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e-lesson #2 – Resistor-type OLTCs
Key focus
To understand the operating principles, switching sequences, construction concepts, and diagnostic considerations associated with resistor-type on-load tap changers.
Lesson description
This lesson examines resistor-type on-load tap changers and explains how resistors are used to limit circulating short-circuit currents during tap transitions under load conditions. It reviews the operation of arcing tap switches, diverter switches, selector switches, and vacuum interrupters, highlighting the differences between oil-switching and vacuum-switching technologies. The session explains make-before-break switching principles, transition timing, synchronization requirements, and the role of resistors in controlling thermal and electrical stresses during operation. Particular attention is also given to maintenance considerations, diagnostic techniques such as DGA, vibration analysis, and DRM testing, together with the operational challenges associated with contact wear, switching performance, and OLTC reliability.
Learning outputs
- Explain how resistor-type OLTCs limit circulating short-circuit currents during tap transitions under load. • Distinguish arcing tap switches, diverter switches, selector switches, and vacuum interrupter designs used in resistor-type OLTCs. • Describe the make-before-break switching principle and explain the importance of fast transition timing and synchronization during operation. • Identify the advantages and limitations of oil-switching and vacuum-switching technologies in modern OLTC applications. • Assess the main diagnostic and maintenance methods used to evaluate OLTC condition, including DGA, vibration analysis, DRM testing, and motor-drive monitoring.