Multilevel Converters for High Voltage Direct Current Transmission Systems

Research Area: Smart grids
Status: In progress  
Jefe de proyecto:  
Proposed start date: 2012-03-01 Proposed end date: 2014-03-01

High voltage direct current (HVDC) systems have become into an attractive alternative for long distance
energy transmission, mainly because they become economically convenient when the distance difficult the
use of ac systems. They also are using to transmit power over underground or submarine cable, when the
ac systems are restricted only to very small distances. Another application is grid interconnection, where
HVDC can connect asynchronous grid or even grid with different frequencies.
Among the HVDC converter topologies modular multilevel converter (MMC) offers several advantages, as
such as high modularity and reduced harmonics.
There are several control objectives in the MMC, like control the ac voltage waveform, dc current, circulating
currents and capacitor voltages, that make the design of the controller very challenging. The capacitor voltage
is a critical control objective because, it affects the performance of the other control objectives.
This work will research control strategies for modular multilevel converter when it is used in HVDC appli-
cations. A theoretical study, which includes the topological analysis and modeling, will be carry out. Linear
and predictive strategies to independently control the input and circulating current will be designed. Ac
voltage waveform control using staircase modulation is proposed. The capacitor voltage will be controlled
using a lookup table taking advantage of the switching redundancies produced in each converter arm. The
power transmission will be modeled and controlled using a energy-based approach.
• Analysis and dynamical modeling of a modular multilevel converter. Including input, output, and
circulating current, and capacitor voltages.
• Study and design control strategies and modulation techniques for AC voltage waveform, DC current,
circulating currents and capacitor voltages. Design control strategies for power transmission and DC
• Simulation of the converter and proposed control strategies.
• Prototype design and implementation of a modular multilevel converter.
• Experimental validation of the MMC control using one converter and a controlled DC source and back
to back MMC-based interconnection.
• Experimental validation of the DC link control.
• Study of performance, reliability and efficiency.
• Guide undergraduate and graduate thesis projects.
• Publish papers in journals and conferences to make this new knowledge available to the academic
Methodology The methodology is based on five consecutively stages: theoretical studies, simulation, design
and implementation of prototype converters, experimental validation and publication of the results.
Expected Outcome
Fulfilling the proposed objectives and goals will generate new knowledge related to control and practical
aspects of HVDC systems based on Modular multilevel converters and how the use of power electronics
converters to improve the stability and power capability of a distributed transmission system.
The theoretical study and simulation will help to understand the control techniques currently used. Exper-
imental results and validation are expected to confirm the proposed hypothesis about the applicability and
performance of MMC controller. Finally, publication of the obtained results on international conferences
and journals should confirm the recognition of the research community about the importance of this topic.

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