Abstract-Advances in power electronics drive systems for variable speed operation has enabled extensive use of such solutions in the propulsion and thruster systems of marine vessels. These solutions however introduce current and voltage distortions that compromises the overall power quality of the onboard electrical system. This paper presents and discusses one approach for generating the harmonic current reference for an active filter based on optimization. Two relevant results are revealed by this study: 1) lower THD values are attained by performing system optimization compared to local compensation of one load, and 2) the lower THD values are achieved with a smaller active filter rating than the one required for local load compensation.
This paper presents a comprehensive control strategy for unified power quality conditioners (UPQCs) to compensate for both voltage and current quality problems. The controllers for the series and shunt components of the UPQC are, equally, divided into three blocks: À main controller, which deals with the fundamental-frequency issues such as active and reactive power flow;`harmonic controller, which ensures zero-error tracking while compensating voltage and current harmonics;´the set-point generation block, which handles the different control objectives of the UPQC. The controller design procedure has been simplified to the selection of three parameters for each converter. Furthermore, the proposed strategy can be implemented measuring only four variables, which represents a reasonable number of sensors. In addition, a pulse width modulation (PWM)-based modulation with fixed switching frequency is used for both converters. The proposed control strategy has been validated experimentally under different conditions, including grid-frequency variations.
The late Spanish grid code established that, to avoid island operation, renewable power sources have to be disconnected from the grid in the case of a voltage sag. This would threaten the system stability in case of massive penetration of renewable sources and motivated a recent revision of the code. The new code establishes strict ratios between active and reactive currents during the sag and requests that the plant must quickly resume its normal operation, when the voltage sag ends. Existing installations are also affected if they want to enjoy the premiums associated with renewable generation. When full upgrading of existing installations is not practical, this study proposes a plug-in shunt-connected power electronics device to help solar plants comply with the new grid code, without modifying the solar inverters. The device and its control algorithm are explained in this study and experimental results using photo-voltaic inverters without low-voltage ride-through capability are presented. Solar inverters remain unaware of the voltage sag and they can be led quickly and smoothly back into the grid when the sag is over.
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