Copyright © SEL 2015

Ultra-High-Speed Relaying for Transmission Lines

Focus for Today

• Benefits of faster line protection

• Limitations of present-day phasor-based protection

• Principles of time-domain protection

Already Pretty Fast – Why Faster?

• Higher power transfers(investment dollars saved)

• Reduced equipment wear (generators and transformers)

• Improved safety

• Reduced property damage

• Improved power quality

How Much Faster?

• Present-day relays♦ Based on phasors

♦ Operate in 0.5–1.5 cycles

• Present-day breakers operate in 2 cycles

• Ultra-high-speed fault clearing♦ Consistent relay operating times

♦ 2 ms (TW) to 4 ms (differential equations)

♦ Subcycle times from future dc breakers

Phasor-Based Protection Makes Sense

• Power systems were traditionally designed and modeled for steady-state operation at system frequency

• “Forcing functions” are at system frequency

• Instrument transformers are rated at system frequency

• CCVTs are band-pass devices

Speed of Present-Day Relays

• Phasors represent steady state

• Determining steady state takes time

This is what we know if we trip in 0.5 cycles

Speed of Present-Day Relays

• Phasors represent steady state

• Determining steady state takes time

Speed of Present-Day Relays

• Phasors represent steady state

• Determining steady state takes time

• Shorter windows are faster but less accurate

1970s and 1980s Designs

• Based on incremental quantities

• Not true TW protection

• Underperformed on security

• No manufacturer follow-through

ASEA RALDA (1976)

BBC LR-91 (1985)

GEC LFDC (1988)

Why Only Now?

• Better technology♦ High-speed ADC

♦ Processing power

♦ High-bandwidth communications

• TWFL experience and new ideas

• Advanced simulation tools

• Simplicity

Introducing the SEL-T400L

SEL-T400L Key Functionality

• Subcycle protection♦ TD21 4 ms for 50% of line

♦ TD32 2 ms + channel time

♦ TW87 1–2 ms + channel time

• Fast MIRRORED BITS® and I/O

• TW fault locator – two-ended and single-ended methods

• 1 Msps DFR and analytics

Phasor and Time-Domain PrinciplesSimilarities and Differences

Algorithm Phasor-Based Differential Equations Traveling Waves

Spectrum 50 / 60 Hz 1 kHz 100 kHz

Filtering

Sampling 16–32 s/c 8 kHz 1 MHz

Line theory

Operating time ~ 1 cycle A few milliseconds 1 ms

Requirements for CTs and PTs Low Moderate High

Traveling Wave Current DifferentialExternal Faults

TW that entered at one terminal…• Leaves at other

terminal

• After line propagation time

• With opposite polarity

Traveling Wave Current DifferentialInternal Faults

Internal fault launches two TWs that…• Are of the same

polarity

• Arrive with time difference, P ≤

Traveling Wave Current DifferentialCorner Case

The principle holds true• TW that entered S

leaves R after

• TW that entered Rleaves S after

TW87 Differential Element

• Operates in 1–2 ms

• Uses current TWs♦ No need for high-fidelity voltage

♦ Will work with CCVTs and CTs

• Communications-based (100 Mbps)

• Not affected by series capacitors

Differential Equation ProtectionIncremental Quantities

Fault

Prefault

And the network simplifies…

Subtract…

0

eS vF

RS LSS

mR mL F

i

v

Differential Equation ProtectionIncremental Quantities

Fault

Prefault

“Source”

And the network simplifies…

eS vF

RS LSS

mR mL F

i

v

Subtract…

Incremental QuantitiesExample

Vol

tage

–0.5 0 0.5 1–50

0

50

Cur

rent

Time, cycles

Incremental QuantitiesExample

1 kHz500 Hz300 Hz

Differential Equation ProtectionIncremental Quantities

Introduce replica current

Even simpler equations…

RS

LS

SmR mL F

vF

i

v

Directional ElementFirst 1 ms of Fault

–60 –40 –20 0 20 40 60–60

–40

–20

0

20

40

60

500 Hz300 Hz

–15 –10 –5 0

0

0.4

0.8

1.2

1.6

500 Hz300 Hz

Directional Element

R L

SF

i

vRR

LR

R

Directional Element

Directional Element

The principle is solid despite transients left in the operating signal. No need for excessive filtering!

300 Hz LPF 500 Hz LPF

Directional Element

–60 –40 –20 0 20 40 60–60

–40

–20

0

20

40

60

500 Hz300 Hz

Reverse fault

Forward faultReplica current makes the element stay picked up

Distance Element

Want to reach up to m0…

Voltage change at the fault:

Therefore, trip if:

SmR mL F

vF

i

v

Distance Element

Fault at 25% of the reach

300 Hz LPF

500 Hz LPF

Trip

Distance Element

m0 = 0.2 – 0.4

Fault at 50% of the line (300 Hz LPF)

m0 = 0.6 – 0.9

Distance ElementSimilar to Zone 1

21 (Z1) TD21Controlled reach

Directionality

Direct tripping

Setting in length units

Independence from SIR

RF impact

Differential Equation 32/21 Elements

• Operate in 2–4 ms

• Use incremental quantities ♦ No need for high-fidelity voltage

♦ Will work with CCVTs and CTs

• Work with any channel

• Not affected by series capacitors

• Inherently secure for LOP

SEL-T400L SettingsNo Short-Circuit Studies Required

• CT, PT ratios, Vnom (nameplate data)

• Line Z1 and Z0 (known for every line)

• Line propagation time (line energization test)

• TD21 reach (user preference)

• Basic channel configuration parameters

Conclusions

• Modern power systems need faster protection

• We have technology for fast line protection

• Time-domain principles are easy to use