US3407826A

US3407826A - Electrohydraulic overspeed control system for a reheat steam turbine

Info

Publication number: US3407826A
Application number: US493827A
Authority: US
Inventors: James B Wagner
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1965-10-07
Filing date: 1965-10-07
Publication date: 1968-10-29
Anticipated expiration: 1985-10-29

Description

oct. 29, 1968 J, WAGNER 3,407,826

ELECTROHYDRAULIC OVERSPEED CONTROL SYSTEM FOR A REHEAT STEAM TURBINE Filed OCt. 7, 1965 4 Sheets-Sheet 2 i 9s lo? 5| 3 s l I}? lo :2 4 r E 3 41 j /7 I fly 65 69 7 I FIG.2

' INVENTOR JAMES B. WAGNER Oct. 29, 1968 J. B. WAGNER 3,407,325

ELECTROHYDRAULIC OVERSPEED CONTROL SYSTEM FOR A REHEAT STEAM TURBINE Filed Oct '7, 1965 4 Sheets-Sheet 5 55 SERVOVALVE M9 TORQUE MOTOR N I46 con. O O

TOROUEM OR ARMAT I B Q FIRBTSTAGE 6 6 JET TUB FORCE FEEDBACK SPRI P R 2 P 21 SECOND H5"0"R|NG I21 9 7 I2 IOOMIORON g g STRAINER OPERATIONAL SCHEMATIC FIG 3 INVENTOR.

JAMES B. WAGNER J. B. WAGNER ELECTROHYDRAULIC OVERSPEED CONTROL SYSTEM Oct. 29, 1968 FOR A REHEAT STEAM TURBINE 4 Sheets-Sheet 4 Filed Oct. '7, 1965 INVENTOR. JAMES B. WAGNER United States Patent 3,407,826 ELECTROHYDRAULIC OVERSPEED CONTROL SYSTEM FOR A REHEAT STEAM TURBINE James B. Wagner, Lynnfield, Mass, assignor to General Electric Company, a corporation of New York Filed Oct. 7, 1965, Ser. No. 493,827 4 Claims. (Cl. 137-31) ABSTRACT OF THE DISCLOSURE An electrohydraulic control system for the intercept valve of a reheat steam turbine, with provision to commence proportionate closing of the steam intercept valve with a servo valve depending on the degree of overspeed, but to close it shut immediately with a dump valve if the servo valve fails to keep up with the proportionate overspeed closing signal.

This invention is directed to a steam turbine control system, and more particularly to a steam turbine in which the steam applied to the steam turbine is automatically reduced as the turbine approaches a dangerous speed.

After a steam turbine is brought up to speed, it is normally maintained at that speed by the main control valves, even when the load is varied. Under abnormal conditions, such as when the generator breaker opens, a load dump may occur so that there is no load on the turbine shaft. Without a load to absorb the energy, the turbine speed may increase rapidly and eventually destroy the turbine and nearby objects.

It is therefore an object of this invention to provide a new and improved steam turbine control system in which the steam applied to a steam turbine is automatically reduced when the turbine speed approaches a dangerous speed.

Accordingly turbine speed, as indicated by a speed transducer, is compared with a reference speed. When a comparison indicates that the. turbine speed has exceeded the reference speed, a steam valve controlling the steam applied to the turbine is automatically closed.

In reheat steam turbines, there is usually a proportionate steam valve called an intercept control valve between the turbine and the reheated steam source so that when the turbine speed reaches the reference speed, the proportionate steam valve starts to close, and continues closing as the turbine speed increases until the steam valve is closed by the time the turbine speed reaches a second overspeed condition. This control of the intercept valve is conventionally carried out to assist in the speed control exercised through the main control valves.

A proportionate steam valve is normally moved through mechanical linkages by a hydraulic ram. The hydraulic ram is in turn controlled by a servo valve.

The steam turbine consistently runs at its normal rated speed, but with varying loads. Under normal circumstances, the steam applied to the turbine through the control valves varies according to the load while the intercept valves normally remain wide open. When the turbine is at full load and the turbine speed increases past the reference speed, an extremely large and expensive servo valve would be required to move the proportionate steam intercept control valve towards closure at a rate determined by the degree of overspeed.

It is therefore an object of this invention to provide a new and improved steam turbine control for controlling the closing of a proportionate steam valve as the speed of the turbine increases past a reference speed.

Accordingly, the command signal applied to the servo valve to control the closing of the proportionate steam ice valve is compared with the actual position of the hydraulic ram to determine if the hydraulic ram is actually closing the proportionate steam valve as fast as the command signal has indicated that it should be closing. When the difference between how fast the hydraulic ram should move and how fast it actually is moving becomes too much, then control over the movement of the ram is removed from the servo valve and turned over to a trip valve to immediately close the proportionate steam valve.

This invention is set forth with particularity in the appended claims. The principles and characteristics of the invention, as well as other objects and advantages are revealed and discussed through the medium of the illustrative embodiments appearing in the specification and drawings which follow.

In the drawings:

FIGURE 1 is a block diagram of a four-section reheat steam turbine.

FIGURE 2 shows a cutaway view of a combined intercept control valve/reheat stop valve shown in block form in FIGURE 1.

FIGURE 3 shows a cutaway view of a servo valve.

FIGURE 4 is a schematic of the control for the servo valve and intercept control valve.

Refer now to FIGURE 1 for a description of a foursection reheat steam turbine. Steam is produced in the steam generator 11, travels through a main stop valve 13 and the main control valves 14 to the first section 15 of the steam turbine. The direction of steam flow is indicated by the direction of the arrows along the steam lines. The steam flows through the first section 15 to the reheat section 17 of the steam generator 11, where it is reheated, then to a first reheat stop valve/intercept control valve 19, and a second reheat stop valve/intercept control valve 21. From the reheat stop valve/

intercept control valves

19 and 21, the steam is applied to the second section 23 of the turbine, through the second section 23, to the third and fourth sections 25 and 27. From the third and fourth sections 25 and 27, the steam goes to the condenser 29 where it is condensed and returned to the steam generator 11. The rotation of the shaft of the turbine turns the generator 31 to generate electric power.

With the turbine operating on steam and the turbine shaft 33 rotating at a constant speed, for instance 3600 r.p.m., the main stop valve 13 is full open and the reheat stop valves/

intercept control valves

19 and 21 are full open. The amount of steam applied to the turbines is specifically controlled by the main control valves 14, according to the load on the turbine shaft 33 by the generator 31.

After the turbine is up to speed, the speed of rotation is kept constant, in this particular instance 3600 revolutions per minute.

When the speed of the turbine begins to exceed 101% of its normal speed, the control system causes steam applied to the turbine from the reheater to be reduced to bring the speed of the turbine back towards normal. If the turbine speed can be controlled, the steam should be reduced, not completely cut off. Therefore, the speed signal from speed transducer 37 indicating the increase in speed is applied to the control 39 and compared with a reference speed. As the speed signal from the speed transducer increases past the reference speed, a valve closing signal is developed to start a servo valve closing the

intercept control valves

19 and 21..

The valve command signal is compared with a signal indicating how fast the intercept control valve is actually closing. If the intercept control valve is not closing as fast as it should, a trip valve is used to immediately close the intercept valves.

Referring now to FIGURE 2 for a description of the combined intercept control valve/

reheat stop valve

19 and 21, the steam flows in through inlet port 51, through the valve seat 53, and out outlet port 55. In the drawing the reheat stop valve 57 and the intercept control valve 59 are shown seated in the valve seat 53 blocking the steam flow from the inlet port 51 to the outlet port 55. The reheat stop valve 57 is connected by a valve stem 61 to a spring retainer 63 which in turn is connected to a hydraulic ram in the cylinder 65. The direction that the hydraulic ram is moved is determined by the fluid applied through the manifold 67 from the reheat valve actuator 69. The fluid is applied by the reheat valve actuator 69 as determined by the electrical control signals applied thereto. Spring 71 is compressed between the spring retainer 63 and the plate 73 which is rigidly connected to the valve body 75 to urge the valve stem 61 downwards, thus bringing the reheat stop valve 57 in contact with the valve seat 53. Thus, to overcome the compression spring 71 and open the reheat stop valve 57, the reheat valve actuator 69 is actuated to move the hydraulic ram in the cylinder 65 upwards, moving the valve stem 61 and the reheat stop valve 57 upwards so that the reheat stop valve is moved up from the valve seat 53. The valve seat 53 is still closed, however, for the intercept control valve 59 is still seated in the valve seat 53.

The intercept control valve 59 is connected by a valve stem 67 to an inverted cup 79. The inverted cup 79 has a cup seat 81 in which a lifting pin 83 rides. The other end of the lifting pin rides in a cup seat 85 in lever 87. The underside of lever 87 bears on stop 86. Lever 87 is pivoted about pin 89 which is supported in support 91 attached to the main valve body. A bearing 93 rests on the other end of the lever 87, secured to a pull-down rod 95 by a bearing cap 97. The pull-down rod 95 is secured in a clevis 99 by a pin 101. Piston rod 103 is connected to the other end of the clevis 99, and to a hydraulic ram in the cylinder 105. A spring 107 is compressed between the clevis and a plate 109 attached to the main body of the valve to urge the clevis 99 upwards. The pull-down rod 95 thus pivots the lever 87 about pin 89 in a counterclockwise direction so that the underside of lever 87 bears on stop 86 to push the intercept control valve 59 downward so that the intercept control valve 59 seats in the valve seat 53.

The hydraulic ram in the cylinder 105 is controlled by a dump valve 111 and a servo valve (not shown, but behind the cylinder 105) connected to the cylinder 105 through manifold 113.

The servo valve is of the conventional torque motor type used in servo valve construction having one or two coils. The servo valve may be of the four-way action type, and of the type in which fluids are supplied thereto under high pressure. Its function is to control the hydraulic ram in the cylinder 105. The flow rate through the servo valve is proportional to the current delivered to it. The size of the ram is chosen such that it can provide the force requirements to operate the stem of the input valve 103. The position of the hydraulic ram is translated to a voltage by means of a feedback transducer. A position transducer rod 115 is connected to the piston rod 103. The feedback transducer may suitably be of the well-known variable reluctance type wherein the position of a magnetic slug determines the inductance of two halves of a continuous winding. When such inductance is measured in a standard bridge circuit, an AC. output is produced therefrom having an amplitude determined by the position of the slug. The bridge circuit is balanced to produce a null output for the fully closed ram position. The completely closed position of a ram signifies the completely closed position of a valve plus any mechanical over-travel provided in the connecting mechanisms between a ram and a valve. The transducer is powered by an oscillator. The ram in the cylinder may be moved by either the servo valve or the dump valve 111. The dump valve 111 will completely close the intercept control valve while the servo valve will move the intercept valve towards closing as the current is applied to it. In other words, the dump valve either opens or closes the intercept valve while the servo valve controls the position at whichthe intercept valve is positioned and the speed at which it moves toward that position.

The servo valve and the dump valve when energized operate to pull down the hydraulic ram in the cylinder, pulling down the piston rod 103, the clevis 99, the pulldown rod 95, rotating the lever 87 in a clockwise direction about pivot point 89. Lever 87, pivoted about pin 89, lifts lift pin 83, lifting lift cup 81, moving rod 67 upwards, and thus moving intercept valve 59 up from the valve seat 53. The intercept valve 59 is moved up from the valve seat 53 a distance determined by the distance that the ram is moved in the cylinder 105.

FIGURE 3 shows a cutaway diagram of a servo valve to control the positioning of the ram in the cylinder to control the opening of the intercept control valve. Such servo valves are commonly used in closed-loop servo systems. They control the flow of fluid to or from a load actuator in proportion to the input differential current signal to the torque motor. Fluid is supplied, under pressure, into cavities and 117. Cavity 119 is connected to the fluid return line. One of the

ports

121 and 123 is connected through a manifold to the cylinder on one side of the ram. The ram is spring loaded so that it is single acting. A relatively small cavity 125 also receives a very small amount of fluid flow which flows through a 100 micron screen 127, through a flexible pipe 129 connected to the armature 131 of the torque motor 134, and out of a projector jet 133. The fluid flowing from the projector jet 133 impinges on two receiver pipes 135 and 137 which are connected to each end of the cylinder chamber 139. At a null approximately one half of the line pressure from the jet 133 is developed in each receiver pipe 135 and 137.

The piston 141 moves in the cylinder chamber 139. The four torque motor leads 143-146 are wound about the torque motor armature 131. The torque motor armature 131 is connected to the projector jet 133 so that when the torque motor armature 131 moves, the projector 133 also moves accordingly.

A differential current is applied to the torque motor leads 143-146 to move the armature 131. Assume that the differential current is applied to the leads 143-146 to rotate the armature 131 in a counterclockwise direction, displacing the jet pipe 133 to the right so that more fluid will impinge on the right-hand receiver pipe 135, and the pressure in the right-hand receiver pipe 135 Will increase. Conversely, less pressure is developed in the left-hand pipe 137, and a differential pressure (and a net force) exists on the piston 141 causing the piston 141 to move to the left.

As the piston 141 moves, a counteracting force is transmitted to the jet pipe 133 by a force feedback spring 136. The piston 141 continues to move to the left until the force created by the force feedback spring 136 is equal to the force developed by the torque motor 134. When this occurs, the jet pipe 133 is again symmetrically positioned over the two receiver pipes 135 and 137, and no differential pressure, and thus no net force, is acting on the piston and it remains at its new position. Thus, the servo piston has assumed a position proportional in direction and magnitude to the input differential current in the torque motor.

Referring now to FIGURE 4, which shows only the portion of the overall control system which operates the intercept valves, speed transducer 147 is a speed sensor which senses the speed of the turbine and produces a signal proportional to the speed of the turbine. Speed transducer 147 is connected through resistor 148 to connection point 152. Potentiometer 154 connected to a zero potential terminal 150 and through resistor 156 to a +30 volt potential terminal 149. The slider of potentiometer 154 is connected through resistor 151 to connection point 152. Potentiometer 154 establishes the reference speed level at which the servo valve 181 will begin to close the intercept control valve. The reference speed level is usually 101% of normal rated speed.

Connection point 152 is connected through amplifier 155 to the anode of rectifier diode 157. Resistors 153 is connected across amplifier 155. The cathode of rectifier diode is connected to the junction of

resistors

153 and 159. Position transducer 161 is also connected through a demodulator 168 to connection point 160. Position transducer 161 is an AC device so that demodulator 168 is needed to insure that AC and DC signals will not get mixed together. The position transducer 161 is the feedback transducer described hereinbefore to indicate the position of the hydraulic ram and thus the intercept control valve.

Connection point 160 is connected through amplifier 163 to the anode of rectifier diode 169 and through amplifier 175 to the dump valve relay 177. The dump valve relay 177 controls the dump valve 111 shown in FIGURE 2. A feedback resistor 166 is connected from the cathode of rectifier diode 169 to connection point 160. The cathode of rectifier diode 169 is connected through resistor 173 to a --16 volt terminal point 171, and through amplifier 179 to servo valve relay 181, to control the servo valve shown in FIGURE 3.

The signal proportional to the speed of the turbine produced by the speed transducer 147 is applied through resistor 148 to operational amplifier 155. So long as the speed signal is below 101% of normal, the output from operational amplifier 155 through rectifier diode 157 is zero so that the servo valve 181 does not begin to close the intercept control valve. The feedback rectifier diode 157 remains back biased because of the positive fixed bias established by the deadband potentiometer 154. When the speed level of the turbine increases past 101%, the speed signal from speed transducer 147 becomes sufliciently negative to overcome the positive fixed bias from the potentiometer 154 and forward bias rectifier diode 157 so that a positive voltage command signal representing a desired (more closed) valve position is applied to connection point 160. The valve closing command signal is amplified by amplifier 163, through diode 169, amplified by amplifier 179, and applied to servo valve 181 to start closing the intercept control valve. As the turbine speed increases, a more positive voltage is applied to connection point 160, causing a momentary voltage of a negative level to appear at the input of amplifier 179. With a negative voltage at the input of amplifier 179, a current signal is impressed on the servo valve 181 to cause the intercept control valve to move in the closed direction at a rate determined by the magnitude of the command signal. The ram position (i.e., actual valve position) signal from the position transducer 161 indicating the resulting ram movement becomes increasingly negative tending to reduce the negative voltage applied to the amplifier 179, i.e., tending to reduce the valve closing rate.

The negative signal from the position transducer 161 increases as ram continues to close the intercept control valve. This increasing negative signal from the position transducer 161 tends to reduce the negative voltage valve closing signal applied to amplifier 179. However, if the ram is unable to close the intercept control valve as fast as the valve command signal indicates that the intercept control valve should be closing, the negative signal from the position transducer 161 will not increase fast enough to reduce the negative voltage valve closing signal below a negative 3 volts. When the negative valve closing signal increases past a negative 3 volts, amplifier 163 snaps down to a negative 7 volts and is held there by Zener diode 165. The minus 7 volt signal is amplified by amplifier 175 and applied to dump valve relay 177 to trip the hydraulic ram to immediately close the intercept control valve.

While the invention has been explained and described with the aid of particular embodiments thereof, it will be understood that the invention is not limited thereby and that many modifications retaining and utilizing the spirit thereof without departing essentially therefrom will occur to those skilled in the art in applying the invention to specific operating environments and conditions. It is therefore contemplated by the appended claims to cover all such modifications as fall within the scope and spirit of the invention.

What is claimed is:

1. In a steam turbine, asource of steam, means for indicating the turbine speed, a proportionate steam valve connected between said source of steam and the steam turbine, means for indicating a reference speed, means for comparing the turbine speed with said reference speed and providing a valve command signal, means responsive to said comparing means for moving said proportionate steam valve towards its closed position, means for sensing the position of said proportionate steam valve and providing a valve position signal, and means responsive both to said comparing means and to said sensing means for immediately tripping said proportionate steam valve if the difference between said signals becomes too great.

2. In a steam turbine, at source of steam, means for indicating the turbine speed, a proportionate steam valve connected between said source of steam and the steam turbine, a hydraulic ram for closing said proportionate steam valve, a servo valve for moving said hydraulic ram to close said proportionate steam valve, means for indicating a reference speed for said steam turbine, means for comparing the turbine speed with said. reference speed and providing a valve command signal, means responsive to said comparing means for controlling said servo valve to move said ram to close said proportionate steam valve, means for sensing the position of said hydraulic ram to provide a valve position signal, and dump valve means responsive to both said sensing means and said comparing means for immediately tripping said proportionate steam valve if the difference between said signals exceeds a predetermined value.

3. In a steam turbine, a source of steam, means for indicating the turbine speed, a proportionate steam valve connected between said source of steam and the steam turbine, a hydraulic ram for closing said proportionate steam valve, a servo valve for moving said hydraulic ram to close said proportionate steam valve, dump valve for immediately moving said hydraulic ram to close said proportionate steam valve, means for indicating a reference speed for said steam turbine, first means for comparing the turbine speed with said reference speed to provide a valve command signal, means responsive to said first comparing means for energizing said servo valve to move said hydraulic ram moving said proportionate steam valve towards its closed position, means for sensing the position of said hydraulic ram to provide a valve position signal, second means for comparing the output of said first comparing means and said sensing means, and means responsive to said second comparing means for energizing said dump valve to close said proportionate steam valve when the difference between said signals exceeds a predetermined value.

4. In a steam turbine of the type having a steam source and a valve for proportionately controlling the flow of steam to a turbine section in response to a predetermined overspeed condition of the turbine, the combination of first means including an operational amplifier arranged to provide an electrical valve position command signal when the turbine speed exceeds a predetermined value,

second means sensing valve position and providing an electrical actual valve position signal,

7 8 third means including a second operational amplifier References Cited arranged to compare the aforesaid signals to provide UNITED 3 ATES A TS Valve slgnfila 1,950,594 3/1934 Bryant 137-27 X a servo valve responsive to sald third means to proporr 3,098,176 7 /1963 Eggenberger 317 5 tionately close the steam valve, and 0 3,288,160 11/ 1966 Eggenberger 13727 a dump valve also responsive to said third means to 3,342,195 9/1967 Wagner 137-29 quickly close said steam valve when the valve closing signal exceeds a predetermind value. CLARENCE R. GORDON, Przmary Exammer.