Megger MTO210, Инструкция по эксплуатации

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Transformer Testing
AVTMTO210 Rev 7 Sept 2013
35
Testing Transformers with Tap Changers
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” and “Off-load”. The "On-
load" 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
"On-load" tap changer is a "Voltage Regulator".
The MTO is ideally suited to test “On-load” tap changers because the instrument
can be left ON while changing from tap to tap. This allows the operator to take
measurements very quickly without discharging, then re-charging the transformer
for every tap. The MTO will re-balance after every tap change. If the tap is
defective (open) or if there is even a fraction of time (1mS) where circuit is open,
the MTO will indicate an open circuit. If the current interruption is longer than
about 10 ms, the MTO will stop the test and discharge the transformer
automatically. This gives the operator a clear indication of a possible fault within
the tap changer. For this OPEN condition, no damage will be done to the
transformer by the MTO’s DC current.
The second type of tap changer is the "Off-load". This is not as common as
“On-load” because in order to change taps, the transformer has to be taken out
of service or at least disconnected from the load. The MTO will test “Off-load”
taps, but the transformer must be discharged between tap changes. If the
transformer is not discharged between tap changes, damage can result in the
transformer or the test unit