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

New complexities in effective DG grounding Larry Conrad January 15, 2015 Conrad Technical Services LLC larry.conrad@conradtechnicalservices.com +1.317.431.1866 1

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 2

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 3

Inside a building and feeder reclosing • Transmission static fell into double circuit distribution • Two phases open on one circuit • One phase open on second circuit with third phase in the clear • Neutral to solar inverter overheated 4

Currents for nearby fault 150 A fuse 480 V GY 13.2 kV 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 5

Fuse coordination with 150 A Slow clearing but no cable damage for a single shot 6

Currents for remote fault • 2-3 seconds Solar farm effectively grounded Circuit may provide as many as four shots of current as circuit goes to lockout Remote fault 7

Consider reclosing • 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 1/0 Phase & Neutral 4/0 Oversize Neutral Phase Neutral I^2*t to I^2*t damage current current lockout threshold Pass/Fail Capacity Pass/Fail 875 2,626 82,750,512 60,000,000 240,000,000 Fail Pass 1,136 3,407 74,288,954 60,000,000 240,000,000 Fail Pass 8

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. 9

Mutual coupling • Urban systems often have many double circuit MV distribution lines • Mutual coupling seldom considered or modeled on 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 10

Should not trip – but it did Fast Trip Neg Distance No Slow Trip Effective grounding causes ground (Fast disabled) current flowing in reverse direction at the substation breaker, but model says it should not trip. Model indicates 251 amperes. Recorder indicated 930 amperes Trip Trip

Mutual coupling makes solar closer Substation Double circuits are very common in urban distribution systems About 1.5 miles and close to larger of common R/W substations. And/or double circuit IØ from sub IØ from solar Approximate location and solar of fault on Feeder 2 Solar farm effective ground 12

Now add mutual coupling 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

Try ungrounded to control fault current 25,000 25 Two 10 kV Arrester Operations 20,000 20 Load rejection 3 MW Site, 200 kW output 15,000 15 10,000 10 5,000 5 Amperes Volts 0 0 -5,000 -5 -10,000 -10 -15,000 -15 Arrester rated low current -20,000 -20 switching surge discharge voltage =~21 kV -25,000 -25 14 CH 1 Pct Cur CH 1

Discussion Thank you larry.conrad@conradtechnicalservices.com +1.317.431.1866 15