Power transformer under short-circuit fault conditions: A multiphysics approach

Power transformer under short-circuit fault conditions: A multiphysics approach

Vol. 7 Issue 3

How to evaluate the robustness of a transformer considering currents and Laplace forces under the short-circuit, fault conditions?

1. Introduction

Power transformers are critical devices in all power systems, and they generally represent the costliest devices [1]. Modeling and designing of transformers by considering multiphysics constraints is a critical task that is nowadays becoming more and more demanding in highly competitive sectors.

While design methodologies for power transformer are well established, especially regarding the active parts (i.e., core and coils) under normal working conditions [2], some additional concerns must also be taken into account at this stage, such as foreseeing malfunctions caused by short-circuit faults. Moreover, electromagnetic and thermal studies are insufficient to predict all possible damage types caused by short-circuits, therefore a complementary structural mechanics approach is necessary [1,3].

Electromagnetic forces arise naturally in any transformer since it is composed of coils carrying currents that are surrounded by a magnetic field. Under normal operating conditions, these Laplace forces occuring on windings are usually modest [4]. However, during short-circuit faults, currents flowing through windings can be significantly increased. Some authors estimate that resulting forces in short-circuit fault conditions can be up to 900 times higher than those experienced under normal operating conditions [5].

Electromagnetic and thermal studies have proven to be insufficient to predict all the possible damages caused by the short-circuit faults, therefore a structural mechanics approach is also required

Consequently, it is not surprising that electromagnetic forces and structural deformations in short-circuit fault conditions are a major concern for designers and manufacturers of transformers. Physical tests are usually carried out to determinate the device’s behavior under such critical conditions, but these tests are expensive, complex, and time-consuming. Also, they can involve testing the device for any potential destruction effects, especially for high-voltage transformers. The risk of destroying this high-value device is a primary concern with a great amount of literature devoted to it [1,6,7].

For such scenarios, an accurate numerical multiphysics simulation can fulfill both aims:

·      Informing designers about the maximum forces and deformations expected during the test to help anticipate the structural impact and to limit the potential damage the transformer may suffer;

·      Determine where simulations can be used instead of otherwise destructive physical tests.

This study describes the analysis of these forces and their structural effects through the coupling of two Finite Element Methods (FEM) software packages: Altair Flux™ with Altair OptiStruct™.