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Rogowski Coil and IED

Rogowski Coils can easily replace conventional current transformers in protection, metering, and control applications. They can be 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.

Rogowski Coils in protective relaying in electric power systems
Rogowski Coils in protective relaying in electric power systems (on diagram: Relay protection zones and RC locations)

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.

Figure 1a shows protective relaying principles using a Rogowski Coil directly interfaced to an Intelligent Electronic Device (IED), and Figure 1b shows solutions described in Standard IEC 60044-8 for Electronic Current Transformers (ECT).

The primary converter module represents signal processing circuitry which may be placed in the immediate vicinity of the Rogowski Coil, and used to amplify, convert, or encode low level signals prior to transmission.

Depending on the design, the primary converter may be located at the high voltage (line) potential, and may use optical fibers for signal transmission and HV insulation. The primary Converter Power Supply from Figure 1 may need to be floated at the HV potential (along with the primary converter). The actual point-of-use of the Rogowski Coil signal may be at or after the secondary converter module.

The link between the primary and the secondary converter may be proprietary.

Application of Rogowski Coils used for Protective Relaying Purposes
Figure 1 – Application of Rogowski Coils used for Protective Relaying Purposes

Current transformers require heavy gauge secondary wires for interconnection to relays and other metering and control equipment (Figure 2). For example, Figure 2 shows a 2000/5 A, C800 class CT connected to a relay. The wire resistance adds to the CT burden and negatively impacts the CT transient response and may cause CT saturation at high fault currents.

In addition, terminal blocks are required so the CT secondary can be shorted. Hazardous voltages can be generated when the CT secondary circuit is opened while load current is flowing.

This CT has the core and winding height of 10 cm and weighs 90 kg.

Current Transformer Connections to Relays
Figure 2 – Current Transformer Connections to Relays

Rogowski Coils may be connected to relays via twisted pair shielded cables with connectors (Figure 3). Terminal blocks are not required since the coil output signal is a minimal voltage from the safety aspect, and this voltage does not increase when the secondary circuit is open.

Figure 3 shows Rogowski Coil width and weight are much smaller than that of a CT. This coil has the same size window as the CT from Figure 2, but can be applied to a significantly larger current range than the CT.

Rogowski Coil Connections to Relays
Figure 3 – Rogowski Coil Connections to Relays

Cable Shielding. Rogowski coils and cabling should be shielded to prevent capacitive coupling to the high-voltage primary conductors and to minimize the influence of high-frequency electromagnetic fields (EMC environment). Cable shielding methods are provided in [2].

Length of secondary cables that can be used for interface of Rogowski Coils with relays depends on the measured signal levels, cable shielding, and environmental conditions. Reported distances used in actual projects are up to 300 meters to transport Rogowski Coil analog signals without amplification.

Voltages in secondary cables are small (even for fault conditions), so any number of cables can be installed in the same conduit without impact from each other.

Volt/Current Sensor Interface to Relays
Figure 4 – Volt/Current Sensor Interface to Relays

Figure 4 shows an example of a system configuration using electronic instrument transformers (EITs). The electronic current transformers are based on the principle of a Rogowski Coil. The electronic voltage transformers are capacitive voltage dividers. Sensing Units (SUs) are arranged near the Rogowski Coils and capacitive voltage dividers on each bay. One Merging Unit (MU) is provided. Each SU is connected to the MU by optical fiber.

The merging unit is connected to the process bus by optical fiber. To ensure high reliability, the system included duplicated Rogowski Coils, SUs, MUs, and process bus. Only capacitive voltage dividers were not duplicated.

The EITs were designed based on IEC 60044-7 and IEC 60044-8.

Title:Why and where Rogowski Coil current sensors are favorable when compared to CTs – IEEE PSRC Working Group members: Ljubomir A. Kojovic (Chair), Robert Beresh (Vice-Chair), Martin T. Bishop, Radek Javora, Bruce Magruder, Peter McLaren, Brian Mugalian and Arnold Offner
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Why and where Rogowski Coil current sensors are favorable when compared to CTs
Why and where Rogowski Coil current sensors are favorable when compared to CTs

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  1. Dragan Tabakovic
    Mar 30, 2021

    If we’re talking about Magnetic potentiometer that was originated by Chattock and for a different application

  2. Senad Resic
    Feb 13, 2021

    Please make the right assessment before making a decision to use the “Magnetischer Spannungsmesser” in important power supply facilities. In laboratories it is okay, but in substations…
    Signal amplitude from the measuring coil is increased by presence of higher harmonics in the primary signal (i.e. increased di/dt) and the output signal of the measuring coil is delayed by ¶/2. To compensate these errors, the output signal from the measuring coil has to be integrated in the integrator. Amplitude/phase characteristic of the “signal integration device” is essential and has to be carefully evaluated, especially if more significant values of higher harmonics are present in primary conductor.
    Unwanted tripping (i.e.) mistripping may cause unnecessary economic loss to electricity suppliers. This loss may be sometimes higher than value of all devices in the substation. Take care!

    • Senad Resic
      Feb 13, 2021

      To the editor: please correct “¶/2” to “90° in 50 [Hz] systems”. Thanks!

    Sep 29, 2020

    I’m very thankful. keep up the good work

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