Dr Stan Zurek, MSc, PhD, MIEEE - Design engineer
In an ideal lossless transformer the input and output voltages and currents are linked to the number of turns of the primary (N1) and secondary (N2) windings, so that: V2 = V1 · n and I2 = I1 / n, where n = N2 / N1 is the transformer ratio.
If there is a single primary turn (N1 = 1) and multiple secondary turns (Fig. 1), then I2 = I1 / N2. Therefore, the output current is directly proportional to the input current but inversely proportional to the number of turns of the secondary coil.
This simple equation is the basis for the operation of current transformers (CT), which are used for accurate reduction of current. For example, if the input current is 1000 A, and the secondary coil has 1000 turns, then the output current will be 1 A. This can be safely and easily measured with an electronic circuit, whereas measuring a current of 1000 A directly would be much more difficult, and potentially much more dangerous.
Principle of operation
In most cases, transformers that are used primarily to step voltages up or down (voltage transformers) should be protected against short circuiting of the output, as this could create dangerous currents in the secondary winding. CTs are however, different – they work continuously under short-circuit conditions.
The current in the single primary turn (Fig. 1) generates magnetic flux F1 in the core. This flux penetrates the secondary winding and generates secondary voltage and current. Then, the secondary current generates magnetic flux F2 with such a direction as to oppose the primary flux, so the resultant magnetic flux in the core is very low. This balance can be sustained over a very wide operating range without entering into non-linear region of magnetic material.
Fig. 1. The concept of a current transformer
However, if the secondary winding were to be left open, then without the secondary current there would be nothing to oppose the primary magnetic flux. This would lead to saturation of the magnetic core with relatively low values of the primary current. Moreover, the saturation would result in a very high voltage generated in the secondary winding. In fact the voltage could easily be high enough to damage the electrical insulation of the winding and to become dangerous to the operator. There are also other harmful effects of saturation like increased power losses, heating, non-linearity and so on.
Therefore, the secondary winding of a CT must be always connected to a low impedance load, usually referred to as “burden”, whose maximum value is stipulated by the designer.
CTs are commonly used in conjunction with an ammeter or with a voltmeter and a suitable shunt resistor. The meter is connected directly into the secondary winding as shown in Fig. 1. Of course, the requirement for a low resistance load must be met at all times. Thus, in those cases where the meter is only connected while a measurement is being made, it is usual to provide a switch which is normally closed, and which is opened only after the meter is connected to the circuit. When disconnecting the meter the actions are reversed – first the switch is closed to short-circuit the CT, and then the meter/load is disconnected.
Fig. 2. 138 kV current transformer
When a CT is used, measurements are made without galvanic connection to the primary circuit. This is especially useful in high-power high-voltage applications. High-voltage CTs are designed to enclose the primary high-voltage circuit, and to incorporate appropriate insulation for the secondary as shown in Fig. 2. The grey bulk at the top of the device contains the actual magnetic core (notice the single conductor passing through it), and the large insulator provides protection against the high voltage of the primary circuit.
The high-voltage CTs are designed for continuous operation. They remain connected in the primary circuit even if not used, because their disconnection would mean powering down the transmission line.
If a CT is designed such that the core can be split, then the current measurement can be performed without disconnecting the primary circuit. One of the best examples of such a solution is the clamp-type CT (Fig. 3).
The passive device shown in the photo has the ratio of 1:1000 and its operating range spans over 6 orders of magnitude – the primary current can be anything from 0.5 mA to 1 kA. Because of this wide range, clamp-type CTs are widely used in applications where the primary circuit cannot be disconnected, typically to safety reasons. The accuracy for high currents can be better than 0.3% with a phase error of less than 0.5 degrees, so they can also be used for accurate measurement of power consumption. A low impedance burden, typically around 1 ohm, must, of course be provided by the external measuring circuit.
On the other hand, currents in the mA range can be also measured, as when testing the performance of an earthing system. The earthing electrode can stay connected and the leakage current can be measured without losing the protection or tripping a residual current device (RCD).
Fig. 3. The clamp-on current transformer (ICLAMP) retains its linearity for over 6 orders of magnitude from 1 mA to 1 kA
To ensure safe operation the CT can be fitted internally with a pair of back-to-back Zener diodes or a triac circuit, which are activated if the output signal exceeds 30 V or so. This is especially important in passive devices like the ICLAMP – the 1 ohm load must be provided by the external device, but the protection circuit is built into the ICLAMP, so it remains safe even if the load is not present.
clamp-on CTs often form an integral part of standalone meters, in conjunction with an LCD and battery power supply. Their size, design, current and frequency range depend on the application.
Sometimes, access to the conductor is difficult or the conductor is of such a large diameter that hand-held CTs cannot be clamped around it. If relatively high currents flow in the system then a flexible probe – a Rogowski coil – can be used. Rogowski coils (Fig. 4) are not typical CTs and their operation is slightly different. Their output must be electronically processed (see the interfacing box in Fig. 4) to obtain information about the primary current.
Nevertheless, their construction and application is very similar to the conventional CTs. The main differences are that they are flexible, and they do not contain magnetic core. The latter feature allows excellent linearity to be achieved, but at the expense of sensitivity, so Rogowski coils are used only for relatively high current applications (from 10 A to 10 kA).
Fig. 4. Flexible CT - Rogowski coil
CTs detect the resultant current enclosed by the core. For example, if a CT is clamped around a three-phase cable supplying balanced three-phase load, then the action of the currents would balance out in the magnetic core. In such a case the CT would not detect any current at all.
This is the principle used in RCDs, which are not activated if the currents are balanced. Leakage currents, however, cause an imbalance that the RCD is able to detect by behaving exactly as any CT would – it measures the primary current resulting from the imbalance. If this measured value is too large then the RCD trips out. Simple but ingenious!