TECHNICAL CONSIDERATIONS AND CHALLENGES REGARDING GRID CONNECTED PV SYSTEMS

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TECHNICAL CONSIDERATIONS AND CHALLENGES REGARDING GRID CONNECTED PV SYSTEMS Tamás Kerekes PhD Department of Energy Technology Aalborg University Overview Introduction Potential of PV PV systems Technical
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TECHNICAL CONSIDERATIONS AND CHALLENGES REGARDING GRID CONNECTED PV SYSTEMS Tamás Kerekes PhD Department of Energy Technology Aalborg University Overview Introduction Potential of PV PV systems Technical challenges Conclusion Department of Energy Technology 2 The resource 150 million kilometres away In one minute, the Earth receives enough energy from the Sun to meet our needs for a year Winds and waves are also consequences of the solar energy Department of Energy Technology 3 The resource Department of Energy Technology 4 Europe potential map Annual total potential: in NW Europe: ca.1000 kwh/m 2 /year in southern Europe ca kwh/m 2 /year Department of Energy Technology 5 World potential map The highest potential is in the Sunbelt (±35 ) Department of Energy Technology 6 World potential map SOLAR IRRADIATION VERSUS ESTABLISHED GLOBAL ENERGY RESOURCES FOSSIL FUELS ARE EXPRESSED WITH REGARD TO THEIR TOTAL RESERVES WHILE RENEWABLE ENERGIES TO THEIR YEARLY POTENTIAL Department of Energy Technology 7 World potential 20 m 2 PV system, 2 3 people home, 1200 kwh/kwp (sunny place) kwh/day Electricity demand can be met by PV during spring and summer COMPARISON OF THE AVERAGE DAILY ELECTRICITY NEEDS OF A 2 3 PEOPLE HOUSEHOLD WITH THE ELECTRICITY OUTPUT OF A 20M² PV SYSTEM Department of Energy Technology 8 World potential AVERAGE HOUSEHOLD CONSUMPTION AND PV COVERAGE AREA NEEDED TO MEET DEMAND IN NINE COUNTRIES Department of Energy Technology 9 Solar resource evaluation How much irradiation/insolation Annual average; monthly average values Choosing optimal orientation and tilt for the PV array Investigate impact of shadows (Sunchart) How does the resource match the demand? Seasonal distribution of loads Department of Energy Technology 10 Solar resource evaluation Seasons and position of Sun/Earth The Sun's position in the sky changes throughout the year due to the 23.5 degree tilt of the Earth's axis The Sun's position in the sky over the course of one year taken at the same time each day (see picture) The shape traced out by the Sun is called an analemma The highest point in the analemma occurs during the summer, the lowest during the winter. Because the Earth's orbit around the Sun is elliptical, the figure eight shape of the analemma is distorted. If the orbit was circular the two loops of the analemma would be equal in size. Analemma photographs taken from different latitudes or at different times of day would appear slightly different. systems.net/~apw/astro/sun.html Department of Energy Technology 11 Solstice: Solar resource evaluation Seasons and position of Sun/Earth Sun's apparent position in the sky reaches its northernmost or southernmost extreme June 20 th and Dec 22 nd (sometimes: June 21 st and Dec 21 st ) Equinox: The tilt of the Earth's axis is inclined neither away from nor towards the Sun, the centre of the Sun being in the same plane as the Earth's equator Mar 20 th and Sep 23 rd (sometimes Mar 21 st and Sep 22 nd ) Department of Energy Technology 12 Solar resource monitoring Irradiation data in [W/m 2 ] If monitoring data is available, then based on these it is easy to follow the performance of the PV system th Jan th May rd May 12:05 01:05 02:05 03:05 04:05 05:05 06:05 07:05 08:05 09:05 10:05 11:05 12:05 01:05 02:05 03:05 04:05 05:05 06:05 07:05 08:05 09:05 10:05 11:05 12:05 01:05 02:05 03:05 04:05 05:05 06:05 07:05 08:05 09:05 10:05 11:05 12:05 01:05 02:05 03:05 04:05 05:05 06:05 07:05 08:05 09:05 10:05 11:05 12:05 01:05 02:05 03:05 04:05 05:05 06:05 07:05 08:05 09:05 10:05 11:05 12:05 01:05 02:05 03:05 04:05 05:05 06:05 07:05 08:05 09:05 10:05 11:05 Monitoring data for AAU Microgrid, Aalborg, DK Department of Energy Technology 13 5 th May 2012 Daily energy production: 11.9MWh Cell temperature reached only 22 deg C due to high wind conditions Department of Energy Technology 14 24 th May 2012 Daily energy production:11.2mwh Cell temperature reached 47 deg C due to low wind conditions (breeze) Department of Energy Technology 15 Solar resource evaluation Daily/Monthly average irradiation [W/m 2 /period] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Monitoring data for AAU Microgrid, Aalborg, DK Department of Energy Technology 16 PV cell characteristic I V and P V characteristics of a PV cell Maximum Power Point Tracking (MPPT) Department of Energy Technology 17 Maximum Power Point Tracking Why MPPT is needed? PV array characteristic is non linear MPP MPP depends on environmental conditions the operating point needs to be adjusted to follow weather conditions MPPT should keep the operating point at MPP in all conditions Department of Energy Technology 18 PV Module selection Many PV modules on the market: Technology: csi, asi, Thin Film, CdTe, etc. Panel surface: Width x Length Panel P mpp Panel V oc 10 Panel efficiency Etc Department of Energy Technology 19 PV Module selection Comparison of Different PV modules Price is based on data from 2006 *Overview of Solar Photovoltaic Markets and Technology Charles Korman, Chief Technologist, Solar Energy, GE Global Research Center Department of Energy Technology 20 Photovoltaic Systems Solar lighting in Mexico Source: Source: Helios PV/Wind in Korea Source: Modular (mw to MW) Noise and pollution free Reliable, long life ( 20 year) Low operation cost Abundant resource (Si) Department of Energy Technology 21 Stand alone PV Systems Solar lighting in Mexico PV/Wind in Korea Solar/FC Source: IdaTech Mobile Solar. Source: Nasa A large variety of applications Source: Helios Department of Energy Technology 22 Grid Connected PV Systems PV Buildings. Source BCIT, Burnaby, British Columbia Residential, not integrated roof Residential PV is developing very fast in Germany, Japan and Spain Power plants Residential, integrated roof Department of Energy Technology 23 Grid Connected PV systems Distributed Generation Department of Energy Technology 24 Grid Connected PV systems Grid Connected PV System Department of Energy Technology 25 Grid Connected PV systems PV Generator Electrical Power Sun Power Converter DC Monitoring Grid AC Residential systems: up to 30kW connected to LV grid (400V) Photovoltaic plants: several MW connected to MV grid (20kV) Department of Energy Technology Source: PoweOne Ultra 1400; tomorrowisgreener.com; danfoss.com 26 Voltage Rise and Overloading Problems 1. What constraints are more influential on LV networks? 2. What is the most effective solution? Available space for PV installation Rural networks Suburban networks Urban networks Load density Critical networks High load density Limited and common areas for PV installation Short distances to the substation Low load density Higher areas for PV installation Longer distances to the substation Department of Energy Technology 27 Simulation Study Low voltage (230/400V) Radial Feeder Model Radial feeder simulation parameters Grid resistance Rg Grid inductance Lg Transf. Primary and secondary resistance Rp, Rs Transf. primary and secondary leak. inductance Lp, Ls Line resistance R Line inductance L Line resistance R1 Line inductance L ohm 0.5 mh 0.5 ohm 1 mh ohm 0.04 mh 0.25 ohm 0.4 mh 11 inverters Average inverter model SOGI PLL Fixed DC voltage at 700 V Stationary frame proportional resonant digital current controller Department of Energy Technology 28 Braedstrup Grid Model Grid characteristics Type of network suburban Type of settlement residential Number of houses 60 Type of houses Transformer rated power Grid topology Average distance between transformer and houses Feeder cable types Service cable types (between houses and feeder connection points) TABLE I Summary of The Grid Characteristic Detached, single- or twofamily 400 kva radial for the simulation, meshed in real m Al 3x240, Al 3x150, Al 3x95 mm 2 (simulation), underground Al 50 mm 2 underground Department of Energy Technology 29 Braedstrup Grid Model Department of Energy Technology 30 The possible solutions to voltage rise problem 1. Adapting tap positions of OLTC transformers (110/20kV) 2. Network expansion Transmission HV/MV OLTC MV/0.4 kv MV/LV Adding new line for PVs Tap position Substation controller Voltage measurements at critical points Existing feeder only with consumers 3. Output power curtailment by PVs V [pu] 4. Reactive power control by PVs 5. PV + storage system 1.1 P Q P distance inverter P during PCC voltage 1.1pu PV Department of Energy Technology 31 State of the art Reactive Power Control (RPC) Methods Main drawbacks of P dependent Q control methods: Absorbing unnecessary reactive power when the produced real power is consumed locally and the grid voltage is in the admissible range Voltage sensitivities are not taken into consideration. The inverters with the leas voltage sensitivity and with the highest voltage sensitivity may utilize the same amount of Q. Drawback of U dependent Q control methods: The inverters closer to the transformer may not react to the overvoltage emergency condition that is occurred at the end of feeders. Following modifications are proposed: Power factor level of the nearest inverters to the transformer is increased at certain amounts for fixed cosφ and cosφ(p) methods Reactive power amount of the inverters nearest to the transformer is forced to be higher for Q(U) method Department of Energy Technology 32 Conclusion PV systems have had a significant increase in the last years Around 70GW of PV installed worldwide High PV penetration can become a challenge for LV grid operators, but solutions are there to overcome these limitations Department of Energy Technology 33
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