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

115 kV

12.47 kV

Put recloser here

DISTRIBUTION

Radial Line

Only one DG: obvious solution to several problems

115 kV

12.47 kV

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

Solution is to use directional overcurrent relays at substation

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

Fault on tap

distance

Voltage Regulation

distance

Feeder trips and reclosesDG disconnects

Low voltage

distance

Feeder trips and reclosesDG disconnects VR steps

distance

Feeder trips and reclosesDG disconnects VR stepsDG reconnects

• 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

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)

Wind generation case study

32 stepV Regswitched

capacitor a b c untransposed lineconstruction (typical)

windfarm

singly-fedinduction gen

• 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

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

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

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.

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.

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

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

DG transformershowing possiblezero-sequence currents

Distributed Generation and Power Quality

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