R. S. Nema

Pulse sequence studies on PD data

Partial discharge (PD) pulses are measured to assess the condition of the insulation under test. The pulse magnitude and its phase position are the most commonly measured parameters. The PD pulse magnitude is used as an index to the quality of the insulation under test. The phase information is used to identify the type of PD. The most common method of PD analysis is statistical analysis on PD distributions. The representation of PD data as distributions all though convenient for representation of magnitude variations, loses the PD pulse sequence information. Recent studies have shown that PD pulse sequence contains information useful for understanding of PD phenomena. The paper presents pulse sequence studies with PD data obtained from model insulation systems representing different types of discharge models. The study brings out different characteristics of discharges and their variations with the applied voltage. The paper discusses the changes in the charge magnitude, the time between pulses and their relation with the phase of occurrence of PD. The use of time information for simple measurement and recognition of PD is also discussed.

Internal partial discharge and breakdown characteristics of thin polypropylene films

The effect of cavity dimensions such as diameter (0.9 mm to 3.2 mm) and depth (40 /spl mu/m to 200 /spl mu/m) on the various PD (partial discharge) characteristics and the related breakdown of thin PP (polypropylene) films was studied experimentally. The inception voltage is found to increase with decreasing cavity diameter for a given depth and with increasing cavity depth for a given diameter. Measurements of discharge magnitudes indicate that a single discharge does not completely discharge the entire cavity. The discharge pulse magnitudes, number, and distribution depend on the diameter as well as the depth of the cavity. Experimental and calculated values of maximum discharge magnitude for different diameters and depths of the cavity indicate that there is an increasing deviation between the calculated and experimental values as the cavity diameter increases. This implies that only a part of the cavity and not the whole of discharges.<<ETX>>

Response of a narrow band PD detector and analyzer in ageing experiments

Partial discharge (PD) pulses are recorded with an analyzer connected to a PD detector. The analyzer presents PD data as distributions that are used to interpret the PD phenomenon. The paper describes results of ageing experiments on oil impregnated pressboard samples subjected to PD measured with a narrow band detector and analyzer system. The main focus of the paper is on instrumentation related with narrow band detector and analyzer. A detailed response study of the narrow band detector is carried out to understand its behavior with pulses appearing with different repetition rate. The paper discusses characterization of PD distributions for meaningful interpretation.

An investigation of the corona and breakdown pulses in uniform field in presence of UV radiation

Occurrence of partial discharge (PD) corona pulses under uniform field conditions and the nondestructive testing of uniform field electrodes is discussed in this paper. The PD pulses occur in presence of UV radiation and appropriate value of current limiting resistance. The effect of grades of polishing, material of electrodes, gases and their mixtures etc. is studied in detail. It is possible to distinguish the PD pulses produced due to partially developed avalanches, rough surface and due to dust or metallic particles. A possible method of confirmation of cleanliness of electrode surface after mounting them in the cell is suggested.

Effect of detector and analyzer settings on partial discharge measurement and representation

Partial discharge (PD) measurement is usually performed with PD detector and analyzer system. The data collected during the test is generally represented as distributions. These distributions are interpreted to reveal the PD phenomenon and the state of the insulation. The paper demonstrates the effect of instrument characteristics on measurement and representation of PD phenomenon with experimental results obtained from short time PD endurance tests on oil pressboard samples. The instrumentation, a combined narrowband detector and multi-channel analyzer (MCA), is analyzed for the effect of the detector resolution and gain settings on the PD distributions. The results show the instrument dependence in the PD characteristics due to differences in measurement ability at various instrument settings. These differences in representation lead to multiple interpretation for the same phenomenon or state of the insulation and hence to wrong classification. The paper emphasis is that interpretation of the results should be made to account for the measurement ability of the instruments in use.

A study on partial discharge degradation of polypropylene films

One of the major causes of failure of polypropylene (PP) film high voltage capacitors is PD (partial discharges). PD occurs in the air gaps in the inter layer spaces present due to variation in tightness of windings. It is also possible that the air gaps present may be at different sub-atmospheric pressures due to vacuum impregnation. Therefore it becomes essential to undertake PD studies at pressures below atmospheric. An attempt is made here to study the PD degradation (on the basis of electric strength measurements) of PP films in short term ageing tests at pressures of 500 and 250 mm Hg in dry air.

A theoretical approach for Townsend's first ionization coefficient in mixtures of gases

Estimation of breakdown voltages of mixtures of gases is usually done empirically due to the incomplete knowledge of the fundamental phenomena which give rise to breakdown in gaseous mixtures. In this paper it is shown that the first ionization coefficient in gaseous mixtures can be obtained theoretically with special reference to electronegative gases and their mixtures with ordinary gases. The calculated values of ∝/p (first ionization coefficient/pressure) according to the developed equation and the values existing in literature are in good agreement. This equation can be used in determining the current growth and in understanding the behaviour of Swarm coefficients in mixtures of gases.

Accelerated aging of high voltage machine insulation under combined thermal and electrical stresses

An attempt has been made in this study, to evaluate systematically the effects of electro-thermal aging (electrical stress from 8.0 kV/mm to 13.3 kV/mm and temperature from 70°C to 155°C) on a resin rich machine insulation system by measuring a number of electrical parameters. Results indicate that when the electric stress and temperature are both high, a breakdown of model samples is achieved very fast. The time to failure for any other situation depends upon the actual combination of electric and thermal stresses applied. The temperature developed in the system appears to play a more significant role in the deterioration of the insulation.