Cost-effective I/O simplifies hydroelectric plant controls upgrade
Lynn Mitchell
Operations Superintendent
Lockhart Power Company
Lockhart, SC 29364
Andy Feimster
President
North Fork Electric
Crumpler, NC 28617
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Lockhart Power Company owns and operates a hydroelectric plant located on the Broad River in upstate South Carolina , and services over 6000 direct customers and 6000 indirect customers over its 90-mile transmission network (Fig. 4). The plant includes an 8-gate dam feeding a canal that channels the water flow to the powerhouse. The powerhouse contains five turbine generators with a combined power capacity of over 17 MW. The dam and turbine control system receives data from power, flow, and level sensing devices to perform monitoring and control of the dam, generators, and associated equipment.
The previous control system, installed in 1987, was a proprietary microprocessor-based system that had become obsolete and was not Y2K compliant. Lockhart required a control system that would meet their Y2K requirements, provide open networking capabilities, and result in a lower total cost of ownership. The new system needed to coexist with an upgraded design of the existing hardwired generator controls, and perform distributed, independent control at the dam and each of the five turbine stations. Each I/O control node would replace the old equipment in the existing control cabinets and be networked to the central control room.
Systems integrator and vendor selection
Lockhart Power contracted North Fork Electric in Crumpler, NC, a provider of hydroelectric products and systems integration services, to lend their expertise to the renovation. North Fork Electric proposed a 3-tier solution. Level 1 included new hardwired controls for manual operation of the system. Level 2 included PLC-based control of the dam and generators in an automatic mode. Level 3 provided PC-based redundant control and monitoring from a central location, and also supplied connectivity to the plantwide Ethernet network.
North Fork Electric was able to source all the control components through Automationdirect.com at less than half the cost of many competing products. Off the shelf hardware and software was available that met the needs of the application and was easy to use and implement.
System components
The system consists of seven DirectLOGIC DL205 micro-modular PLCs using D2-250 CPUs with built in PID functionality. Each of the five systems for generator control includes I/O modules for four 4-20 mA inputs, two ±10 Vdc analog outputs, thirty-two 110 Vac inputs, twelve 110 Vac/dc relay outputs, and an Ethernet communications module. The remaining two PLCs are configured in a master/slave arrangement and control the dam gates-located upriver from the powerhouse-via radio modems.
To reduce maintenance costs, North Fork Electric selected DL205 I/O components to maximize commonality among all systems. The identical analog output modules used on the generators, configured for ±10 Vdc, can be configured for the 0-5 Vdc dam gate control signals by simply moving the module's range selection jumpers. The 12-point ac/dc relay modules are also used for both generator and dam control.
The low-cost Ethernet connection available for the DL205 PLC also weighed heavily in the decision to purchase the DirectLogic product. Additionally, operator interfaces include two DirectTouch 6-in. color touch screen operator panels and a Windows NT-based PC running the LookoutDirect SCADA/HMI software package.
To avoid downtime, North Fork Electric installed the control system one generator at a time. They fabricated five new subpanels, with each controller pre-wired for simple installation into the existing control cabinets.
System operation
In the automatic mode, the PLC can start, stop, and operate the generator via two PID loops that control startup and synchronization of the turbine. Changing the generator gate position varies the flow of water to the turbine, and thus the generator's speed/frequency. To bring a generator online, it must first be synchronized with the line frequency of the power utility. The first PID loop slowly increases the generator speed until it reaches 90% of the setpoint, at which point the logic switches to frequency control. Once online, the generator's power output is varied by changing gate position.
To complete Level 2 control, North Fork Electric then installed the dam control system. This system controls the eight canal gates located at the dam, which regulate the flow of water downstream to the turbines. The operator enters the required gate-position setpoints using the second DirectTouch color touch panel connected to the dam control's master PLC in the control room. Data is transmitted to the slave PLC, located at the dam, via 900 MHz radio modems. The slave PLC monitors individual gate positions using proximity switches that count teeth on the drives' gears, and moves the gates up or down until they reach the desired positions.
North Fork Electric engineers then implemented the third and final tier of control and monitoring. The LookoutDirect SCADA system communicates to all the PLCs over an Ethernet network, performs supervisory control, and collects data that is easily passed to the plant's information system.
North Folk Electric completed the new control system installation in four weeks. Site debugging and programming required an additional four weeks that included several redundant-integrity and Y2K tests, which were performed successfully without any recorded downtime. Lockhart Power now has a powerful, yet simple, redundant control system that uses their familiar hardwired design as the manual control mode.
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