K Sethupathi

The UHF signals are generated due to PD formed by particle movement in liquid nitrogen under AC voltages. The levitation voltage of a particle in liquid nitrogen measured through UHF technique and by conventional PD measurement technique is the same, confirming the sensitivity of UHF technique for identification of PD activity. The frequency content of UHF signal generated due to particle movement in liquid nitrogen, under AC voltages, lies in the range 0.5-1.5 GHz. The characteristics of UHF signal generated due to particle movement between the barrier and high voltage/ground electrode is much similar to the signal generated by particle movement in clean electrode gap. Pseudo resonance phenomena can occur in liquid nitrogen due to particle movement. It is also observed that the partial discharge magnitude, in general, be high when the particle moves between the barrier and high voltage electrode when compared to the barrier and the ground electrode. Percentage of clay in epoxy nanocomposites has not altered the levitation voltage of the particle in the electrode gap. Zero span analysis clearly indicates that pseudo resonance occurs when particle moves (in a short gap) between the barrier and high voltage/ground electrode. © 2010 Elsevier Ltd. All rights reserved.

The shape of electrical trees formed in XLPE cable insulation at low temperature under AC and harmonic voltages are fibrillar type. Life of cable insulation failure due to electrical treeing was analysed through Weibull distribution studies. It is observed that the voltage wave shape and peak factor exhibit high influence on the life of cable insulation due to electrical treeing. Harmonics with higher THD's and the failure rate of cable insulation due to treeing are the same. The presence of 4th and 5th harmonics in supply voltage could cause early failure of cable insulation. Ultra High Frequency (UHF) signal radiated during tree growth process, under harmonic voltages lies in the range 0.5-1.5 GHz. Phase Resolved Partial Discharge (PRPD) analysis using spectrum analyser has revealed that under 50 Hz, discharges occur near zero crossing and in the presence of harmonics, it occurs when the rate of voltage rise is high. The discharge occurs at its peak under low frequency AC voltage of 1 Hz.

In recent times, liquid nitrogen (LN2) is used as an insulant as well as coolant in high temperature superconducting power equipments. One of the major problems in the liquid insulation is the particle contamination, which can generate incipient discharges that can lead to complete failure of the insulation system during operation. An attempt has been made in the present study to understand the feasibility of using Ultra High Frequency (UHF) sensors for identification of partial discharge formation due to particle movement in liquid nitrogen under dc voltages. It is observed that partial discharge formed in LN2 radiates UHF signal. For the purpose of comparison, the studies were also carried out with air as an insulating medium. The UHF signal generated under dc voltage characterized by measurement of its amplitude, rise time, duration of signal and through frequency domain analysis. It is observed that intensity of partial discharge due to particle movement in liquid nitrogen is high under negative dc voltage compared to positive dc voltage. Also, the frequency content of UHF signal generated due to particle movement in liquid nitrogen lies in the entire bandwidth of 1-3 GHz. © 2008 IEEE.

Incipient discharges formed due to particle movement, corona and surface discharges in liquid nitrogen under ac voltages radiates UHF signals. The frequency content of the UHF signal generated due to particle movement at the point of inception lies in the range 0.5 - 2 GHz. Corona and surface discharge radiated UHF signal which vary with applied voltage. At higher applied voltage to the electrode gap, the UHF signal generated due to surface discharges and corona, have increased energy content of the signal in the frequency range 2 - 3 GHz. It is observed that irrespective of type of incipient discharge formation, multiple discharge action occurs, which could be observed through zero span analysis. Ternary diagram can be used as a condition monitoring tool and could help one to classify the type of discharges (partial discharge/corona/surface discharge) that occurs, for any remedial action, during operation. © 2006 IEEE.

Liquid Nitrogen (LN2) is used as an insulant as well as coolant in high temperature superconducting power equipments. Recently it is identified in HTSC transformers; the voltage distribution in the winding is oscillatory in nature. An attempt has been made to generate such unidirectional oscillatory impulse voltage (UOIV) of various frequencies to understand the breakdown characteristics of liquid nitrogen with non-uniform electric field configuration (Needle-Plane configuration). For the purpose of comparison breakdown characteristics under standard lightning impulse voltage were obtained. It is observed that the breakdown strength of UOIV and the lightning impulse voltage (LIV) are nearly same. The discharges initiated by UOIV in liquid nitrogen measured using Broadband sensor. It is observed that discharges occur at every peak of UOIV. It is high when the effective frequency of UOIV is high. It is also observed that number of discharges of various magnitudes occurs in the negative OIV. The magnitude of discharges is almost the same under positive polarity of UOIV. This high and low magnitude discharges can cause early failure of insulation. The signal measured using broadband sensor indicates that the signal lies in the UHF range. The result of the study indicates that PD occurs at the every peak of the applied OIV indicating severity of OIV. © 2013 IEEE.

Liquid nitrogen (LN2) is used as an insulant as well as coolant in high temperature superconducting power equipments. Particle contamination in liquid nitrogen is one of the major cause for formation of partial discharges during operation. An attempt has been made in the present study to understand the feasibility of using Ultra High Frequency (UHF) sensors for identification of partial discharge (PD) formed due to particle movement in liquid nitrogen under AC voltages. It is observed that the partial discharge formed in LN2 radiates UHF signal. The results of the study indicate that the conventional partial discharge measurement and UHF peak amplitude measurement have direct correlation. The Phase Resolved Partial Discharge (PRPD) analysis indicates that the partial discharge formed due to particle movement occurs in the entire phase windows of the AC voltage. The PD magnitude increases with increase in applied voltage. The frequency content of UHF signal generated due to particle movement in liquid nitrogen under AC voltages lies in the range of 0.5-1.5 GHz. The UHF sensor output signal analyzed using spectrum analyzer by operating it in zero-span mode, indicates that burst type PD occurs due to particle movement. © 2009 Elsevier Ltd. All rights reserved.