GE L30 Instruction Manual page 514

CONTROL ELEMENTS
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)
I is the measured overload RMS current at index n
n
E is the accumulated energy at index n
n
E is the accumulated energy at index n – 1
n – 1
The thermal overload protection element removes the
emergency, the thermal memory and
All calculations are performed per phase. If the accumulated energy reaches value 1 in any phase, the thermal overload
protection element operates and only resets when energy is less than 0.05 in all three phases.
Table 5-39: Typical time constants
5
Protected equipment
Capacitor bank
Overhead line
Air-core reactor
Busbar
Underground cable
The figure shows the logic for the thermal overload protection element.
5-300
> 0.97 × k × I
n B
THERMAL PROT 1 OP
output operand can be reset using
THERMAL PROT 1 OP
Time constant Minimum reset time
10 minutes 30 minutes
10 minutes 20 minutes
40 minutes 30 minutes
60 minutes 20 minutes
20 to 60 minutes 60 minutes
> k × I , element starts increasing the
n B
, the element starts decreasing the thermal energy:
output operand when E < 0.05. In case of
L30 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
Eq. 5-31
Eq. 5-32
setting.
THERM PROT 1 RESET