Decoupled steady-state model of the modular multilevel converter with half-bridge cells

Abstract

Modular multilevel converters, based on cascading of halfbridge converter cells, can combine low switching frequency with low harmonic interference. They can be designed for high operating voltages without direct series connection of semiconductor elements. This has led to a rapid adoption within high-power applications such as HVDC, STATCOM and railway interties. Analysing the operation of these converters in the frequency domain poses a few challenges due to the presence of significant low-order harmonic voltages in the cell capacitors. This paper presents a frequency-domain model of the MMC converter with halfbridge cells, based on a two-stage approach. First, the circuit equations are decoupled by a simple linear transformation, whereby the circuit schematic can be separated into a dc-side and an ac-side part. Second, the switching operation within the phase arms is modelled in the frequency domain by iterated convolution. The model is verified against a timedomain simulation of a converter with ratings valid for HVDC applications. It is shown that the proposed methodology, where all calculations are made in the frequency domain, can accurately reproduce the results from the simulation.

Control design of Modular Multilevel Converters in normal and AC fault conditions for HVDC grids

“…The target current is chosen to achieve the desired exchange of power between the AC grid and the DC grid as well as maintaining the internal energy balance of the converter. To represent the converter [37], the circuit equations can be obtained per each phase j (j = a, b, c)…”

Low-voltage fault ride through of the modular multilevel converter in a battery energy storage system connected directly to the medium voltage grid

“…The goal of the steady-state model is to calculate the a-priori unknown arm voltages v 1u , v 1l , v 2u , v 2l , v 3u and v 3l , by which the MMC can be controlled. These voltages are typically split into the parts v 1,line , v 2,line and v 3,line that only have an influence on the line-currents, and the parts v 1,circ , v 2,circ and v 3,circ , that only influence the circulating currents [8]:…”

Single-Stage Control System of I-MMC-Based Island MVDC Link Receiver With Multiple Modulation Freedoms

“…The circulating currents distort the sinusoidal arm currents and increase the RMS value and converter losses. The dynamic behavior of the circulating current and the MVDC current is usually simultaneously governed by a mathematical equation, which is obtained by adding the two arm current equations [30], [31]. Thus, the circulating currents are controlled defining them as a part of the input current in each arm.…”