PMDT is the only manufacturer to utilize all 5 types of online partial discharge sensor technology. Using multiple sensing methods ensures that PD activity is always discovered, located, and characterized.
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.
Internal UHF Sensor
The Internal Ultra-High Frequency (UHF) sensor is installed inside of the power asset to detect the internal signals and analyze them for PD. It employs PMDT's advanced 3rd generation internal UHF sensor technology. It has the following features:
High Detection sensitivity and effectively isolates external noise signals
One-time casting mold and has a life expectancy the same as the power equipment
Reliable and safe to use as all metal parts are grounded
Unique structure and design provide reliable sealing between the sensor and the power equipment
Complies with the running temperature equipment and pressure environment of power equipment
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.
Airborne Ultrasonic Parabolic Dish Concentrator, Internal, and Ultrasonic Extension Wand, 20kHz-80kHz, center frequency 40kHz. Each time a PD event occurs, an ultrasonic emission is transmitted from the site of the defect. The emissions are typically in the ultrasonic range from 20kHz - 300kHz and may also extend lower into the audible range. If an insulation defect is close to the surface, then the ultrasonic signal will spread out and become airborne. The airborne ultrasonic signal may be detected by one or more of the 3 types of airborne ultrasonic sensors which are commonly used.
One type is built into the main handheld PD test unit. Also the extension wand can be used to listen in hard-to-reach seams of Switchgear cabinets or other enclosures. Another type is the Parabolic Dish Concentrator, which is used to extend the distance range of the ultrasonic microphone. The parabolic dish is ideal for checking bushing and insulators for surface tracking and harmful corona. When airborne ultrasonic PD signals are detected on exposed insulator surface, the Parabolic Dish enables very quick and precise pinpointing of the PD location. Airborne ultrasonic is best for detecting corona PD and surface tracking PD. Floating electrode PD will also produce a weak airborne ultrasonic signal. But when airborne ultrasonic signals are detected within enclosures, it is not immediately possible to determine the exact location of the PD. Sometimes you cannot see the insulator which has the PD. By using multi-channel PD Diagnostics, 3 or 4 ultrasonic contact sensors can be used to triangulate the signal origin.
Acoustic Contact (AE)
Ultrasonic Contact Probe (20kHz - 300kHz) The ultrasonic contact probe is ideal for detecting and localizing partial discharges in fluid dielectrics such as oil and SF6 gas compartments. This is sometimes called “in-tank” testing. Each time a PD event occurs, an ultrasonic emission is produced at the site of the defect. If the defect is inside of sealed equipment such as a transformer or SF6 compartment, then the signal will (usually) not become airborne.
Transient Earth Voltage (3MHz -100MHz) Partial discharges in Medium Voltage (MV) power equipment will produce induced radio frequency signals which are sustained and can be detected at the surfaces of MV enclosures, such as Metal-Clad Switchgear, manholes, and metal enclosures. TEV signals spread out quickly on the ground plane. For this reason, it is very difficult to locate the origin of a TEV signal. PD from overhead can induce a TEV signal on an entire line of Switchgear. The TEV sensor is a type of capacitive coupler which is built in to the main PD detection unit.
The TEV sensor is reliable first indicator of possible PD in a Metal-Clad Switchgear, but it also responds to noise from lighting systems, battery chargers, VFD’s, LED’s, and large motors. Void type PD will typically not generate much of a TEV signal due to it being inside a dielectric. Also, PD occurring inside of the fluid dielectrics will not produce a detectable TEV signal due to how the tank is sealed. It’s important to use UHF, HFCT and ultrasonic sensor data in combination with TEV in order to see the whole picture and draw an accurate conclusion about the asset’s insulation health. Don’t rely on a TEV sensor for all of your information. The UHF and acoustic sensors are usually needed in order to locate the PD source.