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Title: Enhancing DC grid reliability by preventing loss of terminals under DC faults
Author: Zaja, Mario
ISNI:       0000 0004 7961 2834
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2019
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Vulnerability of voltage source converters to DC faults is recognized as the main obstacle towards widespread implementation of meshed DC networks. Because of low surge current capability of IGBTs, the converters are blocked under high currents in order to prevent permanent damage to the modules. Converter blocking abruptly interrupts the power flow and causes a major power imbalance in both the AC and DC grid. This thesis aims to improve reliability and security of DC grids by preventing loss of terminals under DC faults. Fault response of a modular-multilevel converter is studied analytically and through simulations. It is identified that the MMC goes through several stages under a DC fault and each stage has different dynamics. The impact of converter modelling is also taken into account. DC grid topology has a major impact on protection system design and performance. If multiple converters are connected to the same bus, fault current is shared between the converters and fault current slope of each individual converter is reduced. Converter blocking can consequently be avoided and protection system requirements can be relaxed. A novel DC grid protection device based on capacitive energy storage is proposed. It is demonstrated that the device reduces the magnitude and frequency of MMC fault current, as well as DC voltage drop. In symmetrical monopole grids, pole voltage deviations under pole-to-ground faults are greatly reduced and converter blocking is avoided. Temporary MMC blocking is studied as another method for avoiding loss of terminals under DC faults. The benefit of this approach is low protection system cost and reduced impact on normal grid operation. A thermal valve model is developed to assess peak thermal stress on antiparallel diodes. Coordination between MMC self-protection and fault detection relays is proposed, as well as the use of converter DCCBs for backup protection.
Supervisor: Jovcic, Dragan ; Hajian, Masood Sponsor: University of Aberdeen
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Electric currents ; Modular-multilevel converters ; Renewable energy ; Electrical engineering ; Power transmission