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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
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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
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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
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Fuel Cells
• Electrochemical cells (not a heat engine)
– Net reaction: 2H2 + O2  2H2O
– PEM (proton exchange membrane) cell:
H2
O2
4e-
4eA
K
A = anode (negative)
K = cathode (positive)
PEM = proton exchange
membrane
2H2O
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Fuel Cells
–
–
–
–
Net reaction: 2H2 + O2  2H2O
PEM (proton exchange membrane) cell
Anode: 2H2  4H++4eCathode: O2+ 4H++ 4e-  2H2O
H2
O2
4H+
4e-
4eA
K
Catalyst 4e-
2H2O
I
4e0.7 V
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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
a
b
c
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a
Vdc
c
b
Passive filter
IGBT or power
transistor, e.g.
Flyback or
free-wheel
diode
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Photovoltaic
– Stacks of solar photovoltaic cells produce DC
which is fed to a power electronic inverter, just
as with fuel cells.
Vdc
a
b
c
– Issue is high installed cost, but breakthrough
may be possible
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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
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Basic Components of Wind Energy Systems
1)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
# 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.
ωr
ωIGref
+
-
Kp =252
Ti =0.3
Kp(1+
1
TiS
10/S
)
1
1+TdS
PI controller
Figure . Pitch control system model.
90

Rate limiter 0
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
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Figure . Fixed speed wind turbine generator (squirrel cage induction generator).
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
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Figure . Variable-speed wind turbine with squirrel cage
induction generator.
Figure . Variable-speed wind turbine with doubly fed
induction generator (DFIG).
– 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
Steam
WF
Hydro
Loads
Tie
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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
Steam
WF
Hydro
Loads
Tie
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Trip
Fault
DG
Neighboring
loads
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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12.47 kV
DISTRIBUTION
Radial Line
115 kV
TRANSMISSION
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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Xut Iut X1
1
1
Vx
Xdg I =0
dg
X2
If
3f SC
No DG:
1
1
If 

 0.833
X ut  X1  X 2 0.1  0.1  1.0
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Xut Iut X1
1
1
Vx
Xdg I
dg
X2
IF
3f SC
With DG, utility sees less current:
1
Vx 
 0.857
X dg X1  X dg X ut
1
X 2 X1  X ut  X dg 
IF  0.857
Idg  0.143
Iut  0.714
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12.47 kV
DISTRIBUTION
Radial Line
115 kV
DG
Put recloser here
Only one DG: obvious solution to several problems
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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 (need voltage polarization for
phase angle reference, which is extra expense)
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