Distributed GenerationDan Harms

La Plata Electric Association

Audience Survey

– 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

• Who has…

Distributed Generation Alternatives

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

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.

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.

Generation Model Demo

Generation Studies

• Voltage Drop / Sag / Flicker• Load Flow• Fault Current• Coordination

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.

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

Voltage Drop / Load Flow Demo

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

Typical Reactances for 40 to 2000 kW Generators

Name Symbol Range in Per Unit

Effective Time

Sub-transient reactance xd’’ .09 - .17 0 to 6 cycles Transient reactance xd’ .13 - .20 6 cycles to 5 secSynchronous reactance xd 1.7 – 3.3 After 5 secondsZero seq. reactance xo .06 - .09Negative seq. reactance x2 .10 - .22

Cummins Power Generation, white paper, “Calculating Generator Reactances”

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, sub-transient, and transient.

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.

Fault Current Demo

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.

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.

Dan HarmsLa Plata Electric Association

970-382-3514dharms@lpea.coop