Instrument power supply issue
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In the control room, for the PLC cabinets and PLC control stations, does the instrumentation department need to provide power? How to supply? Also, I saw a UPS cabinet on the drawings; what is this cabinet used to supply power to?
The power supply for the control station comes from a UPS. The power source for the UPS is derived from the power grid or a power plant.
This is how we use UPS in our factory: first, for individual inputs and outputs, the UPS supplies power to the PLC modules. Second and third inputs are used to invert the voltage to 24V and supply power to the on-site equipment.
Instrument power supply: In our petrochemical industry, a dual-UPS plus mains power supply system is generally used to supply power to PLCs, either in DC or AC form. Dual parallel power supply is also considered, with all power coming from the 220VAC output of the UPS.
OP, this is actually an issue related to the integration between the electrical engineering field and the automatic control field. According to HG/T20636.4-1998, the 380/220VAC AC power supply for instruments (including UPS power supplies, subject to the requirements set by the automatic control team) is to be designed and installed by the electrical engineering team. The electrical engineering team is responsible for delivering wires and cables to the instrument power supply boxes/cabinets, which can be located in the control room, at the local instrument panels, or at any location agreed upon by both parties. The AC power supply below 110VAC is designed and installed by the automation team. In other words, the incoming connection for the UPS power supply before the DCS/PLC control cabinet is definitely to be designed and installed in an electrical manner. As for the connection from the UPS power supply to the DCS/PLC control cabinet, its design and installation can be determined through negotiation between the two parties; it can be done using electrical methods or instrumentation methods, but I recommend an electrical approach for its design and installation.
It was explained in more detail upstairs; the power for PLC or DCS cabinets definitely comes from a UPS, and this is usually handled by electrical engineers. At the same time, the UPS can also supply power to other devices, such as communication cabinets and control panels, depending on the design
One more thing: We added a direct connection to the mains power supply as a precaution in case both UPS units fail and cannot switch over; it indeed worked, ensuring the reliability of the system’s power supply! In the petrochemical industry, a 2-3 power supply connection pattern is generally used! The 2 UPS units are used separately; one of them is connected directly (for a system with 2 connections) or independently (for a system with 3 connections). The 24V system is currently very unreliable, so we have also adopted a dual-input setup to ensure reliability!
Let me add that currently, all power supply designs utilize a dual-power system: from the 380/220VAC power grid to the UPS, and then to the DCS/PLC control cabinets, where the 24VDC power is generated through conversion and rectification – in all cases, there is dual power supply along with automatic, disturbance-free switching. That is, the power supply redundancy method.