Field of the Invention

The present invention relates to the power industry and can be used in electrohydraulic steam turbine control systems using microprocessor technology.

The control over a steam turbine is performed by adjusting the position of riding cutoff valve that displaces the servomotor rod, which, in turn, controls the control valves, i.e. the supply of steam to the turbine. Conventional methods of adjusting the position of riding cutoff valve imply that the riding cutoff valve is placed in a predetermined position corresponding to the established position of the servomotor. However, the above is not true for steam turbines with high-rate servomotors. Due to multiple often unpredictable factors such as temperature, air accumulation, oil pressure, wear-and-tear of friction members etc., the zero position of riding cutoff valve changes, leading to servomotor pulsations and subsequent decrease in positioning accuracy of control valves, as well as the decrease in operating life of steam distribution machinery.

DE 4236846 (Al) discloses a method of adjusting position of riding cutoff valve based on the use of a hydraulic system, wherein an auxiliary valve is used in addition to the riding cutoff valve. In the known method, the auxiliary valve is provided with an electromagnetic device fitted with a position adjuster; using said position adjuster, the auxiliary valve displaces the riding cutoff valve for a distance corresponding to the displacement between the actual and the predetermined positions of the servomotor rod. The known method is easy to implement, however, has insufficient accuracy and operating speed.

Furthermore, the known method has a significant disadvantage inherent to the hydraulic control system: the dependence of spool-type elements with low permutative force on the quality of oil, e.g. on presence of mechanical particles that are always present in the oil, even if it possesses flawless properties as determined by control sampling.

A more advanced method of adjusting the position of riding cutoff valve is disclosed in US 2004081549 (Al). In the electrohydraulic system of automatic turbine rotation speed control, the riding cutoff valve position adjustment contour includes an electromechanical converter, a controller for controlling said converter, and a logical unit processing feedback signals from the external contours of servomotor position and turbine rotation speed. The electromechanical converter is formed using two electromagnetic coils, one of which is the main coil, and the other one is the additional coil. The main coil creates permutative force to displace the riding cutoff valve in accordance with task signal from the controller, and the additional coil is fed separately and creates additional permutative force in accordance with the task signal from the logical unit. The use of additional electromagnetic coil allows to compensate for insufficient displacement of the riding cutoff valve created by the main coil if necessary, and therefore, to increase the accuracy of adjustment. However, a significant disadvantage of said method lies in the fact that the adjustment is performed according to feedback signals, with a delay due to control system response time, with predetermined controller parameters and not taking into account the changes in current work conditions of the control system, which does not allow to minimize-the servomotor pulsations in the established mode. The object of the present invention is to significantly increase the accuracy of positioning of adjustment valves by minimizing the servomotor pulsations in the established mode by correcting the zero position of the riding cutoff valve when current work conditions of the control system are changed, and also to solve the problem of breakdown of automatic control system due to mechanical particle contamination of spool- type devices.

The object is attained by a method of adjusting the position of riding cutoff valve that controls the displacement of servomotor, which, in turn, controls the steam turbine valves in the steam turbine control system with dynamic zero displacement correction of the riding cutoff valve, wherein said method includes determining the value of riding cutoff valve displacement required to provide the predetermined position of the servomotor and displacing the riding cutoff valve in accordance with said predetermined value; wherein at the step of determining the riding cutoff valve displacement value, prior to adjustment step, the values of riding cutoff valve zero displacement are calculated for different positions of servomotor rod over the whole range of said rod displacements; and at the adjustment step, the value of zero displacement dynamic correction for the given servomotor rod position is calculated taking into account the zero displacement measured for the current position of servomotor rod, and the value of displacement of the riding cutoff valve is determined using the dynamic zero correction.

Prior to adjustment, the zero displacement values of the riding cutoff valve are measured in different positions of servomotor rod over the whole range of displacements of said rod; as a result of said measurements, the function of zero displacement of the riding cutoff valve and the servomotor position is obtained; said function reflects the effect of the control valve and the springs connected thereto on the zero displacement of the riding cutoff valve. Further, when steam is supplied to the turbine, the zero displacement occurs due to forces applied by the steam to the control valve, and due to the oil pressure changes in the servomotor line. The zero displacement of the riding cutoff valve leads to decrease in accuracy, and thus, to multiple adjustment iterations, which leads to servomotor pulsations. In order to compensate for said influences, the zero displacement dynamic correction value is calculated for the current position of the servomotor rod. Said value is calculated taking into account the zero displacement values measured prior to adjustment, and the integral term reflecting the influence by factors appearing when the steam is supplied to the turbine.

The calculated dynamic correction value is taken into account when determining the required position of the riding cutoff valve in order to achieve the predetermined servomotor position, thus allowing to compensate for the aforementioned negative influences and to minimize servomotor pulsations, thus increasing accuracy of adjustments.

In the preferred embodiment, the displacement of the riding cutoff valve is performed using an electromechanical converter. In this case, the electromechanical converter rods and the riding cutoff valve rods are rigidly connected to each other, forming a uniform adjustment element. The positioning contour of the riding cutoff valve includes a riding cutoff valve position sensor. The displacement of the riding cutoff valve determines the change in servomotor position. The positioning contour of the servomotor is external with respect to the positioning contour of the riding cutoff valve, and it also includes a position sensor mounted on the servomotor rod. Herein, the term "servomotor position" is used for simplicity, and denotes the position of servomotor rod. Depending on the servomotor position, the measurement of riding cutoff valve zero displacement can be performed manually or using self-tuning. During manual measurement, the servomotor is forcibly placed in different positions over the whole range of adjustment thereof, and the zero displacement of the riding cutoff valve is recorded for each of said servomotor positions based on data provided by the riding cutoff valve position sensor.

The measurement of zero displacements of the riding cutoff valve is performed using a programmable controller in the positioning contour of the servomotor. The servomotor is sequentially positioned in several positions with a predetermined step, said programmable controller records the data from the position sensor mounted on the servomotor rod, and if servomotor vibrations are present, the integral term of the corresponding vibrations is adjusted until the servomotor is stabilized, then the zero displacement of the riding cutoff valve is recorded taking into account said integral term. The measurement of zero displacements of the riding cutoff valve using self-tuning allows to achieve greater adjustment accuracy, but the measurement procedure takes more time than less accurate manual measurement.

The data acquired using manual and automatic measurement is stored as a data array in the programmable controller memory, and is used as a correction when determining the required riding cutoff valve displacement value in order to achieve the predetermined servomotor position.

The determination of riding cutoff valve displacement value is performed by the programmable controller included in the servomotor positioning contour. When determining the required riding cutoff valve displacement value, the dynamic correction of zero displacement of the riding cutoff valve is taken into account; said dynamic correction is calculated taking into account the zero displacement array element component for the current servomotor position calculated in testing mode.

In automatic control systems (ACS) with backup, that include more than one control channel, the measurement of zero displacements of the riding cutoff valve and the formation of measured value array are performed separately for each channel, and the dynamic zero correction of riding cutoff valve is applied when operating any control channel. In ACS with more than one servomotor (SM), the measurement of zero displacements of the riding cutoff valve and the formation of measured value array are performed separately for each SM, and the dynamic zero correction of riding cutoff valve is applied when operating any SM. The positioning of the riding cutoff valve taking into account the zero dynamic correction allows to minimize the vibrational amplitude of the riding cutoff valve in the established mode, and consequently, to minimize the amplitude of servomotor pulsations. Furthermore, the electromechanical converter used in the present method for displacing the riding cutoff valve allows to generate a considerable force, which allows to solve the problem of ACS breakdowns due to mechanical particle contamination of spool-type devices.

The steam turbine automatic control system usually comprises the following adjustment contours listed here in order from the external contour to the internal contour: the adjustment contour of active power of the turbine generator; of turbine rotor rotation frequency adjustment; of steam pressure maintenance; the positioning contour of the servomotor; and the internal contour with respect thereto - the positioning contour of the riding cutoff valve. In order to disclose the present mehod of adjusting the riding cutoff valve position, the positioning contour of the servomotor and the internal contour with respect thereto - the positioning contour of the riding cutoff valve are of particular interest. Thus, the external contours with respect to the positioning contour of the servomotor are omitted in the description of the present adjustment method.

Fig. 1 schematically shows a part of ACS including the positioning contour of the servomotor. As shown in fig. 1 , the positioning contour of the servomotor (SM) is external with respect to the positioning contour of the riding cutoff valve (RCV) in the automatic control system. The positioning contour of RCV comprises internal contours of speed and voltage control, that are not shown in fig. 1 for clarity. On hardware level, the positioning contour of RCV is formed by an electromechanical converter EMC and a position sensor PS1 of thr RCV. The EMC, in turn, comprises an adjustable controller AC, an electromotor EM and the exit rod connected to the rotor via EM mechanism that converts the rotational motion of the rotor into the progressive motion of the rod. The EMC rod is rigidly connected to the riding cutoff valve rod, forming a uniform adjustment contour element. Therefore, the task of positioning the RCV is attributed to the adjustment of electromotor rotation. The AC performs the adjustment of electromotor rotation according to the RCV displacement signal received from the programmable controller (PC) from th external positioning contour of the SM. The AC is formed with possibility of preliminary adjustment of settings according to voltage and speed in accordance with the electromotor used.

The position sensor of SM (PS2) is included in the positioning contour of SM, external with respect to the positioning contour of RCV. PS2 is mounted on the servomotor rod, which is rigidly mechanically connected to the control valve CV of the turbine. The signal from said sensor, corresponding to the current position of the SM, enters the PC, that calculates the displacement value of RCV required in order to reach the predetermined position of the SM. The predetermined position of the SM is also determined by the PC based on signals from external contours of the steam turbine ACS, which are not shown in fig. 1.

The present method of adjusting the RCV position with dynamic zero correction includes the first step of receiving the function of zero displacement of the riding cutoff valve and the servomotor rod position over the whole range of SM adjustments. To this end, the servomotor is forcibly placed in different positions over the whole range of adjustment thereof, and the corresponding zero displacement of the riding cutoff valve is measured for each of said servomotor positions. The measurement can be performed manually (by recording the sensor data visually) or using self-tuning.

The manual measurement is performed when the generator is turned off and the turbine is stopped.

During manual measurement, the automatic positioning contour of the SM is turned off, and the SM is forcibly positioned in several positions with the step determined arbitrarily; the whole range of SM adjustments is thus passed. For each SM position, the corresponding zero displacement of RCV is measured. The measurement of SM and RCV positions is performed visually as indicated by SM abd RCV position sensors. The value array thus received is recorded in the PC memory. The zero displacements of RCV corresponding to intermediate values of SM position are obtained by the way of approximation, thus forming the array of predetermined dimensions. For example, in the SM with adjustment range from 0 to 320 mm, the SM is positioned with a step of 10 mm, and 32 values of zero displacement of the RCV are thus obtained; the intermediate values are then calculated by the way of approximation, and the array of 320 values of zero displacements of the RCV is thus obtained. The manual measurement of RCV displacements is used when high correction accuracy is not required. The self-tuning is performed using PC with the automatic positioning contour of the

SM turned on. The subsequent positioning of the SM is set with a step of 1 mm over the whole adjustment range. In said automatic SM positioning mode, not only the proportional term, but also the integral term is taken into account during the RCV displacement determination. Based on signal from the PS2, the PC determines the presence of SM vibrations, and the integral term is calculated each 10 ms in order to minimize the corresponding vibrations of the RCV position; the PV stores said integral term until the SM position is stabilized. If current SM position maches the predetermined position thereof, and no SM vibrations are detected over 5 s (another waiting time value can be set), the PC records the accumulated integral term in the array element corresponding to the zero displacement of the RCV for the current SM position. Then, the PC decides to switch to the next positioning point, and 1 mm is added to the predetermined coordinate of the SM positioning.

The integral term is calculated as follows: r£ t. = + °' ^{01 * Err or} · ^T 0 ⁾ where is the accumulated integral term,

Error is deviation of the regulated parameter in relative units, calculated as difference between the predetermined and the current positions of the servomotors in absolute units, related to the range of servomotor displacements,

T _a is the integral term amplification coefficient, which is a constant value during this step, and is selected to provide high accuracy of SM positioning. T _u — 0,07 .

Thus, the array for RCV zero displacements correction is formed. For a SM with adjustment range from 0 to 320 mm, the array consists of 320 elements. Values included in said array reflect the influence of control valves and springs mounted thereon on the zero displacement of the RCV. The element values in said array reflect the actual SM characteristics. The RCV zero displacement can be irregular over the SM displacement range. The array obtained during this step is then used in calculating the dynamic correction of RCV zero displacement.

In ACS formed with control channel backup, the measurement of RCV displacements is performed for each control channel separately, and the integral term value array of RCV zero displacements is formed for each channel separately.

In ACS formed with several SMs, the measurement of RCV zero displacements is performed for each SM separately, and the integral term value array of RCV zero displacements is formed for each SM separately.

The next step of RCV stabilization is performed with the generator turned on, when steam is supplied to the turbine. In this case, the zero displacement occurs due to forces applied by the steam to the valves, due to air gaps in the electromechanical converter, and due to the oil pressure changes in the servomotor line. To compensate for said forces, an integral term with a variable amplification coefficient is introduced into the SM positioning contour.

The dynamic correction of the RCV zero displacement is calculated as follows:

«¾, = ⁱ &,(»≤£> + _¾( ^- · - · * (2) where is the dynamic correction of RCV zero,

*7 _{CM :} ^ ^s ^ ^{e correc} ° ⁿ °f RCV zero displacement for the current SM position from the array formed during the previous step, is the current SM position in relative units, ■ ^iS^ ¹ * ^{s me} predetermined SM position in relative units, Ti is the variable amplification coefficient of the integral term. Γ, has the following values: T _u = 0, if Err or = 0,

T _a = 0,1, if Error < 3 ¾¾,

T _u = 0„6, if 3 < Error < 4 Am.

The dynamic zero correction of the RCV calculated using the above equation is taken into account when determining the RCV displacement required to reach the predetermined SM position based on the current SM position. The RCV displacement is determined as follows:

where is the required RCV position, V^ _BB - i ^{s me} dynamic zero correction of the RCV calculated according to (2), k _p is the propotional regulator amplification coefficient determined based on ACS properties.

The value of required RCV displacement calculated by the programmable controller PC in the SM positioning contour, is directed to the RCV positioning contour as the control input.

The present method of RCV position adjustment makes the ACS invariable towards the condition of hydraulic executive devices, which allows to provide the servomotor stability without considering the actual power properties thereof.