Novel Control Algorithms for Power Quality Improvement Using Four-Leg Converter
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In a three-phase four-wire microgrid system, a significant number of end-user devices are unevenly distributed with largely unpredictable switching on or off characteristics both in time and space. Under such circumstances, substantial negative and zero sequence currents are drawn from the utility grid resulting in disrupting the sensitive devices, overheating the neutral conductors, and damaging the power generating sources. Such problems further aggravate by degrading the active power flow capacity of the distributing system with leading or lagging power-factor currents in the presence of reactive loads. A shunt converter, such as a four-leg converter, can be used to mitigate the above-mentioned power quality (PQ) problems in a three-phase four-wire microgrid system. However, the effectiveness of the four-leg converter mainly depends upon the control techniques. Thus, this dissertation focuses on the control algorithms for the four-leg converter to eliminate the PQ problems caused by different loading conditions in a three-phase four-wire system. Also, as an extension of this work, the use of a stand-alone four-leg converter in an aircraft power generation system is investigated to improve the power quality as well as transient and dynamic performance during load-on and load-off operations for aircraft loads. First, a detection technique using reduced order generalized integrator (ROGI) is investigated to achieve accurate and rapid detection of unbalanced currents. Next, a novel decoupling method named double reduced-order generalized integrators (DROGI) is proposed for the four-leg converter. Such a strategy decouples the compensation mode (DC-AC) and rectification mode (AC-DC) of the converter. As a result, more flexibility and selectivity in terms of control methods are achieved. Further, based on DROGI, grid voltage modulated direct power control (GVM-DPC) without PLL is proposed to eliminate the impact of PLL. Moreover, considering unbalanced nonlinear grid/load conditions, a novel method, namely triple-ROGI, is proposed to compensate and absorb the required current, and provide selective harmonic currents to the system. These control algorithms are verified by simulation and experimental results. Also, a control strategy, named dual current control, is proposed for stand-alone four-leg converter in aircraft power generation system. The effectiveness of the control method is validated by simulation results.