About pulsed current transformers

Analysis of magnetic saturation problems

In current test and measurement, pulsed current transformers have long been a good solution for AC current and pulsed current measurement, featuring fast response and high accuracy; problems concerning them often warrant attention. Here we discuss points to note in the use of pulsed current transformers, namely avoiding magnetic saturation that can burn out the transformer.

 Pulsed current transformer test principle

Before discussing magnetic saturation problems, we must first understand the test principle of the pulsed current transformer. The operating principle of the pulsed current transformer is the basic principle of electromagnetic induction; when the measured current passes through the pulsed current transformer, it generates a magnetic field in the soft magnetic core, and the change in the magnetic field generates an induced current. By measuring the induced current, the magnitude of the measured current is indirectly tested.

In this, the soft magnetic core serves as the intermediary that senses magnetic field changes and converts them into induced current, and is the most important part of the pulsed current transformer; therefore, once magnetic saturation occurs in the core, the entire pulsed current transformer will directly become inoperative.

Factors affecting magnetic saturation

There are three main parameters of the pulsed current transformer that affect magnetic saturation:namely current measurement peak value, maximum I·t ampere-second product, and maximum AC RMS value.

1. Current measurement peak value

The current measurement peak value is the maximum pulsed current that the pulsed current transformer can test, and also the maximum range of the transformer.When the measured current exceeds the maximum range of the transformer, magnetic saturation occurs.

2. Maximum ampere-second product I·Tmax

The maximum I·t ampere-second product is the maximum integral value of the amplitude of the current tested by the transformer over time; when the integral value of the amplitude of the measured pulsed current over time exceedsthe maximum I·t ampere-second product, magnetic saturation occurs.

3. Maximum AC RMS value (rms)

The maximum AC RMS value is the maximum AC current tested by the pulsed current transformer, and also the maximum AC range of the transformer.When the measured AC current exceeds the maximum AC RMS value, magnetic saturation occurs.

4. Special case notes

In test applications, pulsed current transformers sometimes encounter test problems; improper handling can cause magnetic saturation, causing them to burn out directly and become unusable. The special cases summarized are as follows:

a.In discharge test application scenarios, the test current exhibits fluctuations of positive and negative pulse currents. Some may think that the magnetic fields generated by the positive and negative currents at this time will cancel each other out and will not affect the magnetic core. However, this is not the case. First, because the soft magnetic material of the core is not an ideal soft magnetic material, it cannot achieve a 100% excitation response;

Moreover, generally speaking, the positive and negative pulse current waveforms cannot be made identical; a residual magnetic field will exist in the soft magnetic material, and magnetic accumulation will occur,When the magnetic accumulation reaches the maximum limit of the soft magnetic material, magnetic saturation will occur.

b.When testing continuous pulses, the duty cycle of each period must be calculated，If the duty cycle is too large, the magnetic accumulation caused by continuous pulses can very easily cause magnetic saturation of the current transformer；If the duty cycle is very small, it can generally be used，However, continuous pulses will also affect the service life of the pulsed current transformer to a certain extent.

If discharging for a long time, or when measuring continuous pulses, it is recommended to select a Rogowski coil,

because the Rogowski coil has a coreless design,it will not exhibit magnetic saturation and is more user-friendly in use.