Eduard Muljadi National Renewable Energy Laboratory Golden CO eduard.muljadi@nrel

NREL is a national laboratory of the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy operated by the Alliance for Sustainable Energy, LLC

Eduard MuljadiNational Renewable Energy

Laboratory Golden CO

[email protected]

Missouri S&T University

October 12, 2009Rolla, MO

New Challenges in High Penetration New Challenges in High Penetration Renewable Energy SourcesRenewable Energy Sources


Conventional vs. Conventional vs. Wind Power PlantWind Power Plant

GSUXfmr

Large Synchronous

Generator

PrimeMover

POI or connection to the grid Collector System

Station

Feeders and Laterals (overhead and/or underground)

Individual WTGs

Interconnection Transmission Line

Other Conv.Generator

LoadLoad

Research needs for wind plant:• Collector system optimization • Reactive power management• Voltage regulation at POI and each turbine • AC vs. DC; OH vs. UG; offshore vs. in land collector systems. • Predictive maintenance.• Wind plant model vs. WTG model.


Power GenerationPower Generation

Conventional Power PlantConventional Power Plant

• Single or multiple large (100 MW) generators.

• Prime mover: steam, combustion engine – non-renewable fuel affected by fuel cost, politics, and pollution restrictions.

• Controllability: adjustable up to max limit and down to min limit.

• Predictability: preplanned generation based on load forecasting, influenced by human operation based on optimum operation (scheduled operation).

• Located relatively close to the load center.

• Generator: synchronous generator• Fixed speed – no slip: flux is controlled via exciter winding.

Flux and rotor rotate synchronously.



Wind Power PlantWind Power Plant• Many (hundreds) of wind turbines (1 MW – 5 MW each) • Prime mover: wind (wind turbine) –renewable (free, natural,

pollution free)• Controllability: curtailment• Predictability: wind variability based on wind forecasting,

influenced more by nature (wind) than human, based on maximizing energy production (unscheduled operation).

• Located at wind resource, it may be far from the load center.• Generator: Four different types (fixed speed, variable slip,

variable speed, full converter) – non synchronous generation• Type 3 & 4: variable speed with flux oriented controller (FOC)

via power converter. Rotor does not have to rotate synchronously.



Conventional vs Wind Power PlantConventional vs Wind Power Plant• Many (hundreds) of wind turbines (1 MW – 5

MW each)

• Prime mover: wind (wind turbine) –renewable (free, natural, pollution free)

• Controllability: curtailment

• Predictability: wind variability based on wind forecasting, influenced more by nature (wind) than human, based on maximizing energy production (unscheduled operation).

• Located at wind resource, it may be far from the load center.

• Generator: Four different types (fixed speed, variable slip, variable speed, full converter) – non synchronous generation

• Type 3 & 4: variable speed with flux oriented controller (FOC) via power converter. Rotor does not have to rotate synchronously.

• Single or multiple large (100 MW) generators.

• Prime mover: steam, combustion engine – non-renewable fuel affected by fuel cost, politics, and pollution restrictions.

• Controllability: adjustable up to max limit and down to min limit.

• Predictability: preplanned generation based on load forecasting, influenced by human operation based on optimum operation (scheduled operation).

• Located relatively close to the load center.

• Generator: synchronous generator

• Fixed speed – no slip: flux is controlled via exciter winding. Flux and rotor rotate synchronously.


Four basic topologies based on grid interface:– Type 1 – conventional induction generator– Type 2 – wound-rotor induction generator with variable rotor

resistance– Type 3 – doubly-fed induction generator– Type 4 – full converter interface

generator

full power

PlantFeeders

actodc

dctoac

generator

partia l power

PlantFeeders

actodc

dctoac

generator

Slip poweras heat loss

PlantFeeders

PF controlcapacitor s

actodc

generator

PlantFeeders


Type 1 Type 2 Type 3 Type 4


Types of Wind Turbine GeneratorTypes of Wind Turbine Generator


Research needs: – Needs a light weight, high efficiency, high power, low rpm, direct drive

generator and the corresponding power converter, suitable for harsh environment (offshore).

– Smarter control strategies to reduce the loads, increase energy yield, and capable of riding through voltage transients and producing high power quality under normal conditions.

– New types of power converters: high power, high efficiency, and good grid interface capability.

generator

full power

PlantFeeders

actodc

dctoac

generator

partia l power

PlantFeeders

actodc

dctoac

generator

Slip poweras heat loss

PlantFeeders


actodc

generator

PlantFeeders


Type 1 Type 2 Type 3 Type 4


Types of Wind Turbine GeneratorTypes of Wind Turbine Generator


Base Generators Adjustable

Generators(conventional

Gen, RE)

VariableGenerators

(RE)

Reserves

Storage

Base Loads

AdjustableLoads

VariableLoads

Power Losses

Real Power

Freq UP

Freq Down

LineCapacitance Switched

CapacitorsSynchronousGenerators/Condensers

Static

Compensation

LineInductance

InductiveLoad

InductionMotors/Generators

Reactive Power

Voltage UP

Voltage Down

Real and Reactive Power BalanceReal and Reactive Power Balance(to keep frequency and voltage constant)(to keep frequency and voltage constant)


Output VariabilityOutput Variability

(Load Center)

Output power to

compensate wind

variability

Power delivered unaffected by wind variability

Output power due to

wind variability

timetime time

Wind Power

Generator

Othergenerators

VARCompensation

to helpregulate voltage

LowWind

PenetrationLevel

Output power to

compensate wind

variability

Output power due to wind variability

timetime time

HighWind

PenetrationLevel

Power delivered can be affected by

wind variability

Othergenerators

Research needs:• Reactive power management • Fast acting generation reserves• Aggregation impacts on power smoothing• Ramp up/down impacts• Storage• Forecast

Voltage and Freq unaffected by wind variability

time

Voltage and Freq can be affected by

wind variability

time

storage

VR


Transmission Transmission ConstraintsConstraints

Stored power in the storage

timetime

Storage (Energy Time

Shifter)

Output power

time

Output power to the load

Powered by the plant Powered by the storage

Output power

curtailed

Output power due to

wind variability

timetime

No-Storage,Curtailment

(Energy Wasted)

Thermal Limit(thin wire)

Stability Limit(high impedance

long distanceweak-grid)

Wind Power

Generator

Wind Power

Generator

Storage

Storage

(Load Center)

Curtailment

Research needs:• Short term storage – stability improvement.• Long term storage – economic/peak-shaving.• Large-scale transmission optimized planning.• HV Power Electronics (FACTS devices).• Smart Grid, DSM, Deferrable Load, LAARS, PHEV• Forecast


Nature of LoadNature of Load

Economic DispatchSpinning/Non Spinning ReservesSub-hour scheduling

Spinning Reserves

Wind plant can only regulate down (curtailment).Generation following/scheduling is based on wind forecast.

Research needs:• Aggregation of ACE control • Reserve sharing among balancing areas• Load Acting as Resource (ERCOT)• Improve accuracy of wind forecast• Look ahead control strategy.


Wind Power Plant Wind Power Plant Output DataOutput Data

Annual Hourly AverageAnnual Hourly Average

Midwest RegionMidwest Region California RegionCalifornia Region

Spring PeakingSpring Peaking Summer PeakingSummer Peaking

Pea

k

S

hif

t

Research needs:Adapt the characteristics the load (DSM) to the local source Understand the regional behavior of wind pattern and other RE Sources.Multiple types of RE sources in parallel mode.


Possible shift by Demand Side Management

Possible shift by Storage or Parallel Operation with other RES

Matching Matching Wind and LoadWind and Load

Research Needs:• Demand Side Management

• Power and information flow (in the same wire or wireless) • Hybrid appliances (gas/electric) activated by electricity pricing.• Price incentives for helping the grid maintain frequency.

• Energy Storage• Short term storage for stability of the power system• Distributed storage close to the load or end use reduces round trip loss, encourages mass production of storage at smaller sizes, thus lowers the manufacturing cost.• Buy low, sell high based on the signal LMP or spot pricing.

• Parallel Operation with other RES

• Generation profile from PV and CSP tends to occur during the day when the price of electricity is high. Thus, higher COE may be offset by the LMP.• Wind and sunlight are two different sources with different time constants. Total output variation may smooth out the total output.


• Wind generation is intermittent and varies during the day and it can cause local congestions.

• Transmission congestions can lead to unequal pricing and vulnerable power system operations.

• Congestion triggers different in prices at different sites throughout the day.

Research Needs:• Economic incentive for V2G or G2V (as energy commodity or spinning reserve) based on transmitted pricing signal over wired/wireless communication.

• Demand side management for customers to get paid to turn on/off the loads to help increasing stability margin.

• Smart devices, smart grid, wireless communication will allow the automation to take place during the day.

• Potential market for customers capable of adjustable VAR production.

Price Triggered LoadPrice Triggered LoadStorage ActivationStorage Activation

6:45PM

7:00PM

2:20PM

6:50PM

1:20PM 6:20PM


Wind Power Plant Wind Power Plant OperationOperation

Normal OperationNormal OperationCumulative vs. Local Ramping Rates One Day of Output Variation

Cumulative Ramping Down 0.4 GW/Hr

Research needs:• Fast acting reserve • Coordination with nearby wind power plants• Short-term energy storage to shape the ramp rates.

Localized Ramping Down 5.25 GW/Hr

Research needs:• Spinning or non spinning reserves• Parallel operation with other RE sources• Long-term energy storage to shift the output and get a better price of electricity.


Voltage Ride Through

0

0.2

0.4

0.6

0.8

1

1.2

1.4

-1 0 1 2 3 4

Time (sec)

Volta

ge (p

.u.)

661

661A

LVRT-WECC Prop.

HVRT-WECC Prop.

Voltage Ride ThroughVoltage Ride Through(during transient events)(during transient events)

• Wind power plant should be able to stay on-line under transient faults/disturbances.

• The voltage should tolerate 0 p.u. for 15 msec (9 cycles).

• The wind power plant should be able to regulate the power factor between 0.95 leading/lagging.

• The wind power plant should have a SCADA system to allow remote access and monitoring.

Research Needs:• Wind plant added value for VAR regulation even when the wind plant is off line.

• Wide Area Monitoring, Protection and Control.

• Wind plant coordination with surrounding other plants.

• WTG with integrated storage.


SummarySummary• Power Quality (Voltage and Frequency) and Energy Management

• Voltage variations can be minimized by adjustable VAR compensation.

• Frequency variations can be minimized by fast acting spinning reserves and storage (short term)

• Load-Wind matching can be improved by including other RE resources (CSP, PV, Geo, Hydro, Bio), cleaner and cheaper conventional power plants, DSM, and storage (long term: CAES, PHEV, Fuel Cell – H2, Battery, Flywheel).

• The forecast error can be minimized by shorter scheduling periods, coordinated nationwide wind measurements, and better forecasting methods.

• Transmission constraints and curtailment can be minimized by improvements on the transmission lines (FACTS devices, Series/Parallel Capacitors, additional lines, dynamic ratings, storage).


SummarySummary• Phasor Measurement Unit (PMU) deployment allows the wind power plant to be

monitored more precisely during normal and transient operation. The stability margin can be measured more accurately, and remedial action scheme can be deployed at the correct time confidently, thus blackout and outage can be prevented.

• Wide Area Monitoring, Protection, and Control (WAMPAC) can help monitor the surrounding power system relative to the stability limit and anticipate the next coordinated control action to protect the wind plant ahead of potential/impending disturbances in the vicinity.

• Use of smart grid, modern control, high speed data communication, and power system intelligence allow the system operates in autopilot and reduces the burden on operators to manage normal operation but allows special intervention in critical events.

• High penetration RE sources may force us to follow a new paradigm (AC vs. DC, centralized vs. distributed, constant f vs. 60 Hz, synchronized vs. floating/island networks, instant delivery vs. stored energy)

Documents

Eduard Muljadi National Renewable Energy Laboratory Golden CO eduard.muljadi@nrel