IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 26, NO. 1, MARCH 2011 235 Flexible Operation Strategy for an Isolated PV-Diesel Microgrid Without Energy Storage A. Elmitwally, Member, IEEE, and Mohamed Rashed, Member, IEEE Abstract—In this paper, a control scheme is proposed for a three- phase isolated photovoltaic (PV)-diesel microgrid without energy- storage element. The scheme aims to: track maximum power from the PVA, regulate the load voltage, compensate the load unbalance viewed by the diesel generator, and to control the diesel-engine speed. The first three tasks are achieved by controlling the pulse width modulation inverter interfacing the PV array to the system. The fourth is realized by a modified fuzzy logic controller of the diesel engine. The obstacles encountered on operating the system under certain probable loading conditions are addressed. Two dif- ferent operation strategies are proposed to provide high-quality power under all loading scenarios, and to achieve the targets of the control scheme. The system operation is investigated under a vari- ety of conditions to prove the aptness of the proposed techniques. Index Terms—Control, diesel, fuzzy logic, hybrid systems, oper- ation, photovoltaic (PV), renewable energy. I. INTRODUCTION A PPLICATION of hybrid photovoltaic (PV)-diesel systems has been widely recognized as a reliable, feasible, and environment-friendly solution to supply power to remote loca- tions. Usually, the system is isolated from the utility grid, as it is difficult and often infeasible to extend lines and feeders to these remote areas. The system is typically sized such that the PV array (PVA) size is almost equal to the expected load power. The diesel-driven generator (DDG) supplies the deficit in the PVA output power due to insolation fluctuations. The coordinated operation of the PV-DDG offers a chance to elim- inate the need for energy-storage device to improve the system economics. Also, the PVA is fully utilized via devoted control that guarantees maximum possible output power from the PVA to make maximum fuel savings of the DDG. Such PV-DDG system without energy storage (see Fig. 1) works well if the load is around its normal level or higher. The DDG supplies at least a minimum power share to the system. However, partial failure of load equipment is probable in these systems that will reduce the load power needs. For classic operation strategies, if the available load capacity is less than the maximally tracked PVA output power for certain climatic conditions, there will be excessive power production. This is supplied to the DDG (the direction of power flow is reversed) and the DDG operates as a motor. Under this condition, the speed-control mechanism will Manuscript received September 18, 2007; revised December 17, 2008; accepted February 13, 2009. Date of publication December 3, 2010; date of current version February 18, 2011. Paper no. TEC-00358-2007. The authors are with Electrical Engineering Department, Faculty of Engineering, Mansoura University, Mansoura 35516, Egypt (e-mail: kelmitwally@yahoo.co.uk, mrashed@mans.edu.eg). Digital Object Identifier 10.1109/TEC.2010.2082090 Fig. 1. System block diagram. fail to keep the DDG speed within the accepted limits. The speed increases steeply to a dangerous level much larger than its rated value. The later out-of-control high speed can have two severe impacts. First, it will cause drastically increasing voltage magni- tude and frequency with abnormal and deleterious values. This, in one hand, detracts dramatically the voltage quality, and hence the performance of the load equipment. On the other hand, it may damage the load equipment and/or the interface inverter resulting in unwanted financial loss. Second, it will greatly in- crease the tear and wear in the diesel engine that may initiate mechanical deterioration to the diesel engine. Therefore, the ap- plied control scheme should be flexible enough to accommodate this possible condition of operation. It must assure a secure and high-quality power from the proposed PV-diesel system under all operating conditions and without switching off/on the DDG. This is what is tackled in this paper. In [1], the dynamic interaction between a DDG, a wind tur- bine generator (WTG), and local load is analyzed. The condition of DDG motoring is manifested, and its impact on voltage mag- nitude and frequency is discussed. In [2], the operation of a DDG integrated to a variable speed WTG is analyzed. A fly- wheel power-smoothing medium is introduced to flatten the power output of the DDG. This aids to prevent the deteriora- tion of DDG if it is operated under fluctuating power produc- tion. The complexity of the control and high cost of the treat- ment system are apparent. Many studies reported the operation and control of hybrid systems under normal operating condi- tions, [3]–[8]. In [3], the development of a predictive artificial neural network (ANN)-based controller for the optimum opera- tion of hybrid renewable-energy-based water and power-supply system is presented. The system consists of PVA, DDG, bat- tery bank for energy storage, and a reverse osmosis desalination unit. The ANN controller is designed to take decision on diesel 0885-8969/$26.00 © 2010 IEEE