Active Harmonic Compensation and Stability Improvement in High-Power Grid-Connected Inverters Using Unified Power Quality Conditioner

Multifunctional features of grid-connected inverters can be used for harmonic compensation of local load voltage and grid-injected current. But, in high-power grid-connected inverters, there is a challenge due to low switching frequency. On the other hand, simultaneous compensation of local load voltage and grid-injected current harmonics is an important issue in grid-connected inverters. Using a Unified Power Quality Conditioner (UPQC) at the Point of Common Coupling (PPC), an improved active harmonic compensation method is proposed which is appropriate for high-power low-frequency grid-connected inverters. The UPQC operates as a combination of a negative shunt virtual admittance and a negative series virtual impedance at the PCC. It suppresses the disturbances caused by local load variation and grid impedance change. Using a low-power, high-frequency UPQC, local load voltage and grid-injected current harmonics up to higher-order components are simultaneously compensated despite grid impedance changes and nonlinear local load variations. The control system is designed according to the impedance-based stability criterion to ensure the system's stability. The theoretical results are validated using different case study simulations in MATLAB/Simulink software

Survey of accidents in suburban Tehran and the prediction of future events based on a time-series model

Background: Car accidents are currently a social issue globally because they result in the deaths of many people. The aim of this study was to examine traffic accidents in suburban Tehran and forecast the number of future accidents using a time-series model. Methods: The sample population of this cross-sectional study was all traffic accidents that caused death and physical injuries in suburban Tehran in 2010 and 2011, as registered by the Tehran Emergency Section. In the present study, Minitab 15 software was used to provide a description of traffic accidents in suburban Tehran for the specified time period as well as those that occurred during April 2012. Results: The results indicated that the average number of traffic accidents in suburban Tehran per day in 2010 was 7.91 with a standard deviation of 7.70. This figure for 2011 was 6 daily traffic accidents with a standard deviation of 5.30. A one-way analysis of variance indicated that the average of traffic accidents in suburban Tehran was different for different months of the year (P = 0.000). The study results showed that different seasons in 2010 and 2011 had significantly different numbers of traffic accidents (P < 0.05). Through an auto-regressive moving average (ARMA), it was predicted that there would be 166 traffic accidents in April 2012 with a mean of 5.53 and maximum of 6 traffic accidents/day. Conclusion: There has been a decreasing trend in the average number of traffic accidents per da