Knowing the peak fault current on their networks is critical to electrical utilities, as it defines the rating of the components such as circuit breakers required to ensure that they can safely withstand the large release of energy that occurs during an electrical fault.
Outram’s patent-pending technique to predict fault current from network measurements has been developed to provide greater accuracy than current approaches. The project, funded by Ofgem’s Innovation Funding Initiative (IFI), consists of three stages. In the first stage of the trials, which measured RMS “break” current, Outram’s solution delivered accuracy to within 2% of the network model. By accurately predicting fault current, utilities can ensure that network components are correctly specified and eliminate the money wasted when over-compensating to provide a safety margin that allows for the existing, possibly inaccurate, approaches to their calculations.
Fault current is measured using an algorithm developed by Outram Research, which runs on their PM7000 power quality analyser. The PM7000 is already widely used by companies wanting to troubleshoot, identify and resolve power quality problems quickly and efficiently. The first stage of the project was to predict the RMS “break” current, which represents the highest possible current that might have to be interrupted in the case of a fault occurring on an already live circuit. Breakers in the network must operate up to this maximum fault current to ensure that the power is safely and quickly disconnected.
“The first stage of the field trials have been very encouraging, demonstrating the huge potential of Outram’s technique to predict fault current,” said Jim Sutherland, SP Energy Network’s Director of Network Development. “We’re optimistic that once all stages are complete, we will be able to use this approach to allow us to accurately specify breakers and other components in the network.”
Stage two of the project, which aims to measure the peak “make” current – the current that flows when a connection is made to a section of the network where a fault is already present, has begun.
Electricity distribution networks are becoming increasingly complex, and can no longer be modelled as a simple “waterfall”, where power flows from the power station to the load. In the event of a fault, components such as motors and embedded generation will deliver power back to the grid. This presents a potentially unknown level of fault current since it is impossible to have prior knowledge of such events without building complex network models and having a full understanding of all customer loads. Local electricity generation from renewable sources presents a similar downstream contribution that should be calculated. The third stage of the project will demonstrate that the Outram approach can elucidate the contribution to fault current from such loads through its measurement technique.
Existing approaches to calculating peak “make” current are less accurate than those for peak “break” current, and they usually ignore the increasingly important downstream contribution.
“ScottishPower has been an excellent partner, allowing us to demonstrate the effectiveness of our new technique in a real-world situation,” said John Outram. “We’re delighted with the results of the first stage of the field trials, and are excited as we move into the second and third stages, where our refined and extended fault current prediction will provide even greater benefits to electrical utilities.”