4. Read Last Resistance Test: Allows population of form from an interrupted test sequence. Software
keeps past results within internal log files in the event of an interruption.
Method 1 - Resistance Test Wizard -
Resistance Test Wizard is designed to automate the winding resistance test, allowing the Transformer Nameplate
Input above is combined with operator selection of taps to complete a test sequence with minimal button presses. This
wizard also allows testing of OLTC taps without discharge of test current during transition (between) of taps, making
testing both more efficient (less time charging and discharging winding and less button pressing) as well as diagnostic
by indication of a make break condition.
NOTE: When selecting ‘All’ phases, multiple phases are tested concurrently. This approach improves test time, BUT
has a potential drawback when testing WyeN and 3-phase Auto windings. The resistance of the neutral bushing
is excluded in the first measurements because test current does not flow through the neutral bushing. This results
in lower than expected readings on the outer 2 phases, which can exceed the “% Difference” limit for testing. To
correct this situation, select individual tests for each phase.
i. Winding Selection: Allows choice of winding(s) to be tested.
ii. Tap Selection: Enables selection of tap(s) to be tested, as well as test direction via tap changer (OLTC) direction in
which test will be conducted.
iii. Phases to Test: Allows choice of which single, or 3-phase windings are to be tested.
iv. Make-Break Transition Recording: For OLTC tap changers can be enabled/disabled within this control. Pass/Fail
criteria are controlled in Resistance Test Wizard Setup above.
Method 2 – Measuring Tap – All Phases Winding Resistance -
i. Test #: Depressing this button commences testing for a specific tap row (circled). Testing is conducted
sequentially (up to 2 windings at once), ensuring optimization of magnetized core.
ii. Test Current: Records the dc current amplitude recorded for the result.
www.megger.com
MTO3XXX-UG-EN Rev 1 Feb 2018
28
Three-phase transformer & winding resistance testing
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