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Power transformer demagnetisation

IEEE Xplore Digital Library

Abstract-Switching operations and direct current applied during routine testing procedures of static winding resistance measurement leave residual magnetization in the core and/or fully saturate the core of the transformer.

When a magnetized power transformer is energized, it will face extremely high magnetizing currents due to the non-linear phenomenon of core saturation. The high level of inrush currents generated upon energization may affect the internal winding geometry of the transformer and also trip harmonic protection devices in the system.

This paper will discuss the demagnetization of the magnetic core of power and distribution transformers. Different algorithms are applied to a variety of transformer designs and the results are validated with excitation current measurements and Sweep Frequency Response Analysis (SFRA) tests. The knowledge acquired and the best practices suggested for transformer demagnetization are summarized for practical application in the field.

By D.M. Robalino

Test d'isolation


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Oil-paper insulation voltage dependency during frequency response analysis

People often believe that power factor/dissipation factor testing at power frequency (50/60 Hz) usually exhibits a flat response as a function of test voltage if the insulation is in good condition. Dielectric Frequency Response, DFR is the extension of power factor testing except that the measurement is performed from 1 kHz down to typically 1 mHz. It is a very useful tool for evaluating the moisture content in solid insulation of HV and EHV components such as power transformers, bushings, instrument transformers and PILC cables. The voltage dependent phenomenon also called “the Garton effect”, caused by paper absorbing electric charges in oil is investigated. The application of DFR in HV and EHV substations required a conceptual analysis of the phenomenon to better interpret the condition of the insulation system while increasing the signal to noise ratio to minimize the effect of surrounding interference. As a result of this work, authors provide practical recommendations regarding test voltages and frequency ranges to be used under high interference environments. The wide application of the method is supported with experimental field data. 

Test et diagnostic des batteries


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La toute dernière ressource

Les centrales électriques, postes de distribution et transformateurs doivent fournir une énergie de secours dans les situations d'urgence. L'outil TORKEL de Megger mesure leur capacité.

Diagnostic de câbles


Sujets (10)

No more cable cutting - Westnetz Case Study

Positive cable identification with suitable devices prior to work on underground cables is a compulsory requirement at Westnetz in Bad Kreuznach, as stipulated by the standard DIN VDE 0105 part 1. However, sometimes problems can arise, as it may not be possible to conclusively identify cables despite the strict compliance with regulations. Traditional cable identifiers work by relying on system properties with many fringe conditions that cannot always be complied with in practice. The cable identifier CI/LCI from Megger is the first to offer a user-friendly and safe solution.

Fault location with the ETF 3

The 3kV energy separation filter (ETF 3) is specially developed for fault location on symmetrical communication and pilot cables. This article covers the technology and best way to use the ETF 3, as well as field examples of the the system in use.

Sheath testing for fault location

Water ingress caused by damaged cable sheaths is a leading cause of faults in plastic-sheathed cables. Proper cable maintenance through sheath testing can extend the life of the cable by preventing the long term damage caused by water ingress. This article covers best practices and best equipment to use when sheath testing.

Pre-location transient methods

Learn more about the behaviour of transiet waves when a breakdown occurs at a cable fault, and understand how to evaluate and measure these transients for a more efficient fault location process. This article covers both DECAY (voltage decoupling) and ICE (current decoupling)



Pre-location pulse reflection process

Learn more about the interaction between pre-locaation and pinpointing to minimise fault location time. pre-location methods referrenced include TDR and HV methods. Additionally, this document discusses the values a fault must exhibit to be located by pulse reflection. 

Pre-location ICE method

The four most effective impulse current (ICE) methods for high resistance and intermittent power cable faults are "Direct mode", "Comparison mode", "Differential-comparison mode", Loop-on/Loop-off mode". The following document discusses the the process of each method.

Pre-location with ARM arc reflection technology

Understanding the most common fault types is critical to cable fault location. More than 80% of cable faults are high impedance faults. This document breaks down the properties of the high impedance fault, and discusses the most effective way to deal with them through Arc Reflection (ARM) technologies. 

Fault location on power cables

Gain insight into specialised fault location and pinpointing proceedures for high voltage power lines. Look at the construction of high voltage cables, as well as the methods and practices that are becoming new the standard, such as condition based preventative maintenance. 

Fault location in low voltage networks

Discover key strategies when conducting fault location on low voltage cables. Inluded are typical LV behaviours, advantages and disadvantages of LV testing, different test methods and best practices. 

Diagnostic de décharge partielle pour le vieillissement des câbles MT en papier imprégné.

Les défauts sur les câbles MT en papier imprégné, dont un grand nombre est en service depuis quarante ans ou plus, sont une préoccupation majeure pour les opérateurs de réseaux de distribution. Se basant sur des documents issus d’une recherche approfondie menée conjointement avec l’Université de Technologie de Varsovie, Piotr Cichecki de Megger montre comment l’analyse de diagnostic de décharge partielle peut aider à résoudre ce problème. 


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