GE C70 Instruction Manual page 377

CHAPTER 5: SETTINGS
The thermal overload protection element estimates accumulated thermal energy E using the following equations
calculated each power cycle. When current is greater than the pickup level, I
thermal energy:
When current is less than the dropout level, I
where
∆t is the power cycle duration
n is the power cycle index
t is the trip time calculated at index n as per the IEC255-8 cold curve or hot curve equations
op(In)
t is the reset time calculated at index n as per the reset time equation
rst(In)
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
Figure 5-135: IEC 255-8 sample operate and reset curves
E = E + ------------- -
n n 1
– t
> 0.97 × k × I , the element starts decreasing the thermal energy:
n B
------------- -
E = E
–
n n 1
t
–
> k × I , element starts increasing the
n B
t Δ
op In ( )
t Δ
( )
rst In
CONTROL ELEMENTS
5
Eq. 5-24
Eq. 5-25
5-253