This chapter discusses the literature survey on the theories, laws and concept that related to the project. The main focuses about this project is do research and study for how to remove noise from impulse signal to get Partial Discharge signal. Based on the research by using the medium like journals, articles, books, e-books and website, almost of them are an analysis about PD occur at HV such as in overhead lines, underground cable and others.Then, how to overcome the problem of signal interference to get PD signal. After that, how to remove noise from impulse signals such as using Wavelet Transform(DWT), Artificial Neural Network (ANN), Fast Fourier Transform (FFT) and others technique. But for this project, it is a chance to make analyze impulse signal of partial discharge and eliminate the noise interference using hardware and software .
2.1 PARTIAL DISCHARGE
Partial discharge is an electrical discharge or small electrical sparks that only occur within the insulation of medium and high voltage electrical assets. Electric discharge is defined as a movement of electrical charges through the liquid, solid or air which is initiated by electron avalanches[4]. PD can occur within gas voids, such as voids in solid epoxy insulation or bubbles in transformer oil. Protracted partial discharge can breakdown of insulation and erode solid insulation. Several studies revealed that, failures due to damage in insulation are most common, it is around 80% of total failures. Then, 90% of total failures have not been detected with common test and maintenance practices. Thefore is not been prevented. PD is the main cause of destruction of insulation quality and its measurement is a nondestructive prime tool which is successfully applied for PD detection in HV power apparatus since many years[5].
Over time, several methods have been developed to detect PD within HV. These can be grouped into four categories, optical detection method, Electrical detection method , Acoustic detection method and UHF detection method.
Optical detection method
In this method of detection appearance of discharge is caused of light which takes the form of ionization during excitation process. The light emitted in this method depends upon temperature, pressure and nature of insulating material. For detection method, insulating material used are of transparent type. But, this method possesses drawbacks in high voltage transformers as mineral oil which it contains is of opaque nature.
Electrical detection method
Electrical method of popular method for measurement of Partial Discharge(PD) in power equipments . Electrical method of detection method is useful for modeling of PD inside the cable insulation, because it emphasizes upon presence of the current as well as voltage pulses which are caused by current streamer due to presence of void and impurities. The duration of pulses are less than one second and range of frequency variation in KHz. Its appearance and shape of pulses its at different phase locations gives information about PD types and information regarding insulation failure. Time domain recording device is employed for recording of partial discharge impulses in this detection method.Detection is useful for online electrical PD detection. During operation of HV power equipment broadband and narrow band electrical pulses are encountered . It is very difficult to classify those electrical noisy signals and PD because, the pulses that appeared in this method of detection depends on the physical dimension of high voltage cables. For this method has several disadvantages but finds wide usefulness in power plants. Although has several disadvantages but It helps the power engineers and technicians by giving necessary as well as vital contents regarding the characteristic, appearance of different type of partial discharge as well as regarding the insulation failure occurance in high voltage power equipment like transformer, generator, cable and others.
Acoustic detection method
Acoustic detection method based on sound waves produced during PD phenomena. This sound can be hearing or not, is due to current streamer formed in the event of PD and the material surrounding the streamer is vaporized. This kind of vaporization leads to release of mechanical energy, which spreading by taking the form of a pressure field. The best example is the occurring of thunder after a lightning strike. Acoustic detection systems divided into two types such as external and internal systems. External acoustic detection systems frequently used these days, uses sensors which are kept outside to power apparatus to detect acoustic signals which are originated in the course of PD. In internal systems sensors are placed inside of power equipment so that sound pressure wave can be measured directly. Acoustic method of detection encounters so many limitations. As the acoustic PD signal propagation are of complex nature, acoustic method of detection finds difficulty to detect it. Sometimes, the acoustic signal detected posses very low intensity so need a highly sensitive sensor which can detect minute changes in signal. Then one other issue which currently limits the usefulness of acoustic PD detection equipment is the high price for these units. So that, the question arises if such detection systems cannot be made cheaper by using mass produced components like commercially available piezoelectric transducers and other off-the-shelf components.
UHF detection method
This method has great advantage like immunity against disturbances as compared to other conventional technique. UHF detection method can be performed for condition monitoring of high voltage equipment both on site and online. After that, UHF detection by means of antenna has no connection to power transformer like other methods[13].
For this project, before get the impulse signal or detect impulse signal, PD obtained from using hardware in experiment. The hardware used to gain the sample of PD of using needle plate electrode.
2.2 FREQUENCY
Frequency is the number of occurrences of a repeating event per unit of time. It also the number of cycles or vibrations undergone during one unit of time by a body in periodic motion. A body in periodic motion is said to have undergone one cycle or one vibration after passing through a series of events or positions and returning to its original state. Therefore, angular velocity it also simple harmonic motion. Frequency is a very important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio (sound) signals, radio waves, and light[6].
If the period or time interval, is required to complete one cycle or vibration is 1/2 second, the frequency is 2 per second. Then, if the period is 1/100 of an hour, the frequency is 100 per hour. So, the frequency is the reciprocal of the period or time interval such as frequency = 1/period = 1/(time interval). The frequency with which the Moon revolves around Earth is slightly more than 12 cycles per year. The frequency of the a string of a violin is 440 vibrations or cycles per second.
In physics and engineering disciplines, the most commonly used symbols for frequency are f and the Greek letters nu (ν) and omega (ω)(see e.g. Planck's formula). Nu is used when specifying electromagnetic waves, such as light, X-rays, and gamma rays. Omega is usually used to describe the frequency of angles-that is, how many objects rotate or revolve around radians per unit of time. Usually, the frequency is expressed in the hertz unit, named in honour of the 19th-century German physicist Heinrich Rudolf Hertz, one hertz being equal to one cycle per second, abbreviated Hz such as one kilohertz (kHz) is 1,000 Hz, and one megahertz (MHz) is 1,000,000 Hz. In spectroscopy another unit of frequency, the wave number is sometimes used[12].