Chapter 2 - Harmonic Models of Transformers

Abstract

This chapter addresses issues related to modeling and operation of transformers under (non) sinusoidal operating conditions. It briefly reviews the operation and modeling of transformers at sinusoidal conditions and highlights the impact of harmonics and poor power quality on transformer losses and discusses several techniques for derating of single- and three-phase transformers including K-factor, FHL-factor, and loss measurement techniques. Extensive application of power electronics and other nonlinear components and loads creates single-time and periodic events that could lead to serious problems within power system networks and its components. Transient models are used for transformer simulation during turning-on, faults, and other types of disturbances. They are based on a system of time-dependent differential equations usually solved by numerical algorithms. Transient models require a considerable amount of computing time. Steady-state models mostly use phasor analysis in the frequency domain to simulate transformer behavior, and require less computing times than transient models. Several transformer harmonic models in the time domain and/or frequency domain are introduced. Important power quality problems related to transformers such as ferroresonance and geomagnetically induced currents (GICs) are explored. Different techniques for system and transformer grounding are explained. Many application examples explaining nonsinusoidal flux, voltage and current, harmonic (copper and iron-core) loss measurements, derating, ferroresonance, magnetic field strength, propagation of surge, and operation of lightning arresters are presented for further exploration and understanding of the presented materials.