Due to the voltage mismatch between the phase legs and the DC bus in Modular Multilevel Converters (MMCs), the differential current in MMCs is inherently subjected to circulating even order harmonics. Repetitive control based active harmonic suppression methods can be adopted to eliminate such harmonics. Nevertheless, conventional repetitive controllers have a relatively slow dynamic response because all the sampled errors in the past one cycle have to be stored, which causes a response delay for one fundamental period. This paper proposes an improved repetitive control scheme that exclusively copes with even order harmonics based on the circulating current characteristics of MMC systems. The design details of the even harmonic repetitive control scheme according to the harmonics characteristics are provided. The proposed even-harmonic repetitive control scheme requires halved data memory to store error samplings and the delay introduced by the repetitive controller is also reduced. According to the frequency domain analysis, the even-harmonic repetitive control features faster convergence rate, greater low-frequency gains, higher crossover frequency, and higher tolerance against system frequency deviation, while possessing the same even-order harmonics suppression capability and stability as conventional ones. Simulation and experimental results are presented to show the steady-state harmonics suppression capability, dynamic response, and disturbance tolerance of the proposed even-harmonic repetitive control scheme.
DC shipboard power systems are gaining popularity primarily because they allow each prime mover to be operated at an optimal speed with respect to fuel efficiency. Another notable advantage is the simplified operation of generators in parallel without concern for frequency synchronization. Careful design of the protection system for dc shipboard power systems is necessary as the continuity of electrical power is required to carry out critical marine missions. In this paper the protection requirements for MVDC shipboard power system with multiple parallel generation system are discussed and the 'Directional Protection' method is presented as an attractive candidate. The backup protection algorithm is also developed for increased redundancy. The protection settings for main and backup protection system are discussed and the results are presented. The various challenges associated with the directional protection are also highlighted.
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