Photovoltaic power plants (PVPPs) typically operate by tracking the maximum power point in order to maximize conversion efficiency. However, with the continuous increase of installed grid-connected PVPPs, power system operators have been experiencing new challenges, like overloading, overvoltages and operation during grid voltage disturbances. Consequently, constant power generation (CPG) is imposed by grid codes. An algorithm for the calculation of the photovoltaic panel voltage reference, which generates a constant power from the PVPP, is introduced in this paper. The key novelty of the proposed algorithm is its applicability for both single-and two-stage PVPPs and flexibility to move the operation point to the right-or left-side of the maximum power point. Furthermore, the execution frequency of the algorithm and voltage increments between consecutive operating points are modified based on a hysteresis band controller in order to obtain fast dynamic response under transients and low power oscillation during steady-state operation. The performance of the proposed algorithm for both single-and two-stage PVPPs is examined on a 50-kVA simulation setup of these topologies. Moreover, experimental results on a 1-kVA PV system validate the effectiveness of the proposed algorithm under various operating conditions, demonstrating functionalities of the proposed CPG algorithm. Index TermsPhotovoltaic systems, single-and two-stage photovoltaic power conversion, constant power generation, photovoltaic panel power-voltage curve, voltage reference calculation I. INTRODUCTIONCurrent-voltage characteristics and output power of photovoltaic (PV) strings vary with changes of solar irradiance, temperature and aging. Accordingly, maximum power point tracking (MPPT) techniques are applied in most of applications in order to maximize the extracted power from a given PV system and increase the overall power conversion efficiency [1]. Several MPPT algorithms, varying in approach and complexity, have been introduced in the literature [2]- [10]. Each method has various advantages and disadvantages in different aspects like computational efficiency, speed of tracking the maximum power point, operation under partial shading and power oscillations during

Multilevel cascade H-bridge (CHB) converters are one of the promising solutions for medium-and largescale grid-connected photovoltaic power plants. However, there is a lack of a complete study about their operation during voltage sags. This paper proposes a flexible control strategy for the operation of photovoltaic grid-connected CHB inverters during unbalanced voltage sags. The key novelty is that the proposed strategy is able to inject both active and reactive powers to the grid with either balanced or unbalanced currents, while ensuring that all dc-link capacitor voltages remain balanced. The simulation and experimental evaluations of a 9-kVA grid-connected sevenlevel CHB illustrate and validate the performance of the proposed strategy for the operation of the grid-connected CHB converter during different unbalanced voltage sags.

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Competition in the aircraft industry market and global warming has driven the industry to think along economic and environmental lines. This has resulted in the emergence of more electric aircraft (MEA). The increase in the power demand of aircraft, especially in the last two decades, coupled with advancement in battery materials and technology has led to the development of many high energy density batteries. This study presents an overview of the battery systems for MEA. In this paper, a study on the battery technologies used in aircraft in the last five decades is being done. A general background of the battery system is presented and the performance of the batteries based on energy densities and low temperature capabilities are evaluated and discussed. Evolution of MEA with its power system architecture and load profile is presented to understand the requirements of the battery system. Weight saving and cost analysis is done for the Li-ion and Ni-Cd batteries with respect to the requirement of an MEA 'Aircraft X'. Battery management system (BMS) for Li-ion batteries is also explored and discussed. Based on the analysis, Li-ion battery is selected and integrated with the power distribution DC network for future MEA.

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