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Distributed Generation and Power Quality
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Distributed Generation• Distributed generation (DG) or distributed
generation resources (DR)
– Backup generation to improve reliability– Economics and/or diversity of fuel sources– Perhaps can relieve T&D system overloads in
short term, especially if load growth is uncertain
- Effect the power quality
3
Interconnection
• Large units 10 MW and up– set up as a small power plant connected to
transmission network– may be steam cycle or combined cycle– may include co-generation
• Medium units 1-10 MW– may connect to distribution or
subtransmission line– may be combustion turbine
4
Interconnection (cont’d)
• Small units (below 1 MW)– connect to distribution– may be reciprocating engine (diesel or natural
gas) or microturbine
• Unconventional generation includes fuel cells, solar photovoltaic, wind turbines– need to be considered separately
5
Fuel Cells
• Electrochemical cells (not a heat engine)– Net reaction: 2H2 + O2 2H2O
– PEM (proton exchange membrane) cell:
A K
A = anode (negative)K = cathode (positive)PEM = proton exchange
membrane
H2 O2
2H2O
4e- 4e-
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Fuel Cells– Net reaction: 2H2 + O2 2H2O
– PEM (proton exchange membrane) cell
– Anode: 2H2 4H++4e-
– Cathode: O2+ 4H++ 4e- 2H2O
A K
H2 O2
2H2O
4e- 4e-
4H+
Catalyst 4e- 4e-
I
0.7 V
7
Fuel cells– Diagrams are oversimplified to illustrate the
basic idea– In practice, stacks of cells must be used for
power level generation– Stacks produce DC which is fed to a power
electronic inverter
Vdc
ab
c
8
Vdc
ab
c
IGBT or power transistor, e.g.
Flyback or free-wheel diode
Passive filter
9
Photovoltaic– Stacks of solar photovoltaic cells produce DC
which is fed to a power electronic inverter, just as with fuel cells.
Vdc
ab
c
– Issue is high installed cost, but breakthrough may be possible
10
Wind turbines• Each turbine may be ~ 1 MW with multiple
turbines in a “wind farm”
• Small farm ~ 5 MW connected to distribution or subtransmission
• Large farm ~100 MW connected to transmission
• Issues are voltage regulation and power fluctuations
Basic Components of Wind Energy Systems1)Turbine blades
2) Turbine hub
3) Shaft
4) Gear box
5) Generator
6) Nacelle
7) Transformer
8) Control
9) Tower
10) Foundation
# Drive train, usually includes a gearbox and a generator
Major Turbine Components
Figure . Major turbine components.
Relationship of Wind Speed to Power Production
# Power production from a wind turbine is a function of wind speed.
# In general, most wind turbines begin to produce power at wind speeds of about 4 m/s (9 mph), achieve rated power at approximately 15 m/s (29 mph), and stop power production at 25 m/s (56mph).
# Cut-in wind speed: The speed at which the turbine starts power production.
# Cut-out wind speed: The speed at which the turbine stops power production.
Pitch Control Method
1+TdS
190
0PI controller
ωr
+
-ωIGref 1
TiSKp(1+ )
Kp =252Ti =0.3 10/S
Rate limiter
Figure . Pitch control system model.
# Usually the main purpose of using a pitch controller with wind turbine is to maintain a constant output power at the terminal of the generator when the wind speed is over the rated speed.
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Machine Type
• Synchronous machine can easily sustain an “inadvertent island” wherein it attempts to supply nearby loads
• Induction generator can also, but is somewhat less likely (unless capacitors in the island temporily supply reactive power, the voltage will tend to collapse)
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Mechanically driven generators
• Synchronous generator directly connected to power system (similar to central station generation)
• Induction or asynchronous generator directly connected to power system– Induction machine driven faster than
synchronous speed will generate real power but still absorb reactive power from electrical system
– Doubly-fed induction generator:
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Wind generators
• Conventional generators are almost all synchronous machines with a wound field
• Wind generators may be induction generators – conventional: fed only from stator so always
draws reactive power from electrical system– doubly fed: feed rotor winding from a power
electronic converter to achieve some var control
Figure . Fixed speed wind turbine generator (squirrel cage induction generator).
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Wind generator interface
• Power electronic converter can be used as an interface between either induction or synchronous generator
• Converter controls may provide significant help with managing voltage fluctuations
Figure . Variable-speed wind turbine with doubly fed induction generator (DFIG).
Figure . Variable-speed wind turbine with squirrel cage induction generator.
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– On a weak system, voltage fluctuations are difficult to manage
– Power fluctuations will “drag” nearby generators on regulation and tie lines (forcing other generators to make up for the fluctuations
WF
Tie
Steam
HydroLoads
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– The steam turbines may be base loaded, so the hydro and the tie line will make up for both load fluctuations and the wind-farm generation fluctuations
– Net effect is that wind is good energy source but not as good for firm power production
WF
Tie
Steam
HydroLoads
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Trip
DG
Fault
Neighboringloads
Inadvertent Island: DG attempts to energize the island, feeding fault, complicating protective relay coordination
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PQ issues affected by DG• Sustained interruptions
– DG can provide backup power for critical loads by operating stand-alone during outage and (perhaps) in parallel during normal conditions
– Voltage regulation limits how much DG a distribution feeder can handle
– Harmonics are a concern with synch generators and inverters (less so with modern inverters)
– Voltage sags: DG helps some but not all cases
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115 kV
12.47 kV
TRANSMISSION
DISTRIBUTION
Radial Line
DG
DG on radial distribution line needs to disconnect early in reclosing interval
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Relaying considerations
• Reclosing on a synchronous machine (motor or generator) directly connected to power system can mechanically damage the unit (e.g., shaft is stressed -> cracks)
• DG infeed may reduce the reach of overcurrent relays– DG feeds fault, so utility current is fault
current minus DG contribution
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11
IutXut X1
Xdg
X2
3 SCIdg=0
If
Vx
No DG:
833.00.11.01.0
1XXX
1I
21utf
28
11
IutXut X1
Xdg
X2
3 SCIdg
IF
Vx
With DG, utility sees less current:
714.0I143.0I857.0I
857.01
XXXX
XXXX1
V
utdgF
dgut12
utdg1dgx
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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 (need voltage polarization for phase angle reference, which is extra expense)