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Correct protection settings of relays are of vital importance to any electrical system, as it ensures that should a fault occur, only the faulted item of equipment is removed form service, and the healthy equipment is kept on line. If the protection study has not been coordinated correctly and there is insufficient grading margin, a simple LV fault can trip a whole HV substation. Aurora has experience of undertaking a wide range of protection grading studies from simple over current earth fault coordination studies for a Ring Main Unit, up to configuring complex differential protection schemes and investigating nuisance protection trips.
Protection studies can be carried out using either ETAP or DIgSILENT software package. These packages allow overall grading to be carried out, but also check of response to system disturbances, through simulation of faults on any part of the network to confirm the exact operating sequence and times for the protective devices.
On larger and more complex networks, the use of differential protection is common. These relays are designed to operate for a specific protected zones and faults, to isolate high value equipment quickly and effectively. Differential relay settings can be difficult to set for engineers unfamiliar with the concepts and requirements, as high impedance schemes need to be set with consideration for the associated Class PX protection CT characteristics. Low impedance protection schemes have a more flexible configuration option for protection CTs, but are more complex to setup and require an extensive communications network to operate correctly.
Protection calculations and analysis are carried out using propriety in house calculation spreadsheets and hand calculations. Where necessary a full model of the protected zone can be setup in DigSILENT Powerfactory or PSCAD and in-zone and out-of-zone faults applied to check stability.
Arc flash studies are becoming increasingly important, as they can help ensure that operational personnel are adequately protected, and provided with suitable PPE, for any switching duties. These studies are typically carried out to IEEE 1584 and look at the typical busbar clearances, working distance, fault levels and protection operating time to identify the total incident energy in Cal/cm2, to ensure it is within safe operating limits, and what level of PPE is required. Studies can be carried out using either ETAP or DIgSILENT as required.
A key point that is often misunderstood in arc flash studies is the interpretation of the results. Reports showing unrealistically high incident energy levels, because of poor assumptions. Or failing to consider that the switchgear has levels of arc containment designed into the equipment. The other thing that is often forgotten is that the high incident energy levels are usually caused by poor protection settings; many companies can carry out an arc flash study, but very few are actually competent to regrade the protection correctly.
Contact: Craig McGlone
Phone: 01305 300 210
Aurora Power Consulting are a leading independent power systems consultancy. We provide specialist power systems analysis services using software such as DIgSILENT, ETAP, PSCAD, EMTP-ATP, CDEGS and XGSLab.
