DCS system UPS power supply configuration

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A chemical enterprise uses a DCS system for chemical operation, monitoring, and control. How to provide high-quality, safe, and reliable power to the DCS system is very important. The plant uses UPS to power the DCS system. The UPS system consists of a rectifier module, a charging module, a battery, an inverter module, an automatic bypass switch, a manual maintenance bypass switch, etc. The UPS can provide high-quality, safe, and reliable power to the DCS to ensure the safety, stability, and reliability of chemical production.

The working principle of the UPS system: The factory power supply with poor power supply quality is first converted into a DC power supply through a rectifier. One way is to charge the battery through a charging module, and the other way uses a high-frequency pulse width modulation (SPWM) method to convert the DC power supply into a pure, high-quality sinusoidal power supply in the inverter to supply power to the load. When the AC working power disappears or components such as the rectifier fail, the battery pack supplies power to the inverter through the battery switch. When the inverter fails and the output voltage is abnormal or overloaded, it switches to the automatic bypass switch to directly supply power to the load from the factory power.

The plant has a relatively small number of DCS system input and output points, and adopts a dual AC input line single UPS power supply mode. For the DCS system with a large number of input and output points, a dual AC input line dual UPS parallel power supply mode is adopted.

1. Dual AC input line single UPS wiring The

dual AC input line single UPS power supply wiring is shown in Figure 1: In normal circumstances, AC input 1 supplies power to the UPS. When AC input 1 fails, it automatically switches to AC input 2 to supply power to the UPS, and AC power supplies 1 and 2 are circuit interlocked. When the UPS is in normal working condition, it operates in the "AC input 1 - rectifier - inverter - load" mode; when the rectifier module fails, it operates in the "battery - inverter - load" mode; when the battery is discharged, it automatically switches to the automatic bypass switch to directly supply AC power to the load; when the inverter module fails, it operates in the "AC input 1 - automatic bypass switch - load" mode to directly supply factory power to the load; when the UPS has a serious fault and needs to be repaired or replaced, it operates in the "AC input 1 - manual maintenance bypass switch" mode to supply factory power to the load. At this time, the UPS is short-circuited and can be handed over for inspection or replacement without affecting the normal power supply to the load.

Figure 1 Dual AC line single UPS power supply wiring

2. Dual AC line dual UPS parallel power supply mode The

dual AC line dual UPS parallel power supply mode is shown in Figure 2: By directly connecting the outputs of two UPS with the same power in parallel, the output voltage, frequency, phase and load current of the two parallel UPS are required to be basically balanced. When the dual UPS parallel system is operating normally, the two UPSs each carry 1/2 of the load current. If one UPS fails, the other will carry all the load, but the load must be reduced. The mean time between failures of this parallel dual UPS system is 7 to 8 times that of a single system, which greatly improves the reliability of the system. However, there is a certain connection between the control of the two UPS in the dual UPS parallel system. When operating, you must understand the control logic of the parallel board, otherwise the parallel board will misjudge due to improper operation, causing the output voltage to sag, which will have an adverse effect on the load.

Figure 2 Dual AC lines and dual UPS parallel power supply mode

3. Precautions for UPS use

(1) When selecting UPS, a certain margin should be left. For example, for a load of 4KVA, the UPS should be configured with more than 5KVA.
(2) UPS should avoid frequent startup and shutdown. It is best to keep it in the startup state for a long time.
(3) Newly purchased UPS should be charged and discharged, which is conducive to extending the service life of UPS batteries. Generally, constant voltage charging is adopted. The initial charging current shall not be greater than 0.5*C5 amperes (C5 can be calculated by the rated capacity of the battery). The voltage of each battery is controlled at 2.30~2.35V to avoid damaging the battery. If the charging current remains unchanged for 3 consecutive hours, it proves that the battery is sufficient. The general charging time is 12~24 hours.
(4) If the factory power supply is normal all the time, the UPS will not have a chance to work. Its battery may be damaged in the long-term floating charge state. The UPS should be charged and discharged regularly. This can not only activate the battery, but also check whether the UPS is in normal working condition.
(5) The UPS should be checked regularly. Check the floating charge voltage once a month. If the floating charge voltage is lower than 2.2V, the entire battery group should be charged evenly.
(6) Wipe the battery with a soft cloth frequently to keep the surface clean.
(7) Temperature control during UPS operation: Keep the temperature range within 20℃～25℃ during UPS operation to extend the service life of UPS battery. Temperature control of UPS is particularly important in an environment without air conditioning.
(8) UPS should be charged immediately after use to restore the battery to normal state.
(9) The distance from the external battery pack to the UPS should be as short as possible, and the cross-sectional area of the wire should be as large as possible to increase the conductivity of the wire and reduce the power loss on the line. Especially when working with high current, the loss on the line cannot be ignored.

4. Daily maintenance and general troubleshooting of UPS

4.1 Daily maintenance

(1) Check the UPS operation display screen every day to confirm whether the UPS power supply operation status simulation indicator light is in normal operation.
(2) Listen to see if there is any abnormal noise from the UPS power supply, mainly the cooling fan of the power module and the isolation output transformer, and whether the transformer has abnormal vibration sound.
(3) Check whether the exhaust vent of the UPS power supply is blocked.
(4) Check the battery floating charge voltage, UPS output voltage and current on the UPS display screen every week, and make a record and compare with the previous data. If the data changes significantly, the cause should be found out in time.

4.2 General fault handling

When the UPS power supply alarms during operation, do not panic and operate any switch or button of the UPS to avoid expanding the accident or even causing UPS output interruption. When the UPS power supply alarms, you should first check the status indicator on the display panel and the prompt on the LCD screen. The UPS has a self-diagnosis function. Once a fault occurs, it will provide information such as the fault type and fault time through the status indicator and LCD screen. After seeing the fault type prompt provided by the status indicator and LCD screen, you can then handle it accordingly according to the different fault types. The following faults often occur during UPS operation.

(1) AC factory power failure, that is, AC input power failure or large-scale fluctuations. When the UPS alarms, the battery discharge alarm information will appear at the same time. At this time, the AC input light of the UPS panel status indicator goes out and the battery discharge light is on. The solution is to first determine whether there is an AC power failure, check whether the UPS input switch is tripped and whether the fuse is blown. If it is indeed a power failure, if the recovery time is unknown, cut off the unimportant loads to ensure uninterrupted power supply to the important loads.

(2) Inverter failure. When this alarm appears, the automatic bypass switch load alarm appears. First, confirm whether the inverter overload alarm appears on the panel at the same time. If so, check the output current on the LCD screen. If it exceeds the rated current, turn off the unimportant loads until the output current is less than the rated output current. Then press the reset button to eliminate the alarm, restart the inverter and output. If there is no overload alarm signal, it means that the inverter is not shut down due to the impact current protection, but the IGBT device or drive control is faulty. The manufacturer should be notified in time for maintenance.

(3) Bypass failure. When this alarm signal appears, the inverter asynchronous alarm signal should generally appear at the same time. This indicates that the voltage and frequency of the input power supply exceed the set range, but meet the rectifier requirements. At this time, the inverter starts to oscillate and no longer tracks the bypass. At this time, large capacity loads cannot be started to avoid impacting the inverter.

5. Conclusion

As long as the UPS power supply is reasonably configured and daily maintenance is paid attention to, and preventive measures are taken, the UPS can run stably and reliably, thereby providing safe, reliable and high-quality power supply to the DCS system, ensuring the safe, stable and long-term operation of chemical production.