Meter And Harmonic Compensation Current Of The Transformer Model Correction Algorithm Research

With wide applications of power electronic equipment harmonics pollution in power system is becoming more and more serious. Normally two methods are used to solve this problem, one is adopting filters and the other is utilizing high power factor rectifier. Emergence of the instantaneous reactive power theory and advances of power electronics technology promote the fast development of APF (active power filter). One key problem with APF application is how to accurately calculate harmonic currents for compensation. High voltage APF normally connects to bus line through a step-down transformer due to restriction of capacity and voltage nomination. However, the step-down transformer renders nonlinear behavior because of ferromagnetic material within the iron. What more, excitation current will also produce new harmonics itself. Therefore, amplitude and phase derivation will be incurred when the compensation currents transfer through the step-down transformer, and thereby the impacts of the transformer must be considered.The models of step-down transformers used for APF is thoroughly discussed in this paper. Firstly, the dynamic characteristic of the step-down transformer is analyzed. With a comparison of the currently available methods for magnetic curve fitting, a simple model is presented with a view to realizing quick and accurate computation. Also, the frequency dependent characteristics of the transformer's equivalent resistance are analyzed, the results of which indicate that the impact of frequency dependence is very trivial and a constant resistance can be used instead in the model.Further, the available harmonics detection methods is not suitable for intensively nonlinear load, so an effective algorithm is presented by comprehensive integration of i_p-i_q and p-q methods to detect harmonic currents with distorted source voltages. Simulation results demonstrate the proposed methodology is effective and correct.A simple circuitry with APF is analyzed, with consideration of harmonic transformer model as well as the new detection methodology. A rectifying algorithm is inserted in the calculation of harmonics currents for instantaneous compensation with...