Research On The Problem Of Harmonic Voltage On Valley Period Of Load In Small Power Networks

In power system, the harmonic which is produced by various kinds of harmonic sources is influencing the electric environment of the whole power system, the danger resulted from this to the electric equipments is serious. With the rapid development of equipment such as the electric electron, the question of harmonic is paid more and more attention to by people. The transformer is one of the traditional harmonic sources, in normal cases, the harmonic it produced is not large, but when the transformer is working in some special operation ways, for example, in the valley period of load during the night, under the voltage of power networks operating on higher situation, the iron core saturation degree of the transformer increases, the harmonic of excitation current it produced increases sharply, therefore, the phenomenon it reflected was that the harmonic voltage of valley period of load in power networks is too high, it is particularly obvious in the small power networks, which have already exceeded the harmonic standard of our country greatly. So, it is much meaningful to research the problem that harmonic voltage is too large on valley period of load in the small power networks.This paper carried on statistical analysis to the data of measurement on a lot of substations, has summarized some characteristics of the harmonic voltage on power networks valley period of load. Taking the specialized electromagnetism transient software EMTDC to study the factors influencing the harmonic current of transformer, and the transformer model adopts UMEC transformer model based on magnetic circuit model principle, for this model has higher precision, this paper also utilizes MATLAB software to deal with analyzing to the emulation data, which was compared and analyzed with the data of measurement.Through a certain amount of between analysis and emulation the paper can get some main conclusions. Harmonic current of transformer mainly relates to saturation degree of the iron core of the transformer, which influences saturation degree of iron core main include the size of the voltage and affects transformer choosing the size of rating magnetic flux density, transformer iron core saturation degree rises while voltage increases, the larger rating magnetic density transformer aggravates the increasing saturation degree; Because of long distance transmission line and little capacity of short circuit, small power networks will make the voltage in the end of power networks rise heavily when on unload or light load, and often there are capacitance banks not removal, which makes the harmonic impedance of the power networks present the capability, may lead to the harmonic amplification phenomenon.