Browsing by Author "Wickramasinghe, Harith R."
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Item Alternate Arm Converter Energy Balancing under Parameter Variation(2019-04) Wickramasinghe, Harith R.; Konstantinou, Georgios; Ceballos, Salvador; Pou, Josep; POWER ELECTRONICS AND SYSTEM EQUIPMENTEnergy balancing of the alternate arm converter (AAC) is a major challenge and overlap period-based circulating current control methods are commonly used. Component parameter variation occurs due to tolerances, component aging, and internal faults leading to stored energy imbalances of modular voltage source converter (VSC) topologies. If not controlled, such variations can severely impact the operation of the AAC. Hence, component parameter variation is a key consideration for energy balancing of all sorts of modular VSC topologies, but has not been addressed in the existing literature for the AAC. This paper investigates the energy balancing capability of the AAC in high-voltage direct current (HVDC) applications under component parameter variations and proposes a compensation method to further improve its energy balancing performance. The effectiveness of the proposed method is verified in a real-Time model of an AAC-HVDC transmission system from widely accepted existing CIGRE benchmark models.Item Comparison of bipolar sub-modules for the alternate arm converter(IEEE Computer Society, 2016-12-21) Wickramasinghe, Harith R.; Konstantinou, Georgios; Pou, Josep; Picas, Ricard; Ceballos, Salvador; Agelidis, Vassilios G.; POWER ELECTRONICS AND SYSTEM EQUIPMENTResearch on dc-fault tolerant multilevel converters has gained noticeable attention over recent years. The alternate arm converter (AAC) is one of such emerging multilevel converter topologies, and a hybrid topology of the two-level converter and the modular multilevel converter (MMC). Bipolar sub-modules (SMs) that can produce both positive and negative voltages are the building blocks of the AAC. This paper analyses the operation of an AAC with the full-bridge SM (FB-SM) and the cross-connected SM (CC-SM). The conduction and switching losses of the two SM configurations are evaluated and compared, in order to identify the suitability of CC-SM for AACs and its performance compared to the FB-SM. The CC-SM with identical semiconductor devices has reduced losses compared to the CC-SM with higher rated devices in the cross-connected path. It is concluded that the CC-SM does not offer advantages in the losses, construction, and application to the AAC, compared to FB-SM.Item Estimation methods and sensor reduction in modular multilevel converters: A review(Institute of Electrical and Electronics Engineers Inc., 2018-07-02) Konstantinou, Georgios; Wickramasinghe, Harith R.; Townsend, Christopher D.; Ceballos, Salvador; Pou, Josep; POWER ELECTRONICS AND SYSTEM EQUIPMENTThe circuit configuration of the modular multilevel converter (MMC) requires a large number of current and voltage measurements as well as communications within the converter arms and between the arm controllers and the main control circuit. In order to reduce circuit complexity and hardware requirements with their associated cost, while adding an additional level of reliability to the operation of the MMC, a number of methods have been proposed for the estimation / observation of the various internal electrical variables or parameters of the MMC. These include estimations for sub-module capacitor voltages, dc-link voltages, arm currents or total arm energy. Such methods aim to eliminate or reduce the sensors in the circuit, reduce communication requirements or both. This paper provides an overview and classification of the different estimation/observation methods for MMCs currently proposed in the literature.Item Offset PWM in modular multilevel converters for stored arm energy reduction(Institute of Electrical and Electronics Engineers Inc., 2016) Konstantinou, Georgios; Wickramasinghe, Harith R.; Ceballos, Salvador; Pou, Josep; POWER ELECTRONICS AND SYSTEM EQUIPMENTModulation techniques for the modular multilevel converter consider that the total number of sub-modules in the phase-leg is, either instantaneously or on average, equal to N balancing the capacitor voltages to a constant reference. This paper proposes an offset PWM technique that utilises the total number of the SMs in the arms of the MMC, particularly under low modulation indices. The result is a reduction in the average voltage of the capacitor and consequently, the total energy stored in the converter. The proposed technique is investigated under open and closed-loop circulating current control and simulation and experimental results are provided to illustrate the operation of the converter.Item Submodule Voltage Balancing and Loss Equalisation in Alternate Arm Converters Based on Virtual Voltages(Institute of Electrical and Electronics Engineers Inc., 2018-10-22) Konstantinou, Georgios; Wickramasinghe, Harith R.; Ceballos, Salvador; Pou, Josep; POWER ELECTRONICS AND SYSTEM EQUIPMENTBalancing of the submodule (SM) capacitor voltages in the alternate arm converter (AAC) exhibits a great amount of similarities with other modular topologies, especially the modular multilevel converter (MMC). However, the requirements of operation at overmodulation range and the alternate conduction of the phase current further complicate the voltage balancing task. In this paper, a voltage balancing and loss equalisation approach based on virtual SM voltages in combination with sorting algorithms is introduced. The developed approach uses the switching states of the SMs as feedback and provides reduction of the switching frequency of the SMs, further addressing excessive switching due to component parameter variation, tolerances and faults. The effectiveness of the algorithm is demonstrated with simulation results from an equivalent full switching model of an AAC for HVDC applications.