Why and where Rogowski Coil current sensors are favorable when compared to CTs

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.

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).

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.

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.

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.

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.

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.

Suggested Guide – Good practices in the design and installation of a facility ground system

Good practices in the design and installation of a facility ground system