Research On The Harmonic Source Parameters

With the rapid development of power electronic technology and the wider attention of energy saving, nonlinear loads with bridge rectifier circuit have been immediately increasing in power system. These loads are widely and randomly distributed in the low voltage distribution system, leading to the increasingly serious harmonic problems, which not only affects the safe operation of power grid but also causes power quality decline. In order to reduce the harmonic pollution and improve the electric environment, with regard to a large number of nonlinear loads, it is an important condition to research the indices of responsibility for multi-harmonic sources and study the harmonic source parameters by establishing appropriate model.It's helpful for the formulation of harmonic suppression strategies and the improvement of incentive mechanism as well as the estimation of harmonic emission levels.First of all, based on the analysis that the existing harmonic pollution indices still have some difficulties in the implementation of the incentive scheme, the ratio of the apparent power of the equivalent harmonic source of a concerned power supply branch at a particular order harmonic frequency to that apparent power transformed in this branch at the fundamental frequency, i.e. Ssi,h* = Ssi,h / Ssi,l=Zsi,h*Isi,h2/Ssi,l is proposed as an indice for harmonic pollution contributions. This indice shows that the harm of harmonic sources to the distribution network mainly depends on the harmonic current emission intensity and harmonic impedance. Meanwhile, it truly and quantitatively reveals the pollution ability of the harmonic source at the PCC, which is useful to the quantitative implementation and accomplishment of the incentive scheme and helpful to guide the harmonic compensation. It is applicable to both equivalent harmonic sources in complex networks and a single nonlinear harmonic source.Secondly, based on Norton equivalent circuit, a new element model i.e."nonlinear load can be described by a controlled current source in parallel with equivalent impedance" is proposed in this thesis. And the influence factors of model parameters are discussed by using fluctuation method. Simulation results under various conditions show that the system in certain cases (with constant ? ) can be represented by Norton circuit, and the equivalent harmonic source of nonlinear component is a controlled source. Besides, this controlled source will be affected by impedance angle. Moreover, in a particular state with constant firing angle a , whether it is a nonlinear component or load group consisting of many nonlinear components,or an equivalent single-port network, it can be described by "Norton equivalent circuit"or “Thevenin equivalent circuit".At last, this thesis pointed out that "non-invasive" methods are not applicable for harmonic source parameters estimation from the perspective of mathematical theory,and explained why.