Transition from AC to DC Distribution Networks

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Abstract

Maximising the capacity of the existing AC distribution network infrastructure by conversion to DC holds significant advantages. In particular it may provide a greater flow of electrical energy within urban areas, allowing a lower investment cost for adoption of electrical vehicles and domestic heating. Integration with Smart Grid applications will require maintained levels of reliability, and improved efficiency and flexibility. The transition of the cable infrastructure from the legacy LVAC (low voltage alternating current) system to LVDC (low voltage direct current) is considered in this work. In particular this thesis investigates the limitations of DC supplied through the existing distribution network without major re- construction, and proposes optimal configurations that could be adopted in a smart-DC distribution network. The implications for power flow in the network are considered with regards to existing cable limitations. The operation of legacy LVAC distribution cables under DC is considered in this work. The electric field distribution in cable insulation under DC voltage is governed by the electrical conductivity of the material unlike the AC case where it is dependent on the permittivity of the materials. Temperature, water ingress and chemical ageing can influence the electrical properties of the insulation.The first major contribution to literature from this work is the demonstration that moisture penetration is the most influential factor in LV cable failure. Local elevated stress further alters the insulation properties leading to a thermal runaway and cable failure. LV cable insulation structures were tested experimentally and by the use of Finite Element Analysis simulations for both AC and DC. DC proved to be the superior option in all tests. Moisture and generated heat cause higher losses under AC due to the impedance being dependent on capacitance which is not as heavily affected as the resistance.The second major contribution of this work is the demonstration that LVDC is a very attractive alternative for future distribution networks. Total system capacity can increase by at least a factor of 1.41. It was shown that LVDC lowers total system losses which in turn lead to a prolonged plant lifespan. A decrease in losses guarantees a more reliable, robust and future proof network. Power electronics will inevitably drop in price allowing for such a transition to take place which will assure the supply of electrical energy for future needs. Rewiring will be inevitable even under AC, following the increase in demand which will require the uprating of the existing lines. A phased implementation will allow for a smooth transition to a LVDC distribution network.

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