Thermal Overload Protection Implementation; Thermal Overload Protection Logic; Application Notes; Setting Guidelines For Dual Time Constant Characteristic - GE MiCOM P40 Technical Manual

P446SV
7.3

THERMAL OVERLOAD PROTECTION IMPLEMENTATION

The device incorporates a current-based thermal characteristic, using RMS load current to model heating and
cooling of the protected plant. The element can be set with both alarm and trip stages.
Thermal Overload Protection is implemented in the THERMAL OVERLOAD column of the relevant settings group.
This column contains the settings for the characteristic type, the alarm and trip thresholds and the time constants.
7.4

THERMAL OVERLOAD PROTECTION LOGIC

IA IA
IB IB
IC IC
Thermal Trip
Characteristic
Disabled
Single
Dual
Time Constant 1
Time Constant 2
Reset Thermal
Thermal Alarm
V00630

Figure 213: Thermal overload protection logic diagram

The magnitudes of the three phase input currents are compared and the largest magnitude is taken as the input
to the thermal overload function. If this current exceeds the thermal trip threshold setting a start condition is
asserted.
The Start signal is applied to the chosen thermal characteristic module, which has three outputs signals; alarm trip
and thermal state measurement. The thermal state measurement is made available in one of the MEASUREMENTS
columns.
The thermal state can be reset by either an opto-input (if assigned to this function using the programmable
scheme logic) or the HMI panel menu.
7.5

APPLICATION NOTES

7.5.1

SETTING GUIDELINES FOR DUAL TIME CONSTANT CHARACTERISTIC

The easiest way of solving the dual time constant thermal equation is to express the current in terms of time and
to use a spreadsheet to calculate the current for a series of increasing operating times using the following
equation, then plotting a graph.
2 (
−
0 4 . I e .
p
I
=
0 4 .
P446SV-TM-EN-1
Max RMS
Thermal
Calculation
445
t / 1 t / 2
) 2 ( )
τ − τ
0 6 . I e . k I
+ −
p
( t / 1 ) ( t / τ 2 )
− τ − −
e 0 6 . e 1
+ −
Chapter 13 - Current Protection Functions
Thermal trip
threshold
2 2
.
FLC
Thermal State
680
Thermal Trip
785
Thermal Alarm
379