Figure 3-1. Voltage inversion
3.2.3 PHASE UNBALANCE
Severe phase unbalance may occur in series-compensated power systems. This unbalance may be caused by
non-symmetrical behavior (flashing or reinserting) of the gaps or other capacitor protective devices. (See Figure
3-2.)
Three-Phase Fault
Flashover in Phase A only
Phase C
Phase B
Phase A
nZL
l-n
ZL
F
Zs
Zc
REMOTE
V
R
If | nZ
L | < | ZC |, then
V
R is inverted
LOCAL
Figure 3-2. Phase unbalance
Phase unbalance causes circulating zero-sequence and negative-sequence currents. These may affect the op-
eration of distance relays, directional overcurrent relays, and current-only schemes that compare composite
quantities. The RFL 9300 is unaffected by phase unbalance because it operates on a per-phase basis; each
phase current and the 3I
0
current are compared separately.
RFL 9300 RFL Electronics Inc.
August 25, 2000 3 - 2 (973) 334-3100