M
AVTMMTO3xx Rev 3 Sept 2013
36
TESTING TRANSFORMERS WITH TAP CHANGERS
F
WARNING
Make sure before changing taps to identify the types of tap
changers on the transformer and test accordingly per
instructions.
Many transformers used today have taps built into them. These taps allow ratio
to be increased or decreased by a few percent. Any of the ratio changes involve a
mechanical movement of a contact from one position to another. It is this
contact that needs to be checked by way of its contact resistance and mechanical
integrity.
The contact may go bad for a number of reasons.
1. Misaligned when manufactured causing insufficient surface contact. Full
load current overheats contact surface causing it to burn.
2. Current passing through contact exceeds full load rating.
3. Tap changing operation failing to have the required "Make Before Break"
sequence will create internal arcing of contact surface.
4. An off-load tap changer is switched while on load. Contact surface
becomes pitted and uneven.
Tap changers are divided into two types; “On-load” {DeEnergized Tap
Changer} and “Off-load” {On Load Tap Changer}. The OLTC tap
changer allows selection of ratio change while the transformer is in
service. This would mean the ratio of a transformer can be changed while
power (current) is still passing through it. The most common example of
this type of OLTC tap changer is a "Voltage Regulator".
The MTO is designed to test both DETC’s and OLTC’s.
For DETC taps, the software will direct the user to discharge the
transformer before changing taps. This will help assure the transformer,
equipment and user to not get damaged inadvertently.
For OLTC’s the software has three methods of testing. The first is
testing the taps in the same manner as a DETC, one tap at a time
stopping and filling in the form after each test. All of the TAPS can be
tested in sequence from 1 to 33 and then transferred to PowerDB form
and the last method is selecting a specific range of taps for testing if all
taps are not required for testing.
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