Study On The Monitoring Protection Device Used In Low Voltage Power System Based On Ethernet

The traditional low-voltage power system, lower than 10kV-level and used in the plant, usually lack fully functional monitoring and protection system. So if there is some fault in the power system, maybe it would cause heavy economic losses and even endanger the life and safety of workers, which can not meet the electrification, mechanization development needs of large industrial and mining enterprises. Based on the actual situation in the plant of large industrial and mining enterprises, applying the latest research achievements in power system protection based on micro-computer, this paper designs an intelligent monitoring protection device, which can communicate based on Ethernet and have the integrated function of monitoring and protection.After detailed analysis of the principle and algorithm of power system protection, this paper considers the Fourier algorithm as the most important and introduces some related algorithms, such as non-periodic decaying DC component filtering algorithm, cosine window and interpolation algorithm for analysis of harmonic. Also a creative algorithm is promoted and used to measure the frequency of the power system. Based on Fourier transform and strict mathematic deduction, the expression of the cosine function for phase difference is carried out. Then using the Fourier transform results of two sampling periods while sampling non-synchronically, the cosine function value of the precise phase difference is shown. Hence, the real signal frequency can be obtained without calculating precise phases of these two periods. In addition, the inverse-time protection is analyzed detailedly .The exponential calculation of the traditional inverse-time over current curves is inherently difficult to realize in the microprocessor. Referring to the IEC standard for inverse-time overcurrent protection, a new inverse-time algorithm suited for microcomputer-based protection system is put forward. A consecutive thermal model, which will perfectly reflect the thermal condition of conductors, is founded with differential equation. Based on the consecutive thermal model and strict mathematic deduction, a discrete algorithm model, which can accurately describe the process of thermal effect, is presented.The monitoring protection device based on microprocessor, designed by this paper, chooses LPC2292 and DSPIC30F6015 to construct dual-processor structure. These two microprocessors realize the man-machine interface and protection algorithm respectively. In the microprocessor LPC2292, designer uses embedded real-time operating systemμC/OS-II, building robust, reliable and scalable software systems. After constructing the hardware and software interface of Ethernet communication and reasonable tailoring of the micro-embedded TCP/IP protocol stack uIP, MODBUS-TCP protocol can be realized. Through the intranet in industrial and mining enterprises, the remote control room can visit the low-voltage power system monitoring protection devices in the plant, realize remote monitoring, control and set the action parameter for protection remotely, without in-depth production site. So this device is particularly applicable to underground low-voltage power system monitoring protection in large mineral production enterprises.