Guide Vane Oscillations

[simboti]

we're a hydro-power plant with a rated output of 750Mw. one of our turbo-alternator's (Francis turbine)guide vanes (GV) have been oscillating arbitrarilly. initially we thought that this was due to the governor actuator loop and we suspected the main servo valve's position transducer and the feedback from the servomotors. however these were tested and re-adjusted as per the commissioning report. dispite these efforts the difference between the guide vane referrence and the actual guide vane opening was approximately 3%. this is unusually high. we therefore replaced the actuator (electrohydraulic convertor),the oscillations temporarily stopped, but the difference between GV ref and GV actual remained the same. we believe that there is no problem with the digital governor (PID controller). since we've checked the programe and the control parameters. how should we therefore proceed? what are the root causes of such oscillations?

 

[simboti]

thanks guys for the quick response. mc5w, the turbine governor that we employee is the HPC 640 GUIDE VANE CONTROLLER. It is digital and is implemented in an ABB Advant Controller 110 (AC110).
Mike,you guessed right the guide vane position is read by a mechanical link connected to one of the servomotors at one end and a position transducer on the other end. these however were checked and necessary adjustments were done.
my worry is that we've operated this machine for two years after refurbishment and we've never attended to the electrohydraulic convertor (actuator, TR10: ALSTOM), though we've opened it for the other machines. Now i'm faced with a situation were i have to check the hydraulic balance, mechanical tendency and the associated electrical compensation. i only have skeletal information and seriosly in search of detailed information.