4–276 889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
MONITORING CHAPTER 4: SETPOINTS
Thermal Overload
Protection (49TOL)
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, the thermal overload protection 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 to 10% of the base current, then the hot
curve characteristic is applied.
The IEC255-8 cold curve operate time is defined as follows (EQN 1):
The IEC255-8 hot curve operate time is defined as follows (EQN 2):
where:
•t
op
= time to operate
•
op
= operate time constant
• I = measured overload RMS current
•I
p
= measured load RMS current before overload occurs
• k = IEC 255-8 k-factor applied to Ib, defining maximum permissible current above
nominal current
•I
b
= protected element base (nominal) current or full load current (FLA)
For example, when this element is used for the generator overload protection, I
b
is the
generator rated current calculate from the Rated MVA and Rated Voltage set under Path:
Setpoints > System > Generator > General.
I
b
is calculated as (EQN 3):
The reset time of the thermal overload protection element is also time delayed using
following formula (EQN 4):
where:
• t
Reset
= time to reset
•
Reset
= reset time constant
•T
min
= minimum reset time setting
In some applications, the shape of the power system element thermal damage curve
substantially deviates from the standard IEC curve. It may be necessary to use a custom
curve in order to incorporate the exact match of the actual overload damage curve
without compromising protection. For these conditions, it is recommended that the
FlexCurves be used. FlexCurves allows the user to program selected operate (t
op
) and reset
time (t
Reset
) for pre-determined current levels.
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