© 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.

6 6 A Solution: Non-Separately Derived

7 7 Separately Derived System: 3-Pole Issue Multiple grounded sources with unswitched neutral share ground current between each source

8 8 A Solution: Separately Derived 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 1Source 2 CT 1 output = I F - 1/2I F = 1/2I F IFIF 1/2 I F IFIF CT 2 output = 1/2I F

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: VLVL Transient Overvoltage developed across inductance LInductance (Henries) diDifferential (change) current (amperes) dtDifferential time (seconds) VLVL i +-

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 Lab Test versus SPICE Model Lab Test: 600 V L-L 10 kA rms available 60 Hz 49.1% PF Equiv. SPICE Model V L-L peak Z = 0.06  60 Hz X/R = 1.77 R =  X L =  60 Hz L = mH For test, both systems are ungrounded

13 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 V L

14 Progression of Arcing Contact Time = 0 Time  1.1 msTime  1.2 msTime  5.5 ms Total Arc Extinguishing Time  8.2 ms Measured:

15 LT-SPICE Model Note: “ungrounded” means capacitively grounded

16 Actual Lab Test ~ 530 V Note: 530V / 848V = 63%

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 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 (I FL = 1804 A rms, I SC = A rms Source 2 – Utility Ratings same as Source 1 Load 600 A rms, 80% PF (X/R = 0.75)

19 Schematic Model Refer to Figure 21, page 20 and pages for calculation of these values

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 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

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

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

24 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 …

25 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 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

MethodAdvantagesDisadvantages 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]

28 Transients What about the reported transients? What does the our model say? 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?

29 Neutral Current Balanced Load Unbalanced Load

30 Test 3: SD 4-Pole Balanced Load

31 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 Voltage across Source 2 neutral Voltage across Source 1 neutral

32 Test 3: SD 4-Pole Unbalanced Load Worst case is full phase current Modeled transient less than phase voltage Peak phase voltage: 277 V rms *1.414 = 391 V peak < 300 V peakVoltage across Source 2 neutral < 300 V peakVoltage across Source 1 neutral

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

34 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

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

36 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

37 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 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 References See page 28 of paper LT Spice is available free of charge from SPICE models used in this paper as well as this PowerPoint can be downloaded from