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Title: Analysis and control of modular multilevel cascaded converter-based flexible AC transmission systems
Author: Huang, Han
ISNI:       0000 0004 7655 1175
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2018
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Power quality issues are becoming more pronounced since the majority of modern day loads draw harmonic currents and consume reactive power. Increasing connection of renewable sourced generators to the utility network exacerbates the situation due to their use of power electronic converters for grid interfacing and their unpredictable generating nature. A particular challenge also lies in unbalanced current drawing, due to large single-phase loads, such as traction drives and renewable energy sourced generators. Such current accumulation may result in the voltage imbalance at the point of common coupling, posing threats to system stability and safe operation. The Flexible AC Transmission Systems (FACTs) such as shunt-connected active power conditioners (APC) can be used not only to eliminate the harmonics, but also compensate reactive power, hence enhance power quality. However for medium/high voltage applications the conventional two-level voltage source converter (VSC)- based types require bulky step-up transformers and high switching frequencies which are costly and inefficient. This thesis investigates the use of Modular Multilevel Cascaded Converters (MMCC) to function as APCs under balanced and unbalanced load conditions. The particular topology explored employs a 5-level full-bridge flying capacitor converter as the submodule, owing to its increased availability of switching states. The benefits of modularity and scalability offered by an MMCC make it an ideal topology to improve the power quality of medium/high voltage power grid. For effective harmonic current mitigation high performance reference current extraction is important. The work proposes a novel technique which uses cascaded notch filters to identify a selection of dominant low-order harmonics in the load current, combining with a predictive + derivative current control scheme the method can offer accurate and fast harmonic current cancellations. This method has shown that it outperforms the conventional synchronous reference frame-based low-pass filtering scheme. For an MMCC-based APC, inner capacitor voltage imbalance may occur particularly under unbalanced and distorted currents which can lead to converter failure. A new PWM scheme, the carrier permutation phase-shift PWM (CP PS-PWM), is proposed. The method rotates multiple triangular carrier waves to each voltage levels in a fixed sequence at the end of each fundamental period. This method has shown to mitigate the MMCC intra-cluster voltage imbalance effectively. The MMCC phase voltage imbalance has been an issue for unbalanced compensation. To mitigate the problem an MMCC-APC in star connection needs a common zero sequence voltage injection and that in delta connection requires a zero sequence current injection. These may limit the operation ranges of the device, particularly when current is distorted. Analysis and comparison of the two configurations under both unbalanced and distorted currents are performed. Experimental verifications are carried out, and the superiority of the delta configured MMFCC-APC is validated. A key contribution of the work also lies in the development of an MMFCC-based Unified Power Flow Controller (UPFC). The new device has a direct AC-AC configuration. It has one terminals of the MMFCC shunt-connected directly to the power lines, and the other terminals are connected to the AC output ends of the twolevel voltage source converter which is serially-connected in the transmission lines through a transformer. The work develops a complete control scheme for this MMFCC-UPFC to achieve transmission line power flow control. Simulation studies of this device on a simple two voltage sourced power network has shown that it performs as desired under varying real and reactive power flow demands.
Supervisor: Zhang, Li ; Chong, Benjamin Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available