Megger MTO106, User Manual

Page: 14 / 20

14
MTO106 ZP-BN01E BN034794AE
5 APPLIcATION EXAMPLES
5
A
pplication examples
5.1 Testing transformers
with tap changers
The tap changer allow ratio to be increased or
decreased. 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 resistance. The contact may go bad for a
number of reasons.
▪
Misaligned when manufactured causing insufficient
surface contact. Full load current overheats contact
surface causing it to burn.
▪
current passing through contact exceeds full load rating.
▪
Load tap changing operation not "Make before break"
creating internal arcing of contact surface.
Winding resistance measurements (WRM) are
normally performed for every tap the same way as
WRM for individual windings. The test instrument is
continuously injecting test current and the resistances
for each tap are measured sequentially as the tap
changer is stepped through its positions. Results are
typically presented as a graph or table with resistance
values for each tap. Resistance changes between
taps should be consistent with only small deviations
between different tap position changes. Fig 1 shows
a typical behavior for a transformer/tap-changer in
as-new condition.
Fig 5.1. Winding resistance vs tap position for a new trans-
former
Measuring the winding resistance for each individual
tap is quite straightforward. The most common
issue is probably that the tester has not waited for a
sufficient time for taking measurements after a tap
change. Monitor the resistance value closely before
storing the value to make sure the resistance value has
stabilized!
5.2 Temperature correction
cold winding resistance measurements are normally
converted to a standard reference temperature equal
to the rated average winding temperature rise plus
20ºc. In addition, it may be necessary to convert
the resistance measurements to the temperature
at which the impedance loss measurements were
made. If winding resistances are to be compared to
factory values, resistance measurements will have
to be converted to the reference temperature used
at the factory (usually 75°c). The conversions are
accomplished by the following formula:
Rs = Rm ( Ts + Tk ) / ( Tm + Tk )
where:
Rs resistance at desired temperature Ts
Rm measured resistance
Ts
desired reference temperature, in °c.
Tm temperature at which resistance was
measured, in °c.
Tk 235 (copper) 225 (aluminum)