Abstract:
An engine control system suited for use with an engine that outputs electrical power to a local load and is electrically connected to an electrical grid. The engine generates power at a power output value and the load consumes power at a load value. The engine control system includes a sensor that measures an electrical parameter indicative of a difference between the power output value and the load value, and a master control system that varies the engine power output value to prevent the electrical parameter from reaching zero.

Description:
RELATED APPLICATION 
   This application claims priority to U.S. patent application Ser. No. 10/795,953, filed Mar. 8, 2004 now U.S. Pat. No. 7,161,953, the entire contents of which are incorporated herein by reference. 

   BACKGROUND 
   The present invention relates to system for inhibiting an islanding condition from occurring. More particularly, the present invention relates to a system and method for inhibiting an islanding condition from occurring in an engine driven generator connected to an electrical grid. 
   Microturbine engines are relatively small and efficient sources of power. Microturbines can be used to generate electricity and/or to power auxiliary equipment such as pumps or compressors. When used to generate electricity, microturbines can be used independent of the utility grid or synchronized to the utility grid. In general, microturbine engines are limited to applications requiring 2 megawatts (MW) of power or less. However, some applications larger than 2 MWs may utilize a microturbine engine. 
   In many applications, microturbine engines are used to supply power for a local load that is simultaneously connected to an electrical grid (i.e., a utility grid). When connected to the grid, it is important to monitor the system for islanding conditions. Islanding exists when the microturbine engine provides power to the local load with the electrical grid connection severed at some point. This condition can be problematic, as the microturbine engine may not be able to react to sudden load changes or may not be able to provide a sudden increase in demand without the support of the electrical grid. As such, an undesirable trip may occur severing all of the power being supplied to the local load. Therefore, it is desirable to detect islanding conditions and to take steps to isolate the local load or remedy the islanding condition when it is detected. 
   While many islanding detection schemes are known, most have difficulty detecting islanding conditions that arise when the microturbine output is closely matched with the local load. 
   SUMMARY 
   In one embodiment, an engine control system suited for use with an engine that outputs electrical power to a local load and is electrically connected to an electrical grid includes a sensor and a master control system. The engine generates power at a power output value and the load consumes power at a load value. The sensor measures an electrical parameter indicative of a difference between the power output value and the load value. The master control system varies the engine power output value to prevent the electrical parameter from reaching zero. 
   In another embodiment, an engine control system is provided for avoiding an islanding condition. The engine control system controls an engine that generates power and is electrically connected to an electrical grid and a local load that consumes power. The engine control system includes a sensor and a master control system. The sensor transmits a signal indicative of the difference between the power generated by the engine and the power consumed by the load. The master control system receives the signal and varies the power generated by the engine to maintain a difference between the power generated by the engine and the power consumed by the load. 
   Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The description particularly refers to the accompanying figures in which: 
       FIG. 1  is a perspective view of a portion of a microturbine engine; 
       FIG. 2  is a schematic illustration of a portion of a power distribution system including the microturbine engine of  FIG. 1 ; 
       FIG. 3  is a chart illustrating a restricted zone; and 
       FIG. 4  is a schematic illustration of a portion of a power distribution system including a plurality of microturbine engines. 
     Before any embodiments of the invention are explained, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof is meant to encompass the items listed thereafter and equivalence thereof as well as additional items. The terms “connected,” “coupled,” and “mounted” and variations thereof are used broadly and encompass direct and indirect connections, couplings, and mountings. In addition, the terms “connected,” “coupled,” and “mounted” and variations thereof are not restricted to physical or mechanical connections or couplings. 
   

   DETAILED DESCRIPTION 
   With reference to  FIG. 1 , a microturbine engine system  10  that includes a turbine section  15 , a generator section  20 , and a control system  25  is illustrated. The turbine section  15  includes a radial flow turbine  35 , a compressor  45 , a recuperator  50 , a combustor  55 , and a gearbox  60 . 
   The engine  10  includes a Brayton cycle combustion turbine with the recuperator  50  added to improve engine efficiency. The engine shown is a single-spool engine (one set of rotating elements). However, multi-spool engines are also contemplated by the invention. The compressor  45  is a centrifugal-type compressor having a rotary element that rotates in response to operation of the turbine  35 . The compressor  45  shown is generally a single-stage compressor. However, multi-stage compressors can be employed where a higher pressure ratio is desired. Alternatively, compressors of different designs (e.g., axial-flow compressors, reciprocating compressors, and the like) can be employed to supply compressed air for use in the engine  10 . 
   The turbine  35  is a radial flow single-stage turbine having a rotary element directly coupled to the rotary element of the compressor  45 . In other constructions, multi-stage turbines or other types of turbines may be employed. The coupled rotary elements of the turbine  35  and the compressor  45  engage the gearbox  60  or other speed reducer disposed between the turbine section  15  and the generator section  20 . In other constructions, the coupled rotary elements directly engage the generator section  20 . 
   The recuperator  50  includes a heat exchanger employed to transfer heat from a hot fluid to the relatively cool compressed air leaving the compressor  45 . One suitable recuperator  50  is described in U.S. Pat. No. 5,983,992 fully incorporated herein by reference. The recuperator  50  includes a plurality of heat exchange cells stacked on top of one another to define flow paths therebetween. The cool compressed air flows within the individual cells, while a flow of hot exhaust gas passes between the heat exchange cells. 
   During operation of the microturbine engine system  10 , the rotary element of the compressor  45  rotates in response to rotation of the rotary element of the turbine  35 . The compressor  45  draws in atmospheric air and increases its pressure. The high-pressure air exits the air compressor  45  and flows to the recuperator  50 . 
   The flow of compressed air, now preheated within the recuperator  50 , flows to the combustor as a flow of preheated air. The preheated air mixes with a supply of fuel within the combustor  55  and is combusted to produce a flow of products of combustion. The use of the recuperator  50  to preheat the air allows for the use of less fuel to reach the desired temperature within the flow of products of combustion, thereby improving engine efficiency. 
   The flow of products of combustion enters the turbine  35  and transfers thermal and kinetic energy to the turbine  35 . The energy transfer results in rotation of the rotary element of the turbine  35  and a drop in the temperature of the products of combustion. The energy transfer allows the turbine  35  to drive both the compressor  45  and the generator  20 . The products of combustion exit the turbine  35  as a first exhaust gas flow. 
   In constructions that employ a second turbine, the first turbine  35  drives only the compressor, while the second turbine drives the generator  20  or any other device to be driven. The second turbine receives the first exhaust flow, rotates in response to the flow of exhaust gas therethrough, and discharges a second exhaust flow. 
   The first exhaust flow, or second exhaust flow in two turbine engines, enters the flow areas between the heat exchange cells of the recuperator  50  and transfers excess heat energy to the flow of compressed air. The exhaust gas then exits the recuperator  50  and is discharged to the atmosphere, processed, or further used as desired (e.g., cogeneration using a second heat exchanger). 
   Turning to  FIG. 2 , a portion of the electrical and control systems of a power distribution system  65  is illustrated schematically. As previously described, the microturbine engine  10  drives the generator  20  to produce an electrical output. The system illustrated herein includes a synchronous generator  20 , with other types of generators (e.g., high-speed alternators, asynchronous generators and the like) also functioning with the present invention. The generator output is delivered to a local load bus  70  via a generator output line  75 . A generator sensor  80 , positioned within the generator output line  75 , measures an electrical parameter of the generator  20  during engine operation. In most constructions, the generator sensor  80  includes a current sensor. The measured current, along with a known voltage, can be used to calculate an actual generator output power. Other constructions can include multiple sensors that measure current, voltage, and/or power directly. The generator sensor  80  can continuously monitor the electrical parameter or can take periodic measurements as desired. 
   The generator output line  75  connects to, and delivers power to, the local load bus  70 . Various local loads  85  (e.g., motors, computers, monitors, robots, welding machines, lights, etc.) may be powered off the local load bus  70 . 
   In some constructions, multiple microturbine engine systems  10 , or other generation systems (e.g., diesel, solar, wind, fuel cell, and the like) are connected to the local load bus  70 , with some or all of them simultaneously providing power to the power distribution system  65 .  FIG. 4  illustrates one possible system that includes multiple engine systems  10 . Each engine system  10  is electrically connected to the local load bus  70  to allow each engine  10  to provide electrical power. Generator sensors  80   a ,  80   b  are positioned to measure the actual output of each engine system  10 . The control system  25  is then able to individually control each engine  10  to produce the desired total output. In many constructions, the control system  25  is able to start or stop individual engines to optimize the system&#39;s operation, while providing the desired amount of total power. 
   A tie line  90  interconnects the local load bus  70  and a utility grid  95 . A transformer  100  may be disposed within the tie line  90  to step-up or step-down the voltage between the utility grid  95  and the local load bus  70 . The tie line  90  facilitates the delivery of electricity from the utility grid  95  to the local load bus  70  and/or from the microturbine engine  10  to the utility grid  95 . The tie line  90  also includes a tie line sensor  105  that measures an electrical parameter (e.g., voltage, current, absolute value of current, power, phase angle, frequency, and the like). In most constructions, the tie line sensor  105  includes a current sensor that measures both the magnitude and direction of current flow within the tie line  90 . However, other constructions may include multiple sensors that measure current, voltage, and/or power flow. The tie line sensor  105  can continuously monitor the electrical parameter or can take periodic measurements as desired. 
   In some constructions, the microturbine engine  10  operates intermittently. As such, the tie line  90  is sized to carry sufficient electricity to power the local loads  85  during periods in which the microturbine engine  10  is inoperative. With the generator  20  synchronized to the utility grid  95  (i.e., the voltage, phase angle, and frequency of the generator output power matched with the utility grid power), both the generator  20  and the utility grid  95  can provide power to the local load bus  70  and the local loads  85 . 
   The microturbine engine  10  includes a control system  25  that controls the operation of the engine  10  (or engines in a multiple engine system). The control system  25  manipulates various components (e.g., valves, pumps, compressors, louvers, switches, relays, and the like) that control various operating parameters of the engine  10 . For example, the control system  25  may control fuel flow to the engine  10  to control engine speed and/or power output. The control system  25  may move or initiate movement of a controller that in turn may manipulate a valve, a compressor, or other control member to control the flow of fuel to the combustor  55 , which in turn controls the speed or the power output of the engine  10 . 
   When the generator  20  is synchronized to the utility grid  95 , the speed of the generator  20  is substantially fixed and the control system  25  controls output power. A power output set point is supplied to the control system  25 , which then maintains the generator output at a value substantially equal to the power output set point. There are many different ways of inputting the power output set point. For example, a manual control could be used. The manual control would allow a user to input a desired value between the engine&#39;s minimum and maximum output. In systems that include more than one engine  10 , individual power output set points for each engine  10  may be used to control the output of each engine  10 . Alternatively, a single power output set point that controls the total output of all the engines may be used. In the later case, the control system  25  would determine the specific output levels of each engine  10  using any one of a number of known schemes. In some constructions, a preprogrammed curve is used to set the power output set point. The curve typically defines a power output set point that varies with the time of day, day of the week, and/or day of the year. However, other parameters (e.g., temperature, pressure, etc.) could also be used to vary the power output set point. 
   During operation of the system  65 , it is possible for a failure of the utility grid  95  to occur, thus producing an islanding condition. Islanding conditions can arise at any time, but are particularly problematic when the power consumed by the local load  85  is very near the power level output by the generator  20 . Under these “perfectly matched” circumstances, the islanding condition is very difficult to detect using known methods (e.g., rate of change of frequency ROCOF, and the like). 
   To improve the likelihood of detecting an islanding condition, the present system  65  inhibits operation of the engine  10  (or engines) in a particular range or restricted zone  115 . The restricted zone is defined as a power flow within the tie line  90  (into or out of the utility grid  95 ) between zero and a predetermined minimum desired power flow  120 , with the minimum desired power flow  120  being a non-zero value. Specifically, the control system  25  receives a signal from the tie line sensor  105  indicating the level of power flow through the tie line  90 . The control system  25  then compares that signal to a predetermined value representing the minimum desired power flow  120  through the tie line  90 . The signal may represent a power flow into the local load bus  70  or a power flow to the utility grid  95 . If the value falls below the minimum desired power flow  120 , the power output set point is automatically adjusted. This process continues until the measured power flow through the tie line  90  exceeds the minimum desired power flow  120 . With the measured power flow out of the restricted zone  115 , islanding detection is much easier and is more reliable. As one of ordinary skill will realize, the actual direction of power flow (i.e. into the local load bus or out of the local load bus) does not significantly affect the ability of the present system to detect islanding so long as sufficient power is flowing. As such, the absolute value of the measured power in the tie line  90  is typically all that needs to be measured. In preferred constructions, a minimum power corresponding to a current flow of 500 amps allows for detection of islanding conditions. In still other constructions, a power flow of 100 amps or less allows for the detection of islanding. As one of ordinary skill will realize, the actual minimum desired power flow may vary greatly depending on the system employed. 
   For example, in one construction, the power output set point is set at 100 kW and the minimum desired power flow is set at 0.5 kW. If the local load  85  is 100 kW, the microturbine engine  10  will supply all of the power to the local load  85  and no power will flow through the tie line  90 . The control system  25  will detect that the flow through the tie line  90  is below the minimum desired power flow  120  and will act to either increase or decrease the power output set point. If the power output set point is reduced (to say 99 kW), power will begin flowing (1 kW) into the local load bus  70  from the utility grid  95 . If on the other hand, the power output set point is increased (to say 101 kW), the microturbine engine output will increase, with the excess power (1 kW) flowing to the utility grid  95 . Under either scenario, the absolute value of the measured power flow through the tie line  90  will eventually exceed the minimum desired power flow  120 . 
   The present system reduces the likelihood of undetected islanding conditions when the engine  10  is driving the generator  20  and producing usable electric power. The generator sensor  80  monitors the current flow from the generator  20  and provides feed back to the control system  25 . The control system  25  adjusts the engine  10  to match the output power level to the power output set point. The tie line sensor  105  monitors the current flow through the tie line  90  and provides an additional feed back loop for the control system  25 . The measured power flow at the tie line  90  is compared to the minimum desired power flow  120  and the power output set point is reset if the measured power flow falls below the minimum desired power flow  120 . The power output set point can be increased or decreased as desired to assure that the measured power flow is above the minimum desired power flow  120 . It should be noted that the minimum desired power flow  120  can be input by the engine user or can be preprogrammed into the control system  25 . The actual value used is a function of many variables (e.g., engine size, instrument sensitivity, instrument accuracy, system load variations, and the like). As such, the values used herein are exemplary and should not be read as limiting in any way. 
   The foregoing describes a microturbine engine  10  that drives a synchronous generator  20 . As one of ordinary skill will realize, other types of generators (e.g., high-speed alternators, asynchronous generators and the like) could be used with the present invention. Furthermore, the system has been described as including a single master control system  25 . As one of ordinary skill will realize, the various control functions could be divided among multiple controllers or multiple control systems as desired. There is no requirement that a single control system perform all of the control functions described herein. 
   Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.