Journal of Reliable Power - SEL

2
II. COMMON ZONE TIMING
In June 2006, a fault occurred just past the remote terminal
on a two-terminal 138 kV transmission line. The directional
comparison blocking (DCB) scheme was disabled at the time
of the fault. Because the fault was within the Zone 2 reach, we
expected a trip within the Zone 2 delay of 25 cycles.
Event data from the local relay are shown in Fig. 3. The
event data capture was triggered at Cycle 4 by the Zone 2
phase distance element (M2P). The relay tripped and reported
Zone 2 and time targets 0.5 cycles later.
IA IB IC
IGMag
VA(kV) VB(kV) VC(kV)
Digitals
2000
0
-2000
750
500
250
0
100
0
-100
Fig. 3.
IN105
IN106
TRIP
52A
Z2T
Z2GT
Z2G
M2PT
M2P
IA IB IC IGMag VA(kV) VB(kV) VC(kV)
.5 cycles
0 1 2 3 4 5 6 7 8
Cycles
Event Data From 138 kV Transmission Line Relay
Unlike the ground fault in Section I, which evolved into a
larger magnitude fault of the same type, this fault evolved
from a single-line-to-ground fault to a phase-phase-ground
fault.
Distance relays support two philosophies of zone timing:
independent or common timing (see Fig. 4). For the independent
timing mode, the phase and ground distance elements for
each zone initiate independent timers. For the common mode,
the phase and ground distance elements for each zone drive a
common timer. The common zone timer is suspended for
1 cycle if the timer input drops out. This feature prevents timer
reset when a fault evolves [3].
Suspend Timing
during the fault transition period, and the common zone timer
was suspended. The M2P element then asserted, the common
zone timer resumed timing, and a trip was issued 0.5 cycles
later. In this example, the benefit of common zone timing was
a total relay response time of 25.5 cycles instead of the
expected 50.5 cycles if we had chosen to use independent
timing.
In this event, the fault location estimate from the relay was
accurate despite the evolving fault data. The contiguous fault
data were roughly from Cycle 4 to 7, which was the window
of time the triggering element M2P was asserted. During that
time, the fault had already evolved, and the fault data were
stable.
In Fig. 5, the fault location estimate is plotted for the two
different fault types experienced. Both estimates produce
roughly the same fault location when data are stable. But from
Cycle 2.5 to 3.5 you can see where both estimates veer wildly
from the actual location because of the fault type evolution.
The reported location from the relay would have been in error
if it had selected data from that time.
m
m
3.5
2
Single-Line-to-Ground Faults (BG)
0
0 1 2 3 4 5 6 7 8
Cycles
2
Phase-to-Phase Faults (BC)
0
0 1 2 3 4 5 6 7 8
Cycles
Fig. 5. MathCad Fault Location Estimates for BG and BC Fault Types
Event data can be replayed as IEEE COMTRADE files
through test equipment into relays. Unique or challenging
fault cases should be archived as IEEE COMTRADE files and
used to test new relays, challenge standard schemes, and
understand relay responses.
M2P
Z2G
Fig. 4.
Z2D
Z2PD
Z2GD
Independent and Common Zone Timing Schemes
0
0
0
Z2T
M2PT
Z2GT
We can deduce from the data in Fig. 3 that the initial fault
had been present, and the Zone 2 ground distance element
(Z2G) had been timing for 24.5 cycles before the fault
evolved. All distance elements dropped out for 0.5 cycles
III. RECLOSE FAILURE
In May 2006, a 34.5 kV line feeding a hospital failed to
reclose for a line fault. The relay was set for two reclose
attempts.
Event data from the relay are shown in Fig. 6. The event
started as an A-phase-to-ground (AG) fault and evolved to a
B-phase-to-ground (BG) fault. This was most likely caused by
a slapping conductor or tree contact. At fault inception, an instantaneous
ground overcurrent element (67G1) asserts and
produces a trip output. At this same time, the relay enters the
reclosing cycle state (79CY), indicating that reclosing openinterval
timing has been initiated. However, we can see that as
the AG fault progresses to a BG fault, the 67G1 element drops
out for 0.75 cycles before reasserting. When the 67G1 element
Lessons Learned Analyzing Transmission Faults | 53