1
TOV Issues in Renewable Plant
Collectors and Application of
Grounding Transformers
Reigh Walling
Walling Energy Systems Consulting, LLC
Overvoltage Definitions
Transient overvoltage impulses and
supersynchronous oscillations lasting less than
a couple of cycles
Examples: lightning, switching, etc.
Temporary overvoltage oscillatory
overvoltage persisting for several cycles to
seconds
Examples: load rejection, single-phase faults,
ferroresonance, etc.
Impacts of Temporary Overvoltage
Primary impact is on surge arresters
MV arresters are inexpensive
Arrester failures are not!
Surge Arresters
Should be applied at open ends of collector
feeders for impulse and switching surge
protection
Surge arresters are designed to limit transient
overvoltages, not intended to limit TOV
Surge arresters are the likely victims of TOV
Grounding
Two different uses of the term grounding in the
power industry
Equipment or safety grounding
Bonding to provide a low impedance path between casing
or other non-energized parts and local ground
System grounding provides a reference point for the
voltages on the three electrical phases with respect
C
B
to ground
I.e., provide a defined zero-sequence
admittance
Defines severity of unfaulted-phase
voltage rise during ground faults
N
1
p.u.
1
p.u.
A
1.73
p.u.
Loss of Ground Scenario
Focus of utility concern regarding DG plants
Ground fault on feeder
Feeder breaker trips; losing normal ground source
DG doesnt trip immediately, continues to energize
If there is no ground source, high TOV may result
C
B
N
1 p.u.
1 p.u.
Phase A faulted
to ground
1.73 p.u.
A
Effectively Grounded Systems
Definition of effectively grounded system is
where the COG < 0.8 (COG = TOV/VL-L)
TOV < 1.39 p.u. in effectively grounded systems
C62.91 states X0/X1<3 and R0/X1<1 generally
results in COG<0.8
but is not the definition of effective grounding
Arresters required to protect 150 kV BIL
equipment on 34.5 kV feeders generally
require effective grounding
Loss of Grounding Scenario
WTGs are not typically grounded,
Y-Y unit transformer does NOT
provide a ground source!
Substation transformer is
normal ground source
Ground fault on
collector cable
WTGs do not see fault
after collector breaker
trips, may run-on
Breaker trip, isolates
collector feeder
Feeder Isolation TOV
Magnitude and character of TOV is highly dependent
on the details of the generators
Type 1 and 2 WTGs can be reasonably approximated
as voltage sources behind impedance
Classical symmetrical component analysis applies
Type 4 WTG and PV inverters behave more like
current sources
Classical analysis and X0/X1 criteria do not apply
Type 3 act like Type 1 & 2 if crowbarred,
like Type 4 if not
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Classic TOV Analysis
Sym. component
network for
single-phase fault
x
Z1
V1
x
I0
x
Z2
V2
x
x
Z0
V0
Unfaulted phase-to-ground voltage
(p.u.)
3
2.8
2.6
2.4
2.2
2
1.8
1.6
1.4
1.2
1
0
5,000
10,000
15,000
20,000
Feet of 500 kcmil 34.5 kV cable per MVA of WTG
Generators conventionally assumed to be
voltage sources behind impedance
Feeder cable capacitance can increase TOV > 3
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Limitations of Conventional Analysis
Voltage source representation only reasonable for
Types 1 and 2 WTGs
Inverters (Type 4 WTG and PV) designed to behave
like positive-sequence current sources
Negative sequence may appear as a large impedance or as
nonlinear impedance
Will not be able to drive ordered current into the high
capacitive impedance of feeder
Modulation index is liked to be maxed out
Unstable control behavior may occur, generating complex
non-sinusoidal waveforms
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Type 3 (DFG) WTG Behavior
Normal operating mode is similar to inverter
Constant power regulation
If solidly crowbarred (rotor protectively
shorted out), will behave like Type 1 & 2
Initial feeder fault is likely to initiate crowbar
Controls designed for fault ride-through may
attempt to recover from crowbar
Complex behavior may result
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Analysis and Modeling of TOV and
Grounding for Inverters and Type 3 WTG
No analytical approach is even close
EMT simulation is the only means to obtain adequate
accuracy
Detailed OEM-supplied model is essential:
Outer loop controls (i.e., power, ac voltage regulation)
Inner loop current regulators
Protective sequences
Protective tripping
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TOV Mitigation
Grounding transformers
Grounding breakers
Direct (fast) transfer trip
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Grounding Transformers
GT
GT
GT
Grounding transformer on each collector feeder
Typically at the substation end
Zig-zag or grounded-wye delta configuration
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Grounding Transformer Application
Zig-zag or Yg-delta no functional difference
Zig-zag theoretically uses less copper and iron
Grounded-wye delta can be a commodity distribution xfmr
Impedance chosen to achieve acceptable TOV
Based on simulations, not calculations!
Current ratings
Continuous very little in a wind or solar plant
Temporary
Ground fault with feeder breaker closed
Sustained energization from WTGs/inverters after breaker opens
Impact on feeder protection
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Conclusions
TOV mitigation is a critical element of wind
and solar plant collector system design
Grounding transformers are a popular solution
to wind plant TOV issues
And what about large-scale PV plants?
Detailed simulation is the only suitable
approach to selection of grounding
transformers for modern renewable plants
