One more systems engineering and analysis focused presentation from Milsoft's 2009 User Conference. It was originally presented by Dan Harms. The Milsoft Electric Utility Solutions Users Conference is the premier event for our users and the vendors who provide interoperable solutions or services that enhance Milsoft Smart Grid Solutions. If you’d like to be on our mailing list, email: missy.brooks@milsoft.com.

1. Distributed Generation
Dan Harms
La Plata Electric Association

2. Audience Survey
• Who has…
– Single residential windmills
– Wind farms
– Single residential solar
electric (1 to 10 kW)
– Commercial solar (10 to
500 kW)
– Solar farms (over 500 kW)
– Biomass
–
–
–
–
–
Micro hydro / hydro
Waste heat recovery
Natural gas turbines
Diesel engines
Algae biofuel

3. Distributed Generation Alternatives

4. Modeling Considerations
• WindMil’s “Generation Model” settings
– Negative Load
• Simply adds a given source of kW and kVAr
– Swing kVAr
• Generator maintains a settable per unit voltage
• kW output held constant
• kVAr swings leading/lagging to hold voltage

5. Negative Load
• Use on solar electric, small windmill, micro
hydro, and other inverter-based systems
• Use on induction generators
• Use on some synchronous generators
– Depends on mode of generation
– If maximum power output is a priority, generator
terminal voltage can be adjusted in lieu of VARs.

6. Swing kVAr
• Use on synchronous generators where VAr flow
can be adjusted through field excitation.
• Requires detailed knowledge of generator’s
capability
– You will need control setting information.
– It’s unlikely that 5 kW generator could bring voltage
up to 1.2 PU.
• Make sure your VAr output is not exceeded.

7. Generation Model Demo

8. Generation Studies
•
•
•
•
Voltage Drop / Sag / Flicker
Load Flow
Fault Current
Coordination

9. Generation – Voltage Drop
• A generator will cause your voltage to rise
– Either by reducing current and therefore voltage drop
or
– By causing reverse power flow and reverse voltage
drop
• The smaller your conductor, the greater your voltage rise.

10. Generation – Voltage Drop (cont.)
• What happens if the generator drops offline?
– If in-line with regulators, they will not be boosting as
much
– If OCR trips and recloses, generation is gone, load is
still there, regulators have to catch up
– Run voltage drop with generation in place, lock
regulators, remove generators, run voltage drop again

11. Voltage Drop / Load Flow Demo

12. Generation – Fault Current
• Generator impedance options for fault
contributions
– Sub-transient (xd’’)
• Determines maximum instantaneous current
• Select AIC rating for breaker.
– Transient (xd’)
• Determines current at short time delay of breakers
– Steady-State (xd)
• Determines steady state current without excitation

13. Typical Reactances for 40 to 2000 kW Generators
Name
Symbol
Sub-transient reactance
xd’’
Range in Per
Unit
.09 - .17
Transient reactance
xd’
.13 - .20
6 cycles to 5 sec
Synchronous reactance
xd
1.7 – 3.3
After 5 seconds
Zero seq. reactance
xo
.06 - .09
Negative seq. reactance
x2
.10 - .22
Cummins Power Generation, white paper, “Calculating Generator Reactances”
Effective Time
0 to 6 cycles

14. Fault Current – Inverter Based Systems
• Inverter based systems
– PV, residential wind, micro hydro, and some bio-mass
– No rotating parts and no inertia
– Fault contribution 1 to 1.2 times rated output
• Go to the impedance calculator in the Equipment Editor for
the generator.
• Enter full load amps x 1.2.
– Use this impedance model for steady state, subtransient, and transient.

15. Fault Current – Large Wind
• Wind Generation
– Reactive power is modulated.
– Capacitive current is injected to maintain voltage.
– Model as Swing kVAr or use negative load with capacitors.
– Fault contribution depends on turbine type and control
settings.
– Work with turbine manufacturer to determine fault
contribution capability.

16. Fault Current Demo

17. When to Model Generators
• Is the generator relatively large to the tap /
feeder / substation it’s on?
– If system has capability of backfeeding about 5% of the load of
the tap it’s on, model it.
• My personal preference
• Net metered?
– If excess generation doesn’t reach grid, don’t model it.
– WindMil doesn’t allocate to negative billing load.

18. Summary
• WindMil can model any generator, but you need
to know how best to model what you have.
– Generator type
• Synchronous, induction, inverter
– Control settings
• VAr control?
– Fault characteristics
• Rotating inertia?
• Talk to manufacturer.

20. Dan Harms
La Plata Electric Association
970-382-3514
dharms@lpea.coop