Function Calculation Principles - ABB REG650 Technical Manual

1MRK 502 043-UUS B
6.3.6.1
6.3.6.2
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 26:
= +
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 26)
(Equation 27)
119