Distributed Generation and
Power Quality
1
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
2
12.47 kV
DISTRIBUTION
Radial Line
115 kV
DG
Put recloser here
Only one DG: obvious solution to several problems
3
12.47 kV
Fault
115 kV
“Sympathetic” tripping
of this circuit breaker (not desired)
due to backfeed from DG
DG
Solution is to use directional overcurrent relays
at substation
4
Fault
on tap
DG
Since DG feeds the fault (backfeed),
it will likely defeat fuse saving
5
Voltage Regulation
V
DG
distance
6
Feeder trips and recloses
DG disconnects
V
DG
Low voltage
distance
7
Feeder trips and recloses
DG disconnects VR steps
V
DG
distance
8
Feeder trips and recloses
DG disconnects VR steps
DG reconnects
V
DG
distance
9
• 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
10
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)
11
Wind generation case study
wind
farm
switched
capacitor
32 step
V Reg
singly-fed
induction gen
a b c untransposed line
construction (typical)
12
• 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
13
DG Transformer Connections
Order used here is HV : LV feeding DG
1. Ygnd : Ygnd interconnects HV and LV
grounds
2. D : Ygnd isolates LV ground from HV
ground
3. D : D used on existing installations
4. Y : D used on existing installations
5. Ygnd : D similar to utility units
14
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.
15
Generator voltage harmonics
• Windings are distributed and often
short-pitched (or chorded):
– each coil spans a pitch of less than 2p/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
DG Transformer Connections
2. D : 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.
23
DG Transformer Connections
3. D : D used on existing installations.
4. Y : D used on existing installations.
Ungrounded, unless DG provides ground.
Some utility faults hard to detect due to
phase shift.
24
DG Transformer Connections
5. Ygnd : D similar to utility units, isolated
grounds, avoids problem with third
harmonic currents.
25
Expected single-phase to ground fault
currents on a radial distribution feeder
26
DG transformer
showing possible
zero-sequence currents
27