Power Research

Dhruvesh Mehta ¹, Prasanta Kundu ¹, Anandita Chowdhury ¹, V. K. Lakhiani ²

Affiliations
1 Department of Electrical Egg. S.V. National Institute of Technology, Surat - 395007, Gujarat, IN
2 Technical Director, Transformers & Rectifiers (I) Ltd., Ahmedabad – 382213, Gujarat, IN

Abstract

Transformer operative life expectancy is governed by its insulation condition which is a complex arrangement of paper and pressboards submerged in mineral oil. It experiences persistent ageing due to steady physical and chemical deprivation being exposed to electrical and thermal stress in-service. The presence and or occurrences of even a minor defect in insulation can develop local concentration of electric field initiating partial discharges under normal operating voltages as well as under different electric excitation conditions. This Paper describes the different overstressed zones inside the transformer which need be taken care at design stage to reduce the chances of generation of partial discharges due to high electric field concentrations. Paper also discusses the methodology adopted by transformer designers to reduce the local concentration of electric field at particular zones and thereby reducing the chances of initiation of partial discharges. ELAX- 2D software based on Finite Element Method (FEM) was used to demonstrate the electrically overstressed zones inside the transformers and appropriateness of different methodologies adopted to preclude the partial discharges from transformer insulation. Partial Discharge test was conducted on analysed transformers and results were discussed to prove efficacy of the methodologies adopted to manufacture Partial Discharge free transformers.

Keywords

Partial Discharge (PD), Finite Element Method (FEM), ELAX -2D, Insulating Kraft Paper, Pressboard, UHV class transformer, Dielectric Insulation

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Tapan M. Rami ¹, Prasanta Kundu ²

Affiliations
1 Manager, Essar Oil Ltd., Refi nery Site, 39 KM, Jamnagar-Okha Highway, Vadinar - 361305, Gujarat, IN
2 Department of Electrical Engineering, S V National Institute of Technology, Surat - 395 007, IN

Abstract

This paper presents the assessment of insulation systems for high-voltage induction motor through measurement of insulation resistance(IR), polarization index(PI) and polarization–depolarization current or charging and discharging current. It is considered that the insulation systems are having different types and conditions of insulation aging. It demonstrates that the IR and PI cannot be used individually to judge insulation dryness, and the combination of IR and polarization and depolarization current (PDC) analysis is a better technique of insulation quality assessment than the insulation resistance alone. The PDC analysis is non-destructive dielectric testing method for determining the conductivity and moisture content of insulation materials in high-voltage motors. On the basis of this analysis, it is possible to take further actions like overhauling, drying process and replacement of the winding of the motor. This paper also presents a description of PDC analysis technique with the practical and theoretical background and some results of IR, PI and PDC measurements on High-voltage motor.

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Pritesh Bhoniya ¹, Prasanta Kundu ¹

Affiliations
1 Department of Electrical Engineering, S V National Institute of Technology, Surat - 395 007, IN

Abstract

With the appearance of deregulation, distribution transformer predictive maintenance is becoming more important for utilities to prevent forced outages with the consequential costs. To detect and diagnose a transformer internal fault requires a transformer model to simulate these faults This Paper presents here an approach has been shown for transformer fault analysis. A transformer model was designed for a 10 MVA, 132/66 kV three phase system. Data for all the parameters essential for designing a transformer were calculated, after studying machine designing. The next task was to calculate the inductance and capacitance (lumped values) which is later on used to make the ladder network which represents the transformer as inductance capacitance model. After dividing the total inductance and capacitance into segments the ladder network is ready to be used for short circuit fault analysis. Artifi cial faults were created by shorting a few segments at a time and the output voltage corresponding to input voltage under different faulty conditions were noted down. These values were used to create a frequency response curve which was compared with the response under normal condition and further analysis was done.

Keywords

Deregulation, Distribution transformer, Transformer model and Frequency response curve.

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Pritesh Bhoniya ¹, Prasanta Kundu ¹

Affiliations
1 Department of Electrical Engineering, S V National Institute of Technology, Surat - 395 007, IN

Abstract

A common failure mode for power transformers is consequent to mechanical deformation of the core or windings. Core damage is more likely as a result of transportation, while winding damage is more likely to be caused by short circuit type forces. Sweep Frequency Response Analysis (SFRA) is an effective low-voltage, off-line diagnostic tool used for fi nding out any possible winding displacement or mechanical deterioration inside the Transformer, due to large electromechanical forces occurring from the fault currents or due to Transformer transportation and relocation. This paper describe through case studies that SFRA test is more sensitive to detect faults compare to conventional testing method.

Keywords

Common failure mode, Sweep frequency response analysis (SFRA) and Frequency response analysis (FRA).

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