Slide 1 Line Current Differential Application on Short Lines Presentation to SSCET October 26 th, 2012

Goals of Protection Definition of Short Lines Challenges Posed by Short Lines Line Current Differential Explained Benefits of Line Current Differential Application Example Content

Goals of Protection Security Dependability: the degree of certainty that the relay will operate correctly. Security: the relay will not operate incorrectly Speed Very high power during fault conditions: delays translate into increased damage: faster protection tends to compromise relay system security and selectivity. Sensitivit y The minimum operating quantities allows the relay to detect an abnormal condition. High-impedance ground faults, voltage unbalance and high source- to- line impedance ratio affect the sensitivity Selectivit y or coordination: ability of the relay system to minimize outages as a result of a fault by operating as fast as possible within their primary zone. Simplicity simple to apply and to obtain maximum protection

Slide 4 What is a short line? Classification of line length depends on:  Source-to-line Impedance Ratio (SIR), and  Nominal voltage Length considerations:  Short Lines: SIR > 4  Medium Lines: 0.5 < SIR < 4  Long Lines: SIR < 0.5

Challenges of Short Lines Sensitivity of Overcurrent Elements

Challenges of Short Lines Coordination of Distance Elements

Challenges of Short Lines Operation Time of Distance Elements

Distance Relay Basics For internal faults: IZ – V and V approximately in phase (mho) IZ – V and IZ approximately in phase (reactance) RELAY ( V,I ) Intended REACH point Z F1F1 I*Z V=I*Z F I*Z - V

Distance Relay Basics For external faults: IZ – V and V approximately out of phase (mho) IZ – V and IZ approximately out of phase (reactance) RELAY ( V,I ) Intended REACH point Z I*Z V=I*Z F I*Z - V F2F2

Distance Relay Basics Reactance comparator [V] power cycles S POL S OP

Distance Relay Basics Lin e System Relay Voltage at the relay: Consider SIR = 0.1 Fault locationVoltage (%) Voltage change (%) 75% % %90.91N/A 110%

Distance Relay Basics Lin e System Relay Voltage at the relay: Consider SIR = 30 Fault locationVoltage (%) Voltage change (%) 75% % %3.2258N/A 110%

Current Differential Relay Basics Unit Protection Communications Channel Required

Current Differential Relay Basics Clock Synchronization Measure channel delay to shift local phasor by angle equal to the half of the round trip delay:

Current Differential Relay Basics Clock Synchronization

Current Differential Relay Basics Communications Channel Noise window time A sum of squared differences between the actual waveform and an ideal sinusoid over last window is a measure of a “goodness of fit” (a measurement error) The goodness of fit is an accuracy index for the digital measurement The goodness of fit reflects inaccuracy due to: transients CT saturation inrush currents and other signal distortions electrical noise The goodness of fit can be used by the relay to alter the traditional restraint signal (dynamic restraint) and improve security

Current Differential Relay Basics Traditional vs. Adaptive Restraint Differential Pickup Restraint 1 Restraint 2 Traditional characteristics Adaptive characteristics

Current Differential Relay Basics Adaptive Restraint Differential Total restraint = Traditional restraint + Adaptive restraint (Error factor) Imaginary (I LOC /I REM ) Real (I LOC /I REM ) OPERATE REST. Error factor is high Error factor is low

Summary SIR, not just line impedance, defines a short line. Overcurrent protection is less secure than alternatives. The sensitivity and speed of distance relaying are adversely impacted, and coordination becomes more complex. Line current differential provides good sensitivity, speed and alleviates coordination issues.

Application Examples

Summary 51 SUB A SUB B SUB C SUB D SUB E time current 51 BLUE relay sees the most current. Coordination time intervals are acceptable. If line between Sub B and Sub C are out of service, coordination time interval between D and C is unacceptable. 87L By eliminating one of the 51 elements, we have increased the coordination time interval and made system coordination easier.

Application Example

Protection Scheme Needs High speed operation Weighted towards security Must protect short line without over- reaching Ability to handle weak source

Application Example POTT Scheme Plus: good security, distance relay, simple comms Minus: Communications channel, weak infeed conditions

Application Example Hybrid POTT

Application Example Line Differential Plus: good security, good for short lines Minus: Complex communications channel

Slide 27 References IEEE C Guide for Protective Relay Applications to Transmission Lines (1999) (draft 2011) Draft contains new information regarding short lines. Relaying Short Lines (Alexander, Andrichak, Tyska) GE Publication GER-3735.

Questions