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UK Design and manufacture of wideband current measurement transformers and wideband voltage measurement transformers

Comparison of transformers with resistive shunts, Rogowski coils and Hall-effect transducers.

a) Resistive shunts

Current flow may be determined by measuring the voltage drop across a resistor connected in series with a circuit. Resistors have a parasitic inductance and stray capacitance associated with them. The ratio of parasitic inductance to pure resistance determines the high frequency limit of the measurement. Increasing the value of resistance to reduce this ratio can cause dissipation and insertion loss problems.It is not easy to produce a low-resistance shunt capable of matching the high-frequency performance of broadband transformer, even with the aid of coaxial techniques. The resistor also suffers the disadvantage of not providing isolation from the circuit under test. If this is not important, then resistive shunts can prove useful for the measurement of low current for frequencies stretching from dc into the MHz region or, for higher currents, to a few hundred kHz.

Coaxial resistive shunts with very low values of parasitic inductance may be used at moderately high currents to frequencies up to about 20MHz. Arguably, the principal advantage of a current transformer is that it provides isolation from the circuit under test.  It also imposes negligible burden on the circuit.Lilco terminated transformers can be operated to 100MHz  and beyond and some unterminated transformers to about 10MHz

A transformer will not measure the dc component of a waveform nor, except in the special case of air-cored transformers, operate in the presence of high dc. However, some designs will operate with dc currents up to several hundred amps.

Thyrtest

Simultaneous measurement of trigger voltage and anode pulse current
in a fast turn-on thyristor

b) Rogowski coils

Rogowski coils can be used to provide isolated measurement of current and can prove useful alternative to a current transformer where the current carrying conductor is not easy to encircle by a cored transformer or where very strong dc component is present.    They can also be useful for measuring pulses with a high I-t product, say greater than 100As. Rogowski coils are not so suitable for application which require a broad bandwidth, high precision and minimal phase shift at high frequency.

Broadband Rogowski coils contain an electronic integrator which requires a power source and the output can be subject to drift. The performance of powered versions of the coil can be exceeded by transformers both at the high and low frequency end of the spectrum. The precision that can be achieved with the Rogowski coil is generally less good than  that obtainable with passive transformers.


c) Hall-effect devices

Current transducers based on the Hall-effect device can measure currents from dc to about 100kHz. They are useful where an isolated measurement of current which includes a dc component is required. They require a power source and precision and phase shift are not as good as can be achieved with a transformer. A ring-type transformer should always be considered for ac current measurement where cost, isolation and absence of a power requirement are of prime considerations.

Updated    October 2017