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889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
PROTECTION
CHAPTER 4: SETPOINTS
Percent Differential
The 889 relay provides one Percent Differential element per protection group. It is intended
for use on the stator windings of the rotating machinery. This element is available only if
the second AC analog input card with the K1-CT bank is properly installed in slot K. For
wiring options, please refer to Installation/Electrical Installation/Differential CT Inputs
chapter of the manual.
For internal summation percent differential, it is recommended that the terminal side CTs
and neutral side CTs have the same ratios but it is allowed for the ratios to be different. The
maximum allowable ratio mismatch is 10:1. In the case of a mismatch, the 889 scales the
currents to the primary of the CT with the higher primary value which is used as the CT
reference for the percent differential element.
Internal summation operates in a per phase basis (phase-segregated). The key variables
used in the element are the Restraining current (I
r
) and the Differential current (I
d
). The way
they are calculated depends on the used differential method as follows:
Internal Summation Method
For internal summation method, differential current Id and restraint current I
r
are defined
below:
Eq. 1
Where: I
ts
is the current phasor of the terminal side CTs (J1-CT bank), in per unit normalized
to the reference CT nominal; I
ns
is the current phasor of the neutral side CTs (K1-CT bank), in
per unit normalized to the reference CT nominal; They are calculated based on the
following equations:
Eq. 2
Where:
The Percent Differential element with the internal summation method selected uses dual
slope dual break point differential/restraint characteristic. The main purpose of the
percent-slope characteristic is to prevent improper operation caused by unbalances
between CTs during external faults. CT unbalances arise as a result of the following factors:
1.
CT accuracy errors
2.
CT saturation
The characteristic allows for very sensitive settings when fault current is low and less
sensitive settings when fault current is high and CT performance may produce incorrect
operate signals. The dual slope characteristic is shown in the following figure.
