Real-time dynamic loading and thermal diagnostic of power transformers

A comprehensive approach to dynamic loading of power transformers is presented. It is based on the Annex G equations in the latest IEEE Loading Guide. The IEEE work is enhanced through a continuous update of rated temperatures. The algorithm was evaluated with nine transformer-months of real-time field data. Field observations and comparison with the IEEE Clause 7 and IEC thermal models are presented. Dynamic loading ratings are determined considering limitations imposed by top-oil and hottest-spot temperatures, load current and loss-of-life as defined by the IEEE for various types of loading. The rating calculations also consider temperatures of bushings, voltage, present value and historical trend of ambient temperature, preload parameters, cooling mode during the overload, moisture content in the paper, presence of bushing enclosures, and the type of oil-preservation system.

I. Introduction

UTILITY managers are charged with optimizing the asset value of power equipment. This involves ensuring optimal utilization in terms of power flow and asset life. The greater use of energy trading means that contracts need to be made and kept, often on a short-term basis, and often for an increased power flow—but without compromising security standards. It should also be made on the basis of an understanding of the consequence on asset life. This requires that system operators have access to an instant or current display of the thermal capability of the system: the dynamic rating. Post fault recovery also presents a similar situation. The interest is therefore to have a variety of tools and systems for a variety of the key system assets [1], [2]. Power transformers represent the most expensive assets in a substation. When they are purchased, the specification identified a thermal rating. This is based on a factory heat run test and Standards such as IEEE C57.91-1995 and IEC 354. Generally, the important aspect for ratings of a power transformer is not that the unit cannot withstand an increased load—but rather that the insulation life reduces as the temperature is increased.