Asset management techniques for transformers

One of the cellulose insulation in high voltage power transformers is paper, and it is widely used in transformers due to low cost and excellent physical/electrical properties. However, the performance of paper could be affected by ageing. Ageing of paper is a complex phenomenon and can be influenced by many factors. This PhD thesis aimed to examine the ageing of paper in transformers based on Tensile Index (TI), dielectric strength and Low Molecular Weight Acid (LMA) measurements.The effect of ageing on the dielectric strength of paper was examined, through studying the influence of paper and oil ageing by-products such as LMA and HMA on the AC breakdown voltage. Meanwhile, the end of life for transmission transformer population in the UK was examined by studying the profile of LMA and TI of paper in scrapped transformers, with phase, winding, and layer locations taken into consideration. The relationship between TI, LMA and transformer age was modelled using previously published formulae in literature. The partitioning of LMA and HMA between mineral oil and paper was also investigated. Since TI is not directly measureable when a transformer is in-service, a TI prediction model was developed based on LMA measured in oil.The results indicated that there is no reduction effect of ageing (moderate towards severe) on the dielectric strength of paper, whereas the dielectric strength of paper is mainly influenced by the oil condition. LMA significantly reduces the breakdown voltage and accelerates the ageing of paper in mineral oil. The end of life based on TI is best represented by the 20 % retention of TI. Based on the case studies, the estimated end of life of transmission transformer population in the UK is around 43 years. The vertical distribution profile of LMA in paper and TI of paper along a winding is mainly influenced by vertical temperature profile. As paper ages, the amount of LMA in paper tends to increase as TI reduces. The percentage of LMA in service aged oil is about one fourth of Total Acid Number (TAN). The partition of LMA in mineral oil and paper favours its stay in paper and is heavily influenced by the weight ratio of paper and oil.The performance of the natural ester under high temperature ageing in open or sealed condition was evaluated based on TAN and viscosity measurements. The ratio of LMA and HMA in aged natural ester was examined. The effect of LMA and HMA on the AC breakdown voltage of natural ester and ageing of paper was also investigated. It was suggested that natural ester is suitable for application in hermetically sealed transformers. High temperature oxidation has significant effect on the physical and chemical properties of natural ester. Due to the polar nature of natural ester, the stay of LMA in natural ester is favoured over in paper which could be the reason why the breakdown voltage of natural ester and the ageing of paper are not significantly affected by LMA as in mineral oil.

Ageing assessment of insulation paper with consideration of in-service ageing and natural ester application

Sufficient insulation between different active parts of the transformer is necessary for its safe operation. The insulation system in a transformer can be categorized as follows: major insulation, and minor insulation. The solid insulation materials widely used in the transformer are paper, pressboard, and transformer board, which are formed from the cellulose found in plants. The main concern with using dry paper as an insulating material is that it is very hygroscopic. The presence of cellulose insulation affects the voltage distribution in heterogeneous dielectric systems: it contributes to the dielectric loss; determines the short???term dielectric strength; and, to a major extent, the life of the total insulated system. Oil is an equally important part of a transformer's overall insulation. The insulation system of a transformer undergoes ageing due to transformer operating conditions. This chapter provides an overview of the key concepts discussed in the subsequent chapters of this book.

Transformer Insulation Materials and Ageing

Study on the mechanical failure of the cellulosic insulation of continuously transposed conductors in power transformers under the influence of short circuits and thermal ageing

Power transformers are essential elements in electrical systems and the end of their useful life is frequently conditioned by the state of the cellulosic insulation materials inside them. These materials are subjected to high temperatures and mechanical stresses, primarily generated by short-circuits or inrush currents. Moreover, both the cellulosic materials and the dielectric oil in which they are impregnated are affected by chemical reactions. In order to achieve a better understanding of ageing kinetics and improve the management and planning in electrical systems from a technical and economical point of view, it is important to better understand the behaviour of cellulosic insulation. The purpose of this review is to analyse the current state-of-the-art with reference to the modelling of the mechanical behaviour of paper materials used as insulation in power transformers, as this topic has been previously analysed in many different publications but only fragmentarily. A suitable model for transformer cellulosic insulation, in combination with engineering measurements, could help to optimise the design of the insulation and manufacturing processes, as well as to accurately predict the end-of-life of power transformers.

State-of-the-art review on the performance of cellulosic dielectric materials in power transformers: Mechanical response and ageing

This paper presents on experimental on cellulose paper insulation and mineral insulating oil in power transformers. These materials were aged in specially designed glass cells at 140,150and 160 centigrade for ageing times. These measurements show that the loss of strength results from level varies of carbon monoxide and carbon dioxide. Paper tensile strength when combined and correlated with the amount of carbon monoxide and carbon dioxide gases in oil provides a good assessment of the condition of transformers cellulose insulation. Also it is suggested that the loss of tensile strength would provide an end of life criterion for predicting insulating paper life. Therefore a new paper expected life time model is developed.

Thermal Degradation of Cellulose Paper Insulation in Power Transformers

Since the life time estimation and prediction of materials/products is very time-consuming, so it is necessary to do accelerated tests. In this paper, accelerated thermal air ageing test has been carried out at three different temperatures, 120, 140 and 160 centigrade in the ageing time. Due to rather coordinated variations and correlate to time, tensile strength has been chosen as a significant indicator to determine the life time of paper insulations. Life time comes to an end when this parameter decreases to the half of its primary amount Therefore tensile strength test on cellulose paper insulation has been done in different thermal stresses and it has been used in new model of life time (or ageing) estimation. New life model has been formulated and called power-exponential model. This model in life time estimations obtained which have good accuracy in comparison to other models such as Arrhenius model. Also we show that is possible to estimation the reliability of paper insulation under thermal stress using this accelerated ageing test. Therefore exponential distribution that it is simple distribution and very common in reliability is used for application coupled with the new model.

Life Time Estimation and Reliability Model for Cellulose Insulation

Hoffa fractures are rare fractures of the femoral condyle that occur in the coronal plane of the bone. In most cases, high‐energy trauma leads to isolated coronal fractures of one of the femoral condyles, medial or lateral. Even with a typical unicondylar Hoffa fracture, our patient sustained a bicondylar Hoffa fracture in his right knee after falling from high and suffering direct trauma as well. The fracture was approached from both the medial and lateral sides of the distal femur. Three‐month follow‐up showed excellent functional scores, no laxity, and no pain.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/ccr3.7466

Bicondylar Hoffa fracture with concurrent medial and lateral collateral ligament avulsion: A case report

Background: Coronavirus disease 2019 (COVID-19) patients who undergo hip fracture surgery are expected to have worse outcomes because they are vulnerable to developing COVID-19-associated complications. The present review attempted to assess the in-hospital and 30-day mortality rates as well as the length of hospital stay in patients with COVID-19 infection who had hip fracture surgery. Methods: Two authors independently searched Google Scholar, PubMed, Web of Knowledge, SCOPUS, and Embase, based on the MeSH-matched scientific keywords. The nine-star Newcastle-Ottawa Scale (NOS) scoring system was employed to assess the methodological quality of all eligible studies. Results: Eleven cohort studies that included 336 patients comprised the study. Three studies reported in-hospital mortality. Eight studies reported 30-day postoperative mortality. The pooled in-hospital mortality rate was 29.8% (95% CI: 26.6%-35.6%). The pooled 30-day postoperative mortality rate was 35.0% (95% CI: 29.9%-40.5%). The mean hospital stay was 11.29 days (95% CI: 10.65 days-11.94 days). Conclusions: The rates of in-hospital and 30-day mortality in COVID-19 patients who undergo hip fracture surgery is high. These data suggest delaying hip fracture surgery until COVID-19 infection of the patients is controlled. Level of Evidence: Level II.

Length of hospital stay and mortality of hip fracture surgery in patients with Coronavirus disease 2019 (COVID-19) infection: A systematic review and meta-analysis

H. Tayyebi

Papers

Thermal Degradation of Cellulose Paper Insulation in Power Transformers

Life Time Estimation and Reliability Model for Cellulose Insulation

Bicondylar Hoffa fracture with concurrent medial and lateral collateral ligament avulsion: A case report

Length of hospital stay and mortality of hip fracture surgery in patients with Coronavirus disease 2019 (COVID-19) infection: A systematic review and meta-analysis