Autonomous Operation of a Hybrid AC/DC Microgrid with Multiple Interlinking Converters

Abstract

Applying conventional dc-voltage based droop approaches for hybrid ac/dc microgrids interconnected by a single interlinking converter (IC) can properly manage the power flow among ac and dc subgrids. However, due to the effect of line resistances, these approaches may create a circulating power as well as overstressing the ICs in the case of employing multiple ICs for interconnecting the ac and dc subgrids. This paper proposes an autonomous power sharing approach for hybrid microgrids interconnected through multiple ICs by introducing a superimposed frequency in the dc subgrid. Hence, a suitable droop approach is presented to manage the power among the dc and ac sources as well as ICs. The outcomes are proportional power sharing, preventing circulating power and overstressing the ICs as well as acceptable dc voltage regulation. Furthermore, the maximum transferred power by the ICs can be improved by employing the proposed approach.

Existing System

Frequency-Based Current-Sharing Techniques.

Proposed System

In this paper, power sharing control issues in hybrid ac/dc microgrids are discussed. The drawbacks of the conventional voltage droop approach are explained, and a new frequency-droop based strategy is proposed for suitable power sharing in hybrid microgrids. In the proposed approach, the Interlinking Converters (ICs) and the dc sources are coordinated by the frequency of an injected ac ripple in the dc grid. Introducing a global variable in the dc microgrid provides an opportunity to accurately coordination the power of the ICs. Employing this method achieves proper load sharing without being affected by the line resistances. The proposed approach, also, prevents the circulating power among the ac and the dc grids as well as overstressing the converters. Furthermore, the maximum transferred power capability by the ICs can be increased by employing the proposed approach. Moreover, due to the lack of virtual resistors as compared to a voltage droop controller, the dc link voltage can be precisely regulated. The effectiveness of the proposed approach is evaluated by the simulations, and the load sharing performance is compared with that achieved if the conventional voltage-based droop approach is used.

Architecture

Block diagram of a hybrid ac/dc microgrid