Backup Considerations for Line
Current Differential Protection
Steven Hodder
Hydro One Networks, Inc.
Bogdan Kasztenny and Normann Fischer
Schweitzer Engineering Laboratories, Inc.
Copyright © Hydro One Networks, Inc. and SEL 2012
Outline
• Line protection redundancy
• Loss of 87L protection
• 87L backup schemes
♦
POTT or DCB
♦
Zone 1 extension
♦
Stepped distance
♦
Overcurrent
• Backup protection strategies
NERC Guidelines
• Protection system ensures that
♦
Fault clearing times maintain system stability
♦
Equipment ratings are not violated
♦
Protection selectivity is maximized
• Protection system performs during any
single contingency (breaker failure, relay
failure, channel loss)
• Protected line is forced out of service
Line Protection Considerations
• Critical fault clearing time – protection type
• Forced outages avoided – protection
availability
• HV / EHV applications
♦
Breaker failure protection
♦
Communications-assisted schemes
♦
Parallel redundancy of protection
Hydro One Example
• Operates Ontario transmission system
(500, 230, and 115 kV)
• Follows NPCC guidelines
• Categorizes BES lines
♦
Impactive
♦
Nonimpactive
Hydro One Example
BES-Impactive Lines
• Fully redundant and independent protection
schemes capable of instantaneous trip
• ZI trip with autoreclose – Z1, POTT or DCB,
87L, and breaker failure plus DTT
• ZT trip without autoreclose – Z2T and 51G
• 87L, POTT, or DCB are acceptable –
schemes functionally identical in A and
B systems
Instantaneous Tripping Lost
• BES-impactive circuit
♦
Independent electricity system operator
(IESO) must be notified within 5 minutes of
second failure
♦
Forced line outage is immediate
• Non-BES-impactive circuit
♦
Line typically removed from service
♦
If removal has adverse impact on customer
load, leave line in service and rely on timed
backup only
Line Current Differential Protection
• Inherently selective
• Very sensitive
• Immune to
system conditions
• Easy to set
• Dependent on
communications
Loss of 87L Protection
• 87L element blocked
• Relay out of service
• Problems with communications
♦
Channel loss
♦
Channel brownout
• Inability to align local and remote
currents – loss of time reference
when using asymmetrical channels
Loss of Time Reference
• IRIG-B connection severed
• IRIG-B signal noisy
• Time source (external clock) malfunction
• Time source not locked
• GPS problems
Using IEEE C37.118-compliant clocks
that report time quality is important
87L Adaptivity to
Channel Problems
Hot Standby Logic
87L Relay
87L
Hot Standby
Logic
Primary Channel
Hot Standby Channel
87L Relay
87L
Hot Standby
Logic
Hot Standby Logic
87L Relay
87L
Hot Standby
Logic
Primary Channel
Hot Standby Channel
87L Relay
87L
Hot Standby
Logic
Three-Terminal Applications
Master
87L
1
Master
Channel 1
Ch
an
87L
2
l
e
n
ne
n
a
h
l2
C
87L
3
Master
3
Three-Terminal Applications
Slave
Slave
87L
1
87L
2
Ch
an
l
e
n
ne
n
a
h
l2
C
87L
3
Master
3
Channel Loss Under Stub Bus
Stub Bus Zone
87L Master
Line Zone
87L
87L
Master
Master
Channel Loss Under Stub Bus
Stub Bus Zone
87L Master
Loss of 87L Protection
87L
87L
Slave
Monitoring actual state of 87L
elements is important
Slave
A87L
Main Path
SONET
Ring #1
B87L
Alternate Path
Add / Drop
MUX
DS1
MUX
DS1
MUX
A87L
B87L
DS1
MUX
DS1
MUX
Communications
Redundancy
Add / Drop
MUX
B87L
Main Path
SONET
Ring #2
A87L
Alternate Path
Hydro One Protection
Philosophy With SONET
DS0 Protection Circuits Static (Pinned)
• Avoid complexity (e.g., testing), especially
as SONET network grows
• Support protection scheme designed to
work despite single communications failure
• Ensure channel symmetry, especially for
business-class SONET equipment
SONET Path Routing for
Three-Terminal Lines
Main
1-2
Main
2-1
2
1
Line
Distance
Line
Distance
Alternate
2-4
Alternate
1-3
3
Alternate
3-1
Line
Distance
Main
3-4
Alternate
4-2
Main
4-3
Communications Contingencies
• Channel redundancy is preferred solution
for POTT or DCB over SONET
• Channel redundancy is limited option for
87L schemes
♦
Available for two-terminal schemes only
when direct point-to-point channels are used
♦
SONET path switching may cause problems
• 87L backup is preferred over 87L channel
redundancy
87L Backup
• Backup required for rare occasions when
87L is unavailable
♦
Channel problems
♦
Timing problems
• Backup integrated within multifunction 87L
relay acceptable and beneficial
• Selectivity requirements can be relaxed if
87L backup engaged only when needed
87L Backup Options
Directional Comparison
• Not practical in many cases – requires
channel in addition to 87L channel
• More difficult to set compared with 87L
• Sensitive if 67Q / 67G used, may
cause sequential tripping
• Less secure compared with 87L
Zone 1 Extension Logic
• Substitute for extra channel at
expense of selectivity
♦
Trips from overreaching Z1
♦
Reduces Z1 reach before reclosing
• Less sensitive compared with 87L
• Susceptible to load and power swings
• Supplemented with 51Q / 51G and
67Q / 67G for high-resistance faults
Stepped Distance
• Time-delayed for selectivity and coordination
• Part of remote backup strategy (if used)
• More difficult to set (infeed)
• Less sensitive compared with 87L
• Susceptible to load and power swings
• Supplemented with 51Q / 51G and
67Q / 67G for high-resistance faults
Time Overcurrent
• Time-delayed for selectivity and coordination
• Torque-controlled with 67Q / 67G
• 51P usage versus 51Q / 51G usage
• More difficult to set (current variability)
• 51P susceptible to load and power swings
87L Backup Strategies
• Fully operational backup
• Backup engaged only upon loss of 87L
• Adaptive backup
Fully Operational Backup
• Parallel with 87L scheme
• Requires no information about 87L state –
standalone implementation
• Biases protection toward dependability
• Must be set conservatively to maintain
security – faces settings challenges
• Not justified if redundant system operational
Backup Engaged if 87L Lost
• Does not adversely impact security
• Can follow simpler operating principles –
stepped distance
• Can be biased toward dependability –
Zone 1 extension
• Faces fewer settings challenges
Backup Engaged if 87L Lost
87L
87L
21
21
• All relays are masters
• Backup inhibited
in all relays
87L
21
Backup Engaged if 87L Lost
87L
87L
21
21
• Master-slave mode upon
first channel failure
• Backup remains blocked
87L
21
Backup Engaged if 87L Lost
87L
87L
21
21
Backup engaged when
second channel fails
87L
21
Backup Engaged if 87L Lost
87L
87L
21
21
Or…
Backup engaged if
any relay is slave
87L
21
Backup Engaged if 87L Lost
87L
87L
21
21
Backup is ready should
second channel fail
87L
21
Adaptive Backup Examples
Zone 2 (Timed)
Zone 1 (Instantaneous)
87L /
21P /
21G
ZAX
X
87L /
21P /
21G
ZCX
ZBX
Terminal A
Terminal C
Terminal B
87L / 21P / 21G
Adaptive Backup Examples
Zone 2 (Timed)
Zone 1 (Instantaneous)
87L /
21P /
21G
ZAX
X
87L /
21P /
21G
ZCX
ZBX
Terminal A
Terminal C
Terminal B
87L / 21P / 21G
Master-Slave Configuration
DTTA-C-B
DTTA-C
21-1
87L
21P
21G
Terminal A
87L
21P
21G
Terminal C
Terminal B
87L, 21P, 21G
DTTA-B
DTTC-B
Stub Bus
DTTRem2_Local_Rem1
Local_DTT_TX
DTTLocal_Rem1
Z2_Pickup (Rem1)
Z2_Pickup (Rem2)
Local_Stub_Bus
DTTRem1_Local_Rem2
DTTLocal_Rem2
Conclusions
• 87L is selective, sensitive, and secure
• Communications paths and equipment
affect overall 87L availability
• Communications and timing contingencies
can render 87L unavailable
Conclusions
• 87L backup covers failures in
communications and timing, not relays
• Regulatory requirements and internal
practices impact selection of backup
• Typical backup schemes include 21T,
Z1EXT, 51, and POTT or DCB
Conclusions
• Settings backup schemes are typically
more involved than 87L
• 87L backup elements integrated within
same protective device are prudent
• Backup only engages upon loss of 87L
♦
Benefits security
♦
Simplifies settings selection
Questions?