Summary Paper for C IEEE Guide for Application of Digital Line Current Differential Relays Using Digital Communication

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1 Summary Paper for C IEEE Guide for Application of Digital Line Current Differential Relays Using Digital Communication by: Neftaly Torres, P.E. 70 th Annual Conference for Protective Relay Engineers, A&M University 04/05/2017

2 D27 Working Group

3 D32 Working Group

4 Table of Contents Overview Current Differential Line Protection Applications Current Differential Operating Methods Communication Scheme Design Application Considerations Testing and troubleshooting

5 Overview - Scope This guide presents practical line current differential schemes using digital communications. operating principles synchronization methods channel requirements current transformer requirements external time reference requirements backup considerations testing considerations troubleshooting It also provides specific guidelines for various application aspects including: multi-terminal lines series compensated lines mutually coupled lines line charging current in-zone transformers and reactors single-phase tripping and reclosing communications channel requirements

6 Operating Principles

Current Differential At any node (junction) in an electric circuit, the sum of currents flowing into the node is equal to the sum of currents flowing out of the node; equivalently, the algebraic sum

7 Current Differential At any node (junction) in an electric circuit, the sum of currents flowing into the node is equal to the sum of currents flowing out of the node; equivalently, the algebraic sum of all the currents at any node in a circuit equals zero. nn kk=1 II kk = 0 I a 1 0 Auto Xfmr Transmission Power Bus Xfmr Line node Black Box (node) I d I b I c II aa + II bb = 0 II aa = II bb Current In = Current Out

8 Current Differential Protection I a +I b I a Ideal Xfmr 1:1 I b I a I b I a I b = 1 0 = II RRRRRRRR II RRRRRRRR 50P II OOOO II aa + II bb = 0 II OOOO = III aa + III bb Basic Operating Signal II RRRRRR = III aa + III bb 22 Basic Restraining Signal

9 Internal Zone Fault I a +I b I a Ideal Xfmr 1:1 I b I a I b I a I b = 1 0 II RRRRRRRR II RRRRRRRR = 0 II OOOO II aa + II bb 0 II OOOO = III aa + III bb Basic Operating Signal II RRRRRR = III aa + III bb 22 Basic Restraining Signal

10 Line Current Differential (87L) I a +I b I a I b SSSSSS AA SSSSSS BB I Local MMMMMMMMMM I Remote I Local I Remote CCCCCCCCCCCCCCCCCCCCCCCCCCCC LLLLLLLL 87 TTTT RRRR TTTT RRRR 87 II LLLLLLLLLL + II RRRRRRRRRRRR = 0 II LLLLLLLLLL = II RRRRRRRRRRRR II RRRRRRRRRRRR II LLLLLLLLLL = 11 Ideal Blocking Point

11 Line Current Differential (87L) LLLLLLLLLL SSSSSS RRRRRRRRRRRR SSSSSS I L MMMMMMMMMM I R I L I R CCCCCCCCCCCCCCCCCCCCCCCCCCCC LLLLLLLL 87 TTTT TTTT RRRR RRRR 87 II LLLLLLLLLL II RRRRRRRRRRRR Current Mismatch Caused by Numerous Factors CT differences, error, and saturation Line charging current Channel time-delay compensation errors (channel asymmetry) Tapped Loads

12 Line Current Differential (87L) LLLLLLLLLL SSSSSS RRRRRRRRRRRR SSSSSS I L MMMMMMMMMM I R I L I R CCCCCCCCCCCCCCCCCCCCCCCCCCCC LLLLLLLL 87 TTTT TTTT RRRR RRRR 87 II LLLLLLLLLL II RRRRRRRRRRRR Current Data Handling and Synchronization Fundamental to LCD. As important as the protection algorithms and logic! Point-to-point communication Channel-based mode: requires no external time source Comm channel tx/rx delays must be nearly identical Internal relay data latencies Algorithm delay Channel delay Delays

13 Current Differential Line Protection Applications Some Advantages Highly sensitive for internal faults and highly secure for external faults Significant selectivity compared to overreaching schemes (e.g. overcurrent and distance relaying) Protects 100% of line without delay Potential devices not required in most cases No need for directional elements in most cases Not susceptible to high loading, power swings, mutual coupling With good comm between terminals LCD can protect regardless of line length, source strength, # of terminals, tap length Insensitive to external faults (no need to coordinate) Some Disadvantages Insensitive to external faults (not a backup) Cost of communication Communication scheme is extremely critical to protection scheme Misoperations could result due to comm failures (i.e. loss of data or jitter) but channel health supervision logic can counter

14 Current Differential Operating Methods Percentage Differential Charge Comparison Alpha Plane Mix of the Above

15 Percentage Differential

16 Percentage Current Differential Protection Idiff I diff max Steady State and Proportional diff current Operating Region Transient diff current from CT saturation I diff min Restraining Region Slope Change Irestraint I a 11 CCCCCCCC Compensation III aa + III bb IIIIIIIIII I b 11 CCCCCCCC Compensation I b I a III aa + III bb 22 IIIIIIIIIIIIIIIIIIII

17 Percentage Current Differential Protection Idiff w/harmonic Restraint I diff max II dddddddd > II dddddddd mmmmmm Unrestrained Trip Steady State and Proportional diff current Operating Region Transient diff current from CT saturation II dddddddd > II rrrrrrrrrrrrrrrrrr SSllllllll xx IIIIIIIIII > IIrrrrrrrrrrrrrrrrrr SSllllllll xx + IIII %HH2 + IIII %HHH Restrained Trip Harmonic Restrained Trip IIIIIIIIII IIIIIIIIIIIIIIIIIIII I diff min ff(ssssss 11, SSSSSS 22 ) IIII 22 IIII 44 IIdddddddd Slope Change + I diff min %HHHH %HHHH - Restraining Region IIdddddddd I diff max IIdddddddd IIrrrrrrrrrrrrrrrr SSllllllll xx Irestraint Trip Unrst Trip Hrst Rst

18 Charge Comparison

19 Charge Comparison Qa[A-s] TTT TTT Q a +Q b TTT TTT Q b[a-s] TTT TTT SSSSSS AA SSSSSS BB I Local MMMMMMMMMM I Remote I Local I Remote CCCCCCCCCCCCCCCCCCCCCCCCCCCC LLLLLLLL 87 TTTT RRRR TTTT RRRR 87 Similar to the % restraint current differential Compares local and remote charges on a half-cycle basis Reduces throughput requirements of the communication channel Allows much greater error in time delay compensation

20 Alpha Plane

Alpha Plane LLLLLLLLLL SSSSSS I L RRRRRRRRRRRR SSSSSS I R II RRRRRRRRRRRR II LLLLLLLLLL = III RR III LL (θθ RR θθ LL ) IIII IIII/IIII II RRRRRRRRRRRR II LLLLLLLLLL = 11

21 Alpha Plane LLLLLLLLLL SSSSSS I L RRRRRRRRRRRR SSSSSS I R II RRRRRRRRRRRR II LLLLLLLLLL = III RR III LL (θθ RR θθ LL ) IIII IIII/IIII II RRRRRRRRRRRR II LLLLLLLLLL = 11 Ideal Blocking Point Internal faults w/outfeed at L Internal faults w/outfeed at R -1 RRRR IIII/IIII II RR = 0 Internal Faults Α-Plane Regions for Ideal Fault and Load Conditions

22 Alpha Plane LLLLLLLLLL SSSSSS I L RRRRRRRRRRRR SSSSSS I R IIII IIII/IIII External faults and load conditions Internal faults w/outfeed at L Internal faults w/outfeed at R Internal Faults -1 RRRR IIII/IIII II RR = 0 Α-Plane Channel Delay Compensation Errors and System Impedance Differences

23 Alpha Plane LLLLLLLLLL SSSSSS I L RRRRRRRRRRRR SSSSSS I R IIII IIII/IIII Internal faults w/outfeed at L Internal faults w/outfeed at R Internal Faults -1 RRRR IIII/IIII II RR = 0 Α-Plane Regions for System Power Angle and Impedance Differences

24 Alpha Plane LLLLLLLLLL SSSSSS I L RRRRRRRRRRRR SSSSSS I R IIII IIII/IIII α R Operate -1 Restrain 1/R RRRR IIII/IIII Traditional Α-Plane Channel Operating Characteristic

25 Communication Scheme Design

26 Protective Relaying Communications

27 Protective Relaying Communications Path

28 Communications Requirements End to End Delay Variable Delay, referred to as jitter or wander; change in delay time from one time period to another Asymmetry; different transmit and receive delay paths Interruptions and re-synchronization delays following a switching operation Protection engineer should define requirements for the relay scheme and work closely with telecom architect

29 Reliability Digital networks are typically designed for high availability (99.98% or better) but not error free Errors caused by: Transients Equipment failures Temp variations Changing atmospheric conditions of microwave link Lack of dependability of comm = lack of availability of protection Relaying needs highly accurate, low latency data path Data needs to be timely, error free, and identifiable by remote relay

30 SONET Network / Normal Operation / Substations A and B have equal delay in their primary communications paths

31 SONET Network / Unidirectional Back-Up Operation / The data being received at Substation A has greater delay than the data being received at Substation B

32 SONET Network / Bidirectional Back-Up Operation / The data being received at Substation A has the same delay as the data being received at Substation B

33 Communications Channel Delay

34 Concept of Current Differential Calculation

35 Configuration of GPS synchronous line current differential relay

36 Communications system based on current network technology

37 Communications system based on future network technology

38 Application Requirements Multi-terminal Line Protection Dual Breaker Applications Setting considerations Open CT Conditions CT ratio compensation Mutually coupled lines Charging current compensation Switch-onto fault Weak Infeed Issues

39 Application Requirements Out-of-step CT saturation detection / compensation Stub bus Single phase tripping Multi-phase autoreclosing Series compensated lines Shunt reactors In-zone transformers and tapped loads Backup protection considerations Communications channel cutout switch

40 Mult-Terminal Line Protection For N Terminal Lines, need N-1 ports for communicating to each relay Solution to reduce complexity: set certain relays as key relays to perform differential calculations receive information from slave relays and send trip signals to slave relays Another solution is to each relay to sum its current with adjacent relay and pass on resultant sum to next relay

41 Close-in external fault for breaker and half bus configuration

42 High Resistance Fault

43 Open CT Conditions Could produce undesirable operation Some manufacturers provide open-ct logic Logic could produce alarms or block trip Important for Protection Engineer to be knowledgeable of how scheme works

44 CT Ratio Compensation and Mutually Coupled Lines Identical scaling of currents at all ends Ratio differences handled by relay Settings need to consider differences in CTs including saturation Mutual Effects do not affect line current differential protection schemes

45 Charging Current Compensation LLLLLLLLLL SSSSSS RRRRRRRRRRRR SSSSSS I L MMMMMMMMMM I R 87 CCCCCCCC LLLLLLLL CCCCCCCC LLLLLLLL 87 I c Charging current is a capacitive leakage current on the transmission line. Can be a very large current on long transmission lines or underground cable Charging current entering local terminal is not exiting the remote Can sacrifice sensitivity to internal faults in order to account for charging current Line discharging current can cause misop for external faults Modern relays have charging current compensation (require voltage measurement)

46 Series Compensated Lines LLLLLLLLLL SSSSSS RRRRRRRRRRRR SSSSSS I L X c MMMMMMMMMM I R 87 CCCCCCCC LLLLLLLL CCCCCCCC LLLLLLLL 87 Series compensation is used to alleviate transmission line loading and/or improve system stability. LCD protection is a good choice for series compensated lines. Immune to voltage inversions Alpha plane principle is tolerant to current inversions and sub-harmonic transients

47 Shunt Reactors LLLLLLLLLL SSSSSS RRRRRRRRRRRR SSSSSS I L X c MMMMMMMMMM I R I R 87 CCCCCCCC LLLLLLLL CCCCCCCC LLLLLLLL 87 Used to compensate cap reactance of long transmission lines or HV underground cable; or voltage when line is lightly loaded or open ended. Pros and cons to including or excluding from differential zone. Pro to inclusion: less complex, less wiring Con to inclusion: line protection will operate for reactor fault, charging current compensation will vary based on reactor being in our out of service Transient behavior of shunt reactors and line capacitances may require dynamic restraint for non-fundamental frequencies in diff current

48 CT Saturation Detection/Compensation Main concern is for external faults and falsely tripping One method of compensation is to decrease sensitivity Some percentage restraint current differential relays include a CT saturation detector that increases the bias

49 In-Line Transformer LLLLLLLLLL SSSSSS RRRRRRRRRRRR SSSSSS MMMMMMMMMM CCCCCCCCCCCCCCCCCCCCCCCCCCCC LLLLLLLL 87 TTTT RRRR TTTT RRRR 87 Magnitude compensation including voltage step compensation and CT ratio matching at both voltages Compensation for transformer phase shifts Zero-sequence removal in case wye winding neutral is grounded Inrush and overexcitation detection to block differential when needed Restrained differential algorithms should be mirrored at both terminals

50 Tapped Transformer LLLLLLLLLL SSSSSS RRRRRRRRRRRR SSSSSS MMMMMMMMMM 87 CCCCCCCC LLLLLLLL CCCCCCCC LLLLLLLL 87 Without measurement or communication from tapped station, line current differential can still be applied with certain considerations: Account for total load current of transformer(s) and lines Coordinate or block for low-side transformer faults Account for magnetizing inrush of transformer(s) and capacitive inrush (diff blocking, 2 nd harmonic restraining, or distance element supervision) External ground faults on high-voltage system causing zero sequence from wye grounded neutral winding (can estimate current or remove zero-sequence diff)

51 Testing and Troubleshooting

52 Loopback Testing LLLLLLLLLL SSSSSS RRRRRRRRRRRR SSSSSS MMMMMMMMMM TTTTTTTT SSSSSS CCCCCCCCCCCCCCCCCCCCCCCCCCCC LLLLLLLL 87 TTTT RRRR TTTT RRRR 87 Connecting transmit and receive ports together Least desirable (limited) Tests minimum pick up points Does not test restraint characteristic, tapped load conditions, correct end-to-end current phasing, etc. If comm channel is available, can loopback at remote terminal and confirm channel integrity

53 Local Relay Back to Back Bench Test LLLLLLLLLL SSSSSS RRRRRRRRRRRR SSSSSS MMMMMMMMMM TTTTTTTT SSSSSS 87 TTTT RRRR RRXX TTXX 87 TTTT RRRR 87 Two or more relays required Use direct fiber or through other communication medium Can be used to test simulated faults Success of testing gives sufficient confidence in relaying, but requires validating communications channel

54 Time-Synchronized End-to-End Testing LLLLLLLLLL SSSSSS GPS GPS RRRRRRRRRRRR SSSSSS MMMMMMMMMM TTTTTTTT SSSSSS TTTTTTTT SSSSSS 87 TTTT RRRR CCCCCCCCCCCCCCCCCCCCCCCCCCCC LLLLLLLL TTTT RRRR 87 Involves testing the entire protection system (except CT if current injection is used) Use GPS time synchronized three phase test sets

55 Troubleshooting an In-Service CurrentDifferential System This subclause on troubleshooting is focused on providing guidance on direction the user toward potential sources o data errors on current differential schemes.

56 Annexes Annex A Differential protection of power lines/cables based on Rogowski coil current sensors Annex B - Bibliography

57 Line Current Differential (87L) I a +I b I a I b SSSSSS AA SSSSSS BB I Local MMMMMMMMMM I Remote I Local I Remote II LLLLLLLLLL + II RRRRRRRRRRRR = 0 II LLLLLLLLLL = II RRRRRRRRRRRR kk = II RRRRRRRRRRRR II LLLLLLLLLL = 11 Ideal Blocking Point

58 QUESTIONS?

Summary Paper for C IEEE Guide for Application of Digital Line Current Differential Relays Using Digital Communication

Summary Paper for C IEEE Guide for Application of Digital Line Current Differential Relays Using Digital Communication Summary Paper for C37.243 IEEE Guide for Application of Digital Line Current Differential Relays Using Digital Communication Participants At the time this draft was completed, the D32 Working Group had