Toshiba GRT100 Series, Инструкция по эксплуатации

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2.11.6 Thermal Overload Element THR
Thermal overload element THR has a characteristic based on thermal replica according to IEC
60255-8 standards (see Appendix N), which evaluates the phase current (A-phase) of CT
secondary circuits. Figure 2.11.4 shows the characteristic of THR element. The element has trip
and alarm stages.
Trip stage:
t =

 Ln
I 2

Ip 2
I 2

(k  I
B
) 2
Alarm stage:
t =

 Ln
(I 2

Ip 2
)
 (1

T
A/

)
I 2

(k  I
B
) 2
where
t : operating time

: thermal time constant
I : load current
k  I
B
: allowable overload current as specified in IEC 60255-8 (refer to Appendix N)
I
B
: basic current of transformer (rated current)
k : constant (allowable overload current / IB)
Ip : prior load current before the overload occurs
T
A : time for alarm
(Alarming time = t (operating time)

TA (setting time))
Ln : natural logarithm
Figure 2.11.5 shows the thermal curve for a range of time constant settings under cold state when
the prior load current Ip is zero.
Trip
I
Alarm
0
K

IB
t
TA
TA
Figure 2.11.4 Characteristic of Thermal Overload Element