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Title: Protection, fault location & control in high voltage multi terminal direct current (HV-MTDC) grids
Author: Tzelepis, Dimitrios
ISNI:       0000 0004 6500 9364
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2017
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With an increased penetration of renewable energy sources, balancing the supply and demand is likely to be one of the major challenges in future power systems. Consequently, there is a growing need for meshed interconnections between countries in order to effectively share the available power capacity and thereby increase operational exibility and security of supply. This has been raised as a major issue in Europe but also in Asia and United States of America. The concept of supergrid has been identifed as a possible solution towards a newback bone transmission system, permitting massive integration of renewable energy sources. High voltage direct current (HVDC) links, utilising voltage-source converters (VSCs),are expected to become the preferred technology for the realisation of such a supergrid. This is due to the fact that such systems offer improvements in terms of system stability, lower cost and operational losses. A natural extension of the existing point-to-point HVDC transmission technology is a multi-terminal direct-current (MTDC) system which utilises more than two VSC stations, effectively forming a DC grid. Such a configuration can provide further technological and economical advantages and hence accelerate the realisation of a supergrid. However, technical limitations still exist, and it is not yet a straightforward task to construct and operate an MTDC grid, as several outstanding issues need to be solved. Consequently, it is essential to study, analyse and address potential challenges imposed by MTDC systems in order to enable widespread adoption. Even though numerous challenges are introduced for the practical implementation of MTDC networks, this thesis deals with the challenges related to the DC-side faults, which is the main issue when considering HVDC technology. DC-side faults in HVDC systems are characterised by large inrush currents caused by the discharge of trapped energy in the system capacitances, escalating over a very short period of time. These include lumped capacitors installed on the DC side of converters, transmission line capacitances, and also the sub-module capacitors contained within modular multi-level converters. When faults occur in multi-terminal HVDC grids, the DC protection system is expected to minimise the detrimental effects by disconnecting only the faulted section while permitting the remaining healthy part of the grid to continue normal operation. Such requirements introduce the need for transient DC fault characterisation and subsequent development of a discriminative, fast, sensitive and reliable DC protection method. Therefore, one of the main objectives of this thesis is to provide demonstrable solutions to the key challenges involved in protecting MTDC grids, and hence enabling the realisation of HVDC-based supergrids. Two alternative, novel protection schemes are proposed, designed and assessed with the aid of transient simulation. The key advantages of the proposed schemes consist in enhanced reliability, fast fault detection, superior stability, and high level of selectivity. To further validate the practical feasibility of the schemes, small-scale laboratory prototypes have been developed to test the performance of the schemes under real-time fault conditions. It should be also highlighted that when a permanent fault occurs in an HVDC transmission system, accurate estimation of its location is of major importance in order to accelerate restoration, reduce the system down-time, minimise repair cost, and hence, increase the overall availability and reliability of HVDC grids. As such, another contribution of this thesis is related to the challenges involved in accurate fault location in HVDC networks, including non-homogeneous transmission media (i.e. the lines which include multiple segments of both underground cables and overhead lines). Two novel fault location methods have been developed and systematically assessed. It is demonstrated that the schemes can reliably identify the faulted segment of the line while consistently maintaining high accuracy of fault location across a wide range of fault scenarios. Further sensitivity analysis demonstrates that the proposed schemes are robust against noisy inputs. In its concluding section, the thesis also outlines a few possible avenues of further research in this area.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral