Wireless Control of Modular Multilevel Converter Submodules With Communication Errors

Abstract

Wireless control of modular multilevel converter (MMC) submodules can benefit from different points of view, such as lower converter cost and shorter installation time. In return for the advantages, the stochastic performance of wireless communication networks necessitates an advanced converter control system immune to the losses and delays of the wirelessly transmitted data. This paper proposes an advancement to the distributed control of MMCs to utilize in wireless submodule control. Using the proposed method, the operation of the MMC continues smoothly and uninterruptedly during wireless communication errors. The previously proposed submodule wireless control concept relies on implementing the modulation and individual submodule-capacitor-voltage control in the submodules using the insertion indices transmitted from a central controller. This paper takes the concept as a basis and introduces to synthesize the indices autonomously in the submodules during the communication errors. This new approach allows the MMC continue its operation when one, some, or all submodules suffer from communication errors for a limited time. The proposal is validated experimentally on a laboratory-scale MMC.

Wireless control of modular multilevel converter autonomous submodules

“…The proposed control method is benchmarked against the existing control methods in [2] and [7]. The simulated circuit diagram is shown in Fig.…”

“…They might end up in steep insertion index changes, which can lead to overcurrents in the arm and ac-side currents and over-voltage in the submodule capacitors when the MMC is connected to an active load. The transition from M1 to M2 is smooth as the same insertion index pattern is continued within the packet loss interval [7]. The transition from M2 to M3 is an open-loop to closed-loop control change.…”

“…Mode 2 (M2): If the submodule experiences two-cycle or longer packet losses (loss train), the submodule extrapolates the previously received insertion index and continues modulation unless the loss train is longer than a predefined safe-operation interval or the measured/estimated local variables, e.g., submodule capacitor voltage, exceed the safety limits. This mode of operation corresponds to the control method proposed in [7].…”

“…Wireless data packet losses up to a few consecutive control cycles are treated by decreasing the closed-loop system control bandwidth. In [7], submodule controllers are proposed to have a set of resonant filters to locally extrapolate the previously received insertion indices in case of experiencing wireless packet loss trains. The extrapolated indices follow the pre-packet-loss pattern.…”

“…In this sense, a sort of open-loop control is provided during the loss period. This control method performs satisfactorily for a limited interval, possibly up to hundreds of control cycles [7]. However, in the case of loss trains with extreme length, i.e., more than hundreds of control cycles, there are two possible problems if this open-loop control continues.…”