Power Management System Helps Plant Reduce Energy Demand

by L.E. Crossley, P.Eng. and M. Demysh, P.Eng.

Diversa Cast Technologies is a manufacturing company specializing in producing aluminum, gray and ductile iron automotive lost foam castings. A power management system is currently installed at their plant in Guelph, Ontario. This power management system is a real time demand management system allowing plant management to reduce operating costs through the automatic control of plant furnace loads with minimum interference to production levels. Based on demand control savings alone the payback for this system is under 8 months.

Experience with this system has proven that the overall demand can be reduced while improving productivity and energy consumption efficiency. The system is fully integrated with the Linamar plants wide area network permitting multiple users to access the system's real time and historical data from their existing desktop computers.

System Design
The Diversa Cast plant has an installed potential load of about 5000 kW made up of two coreless induction melters and supporting equipment. The first melter is rated at 2750 kW with 8000 lbs capacity and is used for iron batch melting in the off peak periods from 11pm to 7am.

The second melter is rated at 1250 kW with a 2700 lb capacity and is used for aluminum heal melting in the on peak periods. The plant operates on 3 shifts 5 days per week. The utility power contract provides power using a time-of-use (TOU) contract with on-peak demand penalties.

The system is shown in Figure 1. Computer equipment is installed in a free standing panel located in the plant's office area and is linked by an RS422 instrument cable to a remote terminal unit (RTU) panel located out in the vicinity of the two melters. The RTU contains data acquisition equipment consisting of a data processor, digital input/output modules and power supplies.

The plant has two utility power in-feeds and two sets of meters. One feed supplies the plant auxiliary equipment while the other supplies the two melters. Each meter has two isolated output contacts providing kWh and kVARh pulse signals proportional to the energy being used. These pulse signals are wired to the RTU panel and are supplied by 24V dc power from the power supply in RTU.

The power consumption of the two melters is monitored by power transducers mounted on the melter power supply units. These transducers provide pulse outputs which are also wired to the RTU panel.

The load on each melter is controlled by the system's computers by effectively changing the operator set points by adjusting the potentiometer load control settings. The degree of control required at any time is calculated by comparing the utility metering input information with the plant's desired demand limit set point.

System Operation
The system's RTU continuously obtains electrical information from the plant's two main utility meters MU1 and MU2, and from the kWh transducers M1 and M2 located on the two melters.

The control software module monitors the energy being used and forecasts the plant's demand in accordance with the power utility billing algorithm used at the plant. By comparing the forecast demand with the desired set point demand, as determined by plant management, the computers issue control command outputs to raise or lower melter loads. These output signals are sent to the RTU panel where they operate digital output modules.

These modules in turn are connected to demand control modules mounted in the control sections of the melter power supply units PS1 and PS2. Resistance is inserted or removed from each of the melters electronic control circuits by the RTU output modules, to control demand.

The system users are provided with a real time graph of overall plant load versus time, with the set target load displayed, together with full analysis tools for historical and cost analysis. In addition to the overall plant total, the load on each individual melter is also shown.

The control of demand is smooth and is applied equally to both melters in three stages. Reloading of the melters is automatic after the load has fallen to the restart level as defined in the system configuration. Every time a control operation takes place the activity is written into a control log together with the time, date and demand at that time.

The load target is password protected and may be changed at any time by authorized users when on the control screen. The output control levels are computed on a minute-by-minute basis using the utility metering signals. The system has several major communication routines.

These routines are:
(a)  The one minute read, on the minute, of all RTU metering counter registers. This information is used for real time load prediction calculations and for load control.
(b)  The 15 minute read and clear, on the 15 minute mark, of all RTU metering counter registers. This information is filed and used for historical analysis purposes.
(c)  The one minute control signal to update the outputs in the RTU in accordance with the load forecasts.

The data values returned to the computers are scaled and stored in files ready for display and analysis. Menu driven routines provide access to the stored data and logs for analysis purposes.

System Software
The software consists of the major modules as shown in Figure 2.

The real time core module contains all the functions necessary to provide data acquisition and analysis as follows:

Communications module: This module provides the data acquisition functions and interface with the control equipment via the RTU.

Display module: Provides the user interface with the system both in graphical and spreadsheet format using Windows 98/NT system software.

Analysis module: Provides graphical and spreadsheet analysis of the stored data including maximums, minimums, coincident demand data, energy use and load factors.

Report module: Provides reports on power use and plant running costs including bill verification by generating utility bill.

Database filer: Stores the data and configuration parameters in ODBC compliant database.

System and cost configuration module: This module allows the number of points to be entered, identified and utility metering algorithms assigned together with energy and demand costs.

Diversa Cast has chosen the automatic demand control option whose operation was described earlier. Other options such as alarm scanning and internet communications may be added to the overall product at a future date.

System Installation and Commissioning
The system was installed by Diversa Cast Technologies electrical staff and commissioned by E2MS. After all wiring was completed, the melter control shedding components were commissioned. Each melter was provided with a three- stage load shed unit that was installed in the melter power supply control section. Each of three output modules in the RTU panel collectively control the shedding and reloading of a melter. Each output module was test energized in turn and its adjustable resistance adjusted for the desired degree of load cut-back.

The cut back in load is progressive so that as each output module is activated, the amount of running load on the melter is reduced. Final checks with each melter running at full load were carried out to verify that the load reduction percentage was as required by the plant management.

Once the melter controls were completed, the utility and melter metering signals were verified and the meter multipliers set in the system configuration software.

Attention was then focused on the commissioning of the computer equipment and the associated software.

Figure 3 shows a portion of the load control screen. The user can set the target that the load control is to operate to. The display, updated once per minute, shows how the load is being controlled using the billing window as a time base.

Plant Experience with the System
The system was commissioned February 14, 2000 and has been in operation for approximately two months. The installation proceeded around production with minimal interference or delays in the melting and pouring schedules. The software has been fully operational without any unscheduled downtime.

The system was started in a monitoring mode for the first two weeks of operation to allow Diversa Cast Technologies to establish a baseline of energy demand and consumption for the plant. The demand control was then turned, on based on initial measurements for the plant melt requirements and base load.

Initial reaction from plant operators was hesitant, as it appeared to limit their ability to meet production melting requirements. This was due to the fact that operators were constantly adjusting power levels of the furnaces to meet a shared combined kW requirement between the two furnaces. Also, power was added to the system until the bath achieved the appropriate release temperature.

By modifying furnace operating procedures and appropriate operator training, Diversa Cast Technologies was able to take advantage of the available power to melt quicker and more efficiently. Operators now utilize the system along with the furnace power management system to input a preset energy kWh count along with a total kW power input. This allows the system to monitor and control the overall power input based on the demand target while allowing the furnace power management system to input the appropriate amount of energy count to the melt bringing the heat to the necessary release temperature. The overall benefit of this procedural change is that operators can now input more power over shorter periods of time to achieve higher melt rates.

Many benefits have been realized beyond the initial project justification based on demand control alone. One of the indirect benefits of limiting demand energy is that the overall Load Factor or efficiency in the way we use our energy has improved. Figure 4 shows a bar graph of the daily load factor since commissioning for the on peak periods which has grown from an average of 70 per cent utilization to an average of over 80 per cent.

Another tangible benefit is that power factor penalties can be significantly reduced by controlling when and how the peak for the plant is set. For instance, if the plant power factor is low, the peak can be reset at the end of the month using the furnaces, which are more efficient, to balance the plant power factor.

This power factor savings also occurs now because the furnace operators are able to apply higher power to the melt without worrying about accidentally resetting the peak for the month.

The software is accessible from anywhere on the WAN which allows anyone with the appropriate permissions to access and view the plant energy, cost and control information. Cost accountants as well as production managers now have the ability to get an instantaneous calculation of the electrical demand and consumption billing for budgeting and planning purposes. This system is also a valuable tool to measure productivity and effectiveness of the plant and melting operations.

As Ontario progresses towards deregulation of electrical energy and as energy costs will continue to escalate, the power management system will be an invaluable tool in helping Diversa Cast Technologies control and manage its energy and production costs.