Stator Differential Logic - GE Multilin 489 Instruction Manual

CHAPTER A: APPENDIX
A.2.2

Stator Differential Logic

489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
Courtesy of NationalSwitchgear.com
In addition, enhanced differential protection algorithm takes care the timing offset
between the main and auxiliary contacts of the breaker. As a result, maximum of 50ms
timing offset between the main and auxiliary contacts of the breaker will block the
differential function.
The differential function uses an internal timer of 130ms as shown in the figure below. This
timer is a common timer for all three phases of the differential function. Normally, the timer
is not engaged ensuring instantaneous operation and backward compatibility with the
previous firmware revisions of the product.
The timer is engaged only when the terminal-side currents in all three phases are zero. If
any of the terminal currents is above 5% of CT nominal, the timer is by-passed. Also, if any
of the neutral-side current is above 5 times CT nominal, the timer is by-passed as well.
In this logic the current magnitudes are filtered fundamental frequency components (T
stands for terminal-side currents, and N stands for neutral-side currents); PKP denotes the
pickup state of the element prior to any user set delay that may or may not be used in a
particular application; A, B and C designate phases.
The differential element works as follows:
With the machine under load, the terminal currents are above 5% of CT nominal and no
delay is applied to the differential function.
With the machine on-line but with no load (below 5%) the delay is applied. However, should
a fault occur at that time, at least one of the terminal current would get elevated
cancelling the delay and resulting in an instantaneous trip.
With the opposite breaker being closed as in the considered dual-breaker application, a
current is drawn (either transformer inrush or load or both). This will activate the
differential characteristic. However, the timer remains engaged because all the terminal
currents (ABC) are zero, and all the neutral-side currents (ABC) are below 5 times CT
nominal. The timer keeps timing out. However, before it expires the relay switches to group
2 and blocks the differential function. This prevents misoperation.
IT mag A > 0.05pu
IT mag B > 0.05pu
IT mag C > 0.05pu
IN mag A > 5pu
IN mag B > 5pu
IN mag C > 5pu
87 PKP A
87 PKP B
87 PKP C
FIGURE A–7: Enhancements to the stator differential logic
Normally, no extra
delay is applied
0ms
100ms
130ms
0ms
With no terminal side currents, a
delay of an ex tra 130ms is applied
to the differenti a l function.
87 PKP
A–9