Bibliotek

Megger’s article “Transformer Winding Resistance Measurement: Field Challenges” was presented at the 2020 NETA PowerTest Conference in Chicago, Il. This article takes an in-depth look at the lesser known facts associated with the DC winding resistance (WR) measurements, diving deeper into topics such as selection of the correct test current, and the importance of compliance voltage during the test. Phenomena as core saturation, current stabilization, the influence of winding inductance on readings, and the effect of temperature, are also explained.

Transformer Turns Ratio (TTR) is one of the most common test used to assess the condition of the transformer’s windings and core.  It is performed as a part of acceptance and maintenance test procedure to determine any problems due to poor design, assembly, handling, overloading, fault conditions or poor maintenance. TTR results are compared against the nameplate ratings to determine any possible insulation deterioration, shorted turns, core heating or any other winding or core abnormalities.TTR is a simple and easy test to perform that is often taken for granted without fully understanding the principle and basis of the test. In cases when measurements are not within expected limits, it becomes a challenging task to determine the root cause and resolve the problem. This paper will focus on some of the unknown facts associated with the TTR test. The paper covers in detail the effect of applied test voltage, comparative analysis of step up vs step down excitation, different vector configurations, differences between nameplate ratio, voltage ratio and turns ratio, sources of ratio and phase angle errors, comparison of per phase testing vs true three phase testing, extreme tap ratios being out of tolerance for On Load Tap Changers (OLTC), and TTR test correlation with other electrical tests. The paper also provides field test results and case examples to explain the above-mentioned unknown facts.

This article describes the application of DFR and HV DFR as a preventive method to evaluate the condition and prioritize maintenance activities on HV and EHV bushings and relates some case studies. 

Power swing which is principally caused by an oscillation in active and reactive power of transmission line, consequent to an enormous disruption in power system, which if not blocked, could cause wrong operation to the distance relay which may lead to tripping the healthy part of the transmission line. In the absence of power swing function it may result in severe damage to the machines or cascading tripping in the grid resulting in blackouts. To prevent such scenarios, intelligent electronic devices (IEDs) have power swing block (PSB) detection and trip logics incorporated with distance schemes. 

Since the invention of the transformer, the design, quality and efficiency of this magnificent machine has been improved thanks to the development of electromagnetic materials, computer design algorithms, insulation materials, cooling systems, structural design, etc. Nowadays, a vast number of transformers operate worldwide aiming to provide reliable and safe operation under the most cumbersome environmental and operational conditions.

Different partial discharge defects show a different distribution in the frequency spectrum. These typical characteristics could be used in combination with the appropriate phase resolved partial discharge pattern for the analysis of partial discharge defects. The classification of the partial discharges allows an assessment of the defects in terms of its criticality.

Improved transformer insulation assessment using dielectric response analysis.

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.

While the share of installed cables without a jacket generally continues to decrease, the unjacketed cables that remain continue to age. As they do so, it is increasingly likely that a fault will develop on these cables. It can be particularly troublesome to locate faults on unjacketed cables as normal Time Domain Reflectometry, or radar, based fault location techniques often do not work, due to the corrosion of the neutral wires. While there is no guaranteed action that would result in locating the fault, this paper will outline some advanced methods, based on field experience, that can help when locating these types of faults. This paper was an article presented at NETA in 2017 and will discuss the Impulse Current method, which does not rely on the neutral wires being intact, in both a theoretical and practical light. 

During recent years a great deal of valuable research work has been done to increase our knowledge of the properties of insulating materials, yet notwithstanding the progress so made the natural laws governing insulation resistance are but little understood.