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

• 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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1.2 p.u.

for 3 ms !!

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

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A

N

B C

1 p.u. 1 p.u.

1.73 p.u. Phase A faulted

to ground

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

• Generators conventionally assumed to be voltage sources behind impedance

• Loads and shunt capacitance usually ignored

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x

x

x

x

x

x

Z1

Z2

Z0

V1

V2

V0

I0

Sym. component

network for

single-phase fault

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

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