Multilevel converters are growing fast, whereas industry needs particular tools to evaluate efficiency and performance of such converters. This study focuses on the analysis and practice of power losses in a three-level neutral-point clamped (NPC) inverter. First, a precise mathematical model for three-level inverters is introduced to be used for simulating power losses of the switches, providing AC voltage and current of each phase, voltage and current of the switches as well as both the conduction and switching losses. Further, an NPC inverter was developed to validate the analytical work by comparing experimental results with those of simulations. Additionally, the thermal modelling of semiconductors is obtained using datasheet parameters. Then, the temperature rise is further modelled using the power losses along with the thermal model in the form of RC ladder. Finally, the lifetime of semiconductors is predicted using the heating curves in line with the power-cycling concept. The measured power losses and temperatures in comparison with the presented model suggest this kind of research as an applicable replacement for expensive measuring devices.

Distributed virtual inertia is an approach of providing synthetic inertia in small-scale modern grids dominated by converter-based generators. In this method, the inertial response of synchronous machines is emulated by the energy stored in the dc-link capacitors of grid-tied converters. Nonetheless, it results in instability of the interfaced converter in weak grids. To overcome this problem and get the most benefit out of the acceptable dc-link capacitor voltage deviations, a new compensation technique is proposed in this paper. The gridinteractive converter in the presented framework is controlled in the current control mode, compositing of two conventional inner and outer control loops, distributed virtual inertia controller and a novel compensator. The detailed small-signal representation of the whole control scheme in state-space form is derived. Then, it is revealed that the coupling between d-and q-axis controllers introduced by the distributed virtual inertia gain and its differential operator gives rise to the system instability in weak grids, which can be eliminated through the ancillary compensator. The time-domain simulation model is built to confirm the efficacy of the proposed control technique. The results depict that the ancillary active power provided by the proposed approach during frequency disturbance is 14% of the converter power rating of 20 kW, which yields the improvement of frequency rate of change by 18.82%.

Integration of renewable energy sources (RESs) into power systems is growing due to eco-friendly concerns and ever-increasing electricity demand. Voltage source converters (VSCs) are the main interface between RESs and power grids, which have neither rotational inertia nor damping characteristics. Lack of these metrics make the power grid sensitive to frequency disturbances and thereby under frequency, to load shedding activation or even large-scale collapse. In this regard, the contribution of this paper is to develop a new control technique for VSCs that can provide virtual inertia and damping properties with the DC-link capacitors inhered in the DC-side of grid-tied VSCs. The applied VSC is controlled in the current controlled model, with the capability of injecting extra active power with the aim of frequency support during perturbations. The dynamics assessment of the proposed platform is derived in detail. It is revealed that the control scheme performs in a stable region even under weak-grid conditions. Finally, simulations are conducted in MATLAB to depict the efficacy and feasibility of the proposed method. The results show that frequency deviation is mitigated under step up/down changes in the demand, and the rate of change of frequency is improved by 47.37% compared to the case in which the synthetic inertia loop is canceled out.

Various modulation schemes have already been developed for conventional three-phase buck-type converters such as sinusoidal pulse-width modulation and space vector modulation (SVM). However, SVM is not applicable to boost-type converters. This study proposes a new concept on sinusoidal modulation for both buck and boost-type three-phase DC-AC converters. A novel optimised modulation technique is introduced by using basic equations of conventional buck and boost. By solving the optimisation problem, pulse widths will be obtained for a given modulation index. The significant point is that the optimisation problem should not be solved for every modulation index because the answer of the optimisation problem obeys a certain pattern. Also, this method has a positive effect on reducing losses, especially in boost-converters. Analysis and simulations are first introduced for three-phase boost-converters, and then is extended to a buck converter. Furthermore, practical verification takes place on two implemented 1 kVA boost and buck-converters in order to confirm the proposed technique as well as theoretical analysis and simulations. The experiments and simulations both verify the suggested optimised modulation technique.

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