By A. Heringer, D. Jahnke and K. Wong
Alexander Generating Station (GS) and Pine Portage GS both had a Nichols electromechanical supervisory control system installed in 1965 and 1968 respectively. In 1985 we replaced the telemetry portion of the Alexander GS system with a telemetry unit. These systems were failing regularly and parts were becoming difficult to find. Cameron Falls is presently the control center for these three stations. We will automate it later this year and supervisory control of all three stations will be moved to Thunder Bay, 100 km to the west.
Need for Automation
There were three main reasons for this work: business unbundling, replacement of failing equipment and an incentive to reduce operating costs.
The new control center in Thunder Bay will ultimately control all ten of Ontario Hydro's hydroelectric plants in northwestern Ontario. To bring these stations into one control room, a SCADA (Supervisory Control and Data Acquisition) system was installed to give operators one HMI (human machine interface) for all stations, including logging, trending, alarm prioritization and all other operating aids provided by these systems. This computer equipment also provides the data needed for the ConEff (Convention Efficiency) system.
We installed a small SCADA system at Cameron Falls to control Pine Portage GS and Alexander GS which will run until the automation of Cameron Falls itself is completed. This system will act both as a backup for control as a local controller for Cameron Falls.
Design Features for Maintainability
As we specify the new equipment, we attempt to incorporate elements of maintainability into the design. We include such criteria as:
The maintainability of the new control equipment- modularity, expandability, reliability, diagnostics, ease of modification, remote accessibility, etc.
The inclusion of features will improve the maintainability of the controlled equipment. The features would consist of such things as better and more thorough information, historical logs, consistency with operating and maintenance procedures, proactive controls that reduce operator intervention.
RTU versus PLC
Pine Portage GS and Alexander GS were already supervisory-controlled with old deteriorating systems. Because of this the stations were easily retrofitted with a modern control system. The governors, AVRs (Automatic Voltage Regulators), start/stop sequences and protection were all in fair condition and a fast inexpensive conversion was done by replacing the old supervisory control. Ontario Hydro decided that either a Unit PLC with communication to the Harris RTU would be added in the future, or a ladder logic within the Harris RTU itself would be used to replace the existing start sop sequencing relays when required. We chose the Harris RTU because it provided closed loop control, enabling us to use setpoint control. It can run ladder logic and provide all the SCADA functions and protocols to ensure secure communication. We viewed functions such as select-check-operate, alarm buffering and acknowledgment as requirements. These functions are all part of the Harris RTU package and could be easily set up with little ladder logic. A redundant design was chosen to provide additional security and allowed us to replace the deteriorating station annunciator at these two plants with a local SCADA workstation.
The alarms, telemetry and control were picked up at existing terminal racks and wired to one redundant RTU. The elimination of duplicate wiring and contact multiplying for local and remote control reduced labor cost substantially.
SCADA Master Functions
The SCADA master used at Cameron Falls was developed by Quindar Products Ltd. The HMI consists of two Digital VAX 4000 workstations that can drive up to four monitors each, and either of these workstations can act as a warm standby for the one running SCADA. The master also contains a local RTU to drive the station bell and buzzer, and several alarm inputs to monitor cabinet temperature and power supply levels. A printer, a terminal server and a modem rack complete the master station. The PC running the ConEff program is also on the LAN (Local Area Network).
Besides the basic functions, the SCADA also determines flows based on unit head and megawatts from lookup tables. The ConEff computer and the operator use the resultant data. We run a computer program that sends the various commands and set points for the control mode selection on each unit in the SCADA master. The program ensures that the unit is on-line before sending commands and looks up unit flow from the flow table.
The MW and VAR control mode enables the operator to enter a desired unit set point and select continuous or single shot operation. In continuous mode a Proportional-Integral-Derivative (PID) loop is turned on and runs constantly, tracking the set point until the operator selects off. This keeps the unit at the desired set point as conditions change. In the single shot mode the same PID loop is also turned on, but is turned off when we reach the set point.
The operator also has the option of selecting wicket gate position mode that moves the wicket gates to the set point entered by the operator. This mode turns on the wicket gate position PID loop and turns it off when we achieve the desire setpoint. We set up some ladder logic to ensure that the MW and wicket position modes are not selected simultaneously. The wicket gate limit control is similar to wicket gate position control. The operator enters a limit setpoint to move the gate limit and the loop is then shut off when they reach setpoint.
There is also ConEff mode for each unit, which takes the setpoint calculated by the ConEff computer and sends it to the RTU. This set point is based on unit net heads and flows obtained from the SCADA to optimize all units as river conditions change.
RTUs (Remote Terminal Units)
We installed three Harris RTUs for this project.
The Alexander GS RTU is equipped with 96 AI's (analog inputs), 256 DI's (digital inputs) and 96 DO's (digital outputs). The AI's are from transducer inputs that we had installed for the Quindar telemetry system in 1968. There are 20 PID loops in this RTU, one for each of the five units MW, MX, wicket gate position and wicket gate limit positions. The RTU communicates with three SCADA masters; the Hydroelectric Control Center presently at Cameron Falls, the GRID Control Center for status and telemetry data only and the local controller. A fourth serial communications port is used to get temperature data from a data logger. This enables the operator to monitor the data logger temperatures and set alarm limits on each point remotely. The old supervisory only had one temperature alarm, forcing the operator to travel to the station to see which point was in alarm.
We equipt the Pine Portage GS RTU with 32 AI's, 192 DI's and 96 DO's. The AI's are from transducer inputs for headwater level, tailwater level, wicket gate position and wicket gate limit position. No transducer for unit MW, MX, Voltage and amp existed, so PML (Power Measurement Limited type ACM3300) digital transducers were installed for each unit and are polled on a serial port. There are 16 PID loops in this RTU. The RTU also communicates with three SCADA masters and a data logger as at Alexander GS.
The Cameron Falls RTU was installed to provide data to the ConEff computer through the SCADA master. The inputs included headwater and tail water levels, unit breaker status and unit MW and MX are obtained from PML digital transducers. This RTU uses three serial ports; one for the SCADA master at Cameron Falls, one for the data logger and one for the PMLs.
PID Application
The original interposing relays used to move wicket gates, gate limits and AVRs where replaced by new relays in the RTU controlled by the PID loops pulse width modulation function. The PID loops were tuned to match the characteristics of each unit that was found to vary greatly. This process took two days for the first unit, but could be done in about a half day per unit as we gained experience.
ConEff (Conversion Efficiency)
The upgrade of the SCADA system from the electro-mechanical system to the computer based system presents an excellent opportunity to incorporate optimization strategies into the control system. Ontario Hydro has long been involved in developing Conversion Efficiency (ConEff) algorithms and programs to optimize the conversion efficiency of our Hydroelectric stations. The Nipigon project is one of the more successful achievements.
The ConEff system in the Nipigon system allows us to load units of different performance characteristics to a set station output as required by the Grid using the minimum amount of discharge. This is a real-time process and it continuously adjusts itself as the operating parameters change. The benefit amounts to 15 GWh per year which is equivalent to 0.25 percent of the total generation from the Nipigon River plants.
The first phase of the Nipigon automation project allows remote operation of Pine Portage GS (upstream) and Alexander GS (downstream) from the control centre in Cameron Falls GS. The control of the Cameron Falls GS is presently achieved through original benchboards in the control room. The ConEff system, with the SCADA system, provides closed-looped optimal load controls for generating units in Pine Portage GS and Alexander GS. In addition, it provides the advisory function to the operators to optimize the unit loading at Cameron Falls GS. That was achieved by installing in Cameron Falls GS a small remote terminal unit (RTU) to collect essential real-time data and feeding them to the SCADA master. Regular adjustments to individual generator output by the operators are required to optimize the station output at Cameron Falls GS. We will change this when we complete the phase II of the Nipigon River plant automation that will allow remote operation of Cameron Falls GS via the SCADA system. The ConEff system will then include closed-looped optimal dispatch for the Cameron units.
The ConEff system was a Windows based program developed in-house using Visual Basic. The Windows-based program brought the familiar graphical user interface to the operators. It also provides a multitasking environment that is important for the effective operation of the system. The ConEff program runs on a Pentium-based computer with a 17-inch monitor as an Operator Interface that is found next to the SCADA master. The decision to use a separate computer to implement the ConEff system was to accommodate a rapid development cycle and at the same time, to provide the maximum flexibility for future upgrade to the ConEff optimizing algorithms without jeopardizing the security of the SCADA system.
The operator interface displays real-time generator and station data. As well, they display optimal loading settings and a graphical indication of any deviations from the optimal settings. We retrieve the input quantities to the ConEff system from the SCADA master through an Ethernet connection. As the Quindar SCADA master is a Digital Alpha based computer with a VMS operating system. Considerable efforts were spent in developing the communication module that communicates with the SCADA master database using the Digital Pathwork networking software and a database access utility provided by the SCADA manufacturer. Some quantities that we retrieve from the SCADA master are:
- individual generator output
- generator operating mode
- station headwater and tailwater levels
- mode selection
The ConEff algorithm approximates unit performance curves with quadratic polynomials. Based on these quadratic performance curves, the algorithm attempts to figure out the unit loading pattern to reduce the discharge to produce a given station output. Incrementally loading the units with units of the least incremental discharge achieve this. The ConEff computer then returns the individual generation (MW) set point to the SCADA master that in turn passes the settings to the RTUs. Control loops (PID) are set up in the RTU to maintain unit outputs to the prescribed settings. Thus maintaining the target station output while keeping the station discharge to a minimum.
With the conventional mechanical governors in Pine Portage GS and Alexander GS, contact closures with variable duration are used to drive the speeder motor to maintain the Megawatt setpoint with satisfactory results. With the introduction of digital governors in the Phase II automation upgrade for Cameron Falls GS, more precise controls on the unit generation will be expected.
Conclusions
The system performance is acceptable. It could be improved by moving some of the programming done in the SCADA to the RTU ladder logic. This will been done on future projects. The implementation of start stop logic as part of the RTU is also being planned for the future, as are retrofit designs. Ontario Hydro has used both RTU and PLC based systems for recent automation upgrades. Each system has it's definite strengths which must be evaluated with respect to business information requirements, maintainability, communications/protocol capability and technical support.
The ConEff module was developed in parallel with the SCADA project and it was completed on schedule. But every project has its share of challenges, some of the experience we gained are:
- the tuning of the PID loop was critical to reduce amount of motoring on the speeder motor
- the implementation of the PID loops within the RTU uncovered a firmware bug since then was corrected by the Vendor
- educating the operator on the concept of optimization and the subsequent "user buy-in" is critical to the success of the project.
A. Heringer, D. Jahnke, and K. Wong are with Ontario Hydro. This paper was originally presented at CEA's Electricity' 97 Conference and Exposition. ET