![]() The process setpoint is controlled by a distributed control system (DCS) with an analog 4-20mA SCR power signal and a 24 VDC digital panel enable. In addition, the stainless steel enclosure provides great outdoor corrosion resistance and reflects much more solar radiative energy than a painted steel panel, lowering the cooling requirements. To keep the SCRs and internal components within the required temperature limits, thermal calculations were performed and a large air conditioner was selected with an integral heater. It is typical for an SCR to generate 1 - 1.5 Watts of heat per switched amp, per switched control leg. With any panel, providing adequate cooling is one of the most important and overlooked design considerations. ![]() These requirements made active cooling and heating absolutely necessary. The panel is located outside at a location that could reach 105 ☏ in the summers and a freezing cold -20 ☏ in the winters. The field maintenance staff does not need to open the large power enclosure to perform control commissioning or maintenance. This allowed the high voltage and power components to be isolated from the control components. Low voltage control power and signals are passed between the power and control enclosure with a small pass-through. While not required by code, we separated the low voltage control components and control connections inside a separate enclosure. Safety is always the highest priority in our designs. The power output from the SCRs is distributed to the 12 heater circuits which are protected by individual 100A circuit breakers. These controllers allow for advanced monitoring of load voltage, current, and power and various control firing strategies and communication options. The power to the heater is controlled by two of Watlow’s new 700A Aspyre Series advanced SCR controllers. Each safety contactor provided a SCR with a positive way of disconnecting power in case of an over-temperature condition of the heater elements or downstream process, an enclosure air conditioner failure, or a plant or local E-Stop button push. Each of the three phases is carried by four 350 MCM conductors and distributed from the main breaker to two safety contactors. The panel is protected by a 1200 amp main circuit breaker with an external disconnect handle. Shift Controls was approached to design and fabricate a heater control panel to deliver power to a large 800 kW, 480 VAC, 3-phase process heater with a full load current of 963 amps with UL508a certification. In addition, the panel also required control and monitoring interfacing with a plant wide DCS system.
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