GE P645

2.2 THE THERMAL MODEL
The simplest implementation of overload protection employs an I
2
t characteristic. Y
ou set time constants such as
the winding time constant at Hotspot location and top oil rise time constant, so that the thermal model can follow
the correct exponential heating and cooling profile. Transformer loads are becoming increasingly non-linear;
therefore the device uses true RMS current values to replicate the winding Hotspot temperature.
Note:
"True RMS" refers to the RMS value of a non-sinusoidal waveform including the fundamental and all other components.
2.2.1 TOP OIL TEMPERATURE CACULATIONS
If the Top Oil temperature is not available as a measured input quantity, it is calculated every cycle by the following
equation:
Q
TO
= Q
A
+ DQ
TO
where:
●
Q
TO
= Top Oil temperature
●
Q
A
= Ambient temperature
●
DQ
TO
= Top Oil rise over ambient temperature due to a step load change
You can either measure the ambient temperature directly, or set it in the Ambient T setting. The ultimate top oil rise
is given by the following equation:
∆Θ ∆Θ
TO U TO R
u
n
K R
R
, ,
= ⋅
+
+






2
1
1
where:
K
u
= the ratio of actual load to rated load
R = the ratio of the load loss at rated load to no load loss (Rat
ed NoLoadLoss setting)
n = Oil exponent (Oil exp n setting)
Q
TO,R
= Top Oil rise over ambient temperature at rated load (Top Oil Overamb setting)
The load current used in the calculations is the RMS value.
2.2.2 HOTSPOT CACULATIONS
The Hotspot temperature can only be obtained by calculation. The following equation is used to calculate the hot
spot temperatur
e every cycle:
Q
H
= Q
TO
+ DQ
H
where:
●
Q
H
= Hotspot (winding) temperature
●
Q
TO
= Top Oil temperature
●
DQ
H
= Hotspot rise above top oil temperature
The ultimate Hotspot rise over top oil is given by:
DQ
H,U
= DQ
H,R
. K
U
2m
Chapter 7 - Transformer Condition Monitoring P64x
146 P64x-TM-EN-1.3