163 International Journal for Modern Trends in Science and Technology Circulating Current Fault-Tolerant Operation using MMC & PWM Compensation K.Navatha 1 | N. Ravi Kumar 2 1 Associate Professor, Department of EEE, SVR Engineering College, Nandyal, Andhra Pradesh, India 2 PG Scholar, Department of ECE, SVR Engineering College, Nandyal, Andhra Pradesh, India To Cite this Article K. Navatha, N.Ravi Kumar, DzCirculating Current Fault-Tolerant Operation using MMC & PWM Compensationdz, International Journal for Modern Trends in Science and Technology, Vol. 02, Issue 11, 2016, pp. 163-169. The modular multilevel converter (MMC) is an emerging and highly attractive multilevel converter topology for high-voltage and high-power applications. This paper proposes the control method of parallel-connected modular multilevel converters (parallel-MMCs), which assumes that the multiple MMCs are directly connected at both ac and dc sides to effectively enhance the power rating as expected. Two key problems were first solved for the parallel-MMCs under the normal operation conditions: voltage balancing of sub modules and mitigation of circulating currents, where the novel transformed third-order harmonic resonant controller in the synchronous reference frame was employed to mitigate the dominant second-order and fourth-order circulating currents and a sixth-order harmonic resonant controller is used to attenuate the zero-sequence sixth-order circulating current existed in all phase currents per MMC. Considering the high risk of switches fault in the parallel-MMCs, the fault-tolerant operation schemes were then proposed in this paper to address the major concerns of open-circuit and short-circuit switch fault in a sub module, respectively. Carefully controlling the healthy sub modules and the corresponding phase arms, the parallel-MMCs can successfully maintain their balanced capacitor voltages and mitigate the circulating currents with the qualified output waveform obtained. In addition, the parallel configuration of MMCs provides the unique solution for the short-circuit switch fault operation which was seldom discussed in the published literature works with respect to the MMC fault-tolerant operation schemes. MATLAB simulations and the constructed experimental prototype have verified the performance of the proposed control strategy. KEYWORDS: Circulating current, fault-tolerant operation, modular multilevel converter (MMC), parallel operation, pulse width modulation (PWM) compensation. Copyright © 2016 International Journal for Modern Trends in Science and Technology All rights reserved. I. INTRODUCTION The modular multilevel converter (MMC), which has been originally presented in [1] and [2], is suitable for the applications of high-voltage dc (HVDC) power transmission [3]–[5], adjustable speed motor drives [6], reactive power compensation [7], etc., mainly due to its inherent advantages of the modular structure and the low output voltage/current harmonics. Inside the MMC, every half-bridge converter is considered as a sub module (SM) as shown in Fig. 1, which can be simply cascaded to increase the dc-link voltage to a desired value. However, as the power increases or in other words when the MMC has to handle the high current, a single MMC is increasingly viewed as inappropriate, restricted mainly by the present semiconductor manufacturing technology without any immediate solution. One intuitive solution is to assume the compact integrated parallel-connected ABSTRACT International Journal for Modern Trends in Science and Technology Volume: 02, Issue No: 11, November 2016 ISSN: 2455-3778 http://www.ijmtst.com