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In recent years, the concept of micro-grid has appeared as an appropriate way for the integration of distributed energy resources (DERs) in the distribution networks. However, micro-grids have encountered a number of challenges from control and protection aspects. One of the main issues relevant to the protection of micro-grids is to develop a suitable protection technique which is effective in both grid-connected and stand-alone operation modes. This study presents a micro-grid protection scheme based on positive-sequence component using phasor measurement units and designed microprocessor-based relays (MBRs) along with a digital communication system. The proposed scheme has the ability to protect radial and looped micro-grids against different types of faults with the capability of single-phase tripping. Furthermore, since the MBRs are capable of updating their pickup values (upstream and downstream equivalent positive-sequence impedances of each line) after the first change in the micro-grid configuration (such as transferring from grid-connected to islanded mode and or disconnection of a line, bus or DER either in grid-connected mode or in islanded mode), they can protect micro-grid lines and buses against subsequent faults. Finally, in order to verify the effectiveness of the suggested scheme and the designed MBR, several simulations have been undertaken using DIgSILENT PowerFactory and MATLAB software packages.

In recent years, renewable energy sources have been considered the most encouraging resources for grid and off-grid power generation. This paper presents an improved current control strategy for a three-phase photovoltaic grid-connected inverter (GCI) under unbalanced and nonlinear load conditions. It is challenging to suppress the harmonic content in the output current below a pre-set value in the GCI. It is also difficult to compensate for unbalanced loads even when the grid is under disruption due to total harmonic distortion (THD) and unbalanced loads. The primary advantage and objective of this method is to effectively compensate for the harmonic current content of the grid current and microgrid without the use of any compensation devices, such as active and passive filters. This method leads to a very low THD in both the GCI currents and the current exchanged with the grid. The control approach is designed to control the active and reactive power and harmonic current compensation, and it also corrects the system unbalance. The proposed control method features the synchronous reference frame (SRF) method. Simulation results are presented to demonstrate the effective performance of the proposed method.

SUMMARY A large amount of renewable energy penetration will cause serious problem in load dispatch in the future power system, where the percentage of controllable generators will decrease, while disturbances will become large, rapid and uncertain. The role of economic dispatch is changing in order to make best use of the ramp‐rate capability of generating units. This paper proposes a concept of feasible operation region (FOR) for dynamic economic dispatch (DED) problem. FOR is defined as the region within that the committed generating units are able to operate while matching the predicted load profile. The computation of FOR can detect possible power imbalance and also can provide accurately the spinning reserves with respect to time. Therefore, the monitoring of FOR with the reserve management on line is quite effective. Then, the paper suggests two step computation method as a new approach for DED. The first step is the computation of FOR for each generator using the most recent real‐time prediction for load as well as renewable energy generation, while preserving adequate reserves. The second step is to determine generation schedule inside FOR, where any conventional real‐time load dispatch method is useful. The method can specify the amount of reserve required to be called into operation to guarantee reliable load dispatch under restricted circumstances in the future. Copyright © 2011 John Wiley & Sons, Ltd.