Ivyland, PA – Placer County Water Agency (PCWA) awarded a contract to American Governor Company (AGC) to provide equipment, engineering, start-up services and spare parts to modernize the Voith Analog Electronic governor at Oxbow. The contract was issued sole-source, based on AGC’s performance on the prior, competitively-bid French Meadow project and reflects the high degree of confidence PCWA has with AGC’s engineering, production, project management and field service personnel. The contract includes digital governor conversion equipment, project engineering and management, spare parts, and start-up services.
The Oxbow powerhouse, located on the Middle Fork of the American River in Placer County, CA, consists of a single 6.1 MW Francis turbine and generator manufactured by Voith Hydro of Germany. Commissioned in 1966, the original governor was a Voith Mechanical system that used links, levers, a dashpot and a separate distributing valve to open the Pressure Relief Valve (PRV) during emergency shutdown conditions. The inclusion of a PRV in the turbine design enabled the wicket gates to close at a faster rate than would otherwise be possible due to water hydraulic (“water hammer”) issues, thus minimizing overspeed.
In the 1980s, the Voith mechanical governor was ‘upgraded’ to analog electronic control, however, the electronic feedback devices at the time were rather crude. Note the lug nuts used to maintain tension on the rotary potentiometer feedback cable in the picture at left. Perhaps the biggest benefit of this system over the original mechanical governor was the inclusion of a color touchscreen to simplify local operation of the unit.
With replacement parts unavailable from Voith, the legacy analog/mechanical Governor and PRV control system was deemed unsupportable and PCWA started down the path of converting the entire system to PLC-based digital control. Early in the project execution phase, AGC personnel inspected the original Voith distributing valves and found them to both be in “very good” condition: a cost-saving decision was made to retain them. AGC would provide a new Electro-Hydraulic Interface pilot control valve manifold assemblies to interface with the existing Wicket Gate distributing valve, while the existing the mechanical actuation method for the PRV distributing valve would be retained, actuated by a new hydraulic cylinder.
AGC provided a new Allen-Bradley™ Control Logix™ PLC Governor control enclosure, new redundant electronic speed sensing, and Wicket Gate and PRV position feedback devices. Due to the compact arrangement of the Voith Actuator Cabinet (which was separate from the Voith Unit Control cabinet shown at right), a standalone PLC Governor cabinet was designed by AGC. Located next to this legacy Unit Control cabinet, the PLC Governor cabinet includes a large color touchscreen HMI for local operator control. A duplicate HMI is provided at the legacy Actuator cabinet, which will be used for tuning, calibration and troubleshooting purposes. Existing field wiring and plant devices will be re-used as much as possible to minimize cutover time, and a DNP3 communication module will simplify the interconnection to PCWA’s SCADA system, which is used to remotely control this plant.
The existing 300psi Hydraulic Power Unit and Pressure Tank provided by Voith in 1966 were examined by AGC personnel and deemed to suitable for many more years of dependable service. The only modification recommended was to replace the difficult-to-adjust spring-operated pump unloaders with solenoid-operated unloader kits that were custom-designed to work with the Voith HPU. These Kits, provided by AGC but installed by PCWA personnel, will simplify the future adjustment of pump loading and unloading,
The PRV adaptation to digital control proved more challenging however, since AGC chose to reuse the existing walking-beam control mechanism and mechanical position feedback for PRV distributing valve operation. This decision was intentional, since the PRV is a plant safety device and most customers are reluctant to ‘put all their eggs in one basket’: relying on digital control for a primary safety device. Melding legacy mechanical technology with modern PLC control proved daunting at first, but the experienced engineering and field service crews at AGC devised a solution that works very well while retaining the fail-safe PRV control that protected the penstock against overpressure since commissioning.
About American Governor Company
American Governor Company provides fully engineered digital governor conversion systems for hydro plants, including PLC-based digital controls, electro-hydraulic valve manifolds, hydraulic power units, electronic feedback devices and unit/plant control and monitoring systems. Installation, start-up, commissioning and field training services are provided by our staff of experienced governor specialists and field service engineers. American Governor also provides full support for legacy governor systems, including a complete line of replacement parts for all types of Woodward governors and a growing inventory of new and reconditioned parts and assemblies for other OEM governors. American Governor offers Field Service, Training, Retrofit kits and Technical Support for legacy governors. The Training and Repair Facility near Stevens Point WI offers factory repair services for governor pumps and permanent magnet generators as well as our annual Governor School and twice yearly Focus Classes. American Governor has provided more than 600 digital governor upgrades and has supported over 1,500 hydropower plants in North America.