Background - GE Multilin 489 Instruction Manual

A.2 Stator Differential Protection Special Application
A.2.1

Background

A–8
Courtesy of NationalSwitchgear.com
The 489 relay is applied in a dual breaker arrangement as shown in the figure below. In this
configuration one breaker is closed at a time eliminating a danger of through fault
conditions. However, the customer prefers not to sum up the two breaker currents to
obtain effectively the terminal-side current of the generator, nor to install an extra CT at
the generator to measure the terminal-side current explicitly. Instead, the customer
applies two 489 relays each spanning its differential zone between the neutral-side CT of
the generator and the CT at the corresponding breaker.
In this application, when a breaker is closed, the other (opposite) relay would measure the
neutral-side current without the matching terminal-side current, as the latter flows via the
other (closed) breaker and it not visible to the opposite relay.
Block 87 when CB closed
489-2
FIGURE A–6: Considered application of two 489s protecting a dual-breaker generator
When both breakers are opened both relays be operational with the differential function
enabled. The application is based on blocking the differential function using the position of
the opposite breaker via the multiple setting group mechanism of the relay.
When both breakers are opened, both relays are in their setting group 1 with the
differential functions operational. When a breaker is closed, its relay remains in group 1 so
that no setting group switching takes place and therefore continuous uninterrupted
protection is provided for the generator. At the same time the opposite relay is blocked by
switching to group 2 in which the differential function is disabled. This prevents mis-
operation. There is no provision for an "advanced close" signal, and the breaker position
signal is used instead.
489-1
G
configuration
489 GENERATOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER A: APPENDIX