Wind Session
Effective Grounding of Wind Farm Collector Circuits
By
B G
Ben
Guth,
th P.E.
PE
Doug Jones, P.E.
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What is transient overvoltage (TOV)?
What is the cause of TOV?
O
How is TOV mitigated?
Do all types of WTGs have problems with TOV?
A dd
And
do all
ll mitigation
ii
i
techniques
h i
work
k ffor all
ll types??
How is TOV mitigating equipment specified?
`
g an economical g
g system
y
Must design
grounding
that:
◦ controls TOV to acceptable levels
◦ limits fault current while enabling secure ground fault
p
protection
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`
`
Transient Overvoltages (TOV) can lead to surge
arrester and other equipment failure
G
Grounding
di
ttransformers
f
or high
hi h speed
d grounding
di
switches usually provide adequate mitigation
Different wind turbine types
yp can mean different
grounding methods
Transformer
69/92/115
MVA
230 kV
34.5 kV
34.5 kV Main Bus
52CAP
DVAR
52F1
Collector
1
52F2
52F3
Collector Collector
2
3
52F4
Collector
4
`
Typically 34.5kV
`
Approximately 30MVA per circuit
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Underground cabling for circuits tend to be highly
capacitive
MOV type
yp surge
g arrestors installed at each end of the
cable system
Pad mount or nacelle mounted generator step up
ttransformers
f
(GSU)
◦ (35kV) solidly GndY primary – (1kV-480V) solidly GndY
secondary
(1kV-480V)
480V) solidly GndY secondary
◦ (35kV) Delta primary – (1kV
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`
`
`
yp 1 – squirrel-cage
q
g induction g
Type
generators
Type 2 – wound rotor induction generator with
controlled
t ll d rotor
t resistance
i t
Type 3 – doubly fed generator commonly called a
doubly-fed induction generator
Type 4 – generator interfaced to the grid totally
through a variable speed power electronic drive
y
system.
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T
i l ffault
l contribution
ib i
Typical
range off 4 to 9 per
unit
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Typical fault contribution range of 4 to 9 per
unit
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`
Typical fault contribution less than 2 per unit
for remote faults before crowbar
Aft crowbar
After
b engages it resembles
bl a Type
T
2
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`
Typical
T
i l ffault
l contribution
ib i
iis very llow and
d
dependant upon pre-fault loading
Usually Type 3 and 4 WTG short circuit
current contributions don’t matter that much
and can be complicated to determine exactly
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Ground fault
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Inadvertent opening of circuit during generation
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Capacitor
p
bank switching
g
`
Collection cable capacitance
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Ground reference primarily derived by Substation
transformer
f
Circuit disconnects from Substation during a
ground fault
Overvoltage of non-fault phases depends on
grounding/generator type
A new ground reference must be derived from
circuit to be “effectively grounded”
`
Very small “Neutral to Ground Impedance”
100
[kV]
75
50
25
0
-25
-50
-75
-100
0 00
0.00
0 05
0.05
(file Cable.pl4; x-var t) v:G5NCA -X0004A
0 10
0.10
v:G5NCB -X0004B
0 15
0.15
v:G5NCC -X0004C
0 20
0.20
0 25
0.25
0 30
0.30
[s]
0 35
0.35
`
Neutral potential & ground potential are coupled
only
l by
b the
h line-to-ground
li
d capacitances:
i
A
A-Gnd Fault on
Ungrounded System
Gnd = 0 V
VL-G = 1 p.u.
A
N
C
VL-G ≈ VL-L
= 1.73
1 73 p.u.
B
Un-faulted System
N
C
B
100
[kV]
75
50
25
0
-25
-50
-75
-100
0 00
0.00
0 05
0.05
(file Cable.pl4; x-var t) v:NGNCA -X0070A
0 10
0.10
v:NGNCB -X0070B
0 15
0.15
v:NGNCC -X0070C
0 20
0.20
0 25
0.25
0 30
0.30
[s]
0 35
0.35
Definition of “Effectively Grounded”
` Effectively grounded when the “coefficient of
grounding” (COG) is less than 80%
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COG = VLG/VLL
`
Effectively grounded: VLG < 80% of VLL
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If nominal
i l VLL iis 34
5 kV
f lt d phases
h
34.5
kV, th
the un-faulted
must be VLG < 27.6 kV
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`
An effectively grounded system can be
approximated
i
d when
h system:
◦ X0 & X1 are both are inductive
◦ X0/X1 ≤ 3
◦ R0/X1 < 1
To obtain effective ground either reduce R0 & X0, or
increase X1
Increase X1 = Increase Load Losses
`
Install high speed ground switch
◦ Requires very fast
switching,
< 1 cycle
f
h
l
◦ Mechanical interlocks do the job well
x Currently only one manufacturer
◦ Electric interlocks have not proven reliable, fast
enough
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Install grounding transformer
◦ Specifications – needs study
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Wye-Delta or Zig-Zag
◦ Equally effective for limiting TOV
◦ Zig-Zag will be physically smaller but are specialty
built transformers available from a limited number
of manufacturers with generally long lead times and
high pricing due to one-off design
◦ Wye-Delta available – pad-mount type are readily
available from multiple manufacturers
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Primary Voltage
Rated KVA (if needed)
Continuous Neutral Current – Continuous “nontripping” current due to unbalance of the system.
tripping
system
Neutral Fault current and duration
Impedance
Primary winding connection
Secondary connection
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Ohms per phase
◦ Requires study/calculation
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Rated time
◦ 10 second, 1 minute, 10 minute, or extended (less then 90
days/year)
◦ Generally a 10 second rating for a WPP with an “effectively
grounded system” due to fast and sensitive protection
equ e e ts
requirements
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Continuous rated current
◦ Based on rated time (IEEE 32)
◦ 10 second rating = 3% continuous current
◦ Example: 3500 amps for 10 sec = 3500 * 0.03 =105 amps
continuous
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Model the system
Choose
some typical
(2000kVA,
+/-7.5%,
Ch
t i l parameters
t
(2000kVA 5.75%
5 75% +/
7 5%
use 6.2%)
Simulate ground fault
D
Determine
i VLG-unfaulted < 0.8
0 8 * VLL (less
(l
than
h 27.6
27 6 kV)
Modify transformer parameters as needed, repeat
Once you’ve reached desired performance, calculate:
◦ ohms per phase
◦ Maximum fault current from grounding transformer
One size does not fit all – must consider the system
y
Check with transformer manufacturer to ensure
buildability/availability
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It’s a “good idea” to limit TOV due to ground faults,
and
transients
due
d reduce
d
i
d to WTG,
WTG resonance, etc.
Debate if the COG method is valid for a
asynchronous system
Must verify surge arresters, cables, and other
equipment will withstand the TOV during ground
fault for expected fault duration
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For Type 1 & 2 WTGs grounding transformers and
fast ground switch breakers are both effective in
mitigating TOV
TOV results whether the step-up transformers are
connected Δ/Yg or Yg/Yg
◦ Generators are ungrounded, therefore do not provide zero
sequence fault current
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Ensure the generator will trip off quickly if the
substation breaker opens inadvertently
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`
`
g transformers and g
Both g
grounding
ground switch
breakers can be effective in mitigating TOV
Ground switch breaker may
y be the p
preferred option
p
as it forces the crowbar to engage and doesn’t
require a full transient analysis
Transient analysis may be needed
Transient analysis adds design time, costs and
complexity
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Type 4 machines should have low fault current
contributions
to a SLG fault
ib i
f l
Verify fault current contribution from WTG
manufacturer
For SLG faults on the system the Type 4 power
electronic controller switches out very quickly
thereby not sourcing voltage to a SLG fault. As a
result no significant energy is provided that would
create TOV
Questions
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Chong Han, Don E. Martin, Member, IEEE, and Modesto R. lezama, “Transient Over-Voltage (TOV) and
Its Suppression For a Large Farm interconnection”.
Eaton Application White Paper, “Transient Over voltages on Ungrounded Systems from Intermittent
Ground Faults.
Pacific Crest Transformers Article, “Grounding Transformers” April, 02 2009.
Steven W
St
W. S
Saylors,
l
P
P.E.,
E ”L
”Large Wi
Wind
d Pl
Plantt C
Collector
ll t D
Design
i
”Wi
”Wind
dF
Farm C
Collector
ll t S
System
t
G
Grounding,
di
IEEE
PES Transmission and Distribution Conference 2008.
Mike Reichard, “Fault Current Contributions From Variable Speed (Type 3 and 4) Wind Turbine
Generators” Texas A&M Protective Relaying Conference Fault Current Contributions from Wind Plants
April
p 1,, 2009.
ANSI/IEEE Std 32-1972 IEEE Standard Requirements, Terminology, and Test Procedures for Neutral
Grounding Devices.
Reigh A. Walling, Michael L. Reichard, “Short Circuit Behavior of Wind Turbine Generators”
EMA VDH/GSMI brochure (Combined 34.5 kV Vacuum Circuit Breaker & Mechanically Interlocked),
available online at: http://www.ema-sa.com.ar
IEEE Standard C62.22-1997, Guide for the Application of Metal-Oxide Surge Arresters for AlternatingCurrent Systems.