IEEE Guide for Application of Digital Line Current ...prorelay.tamu.edu/wp-content/uploads/sites/3/2017/04/1.-Summary... · Digital Line Current Differential Relays Using Digital

Summary Paper for C37.243IEEE Guide for Application of

Digital Line Current Differential Relays Using Digital

Communicationby: Neftaly Torres, P.E.

70th Annual Conference for Protective Relay Engineers, A&M University04/05/2017

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D27 Working Group



D32 Working Group



• Overview• Current Differential Line Protection

Applications• Current Differential Operating Methods• Communication Scheme Design• Application Considerations• Testing and troubleshooting

Table of Contents



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• troubleshootingIt 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

Overview - Scope



Operating Principles



Current DifferentialAt 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.

Ia Ibnode

𝐼𝐼𝑎𝑎 + 𝐼𝐼𝑏𝑏 = 0 𝐼𝐼𝑎𝑎 = −𝐼𝐼𝑏𝑏

Black Box(node)

TransmissionLine

Power XfmrId

Bus

Ic

Auto Xfmr

Current In = Current Out

�𝑘𝑘=1

𝑛𝑛

𝐼𝐼𝑘𝑘 = 0

1∠1801∠0



Current Differential Protection

Ia Ib

𝐼𝐼𝑎𝑎 + 𝐼𝐼𝑏𝑏 = 0

= 1∠180= 1∠0

I’a I’b

Ideal Xfmr1:1

𝑰𝑰𝑶𝑶𝑶𝑶

IbIa

Ia+Ib

50P

𝑰𝑰𝑶𝑶𝑶𝑶 = 𝑰𝑰𝑰𝒂𝒂 + 𝑰𝑰𝑰𝒃𝒃Basic

OperatingSignal

𝑰𝑰𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹 𝑰𝑰𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹

𝑰𝑰𝑹𝑹𝑹𝑹𝑹𝑹 =𝑰𝑰𝑰𝒂𝒂 + 𝑰𝑰𝑰𝒃𝒃

𝑹𝑹Basic

RestrainingSignal



Ia Ib

𝐼𝐼𝑎𝑎 + 𝐼𝐼𝑏𝑏 ≠ 0

= 0

= 1∠0

I’a I’b

Ideal Xfmr1:1

Internal Zone Fault

𝑰𝑰𝑶𝑶𝑶𝑶

IbIa

Ia+Ib

𝑰𝑰𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹 𝑰𝑰𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹

𝑰𝑰𝑶𝑶𝑶𝑶 = 𝑰𝑰𝑰𝒂𝒂 + 𝑰𝑰𝑰𝒃𝒃Basic

OperatingSignal

𝑰𝑰𝑹𝑹𝑹𝑹𝑹𝑹 =𝑰𝑰𝑰𝒂𝒂 + 𝑰𝑰𝑰𝒃𝒃

𝑹𝑹Basic

RestrainingSignal



𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

Line Current Differential (87L)

ILocal IRemote

I’Local I’Remote

𝐼𝐼𝐿𝐿𝐿𝐿𝐿𝐿𝑎𝑎𝐿𝐿 + 𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅𝐿𝐿𝑅𝑅𝑅𝑅 = 0

𝑰𝑰′𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑰𝑰′𝑳𝑳𝑹𝑹𝑳𝑳𝒂𝒂𝑳𝑳

= −𝑹𝑹Ideal

BlockingPoint

𝑆𝑆𝑆𝑆𝑆𝑆 𝐵𝐵𝑆𝑆𝑆𝑆𝑆𝑆 𝐴𝐴

8787

𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀

𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝑆𝑆𝐶𝐶𝑀𝑀𝐶𝐶𝐶𝐶𝐶𝐶𝑀𝑀𝐶𝐶𝐶𝐶𝑀𝑀 𝐿𝐿𝑀𝑀𝐶𝐶𝐿𝐿

𝐼𝐼𝐿𝐿𝐿𝐿𝐿𝐿𝑎𝑎𝐿𝐿= − 𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅𝐿𝐿𝑅𝑅𝑅𝑅

IbIa

Ia+Ib



𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

Line Current Differential (87L)IL IR

I’L I’R

𝑅𝑅𝑀𝑀𝐶𝐶𝐶𝐶𝐶𝐶𝑀𝑀 𝑆𝑆𝑆𝑆𝑆𝑆𝐿𝐿𝐶𝐶𝐶𝐶𝐶𝐶𝑀𝑀 𝑆𝑆𝑆𝑆𝑆𝑆



Current Mismatch Caused by Numerous Factors• CT differences, error, and saturation• Line charging current• Channel time-delay compensation errors (channel asymmetry)• Tapped Loads

𝐼𝐼𝐿𝐿𝐿𝐿𝐿𝐿𝑎𝑎𝐿𝐿 ≠ −𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅𝐿𝐿𝑅𝑅𝑅𝑅

8787




𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇8787



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

Delays• Internal relay data latencies• Algorithm delay• Channel delay

𝐼𝐼𝐿𝐿𝐿𝐿𝐿𝐿𝑎𝑎𝐿𝐿 ≠ −𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅𝐿𝐿𝑅𝑅𝑅𝑅

IL IR

I’L I’R




• 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

Current Differential Line Protection Applications



Current Differential OperatingMethods

• Percentage Differential• Charge Comparison• Alpha Plane• Mix of the Above



Percentage Differential



RestrainingRegion

OperatingRegion

Percentage Current Differential ProtectionIdiff

Irestraint

I diff min

Slope Change

I’a

I’b

𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰

𝑰𝑰𝑰𝑰𝑹𝑹𝑹𝑹𝑹𝑹𝑰𝑰𝒂𝒂𝑰𝑰𝑰𝑰𝑹𝑹

�

Compensation

Compensation

Ia

Ib

𝑹𝑹𝑪𝑪𝑪𝑪𝑹𝑹𝑹𝑹

𝑹𝑹𝑪𝑪𝑪𝑪𝑹𝑹𝑹𝑹

𝑰𝑰𝑰𝒂𝒂 + 𝑰𝑰𝑰𝒃𝒃𝑹𝑹

𝑰𝑰𝑰𝒂𝒂 + 𝑰𝑰𝑰𝒃𝒃

Steady State and

Proportional diff current

Transient diff current from

CT saturation

I diff max



TripHrst

Percentage Current Differential Protection w/Harmonic Restraint

RestrainingRegion

OperatingRegion

Idiff

Irestraint

I diff min

Slope Change

Steady State and

Proportional diff current

Transient diff current from

CT saturation

I diff max

𝑰𝑰𝑰𝑰𝑹𝑹

𝑰𝑰𝑰𝑰𝟒𝟒

∗𝑹𝑹𝟏𝟏𝟏𝟏%𝑰𝑰𝑹𝑹

∗𝑹𝑹𝟏𝟏𝟏𝟏%𝑰𝑰𝟒𝟒

�

+-I diff max

TripUnrst

TripRst

𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰

𝐼𝐼 𝑑𝑑𝑀𝑀𝑑𝑑𝑑𝑑 > 𝐼𝐼 𝑑𝑑𝑀𝑀𝑑𝑑𝑑𝑑 𝐶𝐶𝐶𝐶𝑚𝑚 UnrestrainedTrip

𝐼𝐼 𝑑𝑑𝑀𝑀𝑑𝑑𝑑𝑑 > 𝐼𝐼 𝑟𝑟𝑀𝑀𝑀𝑀𝐶𝐶𝑟𝑟𝐶𝐶𝑀𝑀𝐶𝐶𝐶𝐶 � 𝑆𝑆𝑀𝑀𝐶𝐶𝑙𝑙𝑀𝑀𝑥𝑥Restrained

Trip

𝐼𝐼𝑑𝑑𝑀𝑀𝑑𝑑𝑑𝑑 > 𝐼𝐼𝑟𝑟𝑀𝑀𝑀𝑀𝐶𝐶𝑟𝑟𝐶𝐶𝑀𝑀𝐶𝐶𝐶𝐶 � 𝑆𝑆𝑀𝑀𝐶𝐶𝑙𝑙𝑀𝑀𝑥𝑥 + 𝐼𝐼𝐼𝐼2 �100%𝐼𝐼2

+ 𝐼𝐼𝐼𝐼4 �100%𝐼𝐼𝐻

HarmonicRestrained

Trip

𝐼𝐼𝑟𝑟𝑀𝑀𝑀𝑀𝐶𝐶𝐶𝐶𝑀𝑀𝐶𝐶𝐶𝐶 � 𝑆𝑆𝑀𝑀𝐶𝐶𝑙𝑙𝑀𝑀𝑥𝑥 +-

𝐼𝐼𝑑𝑑𝑀𝑀𝑑𝑑𝑑𝑑


+-I diff min


�

𝑰𝑰(𝑺𝑺𝑳𝑳𝑺𝑺𝑹𝑹,𝑺𝑺𝑳𝑳𝑺𝑺𝑹𝑹 )𝑰𝑰𝑰𝑰𝑹𝑹𝑹𝑹𝑹𝑹𝑰𝑰𝒂𝒂𝑰𝑰𝑰𝑰𝑹𝑹



Charge Comparison



𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

Charge Comparison

ILocal IRemote



8787



Qb[A-s]Qa+QbQa[A-s]

• 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

𝑇𝑇1

𝑇𝑇1𝑇𝑇𝑇

𝑇𝑇𝑇

𝑇𝑇1 𝑇𝑇𝑇



Alpha Plane



Internal Faults

Internal faultsw/outfeed at R

Internal faults w/outfeed at L

Alpha Plane

-1𝑹𝑹𝑹𝑹 𝑰𝑰𝑹𝑹/𝑰𝑰𝑳𝑳

𝑰𝑰𝑹𝑹 𝑰𝑰𝑹𝑹/𝑰𝑰𝑳𝑳

IL IR


𝑰𝑰′𝑹𝑹 = 0


= −𝑹𝑹Ideal

BlockingPoint


=𝑰𝑰𝑰𝑹𝑹𝑰𝑰𝑰𝑳𝑳

∠(𝜽𝜽𝑹𝑹 − 𝜽𝜽𝑳𝑳)

Α-Plane Regions for Ideal Fault and Load Conditions





Alpha Plane



IL IR



Α-Plane Channel Delay Compensation Errors and SystemImpedance Differences

Internal Faults

External faultsand load conditions





Alpha Plane



IL IR



Α-Plane Regions for System Power Angle andImpedance Differences

Internal Faults



Alpha Plane



IL IR


Traditional Α-Plane Channel Operating Characteristic

ROperate

Restrain1/R

α



Communication Scheme Design



Protective Relaying Communications



Protective Relaying Communications Path



• 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

Communications Requirements



• 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

Reliability



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



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

received at Substation “B”



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”



Communications Channel Delay



Concept of Current Differential Calculation



Configuration of GPS synchronous line current differential relay



Communications system based on current network technology



Communications system based on future network technology



• 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

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

Application Requirements



• 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

Mult-Terminal Line Protection



Close-in external fault for breaker and half bus configuration



High Resistance Fault



• 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

Open CT Conditions



• 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

CT Ratio Compensation and Mutually Coupled Lines



IL IR



8787

Charging Current Compensation

𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝐿𝐿𝑀𝑀𝐶𝐶𝐿𝐿 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝐿𝐿𝑀𝑀𝐶𝐶𝐿𝐿

Ic

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)



IL IR



8787

Series Compensated Lines

Xc


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



IL IR



8787

Shunt Reactors

Xc

IR

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




• 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

CT Saturation Detection/Compensation



𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇


8787


In-Line Transformer

• 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




8787


Tapped Transformer


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, 2nd

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)




Testing and Troubleshooting



Loopback Testing

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇


8787


• 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


𝑇𝑇𝑀𝑀𝑀𝑀𝐶𝐶 𝑆𝑆𝑀𝑀𝐶𝐶



Local Relay Back to Back Bench Test

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇8787


• 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


𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇87




𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

𝑇𝑇𝑇𝑇

𝑅𝑅𝑇𝑇

Time-Synchronized End-to-End Testing

8787


• Involves testing the entire protection system (except CT if current injection is used)• Use GPS time synchronized three phase test sets





GPS GPS



Troubleshooting an In-ServiceCurrentDifferential System

This subclause on troubleshooting is focused on providing guidance on direction the user toward

potential sources o data errors on current differential schemes.



Annexes

• Annex A Differential protection of power lines/cables based on Rogowski coil current sensors

• Annex B - Bibliography




ILocal IRemote


𝐼𝐼𝐿𝐿𝐿𝐿𝐿𝐿𝑎𝑎𝐿𝐿 + 𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅𝐿𝐿𝑅𝑅𝑅𝑅 = 0

𝒌𝒌 =𝑰𝑰′𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑰𝑰′𝑳𝑳𝑹𝑹𝑳𝑳𝒂𝒂𝑳𝑳

= −𝑹𝑹Ideal

BlockingPoint


8787


𝐼𝐼𝐿𝐿𝐿𝐿𝐿𝐿𝑎𝑎𝐿𝐿= − 𝐼𝐼𝑅𝑅𝑅𝑅𝑅𝑅𝐿𝐿𝑅𝑅𝑅𝑅

IbIa

Ia+Ib



QUESTIONS?



Documents

IEEE Guide for Application of Digital Line Current ...prorelay.tamu.edu/wp-content/uploads/sites/3/2017/04/1.-Summary... · Digital Line Current Differential Relays Using Digital