Rogowski coils revisited
July 2010
Electrical Power & Protection
Pierre Turpin, project manager, LEM Energy & Automation
A new class of Rogowski coil split-core current transducers: the RT series.
Abstract
The monitoring of electric power consumption has become a key element for managing facilities such as manufacturing, data centres, food processing, retail outlets, hospitals and educational establishments. Three years ago LEM introduced a system called Wi-LEM onto the market which is based on wireless sub-metering components, the EMN, enabling measurement of electricity segmented by activity (lighting, HVAC, motors, heating, etc). Initially the range seemed to be sufficiently comprehensive, with a metering capacity of up to 100 A. However, it was soon found to be too limited for the industrial or heavy duty service domains taking into account the fact that monitoring often begins by measuring global consumption at the point of energy input – requiring a capability to measure up to 2000 A.
LEM therefore developed the RT current sensor adapted to these EMN devices, which provides the same flexibility of installation as split-core current transformers from the lower range, with the same class 1 precision required for the sub-metering field. The Rogowski coil, which has long been noted for its ease of installation, offered the right solution provided that its major drawback could be overcome – inaccuracy caused by the sensitivity to the position of the conductor inside the loop.
With this in mind, LEM engineers decided to revisit the theory in order to find an answer. Thanks to their expertise in magnetic theory they have been able to develop a very simple but effective solution – a sleeve made of magnetic material that makes it possible to mask (magnetically) an entire zone around the coil as well as the imperfections on the closing mechanism and the connections of the sensor’s secondary wires. The sleeve acts as a magnetic short-circuit, ‘virtually’ bringing together the two sections of the coil located on each side. Their approach was a complete success – the error associated with the coil clasp has become almost negligible.
While the major problem with the split-core Rogowski coil had finally been solved, other problems became apparent which diminished the success of the magnetic sleeve. The error associated with the design of the coil clasp system had previously been so important that it had, to some extent, masked the other causes of asymmetry. LEM continued to work to improve this current sensor and after a total of two years has been able to develop the processes and machinery that significantly reduce the symmetry faults, both with regard to the regularity of the coil-winding and creating a uniform section over the entire length of the loop.
Readers wanting more information on the LEM Rogowski coil sensor solution together with an application note please visit: http://instrumentation.co.za/+C13962A
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