Steady-State Analysis of Interaction Between Harmonic Components of Arm and Line Quantities of Modular Multilevel Converters

Abstract

The fundamental frequency component in the arm currents of a modular multilevel converter is a necessity for the operation of the converter, as is the connection and bypassing of the submodules. Inevitably, this will cause alternating components in the capacitor voltages. This paper investigates how the arm currents and capacitor voltages interact when the submodules are connected and bypassed in a sinusoidal manner. Equations that describe the circulating current that is caused by the variations in the total inserted voltage are derived. Resonant frequencies are identified and the resonant behaviour is verified by experimental results. It is also found that the effective values of the arm resistance and submodule capacitances can be extracted from the measurements by least square fitting of the analytical expressions to the measured values. Finally, the analytical expression for the arm currents is verified by experimental results.

Selected aspects of Modular Multilevel Converter operation

“…It means that the assumption v armU + v armL = V dc is no longer valid, and when modulating signals n U and n L are given as (13), the sums of capacitor voltages deviate from their rated values. In other words -the capacitor voltage ripple always exists in the MMC, as per (3).…”

“…The ac component of the difference current, i diff,ac , is minimized (ideally down to zero) by the open loop control method [3,7] -as opposed to the MMC operating under the direct modulation method governed by (13), where the i diff,ac can be large.…”

“…(1)- (13). The peak-to-peak value of the capacitor voltage, ∆V C is treated as a design criterion in this paper.…”

“…Arm currents i armU and i armL in the MMC operating under the direct modulation control consist of circulating current i cc [13] …”

A Hardware-in-the-loop Platform for Modular Multilevel Converter Simulations

“…MMC-HVDC has a lot more advantages than two-level or three-level VSC-HVDCs, such as lower harmonic components at the Alternating Current (AC) side of the converter [4], greater reduction of the switching frequency, fewer losses [5] and operation consistency for the switching devices in series in an arm is not necessary [6]. There has been extensive research on MMC systems, and most of this research has been concentrated on mathematical modeling [7]- [9], modulation mode [10]- [12], and control strategies [13]- [16]. Physical verification plays an important role in testing control devices, planning the primary system and engineering operation, since it is closer to the practical project.…”