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

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

Grid side unbalanced fault detection using soft open point in an electrical distribution network

Soft open point (SOP) is a power electronic device that installed in place of normally open points in distribution networks. This paper investigates the performance of SOP under an unbalanced line to ground (L-G) fault in a medium voltage (MV) distribution network. A method to detect unbalanced grid side AC faults was developed using positive and negative sequence currents injected into the feeder by the SOP. Simulations were carried out on a network model developed in PSCAD/EMTDC. Local measurements at SOP grid connection point were used to define a fault index. Results show that the fault index, based on the values of positive and negative sequence currents, is able to effectively detect the presence of an unbalanced fault in a MV distribution network

Performance of an electrical distribution network with soft open point during a grid side AC fault

Soft Open Point (SOP) is a power electronic device installed in place of normally open points in electrical distribution networks. This paper investigates the dynamic performance of a medium voltage (MV) distribution network with a connected SOP, under grid side AC faults. Use of sequence networks was extended to include SOP, such that conventional fault analysis technique can be used on a distribution network with SOP. A Fault-Index was defined using symmetrical components of voltages measured at the grid connection point of the SOP. The network performance was investigated under a line-to-ground, a line-to-line and a three-phase fault. The behaviour of the network was analysed under different control schemes and various operating scenarios of the SOP. Furthermore, the dependence of the sequence voltages and currents on the SOP set points was investigated. Simulations were carried out on an 11 kV generic UK distribution network model developed in PSCAD/EMTDC. Results show that the convectional fault analysis technique is applicable on a network with SOP, regardless of the SOP control mode. The Fault-Index, defined based on the local voltage measurements, was effective in detecting the presence of an AC fault in the MV distribution network. In addition, the need for a non-current based detection method is illustrated

Prospects and Challenges of 4H-SiC Thyristors in Protection of HB-MMC-VSC-HVDC Converters

Pole-to-pole DC faults on HB-MMC-VSC-HVDC schemes impose significant risk of cascade failure on IGBT/diode pairs. Other novel topologies with fault blocking capability, i.e. AAC converters, and DC circuit breakers are not yet fully matured. Therefore, silicon thyristors are used to bypass the DC faults until AC breakers activate. However, silicon thyristors are also at risk of failure due to the capacitor voltage collapse at high junction temperatures caused due to imbalanced reverse recovery current conduction. Hence, the recovery cycles are included as part of IEC standard 62 501 HVDC type-test program. Emergence of commercial Silicon Carbide (SiC) thyristors has the potential to tackle this risk. This paper investigates such opportunities and challenges by accurately modeling the performance of thyristors at fault. It was seen that SiC thyristors with acceptable surge current and reverse blocking capability can eliminate the failure mode of silicon thyristors due to minimal recovery stored charge, resulting in an equal share of reverse voltage on all thyristors

Operation and performance of a medium-voltage DC link

The operation of a voltage source converter (VSC)-based medium-voltage (MV) direct current (DC) link is described. The use of a DC link in an MV distribution network is presented, considering salient similarities and differences to a high-voltage DC link. A PSCAD/EMTDC model was used to demonstrate the dynamic operation of an MVDC link, under normal and faulted network conditions. The study shows that MVDC can provide flexibility in a distribution network by decoupling real (P) and reactive (Q) power flows through the link, under normal operation. During a fault, however, the current injected from the converter could interfere with the detection and discrimination of AC system faults, in distribution networks with current-based protection scheme

Analysis of cyclic spontaneous switchings in GaN & SiC cascodes by snappy turn-off currents

This paper investigates the crosstalk-induced spontaneous switchings as continuous cycles of turn-on and turn-off transients as a key reliability criterion in SiC and GaN cascode power devices. The paper presents a wide range of measurements to describe the severity of unwanted switching cycles in presence of a few diodes with high turn-off dI/dt which results in a negative gate voltage induced by the source inductance. Modelling is performed which confirms the theory described to explain the root cause of the continued oscillatory transients and comparisons are made with standalone SiC power MOSFETs