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Distributed Generation and Power Quality

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... connect to distribution may be reciprocating engine (diesel or ... are oversimplified to illustrate the basic idea ... 5) Generator 6) Nacelle 7 ... – PowerPoint PPT presentation

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Title: Distributed Generation and Power Quality


1
Distributed Generation and Power Quality
2
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 TD 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 (contd)
  • 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

H2
O2
A anode (negative) K cathode (positive) PEM
proton exchange membrane
4e-
4e-
K
A
2H2O
6
Fuel Cells
  • Net reaction 2H2 O2 ? 2H2O
  • PEM (proton exchange membrane) cell
  • Anode 2H2 ? 4H4e-
  • Cathode O2 4H 4e- ? 2H2O

H2
O2
4H
I
4e-
4e-
Catalyst
4e-
4e-
K
A
0.7 V
2H2O
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

a
Vdc
b
c
8
a
Vdc
b
c
Passive filter
Flyback or free-wheel diode
IGBT or power transistor, e.g.
9
Photovoltaic
  • Stacks of solar photovoltaic cells produce DC
    which is fed to a power electronic inverter, just
    as with fuel cells.

a
Vdc
b
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

11
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
12
Major Turbine Components

Figure . Major turbine components.
13
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.
14
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.
Kp 252 Ti 0.3
10?/S
90
1
1
?
Kp(1 )
TiS
1TdS
PI controller
0
Rate limiter
Figure . Pitch control system model.
15
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)

16
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

17
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

18

Figure . Fixed speed wind turbine generator
(squirrel cage induction generator).
19
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

20

Figure . Variable-speed wind turbine with
squirrel cage induction generator.
Figure . Variable-speed wind turbine with doubly
fed induction generator (DFIG).
21
  • 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
22
  • 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
23
Trip
Fault
DG
Neighboring loads
Inadvertent Island DG attempts to energize the
island, feeding fault, complicating protective
relay coordination
24
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

25
DISTRIBUTION
12.47 kV
Radial Line
115 kV
TRANSMISSION
DG
DG on radial distribution line needs to
disconnect early in reclosing interval
26
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 -gt cracks)
  • DG infeed may reduce the reach of overcurrent
    relays
  • DG feeds fault, so utility current is fault
    current minus DG contribution

27
Iut
Xut
X1
X2
Vx
If
1
3f SC
Xdg
Idg0
1
No DG
28
Iut
Xut
X1
X2
Vx
IF
1
3f SC
Xdg
Idg
1
With DG, utility sees less current
29
DISTRIBUTION
12.47 kV
Radial Line
115 kV
DG
Put recloser here
Only one DG obvious solution to several problems
30
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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