In this paper, we propose a leader-distributed follower-decentralized (LDFD) control strategy for cascaded-parallel microgrids (CPMGs) in the islanded mode, under which the economic dispatch (ED) is achieved with voltage quality guaranteed and frequency synchronization. In this control strategy, a distributed control method is presented for the leader of distributed generators (DGs) and an improved droop control method is designed for other DGs (followers) to optimize the generation cost and frequency control of the system. Furthermore, a unified voltage regulator is proposed to make the power factor of each DG in the same string equal. As only the leader in each string communicates with its neighboring leaders with low data transfer rate, very sparse low bandwidth communication networks are needed in this scheme, which reduces costs and increases the reliability in the event of a communication failure. In addition, the small-signal stability of the system is analyzed and the design of the related parameters is given. Some cases are designed to evaluate the performance of the strategy, and the simulation results verify the effectiveness of the proposed method.
K E Y W O R D Scascaded-parallel microgrid, distributed control, economic dispatch, low bandwidth communication.
| INTRODUCTIONMICROGRID is an effective way of integrating distributed generators (DGs), energy storage units via inverters, and local loads to achieve active distribution networks, 1 which can reduce the impact of large penetration of intermittent renewable energy integrated into the electrical grid, making the power system more reliable, safe, clean, and economical. In recent years, the growing penetration of DGs based on power electronic converters has brought structural changes 2 to microgrids to accommodate a wider range of applications. Combining the ideas of parallel-type 3 and series-List of abbreviations and symbols: LDFD, leader-distributed follower-decentralized; CPMG, cascaded-parallel microgrid; ED, economic dispatch; DG, distributed generator; EDP, economical dispatch problem; PCC, point of common coupling; ICR, incremental cost rate; DG#i.j, the j-th DG in String#i; V P , voltage amplitude of the common bus; δ p , power angle of the common bus; Y 0
L, amplitude of equivalent load admittance; φ L , angle of equivalent load admittance; V i.j , voltage amplitude of DG#i.j; δ i.j , phase angle of DG#i.j; λ i.j , incremental cost rate of DG#i.j; P i.j , output active power of DG#i.j; V G , set of nodes; A G , associated adjacency matrix; D in G , diagonal in-degree matrix; ω * , rated angular frequency; V * , rated voltage amplitude of common bus; Q i.j , output reactive power of DG#i.j; λ i:1 , estimate of the average ICR among leaders;P i:j , estimate of total active power of DGs in String#i; I l , identity matrix; H, ICR estimator transfer function matrix; A, system matrix.