Thermal Overload Protection - Toshiba GRT100 Series Instruction Manual

2.6 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 IEC 60255-8 standards and take into
account the I
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 2.6.1 shows the scheme logic of 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.
Setting
The following shows the setting elements for the thermal overload protection and their setting
ranges.
Element

k
IB
Ip
T A
Scheme switch
THR1 to THR5
THRA
(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.
2
R losses due to the particular operational current and the simultaneous cooling due
S
THR
A
Figure 2.6.1 Scheme Logic of Thermal Overload Protection
Range Step
0.5  500.0min
0.1min
0.10  4.00
0.01
0.50  2.50(*1)
0.01
0.00  1.00(*1)
0.01
0  10min
1min
Off/On
Off/On
[THR1]

"ON"
[THR2]

"ON"
[THR3]

"ON"
[THR4]

"ON"
[THR5]

"ON"
[THRA]

"ON"
Default Remarks
60.0min Thermal time constant
1.30 Constant
1.00 Basic current
0.00 Pre-specified load current
10min Time for alarm (before trip) (*3)
Enable or disable
(*2) Trip
On Alarm
 31 
6 F 2 S 0 7 8 9
&
THR-1
&
THR-2
&
THR-3
&
THR-4
&
THR-5
&
THR-A