This presentation provides an insight into the various considerations about a real case study in which large circulating currents were occurring in a generator neutral earthing resistor, causing significant overheating.

1. MV Generator
earthing in large
cable networks
Jason Mayer
Aurecon Australasia
EESS November 2019

2. 2
 Private commercial and industrial medium voltage
networks commonly employ centralised, direct
connected medium voltage standby diesel
generators
Topic background

3. 3
 Can supply standby power to an entire MV network
using the existing network infrastructure
 Using a MV generator eliminates step up transformer
 No transformer oil containment issues to deal with
 Eliminates large LV cables
 More compact
 Typically less expensive – maybe
 Less maintenance than multiple de-centralised LV
generator systems
Why direct connected?

4. 4
 Neutral earthing is not trivial for direct connected
MV generators
‒ These generators need to be earthed when operating in
islanded mode to provide a system reference
‒ Conversely they should be unearthed when paralleled
with the grid to prevent interference with utility earth fault
protection
Generator neutral earthing
Utility
Transformer Generator
Customer
Network
Utility
Network
other
feeders

5. 5
 Typically generator X0 is lower than X”d so you
need some sort of earthing impedance to limit
generator earth fault current to no more than three
phase fault current – (winding bracing strength)
 Earth fault current limitation may also be required
for step/touch voltage safety reasons
 Typically done with a neutral earthing resistor
Generator neutral earthing

6. 6
 Neutral earthing reactor could be used but certain
rules must be followed
‒ Earth fault current must be at least 25% (preferably 60%)
of three phase fault current to avoid serious transient
overvoltage issues
‒ X0 ≤ 10X1
Generator neutral earthing

7. 7
Typical generator impedances

8. 8
Typical MV network – normal operation
 Intake substation
 11 kV feeders
 Distribution transformers with
400/230V loads
 Standby diesel generators normally
off-line

9. 9
Typical network – loss of grid
 Grid incoming feeders tripped
 Diesel generators on-line
 11 kV system earth provided by
earthing one generator (typically
via neutral earthing resistor)

10. 10
Simplified arrangement
 Generator
supplying MV
distribution network
in islanded mode
Site MV cabling and
substations
Generator
NER
MV Switchgear

11. 11
 Generators do not produce pure
sinusoidal waveforms
 Two common generator types
‒ 5/6 winding pitch
‒ 2/3 winding pitch
 Both generate voltage harmonics
on their output waveform
 5/6 generates lower 5th and 7th but
medium 3rd
 2/3 generates no 3rd but 5th and 7th
Generator harmonics
5/6
2/3

12. 12
Which choice?
 2/3rd Pitch
‒ Non-existent 3rd
harmonic
‒ Higher cost
‒ Larger machine
‒ Low Z0 (higher earth fault current)
 5/6th Pitch
‒ Increased power density
‒ Lower cost
‒ Lower weight
‒ Higher efficiency
‒ Lower 5th and 7th harmonic
‒ Higher 3rd harmonic
‒ Higher Z0 (lower earth fault current)

13. 13
Testing conducted – harmonic voltages
0.00%
2.00%
4.00%
6.00%
8.00%
10.00%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Harmonic Voltages on grid supply
Harmonic Voltage (% of fundamental)
0.00%
2.00%
4.00%
6.00%
8.00%
10.00%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Harmonic voltages on generator
Harmonic Voltage (% of fundamental)
 High levels of 3rd harmonic from 5/6th wound
generators

14. 14
Circulating currents in the generator neutral….
 In some cases you can see significant circulating
current in the generator NER
‒ You need triplen harmonic voltage source – only 3rd and
multiples of 3rd can circulate in the neutral
‒ Need a zero sequence path from the network back to
the generator neutral

15. 15
How can this happen?
 NER circulating current
‒ has to be zero sequence/triplen current
‒ need a triplen harmonic voltage source
 Generators
‒ direct connected at MV
‒ 5/6th winding pitch produces triplen harmonics
 MV cable
‒ if you have long lengths of it you have a capacitive
path to earth

16. 16
 There is a relatively medium
capacitance from conductor to screen in
underground cable
 The screens are connected to the site
earthing system which connects to the
NER earth point
 Lots of cable gives quite low zero
sequence impedance to earth - Xc0
Where is our zero sequence path?

17. 17
Our harmonic current circuit
3rd
Harmonic
Sources
RgjXc
jXc
jXc
-jXc
jXg(Xo)
3xRg
Io
V3
10% of
Vph-e
−𝑗𝑋𝑐 =
1
2𝜋𝑓𝐶

18. 18
Worked example
Parameter Value
Generator Rating 11 kV 2.87 MVA
Generator Winding 5/6th pitch (therefore 10% 3rd harmonic at full load)
Generator X0 9.1% on 2.87 MVA
Cable Size and Length 11 kV 240sqmm XLPE, 30 km, 0.458 uF/km
NER 31 ohms

19. 19
Worked example
-jXc
jXg(Xo)
3xRg
Io
V3
10% or
635V

20. 20
Worked example
𝑋0 =
9.1
100
×
112
2.87
= 3.84𝑖 𝑜ℎ𝑚𝑠 𝑎𝑡 50 𝐻𝑍
𝑋0 = 3.84𝑖 × 3 = 11.5𝑖 𝑜ℎ𝑚𝑠 𝑎𝑡 150 𝐻𝑍
𝑅 𝑔0 = 31 × 3 = 93 𝑜ℎ𝑚𝑠
𝑋𝑐0 =
1
2 × 𝜋 × 150 × 0.458 × 10−6 × 30
= −77.2𝑖 𝑜ℎ𝑚𝑠
𝐼0 =
10% × 6350
93 + 11.5𝑖 − 77.2𝑖
= 5.57/35.2° 𝑎𝑚𝑝𝑠
𝐼 𝑎 = 3 × 𝐼0 = 16.7 𝑎𝑚𝑝𝑠
𝑃𝑜𝑤𝑒𝑟 𝑁𝐸𝑅 = 𝐼2
× 𝑅 = 16.72
× 31 = 8.6 𝑘𝑊

21. 21
On-site validation testing
 Aurecon and Safearth confident of the issue
 Safearth engaged to perform validation testing
of theory
 Using proprietary broad frequency current
injection equipment and calibrated meters
testing was carried out

22. 22
 Off power frequency
and off harmonic
frequency testing
performed
 Meters and source
needed to be
reprogrammed to
frequency in the range
of 150Hz

23. 23
 Measured capacitive
impedance
correlated well with
modelled estimates
 The harmonic
circulating current of
approximately 18A
was validated

24. 24
Where can this go wrong?
 Generator suppliers tend to recommend 5/6th
winding pitch generators (although there is evidence some of
them are changing)
 However they don’t often understand the context
of the site where they were being installed
 The site designer needs to be the one fitting all
the pieces of the design together and checking
this
 NERs typically don’t have a continuous rating

25. 25
 Circulating current in generator NERs during
islanded operation of the network
 Heating, smoke and paint damage
Getting it wrong

26. 26
So what is the correct solution?
 Use NERs with type tested continuous current
rating and install them outside
 Use earthing transformers and leave generator
neutrals floating
 Using LV generators with step up transformers
 Use 2/3rd pitch machines
 Third harmonic filters
 Use NEXs instead of NERs (less I2R heating??)

27. 27
Option selection
Option Technical discussion
Type tested NER Doesn’t eliminate circulating current but deals with
heat more effectively
Earthing transformer Excellent electrical solution but more civil works and
electrical works
Replace alternators Could increase 5th and 7th harmonics, larger machines
in generator hall – need to do some checks
LV generators with step up
transformers
Isolates zero sequence networks. Excellent solution
but more civil and electrical works
Harmonic Filters Not sure a technical solution can be found if there are
a number of switching configurations
Use NEX Due to flexible switching arrangements could cause
resonance issues

28. 28
Type tested NER solution

29. 29
Type tested NER solution

30. 30
Earthing transformer solution

31. 31
Step-up transformer solution
11 kV

32. 32
NEX solution – not recommended

33. 33
Issues with the NEX option - example
-jXc
jXg(Xo)
3jXg
Io
V3
10% or
635V

34. 34
Lessons for large cable networks with generators
 Consider the winding pitch carefully
 Consider your earth fault limitation and the device best
suited to achieve that (typically NER)
 Make sure you talk to the utility about paralleling
 Could use LV generators with Dyn or YNd step up
transformers to isolate zero sequence networks between
generators and the cable network
 Could use MV direct connected generators but with
earthing transformers on the MV busbar and un-earthed
generator neutrals
 Calculate the circulating current and incorporate into
earthing device design and type test it

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