1
Grounding and Overvoltage
Requirements for
Distributed Generation
(Wind and Solar)
Reigh Walling
Walling Energy Systems Consulting, LLC
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Scope of Presentation
We will cover:
– Temporary and transient overvoltages on
distribution feeders
– Related to interconnection of distributed wind and
solar plants
– Ways to avoid or mitigate these overvoltages
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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 many cycles to
seconds
– Examples: load rejection, single-phase faults,
ferroresonance, etc.
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Impacts of Temporary Overvoltage
• Utility equipment – surge arresters
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Surge Arresters
• Surge arresters are designed to limit transient
overvoltages, not intended to limit TOV
• Surge arresters are the likely victims of TOV
• Failure modes of surge arresters
– MOV material fails to a low resistance
– Physical integrity of arrester housing uncertain
– Ground lead disconnects blow off
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Impacts of TOV on Customers
• TOV withstands of
consumer equipment
are poorly documented
• Not defined by any
formal standards
1.2 p.u.
for 3 ms !!
• ITIC (formerly CBEMA)
curve is often cited
• Experience suggests
ITIC curve is excessively
conservative
• TOV can result in large
claims against utility
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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
• Multi-grounded feeders are designed to be
effectively grounded
– Coordinates with arrester TOV capabilities
– Experience indicates customers are ok
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Ground Sources – Desired and Unintended
Ground sources are the zero-sequence
admittances of the circuit
• Primary substation transformer – breaker open
• Cable charging and grounded cap banks
• Grounded-wye loads
• Grounded-wye delta and zig-zag transformers
– May be added to feeder to mitigate loss-of-ground
– Subject to overload from feeder imbalance
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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
• Generators conventionally assumed to be
voltage sources behind impedance
• Loads and shunt capacitance usually ignored
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Wind and Solar Sources
• Solar (PV) inverters – grid interactive
– Controlled current sources, not voltage source
– Three-phase inverters are ungrounded source
– Three-phase inverters may not pass I2
• Wind generation
– Legacy induction generators (Types 1 and 2) approximately
a voltage source until flux collapses
– Full conversion (Type 4) are inverters just like PV
– Doubly-fed (Type 3) are current sources until they crowbar,
then they are like an induction generator
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Conventional X0/X1 only applies to:
• Solar (PV) inverters – grid interactive
– Controlled current sources, not voltage source
– Three-phase inverters are ungrounded source
– Three-phase inverters may not pass I2
• Wind generation
– Legacy induction generators (Types 1 and 2) approximately
a voltage source until flux collapses
– Full conversion (Type 4) are inverters just like PV
– Doubly-fed (Type 3) are current sources until they crowbar,
then they are like an induction generator
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Loading Impact on Type 1 and
Type 3(crowbarred) WTG TOV
• These are the only wind and solar DG that can be
assumed to behave as a voltage source
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Inverter-Interfaced DG
• The following behave as current sources:
– PV
– Type 4 wind turbines
– Type 3 (wind turbines when not crowbarred)
• Conventional X0/X1 criteria do not apply
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Other DG Overvoltage Issues
• Abrupt isolation of inverter into light load
– Known as “load rejection overvoltage”
– Tests and field events have shown voltage > 2 p.u.
– Inverter should trip immediately
• Reclose out of phase of a compensated feeder
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Other DG Overvoltage Issues
• Self-excitation of induction generator
– Light load
– Sufficient capacitive compensation
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Overvoltage Mitigation
• Provision of ground source where needed
– Grounded-wye delta or zig-zag grounding banks
– Grounded-wye delta interconnection transformer
– Grounded wye-wye does not make a ground source
• Adequate load relative to DG
– Load to DG capacity ratio depends on type
• Coordinated transfer trip
– DG disconnected before feeder opens
• Coordinated grounding switch (crowbar)
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Overvoltage Mitigation (cont’d)
• Fast DG overvoltage tripping
– DG may not see TOV on primary side
– Coordination with VRT will be needed in future
• Fast islanding detection
– Increased challenge with evolving ride-through
requirements
• Sacrificial arresters
– Difficult to coordinate with utility arresters
– Coordination with load withstand is uncertain
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Conclusions
• Overvoltages, particularly TOV, can be a major
issue for wind and solar plants interconnected
to distribution
• Conventional X0/X1 criteria are irrelevant for
most wind and solar
• Overvoltage and grounding solutions exist, but
must be appropriately analyzed and applied