Xavier Yang

Power System Flicker Analysis by RMS Voltage Values and Numeric Flicker Meter Emulation

This paper introduces a new methodology of flicker analysis by using root-mean-square (rms) voltage values. The main objective is to simplify flicker assessment in a large-scale grid and propose adequate methods in order to carry out flicker analysis with general power system software. The basis of this paper is that the common flicker frequency of industrial loads is lower than 25 Hz; this makes it possible to use rms values instead of waveform samples to perform flicker computation. The researches are focused on the following three key points in flicker analysis: (1) fast voltage change-dependent distribution load model used for flicker propagation study, (2) IEC 61000-4-15 flicker meter emulators, and (3) simplified dynamic flicker load models. The studied load models and flicker meter emulators have been integrated in power-quality simulation software with the electromechanical transient analysis module. Field measurements have been used to verify and support the proposed methodology. Different case studies show that this approach gives satisfied results in flicker assessment and flicker mitigation. It is also used by utilities to perform flicker assessment prior to the connection of a disturbance load.

Harmonic Summation and Assessment Based on Probability Distribution

At the point of common coupling, there are many cases where the phase-angle difference between harmonic vectors is unknown. In practice, for example, in calculating the steady-state rating of the ac passive harmonic filter, the arithmetic summation method is often used in determining the maximal harmonic level. This summation law is very conservative as it may result in high cost of equipment ratings. Actually, a compromise should be made in taking into account the equipment safety and the risk of overrating as well as excessive costs. Based on uniform distribution of the difference in phase angles, this letter provides an algorithm to calculate the probability upon which the magnitude of summation vector of two harmonics may exceed a given value. This may also result in a recommendation of another harmonic summation method for the standard IEC/TR 61000-3-6.

An Analytical Procedure for Calculating Harmonics of Three-Phase Uncontrolled Rectifiers Under Nonideal Conditions

Detailed analytical procedures (DAPs) are very efficient methods for obtaining the steady-state solution and the harmonic evaluation of three-phase diode bridges with finite capacitive dc smoothing. Until now, these DAPs are developed for balanced operation of the system. This paper presents a new DAP. The consideration of unbalanced operation, the inclusion of dc reactor, and the treatment of the operating point in terms of power are contributions with respect to previous DAPs.

Robust scheduling for microgrid energy management optimization

This paper presents a robust optimization algorithm for scheduling the dispatchable units in a microgrid. Combined economic and environmental optimization problem with strict and soft operational constraints is formulated by Mixed Integer Linear Programming frame. Uncertainties coming from the forecasting error of renewable (RnE) and local load are processed by convex optimization procedure with affine recourse treatment. As a result, the suggested algorithm generates off-line long-term-ahead operation trajectory of the dispatchable units in a microgrid, which minimizes the operation cost while maximally respecting the instantaneous energy balance and has robustness against the uncertainties from RnE as well as consumption prediction errors.

Harmonic summation and assessment based on probability distribution of phase angle

At the point of common coupling (PCC), the summation of two harmonic voltage vectors at same frequency is only certain if their amplitudes and phase angles are well known. Therefore, there are many cases where the phase angle difference between harmonic voltage vectors is unknown. In practice, for example, in calculating the steady state rating of the a.c. passive harmonic filter, arithmetic summation method is often used in determining maximal harmonic level. This summation law is very conservative as it may result in high cost of equipment ratings. Actually, a compromise should be made in taking into account the equipment safety and the risk of overrating as well as excessive costs. Based on uniform distribution of difference in phase angles, the paper provides an algorithm to calculate the probability upon which the magnitude of summation vector of two harmonic voltages may exceed a given value. On the basis of the probability and the given harmonic value, harmonic summation can be assessed. This may also result in a recommendation of another harmonic summation method for the standard IEC/TR 61000-3-6.

Dynamic optimal operation scheduling of microgrid using binary gravitational search algorithm

With the emergence of energy crisis and environmental pollution, the concept of microgrid (MG) is receiving much attention today. The optimal operation scheduling is an important research topic in the field of MG. In this paper, binary gravitational search algorithm (BGSA) is proposed to optimize the operation scheduling of MG dynamically, which includes dynamic optimal unit commitment and power dispatch. At first, according to the characteristics of MG operation and its internal relations between different time periods, a mathematical optimization model is established to achieve the least operation and environmental cost, while meeting the load demand and system operating requirements. In order to solve this complex optimal problem, BGSA is proposed to optimize the unit commitment and power dispatch of MG. The simulation results show that the proposed method has efficient performance in optimizing operation scheduling of MG dynamically, and thus can improve the technical level of MG operation for more economic and environmental benefits.

A Detailed Procedure for Harmonic Analysis of Three-Phase Diode Rectifiers Under Discontinuous Conduction Mode and Nonideal Conditions

Fast and detailed calculations of harmonics, generated by three-phase uncontrolled rectifiers with finite dc smoothing, can be performed by using detailed analytical procedures (DAPs). Several DAPs are developed for continuous- and discontinuous-conduction modes (CCM and DCM) when balanced operation of the system is assumed. The authors of this paper have recently proposed a new DAP (model 1a) in which nonideal conditions, such as unbalances and even harmonic voltages on the ac side, are taken into account. However, only CCM is assumed in model 1a. The main objective of this paper is to overcome this limitation of model 1a so that a new DAP (model 1b) for DCM is presented, which is capable of considering unbalanced operation. Moreover, the boundary of the DCM is also estimated for model 1b under nonideal conditions.

Disturbance assessment in the grid integration of power electronic devices and other non-linear loads

Two main grid access contracts have been set up in France between utilities and customers. These grid connection contracts not only fix power quality commitments for grid operators but also customer disturbance emission limits. The practice of these contracts shows that a good overall power quality can only be achieved by respecting commitments and doing effort from both of grid and customer sides. Since the beginning of the 1990s, a substantial growth of power electronic application has been noticed in electric power systems. More and more power electronic devices have been used in grid and facilities such as low voltage electronic devices (compact fluorescent lamp, switching-mode power supply), adjustable speed drive (ASD), high voltage direct current link (HVDC), etc. The application of power electronic devices in facilities has brought both advantages (energy saving) and side-effects (disturbances). The second part of this paper deals with disturbance load modelling and grid disturbance assessment. For grid disturbance assessment, it is suggested to use multi-phase harmonic injection method with fundamental power flow. This method can give an acceptable results with very short computing time for non linear load modelling. The key point of proposed frequency domain method is that each harmonic source is individually considered as both disturbance source and disturbance receiver in order to take into account interactions between sources. A plug&play HVDC component has been developed and used in one of two case studies in the end of this article.