Case Studies

Location: UK
Industry: Power Transmission

The Challenge

To measure the current in each of the four legs of a transmission tower to determine the total current to earth so that the earth resistance can be determined.

Our Solution

We injected a voltage from a power supply. We configured a Rogowski coil measuring system so that four current measurements could be summed algebraically giving the total current and this enabled the resistance to earth to be calculated. The earth resistance measurement was made at several frequencies.

Location: Finland / UK
Industry: Electrical Installations

The Challenge

To check the calibration of current transformers on site without the need to remove them to a calibration laboratory.

To provide a portable precision current-measurement standard which can be clipped on to the CT wiring without disconnecting any conductors.

Our Solution

Rocoil rigid coil systems can be calibrated to an uncertainty of 0.1%. They can also be produced in a clip-on version without loss of accuracy. These can be used as reference standards for calibration.

Location: Brazil
Industry: Petroleum

The Challenge

To measure the leakage current of a few mA in an SCR when it is turned off. The measurement should survive the current surge of about 60kA when the device is turned on. The same coil to be used for measuring both currents.

Measuring the leakage current requires a sensitive system. The input stages of the sensitive integrator must be designed to withstand the large coil voltage caused by the current surge.

Our Solution

Rocoil engineers produced a solution with two integrators connected to the same rigid coil. The 'leakage current' integrator was designed using resistors that had the watt rating to withstand the coil output at 60kA. During the current surge the integrator would overload and saturate but recover as soon as the current was reduced. The 'current surge' integrator was designed to make accurate measurements at the larger current. This application illustrates the linearity of a Rogowski coil where the same coil can be used to measure a very wide range of currents.

Location: Wales
Industry: Generator Manufacture

The Challenge

To measure a very large current transient with an asymmetric waveform.

Transients are difficult to measure with a conventional current transformer and the usual method is to use a resistive shunt which can be difficult to install and may require modification of the generator output connections.

Our Solution

We designed a Rogowski coil to measure very large transient currents. It was compact and was installed on the output connections with no significant disruption to other work on the generator. It was left in position until it was convenient to make the short-circuit measurements.

Location: Germany / Australia / Tasmania
Industry: Power Station

The Challenge

Currents flowing along the shaft of a generator or motor can cause damage to the bearings. There is a need to monitor these currents.

To measure a low current, less than 1A, in a very large conductor which could be 1m or more in diameter.

Our Solution

Rocoil flexible Rogowski coils can be made in lengths up to at least 12m. A flexible coil round the shaft used with a sensitive integrator can measure currents with a sensitivity of a few mA. The success of these measurements depends on very careful screening of the coil and integrator.

Location: UK
Industry: Power Station

The Challenge

To measure large currents at a low frequency (~1Hz) to check the performance of the regulator which was controlling a boiler feed pump motor at a power station.

Motor problems, such as excessive vibration, can be difficult to investigate because they can have both electrical and mechanical causes. Accurate measurement of the slip-frequency currents can help with the diagnosis.

Our Solution

The slip-frequency currents can be measured accurately using Rocoil rigid coils which have very good low-frequency response. The measurements can be used with a protection system to monitor phase imbalance and earth faults.

Location: Germany / Austria / Spain / Turkey / India / China / Indonesia / USA
Industry: Steel Industry

The Challenge

To measure the current into an arc furnace which can be of the order of 100kA or more. These measurements are used to ensure efficient operation of the furnace.

To measure the actual electrode current where there is limited space and not sufficient room for a current transformer.

Our Solution

Rocoil flexible coils can be installed where there is limited space. Special techniques developed by Rocoil ensure that measuring systems can be calibrated accurately even for the largest currents.

Location: China
Industry: High-Current Test Laboratory

The Challenge

Testing switchgear and fuses to check their correct operation under high-current fault conditions.

To measure large current transients at mains frequency which last only a few cycles. These transients frequently have large 'DC offsets' which need to be measured accurately.

Our Solution

High-current measurements can be made with both flexible and rigid Rogowski coils. Special calibration techniques developed by Rocoil ensure that measuring systems are accurate. Rigid coils are calibrated with an uncertainty of 0.1% even for large currents. The integrators are designed to have very good low-frequency response so that the DC offsets can be measured correctly.

Location: UK / India / Brazil
Industry: Rail

The Challenge

Currents of a few amps in the range 1 - 2 kHz are an essential part of the railway signalling system for determining where the trains are. When there is a fault the trains cannot run and it is important to find the fault with a minimum of delay.

Provide a current transducer to measure signalling currents in a railway line that can be moved around easily to trace the currents.

Our Solution

A U-shaped Rogowski coil was developed which could be simply dropped over the rail to make a measurement. This was a battery-powered unit and the integrator incorporated a high-pass filter that would allow measurement of the higher-frequency signalling currents but reject 50Hz signals which could cause interference.

Location: UK / USA
Industry: Aircraft Test Facility

The Challenge

Certain components, notably aircraft parts, are tested using artificial lightning to check their integrity.

To measure a current pulse of the order of 20kA lasting about 10µs.

Our Solution

For measuring large, fast currents Rocoil has developed special low-output coils. These coils have been used with suitable integrators to provide an accurate and effective measuring system.