NEW DESIGN OF GROUND FAULT PROTECTION

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NEW DESIGN OFGROUND FAULT PROTECTIONThe 71st Annual Conference for Protective Relay Engineers

2018-03-29Page 2

Why do we need ground fault protection?

• widely used to protect transmission anddistribution lines in case of ground faults

• can detect and isolate even high resistiveground faults which are not seen bydistance protection

• often combined with a directional elementand used in a teleprotection scheme

• equipped with phase selection alsosuitable for single pole tripping

2018-03-29Page 3

Basic principle of ground fault protection

CBA IIII ++=03• Ground fault protection is based on zero sequencecurrent

• Ground fault protection picks up if magnitude ofzero sequence current exceeds threshold:

thresholdI >|3| 0

3I0

Threshold

T 0

T - Delay

67N Pickup

67N Trip

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Directional ground fault protection

• directional element based on the anglebetween zero sequence current andzero sequence voltage

3I0

Threshold

T 0

T - Delay

67N Pickup

67N TripAND

3U0

Directionalelement

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Directional ground fault protection

• directional element based on the anglebetween negative sequence current andnegative sequence voltage

3I0

Threshold

T 0

T - Delay

67N Pickup

67N TripAND

3U2

Directionalelement

3I2

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Directional ground fault protection,suited for single pole tripping

• phase selection using the angle of zero sequence currentcompared to the angle of negative sequence current

3I0

Threshold

T 0

T - Delay

67N Pickup

AND

3U2Directionalelement

3I2

67N Trip Phase A

67N Trip Phase B

67N Trip Phase C

Phase selector

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Case 1:Wrong trip using zero sequence polarization

• wrong trip of directionalground fault protection

• Malaysia: 147 km line, 132 kV

• Figure left shows theimpedance trajectoriesin the complex plane

• BG fault, far away from thepolygons of distanceprotection in reverse direction

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• Ground current iE exceeds the sensitivethreshold of 50A primary which leads to apickup of the ground fault protection

• Current of phase B is raising most whichis consistent with the pickup of phase B

• Changes of voltages after fault inception arenot very big but leaving enough quantity ofzero sequence voltage and negative sequencevoltage for the directional element

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• Figure left is showing zero sequencequantities and negative sequencequantities in the phasor diagram

• zero sequence current is leadingzero sequence voltage byapproximately 100° forward fault

• negative sequence current is laggingnegative sequence voltage byapproximately 60° reverse fault

• zero sequence quantities are chosenbecause they are a little larger thannegative sequence quantities.

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Case 2: Wrong polarization due to an error ofvoltage transformer

• Figure left shows the impedancetrajectories in the complex plane

• It can be seen that the fault AGappears in reverse direction(green marked trajectory)

2018-03-29Page 11


• significant decrease in the voltage VAis a clear indication for a fault AG

• voltage VB shows an asymmetrydue to a voltage transformer error

• Wrong reading of VB has a majorimpact on the wrong directiondetermination using negative sequence

• Current IA is raising most which leadsto a pickup of ground fault protectionindicated by the signal “67G2”

• Signal “FSA” indicates that the relaydetects the faulted phase A butunfortunately the wrong directionindicated by the signal “32QF”

2018-03-29Page 12


• Figure left shows the zero sequencequantities and negative sequencequantities in the phasor diagram

• negative sequence current is leadingnegative sequence voltage byapproximately 160° forward fault

• Zero sequence current is laggingzero sequence voltage byapproximately 100°reverse fault

• negative sequence quantities werechosen by setting to detect thedirection to fault

• This leads to the wrong trip of thedirectional ground fault protection

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Case 3: Wrong phase selection

• single phase high resistive fault BG close to Amarilis substation

• Due to this Amarilis was a weak infeed side with a transformer with a deltawinding on its 10 kV side

• At this time the line L-1120 was out of service

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• Figure left shows the impedancetrajectories in the complex plane

• fault BG appears on the real axisof the complex plane in forwarddirection (red marked trajectory)

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• Due to the transformer on theweak side all three phasecurrents are nearly in phase,producing a ground currentwhich is much bigger than eachsingle phase current

• Ground current causes a pickupindicated by the signal“67N Pickup”

• significant decrease in VB butthe faulted phase was notdetected

Instead of a single pole trip command for phase B a three pole trip was issuedindicated by the signal “Trip 3-pole”

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• reason for unselective trip was methodapplied to detect the faulted phase

• Method is based on the relation betweenangle of negative sequence current andangle of zero sequence current

• Zero sequence current should leadnegative sequence current byapproximately 120° for a fault BG

• But zero sequence current is leadingnegative sequence current byapproximately 60° only no clear indication for any type of fault

• Magnitude of the negative sequence currentis very small compared to the magnitude ofthe zero sequence current

L2-E

L3-E

L1-E

I2 = I0

I2 = a2*I0

I2 = a*I0

I0

I2

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New design of directional ground fault protection

• use all available information to detect faulted phase and direction to fault

3I0

Threshold

T 0

T - Delay

67N Pickup

AND

UADirectionalelement

UC

67N Trip Phase A

67N Trip Phase B

67N Trip Phase CPhase

selector

UB

IA

ICIB

AND

AND

AND

2018-03-29Page 18

Multi criteria phase selection

Criteria for phase selection

• Symmetrical components

• Impedance • Phase current• Delta current• Current sample• Current phasor• Phase voltage• Delta voltage• Voltage sample• Voltage phasorC

Criteria 2

Criteria 1

Criteria n

quality

weight 1

quality

weight 2

quality

weight n

Σ

AB

quality phase A

quality phase B

quality phase C

2018-03-29Page 19

Voltage magnitude criteria

quality

ULx [UN]

0.25

0.50

0.75

1.00

0.25 0.50 0.75 1.00

0.90 t/s0.14 0.15 0.16 0.17 0.18 0.19 0.20 0.21 0.22 0.23

U L1-E/ V

-50

0

t/s0.17 0.18 0.19 0.20 0.21 0.22 0.23 0.24 0.25 0.26

U L1-E/ V

-50

0

The lower the voltage, the higher the quality of the result.

2018-03-29Page 20

Current magnitude criteria

quality

ILx [IN]

0.25

0.50

0.75

1.00

1 2 3 4

0.90

0.15

t/s0.15 0.16 0.17 0.18 0.19 0.20 0.21 0.22 0.23 0.24

I L1/ A

-4

-2

0

2

t/s0.16 0.17 0.18 0.19 0.20 0.21 0.22 0.23 0.24 0.25 0.26

I L1/ A

-0.4

-0.2

-0.0

0.2

The higher the current, the higher the quality of the result.

2018-03-29Page 21

Impedance ratio criteria

The lower the ratio between the measured X and the parameterized X, thehigher the quality of the related loop.

quality

|XLx| / Par(X)

0.25

0.50

0.75

1.00

0.25 0.50 0.75 1.00

0.90

R

X

Z1

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Multi criteria directional element

Criteria for directional element

• Zero sequence• Negative sequence• Self polarization• Memory polarization• Cross polarization• Memory cross

polarization• Polarization using

delta quantities of faulted phase

• Polarization using delta quantities of symmetrical components

Criteria 2

Criteria 1

Criteria n

quality

weight 1

quality

weight 2

quality

weight n

Σquality forward

quality reverse

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Multi-criteria directional element applied to case 2

• The binaries show that 7 criteria determine the fault in reverse direction

• Quality of reverse direction: >75%

• Quality of forward direction:<25%

Multi-criteria directionalelement can detect theright direction even ifsome criteria fail

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Conclusion

• It was shown that the reach of the classical impedance calculation method is significantly influenced by resistive faults on heavy loaded lines.

• Using the reactance method this reach error can be eliminated.

• Additionally a new method of loop selection was presented which is optimized for all network topologies.

• The same philosophy is applied for directional element where different algorithm are weighted dependent on network topology.

2018-03-29Page 25

Thank you for your attention!

Jörg BlumscheinPrincipal Key Expert ProtectionEM DG PRO LM&D PPM

Wernerwerkdamm 513629 Berlin

Phone: +49 (30) 386 20135Fax: +49 (30) 386 25158

E-mail:[email protected]

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NEW DESIGN OF GROUND FAULT PROTECTION

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