Modelling of DC-DC converters based on front-to-front connected MMC for small signal studies

Abstract

Abstract-The Multi-terminal DC grid is expected to be built incrementally by interconnecting existing HVDC installations. In doing so, an enabling component is the DC-DC converter which also plays the role of a power flow controller. A number of DC-DC converter topologies, targeting different applications, have been proposed in literature. However, detailed simulation models for system level studies are not developed, yet. This paper will focus on reduced order modelling and control of the Front-to-Front MMC based DC-DC converter for system level studies. The developed model is validated against a full detail average model. A simple approach to investigate poorly damped oscillations is also proposed. The approach is then used to investigate a poorly damped mode in the leg energy state that was excited by a transient in ac power.

Simplified Modelling of the F2F MMC-Based High Power DC-DC Converter Including the Effect of Circulating Current Dynamics

“…n c and n s are the common-mode (dc) and Fig. 1: Per-phase equivalent circuit of the F2F [14] differential (ac) insertion indexes, respectively. The currents are also shown in (1), where i c and i s are the commonmode (circulating) and differential currents, respectively.…”

“…As a starting point, all the ripple components in the arm voltages and the circulating current can be neglected. This model can accurately represent the converter dynamics if compensated modulation is used, as demonstrated in [14]. The resulting dynamic model has only 6 states: 2 × v Σ 0 , 2 × i c0 , and i s .…”

“…However, because of its symmetry, the F2F lends itself to further simplification. This fact was utilized in [14] to develop a simple model of the converter with a single capacitor representing the arm energy dynamics, thus reducing the number of states from 18 to 6. However, as will be shown in this paper, the model is applicable only when using compensated modulation, a special method for calculating the insertion indexes [6].…”

“…When using other types of control, such as direct voltage control, there is an uncontrolled interaction between the different harmonic components in the converter, which leads to a poorly damped oscillation at the converter terminals [15]. The existing simplified model of the F2F from [14] fails to accurately capture such modes of oscillation. Additionally, the model neglects the effect of circulating current ripple and the associated suppression controllers.…”