Abstract
Circulating current has been an inherent feature of modular multilevel converters (MMC), which results in second-order harmonics on the arms currents. If not properly controlled, the circulating current can affect the lifetime and reliability of a converter by increasing the current loading, loss distribution, and junction temperature of its semiconductor devices. This paper proposes controlled circulating current injection as a means of improving the lifetime and reliability of an MMC. The proposed method involves modifying the reference modulating signals of the converter arms to include the controlled differential voltage as an offset. The junction temperature of the semiconductor devices obtained from an electro-thermal simulation is processed to deduce the lifetime and reliability of the converter. The obtained results are benchmarked against a case where the control method is not incorporated. The incorporation of the proposed control method results in a 68.25% increase in the expected lifetime of the converter and a 3.06% increase on its reliability index. Experimental results of a scaled down laboratory prototype validate the effectiveness of the proposed control approach.
Highlights
Since its introduction in 2003 by Marquardt and Lesnicar [1], the modular multilevel converter (MMC) has been considered as a promising candidate for medium-to-high voltage conversion applications such as high-voltage direct current (HVDC) transmission systems [2,3,4], flexible alternating current transmission system (FACTS) [5], medium-voltage motor drives [6], wind energy conversion systems [7], and large-scale photovoltaic generation systems [8]
The accumulated damage is evaluated as 1.1731 × 1 0–9%, which yields an expected lifetime of 270.3077 years
A controlled circulating current injection method has been proposed as a means of improving the lifetime and reliability of an MMC
Summary
Since its introduction in 2003 by Marquardt and Lesnicar [1], the modular multilevel converter (MMC) has been considered as a promising candidate for medium-to-high voltage conversion applications such as high-voltage direct current (HVDC) transmission systems [2,3,4], flexible alternating current transmission system (FACTS) [5], medium-voltage motor drives [6], wind energy conversion systems [7], and large-scale photovoltaic generation systems [8]. Circulating current control (CCC) is one way to minimize thermal stress and improve the lifetime and reliability of an MMC. This paper proposes a modified closed-loop CCC that was inspired by [15] It is used as a means of improving the lifetime and reliability of a MMC. Miners rule [26], which considers damage as a fraction of the life consumed by stress, is used to model the damage as a percentage of the consumed life based on loading history This leads to the evaluation of the remaining useful lifetime (RUL) of the converter and its reliability index.
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