Rogowski Coil based Current Sensing Technology

Abstract: This Technical Note describes Rogowski Coil based current sensing, which serve in the role of current transformers for measuring power system currents for protective relaying or other measurement purposes. Rogowski coils offer a number of application advantages which are being presented here along with comparison with Conventional Iron-core CTs.

Index Terms: Current Transformers, Limitations of Conventional CT, Rogowski Coils, Advantages of Rogowski based sensing technology, comparison of Rogowski coil with Iron Core CTs.

Current transformers are devices used to scale large primary currents to a smaller, standardized, non-hazardous level, easy to measure, secondary currents. Like a traditional voltage transformer, the ratio of the windings determines the relation between the input and output currents. Current transformers of various shapes and sizes are used as an interfacing solution between high currents and instrumentation devices. A CT utilizes the strength of the magnetic field around the conductor to form an induced current on its secondary windings. This indirect method of interfacing allows for easy installation and provides a high level of isolation between the primary circuit and secondary measurement circuits.

Thus, their purpose can be broadly seen as:

• Measurement of AC currents & their metering for energy billing & transaction purposes.
• Protection & Control (for system protection & protective relaying purposes) in power industry.
• Isolating measuring instruments from high voltage/current circuit.

With CTs, relays & measuring instruments can be made relatively small & inexpensive.

Construction-wise CT can be classified into:

1. Bar Type
2. Wound Type
3. Window Type (Ring-type)

Window type CTs are most preferred CT in LV & MV Switchgear owing to their Simple construction & cost benefits.

Construction-wise, a CT consists of

1. A core which can be ferromagnetic (iron) or air core
2. Primary winding
3. Secondary winding of multiple turns

CTs work on the principle of Faraday’s law of Electromagnetic Induction.

In window/ring type, CTs are installed around the main conductor allowing it to pass through its center (window).

Thus, the main conductor acts as a single turn primary winding & secondary winding is wound on the ferromagnetic core & consists of multiple turns.

In many applications the use of traditional iron core transformers (CTs) may be limited by both electrical characteristics and mechanical design. These are listed as follows:

1. Iron core CT uses ferromagnetic core which suffers from the problem of saturation. This in fact is the biggest limitation of conventional Iron-core CTs. Result is nonlinear characteristics in the excitation curve.
2. CT secondary if left open at the time of service induces very high voltage at the secondary terminal which will result in insulation failure & will be detrimental to both the equipment & personnel.
3. In low current measuring systems, CT’s exciting current can become comparable to the main current to be measured. Thus, the CT will start acting as load to the main circuit instead of burden & will create unnecessary loading of the main circuit. Also the measuring accuracy becomes poor.
4. Because of magnetizing current, the O/P is not perfect transformation of I/P current & is phase shifted.
5. This is because of Ratio error & Phase error. Collectively they are called as Transformation errors.
6. Same CT cannot be used for both Protection & Metering without compromising on the Accuracy.
7. It is difficult to install in the crowded panels & tight spaces.

Many of the problems stated above can be overcome by the use of Rogowski Based current sensing Coils.

To overcome Saturation & Linearity problems, Rogowski coils have been used in current measurements. It is named after Walter Rogowski. Rogowski Coils have been used in Power Industry since 1975.

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 ore, 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.

Construction-wise they are classified as:

1. Rigid Rogowski coils (SRC)
2. Flexible Rogowski Coils (FRC)

Rogowski coil consists of a coil, a non-ferromagnetic/air core (rubber/plastic) & an electronic integrator.

Air-cored coil is placed around the conductor in a toroidal fashion & the magnetic field produced by the current induces a voltage in the coil. This is very low owing to the low mutual coupling due to air core.

To complete the transducer, this very low voltage output from coil is Integrated electronically & signal conditioned to provide an output that accurately reproduces the current waveform.

The wire loop (secondary winding) may be configured as single turn, simple helix, toroid, rectangular, etc.

One characteristic of this configuration is coaxial routing of the coil end back to beginning. This ensures that the coil is sensitive to magnetic fields that are produced inside the coil only (i.e. by the main primary conductor) & is free from any interference.

It is important to ensure that the coil winding is as uniform as possible.

O/P from integrator can be used with any form of electronic indicating device that has an i/p impedance > 10kΩ such as voltmeter, oscilloscope or Protection & Control Unit (Release).

Rogowski coil operates on Faraday’s law of emi & Ampere’s circuital law. Operating principle is that if an air-cored coil is placed around the conductor in a closed path, the magnetic field produced by the current induces O/P voltage E which is proportional to the rate of change of current encircled by the coil.

O/P signal from a Rogowski coil is different than in the case of Iron core CT in following manner:

• The O/P from Iron Core CT is secondary current proportional to the primary current
• The O/P signal from Rogowski coil is a voltage proportional to the derivative of primary current

The O/P of rogowski coil is connected to an integrator & signal conditioned to reproduce the current waveform.

Expression for O/P Voltage is given by:

E = -M di/dt, where M is the mutual inductance primary & secondary.

This O/P of rogowski coil is connected to an integrator & signal conditioned to reproduce the current waveform.

Rogowski coils overcome all the limitations of Iron core CTs without compromising advantages of the same:

• Electrical Isolation
• The Rogowski sensor detects current without any contact to the conductor.
• Measurement, Protection & Control of Power System
• Primarily used in metering & protection applications of Electrical Systems.
• Low power consumption & losses
• No significant power consumption is needed for current sensing.
• Low temperature shift
• The output varies very little with changes in temperature.
• No Magnetic core --> No Saturation
• Characteristics are inherently linear --> Wide dynamic range & an excellent transient response.
• Light Weight
• Rogowski is air core coil. Absence of ferrite cores makes it much lighter.
• Low Cost
• Even a small sensor can accurately detect hundreds of Amps of current.
• No core losses
• Due to absence of core, it is free from core losses unlike conventional CTs.
• Easy Calibration
• Can be calibrated at low currents unlike Iron-Core CTs where it was calibrated at rated currents.
• No phase /ratio error (Transformation errors)
• Absence of magnetizing current rules out possibility of transformation errors.
• Higher accuracy & smaller size
• Accuracy is better for wider range. Smaller size makes it easy to install & to transport.
• Non-Intrusive
• Coil does not load the main circuit which is carrying the current to be measured.

• Single CT can be used for both Metering & Protection
• There is no need for calculation of accuracy limit factors owing to the linearity
• Terminal blocks are not required for connection to relays as Rogowski coils can be open-circuited without any danger
• Enables improved switchgear design by reducing switchgear dimensions & improving metering, protection & control performance

APPLICATIONS of ROGOWSKI COIL based CTs:

Accurate measurement of Power, Protection of Electrical Equipment at:

• Power generation, Electricity Transmission & Distribution
• Railways, Industrial Equipment
• Wind & Solar Renewable application

Monitoring operations of Power Grid

Switchgear Industry & Relay Protection

Any industry where current/power measurement is required, Rogowski Coils can be used

In today’s scenario, majority of Industries & Switchgear Manufacturers are using Rogowski Coils

• Ehlert G., “Flexible Air-Core Toroidal Current Transducers Overcome Iron-Core Current Transformers Limitations”, “PCIM” PP 59-63, June 1995
• Using Rogowski Coils for Transient Measurements by D.A Ward & J.La T. Exon
• Rogowski Coils – David Ward, Rocoil Limited
• Application of Rogowski Coils - Lj. A. Kojovic, Senior Member, IEEE
• Current Transformer Application Guide – WECC, June 1989
• Rogowski Coil Current Sensing Technology - David E. Shepard, LEM DynAmp Inc
• IEEE Standard C37.235™-2007, ― “IEEE Guide for the Application of Rogowski Coils Used for Protective Relaying Purposes”