A TURBINE CONTROL DEVICE

Abstract: A turbine control device which compensates for non-linearity of the degree-of-opening/flow-rate characteristic of a regu­ lating valve (14). The device has a turbine rotation speed detector (18), a calculator (20, 24) to determine a desired flow rate based on a predetermined rotation speed and the detected speed, a valve (14) for controlling the flow rate with a variable degree of opening, a flow rate detector (42), a memory unit (48) for memorizing combinations of the detected flow rate and respective degrees of opening of the valve, a converter (44, 46) for converting the desired flow rate into a valve opening command signal based on the memorized combi­ nations of the flow rate and respective degrees of opening of the valve, and a servo controller (28) for the valve.

Description: Field of the Invention This invention relates to a turbine control device which compensates for non-linearity of the degree-of-opening/flow-rate characteristic of a turbine regulating valve. Description of the Prior Art In general, the output of a steam turbine is regulated by using a regulating valve to alter the flow rate of the steam flowing into the turbine. However, the degree-of-opening/flow-rate characteristic of the regulating valve is normally non-linear, with a differential coefficient that is a maximum at the start of opening and which tends to drop as the valve gets closer to being fully open. The turbine control device is, therefore, usually provided with a function generator to compensate for this non-linearity. This will be described with reference to Fig. 1. Inflow of steam generated in a boiler 10 into a turbine 12 is regulated by a regulating valve 14. The steam that flows into the turbine 12 rotates the turbine 12, driving a generator 16 and generating electric power. The actual speed of the turbine 12 is detected by a speed detector 18. A deviation calculator 20 makes a comparative calculation with a speed value set by a speed/load setter 22. The deviation between this set speed and the actual speed is converted into a flow rate instruction by a speed controller 24 and sent to a function generator 26. Based on a preset function form, the function generator 26 converts the flow rate instruction into a valve degree-of-opening instruction, which is then sent to a servo controller 28. The servo controller 28 controls the degree of opening of the regulating valve 14 in accordance with this valve degree-of-opening instruction. Thus, the turbine 12 is controlled to a prescribed speed based on the degree of opening of the regulating valve 14. In such a turbine control device, the function generated by the function generator 16 to compensate for the non-linearity of the degree-of-opening/flow-rate of the valve was conventionally set only with reference to the design data of the regulating valve. As a result, in many cases it would either overcompensate or undercompensate and it was difficult to obtain proper linearity. Furthermore, as this function generator was realized by a mechanical cam or electrical polygonal line function generator, resetting to adjust to operational data was difficult, and could only be carried out after shutting down the turbine. This wastes a lot of time and reduces the life of the turbine rotor. A known device and method for controlling a turbine (US-A-4 267 458) of the kind referred to by the features of the entering clause of claims 1 and 6 respectively, utilizes a degree of opening/flow rate characteristic which is defined by keyboard entered quantities. These quantities may be tuned or adjusted in a maintenance test mode of operation. Due to the entering of quantities by means of the keyboard the control is relatively cumbersome. The control accuracy is furthermore limited since an adjustment of these quantities is performed only at a maintenance test mode. Summary of the Invention One object of this invention is to provide a turbine control device including a function generator which compensates for non-linearity between the degree of opening of the regulating valve and the flow rate. Another object of this invention is to provide a method of controlling a turbine, whereby nonlinearity between the degree of opening of the regulating valve and the flow rate can be easily and accurately compensated. According to one aspect of this invention, there is provided a device for controlling a turbine which is driven by a flowing fluid, the device comprising: means for determining a desired flow rate of the fluid; a valve for controlling the flow rate with a variable degree or opening; means for producing a signal indicative of a degree of opening of the valve; means for detecting flow rate and producing a signal indicative of the flow rate; means for sampling said degree of opening signal and said flow rate signal and memorizing a plurality of points, each indicative of a sampled detected flow rate and an actual degree of opening of the valve for producing the sampled detected flow rate; means for converting the desired flow rate into a valve degree of opening command signal based on the memorized point, and means for operating the valve according to the valve degree of opening command signal. According to another aspect of this invention, there is provided a method of controlling a turbine which is driven by a flowing fluid, the method comprising steps of: determining a desired flow rate of the fluid; controlling the flow rate with a variable degree of opening of a valve; producing a signal indicating the degree of opening of the valve; detecting flow rate of the fluid and producing a signal indicating the flow rate; memorizing a plurality of points, each indicative of a sampled detected flow rate and actual degree of opening of the valve for producing the sampled detected flow rate; converting the calculated desired flow rate into a valve degree of opening command signal based on the memorized points; and operating the valve according to the valve degree of opening command signal. Further objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments that follows, when considered with the attached drawings. Brief Description of the Drawings The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate preferred embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings: Fig. 1 is a block diagram of a turbine control device of the prior art; Fig. 2 is a block diagram of an embodiment of a turbine control device of this invention; Fig. 3 is a detailed block diagram of the sampling memory unit shown in Fig. 2; Fig. 4(a) is a graph showing the variation of the generator output x with respect to time during a load increasing process; Fig. 4(b) is a graph showing the variation of the valve degree-of-opening instruction y with respect to the corresponding time shown in Fig. 4(a); and Fig. 5 is a graph of the valve degree-of-opening instruction y as the ordinate and the flow rate instruction x as the abscissa, showing how to set the function generator in the embodiment shown in Fig. 2, in accordance with the variations of x and y shown in Figs. 4(a) and 4(b). Detailed Description of the Preferred Embodiments Referring to Fig. 2, parts which are the same as in Fig. 1 are indicated by the same numerals. A boiler 10 produces steam, and the steam is introduced to a turbine 12. The flow rate of the steam is controlled by a regulating valve 14. The steam that flows into the turbine 12 rotates the turbine 12, driving a generator 16 and generating electric power. The actual speed of the turbine calculator 20 by a speed detector 18. A deviation calculator 20 makes a comparative calculation with a speed value set by a speed/load setter 22. The deviation between this set speed and the actual speed is converted into a flow rate instruction F by a speed controller 24 and sent to a function setter/generator unit 40. Based on a pre-set function, the function setter/generator unit 40 converts the flow rate instruction F into a valve degree-of-opening instruction signal y, which is then sent to a servo controller 28. The servo controller 28 controls the degree of opening of the regulating valve 14 in accordance with this valve degree-of-opening instruction. Thus, the turbine 12 is controlled to a prescribed speed based on the output of function setter/generator 40 controlling the degree of opening of the regulating valve 14. A power detector 42 is arranged to detect the electric power produced by the generator 16 and produce a power signal x. The produced power is proportional to the steam flow rate. The function setter/generator unit 40 has a first function generator 44, a second function generator 46, a sampling memory unit 48, a first switch 50 and a second switch 52. One of the two function generators, the first function generator 44 in the case of Fig. 2, is in an operational mode, and the other function generator, the second function generator 46 in the case of Fig. 2, is in a setting mode. The two function generators 44 and 46 are used alternately in the two modes, and are switched between modes using the two switches 50 and 52 on a periodic basis. Switches 50 and 52 may be manual switches which are held in the first position during initial acceleration of the turbine and then switched manually to the second position by an operator after the turbine has reached steady state operation. The function generators 44 and 46 receive the flow rate instruction from the speed controller 24. With switches 50 and 52 in the positions shown in Fig. 2, only the function generator 44 is in the operational mode and gives the valve degree-of-opening instruction signal y to the servo controller 28 via the second switch 52. The function generator 46 is in the setting mode and receives a plurality of combinations of points z = (x, y) from the sampling memory unit 48 via the second switch 50, where x denotes the electric power signal from the detector 42 and y denotes the valve degree-of-opening instruction signal given to the servo controller 28. Based on the sampled values of x and y, the setting of function generator 46 is carried out. An embodiment of the sampling memory unit 48 will now be described referring to Fig. 3. A first averaging circuit 60 produces an average of the power signal x produced by the detector 42, and its output is denoted as a mean output x . A second averaging circuit 62 produces an average of the valve degree-of-opening instruction signal y from the function generator 44 in the operational mode, and its output is denoted as a mean degree-of-opening instruction y . It should be understood that the degree-of-opening instruction signal y is being used here to indicate valve position. A signal from a sensor connected to the valve 14 to produce a signal indicating valve position could also be used. The mean power signal x is sent to comparators 641, 642, ..., where it is compared to preset power levels, as discussed below. The outputs of the comparators 641, 642,... are sent, respectively, to one-shot pulse generators 661,662,..., where pulses are produced in response to the outputs of the comparators 641,642,..., respectively. X-registers 681,682,... store, respectively, the sampled output values x₁, x₂,... of the first averaging circuit 60 at the times when the respective pulses are output. Y-registers 701,702,... store the valve opening degree instructions y₁, y₂,..., which are sampled output values of the second averaging circuit 62, at the times the respective pulses are output. A changeover switch 72 is a multiplexer which sequentially transmits the output of the function set value combinations (x₁, y₁), (x₂, y₂),... to the function generator in the setting mode, the second function generator 46 in the case of Fig. 2. The operation of the above device will now be described during a load increasing process when the preset values of the comparators 641, 642 and 643 are respectively set at P/3, 2P/3 and P where the rated power is denoted P. On changeover of switches 50 and 52 to the position shown in Fig. 2, the first function generator 44 is in the operational mode, while the second function generator 46 is in the setting mode. During the process of a load increase, the actual turbine speed detected by the speed detector 18 is compared with the set value of speed/load setter 22, and the speed deviation is converted to a flow rate instruction by the speed controller 24. This flow rate instruction is converted to a valve degree-of-opening instruction signal y by the first function generator 44 and applied to the servo controller 28 which adjusts the degree-of-opening of the regulating valve 14. Thus the amount of steam supplied to the turbine 12 from the boiler 10 is adjusted so that the output of the generator 16 rises. The output x detected by the power detector 42 and the corresponding averaged output x are thus increased from 0. When x reaches P/3, the comparator and the one-shot pulse generator 661 come into action, so that the mean generator output x₁ (⁻ P/3) and the mean valve degree-of-opening instruction y₁ are sampled and stored in the registers 681 and 701 respectively to indicate the valve position required to give a flow rate proportional to P/3. Subsequently, at the time points x₂ = 2P/3 and x₃ = P, the mean generator outputs x₂ and x₃ and the mean valve degree-of-opening instruction signals y₂ and y₃ are likewise successively sampled and stored in the registers 682 and 702, and 683 and 703. Figs. 4(a) and 4(b) show an example of the progress of this operation taking the time axis as a reference, in which the generator output signal x is linear with respect to the time axis, and the valve degree-of-opening signal y is nonlinear. Therefore, for the generator outputs x₁, x₂ and x₃ at practically equal time intervals, the corresponding valve degree-of-opening instruction signals y₁, y₂, and y₃ at unequal intervals are obtained. The multiplexer switch 72 successively transmits the respective function set values of the generator output signals x and the valve degree-of-opening instruction signal y stored in the respective registers 681, 682,... and 701, 702... to be used in the function generator in the setting mode to generate a new function. Function generators, per se, are well-known and will not be discussed in detail here. As shown in Fig. 5, a function curve with a change of gradient at three points can, therefore, be obtained by interpolating between the points (x₁, y₁), (x₂, y₂),.... obtained by correlating the generator output signal x, and the valve degree-of-opening instruction signal y. This curve is updated periodically and used by the function generators to produce a new valve degree-of-opening signal y. After the process of turbine control by adjustment of the degree-of-opening of the regulating valve 14 while the load is being increased has been completed, the first and second switches 50 and 52 are changed over, so that the second function generator 46 is in the operational mode, and the first function generator 44 is in the setting mode. Thus, excellent turbine control can be achieved by accurate conversion from the flow rate instruction to the valve degree-of-opening instruction in accordance with the actual device operation. Although in the above description, sampling using three points at equal intervals has been taken as an example, if required, sampling at unequal intervals or at a larger number of points can be performed by exactly the same technique. Furthermore, the method of obtained a function from the sampled values obtained is of course not restricted to the linear interpolation method shown in this embodiment. A higher order function could be used to interpolate between these points. In the above embodiment, the generator output is used as the signal corresponding to the flow rate. However, apart from this, exactly the same effect and advantages can be obtained by using other signals which are proportional or linear to the flow rate, such as the steam pressure of the first stage of the turbine, the reheated steam pressure, or a mean steam flow rate signal. According to the above embodiments of this invention, accurate non-linear compensation can be carried out and correction of the entire function can easily be achieved all together by means of the sampling memory unit.

Claims: A device for controlling a turbine which is driven by a flowing fluid, the device comprising: means for determining a desired flow rate of the fluid; a valve for controlling the flow rate with a variable degree of opening; means for producing a signal indicative of a degree of opening of the valve; means for producing a signal indicative of flow rate; means for sampling the degree of opening signal and the flow rate signal and memorizing a plurality of points each indicative of a sampled flow rate and an actual degree of opening of the valve for producing the sampled flow rate; means for converting the desired flow rate into a valve degree of opening command signal based on the memorized points; and means for operating the valve according to the valve degree of opening command signal, characterized in that the means for sampling and memorizing operate automatically and during the operational mode of the device. A device according to claim 1, wherein the means for converting the desired flow rate into a valve degree of opening command signal comprises: first and second function generators each having an operational mode for converting the desired flow rate into the valve degree of opening command signal based on a preset function and a presetting mode for presetting a function based on the memorized points; and means for controlling one of said function generators to be in the operational mode and the other function generator to be in the presetting mode, and switching over the first and second function generators between the operational mode and the presetting mode. A device according to claim 1, wherein the means for producing signal indicative of flow rate detects electric power generated by the turbine as an indication of flow rate. A device according to claim 1, wherein the means for memorizing comprises: means for comparing the flow rate signal with a plurality of predetermined values; and means for storing the value of the flow rate signal and the value of the opening position signal at the times when the detected flow rate indication coincides with the predetermined values. A device according to claim 2, wherein said degree of opening signal comprises the valve degree of opening command signal generated by the function generator in the operational mode. A method of controlling a turbine which is driven by a flowing fluid, the method comprising the steps of: determining a desired flow rate of the fluid; controlling the flow rate with a variable degree of opening of a valve; producing a signal indicating the degree of opening of the valve; producing a signal indicative of flow rate; sampling the flow rate signal and the degree of opening signal; memorizing a plurality of points each indicative of a sampled detected flow rate and an actual degree of opening of the valve for producing the sampled detected flow rate; converting the calculated desired flow rate into a valve degree of opening command signal based on the memorized points; and operating the valve according to the valve degree of opening command signal, characterized in that sampling and memorizing are performed automatically and during the operational mode. A method according to claim 6, wherein the step of converting the flow rate into the valve degree of opening command signal comprises the steps of: converting the flow rate into the valve degree of opening command signal based on a preset function in a first function generator; presetting a function in a second function generator based on the memorized points; and switching over the operations of the first and second function generators.

Inventors: TONE, YOUICHI C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1987
Main classification: F01D17
IPC codes: F01D17