The 576-MW Fairfield pumped-storage plant, owned by South Carolina Electric and Gas (SCE&G), utilizes eight reversible pump-turbine units to provide peaking power for the central region in South Carolina. Each unit uses a design where the 20 wicket gates are individually controlled by a hydraulic servo, instead of a more commonly used design linking all the wicket gates together with a gate ring and servo pair. The unique design provided a number of challenges for SCE&G and its suppliers as unit and governor control upgrades were made.
Understanding the problem
The Fairfield plant, located near Columbia, S.C., was originally constructed in the 1970s, with Woodward Cabinet Actuator turbine governor controls put into service in 1978. The control system’s hydraulic portions had significant oil leakage that had been inherent to the design and present from the beginning. These leaks and required a drainage sump located below the gate servomotors.
Meanwhile, its mechanical system design resulted in coarse and sluggish control of the wicket gate servos due to the many control linkages and mechanical restoring system.
Given its poor performance, the original system was unlikely to have met any of the modern Institute of Electrical and Electronics Engineers (IEEE) performance recommendations for governor control systems. After 30 years of service, SCE&G made the decision to replace them due to obsolescence and increasing maintenance costs. The utility also desired updates to utilize modern digital controls for the purpose of remote control, data logging, reporting, trending and alarming.
|South Carolina Electric and Gas’ 576-MW Fairfield pumped-storage plant provides peaking power for central South Carolina.|
Testing the waters
The original project was contracted to two separate vendors; one vendor was responsible for the controls upgrade while another vendor was responsible for the new gate ring design. Units 5 and 6 were selected by SCE&G to be the first pair upgraded, with a conversion from the 20 individually controlled servo design to a more conventional gate shifting ring design with four large servos in a push-pull configuration.
Updates also included Rockwell Automation ControlLogix Redundant PLC processors for unit control, along with a GE Cimplicity human-machine interface for reporting, trending, alarming and daily operation.
During the installation and commissioning of the new shifting ring for Units 5 and 6, the supplier encountered difficulties with the engineering and installation of the ring, along with gate alignment issues. Once completed, the upgrade of the governor and control systems worked as desired, but due to cost overruns and time delays associated with Units 5 and 6, SCE&G decided to upgrade the remaining units with a control system that would incorporate individual servo controls for each wicket gate.
|New proportional and distributing valves installed at Fairfield reduce oil leakage and allow for fast and accurate servomotor timing.|
L&S Electric was subsequently chosen to provide the new control system for the remaining six units. L&S Electric was selected based on the proposed control system solution, prior successful experience, competitive price and overall expertise.
L&S encountered a number of technical challenges in engineering the electrohydraulic valves and associated digital control system to individually control each wicket gate of the turbines, including valve selection and design, valve performance testing, digital control system design and system performance testing.
A two-stage hydraulic control valve system was selected to interface between the new governor control PLC analog outputs and each of the 20 wicket gate servomotors. The two stage valves provide the ability to have integrated dual but independent rate limiters while also providing the required oil flow to meet the servo movement timing needs for the application. The timing adjustments for the valves allow the valve to be setup for a range of both opening and closing maximum servo rates.
The primary (pilot) stage control valve used an off-the-shelf D03 Bosch proportional valve. The second stage or distributing valve was designed specifically for this application to match the oil flow or timing requirements for the Fairfield application.
The two control valves were integrated together along with a fail-safe hydraulic shutdown valve/manifold design with the primary function of delivering a secondary shutdown device required by the IEEE 125-2007 design standard.
Unit control system solution
Controller architecture technical challenges
Within the hydro facility there are two very different types of control requirements that encompass a complete control system: sequential functional control and high-speed process control.
Sequential functional control is best represented at a unit level PLC where functions such as start/stop sequencing, operational control modes and protective trip sequences reside. With this type of application the scan times of the program can approach, and even sometimes exceed, 100ms without degrading unit performance.
High-speed process control is best represented at a governor level PLC where functions such as real time speed, gate or power control reside. With this type of application the program scan time must be actively managed to ensure that the closed loop control on the gate and speed signals can be updated at intervals that do not exceed 10ms to ensure compliance with the performance standards outlined within IEEE Std. 125-2007 (IEEE Recommended Practice for Preparation of Equipment Specifications for Speed-Governing of Hydraulic Turbines Intended to Drive Electric Generators).
While the IEEE and regional regulatory entities are totally independent organizations that do not actively collaborate, it is important to note that the Std. 125-2007 is widely accepted throughout the industry as a minimum design guideline to ensure robust performance within each regulatory jurisdiction.
Compliance with this standard is even more critical with facilities that utilize individual servomotors for each wicket gate. Because of the need for 21 fast closed loop controls, the additional process control associated with a digital governor retrofit increases to a level that creates excessive software overhead for a single PLC-based control system, regardless of brand or supplier.
Controller architecture solution provided
To overcome the dual requirement of sequence control and multiple fast closed loop controls of the servos and speed, the control system for this application has been separated into two independent PLC systems as opposed to a single PLC system design.
The control system includes a dedicated Governor PLC to support real time speed and power process control along with Servo Control to manage the 20 individual servomotor controls and a dedicated Unit Control PLC for sequential functional control of each unit.
This control architecture offers the end user with the following notable cascading benefits throughout the life of the installation:
— Segregation of the control systems allows each PLC to be optimized for a specific function to ensure compliance with industry performance requirements.
— Optimized for a specific function, the overall program size for each PLC is incrementally reduced. This eliminates the need for program interrupts and other complicated methods of task scheduling.
— Simplification of the software design complexity translates directly into increased system troubleshooting efficiency while also minimizing the risk of program changes that can impact the response of the entire unit.
— Segregating the control functions also offers installation benefits that include optimization of the physical location of the controller.
— Optimization of controller physical location reduces the installation cost by minimizing field cable lengths.
— Minimizing field cable lengths reduces the potential for future issues with electrical noise associated with long cable runs.
The control system upgrade project for the six units was completed by L&S Electric at the end of year 2014. Currently, L&S Electric has been contracted to upgrade Units 5 & 6 and the plant common controls system. This upgrade will effectively produce the same design philosophy L&S Electric supplied for the other six units.
Results and lessons learned
Collectively controlling individual gate servos as opposed to controlling a mechanically linked gate servo system does pose many technical challenges. Many of those challenges are the same as or very similar to those faced in engineering distributing valves and closed-loop control systems for shifting ring turbines and even Pelton turbines with deflectors and multiple needle servos.
The Fairfield project resulted in a successful and economical alternative to retrofitting the plant’s turbines with a mechanical gate shifting ring, and also poses new possibilities to potential adaptive controls that could help reduce rough zones, runner wear and maintenance of pump-turbines.
— By Kevin Schultz, lead controls engineer, and Bill Tarter, lead project manager, L&S Electric