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6 Protection Functions P50 Agile P153
6-10 P153/EN M/B
The fact that both EF1 and EF2 elements may be enabled at the same time leads to a number of
application advantages.
2.4.2 Earth Fault Protection Logic
The Earth Fault current is compared with a set threshold ( IN1> (n) Current Set ) for each stage. If it
exceeds this threshold, a Start signal is triggered, provided there are no blocks.
Earth Fault protection can follow the same IDMT characteristics as described in the Overcurrent
Protection Principles section. Please refer to this section for details of IDMT characteristics.
The description also applies to the Earth Fault 2 element (IN2).
The Earth Fault protection trip signal can be blocked by:
•
The Second Harmonic blocking function which is for all three phases. The blocking of Earth
Fault 1 is activated by setting the IN1> (n) 2H Blocking cell to ‘Enabled’ and blocking of Earth
Fault 2 is activated by setting the IN2> (n) 2H Blocking cell to ‘Enabled’, where (n) is the
number of the stage.
•
User defined blocking logic using ‘IO Mask’ functionality.
2.5 Thermal Overload Function
The heat generated within an item of plant, such as a cable or a transformer, is the resistive loss
(I
2
Rt). The thermal time characteristic is therefore based on the square of the current integrated over
time. The device automatically uses the largest phase current for input to the thermal model.
The equipment is designed to operate continuously at a temperature corresponding to its full load
rating, where the heat generated is balanced with heat dissipated. Over-temperature conditions occur
when currents in excess of their maximum rating are allowed to flow for a period of time. It is known
that temperature changes during heating follow exponential time constants.
The device provides single time constant characteristic which is used to protect cables, dry type
transformers (e.g. type AN), and capacitor banks.
Thermal overload protection is designed to prevent the electrical equipment when operating
temperature is exceeded the maximum designed temperature. The fundamental currents are
measured and analysed to monitor the thermal state. In case of thermal overload function the
calculation of the Time to Trip is given by:
t =
τ
ln ((K²- A)/ (K²- Th. Trip))
Where:
t: Time to trip (in seconds)
τ
: Thermal time constant (Te, in seconds) of the equipment to be protected
K: Thermal overload equal to (I
rms
/ k* IFLA)
Where:
I
rms
: RMS current corresponding to the largest phase current
IFLA: Thermal Trip or full load current rating (settable)
k : Settable (1 to 1.5 step 0.01)
A: Initial thermal state. If the initial thermal state is 50% then A =0.5
Th. Trip: Trip thermal state. If the trip thermal state is set at 100%, then Th. Trip is 1
The calculation of the thermal state is given by the following formula: