A Study on New Distortion Power Quality Index in a Distribution Power System

다국어 초록 (Multilingual Abstract)

This thesis presents a study on new distortion power quality index (DPQI), which is directly related to the distortion power generated from nonlinear loads, to apply for a practical distribution power network by improving the performance of the previous DPQI. The proposed DPQI is formed as a specified combination of three factors; the electrical load composition rate (LCR) and total harmonic distortions (THDs) of voltage and current waveforms. The LCR is estimated by the reduced multivariate polynomial (RMP) model, which is a kind of optimization technique with the one-shot training property, and the Euclidean norm of THDs in measured voltage and current waveforms is applied to implement the proposed DPQI. The application of measurement based THD, which makes it possible to remove current estimation procedure of the previous DPQI, plays a major role of performance improvement of the proposed index in this thesis and provides possibility of real-time application.
To implement the proposed index, it is developed that multi-level analysis (MLA) for the exact LCR estimation performance and harmonics detection algorithm based on the least-square principle for Parseval’s theorem based THD calculation. The MLA technique stratifying many loads connected to a single PCC into several levels guarantees constant estimation performance of the LCR in general conditions. Also, the THD based on Parseval’s theorem, which can represent precise information about degree of distortion of waveform including inter-harmonics, helps the proposed index to achieve good performance. Especially, as a least-square method, the RMP model is used to detect harmonic components, and its off-line calculation contributes to reduction of computational effort.
Relationship between distortion power and the formulations of proposed DPQI, which are presented as two types of common form and exact form with compensation term, is verified by mathematical derivations. Based on the proposed DPQI, the harmonic pollution ranking (HPR), which indicates how much negative effect each nonlinear load has on the point of common coupling (PCC) with respect to distortion power, is determined. Its effectiveness and validity are verified by the experimental results from its prototype’s implementation in a laboratory with a single-phase 3 kW photovoltaic (PV) grid-connected inverter, which contributes to a small distortion in voltage at the PCC, and practical nonlinear loads. Then, the harmonic current injection model based time-domain simulations are carried out to prove the effectiveness of the proposed index under the other conditions with different nonlinear loads.