Predictive Strategies in Power Management
Wisconsin Oven Corporation recently designed and manufactured a solution heat-treat system for an aluminum-castings manufacturer in Canada. The customer specified an electrically heated system (oven and quench tank) that required a total of 240 kW of heat input to meet their heating requirements. The customer had a limited amount of electrical power available for this particular piece of equipment and requested that the heating system be limited to a total of 172 amps (575 volts) for the initial production cycles.
Instrumentation and Controls Solution
Wisconsin Oven presented this situation to Invensys Eurotherm (supplier) and asked if they had a product offering that could provide a solution for the customer. Following discussions, the supplier outlined a specific solution for this particular challenge. They presented their standard EPowerTM SCR power controller with the predictive load-management feature. The standard range is capable of operating at any voltage between 120 to 600 volts, and the 250-amp EPower was selected.
The customer set specific limits on each heating circuit, and total amp draw was programmed using graphical configuration software (iTools PC Configuration tool). To complete the control package, the programmable furnace controller (3504), high-limit (3204) and paperless chart recorder (nanodacTM) were supplied. All instruments were programmed using the iTools configuration tool.
Solution Heat-Treating System
The oven itself had 168 kW of total heat input available, and the quench tank had 72 kW of total heat input available. Further features of the EPower controller allowed the oven “priority” of heat input during its initial ramp-up to the normal operating temperature. The control schema was programmed to only allow the quench tank to start its heating once the oven heating system was requesting below 80% heating capacity for a set amount of time. Once the two conditions were met, the system allowed the quench tank any available amperage it requested within the limits the EPower system set for total amps allowed.
The controller solution solved the greatest problem the customer was facing by automatically managing an electrical load with limited available amperage, thus avoiding any potential blackout issues. An added benefit of the control schema also provided a more energy-efficient power controller for the quench tank (the original system had an on-off contactor arrangement).
SCR Power Controllers and Predictive Load Management (PLM)
The PLM feature has two main elements – load balancing and load shedding – that are discussed in the following sections.
Load Balancing: Distributing Power, Balancing Consumption
Load balancing is a strategy of equally distributing power of different loads to obtain an overall power consumption as stable and balanced as possible, thereby eliminating peaks of power. This is in contrast to a random firing system.
Each heating zone controlled by an EPower SCR controller is defined by an output power, cycle time and a maximum power (max capacity), which can be pictured as a rectangle. Rather than letting these rectangles pile up randomly, the EPower controller uniformly distributes them, thereby ensuring that at any given moment the overall power is as stable and balanced as possible.
In the field, up to 64 heating zones can be synchronized together with the help of a fast CAN network, separate from optional fieldbus such as Ethernet or Profibus.
These zones can be monitored on one furnace, across several furnaces or even plant-wide, realizing the most effective results on the main network. The PLM function takes care of the disparity between zones and considers the fact that a zone of 10 kW does not have the same impact as a 100-kW zone when distributing power.
It is important to understand that the PLM function does not change the output power but rather balances and shifts the power evenly, thereby eliminating any sort of disturbance. The result is optimum load management through intelligent load balancing and load sharing, a strategy that will eliminate peaks and flicker and even-out overall power usage.
Another benefit of using the EPower controller with its PLM function is that manufacturers are now able to use zero cross firing for their system without any drawbacks. Eliminating phase-angle firing significantly improves the power factor, which, in turn, results in substantial savings.
Using energy more efficiently (i.e. substantially decreasing the reactive power) results in less power generated by the utility company. In fact, we should consider that consuming reactive power is, in the end, simply a waste of energy. While a bad power factor forces the utility company to generate this extra reactive power, it will be of absolutely no use to the end user. Besides saving costs, implementing a best practice of efficient energy consumption also results in considerably fewer CO2 emissions released in the atmosphere.
Even after implementing a strategy that stabilizes overall power consumption, PLM’s second key feature can come into play to save even more.
Load Shedding: Demand Reduction, Load-Control Strategy
Load shedding is completely embedded with the load-balancing strategy. However, this function will only act when necessary or desired. Load shedding is totally dynamic and fully adjustable while not requiring any additional external hardware (e.g., a PLC or control system). The shedding function allows limiting and shifting the overall energy consumption altogether or with fully adjustable user-defined priorities. Adjustments can be made through fieldbus communication (Profibus, DeviceNet and Ethernet), enabling adjustments in view of on-peak period surcharges.
The load-shedding feature can be used in systems where overall power exceeds the capacity of the main transformer. After the load balance has achieved perfect synchronization across the SCRs, the load-shedding function will limit the power only if power demand would exceed the maximum capacity of the main supply, which avoids a blackout.
We can control the maximum demand charge resulting in additional substantial savings by dynamically adjusting the maximum threshold for the installation.
For example, this can be applied if two furnaces start up simultaneously, thereby significantly increasing the demand in kW. The maximum demand in kW is usually based on usage within a 15-minute moving window. This eventually translates into penalties applied by the utility companies for exceeding or increasing demand.
The maximum allowable demand is now raised by the utility company, and the surcharge will be applied to the current billing month for up to the next 11 following months depending on the contract. So, exceeding the maximum demand once can result in a penalty applied for up to 12 months.
Improving the power factor, controlling the demand charge and reducing peak consumption during ON peak times will result in substantial savings. In addition, the PLM function helps to improve the quality of the main power supply and ultimately reduces CO2 emissions (see whitepaper 1 for more information).
Note: For applications where phase angle must be used due to the nature of the heating elements, Invensys Eurotherm can provide energy cost-reduction solutions with the LTC (load tap changer) function of EPower (see whitepaper 2 for more information). IH
For more information: Please contact or visit www.eurotherm.com/products/power-control/epower. Authors can be contacted as follows: Douglas Christiansen, application engineer, Wisconsin Oven, East Troy, WI; e-mail: firstname.lastname@example.org; web: www.wisoven.com and Peter Sherwin, business development manager, Invensys Eurotherm, Ashburn, VA; e-mail: email@example.com; web: www.eurotherm.com
Whitepapers can be downloaded at http://www.eurotherm.com/products/power-control/epower/
1. Energy Cost Reduction through Load Balancing and Load Shedding
2. Predictive Strategies in Power Management
EPowerTM SCR Power Controller
EPower SCR power controllers have measurement accuracy, functionality and innovation that will provide the necessary power control to help your process, environment and budget.
• Measurement accuracy (< 1%) for better control
• Load management for better distribution of energy and to minimize peak-energy costs
• When necessary, configuration is highly customizable to meet customer requirements.
The photograph shows an EPower SCR power-controller installation. The SCR independently controls both the oven and the quench. With a limited power supply, the application requires a strict control of peak power, utilized load management and software capabilities of the EPower SCR power controller with dynamic shedding priority between the oven and the quench.