Function Calculation Principles - ABB Relion Technical Manual

1MRK 502 048-UUS A
6.4.6.1
6.4.6.2
Technical manual

Function calculation principles

To make a differential protection as sensitive and stable as possible, the restrained
differential characteristic is used. The protection must be provided with a proportional
bias, which makes the protection operate for a certain percentage differential current
related to the current through the generator stator winding. This stabilizes the protection
under through fault conditions while still permitting the system to have good basic
sensitivity. The following chapters explain how these quantities are calculated.
The fundamental frequency phasors of the phase currents from both sides of the generator
(the neutral side and the terminal side) are delivered to the differential protection function
by the pre-processing module of the IED.
Fundamental frequency differential currents
The fundamental frequency RMS differential current is a vectorial sum (that is, sum of
fundamental frequency phasors) of the individual phase currents from the two sides of the
protected generator. The magnitude of the fundamental frequency RMS differential
current, in phase A, is as calculated in equation 30:
= +
Idiff _ A [(Re( IAn IAt ))
ANSIEQUATION2316 V2 EN
One common fundamental frequency bias current is used. The bias current is the
magnitude of the highest measured current in the protected circuit. The bias current is not
allowed to drop instantaneously, instead, it decays exponentially with a predefined time
constant. These principles make the differential IED more secure, with less risk to operate
for external faults. The "maximum" principle brings as well more meaning to the
breakpoint settings of the operate-restrain characteristic.
=
Ibias max( IAn IBn ICn IAt IBt ICt , ,
EQUATION2049-ANSI V2 EN
+ +
2 2
(Im( IAn IAt )) ]
, , , )
Section 6
Differential protection
(Equation 30)
(Equation 31)
137