Simplified control strategies for modular multilevel matrix converter for offshore low frequency AC transmission system

Abstract

The Low frequency AC (LFAC) transmission system is considered as the most cost-saving choice for the short and intermediate distance. It not only improves the transmission capacity and distance but also has higher reliability which makes it more advantageous than the HVDC transmission system. Modular Multilevel Matrix Converter (M3C) is recognized as the most suitable frequency converter for the LFAC transmission system which is responsible for connecting 16.7 Hz and 50 Hz ac systems. In such applications, the ‘double αβ0 transform’ control method is most popular technique that realizes the decoupled control of the input current, output current and circulating current. However, the derivation process of the mathematical model is so complicated that it gives too much burden on the controller of the M3C system. Therefore, this thesis is focusing on simplifying the M3C control strategies when used in LFAC systems and the primary contribution to the knowledge is outlined as follows: (1) A simplified hierarchical energy balance control method which employs an independent control for each of three sub-converters in M3C is proposed in Chapter 5. The output frequency circulating current is injected and utilized to balance the energy between the three arms of the sub-converter. The proposed method achieves a reduced execution time and a simplified control structure, with which a low-cost processor is applicable and the control bandwidth of the system is improved. (2) An improved energy balance control method with injecting both input and output frequency circulating currents is proposed in Chapter 6. The magnitudes of the circulating current responsible for the energy balance control in either frequency are half reduced as compared to the single frequency injection method in Chapter 5. This arrangement alleviates the negative impact of the injected circulating current on the external grid and allows the M3C systems work through larger grid unbalance situations. Finally, the effectiveness of the proposed control strategy is demonstrated by extensive simulation results and validated experimentally using a scaled-down laboratory prototype.