OTI stands for Oil Temperature Indicator and WTI stands for
Winding Temperature Indicator. They are used for
Transformer protection.The temperature of winding increases
as the load increases. Oil is used as coolant for the
winding so as winding heats up oil temp. rises as well.
The life of transformer depends on insulating material
between the windings so it is important to check and
control the temp. OTI and WTI are thus used for alarm or
tripping of the transformers.
A basic or Avg Difference in OTI & WTI are, :- WTI needs a
CT to function , While OTI does not need a CT. WTI has both
Alarm & Trip , Though OTI also has both ALarm & Trip , there
is also a OTI without Alarm & Trip . Its not the cas of WTI.
There is no fix differnce as such.Itdepends upon the
transformer tempareture gradiant at that current, specified
by the manufacturer.the difference between ot&wt varies as
per load and gradient.Normally it can be from 12 to 16
degrees at full load.
The above answer given by me is not true in all cases.Those
were the readings recorded on 220/110 KV auto
transformer.The WT is higher than OT as WTI is close to the
source of heat.The difference between WT and OT depends on
cooling and oil circulation.More the difference worse the
cooling efficiency.The difference is about 20 %(220 KV) to
40% (110KV) of temperature rise in WT ,as observed in case
of the above Tr.But WT of tertiary was found to be less than