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Measuring current with Rogowski coils

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article image Rogowski coil (right) inside OSM bushing housing.

NOJA Power uses Rogowski coil technology in its medium voltage reclosers to measure both phase and residual currents.

These devices have some key advantages over traditional iron core CTs.

These include:

* Increased accuracy over a wider current range.

* Immune to core saturation at high fault currents.

* No need to short outputs when disconnected.

* Lightweight air-core design means a lighter recloser.

Rogowski coils are not a new idea. W Rogowski first presented his design in 1912 which consisted of a helix coil wound around a strawboard band. Basically a Rogowski coil is a helix loop wound around an air core with the two wire ends joined.

Unfortunately, the early applications of this design were limited due to the very small output voltages generated. This characteristic has now been overcome with the advent of sensitive front-end electronics capable of measuring millivolts.

In fact, this low voltage has become a significant advantage in medium voltage applications since the CT does not produce dangerous voltage levels.

Another advantage relevant to recloser design is the elimination of an iron core. This makes saturation of the CT impossible allowing it to continue working correctly even when subjected to high fault currents.

The lack of an iron core also makes the CT considerably lighter. This is part of the reason NOJA Power’s reclosers are so light (<85kg).

Rogowski coils typically have a very wide and linear range of measurement. The coils used in the OSM recloser have been designed with protection functionality in mind and operate from as low as 4A up to 6,000A.

This means the OSM recloser can offer sensitive earth fault (SEF) along with earth fault and overcurrent protection with the single design. No special CTs are required and SEF protection is provided as standard.

The OSM recloser utilises six Rogowski coils. The coils on one side of the recloser are used to measure phase currents. The coils on the other side are connected in series to measure residual currents for earth fault and sensitive earth fault protection.

This arrangement allows Noja to provide a full range of protection and automation features.

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