Partial Discharge (PD) is the phenomenon of partial discharge of electricity in a small part of the electrical material of medium and high-voltage devices.

PD is the result of electrical destruction formed by having air gaps inside the electrical layer.

According to the Fire Protection Association (NFFA 70B), the basic cause of electrical problems in medium and high voltage electrical equipment is damage to the electrical layer, bad contact,…

In medium and high voltage devices, PD is one of the important signals to recognize of damage to the electrical layer.

The truth: 85% of incidents of medium and high voltage electrical devices are related to local electrical generation.

  1. Purpose of PD online testing.

Online PD testing performed in a substation directly on an energy-supplied electrical device is still active.It is non-invasive and can be done safely.This is a reliable method for testing electrically in MV and HV substation equipment.PD tests are highly sensitive for minor electrically edicing defects.It is considered more probed in detecting initial electrical problems than usual offline tests such as the electrolytic edicts.The offline PD methods described in IEC 60270 are the foundation of online testing, however online testing is carried out without service interruption or power off.

As with all condition-based maintenance (CBM) technologies, early detection is critical.Usually by the time you can recognize something by smell or by temperature it is too late.PD testing offers a unique opportunity that in early detection of all kinds of electrical faults without the need for a power shutoff for traditional testing.

Benefits of online launch field measurements:

  • As a predictive test, which in turns out the deterioration of the insymation before the incident occurs.
  • As a non-invasive test, does not require service interruption and is carried out under normal operating voltage and load conditions.
  • As a non-destructive test, it does not test for damage or adversely affect the tested device.
  • As an experiment there is no need to use any over voltage, thus not leaving the tested device subjected to higher voltage pressure than the pressure encountered under normal operating conditions.
  • As a trend experiment can be carried out by storing the results to allow comparison with future trials.
  • In many cases, the location where the local launch occurred may be in the test object, so that the local problem can be corrected.
  • The cost of carrying out a PD survey is relatively cheap compared to offline testing, which allows to carry out annual surveys economically at most facilities.
    1. method

    2.1 Preparation of work

    Perform safe working conditions: Detect any noise/sound – abnormal lighting associated with the faulty condition of the existing equipment, observe a safe distance from the electrical carrying unit to perform measurements

    – Record the structure and electrical parameters of the cutting equipment: Cabinet name, function of cabinet

    – Record operating conditions: load current of each table, operating voltage

    – Check and record environmental parameters

    + Ambient temperature, relative humidity

    – Check and record the noise level inside the cutting equipment room

    + Check and remove (turn off) nearby interference sources as much as possible: mobile wave interference, radio interference from the walkie-talkie system, lighting consy, ventilation fan, Variable Frequency Actuate (VFD), Continuous Power Supply (UPS) or rescoring charger.

    + Ultrasound: Check and record the dB level of the area around the cutting device

    + TEV: Check and record dB levels and pulses/cycles on grounded metal surfaces near the cutting equipment

    2.2 Scope of application

    Metal cutting equipment: Use TEV (Excessive Soil Voltage), UHF (Ultra High Frequency), AE (Ultrasound in Air), Ultrasonic Exposure

    – Use UHF to observe the entire room, then place the UHF sensor at the vents, openings and seams of the closing cabinet.

    – Use TEV sensor: Place the TEV sensor on the front of each cutting device.TEV can also be measured on the back of each cabinet of the cutting device.The normal TEV value is 5-20dB.If you are over 20dB, try powering with the UHF sensor.If TEV reaches 30, 40 or 50 dB, then there may be a PD inside.But TEV signals can be everywhere and come from a number of other sources.So TEV it is very low on all cutting devices and it only reaches very high in a small area.

    – Use ultrasonic chopsticks in the air to check all the seams and vents of the cabinet, where the signal in the air can escape.

    2.3 UHF ultra-high frequency (300MHz – 1500MHz) (RF/radio/microwave frequency range)

    – Every time a PD event occurs, an electrolynthic wave is issued.This emission can also be called RFI or EMI.(Radio frequency interference or electrolytic interference).PD emission is a broadband electromagnetic emission that can stretch from a higher radio frequency range to an extremely short wave range.Research and experience have concluded the UHF range from 300MHz – 1500MHz is the most ideal bandwidth for probing PD defects in MV and HV power systems.

    – All types of local electricity release will generate electromagnetic emission in the UHF range.In most cases, UHF emission can be detected using a UHF sensor.UHF pulses will be released from the site with PD defects and will spread in all directions in the air and through most materials.However, the signal cannot be transmitted through metal.The metal casing will shield uhf emission.The signal can escape through small cracks, gaskets, sealing pads and then spread out into the air.This pulse will occur every time the sine wave reaches the oceanic or negative polar amplite.Therefore UHF pulses will occur with intervals equal to 1/2 of the AC cycle.(for 60Hz: 1/60 = 16.66ms 16.66/ 2 = 8.33ms, for 50Hz a cycle is 20ms and 20ms / 2 = 10ms) So, to prove that a UHF can be from PD activity, there is a distance of 180 degrees (8.33ms or 10ms) between UHF pulses.

    This is the function of (measurements are resolved over time or "phased resolution").These phased resolution measurements are performed in real time and are the basis for rapid PD detection at the scene.

    Test position of UHF sensor:

    + Connectors

    + Windows

    + Metal openings

    TEV Test Process:

    – Before starting to check the PD in the closing device, it is necessary to check the ambient noise level of the room.The test point must be outdoors, metal background values must be obtained from the surface of metal products such as metal doors, metal gates,

    metal fences and other non-cutting equipment…

    – When using TEV detection, the test site is the point on the electrical device where the sensor is in direct contact with the surface.Detection points are different types of electrical equipment, based on the configuration of the power parts.During the INSPECTION of the PD of the cutting device, it is necessary to first determine the location of all electrical equipment, mainly checking the PD of the bus bar (connection, wall bushings, supporting electrics), Cutters, CT, VT, cable joints and other devices, most of which are located in the middle and lower parts of the front panel of the device closed and the upper, middle, and lower parts of the back or side panel.We should conduct local eject detection at these locations of the actant.Checkpoints are displayed in

    As shown below:

    Test location of TEV sensor:

    + Front: Middle and Bottom

    + Back: Top, Middle and Bottom

    + Right or left (if we can): Top, middle and bottom

    Test location of TEV sensor:

    + Front: Middle and Bottom

    + Back: Top, Middle and Bottom

    + Right or left (if we can): Top, middle and bottom

    AE Ultrasonic Sensor Testing Procedure:

    – Each time a PD occurs, an ultrasonic emission is generated at a defective site.Emission is usually in the ultrasonic range from 16 kHz – 300 kHz and can also expand lower in the audio range.If the defect is near the surface, then the ultrasonic signal will spread out and enter the air.Ultrasonic signals in the air can be detected by one or more of the 3 types of ultrasonic sensors that are commonly used.One is integrated into the main handheld PD testing device.

    – Parabola disc set is used to extend the distance range of the ultrasonic microphone.In addition, expansion can be used for listening in hard-to-reach seams of cabinets of cutting equipment or other containers.When ultrasonic PD signals in the air are detected on visiblely electric surfaces, the parabolic disk allows to locate the PD very quickly and accurately.When ultrasonic signals in the air are detected in the enclosure, it is impossible to immediately determine the exact location of the PD without the negative power time of the flight technology and multiple sensors.Three ultrasonic contact sensors can be used to determine the source of the signal.Ultrasound in the air is best for detecting halo PD and PD surface monitoring.The PD floating electrolye will also produce a weak ultrasonic signal in the air.Typical measurement points (red part) as shown below:

    Test location of ultrasonic sensor:

    + The front and back where the sound can escape.

    – Repeat the above AE/TEV/UHF measurement at any location detected with abnormal or high amplit range signals, specific phase-ejection phase sample, synchronized specific waveform.With power cycles or higher/different from adjacent columns at least three times to re-check and confirm measurement results.



    3. Hoang Lien Son Energy Joint Stock Company 3.1.

    Introduction: Hoang Lien Son Energy Joint StockCompany (HLS) was established in 2014 on the basis of a system of electrical testing companies in the building and industrial park industry with experienced shareholders in the field of electrical services and experiments.

    HLS is a pioneer company in Vietnam providing diagnostic testing services for discharge status for medium voltage power systems with a team of well-trained and highly experienced professionals.

    HLS surveys all asset types using the PD Detector to identify sources with high PD and determine their causes and risk levels.

    3.2. Some experimental images of the measurement of local electricity generation of medium voltage cabinets through projects that HLS has participated in: