Introduction to wideband current transformers

	RETURN TO HOME PAGE
INTRODUCTION
Lilco current transformer measuring the short circuit output current of a pulse generator
Current measurement with electrical isolation The principal advantage of using current transformers to measure current in electric circuits is that the measurement circuit can be electrically isolated from the circuit under test. This non-contact approach to current measurement greatly facilitates the simultaneous measurement of current and voltage in different branches of a common circuit. Isolation can be a particular benefit where high voltages are present.
Negligible Burden Current transformers impose a negligible burden on the circuit under investigation and their use has an insignificant effect on the performance of the circuits examined.	Passive Device They are passive devices. Their ability to operate without a power source facilitates their permanent installation in equipment and avoids distortion that could be introduced by electronic amplifiers.
Low-frequency equivalent circuit Is represents the secondary current given by Ip/N where Ip is the primary current and N the turns ratio RW is the resistance of the secondary winding RB is the transformer burden L is the inductance of the secondary winding RM is an hf matching resistor
In its basic form the transformer consists of a toroidal core with a wound secondary which is shunted by a resistor of low inductance. In the terminated model, this resistor is supplied within the case of the transformer. In ring-type models the customer supplies the resistor RB. The primary is made when the conductor is placed through the aperture by the user. No direct connection exists between the primary and secondary electrical circuits. The simplified transformer equivalent circuit is helpful in understanding the operation of the instrument.
The core is selected to have negligible losses under its operating conditions. The winding resistance can have an important effect on the low-frequency cut-off point, though its effect over the major part of the instrument's operating range may be ignored. Current flowing in the primary winding induces a current in the secondary circuit. The current flowing in the load resistor is much lower than the primary current because of transformer action. Over the operating range, the output voltage is the product of Is and RB and is therefore proportional to primary current.	The matching resistor is used to match the output impedance to 50 ohms. The output resistance of the broadband internally-terminated instruments is matched at the time of manufacture. The matching resistor may not be required at the lower operating frequencies of the ring-type transformers. The toroidal structure maximises the magnetic efficiency and reduces electromagnetic pick-up. The case of the terminated transformers forms an electrostatic screen. In the ring-type transformer the primary conductor should be screened if pick-up is likely to be problem
Further technical information on the operation of broadband current transformers may be found in: B. Cordingley, 1998, "Wideband Terminated Current Transformers for Power Electronic Measurements", Power Electronics and Variable Speed Drives, IEE Conf. Pub. No.456, pp 433-436.
Superior accuracy and hf response The company supplies both terminated transformers and unterminated ring-type cts. The terminated transformers have a superior accuracy and higher frequency response than other current transducers which operate on the principle of magnetic coupling, such as Rogowski coils and Hall-effect devices. Accuracy is +/-0.5% over much of the operating range. Lilco terminated transformers are available with frequency responses in the range 0.1Hz to 100MHz.Ring-type transformers are supplied unterminated and are an economically-priced alternative to the terminated range of transformers for some duties. They are suitable for use in applications where the excellent high-frequency response of the terminated transformer is not required. Frequency response is in the range 0.1Hz to 2MHz. In low-current pcb-mounted models the upper frequency limit can be as high as 15MHz.
Comparison of the performances of a transformer and a non-inductive shunt when measuring a pulse current. Upper trace: A current transformer monitors the discharge of a pulse forming network by a fast-switching thyristor, 100A/div., 1us/div. Lower trace: Same measurement but performed with an 0.1 ohm non-inductive shunt terminated for 0.05V/A or 100A/div. Notice should be taken of the fidelity of reproduction and the absence of phase error in the comparison.
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.
Simultaneous measurements 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
TOP OF PAGE	RETURN TO HOME PAGE