Operation of soft open point in a distribution network under faulted network conditions

• Main Author: Aithal, Avinash
• Published: Cardiff University 2018
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.742897

A Soft Open Point (SOP) is a power electronic device, usually implemented using back-to-back voltage source converters (VSCs) installed at normally-open points (NOP) of a distribution network. SOPs are typically utilised to enhance distribution network operation, under normal network conditions. Their applications include power loss reduction, feeder load balancing, network reinforcement, voltage profile improvement and distributed generation (DG) connection support. This thesis investigates the operating principle and applications of a back-to-back VSC based SOP under faulted network conditions. The dynamics of a network with SOP were observed during normal and faulted network conditions. The conventional fault analysis technique using symmetrical components was extended to include SOP, such that it can be applied on distribution networks with SOP. Equivalent sequence networks were developed for different type of faults, including phase-to-ground faults, phase-to-phase faults and three-phase faults. The correlation between the symmetrical components (of voltages and currents) at the SOP grid connection point and the SOP set points were studied. A simple but effective method of fault detection during grid-connected operation was formulated using the sequence voltages at the grid connection point of the SOP. The impact of the SOP dynamics on a conventional distributed feeder automation (D-FA) scheme was investigated. It was found that the current contributed from the SOP during a fault could potentially disturb the protection coordination of the network. Consequently, the sequence of events in a feeder automation scheme is disrupted if the SOP is kept operational beyond the fault ride-through period. A new operating mode was defined to operate the SOP during network faults such that it can be co-ordinated with network protection. Based on the local measurements at SOP grid connection point, methods to determine the presence of a fault, fault type and location of a fault were investigated. A D-FA scheme was proposed, in which the fault diagnostic capability of an SOP was utilised to coordinate the feeder automation sequence. Substantial improvements were achieved in both the restoration time and life of existing switchgear by using SOPs for feeder automation. The fault diagnostic capability of an SOP was validated using a power hardware-in-the-loop (PHIL) experimental setup. A VSC prototype with a constant DC voltage source was used to emulate an SOP. The prototype was integrated with a distribution network modelled in the real-time digital simulator (RTDS), through a power interface. Individual protection features of the SOP including fault detection, fault type identification and estimation of fault location were validated using this experimental setup. The results obtained using the real-time PHIL experiment were consistent with the results obtained using software simulations.