Abstract
Low level modular multilevel converter (MMC) is a promising candidate for medium voltage applications such as MVDC (medium voltage DC current) transmission and megawatt machine drives. Unlike high-level MMC using nearest level modulation (NLM), the low-level MMC using the pulse width modulation (PWM) or NLM + PWM is affected by a common mode voltage (CMV) due to a frequent change of a switching state. This CMV causes electromagnetic interference (EMI) noise, common mode current (CMC) and bearing current leading to a reduction in the efficiency and durability of the motor drive system. Therefore, this paper provides a mathematical analysis on how the switching state affects the CMV and proposes three software based CMV reduction algorithms for the low level MMC system. To reflect the characteristic of MMC modulation strategy for upper and lower reference voltage independently, two separate space vectors are used. Based on the analysis, three different CMV reduction algorithms (complete CMV reduction (CCR), DPWM CMV reduction (DCR) and partial CMV reduction (PCR)) are proposed using NLC + PWM modulation strategy. The performance of the proposed CMV reduction algorithms was verified by both simulation and experimental result.
Highlights
Modular multilevel converter (MMC) is an attractive and advanced multilevel converter topology in the electric power industry
This paper proposes a common mode voltage (CMV) reduction algorithm to a half-bridge (HB) three-phase MMC system that is configured as double star chopper-cells (DSCCs) of [33,34]
In this paper, the common mode voltage analysis based on two separate space vectors was proposed
Summary
Modular multilevel converter (MMC) is an attractive and advanced multilevel converter topology in the electric power industry. The NLM + PWM method can be implemented through a space vector [23] As these modulation techniques make a frequent change of switching states, the CMV becomes an undesirable and critical issue in the medium voltage applications. The paper in [31] deals with the CMV reduction for the flyingcapacitor modular multilevel converter based motor drive system This algorithm successfully eliminates the CMV, it causes a 10% increase in the switching frequency, which will lead to a higher switching loss. The paper in [30] adopts the same CMV reduction algorithm used in [31] and applies to the MMC system connected with an unsymmetrical grid Another CMV elimination scheme has been proposed for an open-end stator winding induction motor [32].
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