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The technical, social, economic as well as environmental multi-benefits of renewable energy sources do not come without certain power quality (PQ) challenges, and deterioration of voltage and current quality is foremost among them. Technically, voltage harmonic distortion, current harmonic distortion, voltage and frequency fluctuations and voltage imbalances are the five decisive issues that judge the PQ performance of renewable energy systems. Healthy PQ, in terms of all the aforementioned phenomena, is assured by monitoring and benchmarking their corresponding indices, but performing the same, only at distributed generation (DG) bus level, is not sufficient since also the grid, as well as load bus PQ, plays an important role which makes the overall PQ assessment of renewable energy systems quite complex. This paper presents an analytic hierarchy process-based novel approach for evaluating and benchmarking the PQ performance of grid-integrated renewable energy systems. The procedure results in a single compound power quality index that is alone ample for benchmarking the overall PQ performance of a renewable energy system above the threshold level as well as for ranking among the PQ performances of individual DG systems differentiated by either renewable energy types or sittings in a distribution network. Such a compound index can ease the PQ performance evaluation and benchmarking process up to a maximum extent. Furthermore, it can be very helpful also for distribution system operators to monitor and penalize the DG owners for poorly operating from PQ aspects and motivate the same for adopting best option among different power quality improvement (PQI) technologies. The proposed approach is verified on a grid-integrated PV-based DG system in MATLAB/Simulink. Besides that, the effect of different PQI techniques, on PQ performance, is tested by the application of the proposed approach. At last, the validation of impending effects of high penetration of renewable energy on PQ of the system is also attempted by the proposed approach.

LIST OF SYMBOLS m Number of criteria n Number of alternatives TVHD Total voltage harmonic distortion VSS Voltage sag score Va, Vb, Vc Post sag RMS voltages of phase A, B and C respectively VUF Voltage unbalance factor Vp Positive sequence voltage component Vn Negative sequence voltage component Vab, Vbc, Vca Three-phase imbalanced line voltages Vabe Difference between the line voltage Vab and the average line voltage VPPI Voltage Profile Performance Index A Pairwise comparison matrix ajk General element of pairwise comparison matrix j, k Names of criterion w Vector of criteria weight S Matrix of PQ phenomena scores v Vector of global PQ phenomena scores Fi Vector of individual corresponding indices of PQ phenomena Fthr Vector of threshold values of indices p Number of PQ phenomena considered UPQIi Unified power quality index for the i th bus Wg, Wd, Wl Weights assigned to grid, DG and load bus group respectively Ag, Ad, Al PQ priority scores of grid, DG and load bus group respectively g Total number of utility grid buses d Total number of DG buses l Total number of load buses wi Power quality importance score of the i th bus of the corresponding bus group UPQIg Unified power quality index of grid bus group UPQId Unified power quality index of DG bus group UPQIl Unified power quality index of load bus group UPQIoverall Unified power quality index of the overall system Iph Photocurrent Is Load current Id Diode current Ish Shunt current Fs Solar radiation To Ambient temperature Vs Load voltage P1, P2, P3 Constants Tj Junction temperature e The charge of an electron a The ideality factor of a diode Ns The number of cells connected in series K Boltzmann's constant Rs The value of series resistance Eg Energy bandgap

In recent years, multilevel inverters (MLIs) have emerged to be the most empowered power transformation technology for numerous operations such as renewable energy resources (RERs), flexible AC transmission systems (FACTS), electric motor drives, etc. MLI has gained popularity in medium- to high-power operations because of numerous merits such as minimum harmonic contents, less dissipation of power from power electronic switches, and less electromagnetic interference (EMI) at the receiving end. The MLI possesses many essential advantages in comparison to a conventional two-level inverter, such as voltage profile enhancement, increased efficiency of the overall system, the capability of high-quality output generation with the reduced switching frequency, decreased total harmonic distortions (THD) without reducing the power of the inverter and use of very low ratings of the device. Although classical MLIs find their use in various vital key areas, newer MLI configurations have an expanding concern to the limited count of power electronic devices, gate drivers, and isolated DC sources. In this review article, an attempt has been made to focus on various aspects of MLIs such as different configurations, modulation techniques, the concept of new reduced switch count MLI topologies, applications regarding interface with renewable energy, motor drives, and FACTS controller. Further, deep insights for future prospective towards hassle-free addition of MLI technology towards more enhanced application for various fields of the power system have also been discussed. This article is believed to be extremely helpful for academics, researchers, and industrialists working in the direction of MLI technology.

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