Study on factors affecting the harmonics of large-scale rectification device in power grid

In recent years, with the rapid development of the power electronic technology, various power electronic devices and nonlinear elements, especially of many kinds of rectifier and switcher, are applied widely in power system. Harmonics have produced serious adverse effects in the equipment of power generators and distribution system, and of their control and protection, as well as the users and the communication fields. Due to the increasing requirements of power quality, the influence of harmonics of large scale power rectifier couldn't be ignored. Harmonic voltage and harmonic current are in a high level relatively because of the large power consumption of the rectifier. This paper mostly studies the factors which affects the content of harmonics. This paper briefly introduces the main structure of the aluminum electrolyzation rectifying system and operating principle, and the reason that harmonics caused and its harm, also analyzes the harmonics of the system about a real rectifying system. On the foundation of the work done above, electromagnetic transient simulation model of the aluminum electrolyzation rectifying system is established at PSCAD/EMTDC. Harmonics of the system either in all units or all except standby unit on is analyzed. In all units on, characteristic harmonics (84n±1, including harmonic voltage and harmonic current) and uncharacteristic harmonics (3rd, 5th, 7th) are negligible. And in all except standby unit on, characteristic harmonics are still almost the same, while many new uncharacteristic harmonics (12n±1, especially 11th and 13th) are generated. In addition, certain uncharacteristic harmonics will be produced because of the asymmetric system and transformer parameters such as leakage resistance and three-phase voltage. Finally, the paper briefly discusses several solutions to decrease harmonics and improve the power quality.