Function Calculation Principles; Fundamental Frequency Differential Currents - ABB REG650 Technical Manual

1MRK 502 034-UEN -
5.3.6.1
5.3.6.2
Technical Manual
Numerical IEDs have brought a large number of advantages and new functionality
to the protective relaying. One of the benefits is the simplicity and accuracy of
calculating symmetrical components from individual phase quantities. Within the
firmware of a numerical IED, it is no more difficult to calculate negative-sequence
components than it is to calculate zero-sequence components. Diversity of
operation principles integrated in the same protection function enhances the overall
performance without a significant increase in cost.
A novelty in GENPDIF, namely the negative-sequence-current-based internal-
external fault discriminator, is used with advantage in order to determine whether a
fault is internal or external. Indeed, the internal-external fault discriminator not
only positively discriminates between internal and external faults, but can
independently detect minor faults which may not be felt (until they develop into
more serious faults) by the "usual" differential protection based on operate-restrain
characteristic.
GENPDIF is using fundamental frequency phase current phasors and negative
sequence current phasors. These quantities are derived outside the differential
protection function block, in the general pre-processing blocks. GENPDIF is also
using with advantage the DC component of the instantaneous differential current
nd th
and the 2 and 5 harmonic components of the instantaneous differential currents.
The instantaneous differential currents are calculated from the input samples of the
instantaneous values of the currents measured at both ends of the stator winding.
The DC and the 2nd and 5th harmonic components of each separate instantaneous
differential current are extracted inside the differential protection.

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 L1, is as calculated in equation 26:
Section 5
Differential protection
103