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2.5 Thermal Overload Protection
The thermal overload protection is applied to protect transformers from electrical thermal damage.
A-phase current is used to detect the thermal overload of a transformer. The characteristics are
exponential functions according to the IEC 60255-8 standard and take into account the I
2
R losses
due to the particular operational current and the simultaneous cooling due to the coolant. In this
way the tripping time during an overload condition takes the pre-load into consideration. An alarm
stage can be set to operate before reaching the tripping condition.
Figure 2.5.1 shows the scheme logic of the thermal overcurrent protection. THR tripping output
can be given to up to five breakers. Any of the five breaker tripping signals THR-1 to THR-5 can
be blocked by the scheme switch [THR1] to [THR5] settings. Alarming signal THR-A can be
blocked by the scheme switch [THRA] setting.
THR
A
S
THR-1
&
+
[
THR1
]
THR-2
&
+
[
TH
R
2
]
THR-3
&
+
[THR3]
THR-4
&
+
[
TH
R
4
]
THR-5 &
+
[THR5]
83
87
Note: Models 203 and 204 are not provided with THR-4 and THR-5.
THR-A
&
+
[
THRA
]
248
THR TRIP
≥1
351
“ON”
“ON”
“ON”
“ON”
“ON”
“ON”
Figure 2.5.1 Scheme Logic of Thermal Overload Protection
Setting
The following shows the setting elements for the thermal overload protection and their setting
ranges.
Element Range Step Default Remarks
τ 0.5 − 500.0min
0.1min 60.0min Thermal time constant
k
0.10 − 4.00
0.01 1.30 Constant
IB
0.50 − 2.50(*1)
0.01 1.00 Basic current
Ip
0.00 − 1.00(*1)
0.01 0.00 Pre-specified load current
TA
0 − 10min
1min 10min Time for alarm (before trip) (*3)
Scheme switch
THR1 to THR5
THRA
Off/On
Off/On
(*2)
On
Enable or disable
Trip
Alarm
(∗1): Multiplier of CT secondary rated current
(*2): Default settings are dependent on the models. See Appendix H.
(*3): Alarming time = THR trip time (operating time) – T
A
(setting time)
Note: Ip sets a minimum level of previous load current to be used by the thermal element, and is
typically used when testing the element. For the majority of applications, Ip should be set to
zero, in which case the previous load current, Ip, is calculated internally by the thermal
model, providing memory of conditions occurring before an overload.