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 – 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

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

Issues 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 – High interference signal – The ratio of the two • • Saturation Arcing • Regulator • Switching oscillations

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