Operational issues associated with grid-connected photovoltaicswith grid connected photovoltaics

ELEC9715: Electricity Industry Operation and Control9 Sept 2008p

Adam Junid, Simin LiStudent IDs: 3170043, z3229153

Photos: •OSEIA, 2006•www.pv.unsw.edu.au/news/past-news/past%20news%202005.asp•ESDEPS, 2002

Small-scale grid connected PV

UNSW Kensington:• By UNSW SPREE

40 kW• 40 kWp• Installed 2005

Source: www.pv.unsw.edu.au/news/past-news/past%20news%202005.asp

Larger-scale grid connected PV

Springerville PV plant, Arizona:• By Tucson Electric

4 6 MW

Floriade Exhibition Centre, Netherlands:• By NUON

• 4.6 MWp• Expandable to 8MW• 8MW limited by transmission line• 3.4MWp commissioned in 2004

• 2.3 MWp• Commissioned 2002

Photos: www.zonnepanelen.wouterlood.com/solar_pages_uk/records_uk.htm

p

Market trends

Source: Antonio et al, 2002

General circuitry

Inverter

DC side isolation switch

PV ArrayAC side isolation switch

Inverter

PV Array(Usually building mounted)

AC mains supplyMeter Main fusebox

To high efficiency applications

Photo: www.solarshop.co.uk

Safety issues

• Major concern is anti-islanding protection and coordinationcoordination– To ensure power line worker safety, workers may need

additional safety precautions, procedures and tools in an area having large amounts of grid-connected PVarea having large amounts of grid-connected PV

– To reduce loads receiving offspec voltages. Increased use of grid connected PV increases likelihood of anti-islanding protection failure; may need new monitoring and controlprotection failure; may need new monitoring and control schemes and procedures

Photo: www.centralpower.ca/Images/Central%20Pictures%20023__Optimized3405by255.jpg

Reliability issues• Nuisance trips may be caused by• Nuisance trips may be caused by

– Many grid-connected PV systems interacting with one another, e.g.• one PV system’s outage may trip another’s• inverters using active phase frequency shift currents (Smith et al 2004) to to

• Lightning potentials due

• inverters using active phase frequency shift currents (Smith et al 2004) to to test for grid frequency shifts may inevitably shift the local grid frequency

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to poor earthing, use manufacturer-recommended earthing Ex

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recommended earthing• Fault, transient surge

and voltage swell E

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• Harmonics immunityissues

Photo: www.southface.org/solar/solar-roadmap/roadmap_art/sample%20pv%20layout.jpg

Prop

Optimal Power issuesOptimal Power issues• Objectives

– Inverter should supply pp ycurrent in phase with the grid for maximum real power injection, unless specified otherwise byspecified otherwise by utility

– The inverter’s MPPT should operate at a voltageshould operate at a voltage point of the PV array at which power output is maximized

– Inverter operating range must match array(s)

Source: Ishikawa, T. 2002

Power Quality issues• Objectives

– Reduce THDv and THDi from– Reduce THDv and THDi from inverter (some inverters have built-in active filtering)

– More sophisticated inverters with pactive filters will be able to monitor THD at the PCC and inject current at the right phase angles to fill in gaps in the sinusoidgaps in the sinusoid

– Reducing DC current leakage into the AC line

– Reducing resonant coupling on theReducing resonant coupling on the DC line due to AC circuitry radiation by manufacturer-recommended earthing practice

• Source: ESDEPS, 2002

Loadflow Integration: AvailabilityAvailability

• Constraints– Industrial PV generation

stations practically only available in the day, unless battery-backed, thus derivatives or swap-options will reflect the PV operator’s pdaytime availability

– If retailers choose to link buy-back rates to NEM spot prices,back rates to NEM spot prices,it may encourage even more use of battery-backed grid-connected PV, which couldconnected PV, which could have beneficial effects

Source: NEMMCO, 2008

Loadflow Integration issues:

CoordinationCoordination

• Future issues:– National standards

to allow for graded and/or coordinated cutoff times for anticutoff times for anti-islanding to minimise future riskminimise future risk of cascading trips

Loadflow Integration issues:C di iCoordination

• Issues:– National standards

to As grid-connected PV

lif tproliferates, ramp-up and ramp-down coordination may also be required to qsupport grid stability and transmission constraints (IEEEconstraints (IEEE 1547.3 draft proposal)

Source: Basso, T. 2003

Loadflow Integration issues: Pricing

• Advantages of buy-back pricing linked to NEM prices (cont’d):– Grid-connected PV operators may install large battery banks to boost their PV system's

power export during high prices– Grid-connected PV manufacturers possibly innovating to design inverter and battery p y g g y

conditioner control to charge batteries during low spot prices, and export more power during high spot prices

– Battery-backed grid-connected PV systems behaving as a natural hedge for retailers against volatile NEM spot prices

Commercial issues:

• Industry restructuring favouring higher buyback rateswould make grid-connected PV both more affordable and reduce NEM peak demand Source: www.nemmco.com.au/met_sett_sra/640-0142.pdf

Thank you

Question time

Adam Junid, Simin LiStudent IDs: 3170043, z3229153Photos:

•OSEIA, 2006•www.pv.unsw.edu.au/news/past-news/past%20news%202005.asp•ESDEPS, 2002