Development of a new multi-graded winding to alleviate resonant characteristic of EHV transformers

Mitigation of winding resonance triggered by oscillatory transient over voltages is one of the key challenges in designing high-voltage transformer windings. Internal resonance is typically an issue with high-voltage transformers with homogeneous winding, manifesting identical natural frequency for the uniform winding sections. Self-capacitance grading can be a possible remedy to resonance issue which incorporates diversification in the natural frequencies of winding disk coils. Practical implementation of this concept is however difficult due to economic and design constraints. This study proposes a practical method of implementing the concept of natural frequency diversification by use of ‘multi-graded inter-winding electrostatic shields’. The concept, theory, design and implementation of the proposed winding method have been presented in this study. A scaled-down prototype of a typical high-voltage transformer has been developed to illustrate the real-time implementation of the proposed method. Both simulation and experimental results established that the proposed winding can effectively suppress oscillatory characteristics under transient over voltages and overcome winding resonance.