PTQ Q4 2026 Issue

Selection method for motor restart in petrochemical plants

Undervoltage restart method fully integrated into motor protection relays uses both voltage and current criteria to reliably identify sag events and avoid misoperation

Wang Xin and Li WenPing CTCI Beijing

V oltage sags and short-duration power interruptions frequently trip low-voltage motors in petrochemi- cal plants, causing unplanned shutdowns, material waste, and economic losses. Existing solutions such as pro- grammable logic controller (PLC)-based restart panels and standalone single-motor restart relays are often expensive, require extensive wiring, or cannot distinguish voltage sags from normal shutdowns. This restart method requires very little additional wiring, draws no external auxiliary power, and supports immedi- ate, delayed, and staged batch restart to maintain stable bus voltage during recovery. As a highly integrated and cost-effective solution, it fits well with the design require - ments of modern petrochemical plants that pursue high reliability and simplified construction. The solution has been deployed in several large petro- chemical facilities, including the 450 KTA ABS and AS pro- ject (Zhangzhou, China), 660 KTA PDH and 450 KTA PP plant (Quanzhou, China), and others. Field operation dur- ing actual voltage sags confirms correct and stable restart performance, effectively reducing unplanned downtime, reducing economic losses, and improving overall plant safety and operational continuity. Stable operations Petrochemical plants are continuous-production facilities that rely on stable operation of hundreds of low-voltage motors. Voltage sags, short-term drops, or momentary losses of supply voltage are common events caused by lightning, grid faults, start-up of large equipment, or exter- nal network disturbances. Such sags often cause contac- tors to drop out and motors to trip. The resulting production interruptions can lead to pipeline blockages, equipment damage, and significant economic losses. Two restart methods are widely used in industry today. The first uses a dedicated PLC-based self-starting panel. It offers flexible logic but has high hardware cost, complex on-site wiring, large space requirements, and a single point of failure. If the PLC or its power supply fails, all motors lose restart capability. The second restart method uses standalone single- motor restart relays. These are cheaper but still require extra wiring, depend on an external auxiliary power supply,

and typically use only voltage criteria. They cannot reliably distinguish a voltage sag from a normal manual shutdown, posing safety risks if a motor restarts unexpectedly after maintenance. The undervoltage restart method in this discussion is embedded directly into a motor protection relay (MPR), applying dual voltage-current criteria, requiring no exter- nal power supply for timing, and adding very few wiring changes. It supports immediate restart, delayed restart, and staged batch restart of motor groups. This method has been ver- ified in practical engineering and can effectively improve the stability of power supply and production continuity. Field experience from multiple large petrochemical projects demonstrates its reliability and economic benefits. The undervoltage restart method is embedded directly into a motor protection relay, applying dual voltage-current criteria Existing motor restart methods A PLC-based restart panel uses a PLC to detect voltage sags and issue closing commands after power returns. The PLC is usually installed centrally in the distribution room and powered by an uninterruptible power supply (UPS) to keep logic running during short interruptions. Depending on the number of motors, single or multiple PLCs can be used. PLC limitations to consider include high hardware cost, complex wiring between the PLC and each motor starter, large installation footprint, and centralised control risk. Failure of the PLC or its power supply disables all motor restart functions. Standalone single-motor restart relays are external devices that connect to the contactor auxiliary contacts and bus voltage. They judge power loss only by voltage level and contactor position, and they need a separate auxiliary power supply to maintain timing during a sag. Other notable limitations include that they are a voltage-

PTQ Q4 2026