| Worldwide, driven by economic technical and environmental reasons, there is an increasing interest in intelligent and flexible microgrid. Microgird is a key point to integrate Distributed Generation into the grid. Then, the energy can be generated and stored near the consumptions points, at the same time, the reliability can be increased and losses produced by the power lines will be reduced.Microgrid is a low voltage system since the generation is based on small generation machines and small prime movers which is requires for power electronics equipments. These power electronics equipments work very fast, but they do not have inherent inertia which ensures the stability of the system. Then the droop control is used to achieve this performance. Microgrid needs to control the frequency and the amplitude of the inverter output voltage, which is proportional to the active power and reactive power respectively. However, this control strategy may be changed when the line impedance is highly resistive which is true for low-voltage microgrid. Thus, the opposite droop control is adapted in this paper.In this paper, both the conventional droop control and opposite droop control is studied. The line impedance influences the control strategy of DGs in a large extent. The conventional droop control has a good performance when the line impedance is mainly inductive, while the opposite droop control is more suitable when the line impedance is mainly resistive. This paper will pay more attention to the opposite droop control strategy. And to achieve good power sharing and reducing current circulation, virtual resistor is adapted in the control loop. However, this virtual resistor brings serious voltage deviation. Therefore, to deal with this problem, two methods are proposed in this paper. The voltage can be restored through increasing the voltage reference or adding voltage control loop around the droop control. The simulations and experiments show the effectiveness of the both methods. |
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