New complexities in effective DG grounding Larry Conrad January - - PowerPoint PPT Presentation

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New complexities in effective DG grounding

Larry Conrad January 15, 2015 Conrad Technical Services LLC larry.conrad@conradtechnicalservices.com +1.317.431.1866

Background

• Utility serving major Midwest city
• Combination of a renewable tariff and the

American Recovery and Investment act flooded the utility with solar requests

• Regulatory rules encouraged DR to be

effectively grounded

• Transition from zero to about 100 MW solar in

about 2 years

– 20 kW to 10 MW sites – All on 13 kV distribution feeders

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Two types of effective grounding

• EG – Effectively grounded (preferred 1547.8)

– Just enough to meet the criteria – Power supply transformer ungrounded – Small grounded wye – delta transformer – Zig – zag transformer – Impedance in the transformer neutral connection

• SEG – Strong effective grounding

– Major source of ground fault current – Grounded wye to delta power supply

• Regardless, developers were not familiar with the effective

grounding concept.

• Nearly all utilities have avoided additional ground sources for a

very long time

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Inside a building and feeder reclosing

• Transmission static

fell into double circuit distribution

• Two phases open on
• ne circuit
• One phase open on

second circuit with third phase in the clear

• Neutral to solar

inverter overheated

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Currents for nearby fault

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13.2 kV GY 480 V GY Solar farm effectively grounded Higher current in neutral as expected but different from what building designers planned for. Two cases where designers did not anticipate 3X 150 A fuse

Fuse coordination with 150 A

Slow clearing but no cable damage for a single shot

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Currents for remote fault

• 2-3 seconds

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Solar farm effectively grounded Remote fault Circuit may provide as many as four shots of current as circuit goes to lockout

Consider reclosing

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• Fast or slow trip
• Fast reclose and trip
• Reclose in 20 seconds and trip
• Reclose in 20 seconds and trip
• Full size neutral survives 2, maybe 3 current shots but

not 4.

• Neutral must be oversized or go ungrounded

Phase current Neutral current I^2*t to lockout I^2*t damage threshold Pass/Fail Capacity Pass/Fail 875 2,626 82,750,512 60,000,000 Fail 240,000,000 Pass 1,136 3,407 74,288,954 60,000,000 Fail 240,000,000 Pass 1/0 Phase & Neutral 4/0 Oversize Neutral

Other observations from event

• There was a period of time where only one

phase was energized. Grounding transformer tried to supply reduced voltage to other phases

• Had it been on the other circuit, it would have

tried to carry the third phase

• A line open could occur anywhere upstream

leaving just the “right” amount of load for the grounding transformer to “carry” the third phase

• Grounding transformer presents a separate

public safety issue from the inverter.

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Mutual coupling

• Urban systems often have many double circuit

MV distribution lines

• Mutual coupling seldom considered or modeled
• n distribution systems.
• Mutual coupling may be a required part of

distribution fault evaluation in the presence of additional grounding sources.

• Multiple sources and low impedance sources

exacerbate the situation

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Fast Trip Neg Distance Trip Trip No Slow Trip (Fast disabled)

Effective grounding causes ground current flowing in reverse direction at the substation breaker, but model

says it should not trip. Model indicates 251 amperes. Recorder indicated 930 amperes

Should not trip – but it did

Mutual coupling makes solar closer

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Substation Solar farm effective ground About 1.5 miles

• f common R/W

And/or double circuit Approximate location

• f fault on Feeder 2

IØ from sub and solar IØ from solar Double circuits are very common in urban distribution systems and close to larger substations.

251 Amp without mutual coupling 975 Amp with mutual coupling – trip was correct

(Event recorder reported 930 Amps) As more significant sources of ground current are added to distribution systems, distribution models will need to include mutual coupling

Now add mutual coupling

Try ungrounded to control fault current

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• 25
• 20
• 15
• 10
• 5

5 10 15 20 25

• 25,000
• 20,000
• 15,000
• 10,000
• 5,000

5,000 10,000 15,000 20,000 25,000

Amperes Volts

CH 1 Pct Cur CH 1

Arrester rated low current switching surge discharge voltage =~21 kV

Two 10 kV Arrester Operations

Load rejection 3 MW Site, 200 kW output

Discussion

Thank you larry.conrad@conradtechnicalservices.com +1.317.431.1866

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