Power management

Republic Technologies International recovered the cost of a new Rockwell Automation Power and Energy Management Solutions (PEMS) system in under six months.

Steel bar producer Republic Technologies International (RTI) of Canton, Ohio, recovered the cost of a new Rockwell Automation Power and Energy Management Solutions (PEMS) system in under six months as a result of savings in energy costs and improved productivity.

Electricity costs of £1.7 million per month make up a significant share of operating expenses for RTI. The company’s contract with its electricity supplier includes a demand limit, above which the price of power significantly rises.

The PEMS system provided by Rockwell Automation inhibits or sheds electric furnace loads to control power demand. The system sums electric meter pulses, and forecasts demand within a 30 minute fixed demand interval. Furnace operators select priorities to determine the system’s actions based on the demand forecast.

To provide redundancy, the system uses two Allen-Bradley PLC-5 controllers with backup communications modules, tied into the existing control Ethernet network. Remote I/O panels at the three furnaces use a fibre optic loop to communicate with the PLCs. An Allen-Bradley Powermonitor II power meter on the plant mains provides additional power quality data.

The PEMS system provides four dedicated operator HMI stations, including one main and one backup workstation, and a dial-in server located in one of the electricity substations.

Plant energy consumption is regulated to preset limits by the system, which calculates energy projections every five seconds at the plant mains and the three furnaces. The algorithm permits the shedding of furnace loads to be delayed as long as possible. The system also predicts when the next furnace will be shed.

Operators classify each quarter of an hour of the day as super peak, on peak, intermediate peak, or off peak. The operator enters contract, shed and restore level setpoints, and the system automatically manages setpoints based on utility status, day of week and time of day. The system dynamically responds to real time spot power purchases, and HMI screens are designed to assist operators in managing purchases of power at spot market prices on an hour by hour basis.

The Powermonitor II is used to provide valuable real-time power quality data. Besides voltage and current, MW, MVAR, MVA and power factor, the PMII tracks and triggers alarms on transient voltages that are hard to pinpoint with a chart recorder. The monitor provides detailed power quality information, such as voltage harmonic distortion.

The system also produces a variety of tabular and graphical reports, including daily reports, and weekly and monthly summaries of demand, delays and energy usage. The demand management system has been designed with ‘hooks’ for a direct interface so, in the future, RTI can communicate directly with its electricity supplier. The supplier will then download its system status and market prices, and the demand management system will use this information to automate the purchase decision. Purchased quantities will also be uploaded directly to the utility.

The total installed cost was recovered in less than six months through greatly reduced manpower, required to manage energy demand, while production has increased as a result of cutting furnace delay by 78% per heat. The system has also reduced voltage sags through better utilisation of existing capacitor banks, while RTI anticipates saving an additional £44,000 per month by reducing its contracted demand level.

The opportunities for savings on all types of energy costs fall into three main categories: Monitoring and cost allocation; power system monitoring and control; demand management and load shedding. Monitoring and cost allocation can reduce energy consumption by 3-10% through raising awareness of where energy is being used and so encouraging a reduction in energy wastage.

Power system monitoring and control identifies and corrects power quality problems such as harmonics and transient voltages that can damage sensitive equipment and increase energy bills through poor power factors.

Demand management can save 5-25% of energy bills by automatically capping peak energy demands at the level agreed with the electricity supplier and so avoiding financial penalties. As in the RTI case, this can involve emergency shedding of non-essential loads to ensure that critical plant and equipment are continuously supplied, without exceeding the agreed maximum power demand.

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