DCS system UPS power supply configuration

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 is to use 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 the 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 and the factory power is directly supplied to the load.

The factory 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 DCS system input and output points, the factory adopts a dual AC input line dual UPS parallel power supply mode.

1 Dual AC input line single UPS wiring

Dual AC input line single UPS power supply wiring is shown in Figure 1: In the figure, under normal circumstances, AC input 1 supplies power to UPS. When AC input 1

When there is a fault, it automatically switches to AC input 2 to supply power to the UPS, and AC power sources 1 and 2 are interlocked. When the UPS is in normal working condition, it works in the operating mode of "AC input 1-rectification-inversion-load"; when the rectifier module fails, it works in the operating mode of "battery-inversion-load"; 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 directly supplies factory power to the load in the operating mode of "AC input 1-automatic bypass switch-load"; when the UPS has a serious fault and needs to be repaired or replaced, it supplies factory power to the load in the operating mode of "AC input 1-manual maintenance bypass switch". At this time, the UPS is short-circuited and can be handed over for inspection or replacement without affecting the normal power supply of the load.

[page]2 Dual AC input dual UPS parallel power supply mode

The dual AC input dual UPS parallel power supply mode is shown in Figure 2: By directly connecting the outputs of two UPSs with the same power in parallel

In the past, the output voltage, frequency and phase of the two UPS in parallel were the same, and the load current was basically balanced. When the dual UPS parallel system was operating normally, the two UPSs each carried 1/2 of the load current. If one UPS failed, the other would carry all the load, but it must be used at a reduced load. The average failure-free time 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, due to improper operation, the parallel board will misjudge and cause the output voltage to sag, which will have an adverse effect on the load.

3 UPS use precautions

(1) When selecting UPS, leave a certain margin. 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 the UPS battery. Generally, constant voltage charging is adopted. The initial charging current shall not be greater than 0.5*C5 amperes (C5 can be calculated from 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 fully charged. The general charging time is 12~24 hours.

(4) If the factory power supply is normal, 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. The floating charge voltage should be checked once a month. If the floating charge voltage is lower than 2.2V, the entire battery group should be charged evenly.

(6) The battery should be wiped with a soft cloth frequently to keep the battery surface clean.

(7) Temperature control during UPS operation. The temperature range during UPS operation should be controlled within 20℃~25℃ to extend the service life of the UPS battery. In an environment without air conditioning, UPS temperature control is particularly important.

(8) The UPS should be charged immediately after use to restore the battery to normal condition.

(9) The distance between the external battery pack and 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 is in normal operation.

(2) Listen to see if the UPS power supply makes any abnormal noise, 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 power supply display screen every week, and make a record and compare it with the previous data. If the data changes significantly, the cause should be found out in time.

[page] 4.2 General troubleshooting

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 alarm occurs, 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 perform corresponding processing according to different fault types. During the operation of the UPS, the following faults often occur.

(1) AC factory power failure, that is, the AC input power outage or large-scale fluctuations. When the UPS issues this alarm, the battery discharge alarm information will also appear. 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 it is an AC factory power failure, check whether the UPS input switch is tripped, and whether the fuse is blown. If it is indeed a factory power failure, if the recovery time is unknown, cut off unimportant loads to ensure uninterrupted power supply to 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 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 factory power 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, daily maintenance and care are taken to prevent problems before they occur, the UPS can operate stably and reliably, thereby providing safe, reliable and high-quality power to the DCS system, ensuring the safe, stable and long-term operation of chemical production.


References

1. SALICRU UPS User Manual.

2. Zhang Guoguang. Precautions for using UPS. Modern Communications [J], 2004.8