Neutral Switching of Grounded Sources
Dave Loucks, PE
© 2010 Eaton Corporation. All rights reserved.
Switching Methods
• 3-Pole
• 3-Pole with Overlapping Neutral
• 4-Pole
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2
Issues
1. Ground Discrimination
2. Transient Voltages
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3
Ground Discrimination
• Identify source(s) feeding ground fault and
clear
• Two scenarios:
• Single Ground: Non-Separately Derived
• Multiple Ground: Separately Derived
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4
Single Ground: Non-Separately Derived
CT 1output = IF
CT 2output = 0
Source 1
IF
Source 2
IF
IF
IF
IF
• Issue
• Ungrounded sources cannot be detected as
sources of ground current using CT at source.
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5
A Solution: Non-Separately Derived
Reliable Tripping Power
S1A
S1T
GF
Relay
S2A
Source 1
IF
S2T
Source 2
IF
IF
IF
IF
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Separately Derived System: 3-Pole
CT 1output = IF - 1/2IF = 1/2IF
CT 2output = 1/2IF
Source 1
Source 2
1/2 IF
1/2 IF
IF
• Issue
1/2 IF
IF
1/2 IF
1/2 IF
• Multiple grounded sources with unswitched neutral
share ground current between each source
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A Solution: Separately Derived
CT 1output = IF
CT 2output = 0
Source 1
Source 2
IF
IF
• Solution
• Switch neutral to break “cheat” path for ground
current.
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Separately Derived – Overlapping Neutral
CT 1output = IF - 1/2IF = 1/2IF
CT 2output = 1/2IF
Source 1
1/2 IF
Source 2
1/2 IF
IF
1/2 IF
IF
1/2 IF
1/2 IF
• 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
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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.
VL
+
i
-
• We can calculated the magnitude of voltage of this
transient from:
di
VL  L
dt
VL
Transient Overvoltage developed across inductance
L
Inductance (Henries)
di
Differential (change) current (amperes)
dt
Differential time (seconds)
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10
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.
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11
Lab Test versus SPICE Model
Lab Test:
Equiv. SPICE Model
• 600 VL-L
• 848.428 VL-L peak
• 10 kA rms available
• Z = 0.06 
• 60 Hz
• 60 Hz
• 49.1% PF
• X/R = 1.77
• R = 0.0295137 
• For test, both systems are
ungrounded
• XL= 0.0522393 
•  @ 60 Hz
L = 0.1385 mH
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Problem: We Still Have an Unknown
VL  L
di
dt
• 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
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Progression of Arcing Contact
Measured: Time = 0
Time  1.1 ms
Time  1.2 ms
Time  5.5 ms
Total Arc Extinguishing Time  8.2 ms
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14
LT-SPICE Model
Note: “ungrounded” means capacitively grounded
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15
Actual Lab Test
Note: 530V / 848V = 63%
~ 530 V
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Model Matches Very Closely
Note: 801V / 848V = 94%
Fault
current
~ 801 V
~ 530 V
Current zero,
arc
extinguishes
Arcing
voltage
System
voltage
• In fact, our model is “conservative” since
model predicts higher transient than lab test
• Raises confidence that transients will be less
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17
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
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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
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20
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
Reliable Tripping Power
S1A
GF
Relay
S1T
S2A
S2T
Source 1
Source 2
IF
IF
IF
IF
IF
• 3-Pole switching means no neutral switching
•  no neutral transient
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21
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
0.001847
0.001847
IF
0.5 
24.01 H
24.01 H
0.001847
0.001847
24.01 H
24.01 H
IF
0.5 
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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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24
Test 2: SD 3-pole switching (Page 21-22)
A, B and C phase current = 0
Neutral current = 733 A peak
(533 A rms)
• GF current is detected as flowing through deenergized source!
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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
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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 desensitizing 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 deenergized 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 deenergized and their CT circuits
paralleled with the CTs of active
sources [11]
Transients
• What about the reported transients?
• What does the our model say?
VL  L
di
dt
• 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
Voltage across Source 2 neutral
0.1 V peak
Voltage across Source 1 neutral
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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
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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
100 ms open transition
Source 1 detected GF current
Total GF current flowing
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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 deenergized 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
~ 1/2 peak GF current at source feeding fault (Source 2)
Balance of GF current flowing through overlapping neutral
Actual GF current increases higher than with 4-pole switching!
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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
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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
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