Download 345kV Lines

Competitive Renewable Energy Zones
Protection & Control Issues -
345kV Lines
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NERC Press Release on April 24, 2009
“ Misoperations of system protection and control systems
have been a leading cause of bulk power system disturbance
in North America for a number of years, causing nearly 45% of
category two and higher disturbances in 2007 ” -NERC
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NERC Statistics
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NERC Standards, Papers, Reports etc.
 PRC-001-01: System Protection Coordination
 PRC-004-01: Analysis and Mitigation of T and G Protection System
 Reliability Paper: Protection System Reliability - Redundancy
 Special Report: Accommodating High Levels of Variable Generation
 And more …..
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Wind farm LVRT curve from FERC 661A and PRC-024-1
0.65pu
18 cycles
Can backup protection
trip within 18 cycles ?
0.45pu
Can breaker failure prot.
trip within 9 cycles ?
9 cycles
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Transmission Line High-Speed Protection
High-Speed Line Protection:
Communication-based protection scheme that can cover
100% line and trip in 1-2 cycles
 Power line carrier-based schemes - DCB or POTT
 Optical fiber-based current differential scheme (87L)
Which should be used for CREZ lines ?
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Power Line Carrier (PLC) - Based Schemes
 Utilizing power line for communication. Consist of wave traps,
coupling capacitors, line tuners, transmitters, receivers, etc.
 Two prevailing PLC-based schemes:
• Directional Comparison Blocking Scheme (DCB)
• Permissive Overreach Transfer Trip (POTT)
 Dependent upon distance (21) and directional (67) elements
Z2/3
21
Z1
21
Z2/3
Z1
PLC Channel
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Line Differential Protection (87L)
Based on digital communication channels such as optical
fibers, SONET network, etc.
Compare current flow in & out line terminals
Easy to discriminate internal & external faults
High dependability – instantaneous trip for all internal faults
High security – no operation for external faults
I_Local
I_Remote
87L
87L
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PLC Schemes vs. 87L : Mutually Coupled Lines
 Many CREZ 345kV lines are double circuits in long distance
 Zero sequence mutual coupling effects 21G / 67G
 21G may overreach or underreach
 Wrong direction from 67G due to zero seq. voltage reversal
 Mis-operations occur often from DCB or POTT due to
communication errors
 87L is not affected by mutual coupling
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PLC Schemes vs. 87L: Current Reversal
 Cause: one end tripped prior to the other end
 21 / 67 may give wrong directions
 DCB or POTT may trip the healthy line due to scheme error
 87L is not affected by current reversal
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PLC Schemes vs. 87L : Power Swings
 Power system dynamic balance is disturbed after loss of
critical generators or transmission lines
 Swings could be damped by automatic & manual system
controls, or aggravated towards out-of-step and blackout
 21P is susceptible to power swings
 87L is not affected by power swings
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PLC Schemes vs. 87L : CCVT Transients
 21 and 67 protection are relying on voltage signals
 CCVT transients may cause overreach, underreach, delayed
trip or wrong direction
 DCB or POTT may mis-operate for wrong direction
 87L is not affected by CCVT transients at all
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PLC Schemes vs. 87L: Series-compensated Line
 Protection Issues brought by series-compensation (SC)
• Uncertainties caused by SC, MOV and bypass switch
• Voltage inversion and current inversion effect 21/67
• 21 over-reach or under-reach
• Sub-synchronous resonance effects phasor measurement
• Other interactions with system components
 DCB or POTT schemes have high probability of mis-operation
for SC line and the adjacent lines as well
 87L is immune to all above issues except current inversion,
that can be overcome by proper 87L settings
X
remote bus
I0
RF
(1-K)*Z0L
Z0S2
Z0S1
ZSC&MOV
ZAPPARENT
K*Z0L
V0
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R
A zero seq. network example
PLC Schemes vs. 87L : Weak Feed Applications
 Wind farm is regarded as weak source or no source
 21/67 at weak terminal may not see the fault
 DCB and POTT need special logics to handle weak feed
applications, at a price of increasing the complexity of
coordination and increasing possibility of mis-operation
 Weak feed has no impact on 87L
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PLC Schemes vs. 87L : High Impedance Faults
 21G cannot detect ground faults with high impedance
 67G has to compromise between the sensitivity and
security against system unbalance under heavy load
 87L is sensitive to internal high impedance faults and not
affected by load
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PLC Schemes vs. 87L:Cross Country Faults, Evolving Faults
 Cross Country Faults: two faults at the same time but
different locations, one internal and one external
 Evolving Faults: 1LG fault evolves to 2LG or 3LG fault
 21 or 67 could be confused, DCB or POTT may mis-trip for
external fault or no-trip for internal fault
 87L can discriminate if there is internal fault, and not
confused by evolving faults
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Summary on PLC-based schemes vs. Fiber-based 87L
PLC-based
schemes
Optical fiberbased 87L
Double Cir. - Mutual Coupling
Double Cir. - Current Reversal
Power Swings
CCVT Transients
Series-Compensated Line
Weak Feed Application
High Impedance Faults
Cross-country Faults
Evolving Faults
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Proposed 345kV Line Protection Schemes for CREZ
Redundant protection system including:
 Dual high speed primary protections
 Redundant communication channels using optical fibers
fibers
MUX
Self-healing SONET
87L
87L
fibers
MUX
87L
Direct fibers
87L
 Primary A: Line differential protection (87L) over direct fibers
 Primary B: 87L over self-healing SONET network
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OPGW for Every New CREZ 345kV Line
 Optical Fiber Composite Overhead Ground Wire (OPGW)
 Lower cost for new line construction
 Expensive to put in afterwards (factor of 5-7)
 Other benefits of optical fiber network utilizing OPGW
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




Wide Area Control
Synchronous phasor data stream
SCADA network communication
Disturbance monitoring system
Video surveillance system
Smart grid, voice communication etc.
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Breaker Failure Protection
 Since over 20% generation may be from wind / sun, system
stability has high requirements on backup protection
 Wind farm LVRT curve implies fault clearance within 9 cycles
even if breaker fails
BF trip
86 trip
DTT trip
Fault cleared
Fault Inception
Relay time
Breaker clearing
time
Fault detector
drop out time
Safety Margin
Local & remote
breakers clearing time
t (cycles)
General
Better breakers
& better schemes
2
3
1-2
2
5
Total 13-14
1
2
<1
2
3
Total 8-9
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Summary
 Redundancy is necessary for CREZ 345kV lines
 87L is superior to carrier-based schemes
 Communication channels using optical fibers and selfhealing SONET network
 OPGW for every new CREZ 345kV line
 Breaker failure and backup timing
 Joint Development of CREZ
Common approach /
design specification on system protection and control
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Discussion
Questions?
Comments?
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