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Low-frequency cut-off
The frequency at which the output falls to a level which is 3dB below the true signal value determines the low frequency limit of performance. This point is a function of the total secondary load resistance. In a terminated transformer the use of a 50R terminating resistor will extend the low-frequency cut-off point. Reducing the value of the secondary load resistor in an unterminated design will similarly extend the performance of unterminated models.
If low-frequency signals are to be monitored on an oscilloscope then dc coupling should be selected to ensure faithful reproduction. It should be noted that the level of any dc component in the signal is not recorded by the instrument. In many applications the output corresponding to zero current is readily apparent
High-frequency cut-off
The position of the upper 3dB point is determined by the transformer’s stray inductance and capacitance. The action of these parasitic components varies with model. The cut-off point is marked by a 3dB deviation of the signal from the true reading. Unlike the lf point which invariably corresponds to a reduced signal, the upper cutoff is often marked by an enhanced output.
For accurate measurements at high frequency, or where long cables are in use, it is desirable to terminate the receiver end of the cable with 50R. The terminated broadband transformers are screened against electrostatic pick up. At higher frequencies and at large potential differences, electrostatic pickup may cause measurement difficulties in unscreened designs. Any interference resulting from pick up may be reduced by screening the primary conductor.
Problems may be experienced as a result of current flowing in the screen of the coaxial cable under some circuit conditions. This current flow may be attenuated by clamping a ferrite core around the cable.
Rectangular waveforms are rich in high-frequency harmonics. If it is required that the fast leading and trailing edges of the waveform are displayed with precision a current monitor of adequate bandwidth must be selected.
Biasing
The dc component of a signal increases the flux within the core and is capable of affecting its permeability. At a sufficiently high level it will reduce permeability and degrade low frequency performance. A dc component can cause signal distortion, an increase in low-frequency cut-off point and a reduction in I-t capability and droop performance. The core recovers once the dc component is removed or reduced.
A dc bias may be applied to the core in such a manner as to offset a dc component of current in the primary conductor. Bias may also be used to increase the available flux swing and thereby increase I-t capability.
An offset dc bias is applied by adding one or more turns through the aperture of the transformer and setting the current to an appropriate value. It should be noted that the ac component of the signal to be measured will induce a voltage in the auxiliary winding. The impedance of the current source should be great enough to prevent a significant level of induced current flow. An inductor or resistor of greater than two ohms should prove adequate for all terminated models. Bias current may be reduced by increasing the number of turns on the auxiliary winding. This will necessitate a proportionate increase in the impedance of the bias supply.
Burden
Defines the VA rating of the design secondary load at rated frequency. The performance of a current transformer is likely to be satisfactory at lower values of burden. See also Resistive load.
Class
This term is used to describe the precision of current transformers primarily designed for power frequency operation and is described in BS7626:1993.
Current, maximum rms current, Irms
The maximum rms current is limited by heating effects in the monitor. Exceeding this value can cause a temporary loss of calibration accuracy, though permanent damage may result from extreme or prolonged overloading.
Current, Peak, Ipeak
Peak current capability is dictated either by the voltage breakdown limit of the secondary winding, or in the case of terminated transformers, the current that will damage the terminating resistors. Currents in excess of Ipeak can cause permanent damage to the monitor.
Current, primary, Ip
The design rms primary current carried by a conductor passing through the aperture of the transformer.
Current, saturation, Isat
DC current has the tendency to saturate the core. This can have an adverse effect on low-frequency parameters such as droop and I-t capability, though it may have comparatively little effect on high-frequency performance. The value given for Isat is the approximate dc current that will cause the lower 3dB cut-off point to double in frequency. Some Lilco transformers are designed to operate satisfactorily in the presence of high levels of dc. Saturation of itself does not permanently damage the transformer.
Current, secondary
The current flowing in the secondary at rated primary current. The flow of secondary current for a given primary current remains essentially constant for burdens or load resistance values from zero to rated value.
Droop
When measuring rectangular current pulses, the recorded current decays exponentially from its initial peak value. Droop is a measure of the deviation of the recorded output from the true signal. It is defined as the initial percentage fall of current per unit time.
In many applications the rate of decay of current with time will be too small to be detected. Droop characteristics may be improved, albeit at the expense of sensitivity, by use of a terminating resistor or reducing the secondary burden.
Ipeak/f
Ipeak/f is the maximum ratio of peak sinewave current to frequency that can be handled without distortion. It is the sinewave equivalent of the I-t restriction imposed in the case of rectangular waveforms and is limited by core saturation.
I-t Capability
This value, the product of current and time for a rectangular waveform, serves as a guide to a transformer’s ability to withstand saturation effects. If this value is exceeded then there is a possibility that the core will start to saturate. Under saturation the low-frequency characteristics of the core become impaired and the output waveform will be seen to drop rapidly. The transformer recovers once the magnetic flux is reduced to pre-saturation level. This parameter need only be considered for currents is excess of Isat.
I-t capability may be increased by applying a dc bias so as to oppose the saturating effect of the current being measured. Setting the bias level approximately equal to Isat will produce optimum results. I-t capability may also be improved by reducing load resistance. In many internally-terminated broadband transformers the use of a 50R terminating resistor will effectively double I-t capability. Similarly, the I-t performance of unterminated transformers may be improved by reducing the value of the resistor or load connected across the secondary.
In some applications, the pulse repetition rate is such that the core does not recover sufficiently after each pulse to possess sufficient I-t capability to monitor a following pulse. The use of an asymmetrical load with an unterminated transformer can extend the instrument’s capability.
Insertion resistance
The presence of a current transformer places a burden on the circuit being monitored which can be thought of conveniently as inserting a series resistor, The value of the resistor is usually too small to have any noticeable effect. For example, in 50 ohm terminated models with 1V/A sensitivity the maximum value of insertion resistance is less than 0.04 ohm and a model of 0.01V/A sensitivity the equivalent insertion resistance is less than 0.00001 ohm.
Phase shift
The phase shift between signal and current for frequencies in the range of one decade of the lf cut-off point is less than 6 degrees. Within two decades of this point the phase error drops to one tenth of this value and may be neglected for most practical purposes. Another source of phase error can arise at high frequency due to transit times for signals in connecting cables. Cables of equal length should be used where phase relationships are important.
Price
The small quantity price of the transformer before carriage and taxes.
Resistive load
The value of resistance that may be used as a burden to achieve the described sensitivity and low-frequency characteristics. The current in the secondary circuit may be considered constant at values of load resistor below that specified and reduction of load resistance can enhance some low-frequency characteristics, such as I-t capability, droop and lf cut-off point at the expense of sensitivity. Reducing the value of load reduces power dissipation. For high-frequency use the ratio of parasitic inductive reactance to resistance of the load needs to be considered, planar resistor structures found in some thin film, thick film and bulk metal foil technologies are preferred to spiral and wire wound structures. Suitable resistors of inductance less than 10nH are generally available and can be supplied by Lilco on request
Note: Current transformers should not be operated with the secondary open circuit as very high terminal voltages may be present. Operation under this condition may permanently damage the transformer and constitute a shock hazard.
Rise-time
At its high frequency limit, the current may exhibit ringing or roll-off effects. If the leading edge of a fast-rising waveform exceeds the rise-time specification, there is a risk that the output signal will deviate from the correct value and cause an error in the transient reading. The figure given in the published data is the 10% to 90% rise time of the current which will not cause more than a 10% deviation.
Overshoot may cause ringing in some circumstances. The effects of ringing may be removed by reducing the bandwidth of the receiver below that of the high frequency response cut-off point. Alternatively, bandwidth may be reduced by inserting a low-pass filter in the signal circuit.
Sensitivity
Sensitivity is a measure of the instantaneous voltage achieved per unit current in a conductor passing through the aperture.
In terminated transformers the sensitivity is calibrated to +/-0.5% at a frequency which is the geometric mean of the low- and high- frequency cut-off points. The accuracy at frequencies within one decade of the 3dB cut-off points is +/-2% The tolerance is valid when the output is fed into a receiver of high in put impedance (>50k). If the receiver has an input impedance of 50R, the sensitivity will be half that recorded in the data. It should be noted that the accuracy of the resistance value of the termination and transmission losses in the connecting cable will have an effect on precision.
In unterminated units the sensitivity is determined by the ratio of measurement resistance to current transformation ratio.
Sensitivity may be effectively increased by threading the conductor through the aperture several times.
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