GE N60 Instruction Manual page 474

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 estimated I
10% of the base current. If I
is applied. I current is estimated with a fixed time constant for both cooling and heating that reaches to the final value in
p
two seconds on a step change (either step up or step down) signal.
The IEC255-8 cold curve is defined as follows:
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
5 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
5-280
current is greater or equal than 10% than the base current, then the hot curve characteristic
p
, defining maximum permissible current above nominal current
B
N60 NETWORK STABILITY AND SYNCHROPHASOR MEASUREMENT SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
current is less than
p
Eq. 5-24
Eq. 5-25
) is limited to 8, even when
B
Eq. 5-26