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
• 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
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
• 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
• 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
• 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
• 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
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
• 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
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 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
• 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 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