Automated Steam Turbine Startup Method and Apparatus Therefor

Steam turbines are commonly used to drive electrical generators in power plants. A typical steam turbine is a massive yet intricate piece of machinery that must be started up in a controlled manner in order to protect the many turbine components from damage from stresses and distortion that would result from immediate exposure to high temperature and pressure steam. One part of the turbine startup process is the prewarming procedure, which includes the steps of steam chest warming and turbine rotor prewarming. The chest warming procedure warms the control valve steam distribution chest to the stop valves; this step is accomplished before the control valves are opened during turbine acceleration and loading. The goal of the rotor prewarming process is to raise rotor bore temperatures without exceeding allowable rotor stress limits; rotor stress is controlled by bringing temperatures of critical rotor locations to specified values before the turbine is accelerated to its nominal operating speed.

When a steam turbine is being started up from a cold-shutdown condition, necessary auxiliary systems (such as lubrication oil, auxiliary steam systems, and the like) are placed in service. The turbine is placed on a turning gear, which keeps the turbine spinning slowly (e.g., 2-3 rpm) during the prewarming procedure. The steam chest of the turbine must be warmed in a manner such that differential temperatures between inner and outer walls of the steam chest do not cause excessive stress in the metal and such that the turbine steam control valve(s) are warmed prior to the unit being exposed to nominal steam system pressures and temperatures.

In prewarming the rotating components of the turbine, small amounts of steam are admitted to the high pressure side of the turbine to cause the turbine rotor to warm up, both through direct exposure to the steam and conduction of heat through turbine components, such as the rotor shaft: During the heating process, condensate from the steam admitted to the turbine must be drained off to avoid buildup of liquid in the turbine housing to prevent the turbine buckets from passing through water thereby causing cavitation damage. This procedure is continued until specified temperatures are reached on key turbine components and the steam pressure in the turbine reaches a selected level, at which time the turbine is ready to be accelerated and loaded.

It is desirable from an operational standpoint to complete the prewarming procedure in the shortest time consistent with turbine limitations such as allowable rotor stress. The prewarming procedure is typically accomplished by operators manually checking various indications of critical turbine parameters, such as rotor temperatures and stresses, and then implementing actions (such as opening or closing the steam supply valve) to increase or decrease, as appropriate, the rate of the turbine warm-up. There are several constraints on the rapidity with which the turbine can be warmed, however, such as the amount of steam admitted should not cause the turbine to "roll-off" the turning gear and accelerate prematurely; rotor and steam chest stress limits cannot be exceeded, and the turbine pressures and temperatures must be maintained within specified limits. In some situations, however, such as when there is an immediate need to have the turbine-generator in service, it is necessary to complete the prewarming procedure more rapidly than normal. In such circumstances, the operator must optimize the procedure by accepting some deviation from normal warm-up procedures, such as increasing the steam flow and allowing the turbine to periodically "roll off" the turning gear while ensuring that the turbine is not damaged by premature acceleration.

Efficiency and ease of operation make it advantageous to automate as many turbine operating control procedures as possible. There are, however, few good models of turbine operation at low temperatures and pressures, and thus traditional control oriented approaches to automation are not readily adapted to control the turbine prewarming procedure.

It is therefore an object of this invention to provide an automated method of prewarming a steam turbine.

A further object of this invention is to provide a turbine operation method employing fuzzy logic to generate signals to control the prewarming procedure for a steam turbine.

A still further object of this invention is to provide an automated prewarming procedure for a steam turbine that allows the operator to select a warm-up profile ranging between a slow warm-up rate and a fast warm-up rate.

In accordance with this invention, a method for automated control of a steam turbine employs fuzzy logic and includes the steps of producing a control signal for a turbine regulatory control function by means of a fuzzy inference calculation in accordance with a selected fuzzy-logic rule base and a selected turbine startup profile, and applying the control signal for a turbine regulatory control function to a respective function controller such that a selected turbine startup profile is implemented.

In particular, a method for prewarming a steam turbine includes the steps of determining turbine performance parameters such as turbine bore temperature, turbine speed, turbine bore stress, and steam line condensate level. For each turbine performance parameter, a respective regulatory control direction is generated by a respective control loop. Further, weighting factors are produced by means of a fuzzy inference calculation based on selected performance parameters, the operator-selected warm-up profile, and a fuzzy logic rule base; a weighting factor is applied to a respective one of the regulatory control directions to produce the control signal for the turbine regulatory function. The operator-selected warm-up profile comprises at least a slow warm-up profile and a fast warm-up profile for the turbine to place the turbine in condition for operation (e.g., acceleration and loading). The weighted steam plant control signal is then applied to a respective steam plant controller. Typically, the step of producing the weighted steam plant control signal includes the steps of producing a weighted steam supply valve control signal and a weighted drain valve control signal. The step of producing these weighted control signals each respectively include assigning weights to respective control directions generated by respective regulatory loop processors for turbine bore temperature, turbine speed, turbine bore stress, and condensate level.

An apparatus for automated control of a steam turbine warm-up procedure includes a turbine warm-up control circuit coupled to the steam plant to receive selected sensing signals and to drive turbine regulatory control actuators, such as a steam supply valve actuator. The turbine warm-up control circuit comprises a plurality of control loop regulators, a fuzzy logic controller, and a regulatory output weighting component. The fuzzy logic circuit comprises means for generating a plurality of weighting factors. The weighting component is coupled to receive the respective steam system regulatory directions from the control loop regulators and to receive the weighting factors from the fuzzy logic circuit so as to generate the weighted steam plant control signal.

The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description in conjunction with the accompanying drawings in which like characters represent like parts throughout the drawings, and in which:

FIG. 1 is a schematic diagram of steam plant having a steam turbine that can be prewarmed in accordance with the method of the present invention.

FIG. 2 is a block diagram illustrating the steam turbine prewarming control architecture used in accordance with the method of this invention.

FIG. 3 is a table presenting an example of fuzzy logic mode selection rules used in accordance with the method of the present invention.

A steam plant 100 used in the generation of electricity commonly comprises a steam generator 110 coupled via steam piping 120 to deliver steam to a steam turbine 130, which in turn is mechanically coupled to turn an electrical generator 190. Steam generator 110 typically comprises a steam drum 112 in which water is converted to steam by heat from a thermal energy source such as an oil or coal-fired boiler or a nuclear reactor. The steam passes from steam generator 110 through steam piping 120 so as to be directed to components in steam plant 100, such as turbine 130, in which energy in the steam is extracted. Steam that has passed through such steam plant components is exhausted to a condenser 180 in which the steam is condensed and the resulting water is typically fed back into the steam generator.

Steam piping 120 comprises a main stop valve 122, a main stop valve bypass valve 124, and a plurality of drain valves 126 (typically relatively small valves used to drain condensed steam out of the steam lines). Steam piping 120 is coupled to turbine 130 such that steam passing from steam generator 110 enters turbine 130 through a steam chest 132 and a control valve 134. As illustrated in FIG. 1, steam turbine 130 comprises a high pressure turbine portion 140, a reheat turbine portion 150, and a low pressure turbine portion 160; each of these turbine portions are mounted on a common turbine shaft 135. In this arrangement, high pressure turbine portion 140 is typically coupled via steam piping 120 to steam generator 110 such that steam that has passed through high pressure turbine portion 140 is piped back to steam generator 110 (where it is reheated). Reheat turbine portion 150 is coupled via steam piping 120 and combined reheat valves 151 to steam generator 110 such that reheated steam is directed into the high pressure end of reheat turbine portion 150. Reheat turbine portion 150 is further coupled to low pressure turbine portion 160 such that steam exhausting from reheat turbine portion 150 is directed into the low pressure turbines. Low pressure turbine portion 160 is coupled to condenser 180 such that steam exhausting from the low pressure turbine(s) passes into condenser 180. Common turbine shaft 135 is coupled to drive electrical generator 190.

Further, a turning gear 170 is detachably coupled through a clutch type mechanism to turbine shaft 135. The turning gear comprises a motor and associated gearing that is adapted to rotate the turbine at a slow speed, e.g., 2-3 rpm, during periods when steam is not being used to drive the turbine, such as during maintenance and start-up operations.

After a shutdown period when turbine components have cooled from their normal operating pressures, a deliberate warm-up procedure must be followed to avoid excess stress on turbine components. In particular, care must be taken to not cause stress-induced damage of the steam chest and the rotor of the turbine. For example, the critical locations to be warmed comprise a first stage rotor bore 142 of high pressure (HP) turbine 140, an inlet section 152 of reheat turbine (RHT) portion 150, and an exhaust section 154 of RHT portion 150. In the prewarming procedure the heating of the HP rotor bore is controlled by the application of steam to the turbine (typically through control of main steam bypass valve 124). The heating of reheat turbine portion 150 results from conduction along the rotor (common shaft 135) of the HP turbine and from steam drawn in from steam seals around the rotor.

The following temperatures are exemplary of minimum temperatures required prior to acceleration and loading of a large (e.g., 500 MW)turbine generator:

HP rotor bore (1st stage area)temp.:>300.degree. F.

RHT bowl (inlet area)temp.:>130.degree. F.

RHT exhaust area temp.:>130.degree. F.

In the event the temperature at any of these selected locations is less than the specified minimum, the turbine must be prewarmed while it is "on turning gear." The clutch arrangement of turning gear 170 allows the turbine to "roll off" if sufficient steam is admitted to the turbine, although such "rolling off" is generally undesirable, and typically is not provided for in standard manual warm-up procedures. Some roll off is tolerated when rapid warm-up of the turbine is required, although the turbine should be brought back onto the turning gear promptly.

The turbine warm-up procedure typically has two phases. The first phase of the warm-up procedure is referred to as the condensation phase, and is normally performed manually by the turbine operator. In this phase, main steam bypass valve 124 is opened a small amount to pressurize steam chest 132 HP turbine 140, and reheat pipes 120; combined reheat valves 151 are kept closed during this phase. During the initial stages of pressurizing turbine 130, drain valves 126 must be opened frequently to remove condensate from steam pipe 120 and components of turbine 130. The amount of steam applied (controlled by the position of main steam bypass valve 124)is gradually increased as the turbine components begin to warm and less condensate is formed.

At the point that the temperature of the interior metal turbine surfaces reach the saturation temperature of the steam, pressure in the turbine begins to rise, at which point the pressurization phase of the prewarming procedure begins. Typically turbine 130 is pressurized to about 70 psig and maintained at this pressure until the desired turbine temperatures (noted above) are reached and the turbine is ready for acceleration and loading. In current practice, the operator has no automated way of specifying a warm-up profile that can consistently result in a predetermined warm-up rate.

In accordance with this invention, a method for prewarming a turbine uses fuzzy logic such that the warm-up is accomplished maintaining selected turbine specifications and in accordance with an operator-selected warm-up profile. With the use of fuzzy logic the method of the present invention provides an automated start-up procedure that optimizes the prewarming process by minimizing its duration for a given warm-up profile selected by the operator, such as a "normal" (that is, slow) warm-up or a fast warm-up (as in when there is an immediate need for use of the turbine).

The prewarming architecture in accordance with this invention comprises a turbine control circuit 200 (FIG. 2) which provides both regulatory control functions and supervisory control functions. Regulatory control refers to sensor-based closed loop control of various process parameters in the turbine, and the supervisory control utilizes fuzzy logic to weight the control signals generated by the regulatory control loops in order to optimize the prewarming process. Turbine control circuit 200 comprises a plurality of regulatory control loops for generating regulatory control signals for respective steam plant systems; a fuzzy logic controller 250; and a regulator output weighting component 260, which components are coupled together as described below to generate, using an operator-selected warm-up rate and fuzzy inference calculations, weighted control signals to drive components of steam system 100 to effect the desired turbine warm-up profile.

Turbine controller 200 comprises a plurality of regulatory control loops (or regulators) such as a temperature control regulator 210, a speed control regulator 220, a stress control regulator 230, and a condensate control regulator 240 (as used herein, "condensate control regulator" is used to denote control of condensation collected in the steam space of supply lines and the turbine steam spaces as opposed to condensate level in condenser 180). In the block diagram of FIG. 2, for ease of illustration, single boxes are used to denote each type of regulatory control loop discussed; for a given type of regulatory control, however, such as drain valve control or temperature control loops, the illustrated control loop may comprise multiple respective control loops, e.g., one for each drain valve. Each regulatory control loop is respectively coupled to a reference value (or set-point) data base 205 so as to receive the respective selected reference value for the parameter being controlled by the regulatory control loop. Data base 205 typically comprises an electronic memory device such as is commonly used in computers.

In the method of this invention selected turbine condition parameters are determined. For example, each regulatory control loop is coupled to sensors (not shown)in steam plant 100 to receive respective signals providing information on the actual state of the turbine for a respective parameter, such as the temperature at a selected location in the steam system, turbine speed, or the like. The sensor signal used in a particular regulatory control loop may comprise a measured value of the controlled parameter (e.g., turbine speed) or, alternatively, a calculated value of the controlled parameter based upon measured conditions in the steam turbine (e.g., rotor bore stress is determined based upon temperature measurements; condensate level is determined based upon drain valve temperature and steam system pressure measurements).

In each regulatory control loop a reference value of the respective controlled parameter (obtained from reference value data base 205)is compared with actual state of that turbine parameter and a corresponding error signal is produced. For example, in temperature control loop regulators 210 a reference temperature for a given turbine component is compared with the determined value of that parameter in the warming turbine; the reference temperature in respective temperature control loops comprise the predetermined minimum temperature for the HP bore, RHT bowl, and RHT exhaust turbine rotor areas that must be reached prior to turbine acceleration and loading. Similarly, in speed control loop regulator 220 a reference turbine speed (such as the turning gear speed) is compared with the measured actual turbine speed; in stress control loop regulator 230 the stress reference value (e.g., the turbine specification value for maximum allowable rotor bore stress)is compared to the calculated actual bore stress based upon the measured surface temperature values at selected points in the turbine; and in drain control loop regulators 240 a reference signal for allowable condensate is compared with a determined signal of condensate amount (determined for example, by use of temperature sensors in the drain valve piping)to generate a condensate error signal. Each control loop regulator is respectively coupled to fuzzy logic controller 250 to supply the respective error signals generated to the fuzzy logic controller.

Each control loop regulator typically comprises a proportional-integral algorithm controller that generates a respective regulatory control direction based upon the error signal produced through comparison of the determined turbine performance parameter with the respective reference value of that parameter. Further, each regulatory control loop generates a respective regulatory control direction (that is, a signal to direct action to achieve an effect in the steam plant). Temperature control regulators 210, for example, use the temperature error signal to generate a regulatory control direction to position the steam supply valve so as to minimize the error between the reference temperature and the determined temperature. Typically the steam supply valve controlled (via an actuator) by turbine warm-up control circuit 200 is the main steam bypass valve 124 (FIG. 1); such a drive signal is denoted in FIG. 2 with the terminology ".DELTA.MSBV".

Speed control loop regulator 220 also generates a respective steam supply valve regulatory control direction .DELTA.MSBV from the signal corresponding to the speed error between the measured speed of the turbine with the turning gear speed (i.e., the nominal warm-up speed). Similarly, in stress control loop regulator 230 the stress error signal is used to generate another regulatory control direction .DELTA.MSBV for the steam supply valve. In drain valve control loop regulator 240 the condensate error signal is used to generate a regulatory control direction .DELTA.DV for opening or closing respective drain valves.

Each control loop regulator is respectively coupled to regulator output weighting component 260 so that the respective regulatory control directions generated are supplied to weighting component 260 to be further processed as discussed below.

In accordance with this invention, fuzzy logic controller 250 provides the supervisory control function to optimize the turbine warm-up in accordance with the operator selected warm-up profile through the application of a selected fuzzy logic rule base 257. Fuzzy logic controller 250 comprises a warm-up profile selector 255 that provides for selection of a slow (or normal) warm-up profile and a fast warm-up profile; alternatively, additional (e.g., intermediate timeframes for the warm-up) can be provided for in the fuzzy logic controller. The fuzzy inference calculation generates a plurality of weighting factors "W.sub.n ", with "n" corresponding to the number of regulatory control directions to be weighted in regulatory output weighting component 260. For example, in FIG. 2 four weighting signals "W1" to "W4", inclusive, are represented and correspond respectively to the regulatory directions from temperature control regulator 210, speed control regulator 220, stress control regulator 230, and condensate control regulator 240.

The fuzzy inference calculation applies predetermined fuzzy inference rules from rule base 257 to generate the respective weights for the regulatory control directions generated by the control loop regulators. One example of a fuzzy logic rule base for controller 250 is set forth in FIG. 3. For this example, the rule set for the fuzzy inference calculation is determined by three inputs to fuzzy logic controller 250: the operator-selected warm-up profile can be either "slow" or "fast"; the turbine speed error is "zero" or "high"; and the turbine stress error is "zero" or "high". The various weights assigned to the respective regulatory control directions are qualitatively presented in the rule table of FIG. 3. In accordance with known practices in the field of fuzzy logic, such semantics correspond to quantitative values between 0 and 1, with the application of particular rules and the exact values applied being determined by the system designer to provide the optimal system performance (e.g., to mimic and improve upon heuristically-determined operator control actions in the plant).

Fuzzy logic controller 250 is coupled to regulatory output weighting component 260 so that the weighting factors "W1" through "W4.revreaction. are supplied to component 260. Weighting component 260 comprises means for weighting the respective regulatory control direction signals from the control loop regulators by the amount of the respective weighting signal from fuzzy logic controller 250. For example, such weighting may be accomplished by electronically multiplying the respective weighting factor and regulatory direction. Further, respective regulatory control directions for control of the same steam plant parameter, such as steam supply as controlled by the main steam bypass valve position, are combined (such as by electronically summing the signals)to produce control signals for the selected turbine regulatory function. Thus, the final combined output control signal for the steam supply valve ".DELTA.MSBV" can be expressed as follows: ##EQU1##

(In this example this relationship provides a combined output weighted average value that does not exceed 1, which would represent the maximum opening of the steam supply valve. Similarly, the the control signal for a respective drain valve control ".DELTA.DV" can be expressed as: W4*.DELTA.DV.sub.cond Weighting component 260 is coupled to actuators in steam plant 100 so that control signals are applied to the respective actuators in the steam plant to implement the selected warm-up profile.

For example, the fuzzy inference calculation of the rule in the first line of FIG. 3 provides that for a "slow" prewarming rate selected by the operator, zero speed error, and zero stress error, the weight assigned to all regulatory controller directions would be low except that for condensate control. The turbine would thus be warmed very slowly, with a heavy emphasis on keeping the turbine on the turning gear and reducing condensate accumulation. If the speed error and the stress error became high, however, the rule appearing in the fourth line of FIG. 3 begins to take effect and the weights of the regulatory controllers for speed and stress are increased in order to reduce the error between the determined value and the reference value for each of these parameters. In the same situation (high speed error, high stress error), if the operator-selected warm-up profile calls for a "fast" warm-up (that is, the rule of line 8 of FIG. 3 applies), the weighting assigned to speed control, stress control, temperature control, and condensate control is different than when the "slow" warm-up profile has been selected, and as a result, the turbine may periodically roll-off the turning gear.

In another embodiment of the present invention, fuzzy logic controllers can be used for one or more of the above mentioned control loop regulators in lieu of conventional numerical regulatory controllers using proportional-integral algorithms.

Thus in operation, the method of the present invention provides for determining the selected turbine performance parameters (that is, parameters selected to provide the desired control of the prewarming procedure for the turbine) and, for each of these parameters, generating a respective regulatory control direction. Weights to be assigned to respective regulatory control directions are produced by means of a fuzzy inference calculation using a fuzzy rule base and an operator-selected warm-up profile; respective weights are applied to corresponding regulatory control directions to generate weighted steam plant control signals. The weighted control signals are then applied to a respective steam system actuator (such as the actuator for the valve used to control steam supply to the turbine) so as to implement the desired warm-up profile.

While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.