BECKWITH ELECTRIC M?3311A, Instruction Book

Page: 186 / 450

4–59
System Setup and Setpoints – 4
49 Winding Thermal Protection
The thermal overload function provides protection
against possible damage during overload condi-
tions. Temperature and overload monitoring of
oil-filled transformers are carried out with the use
of indicating thermostats (standard). The oil ther-
mometer, which measures the top oil temperature,
cannot be relied upon to detect short-time overloads
beyond permissible limits.
Transformers without winding thermometers should
have a thermal current protection with operating
current/time characteristics that correspond to the
current overload characteristic of the transformer
windings. For transformers with winding thermom-
eters, a thermal current protection will provide a
back-up function for this monitoring device.
The 49 function uses the demand current as
pre-load current, to protect the transformer following
the IEC-255-8 standard:
Where: t = time to trip
τ
= time constant
I
load
= relay current
I
preload
= pre-load current
I
max
= maximum allowed continuous overload current
The pre-load current “
I
pre-load
” is the previous average
current for the last 15 minutes, 30 minutes, or 60
minutes programmable into the demand metering.
The M-3311A includes four setpoint groups that
can accommodate a power transformer’s different
MVA requirements. One setpoint group can be used
for basic rating setpoints and others can be used
to change to a second group of setpoints for use
with higher ratings when forced cooling is required.
Example: If we consider that the transformer was
working with 80% of its rating power prior to over-
load, then the current goes up to 2.0 times the
maximum current (
I
load/
I
max=2.0). Selecting the
curve P=0.8 (see Figure 4-33), we have t/
τ
=0.1133.
If
τ
=30 minutes, then the time delay for this condi-
tion would be: t=0.1133 x 30=3.3999 minutes.

NOTE:
Winding Four is not available for selection
in Two or Three Winding applications.