GE C70 Instruction Manual page 376

CONTROL ELEMENTS
The thermal overload protection element corresponds to the IEC 255-8 standard and is used to detect thermal overload
conditions in protected power system elements. Choosing an appropriate time constant element can be used to protect
different elements of the power system. The cold curve characteristic is applied when the previous averaged load current
over the last 5 cycles is less than 10% of the base current. If this current is greater or equal than 10% than the base
current, then the hot curve characteristic is applied.
The IEC255-8 cold curve is defined as follows:
5
The IEC255-8 hot curve is defined as follows:
where
t = time to operate
op
τ = thermal protection trip time constant
op
I = measured overload RMS current
I = measured load RMS current before overload occurs
p
k = IEC 255-8 k-factor applied to I
I = protected element base (nominal) current
B
To ensure element accuracy for high overcurrent conditions, the maximum value of I/(k x I
realistically it is exceeding this value.
The reset time of the thermal overload protection element is also time delayed using following formula:
where
τ = thermal protection trip time constant
rst
T = a minimum reset time setting
min
5-252
THERM PROT 1 TRIP

TIME CONST: 45 min.
THERM PROT 1 RESET

TIME CONST: 45 min.
THERM PROT 1 MINIM

RESET TIME: 20 min.
THERM PROT 1 RESET:

Off
THERM PROT 1 BLOCK:

Off
THERMAL PROTECTION 1

TARGET: Self-reset
THERMAL PROTECTION 1

EVENTS: Disabled

---------------------- -
t = τ × ln 
op op
2

I
–
2

I
---------------------- -
t = τ × ln 
op op
2

I
–
, defining maximum permissible current above nominal current
B
 ( kI )
B
t = τ × ln ------------------------ -

rst rst
2

I ( kI
–
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
Range: 0 to 1000 min. in steps of 1
Range: 0 to 1000 min. in steps of 1
Range: 0 to 1000 min. in steps of 1
Range: FlexLogic operand
Range: FlexLogic operand
Range: Self-reset, Latched, Disabled
Range: Disabled, Enabled
2 
I

2

( )
kI
B
2

I
–
p

2

( kI )
B
) is limited to 8, even when
B
2

+ T

min
2

)
B
CHAPTER 5: SETTINGS
Eq. 5-21
Eq. 5-22
Eq. 5-23