Three phase voltage source inverters typically feature a significant amount of dc energy storage to maintain a stiff dc bus. In these cases, the dc link capacitor is sized to store enough energy to maintain several tens of cycles of ac output at the rated power. In this paper, we propose to reduce the size of the dc link capacitor dramatically, to store enough energy to provide just one high frequency switching cycle of ac output power. The dc bus is no longer stiff and hence classical sinusoidal pulse width modulation cannot be used. But, the stored energy modulation (SEM) concept that we propose herein synthesizes high quality sinusoidal output voltage waveforms even with such tiny dc link capacitors. In SEM, the switching intervals of the interconnecting switches are carefully determined nonlinear functions of various operating parameters such as reactive component sizing, switching frequency, load levels, etc. This paper presents the analytical and detailed design development of the stored energy modulation approach along with circuit simulation, experimental results and comparative evaluation of a dc to three phase system example.

During IGBT breakdowns, the grid side converter (GSC) of a renewable power plant must be disconnected from the grid. In the recent past, topological modifications propose the faulty phase to be connected to the mid-point of the DC bus as a solution. In this configuration, the converter can only generate four vectors. Moreover, since the capacitor voltages carry a voltage ripple at grid frequency, the magnitude and angle of the vectors become variable during post-fault scenarios. Recent model predictive control (MPC) approaches select the vectors to be used in the cost function based on the location of the reference vector in a post-fault unsymmetrical vector diagram or by checking the effect of all of them, which is time consuming. This paper proposes a simplified approach which reduces the computing time by avoiding successive iterations to obtain the correct vector pairs. Furthermore, an approach to rebuild a regular hexagon from the four post-fault vectors is proposed. Hence, the proposed controller retains the advantages of the conventional space vector modulation (SVM) approach with minimal additions to handle post-fault scenarios. Lastly, the paper studies the limitations in the VSC output voltage and power magnitudes during post-fault scenarios. The approaches presented in this paper have been verified using simulation and experimental results.

Today's semiconductor devices are accompanied by high switching frequencies (> kilo-hertz) and small transition times (< micro-seconds). Such fast transition times are accompanied by undesirable effects such as voltage overshoots at the load terminals, ground leakage currents, wide-band electromagnetic noise, etc. With the advent of wide band-gap devices, several applications are moving towards higher switching frequency operation with at-least an order of magnitude reduction in transition times. While these characteristics are considered necessary to break the next-generation barriers in power density, efficiency and applicability, the undesirable effects due to faster transitions are expected to present obstacles. This work proposes a PWM approach to modify the shape of the switching voltages to overcome the disadvantages of the fast transition times without any increase in switching losses. In fact, several of the switching transitions feature ZVS operation, resulting in reduced switching losses. The paper discusses the analytical details of the approach using a simple DC-DC boost-buck converter and extends it to a DC to three-phase AC converter using the principles of space vector modulation. The paper presents detailed simulation and comparative results in terms of voltage over-shoots over long cables, loss calculations and electromagnetic noise. Results from a laboratory-scale working prototype confirm the benefits of the proposed approach in terms of EMI and loss reduction.

Cascaded dc voltage link power converters typically employ a significant amount of energy storage at the intermediate link. The dc bus voltage is assumed to be stiff enough to provide decoupling between the source and load, allowing dynamic regulation functions to be implemented at the load and source terminals independently. In these cases, size of the dc bus capacitance is dictated by requirements of steady state ripple voltage, ripple current rating, ride-through considerations and more importantly, voltage overshoot during source and load transients. Design trade-offs in realizing a stable system with acceptable transient performance often place a lower bound in the value of dc bus capacitance. In this paper, a robust predictive controller based on integrated and coupled control at the intermediate dc link, source and load terminals is presented. The approach permits the use of a small amount of intermediate energy storage compared to conventional decoupled designs. The predictive control aspect of the proposed approach features cost function minimization and accurate duty ratio calculations based on converter model that accounts for small values of dc link capacitance. While this allows stable operation of the system, it leads to persistent steady state error and transient overshoot voltages. The robust aspect of the proposed approach is based on the theory of variable structure systems. It introduces a capacitor charge restoration function that overcomes the drawbacks of the predictive control by squarely addressing uncertainty of model parameters and operating conditions. The resulting significant reduction in size of the capacitive reactive components opens the prospect of replacing electrolytic capacitors that have a limited life span with film capacitors with much longer life in emerging applications. The proposed approach is presented in a step-by-step manner using a dc to dc converter example and further extended to a dc to ac inverter case. Extensive computer simulations and experimental results from a laboratory scale prototype of an example system are used to demonstrate the viability of the proposed controller.

Recently, the falling costs of photovoltaic (PV) and battery storage (BS) technologies have raised interest in the creation of hybrid PV+BS power plants. Together with increasing energy storage capacity by storing clipped energy, hybrid plants broaden a power plant’s grid services by featuring fast dispatch flexibility and voltage-ampere reactive support. Although many proposed PV plants are being developed with colocated batteries, the dominant architecture of ­utility-scale PV+BS power plants is uncertain. Moreover, power architectures are expected to evolve over time based on future trends. The primary goal of this article is to critically examine and compare several dc–dc and dc–ac power converter architectures that are suitable candidates for hybrid PV+BS power plants. In particular, the article presents characteristics of several power conversion architectures from the point of view of power semiconductor requirements, efficiency, reactive component requirements, modularity, control complexity, and so forth. Detailed analytical models are utilized, along with a benchmark design example to present a comparative evaluation of the alternatives for performing engineering tradeoff studies.

Modular multilevel power electronic converters are considered an increasingly critical family of converters for myriad high voltage high power applications. With the ever-growing emphasis on electrification of the economy, they play a crucial role in serving energy sources and loads whose electrical ratings go beyond the ratings of the conventional power electronic building blocks. In particular, modular multilevel converter (MMC) topology enjoy its dominance in such applications due to modularity, scalability, performance and fault-tolerance capability. However, the MMC topology design imposes low-frequency ac components on the module capacitors and thus is inhibited by the capacitor size. Capacitor sizing plays a significant role in the overall system’s size, cost and reliability. This paper introduces a minimal capacitor module based topology for DC to three-phase AC conversion. The unique design feature of the module includes minimal capacitor requirement due to elimination of single-phase ac power processing requirements. Together with improved power density, reduction of capacitor size permits the use of only film capacitors thus eliminating the weakest link of the overall system. Along with the step-by-step analytical derivation of the proposed approach, the paper presents detailed simulation studies, comparative analysis and experimental results from a proof-of-concept laboratory-scale prototype.