Abstract
Ultra-capacitors are increasingly becoming important in onboard energy storage applications in electric ship, and electric and hybrid electric vehicles where they provide peak power demands during acceleration and also absorb regenerative energy during breaking, thereby improving on power quality and reliability. While active research is continuously being pursued in the use of ultra-capacitors as energy storage in motor drive applications, the topologies reported so far in literature have drawbacks such as limited real power compensation with no reactive power compensation, reactive power compensation with no real power compensation, and the use of DC-DC converters for interfacing, which have significant advantages in low to medium power applications, but degraded performance for high power applications such as in electric ship. Cascaded multilevel converters have been at the center of research for the past several years due to their inherent advantages in medium and high power applications, especially in motor drive applications. A cascaded multilevel converter is a power electronics device designed to synthesize a staircase AC voltage output from several DC sources. The advantages of cascaded multilevel converters include: (1) higher voltage levels can be obtained, thus eliminating the need for costly, bulky, and weighty transformers, (2) lower dv/dt leading to lower electromagnetic interference (EMI), (3) fundamental switching scheme resulting to lower switching losses, (4) lower harmonic distortion with increase in the number of DC voltage levels, thus leading to a lower requirement for output filter. The traditional cascaded multilevel converter interfaces DC energy sources. This research proposes two hybrid cascaded multilevel converter (HCMC) topologies and corresponding control strategies applied to motor drive, interfacing both dc sources and capacitor energy storage elements. In the first topology, the capacitor is controlled to provide reactive power to cancel lower order harmonics through a conditioning converter. The main converter supplied by a DC source can therefore be controlled to provide real power with reduced switching loss and improved efficiency and EMI. Analysis approach for the interaction between modulation index and displacement power factor has been developed to establish the capacitor charging and discharging conditions for induction machine load. A dynamic control strategy for energy storage elements is also developed. A major advantage of the proposed control method is the ability of the capacitor voltage to be successfully maintained at the desired value when the machine is in transient state, which is a key contribution. In the second proposed HCMC topology, the capacitor energy storage can be controlled not only to provide reactive power compensation to improve on power quality, but also to provide real power during acceleration, and absorb regenerative power during deceleration or braking period. Each phase of the proposed converter topology splits the supply of real power between a “main” converter supplied by a dc source, and two “auxiliary” converters supplied by energy storage elements. A hybrid modulation strategy combining sinusoidal pulse width modulation (SPWM) with phase-shift control is adopted for this topology. The motor drive control scheme is divided into three main parts: (1) Indirect or feed-forward vector control of the induction motor to generate the output voltage reference, (2) power distribution control to split the supply of real power between the main and auxiliary converters, (3) voltage balancing control of energy storage elements, firstly to ensure that each capacitor voltage is regulated to the desired value, secondly to ensure voltage balance between all the capacitors in each cascaded converter phase or cluster, thirdly to ensure voltage balance between the three clusters of single-phase cascade converters, and lastly to obtain balanced ac output current by forcing the converter neutral point current to zero. The two multilevel converter topologies and their respective control strategies each have some inherent advantages that make them suitable for different motor drive applications. System level simulation results are provided to verify the proposed converter topologies and their corresponding control methods.
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