SE544489C2 - Method for controlling rotational speed of a turbine and a controller and system therefor - Google Patents

Abstract

A method and a controller (101) is disclosed for controlling rotational speed of a turbine (122) in a turbine system (105). The turbine system (105) comprising the turbine (122), a pressure sensor (130), a filter (132) and a drive unit (126). The controller performs the method and collects (S100) sensor data from the pressure sensor (130) and the drive unit (126), compares (S102) the collected sensor data with reference values to determine if the collected sensor data is valid, calculates (S104) the current operating point of the turbine system (105) based on the collected sensor data and system parameters of the turbine system (105), determines (S106) a current optimal rotational speed of the turbine based on the current operating point, checks (S108) the validity of the current optimal rotational speed by using the filter and controls (S112) the drive unit (126) to drive the turbine at the determined current optimal rotational speed.

Description

METHOD FOR CONTROLLING ROTATIONAL SPEED OF A TURBINE AND ACONTROLLER AND SYSTEM THEREFOR Technical field 1. 1. 1. id="p-1" id="p-1"

[0001] The present invention relates generally to a method performed by acontroller for controlling rotational speed of a turbine and to the controller and aturbine control system for performing said method, and more particularly to a method for optimization of power output in a turbine control system.

Background art 2. 2. 2. id="p-2" id="p-2"

[0002] A turbine wheel design is typically optimal for its chosen design point.For a typical turbine the design point is characterized by several parameters, mostnotably pressures and temperatures of a fluid and a desired power output. ln areal application, i.e. when implemented, there are practical considerations thatplay very important roles in the design process. For example, there could be spaceconstraints and maximum rotational speeds, as well as environmental aspects toconsider. Furthermore, the design is often considering so-called “off-design”operating points that are meant to help the designer to understand the range of operating conditions that may occur. 3. 3. 3. id="p-3" id="p-3"

[0003] lnevitably, when moving the operating conditions off the design point, aloss of efficiency will occur. This could for example be operating at differenttemperatures and/or pressures than the design point conditions. Thus, it is important to keep the maximum power output without losing efficiency. 4. 4. 4. id="p-4" id="p-4"

[0004] There are known methods that use pumps with varying speeds to keepthe power output near the maximum value. One such method is described in USpatent no. 8,813,498, wherein pressure sensor measures an Organic RankineCycle (ORC) working fluid pressure in front ofa radial inflow turbine, while atemperature sensor measures an ORC working fluid temperature in front of theradial inflow turbine. A controller responsive to algorithmic software determines asuperheated temperature of the working fluid in front of the radial inflow turbine based on the measured working fluid pressure and the measured working fluid temperature. The controller then manipulates the speed of a working fluid pump,the pitch of turbine variable inlet guide vanes when present, and combinationsthereof, in response to the determined superheated temperature to maintain thesuperheated temperature of the ORC working fluid in front of the radial inflowturbine close to a predefined set point. The superheated temperature can thus bemaintained in the absence of sensors other than pressure and temperature SenSOFS. . . . id="p-5" id="p-5"

[0005] One problem of the solution in US patent no. 8,813,498 is that themethod needs many steps for measuring and controlling the speed of the pumpand/or the pitch of turbine variable inlet guide vanes to maintain the working fluidsuperheated temperature at the inlet side of the radial inflow turbine at apredefined set point. This increases the complexity of the solution. Another problem is that the solution is only suitable in superheated turbine systems. 6. 6. 6. id="p-6" id="p-6"

[0006] Yet another problem in many prior art solutions is that the design point ofa turbine will be at maximum rotational speed. This rotational speed is oftenmaintained over a broad variety of operating conditions, which results in damagesof the turbine and shortens the lifespan of the turbine.

Summary of invention 7. 7. 7. id="p-7" id="p-7"

[0007] An object of the present invention is to provide a method and a controllerfor controlling rotational speed of a turbine in a turbine system to improve thepower output without losing efficiency. 8. 8. 8. id="p-8" id="p-8"

[0008] Another object is to provide a turbine control system for controlling rotational speed of a turbine to improve the power output without losing efficiency. 9. 9. 9. id="p-9" id="p-9"

[0009] A third object is to provide a computer program comprising non-transitorycomputer readable code means to be run in a control node for controlling therotational speed of a turbine in a turbine system to improve the power outputwithout losing efficiency. . . . id="p-10" id="p-10"

[0010] A fourth object is to improve the durability of a turbine thus to extend theIifespan of the turbine. Furthermore, the invention provides a solution suitable inanything that has a turbine installed, which means, it is suitable in many fields. 11. 11. 11. id="p-11" id="p-11"

[0011] The above objectives are wholly or partially met by the method, controllerand system described in the appended claims. Features and different aspects areset forth in the appended claims, in the following description, and in the annexed drawings. 12. 12. 12. id="p-12" id="p-12"

[0012] According to one aspect, a method is provided for controlling rotationalspeed of a turbine in a turbine system. The turbine system comprises the turbine,a pressure sensor, a filter and a drive unit, the method comprises collecting sensordata from the pressure sensor and the drive unit, comparing the collected sensordata with reference values to determine that the collected sensor data is valid,calculating the current operating point of the turbine system based on the collectedsensor data and system parameters of the turbine system, determining a currentoptimal rotational speed of the turbine based on the current operating point of theturbine system, checking the validity of the current optimal rotational speed byusing the filter to sort out rotational speed values having predefined properties andcontrolling the drive unit to drive the turbine at the determined current optimal rotational speed. 13. 13. 13. id="p-13" id="p-13"

[0013] ln another embodiment, the step of checking the validity of the currentoptimal rotational speed further comprises the step of applying limits, dead bandsand rate of change limitations to the current optimal rotational speed. 14. 14. 14. id="p-14" id="p-14"

[0014] ln yet another alternative, the method further comprises continuously or at a specified rate repeating the steps of the method. . . . id="p-15" id="p-15"

[0015] ln yet another alternative, the current optimal rotational speed is determined based on an empirically validated model. 16. 16. 16. id="p-16" id="p-16"

[0016] ln yet another alternative, the sensor data is transmitted from the turbinesystem to the controller through a first interface. 17. 17. 17. id="p-17" id="p-17"

[0017] According to another aspect, a controller is provided for contro||ing therotational speed of a turbine in a turbine system. The turbine system comprisesthe turbine, a pressure sensor, a filter and a drive unit and the controller comprisesprocessing circuitry and a non-transitory computer-readable medium, configured tostore instructions, which when executed by the processing circuitry, cause thecontroller to collect sensor data by means of the pressure sensor and the driveunit, compare the collected sensor data with reference values to determine thatthe collected sensor data is valid, calculate the current operating point of theturbine system based on the collected sensor data and system properties of theturbine system, determine a current optimal rotational speed of the turbine basedon the current operating point of the turbine system, check the validity of thecurrent optimal rotational speed by using the filter to sort out rotational speedvalues having predefined properties and control the drive unit to drive the turbineat the determined current optimal rotational speed. 18. 18. 18. id="p-18" id="p-18"

[0018] According to another aspect, a turbine control system is provided forcontro||ing the rotational speed of a turbine, comprising a turbine system, acommunication system and a controller as defined above, wherein the turbinesystem comprises a turbine, a drive unit, a pressure sensor, a filter and a secondinterface, wherein the turbine system communicates with a first interface of thecontroller through the communication system and the second interface. 19. 19. 19. id="p-19" id="p-19"

[0019] According to another aspect, a computer program comprising non-transitory computer readable code means being adapted, if executed onprocessing circuitry, to implement the method described above. . . . id="p-20" id="p-20"

[0020] By implementing this solution, the turbine will output the most optimalpower depending on the operating conditions and the turbine design itself, i.e. themethod aims to minimize the loss caused by different operating conditions bymeans of adjusting the rotational speed of the turbine wheel. 21. 21. 21. id="p-21" id="p-21"

[0021] By characterizing the efficiency of the turbine across different operating conditions, using an empirically validated model of the turbine, and tracking the current conditions by collecting relevant sensor data, the speed of the turbinewheel may be instantaneously adjusted to its most efficient value. 22. 22. 22. id="p-22" id="p-22"

[0022] ln many cases the design point is at, or close to, the maximum allowedoperating rotational speed. This invention will therefore also improve durability byoperating at lower, on average, speed and thus extend service life and cost. Allelse being equal, the service life is directly proportional to the rotational speed ofthe turbine in cases of roller bearings. For example, a 20% reduction in rotationalspeed would incur a 20% longer service life which could lead to a substantialsaving in service cost for a fleet of machines globally. Furthermore, a reduction inrotational speed is almost always beneficial to lubrication systems for rollerbearings. ln some cases, the improvement is larger than from the rotational speed factor alone in terms of service life.

Brief description of drawinqs 23. 23. 23. id="p-23" id="p-23"

[0023] The invention is now described, by way of example, with reference to the accompanying drawings, in which: 24. 24. 24. id="p-24" id="p-24"

[0024] Fig. 1 is a flow chart showing an example of a method for controllingrotational speed of a turbine in a turbine system . . . id="p-25" id="p-25"

[0025] Fig. 2 describes an example of a turbine control system. 26. 26. 26. id="p-26" id="p-26"

[0026] Fig. 3 describes an exemplary structure of a controller for controllingrotational speed of a turbine in a turbine system. 27. 27. 27. id="p-27" id="p-27"

[0027] Fig. 4 describes an example of comparison of turbine model withexperimentally fitted data for a few variations of the input state to the turbine. 28. 28. 28. id="p-28" id="p-28"

[0028] Fig. 5 describes an example of empirical fit to net power output. 29. 29. 29. id="p-29" id="p-29"

[0029] Fig. 6 describes functional relationship of current optimal rotationalspeed limited to dependence on pressure ratio between inlet and outlet of the turbine.

Description of embodiments . . . id="p-30" id="p-30"

[0030] A detailed description of particular embodiments of the presentdisclosure are described herein-below with reference to the accompanyingdrawings; however, the disclosed embodiments are shown merely as examples ofthe disclosure and may be embodied in various other forms. Well-known functionsor constructions are not described in detail to avoid obscuring the presentdisclosure in unnecessary detail. Therefore, specific structural and functionaldetails disclosed herein are not to be interpreted as limiting, but merely as a basisfor the claims and as a representative basis for teaching one skilled in the art tovariously employ the present disclosure in virtually any appropriate detailedstructure. Like reference numerals may refer to similar or identical elements throughout the description of the figures. 31. 31. 31. id="p-31" id="p-31"

[0031] Fig. 1 shows an example ofa method for controlling rotational speed of a turbine in a turbine system, which is shown in Fig.[0032] The method is performed by a controller 101 for controlling rotationalspeed ofa turbine 122 in a turbine system 105. The method comprises collectingS100 sensor data from sensors in the turbine system 105. The sensors may bepressure sensors 130 and drive sensors in the drive unit 126. The sensors mayfurther be temperature sensors 128. The temperature sensors 128 and pressuresensors 130 may be arranged at the inlet side of the turbine, the drive sensors arearranged in the drive unit 126. The collected sensor data may be real-time data orprocessed data, such as calculated or estimated data. The method furthercomprises comparing S102 the sensor data with reference values to determinewhether the collected sensor data is valid or not. The reference values may bepreviously collected sensor data and/or theoretically determined values. Thereference values typically define a range of values within which the collectedsensor data must fall in order to be determined as valid. lf it is determined that thecollected sensor data is not valid, step S100 is repeated and new sensor data iscollected and then compared with reference values in step S102 to determine the validity. 33. 33. 33. id="p-33" id="p-33"

[0033] The method further comprises calculating S104 the current operatingpoint of the turbine system 105 and determining S106 a current optimal rotationalspeed of the turbine 122 based on the current operating point of the turbinesystem 105. The current optimal rotational speed is determined from anempirically validated model of the turbine 122. The empirically validated model willbe closer defined and described in conjunction with Fig. 4 to Fig. 6. The underlyingfunctional relationship of the model used may be arbitrarily complicated but mustdepend on different system variables that are accurately measured by sensorsused by the present method. Such sensor may for example be temperaturesensors 128, pressure sensors 130, different sensors in the drive unit 126 or othersensors capturing different properties of the turbine system[0034] Fig. 4 describes an example of comparison of turbine model with experimentally fitted data for a few *=í;__variations of the input state to the turbine 122, in this case variations of the input temperature to the turbine. Theexperimental data is created by using a CFD (Computational Fluid Dynamics)model to simulate optimal speed for a turbine at different operation conditions. ln this example the simulation varies the input temperature to the š:~.;'::.:í.=“::::*t;;::.si ;_1.:~':;::::::~f:: :““:,..ï::iï~':fw::š>:ip. The dotted line is the simulated optimal speedtaken into account the different simulated operating temperatures. . . . id="p-35" id="p-35"

[0035] Turning now to Fig. 5 an empirical fit to net power output is shown. TheCFD simulated optimal speed curve in Fig. be used as a basis whenperforming the empirical test. The empirical test is performed on the turbinesystem 105 which also may be seen as a module, i.e. including pumps, powerelectronics drivetrain, etc. and not the turbine 122 itself. ln the case depicted inFig. 5, module M38 is tested for different rotational speeds at a constant pressureratio Pin/Poul. The net power output is measured at intervals of 1000 rpm. A curveis plotted with straight lines between the measured values and any known curvefitting technique may be used to a obtain an optimal curve fit. ln this case thehighest net power output was found at 16 000 rpm and when using the obtainedoptimal curve fit the highest net power output is at 15 790 rpm. By using thisempirical definition of the optimal rotational speed for the turbine 122, any external losses due to pumps, power electronics drivetrain, etc. of the turbine system aretaken into account when calculating the optimal rotational speed for the turbine122. This also means that the deviation from the turbine model based onexperimental data is taken into account when calculating the optimal rotationalspeed. Thus, the calculation of the optimal rotational speed of the turbine 122 isbased on the current operation conditions of the turbine system[0036] Fig. 6 describes functional relationship of current optimal rotationalspeed limited to dependence on pressure ratio between inlet and outlet of theturbine. ln Fig.6 many empirical tests are summarized for different pressure ratiosand also for different modules, i.e. M38 and M6. There is also shown a fittedcurve, one point of which (encircled) is the optimal point found during the empiricaltest described in conjunction with Fig. 5. The dotted line in Fig. 6 represents thecurrent optimal rotational speed and is based on the obtained empirically validated model. 37. 37. 37. id="p-37" id="p-37"

[0037] After that the current optimal rotational speed has been determined, thevalidity of the current optimal rotational speed is checked in step S108. This isdone by applying a filter 132 to the determined current optimal rotational speed,which filter 132 sorts out rotational speed values have predefined properties, i.e.values that are outside a specified range. Such a range may be defined byapplying limits, dead bands and rate of change limitations to the determinedcurrent optimal rotational speed values. lf the optimal current rotational speed isdetermined to be valid in step S108 the method continues with step S112, in whichthe drive unit 126 is controlled to drive the turbine 122 at the determined current optimal rotational speed. 38. 38. 38. id="p-38" id="p-38"

[0038] ln an exemplary embodiment, the above steps (S100-S112) mat beperformed continuously or at a specified rate. The refreshment rate may varbetween 0,5 to 30 seconds and is preferably set to once per second, which willgive a responsive system. The above described functional relationship may betracked by the controller 101, in which a computer program 132, stored there onimplements the method. The controller 101 may also have stored thereon theturbine model used to determine the current optimal rotational speed 39. 39. 39. id="p-39" id="p-39"

[0039] Now a non-limiting overview of the turbine control system 100 will bedescribed in conjunction with Fig.2. Details of the controller 101 are illustrated inFig.3. The turbine control system 100 comprises a controller 101, a turbine system105 and a communication system 103. The controller 101 is arranged to performthe method according to steps S100-S112, which were described above inconjunction with Fig. 1. The controller 101 may be arranged as an independentunit. The controller 101 may also be part of an independent system outside theturbine system 105, for example be provided in a cloud computing service andwith a communication channel to the turbine system 105 to exchange data with theturbine system 105. ln another exemplary embodiment the controller 101 may beintegrated in the turbine system 105 or be arranged as part of the turbine system105. The controller 101 comprises at least one processing circuitry 118 and atleast one non-transitory computer readable medium 120, the non-transitorycomputer readable medium 120 containing instructions executable by the processing circuitry[0040] The instructions executable by said processing circuitry 118 may bearranged as a computer program 132 stored in the non-transitory computerreadable medium 120. The processing circuitry 118 and the non-transitorycomputer readable medium 120 may be arranged in an arrangement. Thearrangement may alternatively be a microprocessor and adequate software andstorage therefore, a Programmable Logic Device, PLD, or other electroniccomponent(s)/processing circuit(s) configured to perform the actions, or methods,mentioned above. The non-transitory computer readable medium 120 may be amemory. The memory may be realized as for example a RAM (Random-accessmemory), ROM (Read-Only Memory) or an EEPROM (Electrical ErasableProgrammable ROM). Further, the computer program 132 may be carried by aseparate computer-readable medium, such as a CD, DVD or flash memory, fromwhich the program could be downloaded into the at least one memory. 41. 41. 41. id="p-41" id="p-41"

[0041] The computer program 132 may comprise computer readable codemeans, which when run in the controller 101 causes the controller to perform thesteps described in the method as illustrated in Fig. 1. Analogously, the computer program 132 may comprise computer readable code means, which when run inthe turbine system 105 or in a separate unit causes the turbine system 105 or theseparate unit to perform the steps described in the method as i||ustrated in Fig.[0042] Although the instructions described in the embodiments disc|osed aboveare implemented as a computer program 132 to be executed by processingcircuitry 118, at least one of the instructions may in alternative embodiments beimplemented at least partly as hardware circuits. 43. 43. 43. id="p-43" id="p-43"

[0043] The controller 101 may comprise an interface 116 for communicatingwith the turbine system 105 via the communication system 103. The interface 116can be software, a mix of software and hardware in the form of hardwiredconnections, different field bus solutions and communication protocols. 44. 44. 44. id="p-44" id="p-44"

[0044] The controller 101 further comprises at least one functional unit, thefunctional units are shown in Fig.3. The controller 101 may comprise a collectingunit 102 for collecting sensor data from the temperature sensor 128, the pressuresensor 130 and the drive unit 126, a first checking unit 104 for comparing thecollected sensor data with reference values to determine that the collected sensordata is valid, an evaluating unit 106 for calculating the current operating point ofthe turbine system 105 based on the collected sensor data and systemparameters of the turbine system 105, a first determining unit 108 for determininga current optimal rotational speed of the turbine based on the current operatingpoint of the turbine system 105, a second checking unit 110 for checking thevalidity of the current optimal rotational speed an adjusting unit 114 for controllingthe drive unit 126 to drive the turbine at the determined current optimal rotational speed. 45. 45. 45. id="p-45" id="p-45"

[0045] ln an exemplary embodiment, the controller 101 further comprises asecond determining unit 112 for determining the current optimal rotational speedby applying limits, dead bands and rate of change limitations to the current optimalrotational speed.[0046] Returning to Fig. 2, the turbine system 105 comprises a turbine 122, adrive unit 126, a pressure sensor 130 and a filter 132. At least one sensor mayfurther be arranged in the drive unit 126. ln some embodiments, a temperaturesensor 128 may also be arranged in the turbine system 105. As described above,the temperature sensor 128 and the pressure sensor 130 may be arranged at thein|et side of the turbine. The number of sensors may be adapted depending onwhich empirically validated model of the turbine that the method used. The turbinesystem 105 may comprise a second interface 124 arranged for communicating with the controller 101 through the communication system[0047] Data exchanged through the second interface 124 from the turbinesystem 105 to the controller 101 may comprise of sensor data and system status.Data exchange through the first interface 116 from the controller 101 to the turbine system 105 may comprise commands and setpoints. 48. 48. 48. id="p-48" id="p-48"

[0048] lt will be appreciated that additional advantages and modifications willreadily occur to those skilled in the art. Therefore, the disclosures presentedherein, and broader aspects thereof are not limited to the specific details andrepresentative embodiments shown and described herein. Accordingly, manymodifications, equivalents, and improvements may be included without departingfrom the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

Claims (12)

1. A method performed by a controller (101) for controlling rotational speedof a turbine (122) in a turbine system (105), said turbine system (105) comprisingthe turbine (122), a pressure sensor (130), a filter (132) and a drive unit (126), themethod comprising: collecting (S100) sensor data from the pressure sensor (130) and the driveunit (126), - comparing (S102) the collected sensor data with reference values todetermine that the collected sensor data is valid, - calculating (S104) the current operating point of the turbine system (105)based on the collected sensor data and system parameters of the turbinesystem (105); - determining (S106) a current optimal rotational speed of the turbine based on the current operating point of the turbine system (105); - checking (S108) the validity of the current optimal rotational speed by usingthe filter (132) to sort out rotational speed values having predefinedproperties; - controlling (S112) the drive unit (126) to drive the turbine at the determinedcurrent optimal rotational speed.

2. The method according to claim 1, wherein the step of checking (S108)the validity of the current optimal rotational speed further comprises the step ofapplying (S110) limits, dead bands and rate of change limitations to the currentoptimal rotational speed values.

3. The method according to claim any of claims 1-2, further comprisingcontinuously or at a specified rate repeating the steps of claim

4. The method according to any of claims 1-3, wherein the current optimalrotational speed is determined based on an empirically validated model.

5. The method according to any of claims 1-4, wherein the sensor data istransmitted from the turbine system (105) to a controller (101) by means of acommunication system (103) from a second interface (124) of the turbine system(105) to a first interface (116) ofthe controller (101 ).

6. A controller (101) for controlling the rotational speed of a turbine (122) ina turbine system (105), said turbine system (105) comprising the turbine (122), apressure sensor (130), a filter (132) and a drive unit (126) and the controller (101)comprising processing circuitry (118) and a non-transitory computer-readablemedium (120), configured to store instructions (132), which when executed by the processing circuitry (118), cause the controller (101) to: -collect sensor data by means of the pressure sensor (130) and the drive unit(126), -compare the collected sensor data with reference values to determine that thecollected sensor data is valid, -calculate the current operating point of the turbine system (105) based on thecollected sensor data and system properties of the turbine system (105), - determine a current optimal rotational speed of the turbine (122) based on thecurrent operating point of the turbine system (105); - check the validity of the current optimal rotational speed by using the filter (132)to sort out rotational speed values having predefined properties; - control the drive unit (126) to drive the turbine (122) at the determined current optimal rotational speed.

7. The controller (101) according to claim 6, wherein the controller (101) isfurther caused to determine the current optimal rotational speed by applying limits,dead bands and rate of change limitations to the current optimal rotational speed.

8. The controller (101) according to any of claims 6-7, wherein thecontroller (101) is further caused to continuously or at a specified rate repeat thesteps of claim

9. The controller (101) according to any of claims 6-8, wherein thecontroller (101) is further caused to determine a current optimal rotational speedbased on an empirically validated model.

10. The controller (101) according to any of claims 6-9, wherein thecontroller (101) is further caused to transmit sensor data from the turbine system(105) to the controller (101) through a first interface (116).

11. A turbine control system (100) for controlling the rotational speed of aturbine (122), comprising a turbine system (105), a communication system (103)and a controller (101) according to any of claims 6-10, wherein the turbine system(105) comprises a turbine (122), a drive unit (126), a pressure sensor (130), a filter(132) and a second interface (124), wherein the turbine system (105)communicates with a first interface (116) ofthe controller (101) through thecommunication system (103) and the second interface (124).

12. A computer program (132) comprising non-transitory computer readablecode means being adapted, if executed on processing circuitry (118), to implementthe method according to any one of the claims 1 to 5.