1
TOV Issues in Renewable Plant
Collectors and Application of
Grounding Transformers
Reigh Walling
Walling Energy Systems Consulting, LLC
2
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.
3
Impacts of Temporary Overvoltage
• Primary impact is on surge arresters
• MV arresters are inexpensive
• Arrester failures are not!
4
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
5
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.
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Loss of Ground Scenario
• Focus of utility concern regarding DG plants
–
–
–
–
Ground fault on feeder
Feeder breaker trips; losing normal ground source
DG doesn’t 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
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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
8
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
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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
x
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
15
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