Current Balancing Control for Interleaved Half-Bridge Submodules in Modular Multilevel Converters

Abstract

This paper presents a closed-loop current balancing control for Modular Multilevel Converters with Interleaved half-bridge Sub-Modules (ISM-MMC). The new control loop solves the well-known problem of proper current balancing among interleaved half-bridge legs in each ISM-MMC submodule while preserving a simple and reliable structure. In addition to that, a novel capacitor voltage balancing strategy for MMCs is developed. The new algorithm contains main advantages of the classical capacitor voltage balancing methods while provides an opportunity to decouple two balancing tasks of ISM-MMC, namely capacitor voltage and interleaved legs current balancing. The proposed control methods feature good dynamic performance and are compliant with a digital processor's operational constraints. The effectiveness of the new balancing methods was verified during extensive numerical simulations and experimental tests on a laboratory prototype by the corresponding system response under the input/output characteristics variation and interleaved current control perturbation.

Interleaved Half-Bridge Submodules with Sensorless Leg Current Balancing in Modular Multilevel Converters

“…This section explains a few modifications of the conventional interleaved current balancing method in ISM-MMC that has been introduced in [13]. Fig.…”

“…In addition, variation of real circuitry parameters is generally limited by manufacturing tolerances, which are in the range of few tens of percent. Therefore, the reconstructed SM voltage has quite a good accuracy with respect to the actual value, and it can be treated as an independent variable in (13).…”

“…Considering definition of the estimated model (capacitor voltage and current) given by ( 12), (13) and subtracting it from the plant's model formulated by ( 5) and ( 6), the observation error dynamics has the following form: where 𝑣 ̃𝑐 = 𝑣 𝑐 − 𝑣 ̂𝑐 and 𝑖̃𝑐 = 𝑖 𝑐 − 𝑖̂𝑐 are observation errors of capacitor voltage and current, respectively. The stability of the designed observer (i.e., the trajectories of estimates do not diverge from their corresponding real values) can be formulated by exhibiting a Lyapunov function that corresponds to the energy in the increment with respect to an arbitrary, nominal state trajectory.…”

“…Once it is deactivated, these currents start getting instantly imbalanced. The reason is that the capacitor voltage balancing control forces switches commutate to satisfy its objective, balance capacitor voltages around their mean [13]. The equivalent resistance of individual HB-legs acts as a damping factor limiting this unbalance.…”

Interleaved Half-Bridge Submodules with Sensorless Leg Current Balancing in Modular Multilevel Converters

“…This approach is well established for dc/dc [10], [11] and some ac/dc converters [12]. A similar method has been proposed in [13] for an ISM-MMC. An obvious drawback of these current balancing methods is the large number of current sensors that is almost proportional to the number of interleaved units.…”