Control of a hybrid modular multilevel converter during grid voltage unbalance

Abstract

The recently proposed parallel hybrid modular multilevel converter is considered to be a low loss, low component count converter with soft switching capability of the 'main' bridge. The converter has similar advantages to other emerging modular multilevel converter circuits being considered for HVDC power transmission. However, during ac network unbalance the individual 'chain-links' exchange unequal amounts of power with the grid which requires appropriate remedial action. This paper presents research into the performance of the converter and proposes a suitable control method that enables the converter to operate during grid voltage unbalance. The proposed control concept involves the use of asymmetric third harmonic voltage generation in the 'chain-links' of the converter to redistribute the power exchanged between the individual 'chain-links' and the grid. Mathematical analysis and simulation modelling with results are presented to support the work described.

Impact of Grid Unbalances on Electric Vehicle Chargers

“…However, control strategies can be used to address these drawbacks and enable the use of certain topologies in designing fault-tolerant chargers. For instance, a solution for the problem of unequal power exchange between the grid and a modular multilevel converter (MMC) chain-links during voltage unbalance is suggested in [46]. The proposed solution involves injecting triple harmonics current into the chain links in proportion to the degree of voltage unbalance.…”

Research and application on multi‐terminal and DC grids based on VSC‐HVDC technology in China

“…The advantages of a DC grid are that it increases system flexibility and reliability, and provides redundancy by sharing resources that result in lower power losses and interference [5]. Therefore, DC grids are considered to be the most effective and promising technical solutions for the collection and integration of renewable onshore and offshore wind generation, collection and transmission of remote renewable energy resources to load centres, ocean archipelago power supplies, the construction of new types of urban/distribution power networks, and interconnections of AC power systems [10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Thus, the development and application of DC grids have become an important direction for the development of smart grids and the energy internet.…”

“…Although there are many different control technologies at various levels which have been invented and studied around the world [14][15][16][17][18][19][20][21][22][23], the voltage margin control, the voltage droop control, the voltage droop control with a voltage margin, a voltage droop control as a backup and so on are the basic control methods as described in [5].…”