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Distributed Generation and Power Quality

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Relaying considerations

• DG infeed may reduce the reach of overcurrent relays– DG feeds fault, so utility current is fault

current minus DG contribution

– Sympathetic tripping of feeder breakers

– Defeat fuse saving

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115 kV

12.47 kV

Put recloser here

DISTRIBUTION

Radial Line

DG

Only one DG: obvious solution to several problems

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115 kV

12.47 kV

DG“Sympathetic” tripping of this circuit breaker (not desired) due to backfeed from DG

Fault

Solution is to use directional overcurrent relays at substation

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DG

Since DG feeds the fault (backfeed), it will likely defeat fuse saving

Fault on tap

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DGV

distance

Voltage Regulation

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DGV

distance

Feeder trips and reclosesDG disconnects

Low voltage

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DGV

distance

Feeder trips and reclosesDG disconnects VR steps

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DGV

distance

Feeder trips and reclosesDG disconnects VR stepsDG reconnects

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• Sequence– First we see interruption and reclosure with a

voltage sag due to DG disconnect– Then voltage returns to normal (station step

regulator or LTC)– Then DG reconnects, and we see a voltage

swell– Then voltage returns to normal (station step

regulator or LTC)• Obviously needs to have coordinated

voltage control– Limits to how much DG one feeder can stand

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Power flow reversal in voltage regulators

• If excess DG during low load causes power to flow in reverse direction through voltage regulators misoperation is possible

– Modern controls recognize this and change to reverse power mode (regulate in opposite direction so that controls operate correctly)

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Wind generation case study

32 stepV Regswitched

capacitor a b c untransposed lineconstruction (typical)

windfarm

singly-fedinduction gen

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• Detailed 3-phase study shows:– as wind generation increases, one outside

phase voltage rises while the other drops– feeder has problems with frequent capacitor

switching and/or votlage regulator stepping events

– Cases like this may need dedicated feeder or doubly-fed generator to avoid power quality problems for other customers

– If DG can generate reactive power, capacitor controls needs coordination with reactive generation

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DG Transformer Connections

Order used here is HV : LV feeding DG1. Ygnd : Ygnd interconnects HV and LV

grounds 2. : Ygnd isolates LV ground from HV

ground3. : used on existing installations4. Y :used on existing installations5. Ygnd : similar to utility units

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DG Transformer Connections

1. Ygnd : Ygnd interconnects HV and LV grounds. DG may need a neutral reactor to limit 1 phase to ground short circuit currents. DG may need 2/3 pitch winding to avoid large third harmonic votlages (which can cause third harmonic currents on HV and LV sides).

- No phase shift between primary and secondary voltage.

Generator voltage harmonics

• Windings are distributed and often short-pitched (or chorded):

– each coil spans a pitch of less than 2/p where p is the number of poles, to reduce the time harmonic voltage induced

3-phase 6-pole 36-slot full-pitch stator winding

Development of stator winding

One coil of the winding: conductors in top of slot 1 return in bottom of slot 7, coil pitch = pole pitch, so a full-pitch winding

One coil of the winding: conductors in top of slot 1 return in bottom of slot 6, coil pitch = 5/6 pole pitch, so a 5/6-pitch winding

What’s so great about a 2/3-pitch coil anyway?

Third harmonic flux linking a full-pitch (blue), 5/6-pitch (green), and 2/3-pitch (red) coils.Notice that the flux linkage (net area enclosed) is zero at the 2/3-pitch coil.

Third harmonics

• The third harmonic flux will not link a stator winding with a 2/3 pitch

• If third harmonic voltages are present, third harmonic circulating currents can be quite large if generator is solidly grounded

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DG Transformer Connections

2. : Ygnd isolates LV ground from HV ground. DG may need 2/3 pitch winding to avoid third harmonic voltages, but third harmonic currents are contained on LV side. Some utility faults hard to detect due to phase shift.

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DG Transformer Connections

3. : used on existing installations. 4. Y :used on existing installations.

Ungrounded, unless DG provides ground. Some utility faults hard to detect due to phase shift.

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DG Transformer Connections

5. Ygnd : similar to utility units, isolated grounds, avoids problem with third harmonic currents.

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Expected single-phase to ground fault currents on a radial distribution feeder

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DG transformershowing possiblezero-sequence currents