PD (partial discharge) Assessment Techniques For Electrical Equipment

Condition Assessment Techniques For Electrical Equipment

Condition assessment techniques are the very famous method to look after the electrical equipment. These are used to identify the significant changes which are caused by many problems and prevent the failure. Due to these techniques, we check the tool as scheduled. There is a number of the method but here I explain two techniques.

•            PD monitoring

•           Transformer moisture measurement

PD monitoring

The PD (partial discharge) is used many years for test and investigates the insulation system design and check the level of quality of new equipment. In the last 20 years the use of partial discharge increase dramatically.Now a day these are used to diagnose the present condition of electrical insulation in the using apparatus. It is used in like switchgear, transformer, cables, windings of the stator in the generator and as well as in the motors. PD has many qualities so that the use of partial discharge increase day by day due to the sensors which are used in it is low cost, electronics as well as the memory is effected its popularity. The use of ultra wide band detection and the acutance in the measurement of the partial discharge in the presence of noise are the methods that cause its (PD) popularity.    

Partial discharge is produced due to electrical breakdown of a gas (e.g. air) in the highly un specified electrical field. Partial discharge is like a small electrical spark. The partial discharge will degrade the level of organic material and sometimes cause the big fault and stop working of electrical insulation if the void is present in the liquid or solid of organic. The nature of partial discharge is destructive for over previous 70 years in the oil-paper insulated high voltage cables as well as the other high voltage equipment. The insulation of different instrument is made of organic materials like oil, paper, polyester .polyethylene, rubbers, polyester.We have to pay full attention to formed the insulation system of high voltage cables, transformers, capacitors and switchgear that not include the little amount of void in the region of electric high stress.

PD is also caused to destroy earlier the electrical apparatus. At the result of the high-temperature operation, voids mix within the epoxy mica insulation or oil-paper insulation due to this localized electrical stress on insulation surface and discharge in the air.  The presence of Partial discharge cause the environmental mechanical and thermal aging processes.  

There is no doubt that the partial discharge measurement method is devolved since 1930. There are many reasons to measure the PD design test. It is simple to find the presence of PD for new insulation system under the normal operation.

Quality Assurance: it is done to believe that partial discharge is not the presence in the newly formed insulation system.

Diagnostic test: this is suing to identify the fault due to thermal, cables the electrical and mechanical stress during the working of insulation system in the transformer, switch cables the and aim and purpose of these tests to find the PD level. If the amount of PD Level is too high then it indicates of failure of system insulation permanently.

There is a number of a researcher who finds the method of investigating the partial discharge during 1950 to 1980 like Bartnikas, Kelen, Kreuger. These top researcher introduced a different method to show the data and identify the PD. After a long time, these methods are used to measure and detect the PD in laboratories and industries no a days.

the most famous researcher  Bartnikas, Kelen,  Kreuger, Gulski and other prolific researcher take a step forward and formed the advanced technology to measure the partial discharge.to the point that off the line and on line. These online techniques are used in high percentage transformers machines cables and switchgear that are working in the world industrial countries.

Features of PD measurement systems

A famous researcher Bartnikes recently summarized the theory of PD, calculation technology, and a brief overview of the partial sensor.

Sensors (Coupling Devices)

electron move very fast form one side of gas-filled void and pass another side when the partial discharge waves formed. The time of these pluses is nanosecond and the electron moves at the speed of light. The e- has charged and each discharge generates current waves I = DP/DT. A positive charge also flows in opposite direction .the current generate a disturbance and at conclusion current and voltage flow outside the PD site.  At this time the frequencies reach at several megahertz the Fourier transform of a current shown.

PD pulse current can easily identify by any sensor that can sensitive the high frequency. When we did an off line test then we use a very high voltage capacitor which is connected to the terminal of high voltage of the test object.

Partial discharge can also be detected by the following

Electromagnetic Radiation

Acoustic noiseVisible and UV light

Electromagnetic radiation can be measured by RF antennae.

Due to Discharge on insulation system electron and ion formed a gas pressure that can be identified by the acoustically.

The final famous process is detected PD uses the fact that the light that’s transparent the energize molecule and emit energy after the discharge. When they pass through the spectrum a light is produced and a naked eye can easily detect these rays of light on the surface f PD.

Detectors

partial discharge is detected by the voltage signal, PD pulses current convert into the voltage signals because it is easy to detect the voltage signal rather than current. The conversion takes place via a resistor or R L C detection impedance is used.RLC identify the method of resistance is very famous, the total number of ions and electrons and ions that are bombarding the insulation surface is directly proportion the failure or decay of organic insulation surface and if the rate of discharge is greater than the decrease of insulation is faster.

Detectors

Many types of instruments are in the market to calculate the signals which are from the Partial detector and sensors combinations.

Analog oscilloscope was most famous and reliable tool to calculate the PD showing signals it’s about last twenty years ago. The pluses were calculated by the PC (vertical scale) versus the AC phase position. The bandwidth of the oscilloscope at that time is very limited and the pulses of PD appears on the screen of the oscilloscope for very minimum time so it was very difficult to calculate the accurate magnitude of the pluses. The rate of repetition of partial discharge pluses is not measured able, last 20 years are very important to the development of the instruments in the field of PD deduction. Now a days all the instrument are digital infect the instruments used before was totally analog. Due to the digital world, we can easily enable to the separation of noise, continuous online PD monitors, PD site Location and practical. 

Statistical Analysis

This approach uses a skew, standard deviation and kurtosis to identify the figure and location of positive and negative partial pulses with respect to the AC phase angle. Every partial discharge mechanism and each type of noise have unique set of mean phase angle, standard deviation, skew, and kurtosis. (G.C.Stone, October 2005) This was the first researched automates pattern to identify the method.PPA plot gives all raw data .this is shown in the figure.

        Pulse phase analysis plot of the pulse magnitude versus AC phase position   

  with pulse count rate in pulses per second represented as color.

Partial discharge ratio changes as the pattern of PD. The pattern shape differs in the mean skew and kurtosis with respect to the phase angle. Which is shown in the figure.