Harmonic AC Voltages Research Articles

Influence of harmonic AC voltage on surface discharge formation in transformer insulation

The Ultra High Frequency (UHF) signal radiated during the surface discharge process under harmonic AC voltages with different Total Harmonic Distortion (THD) shows a dominant frequency at 0.8 GHz. The surface discharge inception voltage decreases with an increase in harmonics and THD as observed in the present work. The peak factor of the harmonic voltage and the surface discharge inception factor show inverse relationship. The rise time of injected current pulse due to surface discharges measured using High Frequency Current Transformer (HFCT) is about 600 ps and 900 ps for the positive and negative current signal, respectively. Cumulative energy spectra and Ternary plot confirms that UHF signal generated due to surface discharge process does not significantly vary under harmonic AC voltage with different THD's. The magnitude of UHF signal formed due to surface discharge under 50 Hz AC voltage is higher than that formed under the harmonic AC voltage. Surface charge measurement after corona charging process with the oil impregnated pressboard material indicates that the amount of charge deposited is high with the specimen damaged under harmonic AC voltage. Degraded zone of oil impregnated pressboard insulation due to surface discharges was analyzed using Attenuated Total Reflectance Fourier Transform Infra-red (ATR-FTIR) spectroscopy. Phase resolved partial discharge (PRPD) studies indicate that surface discharge activity predominates when the rate of rise of voltage is high.

Understanding water droplet initiated discharges on epoxy nanocomposites under harmonic AC voltages adopting uhf technique

In the present work, epoxy nano composite samples of different clay content are subjected to water droplet initiated discharge tests using controlled harmonic content high voltage supply. The effect of a clay additive up to 1% by weight is shown to increase droplet contact angle by 8% with a subsequent reduction beyond this content. Using a UHF technique to identify the corona inception voltage, it is difficult to identify correlation with harmonic content. However, it was found that the corona inception factor (CIF) and the peak factor of the applied AC/harmonic AC voltages showed an inverse relationship. The rise time of the current pulse due to water droplet initiated corona discharge under AC/harmonic AC voltage was found to be about 700 ps and from FFT analysis, the corresponding UHF generated signal has a dominant frequency of about 1 GHz. The characteristics of the captured UHF signals are similar for all investigated clay contents and harmonic AC voltages. It was found that the discharges caused damage near the high voltage electrode and that the surface carbonization is related to the accumulated dissipated energy during the discharge process. Analysis of the voltage, current signals and phase resolved partial discharge (PRPD) studies indicate that discharges occur around the voltage peak and also when the rate of rise of voltage is high. The number of discharges is higher in the positive half cycle compared with the negative half cycle.

Investigation of partial discharge activity by a conducting particle in transformer oil under harmonic AC voltages adopting UHF technique

A methodical experimental study was carried out to understand the partial discharges initiated due to particle movement in transformer oil under high frequency ac voltage and under harmonic ac voltages (with different THDs), by adopting UHF technique. It is also observed that irrespective of order of harmonics, increase in THD of the supply voltage shows reduction in levitation voltage. The rise time of the partial discharge current pulse formed due to particle movement is few ns. It is observed that the frequency content of the signal radiated due to partial discharges generated due to particle movement in the electrode gap lies in the UHF signal range, with its dominant frequency near 1 GHz. The supply voltage frequency and the time required for 100 UHF signals to form shows inverse relationship. When the applied voltage is increased above levitation voltage, the UHF signal magnitude formed due to particle movement increases, irrespective of supply voltage frequency. Ternary diagram clearly indicates that irrespective of frequency of the supply voltage, the UHF signal location in the ternary diagram is the same.

Understanding electrical treeing phenomena in XLPE cable insulation under harmonic AC voltages adopting UHF technique

In the present study, impact of high frequency ac voltage and harmonics in ac voltages on treeing were studied. When the applied voltage is high, a bush type tree forms irrespective of frequency of the supply voltage. Influence of harmonics on shape of tree formed indicates that a voltage wave with high total harmonic distortion have characteristics much similar to high frequency ac voltage. It is observed that the tree inception time is much lower with 11th harmonic ac voltages with higher THD. Presence of odd harmonics of equal magnitude, irrespective of amount of total harmonic distortion, the shape of the tree formed is almost same as the tree generated with 50 Hz ac voltage. The injected PD current pulses during treeing process have rise time of few nanoseconds. The UHF signals generated during tree growth (at the point of inception and propagation) have signal bandwidth in the range of 1-2 GHz. Measure of UHF signal magnitude during treeing process could help one to classify the shape of tree formed. Weibull distribution studies were carried out to estimate the life of XLPE cable insulation failure due to electrical treeing under different harmonic ac voltages. Independent higher order harmonic ac voltages with increase in total harmonic distortion reduce the life of cable insulation due to electrical treeing.

A double microbeam MEMS ohmic switch for RF-applications with low actuation voltage

In this paper, we propose the design of an ohmic contact RF microswitch with low voltage actuation, where the upper and lower microplates are displaceable. We develop a mathematical model for the RF microswitch made up of two electrostatically actuated microplates; each microplate is attached to the end of a microcantilever. We assume that the microbeams are flexible and that the microplates are rigid. The electrostatic force applied between the two microplates is a nonlinear function of the displacements and applied voltage. We formulate and solve the static and eigenvalue problems associated with the proposed microsystem. We also examine the dynamic behavior of the microswitch by calculating the limit-cycle solutions. We discretize the equations of motion by considering the first few dominant modes in the microsystem dynamics. We show that only two modes are sufficient to accurately simulate the response of the microsystem under DC and harmonic AC voltages. We demonstrate that the resulting static pull-in voltage and switching time are reduced by 30 and 45%, respectively, as compared to those of a single microbeam-microplate RF-microswitch. Finally, we investigate the global stability of the microswitch using different excitation conditions.