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 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-

6

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.

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

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

Figure . Variable-speed wind turbine with doubly fed induction generator (DFIG).

Figure . Variable-speed wind turbine with squirrel cage induction generator.

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

WF

Tie

Steam

HydroLoads

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

WF

Tie

Steam

HydroLoads

23

Trip

DG

Fault

Neighboringloads

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

115 kV

12.47 kV

TRANSMISSION

DISTRIBUTION

Radial Line

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 -> cracks)

• DG infeed may reduce the reach of overcurrent relays– DG feeds fault, so utility current is fault

current minus DG contribution

27

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

29

115 kV

12.47 kV

Put recloser here

DISTRIBUTION

Radial Line

DG

Only one DG: obvious solution to several problems

30

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)