Partial Discharge Test Equipment and Field Testing Service
Partial discharge testing of electrical power systems enables electrical insulation to be tested while it is online and in service. This convenience lowers the cost and impact of testing while providing useful information about electrical asset condition.
Insulation failures are the leading cause of electrical failures and outages. Insulation systems are designed to isolate electrical potential from ground. When these insulation are damaged they begin to leak small amounts of energy which then damages the insulation further. Most insulation systems are manufactured and installed perfectly with no issues. However, the conditions on most modern power systems give rise to harmonics and voltage spikes which cause damage to healthy power systems. PD testing gives engineers and managers target list of failing components that constitute priority for repair or replacement. PD test results will allow you to intervene during a scheduled maintenance rather than experiencing failure at some unexpected time. Furthermore, we can provide surge supression and harmonic mitigation solutions which will protect and preserve your power system components from voltage spikes and harmonics while providing additional safety and operational benefits. see VSGR
PD Test Technology is able to be performed both on live power systems or on de-energized power systems. For live power systems, we call this Online Partial Discharge or OLPD. For online PD testing, we employ the use of a combination of PD sensors. This allows us to pinpoint the location of the defect, and to characterize the severity and type of discharge. Please contact us to get more information such as data sheets for portable PD test sets and PD monitors..
Electromagnetic Sensors: UHF HFCT TEV
Acoustic Sensors: Acoustic Contact Probe , Airborne Ultrasonic Dish, Ultrasonic Mic
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 grounds 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.
High Frequency Current Transformer (0.5MHz - 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 distances up to 1km away (this limit depends on the type of ground shield and the stregnth 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 grounding 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 bounded 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 instrumets have the sampling rates required for this function.
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, man holes, and metal enclosures. TEV signals spread out quickly on the ground plane. For this reason it is very difficult to locate 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 for indicating most types of PD in 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 TEV range signal. Also PD inside of fluid dielectrics such as transformer oil will not produce a detectable TEV signal. It’s important to use UHF, HFCT and ultrasonic sensor data in synthesis 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.
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.
Fluid and solid dielectrics such as oil, SF6, and resins, will permit ultrasonic PD signals up to around 300kHz. However, air cannot permit ultrasonic signals higher than roughly 80-100kHz. To use the AE/ultrasonic contact probe, we must apply a high-vacuum silicon gel to the contact probe to bridge the air gap between the exterior of the tank and the contact sensor. This gel will conduct the PD signal from the exterior of the tank onto the ceramic head of the AE contact sensor. Precise PD localization can be achieved by using 3 or more ultrasonic contact sensors to triangulate the exact source. This is especially useful when a transformer is producing acetylene and the engineers need to know exactly where the problem is before a maintenance outage to drain oil and go inside. Another important application of ultrasonic localization is in SF6 gas insulated equipment.
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 surfaces. 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.