© 2010 Eaton Corporation. All rights reserved.

Neutral Switching of Grounded Sources

Dave Loucks, PE

2 2

Switching Methods

• 3-Pole

• 3-Pole with Overlapping Neutral

• 4-Pole

3 3

Issues

1. Ground Discrimination

2. Transient Voltages

4 4

Ground Discrimination

• Identify source(s) feeding ground fault and clear

• Two scenarios:• Single Ground: Non-Separately Derived

• Multiple Ground: Separately Derived

5 5

Single Ground: Non-Separately Derived

• Issue• Ungrounded sources cannot be detected as

sources of ground current using CT at source.

Source 1 Source 2

CT 1output = IF

IF

IF IF IF

CT 2output = 0

IF

6 6

A Solution: Non-Separately Derived

Source 1 Source 2

IF

IF IF IF

IF

S1A

S2A

GFRelay

S1T

Reliable Tripping Power

S2T

7 7

Separately Derived System: 3-Pole

Source 1 Source 2

CT 1output = IF - 1/2IF = 1/2IF

IF

1/2 IF 1/2 IF

IF

1/2 IF 1/2 IF 1/2 IF

CT 2output = 1/2IF

• Issue• Multiple grounded sources with unswitched neutral

share ground current between each source

8 8

A Solution: Separately Derived

Source 1 Source 2

CT 1output = IF

IF

CT 2output = 0

IF

• Solution• Switch neutral to break “cheat” path for ground

current.

9 9

Separately Derived – Overlapping Neutral

• During the transition, this design looks the same as a 3-Pole device

• If a GF were to occur during this transition, same problems as with 3-Pole switching

Source 1 Source 2

CT 1output = IF - 1/2IF = 1/2IF

IF

1/2 IF 1/2 IF

IF

1/2 IF 1/2 IF 1/2 IF

CT 2output = 1/2IF

10 10

Transients

• So maybe an overlapping neutral has some problems with GF discrimination, but doesn’t 4-pole switching the neutral create transient overvoltages on the neutral?• After all, whenever current through an inductive circuit is

interrupted, you have the potential for transient overvoltage.

• We can calculated the magnitude of voltage of this transient from:

dt

diLVL

VL Transient Overvoltage developed across inductance

L Inductance (Henries)

di Differential (change) current (amperes)

dt Differential time (seconds)

VL i+ -

11 11

Can We Model Transients Accurately?

• To prove our model was accurate, we would need to compare our calculated transients on a switched neutral with measured transients from an actual lab experiment

• If our modeled values are equal to or greater than the values measured in the lab, then we could have confidence in our model• After all, if our model estimates higher transients

than what we measure in the lab, we can call our model “conservative” since actual transients will not be higher than our model.

12 12

Lab Test versus SPICE Model

Lab Test:

• 600 VL-L

• 10 kA rms available

• 60 Hz

• 49.1% PF

Equiv. SPICE Model

• 848.428 VL-L peak

• Z = 0.06 • 60 Hz

• X/R = 1.77• R = 0.0295137

• XL= 0.0522393

@ 60 HzL = 0.1385 mH

• For test, both systems are ungrounded

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Problem: We Still Have an Unknown

• While we know L, di (change of current) …

• … we don’t know dt (time to change current)• How fast does a switched neutral operate?

• The faster it operates, the smaller the value of dt

• The smaller the value of dt, the higher VL

dt

diLVL

14 14

Progression of Arcing Contact

Time = 0 Time 1.1 ms Time 1.2 ms Time 5.5 ms

Total Arc Extinguishing Time 8.2 ms

Measured:

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LT-SPICE Model

Note: “ungrounded” means capacitively grounded

16 16

Actual Lab Test

~ 530 V

Note: 530V / 848V = 63%

17 17

Model Matches Very Closely

• In fact, our model is “conservative” since model predicts higher transient than lab test

• Raises confidence that transients will be less

Current zero, arc

extinguishes

~ 530 V

Fault current

Arcing voltage

System voltage

~ 801 V

Note: 801V / 848V = 94%

18 18

Increase Size of Model

• Now that we have our switching model, we can change our source and see how the transients change

• New Test:• Source 1 – Utility

• 1500 kVA transformer, Z = 5.75%, 480Y/277 Vrms, 60 Hz, X/R = 6.6 (IFL = 1804 Arms, ISC = 31377 Arms

• Source 2 – Utility• Ratings same as Source 1

• Load• 600 Arms, 80% PF (X/R = 0.75)

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Schematic Model

• Refer to Figure 21, page 20 and pages 14 - 19 for calculation of these values

20 20

GF and Transient Tests

• Non-separately derived (NSD) sources• Test 1: Open transition 3-pole switching (Figure 21)

• Separately derived (SD) sources• Test 2: Open transition 3-pole switching

• Test 3: Open transition 4-pole switching

• Test 4: Open transition phase 3-pole, closed transition neutral (overlapping) switching

21 21

Test 1: NSD 3-pole switching (page 20)

• NSD means only one source grounded• Assume GF system connected as shown in Figure

5 (page 4), so no GF discrimination problem

• 3-Pole switching means no neutral switching no neutral transient

Source 1 Source 2

IF

IF

IF

IF

IF

S1A

S2A

GFRelay

S1T

Reliable Tripping Power

S2T

22 22

Test 2: SD 3-pole switching (Page 21-22)

• SD means multiple grounded sources• Unswitched neutral means cheat path allows GF

currents to flow even through sources with 3-pole switching device open

Source 1 Source 2

IFIF

0.001847 24.01 H

0.001847 24.01 H

0.5 0.5

0.001847 24.01 H

0.001847 24.01 H

23 23

Test 2: SD 3-pole switching (Page 21-22)

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Test 2: SD 3-pole switching (Page 21-22)

Actual GF current magnitude (1472

A peak)( 1041 A rms)

GF current measured at source 1 zero-sequence

CT (671.8 A peak)(475 A rms)

• Measured GF current is only fraction of actual

• … but that isn’t the only problem …

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Test 2: SD 3-pole switching (Page 21-22)

• GF current is detected as flowing through de-energized source!

A, B and C phase current = 0

Neutral current = 733 A peak (533 A rms)

26 26

Switch Neutral

• These problems are well known and are the reasons why we switch the neutral of separately derived sources

• Two main methods:• 4-Pole

• 3-Pole with Overlapping Neutral Switching

• 3-Pole with Customer GF wiring

Method Advantages Disadvantages

4-Pole Switching No circulating current, so no possibility of desensitizing energized source GF relay and no possibility of nuisance tripping a GF relay protecting a de-energized source

Higher cost Larger footprint (size) Reported neutral transients*

3-Pole Switching with Overlapping Neutral

May be less expensive than true 4-pole since overlapping neutral typically is not rated for fault duty switching

During the time when both neutrals are connected, the same disadvantages as a 3-pole switch (nuisance tripping of GF relay on de-energized source and de-sensitizing energized source GF relay) exists

Added complexity and reduced reliability from an external switch controlled by levers and interlocks connecting to main switch

Added complexity to add GF relay switching as shown in Figure 5 to prevent nuisance tripping of de-energized source.

3-Pole Switching with Special GF Sensing Scheme

Less expensive than 4-pole or 3-pole with overlapping neutral

More complex wiring as de-energized sources have their trip circuits de-energized and their CT circuits paralleled with the CTs of active sources [11]

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Transients

• What about the reported transients?

• What does the our model say?

dt

diLVL

• Equation of transient says it will be proportional to the current flowing in the neutral at the time of the interruption

• How much current flows in the neutral?

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Neutral Current

• Balanced Load

• Unbalanced Load

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Test 3: SD 4-Pole Balanced Load

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Test 3: SD 4-Pole Balanced Load

• Very low or no current flows in neutral

• Modeled transient less than a 0.1 volt

0.1 V peak

0.1 V peak

Voltage across Source 2 neutral

Voltage across Source 1 neutral

32 32

Test 3: SD 4-Pole Unbalanced Load• Worst case is full phase current

• Modeled transient less than phase voltage

• Peak phase voltage: 277 Vrms*1.414 = 391 Vpeak

< 300 V peak Voltage across Source 2 neutral

< 300 V peak Voltage across Source 1 neutral

33 33

Unbalanced Load

• Need to confirm that GF current correctly isolates ground currents to only active sources• Notice that regardless of which source is switched,

only the active source detects GF current

Source 2 detected GF current

Source 1 detected GF current

Total GF current flowing

100 ms open transition

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4-Pole “Non-Issues”

1. Worst case unbalanced load switching means neutral must switch load current• But it is a fully rated pole

2. Worst case, it must switch peak phase voltage• But it is a fully rated pole

3. Absolute worst case is that it might need to interrupt fault current• But it is already a fully rated, fault duty interrupting

pole

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Test 4: SD 3-pole OL neutral (Page 25-26)

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Test 4: SD 3-pole OL neutral (Page 25-26)

• Any transient from the switching only occurs if there is current flowing through the de-energized source.• That can occur during a GF

• Also, the problem is that a 3-pole OL switch is essentially a 3-pole switch during the switching time, so it suffers from the same problems of GF discrimination

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Test 4: SD 3-pole OL neutral (Page 25-26)• Here a GF occurs during transition, but fault current is

divided between sources• Reduces current to relay that supposed to see it

• Nuisance tripping of relay that isn’t supposed to see it

Actual GF current increases higher than with 4-pole switching!

~ 1/2 peak GF current at source feeding fault (Source 2)

Balance of GF current flowing through overlapping neutral

38 38

Summary

• 4-Pole accurately measures GF currents

• 3-Pole with OL neutral does not accurately measure GF currents

• 4-Pole does not generate voltages that exceed normal phase voltage

• 3-Pole with OL neutral could create a transient during a GF

• 4-Pole does not increase magnitude of GF

• 3-Pole with OL neutral can increase magnitude of GF

39 39

References

• See page 28 of paper

• LT Spice is available free of charge from http://www.linear.com/ltspice

• SPICE models used in this paper as well as this PowerPoint can be downloaded from http://pps2.com/files/xfer/spice