PREPARED BY

S.K.ROY /DGM(C&I)

MEJIA SITE

First time BHEL has introduced Proportional valve for electro hydraullic governing control supplied

by M/S Rexroth Bosch at Mejia-7 (500 MW) , replacing the age old moving plunger coil system with Collins feedback transmitters.

The EHTC system with Proportional valve is used for the control of direct operated high response valves STW and uses two stage position control. In first stage demand signal from speed /load

controller is compared with EHC piston position and differences is applied as differential input command value to Amplifier module. In second stage actual value of pilot valve spool position is

measured and compared with the externally applied command valve . Any differences between the actual and command values are compensated for, through movement of proportional valve.

The movement of proportional valve allows control fluid to admit in the space above or below the piston with the opposite side of the piston opened to the fluid drain. The resulting motion of the piston

is transmitted to the sleeves via lever which in turn slides on the follow-up pistons , as a result secondary oil pressure varies and changes the control valve position accordingly.

Advantages:

1) Response is very fast as compared to moving plunger coil system. 2) Easy calibration, adjustment and maintenance . 3) Open architecture , as easily available in the market.

Whole system consist of –

1) Proportional valve. - (MAKE - Rexroth Bosch ) 2) Valve spool position Transmitter. - (MAKE - Rexroth Bosch )

3) EHC Position feedback Transmitter. - (MAKE - Baluf ) 4) Amplifier module. - (MAKE - Rexroth Bosch )

5) Capacitance module. - (MAKE - Rexroth Bosch ) 6) Position measurement module-APM .- (MAKE-EDN) 7) Positioner Module.- AVB + ARS. - (MAKE-EDN)

REPORT ON COMMISSIONING OF FIRST 500 MW TG

ELECTRO-HYDRAULLIC GOVERNOR CONTROL WITH

PROPORTIONAL VALVE SYSTEM.

EHTC SCHEME WITH PROPORTIONAL VALVE

PRPORTIONAL VALVE: The proportion valve consists of a current solenoid and an inductive type position transducer. A positive command value causes an increase in current in solenoid and hence a flow in the valve from

P to A and from B to T. A negative command value causes an increase in current in solenoid “a“ and hence a flow in the valve from P to B and from A to T.

AMLIFIER MODULE: The amplifier module contains the electronics that are required to control the STW high response valves with electrical feedback. The valve spool position is measured through inductive position

feedback transmitter and is compared with the differential input or externally applied command valve. Any differences between the actual and command values are compensated for and command issued to

proportional valve for opening or closing. It has following features:

– Ramp generator – Self clocking output stage

– Oscillator and demodulator for inductive position measurement – Valve spool position controller

– Enable circuit Externally it is possible to adjust 1)the ramp time up to approx. 1 s (tmin approx. 1 ms), 2) the

command value reduction up to approx. 20% and 3) the actual value zero point with in the range ±

30% through trimming potentiometers.

Terminal 3 is used as the enable input.

INTERNAL CONFIGURATION OF AMPLIFIER MODULE

SPOOL POSITION FEEDBACK TRANSMITTER

PROPORTIONAL VALVE SOLENOID

SYSTEM DESCRIPTION

CAPACITANCE MODULE: This capacitor module is used for smoothing operating voltages for supplying amplifier modules that control proportional and servo-valves.

Capacitance module has following features-

– Capacitors – Polarity reversal diode

– Fuse protection – Overvoltage protector

– LED indicator for output voltage

Position measurement module-APM : The position measurement module is used in position control application for measurement of EHC piston position for Steam Turbines with EHTC equipment having high /low pressure governing system. This module will provide 24V dc excitation supply to the field mounted EHC position transmitter thro’ fuse and receive position signal. The received signal is converted into 0-10Vsignal (for the actual value scaled down to 0-100%. The converted voltage signal is monitored for lower limit and then conditioned. The output value can be calibrated with respect 0-100% position through potentiometers mounted on the front panel. Positioner Module.- AVB + ARS. Proportional valve Driver module is a PROMAX module developed to be used in the Turbine control system. It is a matching position controller(PID) for the alternate design of the Electro-Hydraulic Converter. The position controller consists of a PD-T1 and PI elements connected in series. Value of gain, derivative gain and time constants can be set from front panel as per system requirement.

INTERNAL CONFIGURATION OF CAPACITANCE MODULE

Construction

The principal components of the converterare proportional valve (1), Manifold plate(12), EHC Piston (7), Casing (4), positiontransmitter (2), follow-up pistons (21) andactuator (17). Bushings and follow-up pistons ‘A’ are connected to each other via the adjusting screws (24), spring end pieces and the springs (22). The control signals from the electro-hydraulic controller operates on proportional valve(1) through a matching amplifier module. The control fluid from connection ‘a’ is admitted to the space above or below the piston (7)with the opposite side of the piston opened to the fluid drain. The resulting motion of the piston (7) is transmitted via lever (13) to the sleeves (20) which in turn can slide on the follow-up pistons (21). The secondary fluid circuits, which are fed from the trip fluid circuit via throttles and supply the various actuating devices, are connected at point ‘b’. The secondary fluid pressures are determined by the tension of springs (22) which counter balance the fluid pressure acting on the follow-up pistons (21). Each follow-up piston and sleeve (20) has ports, which control the secondary fluid flow according to their overlap. When the throttling area is changed by the movement of the sleeve (20), it also changes the pressure in the follow-up piston causing it to follow the movement of the sleeve. This varies the tension of springs (22) until equilibrium is regained between the spring force and the new secondary fluid pressure. Each position of the EHC piston (7) thus corresponds to a specific position of the sleeves (20) and, therefore, the follow-up pistons (21). The position of the follow-up piston is the determining factor for the secondary fluid pressure at point ‘b’. The initial tension of the follow-up piston springs can be varied by means of the setscrews (24).

MECHANICAL STRUCTURE

EHC POSITION FEEDBACK TRANSMITTER

EHC POSITION FEEDBACK TRANSMITTER

SPOOL POSITION FEEDBACK TRANSMITTER

PROPORTIONAL VALVE

Controlling action with the Electro hydraulic Converter When the electric controller gives a command to open the turbine control valves, the proportional valve enables the control fluid to flow through port ‘B’. The fluid flows to the space below the piston (7) via manifold plate (12). This moves the piston (7) upwards. The following movement of the piston (7) then slides the sleeves (20) downwards over the levers (13,19) reducing the drain area between the sleeves and the follow-up pistons (21), causing the pressure in the follow-up pistons and secondary fluid circuits to rise. The motion of the piston (7) produces a simultaneous feedback action on the proportional valve (1) via the position transmitter (2). The control command gets modified and in the equilibrium position there is no flow through the proportional valve neither to port ‘B’ nor to port ‘A’. When a command is given to close the control valves, the controlling action is similar but in the reverse sequence

Adjusting Device for Valves

The follow-up pistons ‘B’ and the lever (14) of the rotary shaft (15) situated above it are connected by the springs (22) of follow-up pistons ‘B’, the guide pin (16) and the setscrews (30). During normal operation an energized solenoid valve allows control fluid ‘a1’ to flow under the piston (25) of the actuator (17). The piston (25) is moved upwards against the forces of the spring (26). Stop (34) locked in normal position by pin and the initial tension of the tension springs (22) of the follow-up pistons ‘B’ is adjusted by means of the lever (28, 14) which results in the IP control valves opening in relation to the HP control valves as intended for this operation. If the condition turbine load less than a set minimum load and the ratio of HP exhaust steam pressure to main steam pressure greater than the set pressure ratio is fulfilled, for example after load shedding, the solenoid valve will be de energized. This blocks the flow of control fluid to the actuator (17) and allows control fluid under the piston (25) to flow into the return pipe. The force of the spring (26) moves the piston into the lower end position and the tension springs (22) of the follow-up pistons ‘B’ are adjusted so that the IP control valves do not begin to open until the HP control valves are wider open. The lever (28) then rests on the precisely set stop (34). Limit switch (29) indicates:

Setting device engaged.

Setting Device for start-up without Bypass Valves

If the plant is started up without bypass system, the IP/reheat stop and control valves must open before the main steam stop and control valves. For this purpose, the hand wheel (32) is set in the upper end position. Signal from limit switch (33): Setting device in operation without bypass system position.

Problem faced during Erection & Commissioning :

1) As per drawing /engineering , the Amplifier and capacitance module to be housed in EHTC cabinet in control

room , and the length of signal cable between EHTC cabinet and Proportional valve should be less than 60 meters. The distance between EHTC cabinet and Proportional valve located near Turbine at Mejia -7 is more than 60meters . Therefore it was decided to house the Amplifier and capacitance module in a JB and located near the Governing rack with a consideration that there will not be any signal loss as milli amp signal is transferred from position module to amplifier module. (see fig-1)

2) EHC Valve was not operating with respect to command issued from EHTC cabinet and there was hamming noise from the Proportional valve solenoid and hunting in EHC piston. Problem was analysed and modification was carried out. As per original scheme/engineering, supply of enabling signal and supply of Capacitance module is extended from floating voltage of max EHTC cabinet. Since the same floating voltage is used for field signals also and any change in fault level due to leakage current or grounding ,the voltage level (+) and (-) of floating voltage varies with respect to ground potential , which results in disruption of enabling signal and causes hunting of EHC piston. To over come the problem , supply of enabling signal and supply of Capacitance module was extended from system supply having zero ground potential through an isolator /MCB. With the above modification we could successfully achieve the smooth operation of EHTC.

Points to be remembered during commissioning: 1) Command output from EHTC cabinet is 20 to 4 ma with respect to 0 to 100 % command. 2) Supply to amplifier module and enabling signal should be 24V dc, with M having Zero potential.

REXROTH AMPLIFIER MODULE

REXROTH CAPACITANCE MODULE

Amplifier Module and Capacitance Module housed in JB,

located near the Governing rack.

PROPORTIONAL VALVE

SPOOL POSITION FEED BACK TRANSMITTER

PROPORTIONAL VALVE MOUNTED ON GOVERNING RACK

AMPLIFIER MODULE

Fig-1

GOVERNING RACK

REXROTH AMPLIFIER AND CAPACITANCE MODULE MOUNTED IN SIDE THE JB

GOVERNING CONSOLE

ACKNOWLEDGEMENT :- My sincere thanks to Shri H.N. Tagore, AGM/TSX and Shri D. Mahapatra, SDGM/Mejia

site for their assistance during commissioning . I would like to appreciate contribution of Shri Kunal /Engineer (C&I) and Ms. Diptimayee Sar /AE – II(C&I) ,who were associated with me during erection and commissioning of governing system and the help extended for the

preparation of report. My appreciation to Haridwar Unit also for introducing Proportional valve system for Governer control, which is better than earlier Plunger coil system.

END REPORT