1 Distributed Generation and Power Quality
Feb 07, 2016
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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