PMDT HFCT Sensor
High Frequency Current Transformer (500KHz - 50MHz) When PD occurs, there are small current pulses that are induced onto the ground shield or case ground. These pulses will travel dozens of meters along the ground grid in the form of high frequency current pulses in the range from 500kHz to 50MHz, Usually centered near 10MHz. High Frequency Current Transformers are a reliable method to measure these high frequency PD pulses. These pulses spread out onto the ground grid like ripples in the water. They are especially useful for quickly testing for internal PD in a large area such as an entire power transformer or an entire cable, or even an entire substation.
The HFCT sensor has a split core and so it’s simply clamped around a low resistance grounding lead. HFCT sensors have a distinct advantage of being able to detect PD signals on cables from long distance up to 1km away (this limit depends on the type of ground shield and the strength of the PD pulse). This means multiple PD signals and noise signals can also be detected from many points on the ground grid where a ground lead is exposed. In cable PD testing, the localization of PD via the HFCT sensor is achieved by determining the time difference between the arrival of the initial PD pulse, and the reflected pulse which has bounced off of the opposite end of the cable from where you are testing. The speed of the pulses on the ground shield is similar to the speed of light. Exact signal speeds are known for various cable types. Knowing the speed, the cable length, and the time difference of arrival between the initial pulse and the reflected pulse enables the calculation of the distance to the PD. Online Cable PD testing instruments have functions to identify these pulses and perform the distance calculation. PDiagnostc instruments have the sampling rates required for this function.