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3-Level Voltage Source Inverters (3L-VSI) have emerged as effective approach to achieve high efficiency and better harmonic distortion performance. As a State-of-Art (SoA), two types of carrier arrangement methods are used for carrier-based pulse width modulation (PWM), namely, phase disposition (PD) and alternate phase opposite disposition (APOD). PD PWM gives better performance in terms of current quality and Total Harmonic Distortion (THD) whereas, APOD PWM is more effective for suppression of Common-Mode Voltage (CMV) with the expense of increased THD. In this paper, a novel Quasi-reference PWM method for 3L-VSIs is presented which results in low CMV as that of the APOD along with the reduced THD. The proposed method can be considered as a distinct trade-off between the two SoA methods, which maintains the merits of both. This paper presents the detail analysis and experimental verification of the proposed PWM technique.

High penetration of distributed generation (DG) has led to frequent voltage fluctuations in the distribution network. This paper describes the design of a partially rated, power-electronic-assisted onload tap-changing (OLTC) autotransformer. Positive and negative compensation of the grid voltage can be achieved on feeders that have high distributed generation and/or loading. A novel design of taps comprised of several no-load switches and a single semiconductor-mechanical hybrid switch have been proposed, that requires reduced voltage rating and a number of switches. In steady state, the mechanical switch in the hybrid switch conducts the load current resulting in low steady-state losses. During the tap change process, the OLTC uses semiconductor switches, namely insulated-gate bipolar transistor /metal-oxide semiconductor field-effect transistor, thus achieving arc-free tap change and long lifetime of switches. The OLTC system has been customized for both low-voltage and medium-voltage three-phase distribution networks. An open-delta configuration for the medium-voltage application has been proposed that requires only two OLTC units to control all three line voltages. Simulations are carried out to verify the steady-state and transient operation of the proposed OLTC.

Matrix converters have no boost capabilities. Their output voltage amplitude is therefore reduced compared to back-to-back inverter topologies. Moreover, they are sensitive to grid voltage fluctuations and disturbances and the voltage transfer ratio is further reduced under such grid perturbations. The present paper analyses the behaviour and limits of the two-stage matrix converters. It proposes and investigates three alternative topologies that modify the DC-link voltage of the two-stage matrix converter, by the means of additional circuitry, in order to increase the voltage transfer ratio above unity and to obtain immunity to grid disturbances. A high-performance control of the grid current is required to obtain a power factor close to unity. An effective control method is proposed for each converter. Feedforward model predictive controllers allow obtaining good performances. The performances of the topologies are finally analysed and compared.

Unbalanced supply voltages can be fully compensated in boost type power converters with active front end if sufficient energy storage is provided by the DC-link capacitor. In converters that have no energy storage, for example in a matrix converter, the maximum voltage transfer ratio will be reduced if one or more input lines is at a reduced voltage. Control techniques to compensate the effect of unbalanced voltage supply for matrix converters allow for a limited output voltage and power capabilities, which depends on the level of unbalance. This paper proposes the utilization of the clamp capacitor, which is normally needed only to protect a direct power converter (DPC), to extend the operating range of a two-stage DPC during unbalanced supply conditions preserving its theoretic maximum voltage transfer ratio capability. This solution is validated through simulations

The Hybrid ac/ac Direct Power Converter (HDPC) has the advantage of improved voltage transfer ratio and higher robustness against supply voltage unbalances over the Conventional Matrix Converters (CMC). The HDPC also has the inherent advantages of the CMC such as: adjustable input power factor, sinusoidal supply currents and no bulky energy storage elements which reduce the life time of the converter. This paper proposed a predictive current controller scheme for a HDPC based on 2-stage matrix converter providing unity voltage transfer ratios; even in the case the supply voltage is highly unbalanced. The proposed predictive current controller is simple in digital implementation, provides higher bandwidth control, requires no additional circuitry and maintains a constant switching frequency which allows synchronizing the switching of the HDPC for sinusoidal supply currents.

Direct power converters (DPC), which has no energy storage, the converter output voltage capability is affected when the input voltage is unbalanced. This paper proposes the utilization of the clamp capacitor, which is normally needed to protect a DPC, to extend the operating range of a two stage DPC during unbalanced supply, preserving its theoretical output voltage capability. A comprehensive simulation study with analytical explanations justifies the proposed method. Experimental results using a laboratory prototype confirms the viability of the proposed active compensation technique