Backup Considerations for Line Current Differential Protection
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?
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