Case Study

for
Review of PLCC scheme for
distance protection relays
From, BNC
Overview of BNC-Balipara Ckt#2 Tripping

 On 02.10.2021, at 10:55hrs, 400kV BNC-Balipara Ckt-2

tripped due to L-G fault in B-Ph of the circuit.
 Zone-2 Start and Zone-2 trip was observed in MiCOM P444
relay at both ends whereas REL670 shown only zone-3
start indication.
 The circuit tripped in zone-2 timings without any Auto
reclose attempt.
 The relay calculated fault distance was 42.4 km from BNC
end and actual fault was found at 38km during post fault
patrolling.
Line Detail and Relay Installed in BNC-
Balipara Ckt-2
Name of line BNC-BALIPARA Ckt-2
Line Length(km) 59.995
Type of Conductor ACSR Twin Moose
Voltage Level 400kV
Line Impedance 10.19
Line Angle 84.70

Name of the End Main-I Main-II

BNC ABB REL670 MiCOM P444
Balipara MiCOM P444 ABB REL670
Related Setting at BNC end
Main-I (ABB REL670) Main-II (MiCOM P444)
Zone-1 Zone-2 Zone-3 Zone-4 R1G(Ω) 27.5
R1PH(Ω) 16.5
R1PE (Ω) 1.38 2.59 12.06 0.35
tZ1(msec) 100
R0PE (Ω) 12.84 24.07 112.01 3.21
R2G(Ω) 41.25
X0PE (Ω) 51.54 96.56 449.60 12.88
R2Ph(Ω) 33
RFFwPE(Ω) 50 75 125 75
tZ2(msec) 350
tPE (Sec) 0.1 0.350 1.5 0.5
R3G-R4G(Ω) 41.29
RLdFw(Ω) 48.72
R3Ph-R4Ph(Ω) 41.25
ArgLd(deg) 30
tZ3(sec) 1.5
PLCC POR
PLCC Scheme POR
Scheme
REL670 Zone characteristics for LE Faults
Observation from REL670 setting

 The set value of RFFwPE in REL70 is 50 ohm for Zone-1, 75

ohm for Zone-2 and 125 ohm for Zone-3, whereas, the
minimum resistive reach of the Load encroachment
characteristic, RLdFw is 48.72 ohm.
 Hence, the Load Encroachment characteristic enters some
portion of Zone-1 and significant portion of Zone-2 and Zone-
3 characteristics for L-G fault in forward direction.
MiCOM P444 Load Encroachment
characteristics
• In MiCOM P444, R3G-R4G is set with
some suitable margin from minimum
load impedance Z.
R3G-R4G ≤ 80% of Minimum Load Imp
𝑈𝑚𝑖𝑛
ൗ 3
≤ 80% ,
1.2 𝑥 𝐼𝑚𝑎𝑥

where Umin and Imax are Min Line

Voltage and max load current in
normal condition without fault.

• It can be observed that load

encroachment binder in MiCOM P444 is
well beyond the zone-3 reach.
Relay indication at BNC end for BNC-Balipara
Ckt-2 tripping
MAIN – I MAIN-II BCU
(REL 670) (MICOM P444) (REC 670)
PROTECTION START DEF , Z3 DEF, Z2, Z3 --
PROTECTION OPERATED -- Z2 --
TRIP SIGNAL -- Z2 Trip
FAULT DURATION 350 ms 350 ms --
FAULTED PHASE B – Phase B – Phase --
FAULT TYPE Phase – Earth Phase - Earth
FAULT CURRENT 4 KA 4 KA --
AR START -- -- NO
AR SUCCESSFUL -- -- NO
DISTANCE TO FAULT (Km) -- 42.4 --
DR Analysis
DR Analysis
 The fault Angle between VR and IR is 20° , followed by
increasing trend for current, indicating high resistive
vegetation faults.
 Fault angle is less than load encroachment characteristic
angle, ArgRLd setting in REL670 relay. The fault is in Zone-2,
as indicated by MiCOM P444 indications and since the
Minimum load resistance setting in REL670,RLdFw is less
than Maximum resistive reach of even Zone-1 for P-E faults,
so the relay senses the fault as load.
 But, as discussed, load characteristic in MiCOM P444 is well
beyond Zone-3 and thus it senses the fault in Zone-2. It thus
waits for the carrier signal for ZCOM trip.
BNC-Balipara ckt-2 PLCC Scheme and Carrier
Exchanged during fault
 PLCC reading in BNC PLCC panel
PRE-FAULT POST-FAULT
CHANNEL-I CHANNEL-II CHANNEL-I CHANNEL-II
A B A B A B A B
Tx 23 45 42 21 23 46 43 21
Rx 09 56 55 90 10 56 55 91

• As per POR scheme, MiCOM P444(Main-II) at BNC has “Zone-2 start”

indication, and sends carrier signal via channel-2 A and Channel-1 B which
is evident by increment in Tx counters of Channel-2A and Channel-1B.

• Similarly, MiCOM P444(Main-I) at Balipara end with similar “Zone-2 start”

indication sends carrier signal via channel-1 A and Channel-2 B and Rx
counter of these channels at BNC has increased by 1.
Failure of ZCOM trip
 As per the existing PLCC scheme, carrier exchange happens only
between
i. Main-I of BNC(REL670) and Main-I of Balipara (MiCOM P444).
ii. Main-II of BNC(MiCOM P444) and Main-II of Balipara
(REL670).
And there is no communication between Main-I of BNC and
Main –II of Balipara and vice-versa.

 The MiCOM P444 relays at both ends has Zone-2 Start indication
and were waiting for Carrier signals for Zcom trip.
 But, since there is no communication between MiCOM P444
relays of two ends, they did not receive the carrier signal
required for ZCOM trip and hence breaker at both ends tripped
in Zone-2 timing.
Proposed Solution
 The communication between Main-I and Main-II may be
established in soft logic as well hard wiring may be done
between the concerned BI and BO so that the carrier
received from other end will be received by both Main-I and
Main-II relays.
 This will ensure ZCOM trip of at least one of the relay in
case one of the relay fails to detect the fault which further
will ensure start of Auto reclose function at both ends.
 The Load Encroachment setting of REL670 relay for
vegetative faults may be reviewed.
Proposed solution-Cable laying
Proposed Change in Soft Logic
Proposed change Outcome
 As observed in this case, the unavailability of
communication between the two distance relays led to
failure of Auto reclose which eventually led to permanent
outage of the line.
 But with the solution proposed, we can avoid any such
outages in future where installed Main-1 and Main-2 relays
at both ends are of different make.
THANK
YOU