4.1 PHASE CURRENT DIFFERENTIAL TRIPPING CRITERIA
The phase current differential characteristic is defined by four settings:
● Phase Is1: The basic differential current setting which determines the minimum pick-up level of the
protection.
● Phase k1: The lower percentage bias setting used when the bias current is below the Phase Is2 setting. This
provides stability for small CT mismatches, while ensuring good sensitivity to resistive faults under heavy
load conditions.
● Phase Is2: A bias current threshold setting, above which the higher percentage bias setting Phase k2 is
used.
● Phase k2: The higher percentage bias setting used to improve protection stability under heavy through fault
current conditions.
The tripping criteria is defined by the bias characteristic graph. Basically if the differential current is above the line,
the protection will trip. If it is below, the protection will restrain.
When the phase-differential protection issues a trip command, it sends a per-phase ‘differential intertrip’ signal to
the remote terminals to instruct them to trip their circuit breakers. This is to ensure that all ends of the protected
line trip, even for marginal fault conditions.
For grading with other protection, the phase differential protection trip signal can be time delayed using either a
definite time, or an inverse time characteristic. By default, the delay characteristic is set to definite time with a zero
second delay, resulting in instantaneous tripping.
Since differential and bias current calculations are made on a per-phase basis, the values vary on a per-phase
basis. For optimum stability the highest value of the three-phase bias currents is used to restrain all three phases.
Minimum Trip Level
The minimum trip current level (Idiff min) is a function of Phase Is1 and Phase k1 as shown in the following
calculation (note: Phase Is1 and Phase k1 have been shortened to just Is1 and k1 for clarity).
I
di
= k1(Ibias) + Is1 ...
I
di
= (k1)(0.5I
di
) + Is1 ...
Is1 = I
di
- k1.0.5I
di
= I
di
(1 - 0.5k1) ...
I
di
= Is1/(1 - 0.5k1)
if k1 has a value of 30% for example, this means that the minimum trip current is 1.176Is1.
Chapter 6 - Current Differential Protection P543i/P545i
106 P54x1i-TM-EN-1
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