Copyright © SEL 2016

Negative Sequence: How to Use It

Greg SmelichSchweitzer Engineering Laboratories, Inc.

Copyright © SEL 2016

• Include voltage, current, and impedance

• Can be used for overcurrent, differential, directional, and fault location elements

Negative-Sequence Quantities

Negative-Sequence Overcurrent Element

Long Clearing TimeFebruary 2014

Long Clearing TimeFebruary 2014

• Phase-to-phase fault was right at pickup of phase time-overcurrent element (512 A)

• Fault time was long enough to burn line down (43 seconds)

• Phase-to-phase fault current was approximately 0.86 pu of three-phase fault

The Perfect Storm

Three-Phase Fault

2 pI , I

1I

1Z

E

p 1 1

2

I I E / Z

I 0

Phase-to-Phase Fault

2 pI , I

1I

1Z

E

2I

2Z

1 2

1 2 1

2p 2 2

p

phase-to-

1 1

phase three-pha

1

1

se

p

p

Z Z

I I E / 2Z

I a a • I j1.732• I

I 1.732• I E •1.732 / 2 • Z

I 0.866 • I

I 1.732 / 2 •E / Z

I 0.866 •E / Z

• Traditional protection Time-delayed backup

Limited protection features

• Enhanced protection High-speed backup

High-speed bus protection

No additional relays

Traditional Versus Enhanced Distribution Protection

Traditional Distribution Protection

Enhanced Feeder Protection

Traditional Overcurrent Protection

Traditional Overcurrent Protection

Phase Overcurrent Protection

Phase and Ground Overcurrent Protection Three-Line Diagram

• Advantages Is more sensitive than phase overcurrent

protection for ground faults

Is easy to make connection to detect ground current

Improves protection for most common fault type

• Disadvantage – no protection for phase-to-phase or three-phase faults

Ground Overcurrent Protection Not a Complete Solution

Introduction to Negative-Sequence Overcurrent Elements

Phase, Ground, Negative-Sequence Overcurrent Protection

2 2 2

2

• Advantages Is more sensitive than phase overcurrent

protection for phase-to-phase faults

Is easy to calculate negative sequence in relay

Improves protection for second most common fault type

• Disadvantage – not sensitive tothree-phase faults

Phase-to-Phase Fault DetectorNegative-Sequence Overcurrent Element

Setting Negative-Sequence Overcurrent Elements

• Usually next downstream overcurrent device

• Typically no downstream negative-sequence overcurrent device

Step 1: Find Downstream Phase Overcurrent Device of

Greatest Coordination Concern

• As equivalent phase time-overcurrent element backup

• As being located at downstream device

Step 2: Think of Negative-Sequence Element

• Same curve shape

• Coordination margin of 12 to 30 cycles

• Same pickup plus 10%

Step 3: Perform Typical Phase Coordination With Downstream

Phase Overcurrent Device

3

Step 4: Transform Equivalent Element Settings to Negative-Sequence

Overcurrent Settings

• (equivalent element pickup)

Negative-sequence element pickup =

Time dial and curve type are unchanged

Equivalent Phase Overcurrent ElementTi

me

(sec

onds

)

Tim

e (s

econ

ds)

Negative-Sequence Overcurrent Element

Enhanced Overcurrent Protection

• Are immune to balanced load conditions

• Improve sensitivity to phase-to-phase faults

• Can be easily implemented in microprocessor-based relays

• Are easy to set

Negative-Sequence Elements

Customer Negative-Sequence Settings Coordination

p

p

p

p

51QP = 5.00 51QC = U3

51QTD = 3.00 51QRS = N

51QCT = 0.00 51QMR = 0.00

Customer SolutionAdd Negative-Sequence Protection

• Old TR = 50P1 + 51P1T + (51G1T + 50G1) * LT1 + OC + 81D2T + (51P1 + 51G1) * !LT5 + (PB8 * SV2T)

• New TR = 50P1 + 51P1T + (51G1T + 50G1 + 51QT) * LT1 + OC * LT3 + (51P1+ 51G1) * !LT5 + (PB8 * SV2T)

Customer SolutionAdd Negative-Sequence Protection

Déjà Vu in March 2014

Déjà Vu in March 2014

• 51QT clearing time of 4 seconds

Line was not damaged

• Phase fault current

540 A (1.125 multiples of pickup)

• Negative-sequence fault current

3I2 = 981 A (2.45 multiples of pickup)

Noticeable Improvement

Negative-Sequence Differential Element

87Q Principle

• Sensitive

• Fast

• Inherently secure

1Q 2Q

The Secret to SensitivityDifferential Current?

ABC

Q

ABC

Q

I1A

I1B

I1C

I2A

I2B

I2C

I1Q

I2Q

IDIF(Q) ABC

Q

I1A

I2A

I1B

I2B

I1C

I2C

IDIF(A)

IDIF(B)

IDIF(C)

IDIF(Q)MathematicallyEquivalent

No 87P differential = no 87Q differential

1 2DIF Q Q Q N Q

The Secret to SensitivityRestraining Current?

87P 87Q

Lower restraint provides sensitivity but challenges security

RST(Q)

DIF(Q)

1 2RST Q Q Q N Q

Restraining Current Purpose

Reflect the stress on protection system components, CTs in particular, that can cause

spurious differential current

Negative-sequence restraining current does not meet this requirement for

balanced faults / events

• Security problems under CT saturation

• Need for external fault detection logic

The Secret to SensitivityAddressing CT Saturation

• Is excellent for protecting lines and transformers

• Needs security for CT saturation

Negative-Sequence Differential

Negative-Sequence Directional Element

Negative-Sequence EquationsVoltage and Current

22

22

2

3

3where 1 120

and 1 240

A B C

A B C

V V a V aV

I I a I aIa

a

• Positive torque indicates forward direction

• Negative torque indicates reverse direction

Traditional Negative-Sequence Directional Element

2 2 2 2

Phasor Diagram Shows Maximum Torque Angles for Traditional Negative-

Sequence Directional Element

3V2

3I2 Forward

3I2 Reverse

MTA

Boundary

• Torque produced is proportional to magnitude of V2 and I2, limits sensitivity of directional element

• Direction of very short phasor is difficult to determine, incorrect directional declarations may result

Traditional Negative-Sequence Directional Element Limitations

2 2 2 2

• Negative impedance indicates forward direction

• Positive impedance indicates reverse direction

• Forward and reverse thresholds can be applied for security

Negative-Sequence Impedance Directional Element

2 22

2 2

Negative-Sequence Impedance Directional Element

22

2

3VZ3I

2

2

2

• Direction can be determined for faults with essentially zero negative-sequence voltage

• Directional element has greater sensitivity

Negative-Sequence Impedance Directional Element

2 22

2 2

Negative Sequence for Fault Location

Negative Sequence for Fault Location

Double-Ended

Negative Sequence for Fault LocationDouble-Ended

2

2S

2S

2S 2R

2R

2Rm 2L m) 2L2F

2 2 2 2

2 2 2

S R R L

L S R

• Include voltage, current, and impedance

• Can be used for overcurrent, differential, directional, and fault location elements

• Are already built in to many digital relays; just need to be enabled

Negative-Sequence Quantities

Understand elements and associated settings before applying

Questions?