Dr. Zuher Alnasir and Professor Mehrdad Kazerani established a focus on developing a Small-scale Standalone wind energy conversion system (WECS) based on current-source inverter.
The study published in Renewable Energy journal made attempts to verify the feasibility of the proposed system by addressing the challenges associated with maximum power tracking, energy storage integration, power management and dc-link inductor design.
As a robust, low-cost and low-maintenance machine, Squirrel-Cage Induction Generator (SCIG) has been employed in the proposed small-scale standalone WECS.
Voltage-Source Inverter (VSI) has been dominantly employed in variable-speed wind turbine systems for converting DC to desired AC output voltage. Current-Source Inverter (CSI) has also been widely used in medium-voltage and high-power applications. To investigate the potential of CSI in low-power off-grid wind turbine applications, the authors conducted a comparison between Pulse-Width Modulated Voltage-Source Inverter (PWM-VSI) and Pulse-Width Modulated Current-Source Inverter (PWM-CSI) in terms of reliability, cost, efficiency, and protection requirements.
Although IGBT-based PWM-VSI offers some advantages in small-scale off-grid wind energy conversion systems in terms of capital cost, overall efficiency and open-circuit fault requirements, IGBT-based PWM-CSI is the winner in the comparison, in terms of reliability, operation and maintenance costs, short-circuit fault requirements and inherent voltage-boost capability.
The wind energy conversion system proposed by Alnasir and Kazerani (2016) consists of rotor blades, a geared-drive self-excited squirrel-cage induction generator, a three-phase diode bridge rectifier, a dc/dc buck converter , a dc-link inductor, a three phase pulse-width modulated IGBT-based current-source inverter, a capacitor filter, a delta/star transformer, a Y-connected three-phase R-L load, a battery-based energy storage system featuring an H-bridge converter with reduced number of switches, and a dump load controlled by an IGBT switch. The dc-link inductor, shared by three power electronic converters, was systemically designed. The current-source inverter-based wind energy conversion system exchanges power between the battery pack and dc bus through the interfacing H-bridge converter with bipolar-voltage and unidirectional-current capabilities.
Control strategies for controller blocks include: (i) maximum power point tracking (MPPT) control for extraction of maximum power from wind by regulating the generator shaft speed at the optimum value corresponding to present value of wind speed (Vw), (ii) power management in dc-link among battery and dump load, as battery absorbs excess power during high-wind-speed or low-load periods to compensate for shortage of power during low-wind speed or high-load period, and (iii) load-side control to compensate for voltage imbalance at the point of common coupling so that Voltage Unbalance Factor (VUF) does not exceed permissible limit of 1%.
Performance of the system was evaluated under variable wind speed and unbalanced three-phase R-L load. Rated values for unbalanced load in phases a, b and c were 9.3kW/2.7kVar, 4kW/6kVar and 6.7kW/1.3kVar, respectively.
From simulation results, it can be observed that maximum power at various wind speeds was extracted through the MPPT controller, dc-link inductor current was controlled at the desired values through the H-bridge converter, and voltage and frequency were regulated at the desired values through the current-source inverter. Due to unbalanced load, different modulation indexes were produced for three phases. RMS values of line voltages exhibited negligible imbalance with the largest deviation of -2.72% observed in phase voltage magnitude during load changes. The highest value noticed for VUF was 0.7% which is below the permissible limit of 1%. 3.4%, 3.2% and 2.8% were the highest Total Harmonic Distortions (THDs) detected in line voltages Vab, Vbc and Vca, respectively, which are lower than the typical limit of 5%. The highest THDs of load currents were 1.1%, 0.82% and 0.98% for ia, ib and ic, respectively, implying close-to-sinusoidal currents.
Generator response, when the proposed system was run under two different wind speed profiles, showed that maximum power point tracking controller under a wind speed profile with lower rate of wind speed change (i.e., lower dVW/dt) performed better than in the case of higher dVW/dt. The negative impact of system inertia on MPPT controller was also demonstrated.
Despite achieving high quality voltage and current waveforms at the load bus, Alnasir and Kazerani (2016) emphasized the possibility of incorporating additional features to modulate neutral currents.
Zuher Alnasir, Mehrdad Kazerani, A Small-Scale Standalone Wind Energy Conversion System featuring SCIG, CSI and a Novel Storage Integration Scheme. Renewable Energy, Volume 89, 2016, Pages 360–370.
Electrical and Computer Engineering, University of Waterloo, 200 University Avenue West, N2L 3G1, Ontario, Canada.
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