supplied by a small transformer or a diesel generator with limited capacity, restarting a full group of motors, even staged, may still cause excessive voltage drop. In such cases, a power system study (e.g., dynamic simulation using software such as Electrical Transient Analysis Program [ETAP]) is recommended before applying the method. This is not a weakness of the method itself but a constraint of the power supply. Conclusion This article presents an under- voltage restart method embed- ded directly in MPRs for low-voltage motors (typically ≤690 V) in petrochemical plants. The method uses voltage and current dual criteria to accurately distinguish voltage sags from normal shutdowns. It requires very little additional wiring, consumes no external auxiliary power, and supports immedi- ate, delayed, and staged batch restart modes. The integrated design simpli- fies engineering implementation, reduces system complexity, and improves the stability and avail- ability of motor power supply systems. It provides a practical and economical way to address voltage sag problems in pet- rochemical plants and can be
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Figure 3 Control circuit diagram of the motor. The control circuit uses the built-in restart output contacts of MPR. It realises immediate restart and delayed restart without addi- tional components
timing during a complete power loss, most MPRs include an internal lithium-manganese battery or supercapacitor. This component accurately measures the power-loss duration without external power. Under normal operation, its service life reaches up to 15 years, and it requires no maintenance, a significant advantage over external relays that need separate power and periodic battery replacement. Limitations and practical considerations While the methodology in this discussion has proven suc- cessful, several limitations should be noted: Parameterisation requires expertise: Correct operation depends on appropriate settings for recovery voltage, dropout voltage, and timing delays. If the recovery voltage is set too high (e.g., >90% of rated voltage), the MPR may never issue a restart command after a deep sag. If set too low (e.g., <70% of rated voltage), the motor may restart on a severely depressed bus, causing excessive current and voltage drop. Plants must ensure that engineers under- stand the logic before commissioning. Large motor groups on weak power supplies: For plants
widely popularised in engineering projects. Field applications in several large petrochemical plants, such as the previously mentioned facilities, the 40 KTA EAA project (Lianyungang, China), and the 200 KTA HMD pro- ject (Lianyungang, China), have demonstrated correct and stable operation during actual voltage sag events. The solution significantly reduces unplanned downtime and associated economic losses while improving plant safety. Given its low cost, high reliability, and ease of implementa- tion, the method has strong potential for wider adoption in continuous-process industries. With the increasing demand for stable operation in modern petrochemical facilities, this method can provide reliable technical support for voltage sag resilience and continuous production worldwide. Wang Xin is a Senior Engineer at CTCI Beijing, a CTCI company. He is also a registered electrical engineer (power, distribution), a registered fire engineer, and a registered consulting engineer. He is primarily engaged in electrical design for petrochemical plants. Li WenPing is an Assistant Chief Engineer at CTCI Beijing, a CTCI company, primarily engaged in electrical design for petrochemical plants. She is also a registered electrical and fire engineer.
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PTQ Q4 2026
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