IEEE PSRC Committee

Practical Aspects of Rogowski Coil Applications to Relaying

The IEEE PES Power System Relaying Committee has recognized the impact of non-conventional instrument transformers on protection and control systems and has established working groups to analyze the issues and guide our community regarding their applications to relaying.
This work has resulted in the publication of IEEE C37.235-2007 - IEEE Guide for the Application of Rogowski Coils Used for Protective Relaying Purposes and IEEE C37.241-2017 - IEEE Approved Draft Guide for Application of Optical Instrument Transformers for Protective Relaying. Both guides can be purchased from the IEEE.

Another working group was created with the assignment to produce a special report describing applications of Rogowski Coils used for protective relaying in electric power systems. The following is a summary of the report which is available at the IEEE PES PSRC web site at www.pes-psrc.org/kb/published/reports.html
Rogowski Coils operate on the same principles as conventional iron-core current transformers (CTs). The main difference between Rogowski Coils and CTs is that Rogowski Coil windings are wound over an (non-magnetic) air core, instead of over an iron core. As a result, Rogowski Coils are linear since the air core cannot saturate. However, the mutual coupling between the primary conductor and the secondary winding in Rogowski Coils is much smaller than in CTs.
Therefore, Rogowski Coil output power is small, so it cannot drive current through low-resistance burden like CTs are able to drive. Rogowski Coils can provide input signals for microprocessor-based devices that have a high input resistance; therefore, these devices measure voltage across the Rogowski Coil secondary output terminals.

In general, Rogowski Coil current sensors have performance characteristics that are favorable when compared to conventional CTs. These characteristics include high measurement accuracy and a wide operating current range allowing the use of the same device for both metering and protection.
This can result in reduced inventory costs since fewer sensors are needed for all applications. Less variation in inventory requirements should also improve installation time when replacements are needed, reducing equipment downtime; thereby, lowering overall sensor costs to the utility or industrial company.
This is achieved through higher unification and standardization of products, which benefits manufacturers and users.

In addition, Rogowski Coils make protection schemes possible that were not achievable by conventional CTs because of saturation, size, weight, and/or difficulty encountered when attempting to install current transformers around conductors that cannot be opened.
An additional advantage of Rogowski Coil current sensors is significantly lower power consumption during operation. Rogowski Coils are connected to devices that have high input resistance, resulting in negligible current flowing through the secondary circuit. Conventional CTs contain a ferromagnetic core that also consumes energy/power due to hysteresis losses.

Rogowski Coils have no core losses. In fact, an operating Rogowski Coil has much smaller power loss than conventional CTs - which leads to significant savings of energy and ultimately reduced lifecycle costs.
Rogowski Coils can replace conventional CTs for protection, metering, and control. Rogowski Coils have been applied at all voltage levels (low, medium, and high voltage).

However, unlike CTs that produce secondary current proportional to the primary current, Rogowski Coils produce output voltage that is a scaled time derivative di(t)/dt of the primary current. Signal processing is required to extract the power frequency signal for applications in phasor-based protective relays and microprocessor-based equipment must be designed to accept these types of signals 

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