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Series current flow controllers for high voltage direct current transmission grids

Balasubramaniam, Senthooran 2017. Series current flow controllers for high voltage direct current transmission grids. PhD Thesis, Cardiff University.
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Large scale grid integration of renewables and cross country border energy exchange may be facilitated by multi-terminal high-voltage direct-current (MTdc) grids. However, as the number of terminals and dc lines increases, power flow management between dc nodes becomes amajor challenge. The current carrying capability of dc lines is limited by their thermal and electric stress limits. Thus, the line current must be maintained within the permissible operational region to protect the lines fromdamages. This thesis addresses this fundamental issue through the introduction of inter-line current flow controllers (CFCs) into MTdc grids. An inter-line CFC is a low power rated controllable voltage source that can enhance system performance by suitably redirecting the current flow at the point of connection. It enables regulation of the dc line current flow by changing the voltage at the dc terminals where it is inserted. The research work presented in this thesis is aimed to realise the most feasible CFC topologies to facilitate flexible power flow between dc nodes. Themain contributions of this research work comprise of four parts, namely, (1) design and development of dc CFC topologies, (2) prototyping of the proposed CFCs (3) implementations of centralised and communication free control schemes for densely meshed dc grids, and (4) protection of inter-line CFCs. In the first phase, the characteristics, control and operation for five configurations of interline CFCs are studied, namely, resistive, RC circuit, capacitive, dual H-bridge, and single H-bridge based CFCs. A multi-port CFC is proposed to facilitate current regulation on multiple lines simultaneously. An experimental platform consisting of a three-terminal dc grid and small scale prototypes of the proposed CFCs have been developed to validate the concepts. It is clearly shown through experiments and time-domain simulations that all devices are capable of improving the system performance. A centralised hierarchical control system is proposed to coordinate the operation between multiple CFCs. A novel voltage sharing control scheme is demonstrated. It is shown that such ii Abstract scheme reduces the workload on a single CFC by sharing the required control voltage between multiple CFCs, and, additionally, can be used to avoid control conflicts among active CFCs during communication failure. New protection methodologies are implemented to protect the CFC during internal failures and dc faults. Small-scale dc circuit breakers have been developed to study the performances of 1B and 2B-CFCs under a pole-to-pole fault.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Current Flow Controllers; Multi-terminal HVDc Grids; High Voltage Direct Current; DC Circuit Breaker; Voltage Source Converter; Power Flow Controllers.
Date of First Compliant Deposit: 21 September 2017
Last Modified: 05 Nov 2021 15:50

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