PMDT UHF Sensor
UHF Ultra High Frequency (300MHz - 1500MHz) (RF/radio/microwave range) - Every time a PD event occurs, electromagnetic waves are transmitted from the site of the defect. This emission may also be called RFI or EMI. (Radio Frequency Interference or Electro-Magnetic Interference) The PD emission is a broadband electromagnetic emission which can range from the upper radio frequency range up into the microwave range. The UHF range from 300MHz – 1500MHz is the most ideal bandwidth to probe for PD defects in MV and HV power systems.
All types of partial discharges will produce electromagnetic emissions in the UHF range. In most cases, the UHF emission is detectable with the UHF sensor. The UHF pulses are going to be emitted from the site of the PD defect and will spread in all directions through air and through most materials. However, the signal cannot travel through metal. Metal enclosures will shield the UHF emission. The signal can escape through tiny cracks, gaskets, and seals and then spread out into the air. This pulse will occur at or near the positive or negative peak amplitudes of the AC sine wave. Thus the UHF pulses will occur with intervals roughly equal to ½ AC cycle time. (for 60Hz : 1/60 = 16.66ms 16.66/2 = 8.33ms , for 50Hz one cycle is 20ms and 20ms/2= 10ms) To prove that a UHF could be from a PD activity, we can show that there is a 180 degree separation (8.33ms or 10ms) between the UHF pulses. This is the function of (time resolved or “phase resolved” measurements). These phase-resolved measurements are made in real-time and serve as the basis for quick PD detection in the field.
In some cases, such as inside of a power transformer, the UHF emission may be completely shielded by sealed metal enclosures. Fortunately, the UHF signal will also induce a high frequency current pulse onto the ground straps of the apparatus. The high frequency current pulse will be detectable with a special High Frequency CT (HFCT). Learn more about HFCT detector is the HFCT section.