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Competitive Renewable Energy Zones Protection & Control Issues - 345kV Lines 1 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 2 NERC Statistics 3 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 ….. 4 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 5 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 ? 6 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 7 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 8 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 9 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 10 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 11 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 12 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 13 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 14 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 15 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 16 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 17 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 18 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 Wide Area Control Synchronous phasor data stream SCADA network communication Disturbance monitoring system Video surveillance system Smart grid, voice communication etc. 19 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 20 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 21 Discussion Questions? Comments? 22