RCTi

Rogowski coils provide a simple and easy way to retrofit current measurement. When the Rogowski measurement is connected to a power meter or energy monitor analysis of basic power consumption in an industrial process, or even more sophisticated analysis such as a power quality audit, can be undertaken.

The single channel RCTi or three channel version provides a 0-5Vac output, however PEM Ltd have supplied many 0 to 0.333Vac versions often the standard input requirement for power meters. Alternatively the RCT1A provides an isolated 0-1A ac output capable of replacing standard current transformers.

Rogowski coils can be used to measure relatively small currents at 50/60Hz and as well as higher harmonics from a few 100mA to several 10A’s. 

For example, in very large machine shaft diameters (up to 3m in some cases), Rogowski coils can be used to measure leakage currents that result from non-symmetrical magnetic fields in the motor (or generator) due to rotor asymmetry, or arising from capacitive coupling from the VSD or even electrostatic events close to the motor such as lightning or discharge from local coupled machines.

The RCTi can be used in a wide variety of HVDC operations. Ostensibly it is used to monitor the steady state AC current inside the valve/switch enclosures or on switch modules but it is also capable of monitoring large overloads with very high di/dt (over 1000A/µs) for protection purposes. The RCTi probe is particularly suitable for this application as there is no loss of accuracy or damage when monitoring these over-load surges and the coil can be small.

In several HVDC applications fully customising and miniaturising both the Rogowski coil and electronics provides for a low cost sensor.

In sub-stations connecting wind-farms to the grid Rogowski sensors can be used to continuously monitor current magnitude, power factor, harmonic strength and fault conditions. They can be sized to suit the installation and designed for outdoor use. They have accurate and predictable phase response over a wide frequency range. Similarly in grid connections to solar generation they can measure the fundamental with good accuracy, both current magnitude and phase but also have the ability to monitor wider frequency effects including load shifting, peak shaving and solar firming. 

Rogowski sensors can be permanently installed for long-term grid planning studies in places where it would be prohibitively expensive and unsafe to install conventional CTs on live circuits. 

For example Fort Collins Utilities, Light and Power Department has initiated an automation project to monitor co-generation facilities and control and monitor distribution load centres (switches and fuse locations) using the RCT probes in numerous locations ensuring exact real time currents may be obtained for long range planning studies and for analysing switching operations during outage restoration.

Although not able to measure the DC component of the current waveform Rogowski coils can accurately measure the shape and peak to peak magnitude of the input current waveform to a rectifier. This is achieved with a Rogowski sensor having a low frequency capability capable of minimising droop distortion and Rogowski coils with a predictable temperature drift and good positional accuracy.  

High-power rectifier applications are often bespoke so require a coil circumference, cable length to recording electronics and overall sensitivity (V/A) to be tailored to suit the installation.

The RCTrms is ideal for arc furnace applications where accurate measurement of large load currents, in the 10kA to 100kA range, are required. Conductors are often large and the RCTrms can be supplied with coil lengths up to 5.0m if necessary. This offers an attractive alternative to current transformers, which are impractical and expensive.

In one such application the RCTrms is used to monitor three individual secondary currents of a  submerged arc furnace transformer.  The output of the RCTrms 4-20mA signal is connected to a PLC for continuous monitoring. The secondary windings of the transformer are connected via multiple contact clamps to the electrodes. Continuous monitoring of the secondary currents using the RCTrms is important to ensure that the secondary currents are all of the same magnitude.  An imbalance in the transformer secondary currents can be used to immediately identify a failure in the electrode contacts. Without current monitoring, the contact failure may not be identified and can lead to reduced transformer lifetime due to localized overheating of the transformer windings.

Unlike other forms of current measurement the size of the sense coil (the Rogowski coil) is independent of the size of the measured current. Thus for large currents the size of the Rogowski coil can be chosen to suit the conductor diameter, whereas probes based on magnetic materials such as hall effect become increasingly bulky and expensive at larger current ratings. 

Over a 1000A range there really is no better alternative to a PEM Rogowski current probe.

In industrial induction heating applications a current sensor is often used to monitor both current amplitude and zero-crossing detection, typically providing the feedback signal to a voltage-fed medium frequency inverter. Measured currents range from 500A to 10,000A and the frequency varies from a few hundred Hz to 10’skHz. The phase error of the measurement needs to be tightly controlled for accurate zero crossing detection. The RCTi has a very small and highly predictable phase error over this frequency range and fits the application perfectly due to its easy installation, high reliability and small size.

Additionally Rogowski probes are useful for diagnostic work in developing and maintaining induction heating coils and power converters, where fast switching frequencies, or hf resonance demand higher frequency performance than the RCTi.