Specifications

7SR242 Duobias Applications Guide
49 Capacity Alarm Setting
This setting can be used to provide an alarm prior to a thermal trip occurring and is typically set to about 80 to 90
% of thermal capacity. The thermal capacity alarm can be mapped to a binary output wired to the control system.
Example
Figure 2-19 Thermal Overload Settings
1. In fig. 2-19 the direction of power flow is HV to LV. If W1 input is connected to HV CTs (as is usual) then
set 49 Select = W1. The transformer loss current and harmonic currents are then included in the thermal
calculation.
2. FUNCTION CONFIG>Gn Thermal: Enabled
CURRENT PROT’N>THERMAL>Gn49 Thermal Overload: Enabled.
3. CURRENT PROT’N>THERMAL>49 Overload Setting:
Maximum Primary Full Load current =
A5.231
85.01323
45000
uu
Secondary Current = 231.5A/300 = 0.772A. The thermal function should not trip for currents below this
value.
A setting of 110% is used to include a margin of safety.
Therefore 49 Overload Setting = 0.85 x I
n
(I

) (1.10 x 0.772)
4. The time constant to apply will depend upon the transformer overload specification, but in this case it
was decided to set a time constant of 178 minutes. This will allow an overload of 150% from ambient for
about two hours before a trip is issued.
5. The capacity alarm is a useful function and therefore it is set to 90%. The current required to reach this
90% figure should be calculated. It is important not to alarm for current within the normal loading range of
the transformer.
The steady state thermal capacity = I
2
/ I

2
x 100%
For this example: 90% = I
2
/ I

2
x 100%. I = 0.806 x I
n
and this level is above the maximum full load
current of 0.772 x I
n
.
The above provides guidelines only as setting philosophies differ. Alternative protection setting groups may be
used to match transformer loading for temporary or emergency overloads, wide variations in winter/summer
loading or if a cooling failure (pump or fan) occurs. The thermal settings applied will differ in each Setting Group
and will be made appropriate to the specific load conditions.
©2010 Siemens Protection Devices Limited Chapter 7 Page 33 of 56