Geothermal Wellfield Control Using Foundation Fieldbus

Abstract

The Western GeoPower plant is a new 38.5 MW steam dominated geothermal power project being installed at the Geysers area north of San Francisco California. California’s H2S emissions standards are extremely strict and the wellfield must be brought online very carefully in order to minimize atmospheric venting of steam. This paper describes a unique control strategy in which the central DCS (distributed control system) provides supervisory control flow setpoints to local PID (proportional, integral, derivative) control algorithms which reside in the local wellhead flow control motor operated valves (MOVs). This Foundation Fieldbus (FF) enabled MOVs execute a PID control algorithm internally and gather their process variable information directly via FF communications from their local instrumentation peers. The result is a truly distributed control system in which the control logic resides inside the wellhead MOVs and instrumentation with no need for a stand-alone wellhead PLC or other traditional control system.