Abstract
The arm inductors of a modular multilevel converter (MMC) system are the necessity for the operation of the system, and the two kinds of arm inductors exist: separated inductor and coupled inductor (CI). The impact of series connection CIs on the MMC has not been fully explored. This paper provides an accurate estimation of the circulating currents of the CIs based MMC with a variety of two-stage configurations by using the combinations of positive- and negative-coupled inductors. In addition to this, a decoupling equivalent method is developed and equivalent circuits of each CI configuration are proposed. The influences of two-stage CIs on the harmonic components in circulating currents are discussed with emphasis on the even-order harmonic through the proposed estimation methodology. Simulation results are presented to demonstrate the effectiveness and accuracy of the proposed estimation as well as decoupling equivalent method. Finally, the correctness of theoretical analysis is validated with experiments.
Highlights
As a new generation of DC transmission technology, the high voltage DC transmission (HVDC) has recently undergone rapid growth on the global scale [1]–[4]
SIMULATION RESULTS In order to explore the effect of different kinds of connection on the circulating current, a three-phase modular multilevel converter (MMC) with coupled inductor (CI) is built in the MATLAB/Simulink environment
Assessment of impacts of the total four kinds of two-stage connections on the circulating current can be implemented by performance observation of the MMC with a single equivalent dotted terminal (DT) or non-dotted terminal (NT) connected CI
Summary
As a new generation of DC transmission technology, the high voltage DC transmission (HVDC) has recently undergone rapid growth on the global scale [1]–[4]. The effect of the coupled arm inductor on the transient response, the capacitor voltage ripple, the circulating current, the DC short-circuit current and the switch power losses of the MMC has been examined in [33]. According to the equivalent circuit of MMC, each harmonic of circulating currents equals to the corresponding harmonic of SM output voltage in one phase divided by the reactance of arm inductor as shown in (20). B. DECOUPLING EQUIVALENT METHOD FOR TWO-STAGE CI CONNECTIONS IN MMCS As stated above, the unique character of the CIs determines that replacing separated inductors with CIs will have an effect on circulating currents. The NT connection can effectively supress the circulating current, whule the DT connection will amplify the circulating current
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