Abstract
Due to the demand for high reliability, modular multilevel converters (MMCs) are designed with redundant submodules. Redundant submodules can be integrated into the converter by employing different redundancy schemes: the conventional active scheme, the load-sharing active scheme, and the passive scheme. Different schemes have different impacts on the improvement of converter reliability. The contributions of this paper include that an analytical method is proposed to evaluate the reliability of MMCs under different redundancy schemes and the factors’ influence on the converter reliability is analyzed to determine the proper redundancy scheme. Reliability models of MMCs under different redundancy schemes are built using Markov chains and the iteration method. Based on the proposed models, the effects of redundant schemes are evaluated in terms of the converter reliability. A case study is conducted to validate the feasibility and robustness of proposed models and to specify the conditions in the favor of each redundancy scheme. The benefits of sharing redundancy among arms are also explored from the reliability point of view. If insulated-gate bipolar transistors (IGBTs) and capacitors are dominant components in a submodule in terms of failure rates, the load-sharing active scheme performs better; otherwise, setting the redundant submodules in an idle state is more effective. It is also found that the number of required redundant submodules is greatly reduced by sharing redundancy among arms.
Highlights
M ODULAR multilevel converter (MMC) shows attractive features in operational power losses, industrial scal-Manuscript received December 9, 2016; revised May 12, 2017; accepted June 7, 2017
MMCs in the High Voltage Direct Current (HVDC) applications are usually comprised of hundreds of SMs
Pn∗−k+1 (t) = kλs Pn∗−k (τ )dτ where Pj∗(t) is the probability of the arm being in state j when passive redundancy scheme is applied; λs is the failure rate of operating SMs (in (4)); λsd is the failure rate of redundant SMs (in (7)); k is the minimum number of SMs in each arm; n is the number of assembled SMs in each arm
Summary
M ODULAR multilevel converter (MMC) shows attractive features in operational power losses, industrial scal-. MMCs in the HVDC applications are usually comprised of hundreds of SMs. existing methods can not be applied to the redundancy analysis of MMCs. To evaluate the effects of different redundant schemes on the converter reliability, detailed mathematical models suitable for the redundancy analysis of large scale systems are proposed, which is the main contribution of this paper. Between the two configurations, the sharing redundancy between arms in the same phase leg is more possible to be used in industry as it does not increase the circuit complexity too much Note that this could trigger new circuit design and patents which are worthy of further investigation, this paper only analyses the reliability benefits and presents a potential trend of research topic
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