Speed of Line Protection

Copyright © SEL 2015

Speed of Line Protection – Can We Break Free of Phasor Limitations?

66th Annual Texas A&M Conference for Protective Relay Engineers

College Station, TXApril 1, 2015

Edmund O. Schweitzer, III Bogdan KasztennyArmando GuzmánVeselin Skendzic Venkat Mynam

Schweitzer Engineering Laboratories, Inc.

Progression of Speed• Operators tripped breakers by hand

• Fuses and breakers: time overcurrent

• Distance relays

• Pilot wires and protection channels

Time-domain elements…based on traveling waves

and differential equations…promise 2 to 4 ms performance

Phasor-Based Protection Makes Sense

• Traditional models are steady-state

• The “forcing functions” are at 60 Hz

• Instrument transformers are rated at 60 Hz

• CCVTs are “band-pass” devices at 60 Hz

Attracted-Armature Relays• Respond to AC, DC, and transients,

but calibrated for AC at 60 Hz

• Overreach for exp(-at)

• Remnant flux delays dropout

• Wide margin between pickup and dropout

Induction Disk and Cylinder Relays

• Torque ≈ k I2

• Transients andremnant fluxhave less effect

• Pickup ≈ dropout

It Takes a Cycle to Catch a Cycle• Phasors: sinusoidal steady state• Faults change the network• We want to determine where…fast!• Determine the sinusoidal steady state

→ filter out everything else• Shorter windows: faster, less accurate

Digital Filter Passes Cosine, Rejects exp(-at)

y = x1 – x2 – x3 + x4

DC = 1 – 1 – 1 + 1 = 0

Ramp = 1 – 2 – 3 + 4 = 0

Cosine = 1 + 1 + 1 + 1 = 4

Speed of Present-Day RelaysDetermining steady state takes time.

Shorter windows are faster but less accurate than full cycle.

–1 0 1 2 3 4 5–20

–15

–10

–5

0

5

10

15

Time, cycles

Cur

rent

, pu

Ope

ratin

g Ti

me

in C

ycle

s

Fault Location in Percent of Set Reach

Line-to-ground fault, SIR = 1.0

Modern Distance Relays: 8–16 ms

0 10 20 30 40 50 60 70 80 900

0.5

1

1.5

100

15 MW more per millisecond savedR. B. Eastvedt, BPA, 1976 WPRC

The Need for SpeedMoving Energy at the Speed of Light

Safer • Less Damage • Improved Dynamics

ASEA RALDA (1976)5 ms Directional Wave Relay

−

B

Di Dv Di Dv− + −

+ + − +

A

BBC LR-91 (1985) 5 ms UHS Directional Relay

Fault is forward when the operating point enters the 2nd or 4th quadrant 

Why Today? The Need for Speed

Faster communicationsPowerful processors

Better simulationsMay be simpler

Waves Travel Toward Line Terminals

vRvSiRiS F

Currents and Bewley Lattice DiagramBG Fault, June 04, 2013

Mic

rose

cond

s

-20 0 20 40 60

Goshen B (A)0 20 40 60 80 100

km-10 0 100

100

200

300

400

500

600

700

800

900

1000

Drummond B (A)

A SOLID Three-Phase FaultABC Fault, Aug 28, 2013

-500 0 500

Goshen A (A)0 20 40 60 80 100

km

Mic

rose

cond

s

-100 0 1000

200

400

600

800

1000

1200

1400

1600

1800

2000

Drummond A (A)

Practical Traveling Wave RelayingBuild on TWFL Experience

Single-ended: sort out reflections; easier with voltages

Two-ended:

Directional comparison

Current differential

Finding Incident and Reflected Waves

I R I R

I RI R

c c

I c

R c

v v v i i i

v vi i

Z Z

1Thus : v v Z i2

1 v v Z i2

Speed of Light Limits Relay Time

The fastest communications path is the line

S R100-mile line ≈ 600 µs X

300 μs 300 μs

600 μs by line or 1,000 μs by fiber

900 μs or 1,300 μs

Propagation Adds Up to 1.6 msWhat Are Other Delays?

Propagation 1.6 msSignal processing 0.5–2 msPOTT processing 0.1 msTRIP output 0.1 msTOTAL: 2.3–3.8 ms

Then, it’s up to the circuit breaker

Speed Limiters and OptionsCircuit breaker 17 ms Solid state?

Auxiliary relay 4 ms Don’t use

Contact input delay 1 ms Improve signaling

Process bus 0.4 ms Hard wire

Time-Domain Directional Element (TW)

vF

v i vF

vi

V I

+–

–+ Forward

+–

+– Reverse

Time-Domain Directional Element (R-L)Incremental Quantities

Time

Cur

rent

Time

Cur

rent

TimeCur

rent

F

vF

F

vPRE

iPRE

–vF

Fi

v

+

=

Time-Domain Directional Element (R-L)

Assume R 0

v

i

v i

+–

–+ Forward

+–

+− Reverse

RS LSv mR mL

i vF

Fast Directional Element (R-L)

-40 -20 0 20 40-20

-10

0

10

20

-40 -20 0 20 40-40

-20

0

20

40 

LS mL (1−m)L LR

Fast and Secure Directional Element 

0 2 4 6 8-50

0

50

0 2 4 6 8-2000

0

2000

Time (msec)

Time (ms)

Time-Domain Distance Element

m0 = Set Reach

  LSv mL

i vF

-5 0 5 10 15-50

0

50

-5 0 5 10 15-100

0

100

-5 0 5 10 150

75

150

Time (msec)

Fast and Secure 21Fault at 25% of the line with m0 set to 50%

Time (ms)

New TW Differential PrincipleCurrent Only

• Internal fault surges: same polarity• External fault surges:

♦ Generally of opposite polarity♦ Spaced one travel time T apart

Σ of aligned surges = OPERATE

Δ of surges T apart = RESTRAIN

300 μs 300 μs

Internal Mid-Line Fault

S R

IF(0)

Σ = Is(300) + Ir(300) = BIGΔ = Is(300) − Ir(300 +/− 600) = small

200 μs

400 μs

IF(0)

Internal Fault Closer to SΣ = Is(200) + Ir(400) = BIG

Δ = Is(200) − Ir(200 +/− 600) = small

S R

600 μs

S

IF(0)

External Fault Travels the Entire LineΣ = Is(50) + Ir(650) = small

Δ = Is(50) − Ir(50 +/− 600) = BIGR

Traveling Wave Current DifferentialCorner Case

The principle holds true• TW that entered S

leaves R after T

• TW that entered R leaves S after T

T

T

S R

30 30.5 31 31.5 32 32.5 33–500

–300

–100

100

300

500

Time, ms

Prim

ary

Ampe

res

Fast Hardware for < 4 ms Tripping

The time to trip is determined by propagation and processing… NOT by the 16,667 μs duration of a cycle

Component Time Domain, μs Today, μsSampling period 1 > 100 Processing interval 100 1,000–2,000 Trip outputs 10–100 4,000Digital trip outputs 100–1,000 1,000–4,000 Channel interface 50–1,000 2,000–8,000 Data sharing 50–1,000 100,000

Breaking Free of Phasor LimitationsEnergy Moves at the Speed of Light

• Inherently FAST principles for 32, 21, 87 for 2 to 4 millisecond trip times

• Easier to set and understand• Inherently secure for LOP• Suitable for single-pole tripping• Inherently suitable with series compensation• Addresses the need for speed