Low-inertia power generation units make nanogrids vulnerable to the voltage and power fluctuations caused by pulse loads and abnormal grid conditions. The conversion of critical loads to smart loads is a potential solution for improving the stability and power quality in nanogrids. This paper investigates the effects of utilizing smart loads on the performance of nanogrids. A smart load can compensate for sudden deviations between supply and demand and therefore can mitigate voltage and power oscillations in lowinertia nanogrids. The conversion of critical loads to smart loads can reduce the stress on the energy storage units and minimize the required battery banks in nanogrids. In this paper, several case studies are considered to verify the stability and power quality improvement of nanogrids when some loads are converted to smart loads. INDEX TERMS Central battery bank, critical loads, grid of nanogrids (GNG), small-signal stability, smart city, smart loads.

Phase-shift control is a common approach to voltage control of traditional H-bridge inverters, but this method has not been used to voltage control of H-bridge Z-source inverter (ZSI) so far. This study proposes a new phase-shift control method for H-bridge ZSI, which its main difference with phase-shift control of traditional H-bridge inverters is existence of four shoot-through states (short circuit of the H-bridge) in each switching cycle. One of the advantages of the proposed method is simultaneous elimination of several harmonics by choosing the appropriate switching angles. Moreover, by using this phase-shift control method, the amplitude of low-order harmonics for many values of switching angles is less than the amplitude of low-order harmonics in traditional H-bridge voltage source inverter (VSI). Total harmonic distortion and distortion factor, as two important indexes for quantifying the merits of the proposed method, are calculated and compared with their values in traditional H-bridge VSI. A detailed description of suggested method and comparison with the traditional method are presented in this study. Simulation and experimental results are also given to demonstrate the new features of the proposed method.

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Multilevel inverters have recently been received more attention in the power industry. Two traditional types of multilevel inverters are multilevel voltage-source inverter and multilevel current-source inverter. Despite this widespread use, multilevel Z-source inverter (ML-ZSI) has not been regarded enough, because a unique approach to voltage control of ML-ZSI has not yet been presented. Besides, the phase voltage and its total harmonic distortion (THD) have not been calculated for ML-ZSI so far. This study proposes two new methods for the formulation of phase voltage in ML-ZSI. The first method is based on Fourier series analysis of the phase-voltage waveform, and the second utilises the staircase waveform of the phase voltage. In addition, two new methods are introduced for computing the phasevoltage THD in ML-ZSI: approximate method and waveform-integration method. Although the waveform-integration method leads to the accurate answer, using this method is more difficult than an approximate method. The detailed description of suggested methods and comparison with each other are presented in this study. Simulation and experimental results are also given to demonstrate the new features of the proposed methods.

This paper presents a piecewise linear-elliptic (PLE) droop control scheme to improve the dynamic behavior of islanded microgrids. Islanded microgrids are typically vulnerable to voltage and frequency fluctuations, particularly if a combination of high- and low-inertia power generation units are used in a microgrid. The intermittent nature of renewable energy sources can cause sudden power mismatches, and thus, voltage and frequency fluctuations. The proposed PLE droop control scheme can be employed in a battery energy storage system (BESS) to effectively mitigate voltage and frequency fluctuations in an islanded microgrid. Though the PLE shape can be implemented for any droop control scheme, it has been applied for active power-frequency (P-f) and reactive power-voltage (Q-v) droops in this paper. In addition, the dynamic response of a battery-fed smart inverter equipped with the proposed PLE droops has been compared with the results obtained from a linear droop control scheme in an islanded microgrid containing high- and low-inertia power-generation units. In this paper, the results of several case studies are presented to confirm the capability of the PLE droop control in mitigating voltage and frequency fluctuations in islanded microgrids.