset speed and output voltage are observed and used subsequently to redefine the transition throttle setting and the transition excitation voltage for subsequent use. Specifically, the genset controller  22  monitors the voltage and current levels at the generator outputs  15  which levels are indicated by signals from the output sensors  26 . The genset controller  22  also receives data from the engine control subsystem  24  that indicate the speed of the engine  12  as detected by speed sensor  35 . These measurements then are stored in the memory of the genset controller at step  122 . 
     The genset controller  22  also monitors the engine speed and generator output measurements to determine when the transient conditions caused by the new load have ceased. For example, that cessation may be indicated by an increase in engine speed, output voltage, or both. After the transient period has elapsed and the load demand has settled into the steady state condition, the execution of the load control routine  100  advances to step  124 . Hereafter, the normal governor and regulator settings, i.e. the normal throttle setting and the normal excitation voltage, are used to control the engine throttle and excitation of the generator  16  to produce the nominal output voltage from the genset  10 . 
     Then at step  125 , the genset controller  22  analyzes the measured transient response characteristics to determine whether the adapted governor and regulator setting need to be redefined. Specifically, the genset controller examines the magnitude of any deviation of the generator output voltage from the nominal voltage. If the output voltage and the engine speed remained within a predefined acceptable range, execution of the load control routine  100  terminates. 
     Otherwise, if either one of the generator output voltage and the engine speed deviated outside its predefined acceptable range, the control process branches to step  126 . At this juncture, the transition excitation voltage is changed to compensate for any such deviation. For example, if the actual generator output voltage during the transient period was greater than the nominal output voltage, the adapted regulator setting for the associated load is redefined to produce a lesser transition excitation voltage for the generator field coil. Inversely, if the actual generator output voltage was less than the nominal output voltage, the adapted regulator setting is configured for a greater transition excitation voltage. Thus the new setting designates a new value for the excitation of the generator  16  to compensate for the transient conditions that are expected to occur the next time that this same load is initially activated. 
     Similarly at step  128 , the new transient response characteristics are employed to calculate a new adapted governor setting for use in controlling the engine speed the next time that this load is applied to the genset output. For example, if the actual engine speed during the transient period was greater than the nominal speed, the transition throttle setting is adjusted for a slower speed. Inversely, if the engine ran slower than the nominal speed, the transition throttle setting is redefined for a faster speed. Then the new adapted regulator and governor settings are stored within the memory of the genset controller  22  at step  130 . The execution of the load control routine  100  then terminates until the next time a load change request is received. 
     A similar process may be used to anticipate and counteract transient effects on the genset output that occur when a large load is disconnected from the output of the genset  10 . Removal of a significant load typically causes an increase in engine speed and the generator output voltage. The inverse process is used to disconnect a load and maintain the engine speed and generator output voltage at their nominal levels. 
     The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.