Dynamic Resistance Measurement As a tool for OLTC Condition assessment

The dynamic LTC function provides Graphical results on a PC that makes easy identification of LTC faults in contact resistance, contact timing, make before break mechanism timing and diverter issues, etc. The LTC does not have to be drained or opened up to achieve this diagnostics.

Content of this presentation Introduction History + WG Tap changers + failures + why test Test method Case studies Good OLTC Bad OLTC Demagnetization

What is an OLTC? Standard defines OLTC as 3.14 load tap changer (LTC): A selector switch device, which may include current interrupting contactors, used to change transformer taps with the transformer energized and carrying full load.

BASICS What is DRM? Dynamic vs Static

Static – dynamic : golf ball Click for video

Dynamic resistance measurement principles Fast sampling record of the test current Transition time

The result is a graph

Basic test issues Types of regulation Static vs. dynamic Types of switching cycle Static vs. dynamic Continuous vs. step by step Why a sudden jump in step by step mode Instrument and computer Presentation of RMO TD

Presentation on References and Working group activities History and purpose History and purpose History and purpose Presentation on References and Working group activities

Presentation on DC testing Parameters variation Testing transformers Presentation on DC testing Parameters variation

CASE STUDIES GOOD and BAD Diverter switch Four resistors Vacuum / One resistor Arcing tap switch Series transformer Reactance type Wavering trace a contact problem

Specific types MR type M MR type R, T, D MR type VRC MR type V MR type VV MR type RMV ABB UZ ABB type VUCGRN ASEA UZB Fuller UK type? English Electric UK type? Waukesha type UZD

Interesting cases ELPROM Bulgaria, see > motor current issue SMUD PG&E Bellehaven Gill Malaysia discontinuity bad spring UK change over switch mis-adjustment MR type RMV gassing

Presentations on various problems and issues when testing

This section deals with: Simulation of defects in Ireland Testing OLTC w/o transformer Motor current analysis Neutral operation (2+2/3+1 positions) Open or shorted secondary Main contact vs. Main switching contact Powering the instrument (motor influence)

Various problems Runaway – stabilization Small ripple Saturation High interference signal The ratio of the two Saturation Arcing Regulator Switching oscillations

Presentation on removal of remanent magnetism Demagnetization Presentation on removal of remanent magnetism

Conclusions Not for everyone Need to know what one is doing Quick Straight forward Visible problem indication

The end