2010 PV Inverter Systems
Prepared by: Joss K. Mankad
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Contents Preface ................................................................. ....................................................................................................................................... ......................................................................................... ................... 3 1.
PV inverters introduction .............................................................. ...................................................................................................................... ........................................................ 4
2.
PV cell types ............................................................ ............................................................................................................................... .............................................................................. ........... 5
3.
Grid tie PV inverter design ............................................................ .................................................................................................................... ........................................................ 5 Key terms for a pv inverter ....................................................................................................................... ....................................................................................................................... 5 Layout of a grid tie PV inverter system ......................................................... ..................................................................................................... ............................................ 7 Inversion function ..................................................................................................................................... ..................................................................................................................................... 7 Filters........................................................................................................................ Filters..................................................... .................................................................................................... ................................. 8 Maximum power point tracking ................................................................... ............................................................................................................... ............................................ 8 Auto wake-up system ............................................................ ............................................................................................................................. ................................................................. 10 Grid disconnection (islanding mode detection)..................................................................... ...................................................................................... ................. 10
4.
Other protections and controls ............................................................... .......................................................................................................... ........................................... 11 Ground fault protection ................................................................................ .......................................................................................................................... .......................................... 11 Ac over current detection and protection .............................................................................................. .............................................................................................. 12 Single phase open detection ............................................................. ................................................................................................................... ...................................................... 13 Thermal requirements ............................................................................................................................ ............................................................................................................................ 13 Dc bus over voltage and over current protection ............................................................... ................................................................................... .................... 13
5.
Monitoring software and metering .......................................................... .................................................................................................... .......................................... 13
6.
Testing of a PV system ................................................................... ........................................................................................................................ ..................................................... 14
7.
Future trends ...................................................................... ...................................................................................................................................... ................................................................ 15
8.
Conclusion ............................................................... ..................................................................................................................................... ............................................................................ ...... 17
9.
Appendix ................................................................. ....................................................................................................................................... ............................................................................ ...... 18 Various standards ................................................................................................................................... ................................................................................................................................... 18 Voltage requirements ............................................................................................................................. ............................................................................................................................. 19 Frequency requirements.................................................................... ......................................................................................................................... ..................................................... 19 Synchronization( IEEE 1547) ............................................................. ................................................................................................................... ...................................................... 20 Ac current harmonics ............................................................. .............................................................................................................................. ................................................................. 20 Efficiencies and costs of PV module ............................................................. ....................................................................................................... .......................................... 21 Solar map of India .................................................................................................... ................................................................................................................................... ............................... 22
10.
Index.................................................................... .......................................................................................................................................... ............................................................................ ...... 24
11.
References ...................................................................... ...................................................................................................................................... ................................................................ 25
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Preface The Indian economy has been growing at more than 8% since last three years. In spite of the expected slow down in the global economy the Indian GDP is likely to grow at about 7%. To grow at this rate one of the major requirement is to have uninterrupted supply of energy. The primary use of energy is for electricity generation followed by fuel for transportation. An Indian consumer consumes 512 kg of oil equivalent of energy for power, transportation, industrial and domestic application in cities and rural areas. The major source of energy is coal and oil other fuel sources contributing to produce energy are gas, hydro, nuclear and renewable sources. Government of India has a mission of “power for all” by 2012. Currently 44% of the 200 million Indian house holds do not use
electricity and 75 million rural households still use kerosene for lighting. This means additional energy through grid interactive and distributed sources are required. India has about 250 to 300 sunny days in most parts of the country. Annual solar energy received approximately 5000 trillion KWh per year and the daily average solar energy incident varies from 4.6-6.4 KWh per square meter. Per day, conductive arid condition and minimal sun tracking, making it an ideal place for solar energy power plan. Solar energy can be harnessed through two routes, namely photovoltaic and solar thermal, by direct conversion to electrical energy and heat energy respectively. A variety of photo voltaic technology enables the direct conversion of solar radiation into electricity. A variety of PV systems have been developed for centralized applications like street lights home lighting, stand alone PV powe r plants, building integrated PV systems…etc. solar photovoltaic has a potential for grid interactive power plants. The presented work reviews the working of PV inverters as a grid interactive plant, their advantages and drawbacks plus recent and future trends in PV inverter system.
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1. PV inverters introduction PV cells were first used to power satellites. Through the middle of the 1990s the most common terrestrial PV applications were stand-alone systems located where connection to the utility grid was impractical. By the end of the 1990s, PV electrical generation was costcompetitive with the marginal cost of central station power when it replaced gas turbine peaking in areas with high afternoon irradiance levels. Encouraged by consumer approval, a number of utilities have introduced utility-interactive PV systems to supply a portion of their total customer demand. Some of these systems have been residential and commercial rooftop systems and other systems have been larger ground-mounted systems. PV systems are generally classified as utility interactive (grid connected) or stand-alone. Orientation of the PV modules for optimal energy collection is an important design consideration, whether for a utility interactive system or for a stand-alone system. Best overall energy collection on an Annual basis is generally obtained with a south-facing collector having a tilt at an angle with the horizontal approximately 90% of the latitude of the site. For optimal winter performance, a tilt of latitude +15° is best and for optimal summer performance a tilt of latitude –15° is best. In some cases, when it is desired to have the PV output track utility peaking requirements, a west-facing array may be preferred, since its maximum output will occur during summer afternoon utility peaking hours. Stand-alone PV systems are used when it is impractical to connect to the utility grid. Common standalone systems include PV-powered fans, water pumping systems, portable highway signs, and power systems for remote installations, such as cabins, communications repeater stations. The PV modules must supply all the energy required unless another form of backup power. Stand-alone systems also often incorporate battery storage to run the system under low sun or no sun conditions. Utility-interactive PV systems are classified by IEEE Standard 929 as small, medium, or large (ANSI/IEEE, 1999). Small systems are less than 10 kW, medium systems range from 10 to 500 kW, and large systems are larger than 500 kW. Each size range requires different consideration for the utility interconnect. In addition to being able to offset utility peak power, the distributed nature of PV systems also results in the reduction of load on transmission and distribution lines. Normally, utility-interactive systems do not incorporate any form of energy storage — they simply supply power to the grid when they are operating. In some instances, however, where grid power may not be as reliable as the user may desire, battery backup is incorporated to ensure uninterrupted power.
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2. PV cell types
A solar panel (photovoltaic module or photovoltaic panel) is a packaged interconnected assembly of solar cells, also known as photovoltaic cells. The solar panel is used as a component in a larger photovoltaic system to offer electricity for commercial and residential applications. Because a single solar panel can only produce a limited amount of power, many installations contain several panels. This is known as a photovoltaic array. A photovoltaic installation typically includes an array of solar panels, an inverter, batteries and interconnection wiring. Photovoltaic systems are used for either on- or off-grid applications, and for solar panels on spacecraft. Solar panels use light energy (photons) from the sun to generate electricity through the photovoltaic effect. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate). The majority of modules use wafer-based crystalline silicon cells or a thin-film cell based on cadmium telluride or silicon. Crystalline silicon, which is commonly used in the wafer form in photovoltaic (PV) modules, is derived from silicon, a commonly used semi-conductor. In order to use the cells in practical applications, they must be:
connected electrically to one another and to the rest of the system Protected from mechanical damage during manufacture, transport, installation and use (in particular against hail impact, wind and snow loads). This is especially important for wafer-based silicon cells which are brittle. Protected from moisture, which corrodes metal contacts and interconnects, (and for thin-film cells the transparent conductive oxide layer) thus decreasing performance and lifetime.
Most modules are usually rigid, but there are some flexible modules available, based on thin-film cells. Electrical connections are made in series to achieve a desired output voltage and/or in parallel to provide a desired amount of current source capability.
3. Grid tie PV inverter design Key terms for a pv inverter
PV Production – Electricity generated by a PV System. May be stated in terms of kW
(Point in time power) or kWH (Power produced in a given period of time).
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PTC – Performance Test Conditions. Testing standard for modules that was
developed to more closely represent conditions likely to be found in the field. 1000 2
W/M , 20°C Ambient Temp. (AKA – PVUSA Test Conditions)
STC – Standard Test Conditions. Laboratory testing standard for modules. 1000 2
W/M , 25°C Cell Temp. “Nameplate Rating” of Module or Array. Manufacturers advertise and sell modules based on this value. (AKA – Peak Power Rating)
Combiner – Circuit Termination Equipment that connects multiple PV Source
Circuits, generally in Parallel. Have fuses or breakers for over current protection of individual circuits.
Disconnect – Switch. Turns electrical circuit on & off.
Inverter – Converts DC to AC.
Module – An individual assembly of PV cells. Often called a “panel” by unfamiliar
folks.
Voc – Open Circuit Voltage at STC.
Vpeak – (Vmpp) Voltage at Maximum Power Point at STC.
Isc – Short Circuit Current (Amps) at STC.
Ipeak – (Impp) Amperage at Maximum Power Point at STC.
Max Power – (Wpeak) Watts at STC.
Max DC Short Circuit Current – Maximum Array Isc at STC.
DC Voltage Range – Minimum to Maximum DC Voltage Input for the Inverter to
operate. Array must be electrically connected to provide voltage in this range.
Output Power – Rated AC output capacity in Watts.
AC Voltage Range – Minimum to Maximum AC Voltage Input for the Inverter to
operate. Grid Voltage.
Efficiency – Efficiency of the DC-AC conversion in terms of watts. Ratings are based
on “X” % of capacity.
Maximum Input Current – Maximum current of the Array at Max Power Point.
Max DC Short Circuit Current – Maximum Array Isc.
BOS – Balance of System. Equipment & hardware required to build a PV system
other than Modules & Inverters.
AC Disconnect – Switch in the AC Circuit. Disconnects AC source from Inverter. May
be integral or external to Inverter. Multiple AC Disconnects are often required.
DC Disconnect – Switch in the DC Circuit. Disconnects DC source from Inverter. May
be integral or external to Inverter.
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Layout of a grid tie PV inverter system
Above figure shows the basic layout of a grid tie pv inverter system. The main components of this system are PV arrays, inverter and other control accessories. First the PV arrays convert the solar power in to dc electricity. Then the DC output of PV arrays is given to the inverter which converts the DC form into AC. As most of the inverters are based on PWM technique their output is not purely sinusoidal. So a filter is connected with the inverter to produce purely sinusoidal electricity. To utilize the output of this system more efficiently other control systems like maximum power point tracker, auto wake up systems and other control accessories are also connected with this system which are discussed in detail further. Now, let us understand the functions of PV inverter system in detail.
Inversion function
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The method by which dc power from the PV array is converted to ac power is known as inversion. Other than for use in small off-grid systems and small solar gadgets, using straight dc power from a PV array, module or cell is not very practical. Although many things in our homes and businesses use dc power, large loads and our electrical power infrastructure are based on ac power. This dates back to the early days of Edison versus Tesla when ac won out over dc as a means of electrical power distribution. An important reason that ac won out is because it can be stepped up and travel long distances with low losses and with minimal material. This could change in the distant future if more of our energy is produced, stored and consumed by means of dc power. Today, the technology exists to boost dc electricity to high voltages for long d istance transfer, but it is very complex and costly. For the foreseeable future, ac will carry electricity between our power plants, cities, homes and businesses. In an inverter, dc power from the PV array is inverted to ac power via a set of solid state switches —MOSFETs or IGBTs —that essentially flip the dc power back and forth, creating ac power. Diagram shows basic operation in a single-phase inverter.
Filters
As most of the inverters work on the bases of PWM technique filters are essential part of a PV inverter systems. These convert PWM output of an inverter into pure sinusoidal output. Filters are also used to filter the noise and spikes from semiconductor components. Other filters required in this system are EMI/RFI (electro magnetic interference/ radio frequency interference).
Maximum power point tracking
Maximum Power Point Tracking, frequently referred to as MPPT, is an electronic system that operates the Photovoltaic (PV) modules in a manner that allows the modules to 8
produce all the power they are capable of. MPPT is not a mechanical tracking system that “physically moves” the modules to make them point more directly at the sun. MPPT is a fully electronic system that varies the electrical operating point of the modules so that the modules are able to deliver maximum available power. Additional power harvested from the modules is then made available as increased battery charge current. MPPT can be used in conjunction with a mechanical tracking system, but the two systems are completely different. PV modules have a characteristic I-V curve that includes a short-circuit current value (Isc) at 0 Vdc, an open- circuit voltage (Voc) value at 0 A and a “knee” at the point the MPP is found—the location on the I-V curve where the voltage multiplied by the current yields the highest value, the maximum power. Diagram shows the MPP for a module at full sun in a variety of temperature conditions. As cell temperature increases, voltage decreases. Module performance is also irradiance dependent. When the sun is brighter, module current is higher; and when there is less light, module current is lower. Since sunlight intensity and cell temperature vary substantially throughout the day and the year, array MPP current and voltage also move significantly, greatly affecting inverter and system design.
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The above figure shows the characteristics of current and power versus the terminal voltages. MPPT shall be fast enough to increase the energy extraction by following the isolation level closely but not too fast to cause the instability. Tracking along the left slope of the power curve is not stable so should be avoided. MPPT shall take the grid voltage into account while tracking lower. If DC bus voltage drops too low but grid voltage is higher, the quality of the current to the grid will suffer.
Auto wake-up system The PV inverter shall be able to wake up when the available power is more than the total loss of the inverter system. Pre-mature wake up will cause negative power flow from the grid thus hampers the energy production. Delayed wake up also hampers the energy production. Since the available power changes with weather condition, the wake up algorithm has to be adaptive in nature.
Grid disconnection (islanding mode detection)
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As required by UL 1741 and IEEE 1547, all grid-tied inverters must disconnect from the grid if the ac line voltage or frequency goes above or below limits prescribed in the standard. The inverter must also shut down if it detects an island, meaning that the grid is no longer present. In either case, the inverter may not interconnect and export power until the inverter records the proper utility voltage and frequency for a period of 5 minutes. These protections eliminate the chance that a PV system will inject voltage or current into disconnected utility wires or switchgear and cause a hazard to utility personnel. If an inverter remained on or came back on before the utility was reliably reconnected, the PV system could back feed a utility transformer. This could create utility pole or medium voltage potentials, which could be many thousands of volts. A significant battery of tests is performed on every grid-tied inverter to make ce rtain that this situation can never occur. The figure shown here represents the islanding operation of an inverter. Upper waveform shows the output voltages of inverter and the waveform shown below represents the line or grid voltages.
Scale: Vertical- 200V/div. Horizontal -100ms/div. After 650 ms the inverter shuts down this is in the permissible limits declared by IEEE 1547.
4. Other protections and controls Ground fault protection
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For a grounded system, the ground fault occurs when DC terminal of PV array touches the chassis. Current flows through the ground to the negative terminal first then goes to the positive through the PV cells. The current sensor on the wire between the negative and the ground terminal will pick up the fault. Inverters shall shutdown and disconnect itself from the grid as well as the PV array as soon as the ground fault is detected. The ground fault current shall be interrupted.
Ac over current detection and protection Grid Transients or grid faults can cause over current condition. Fault in transformer or line filter can also cause over current condition. Faulty switching devices can cause dead short circuit. Over current fault shall shutdown the unit and disconnect from the grid. For this purpose proper protective devices must be connected with the system.
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Single phase open detection Loss of any phase of the grid shall be detected and the inverter shall be shutdown to avoid possible safety hazard. Loss of a phase can be detected by using current sensors on each phase. If the output transformer is external to the inverter, the single phase open shall be detected on both sides of the transformer.
Thermal requirements The inverter system and components shall not attain a temperature at any operating condition so as to result in risk of overheating, fire, or insulation damage. Junction temperature of switching devices shall be maintained below maximum allowed value (e.g. 120 C). Solar inverters are generally installed outdoor so shall handle extreme temperature conditions like –40 C to 50 C. Solar power is not generated uniformly through out the day so the cooling system does not have to be turned ON all the time. A power output based cooling system control shall be designed to improve the efficiency.
Dc bus over voltage and over current protection When DC bus exceeds the designed maximum voltage (e.g. 600V), the inverter shall shutdown and array shall be disconnected from the DC bus to protect the DC bus Capacitors, Disconnects and Wire Insulations. Also, inverter shall be able to identify the reverse polarity of the DC connection and warn the installer and remain in sleep state. Over current can be caused by too low DC bus voltage, higher AC grid voltage transient, and enhanced solar condition or due to some hardware failure/short circuit in DC bus circuitry or wrong DC bus connection. The inverter shall be shutdown and isolated from the grid as well as the PV array when the over current condition occurs.
5. Monitoring software and metering To reliably control the inverter, the software designed to run on the inverter’s digital signal processor or microcontroller is developed over years of code writing and debugging. The most critical control is the one driving the power stage. This creates the PWM waveforms that generate the sine waves ending up on the utility grid and at the building’s loads. Software also controls the inverter’s interaction with the grid and drives all the appropriate UL 1741 and IEEE 1547 required controls and events. Another part of the software controls the MPPT function that varies the dc voltage and current level as required 13
to accurately and quickly follow the moving MPP of the PV array. All of these major functions, as well as a multitude of others, are carried out in unison like an orchestra. Software is used to drive the contactor that places the inverter on the grid in the morning and off the grid at night. Software controls temperature limits and optimizes cooling system controls. Software, its development history and robustness, is a critical element in any inverter. The metering aspect will form part of the Connection Agreement. "Net metering", where exported units run the meter backwards (making the PVgenerated unit selling price effectively the same as the purchase rate for utility- supplied electricity), is seen as the most attractive option for PV system owners and gives quite an attractive rate of return (some estimates have shown that this method can contribute approximately 15% of the PV system cost recovery). However, for any electricity import/export arrangement, a meter with two separate metering registers is required. It is not satisfactory to allow the meter to simply run backwards for exported units, since this leaves the meter open to fraudulent abuse. In any case, the commonly used Ferraris electro-mechanical meter is mechanically damped to prevent the meter from rotating backwards. Also, modern electronic meters can be programmed to run only in one direction (i.e. only forwards), again to combat meter fraud. A more conventional approach is to use a PV-generated unit selling price equivalent 1 to the electricity pool price (approximately /3of the domestic purchase rate). However, the cost of the two-way import/export meter becomes a major cost factor for smaller PV installations. For smaller systems the value of units exported is likely to be small and so it may be beneficial to ensure that as much of the PV output as possible is used within the house.
6. Testing of a PV system Various tests on PV systems are performed to check its performance under various conditions. Here some of the tests recommended by underwriter’s laboratory are
mentioned. The main tests are
a) Normal safety related tests 14
b) Abnormal safety related tests c) Performance related tests
In normal safety related tests the dielectric strength,
over
current
capacity,
thermal
stress
capacity, ground fault detection, anti islanding mode tests…etc are conducted . For the testing of islanding
mode a dummy load of parallel R-L-C network is connected with the pv inverter systems.
In abnormal safety related tests overload capacity tests, short circuit tests, single phase tests; dc reverse polarity tests….etc are conducted. And in performance related tests harmonic tests, maximum power point tracking tests and wake-up strategy tests are done.
7. Future trends Future advances in central grid-tied inverters —and eventually string inverters as well — will likely also include some type of utility interaction to support the grid in times of distress. This might include the inverter remaining online during a brownout or other voltage events, even during frequency events, giving the utility more time to make adjustments or isolate circuits to remedy the problem and preventing even larger grid instability or a propagated blackout. Currently, if the grid voltage or frequency goes outside of the windows specified in IEEE 1547, all grid-tied inverters must go offline, which likely accelerates grid failure. Utility control of PV inverters and other discontinuous sources might make it possible to remedy some grid problems. Because inverters draw their sine wave current waveforms in many increments, the differential control of these increments can help specifically to adjust the grid voltage waveform, minimizing or correcting power factor or other problems created by certain loads in the area. Another area for the advancement of inverters and grid tied 15
renewable energy systems will likely involve energy storage. As a larger portion of our power generation capacity comes from discontinuous sources of energy, like PV and wind, storage becomes more important. A future component of large commercial PV systems might include on-site energy storage with utility control or based on time of use costs for electricity. With increasing amounts of PV power processed by DSP-controlled inverters, there are many critical functions that inverters can incorporate as the industry progresses. As these potentials are realized, PV power will become an increasingly widespread and important portion of our energy infrastructure.
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8. Conclusion We can conclude that in the rising era of usage of renewable energy sources, solar energy is abundantly available energy, so usage of solar as a renewable will increase in future. PV inverters are the easiest method to convert solar energy than the other methods. The installation of a PV system is easy and it can be installed near the load, so negligible transmission losses. Even PV systems can be installed inside the houses so we can reduce the electricity bills and if we are connecting these systems to grid then we can get compensation in electricity bills. The costs of PV cells are higher but they are reducing gradually. Research work in various organizations is in progress to increase the efficiency and reduce the cost of PV cells.
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9. Appendix Various standards
IEEE 1547 – 2003 ~ Standard for Interconnecting Distributed Resources with Electric Power Systems
IEEE Std. 519 – 1992 ~ IEEE recommended practices and requirements for harmonic control in electrical power systems
PV systems
IEEE 929 – 2000 ~ Recommended Practice for Utility Interface of Photovoltaic (PV) Systems (Superceded by IEEE 1547 )
IEEE 1374 – 1998 ~ IEEE guide for terrestrial photovoltaic power system safety
US National Electrical Code (NEC) Article 690 ~ Solar Photovoltaic Systems
Power factor (IEEE 1547-2003) - The PV system should operate at a power factor > 0.85 (lagging or leading) when
output is > 10% of rating - PV inverters designed for utility-interconnected service operate close to unity power factor NEC Code 690 - 690.13(A) ~ Disconnect means shall be provided between photovoltaic power system output and other building conductors, no disconnect in grounded conductor - 690.2 ~ DC operating voltages of 12 volts up to 600 volts, with AC outputs from 120 V single phase to 480 V three phase - 690.2 ~ Inverters for PV systems in sizes from 100 W to custom designs of up to 1 MW or more - 690.8 ~ Conductors and overcurrent devices to be sized such that over current devices shall not be operated continuously at more than 80%
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Voltage requirements - For normal conditions and the required regulation is +/- 5% on a 120-volt base at the service entrance
Frequency requirements
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Synchronization( IEEE 1547)
Ac current harmonics
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Efficiencies and costs of PV module
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Solar map of India
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10.
Index
AC over voltage-15 Auto wake-up-9,12 DC over current-15 DC over voltage-15 Disconnects-8,15 Filters-10 Harmonics-22 Islanding-13,17 MPPT-11,12,16 PV inverter-5,6,8,9,10,17,18,19,20 PV module- 6,12
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11.
References
Papers
1. Grid inter active solar inverters and their impact on power systems, Dr. Anil Tulafhar 2. How Inverters Work,By James Worden and Michael Zuercher-Martinson 3. Islanding of grid connected PV inverters-by H. haeberlin andCh. Beutler
4. An improved Active Islanding Detection Technology for Grid-connected Solar Photovoltaic System by H. T. Yang, P. C. Peng, T. Y. Tsai, and Y. Y. Hong 5. Scenario of renewable energy potential in india by Sagar N. Patel, Ishan P. Joshi, B.S.Pillai
Website links
1. http://www.sandia.gov/pv/docs/Design_and_Installation_of_PV_Systems.htm 2. http://photovoltaics.sandia.gov/docs/Design_and_Installation_of_PV_Systems.htm 3. http://www.teriin.org/index.php?option=com_content&task=view&id=62 4. http://www.teriin.org/images/Solar-desalinationMAP.jpg 5. http://www.india-reports.com/summary/map-solar.aspx 6. http://www.energy.ca.gov/reports/2001-09-04_500-01-020.PDF
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