Cascaded modular flying capacitor converter using wavelet-energy operator synchronisation for unbalanced grid voltages

Conventional two-level converters synthesize an output by connecting two switches to the VDC rail at a high switching frequency in order to produce an acceptable output. The associated voltage stress and switching loss per device, makes them unsuitable for medium to high voltage applications (FACTs and interfacing renewable sources).
This paper presents a cascaded modular multilevel (CMMC) converter structure, specifically the cascaded floating capacitor modular multilevel converter (CFC-MMC) which has the ability to operate at a lower device switching frequency and is easily extended to higher voltage levels. The basic unit in this structure is a module, which consists of a 5-level full bridge FC [1]. Depending on power rating (or number of voltage levels) required, modules are cascaded (series-connected) to form a single phase or three single phases are connected to form a three-phase structure. Fig. 1(a), shows the output from a simulated 6-module (i.e 2 modules-per-phase) configuration with nine voltage levels. Also, the structure of the CFC-MMC module presents other redundant topology configurations as it can be reconfigured to the hybrid FC of conventional h-bridge and this increases the reliability of the converter. A wavelet-energy operator synchronisation technique is also introduced with the ability to handle each phase of the converter as a single phase which is important when dealing with amplitude based grid unbalances.
Wavelet transforms are shown to have good phase tracking property [2] in power signal processing. An online lifting wavelet transform is used to extract unwanted voltage harmonics in the system and energy operator [3] to extract the positive sequence and estimate the amplitude and the phase angle used for synchronisation to the CFC-MMC structure. The energy operator overcomes the issue (ability to on-line measure the changes in amplitude) seen by both basic and conventional synchronisation methods under abnormal grid conditions. One unique characteristic is that the synchronisation signals of each phase of the grid voltage can be generated through this scheme. The wavelet transform enhances this scheme allowing synchronisation with unbalanced voltages and harmonics (Fig. 1b).