DC link capacity enhancement for dynamic operation of back-to-back modular multilevel converters

Abstract

With the increasing appearance of DC loads and DC sources in the MVAC distribution grids, the need for DC power systems is getting more and more pronounced. As a result, the MMC has become a promising and innovative technology that can serve as the interface between a DC and AC network. The MMC-based MVDC distribution link is one of the solutions to incorporate DC in power grids. This link consists of two back-to-back configured MMCs connecting two possibly asynchronous AC networks. Generally, an MVDC distribution link is operated at a fixed rated DC voltage set through the MMC. This work explores the possibility of enhancing the DC link voltage beyond the nominal value to improve the power transfer capacity of the distribution link. The enhancement is achieved while preserving the average energy stored in the MMC and maintaining the AC-side harmonic performance.
First, a control strategy was developed that qualified for the static operation of back-to-back configured MMCs. This control strategy was designed using multiple sub-control modules, each regulating one or multiple circuit quantities using a closed-loop PI or PR control strategy. Second, the analytical limitation to the DC link voltage enhancement was determined. This provided the enhancement boundary expression, which revealed the dependence between the enhancement limit and the grid-injected reactive power. Third, the MMC control strategy was adapted to dynamically incorporate the DC link voltage enhancement. This was performed using the open-loop DC link voltage control, which dynamically enhanced the link voltage by using the reactive power input reference. The controller is realised using a finite state machine that provides an event-driven control system with conditional state transitions. Simulations verified the performance of this controller. Finally, the dynamic DC link voltage controller is implemented for a lab-scale MMC and optimally tuned using the MO tuning method. This is to provide experimental verification of the DC link voltage enhancement. Three experiments were performed to: demonstrate the workings of the enhancement concept, verify the analytical enhancement boundary expression, and show the performance of the dynamic DC link voltage controller. The experiments concluded that the open-loop dynamic control successfully achieved the DC link voltage enhancement and limited the grid injection of harmonic elements during system transients.
So it is concluded that the power transfer capacity of an MMC-based MVDC distribution link can be enhanced by applying dynamic DC link voltage control. This while operating with the same submodule stresses and preserving the AC-side harmonic performance. When the DC current of the distribution link is fixed at the rated value, the power transfer capacity of the link increases linearly with the controller enhancement function. For system transients, the dynamic controller uses smart control logic to avoid harmonic element injection. Combined, this concludes that the dynamic DC link voltage control achieves a proper DC link capacity enhancement that meets the requirements for both steady state and dynamic conditions.