Abstract : This article provides a comprehensive discussion of the characteristics, functions, principles, and applications of the 7SJ551 multi-functional protection relay in the Inner Mongolia Fertilizer Plant. Keywords : Relay, Microprocessor, Protection, Measurement. The Inner Mongolia Fertilizer Plant is a large-scale fertilizer plant built in the 1990s. Its 6KV high-voltage electrical cabinets for the ammonia synthesis and power boiler systems are imported equipment. The secondary protection for AC high-voltage motors, transformers, and cables all utilize Siemens 7SJ551 multi-functional relays. This relay protection system overcomes many shortcomings of traditional mechanical and transistor relays, employing microprocessors, LCD displays, and other new technologies. It eliminates the need for many measuring instruments, signal fault alarms, optical displays, and auxiliary relays in the protection process, and simplifies wiring and calibration. Currently, it is indeed an advanced new technology in the electrical relay protection of large fertilizer enterprises, improving technical levels and reliability indicators, and enhancing the performance of protection devices. The 7SJ551 multi-functional relay is introduced below. 1. Overview of the 7SJ511 Multifunctional Relay It is a general-purpose, precision protection device, roughly the size of a GL-10 series overcurrent relay. Its protection applications include motors, transformers, and cables. The panel features a 32-character LCD display that shows all useful information, such as various operating data, set device data, protection data, current operating date and time, various electrical parameters during operation, fault alarm and trip date and time and electrical parameters, total equipment operating time, manufacturer data, etc. LEDs 1, 2, 3, and 4 on the panel can be used to group alarm and circuit breaker signals, arrange them in three columns, control and manage measured values, and handle actual protection functions, such as measurement filtering, continuous calculation of fault detection values, fault state determination during faults, limit value and time sequence scanning, remote control protection and automatic access signal sequence control, trip command judgment, storage of instantaneous current and voltage values ​​during faults, and processing of input and output functions from binary inputs. [b][align=center]For details, please click: Application of 7SJ551 Multifunctional Protective Relay in Fertilizer Plants[/align][/b]