Solar inverter

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Internal view of a solar inverter. Note the many large capacitors (blue cylinders), used to store power briefly and improve the output waveform.

A solar inverter, or PV inverter, converts the variable direct current (DC) output of a photovoltaic (PV) solar panel into a utility frequency alternating current (AC) that can be fed into a commercial electrical grid or used by a local, off-grid electrical network. It is a critical component in a photovoltaic system, allowing the use of ordinary commercial appliances. Solar inverters have special functions adapted for use with photovoltaic arrays, including maximum power point tracking and anti-islanding protection.

Classification[edit]

Simplified schematics of a grid-connected residential photovoltaic power system[1]

Solar inverters may be classified into three broad types:[citation needed]

  • Stand-alone inverters, used in isolated systems where the inverter draws its DC energy from batteries charged by photovoltaic arrays. Many stand-alone inverters also incorporate integral battery chargers to replenish the battery from an AC source, when available. Normally these do not interface in any way with the utility grid, and as such, are not required to have anti-islanding protection.
  • Grid-tie inverters, which match phase with a utility-supplied sine wave. Grid-tie inverters are designed to shut down automatically upon loss of utility supply, for safety reasons. They do not provide backup power during utility outages.
  • Battery backup inverters, are special inverters which are designed to draw energy from a battery, manage the battery charge via an onboard charger, and export excess energy to the utility grid. These inverters are capable of supplying AC energy to selected loads during a utility outage, and are required to have anti-islanding protection.

Maximum power point tracking[edit]

Solar inverters use maximum power point tracking (MPPT) to get the maximum possible power from the PV array.[2] Solar cells have a complex relationship between solar irradiation, temperature and total resistance that produces a non-linear output efficiency known as the I-V curve. It is the purpose of the MPPT system to sample the output of the cells and determine a resistance (load) to obtain maximum power for any given environmental conditions.[3]

The fill factor, more commonly known by its abbreviation FF, is a parameter which, in conjunction with the open circuit voltage and short circuit current of the panel, determines the maximum power from a solar cell. Fill factor is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc.[4]

There are three main types of MPPT algorithms: perturb-and-observe, incremental conductance and constant voltage.[5] The first two methods are often referred to as hill climbing methods; they rely on the curve of power plotted against voltage rising to the left of the maximum power point, and falling on the right.[6]

Anti-islanding protection[edit]

In the event of a power failure on the grid, it is generally required that any grid-tie inverters attached to the grid turn off in a short period of time. This prevents the inverters from continuing to feed power into small sections of the grid, known as "islands". Powered islands present a risk to workers who may expect the area to be unpowered, but equally important is the issue that without a grid signal to synchronize to, the power output of the inverters may drift from the tolerances required by customer equipment connected within the island.

Detecting the presence or lack of a grid source would appear to be simple, and in the case of a single inverter in any given possible physical island (between disconnects on the distribution lines for instance) the chance that an inverter would fail to notice the loss of the grid is effectively zero. However, if there are two inverters in a given island, things become considerably more complex. It is possible that the signal from one can be interpreted as a grid feed from the other, and vice versa, so both units continue operation. As they track each other's output, the two can drift away from the limits imposed by the grid connections, say in voltage or frequency.

There are a wide variety of methodologies used to detect an islanding condition. None of these are considered fool-proof, and utility companies continue to impose limits on the number and total power of solar power systems connected in any given area. However, many in-field tests have failed to uncover any real-world islanding issues, and the issue remains contentious within the industry.

Since 1999, the standard for anti-islanding protection in the United States has been UL 1741, harmonized with IEEE 1547.[7] Any inverter which is listed to the UL 1741 standard may be connected to a utility grid without the need for additional anti-islanding equipment, anywhere in the United States or other countries where UL standards are accepted.[8]

Similar acceptance of the IEEE 1547 in Europe is also taking place, as most electrical utilities will be providing or requiring units with this capability.[9]

Solar micro-inverters[edit]

A solar micro-inverter in the process of being installed. The ground wire is attached to the lug and the panel's DC connections are attached to the cables on the lower right. The AC parallel trunk cable runs at the top (just visible).

Solar micro-inverter is an inverter integrated to each solar panel module. The inverter converts the output from each panel to alternating current They're designed to allow parallel connection of multiple units connected in parallel.[10]

Each integrated module provides AC output and are connected together in parallel. This arrangement provides easier installation, redundancy and more effective capture of energy when they're partially shaded. As of 2010, they're mainly used for single phase applications and most units in production relied exclusively on electrolytic capacitors for buffering and there is a concern of long term reliability of these capacitors in each module.[11] A 2011 study at Appalachian State University reports that individual integrated inverter setup yielded about 20% more power in unshaded conditions and 27% more power in shaded conditions compared to string connected setup using one inverter. Both setups used identical solar panels.[12]

Grid tied solar inverters[edit]

An industrial grid-tied solar inverter
A PV inverter installed in a porch

Solar grid-tie inverters are designed to quickly disconnect from the grid if the utility grid goes down. This is an NEC requirement that ensures that in the event of a blackout, the grid tie inverter will shut down to prevent the energy it produces from harming any line workers who are sent to fix the power grid.

Grid-tie inverters that are available on the market today use a number of different technologies. The inverters may use the newer high-frequency transformers, conventional low-frequency transformers, or no transformer. Instead of converting direct current directly to 120 or 240 volts AC, high-frequency transformers employ a computerized multi-step process that involves converting the power to high-frequency AC and then back to DC and then to the final AC output voltage.[13]

Historically, there has been concerns about having transformerless electrical systems feed into the public utility grid. The concerns stem from the fact that there is a lack of galvanic isolation between the DC and AC circuits, which could allow the passage of dangerous DC faults to be transmitted to the AC side.[14] Since 2005, the NFPA's NEC allows transformerless (or non-galvanically) inverters. The VDE 0126-1-1 and IEC 6210 also have been amended to allow and define the safety mechanisms needed for such systems. Primarily, residual or ground current detection is used to detect possible fault conditions. Also isolation tests are performed to insure DC to AC separation.

Many solar inverters are designed to be connected to a utility grid, and will not operate when they do not detect the presence of the grid. They contain special circuitry to precisely match the voltage and frequency of the grid. See the Anti-Islanding section for more details.

Solar charge controller[edit]

A typical solar charge controller kit

A charge controller may be used to power DC equipment with solar panels. The charge controller provides a regulated DC output and stores excess energy in a battery as well as monitoring the battery voltage to prevent under/over charging. More expensive units will also perform maximum power point tracking. An inverter can be connected to the output of a charge controller to drive AC loads.

Solar pumping inverters[edit]

Advanced solar pumping inverters convert DC voltage from the solar array into AC voltage to drive submersible pumps directly without the need for batteries or other energy storage devices. By utilizing MPPT (maximum power point tracking), solar pumping inverters regulate output frequency to control speed of the pumps in order to save pump motor from damage.

Inverter failure[edit]

Solar inverters may fail due to transients from the grid or the PV panel, component aging and operation beyond the designed limits. Following are some common reasons specific components of inverters age quickly or fail:

Capacitor failure[edit]

Inverter bridge failure[edit]

Electro-mechanical wear[edit]

  • Component stress
  • Contamination at contacts
  • Extreme temperature conditions
  • Ultrasonic vibration originating in (magnetic cores of) inductive components

See also[edit]

References[edit]

  1. ^ Solar Cells and their Applications Second Edition, Lewis Fraas, Larry Partain, Wiley, 2010, ISBN 978-0-470-44633-1 , Section10.2.
  2. ^ "Invert your thinking: Squeezing more power out of your solar panels". scientificamerican.com. Retrieved 2011-06-09. 
  3. ^ Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques
  4. ^ Benanti, Travis L.; Venkataraman, D. (25 April 2005). "Organic Solar Cells: An Overview Focusing on Active Layer Morphology". Photosynthesis Research 87 (1): 77. doi:10.1007/s11120-005-6397-9. Retrieved 27 August 2013. 
  5. ^ "Evaluation of Micro Controller Based Maximum Power Point Tracking Methods Using dSPACE Platform". itee.uq.edu.au. Retrieved 2011-06-14. 
  6. ^ Comparative Study of Maximum Power Point Tracking Algorithms. doi:10.1002/pip.459. 
  7. ^ "How Inverters Work". solar.gwu.edu. p. 3. Retrieved 2011-06-10. 
  8. ^ "UL 1741 UPDATE A SAFETY STANDARD FOR DISTRIBUTED GENERATION". eere.energy.gov. Retrieved 2011-06-10. 
  9. ^ "Hi‐MW Electronics – One key to a Future Grid which is Smarter, Greener, more Robust and more Reliable". nist.gov. Retrieved 2011-06-10. 
  10. ^ http://dspace.mit.edu/bitstream/handle/1721.1/61308/702674669.pdf
  11. ^ Pierquet, Brandon J., and David J. Perreault. (2010). "A Single-Phase Photovoltaic Inverter Topology with a Series-Connected Power Buffer". IEEE: 2811. doi:10.1109/ECCE.2010.5618166. Retrieved 27 August 2013. 
  12. ^ "A SIDE-BY-SIDE COMPARISON OF MICRO AND CENTRAL INVERTERS IN SHADED AND UNSHADED CONDITIONS". Retrieved 27 August 2013. 
  13. ^ Photovoltaics: Design and Installation Manual. Newsociety Publishers. 2004. p. 80. 
  14. ^ "Summary Report on the DOE High-tech Inverter Workshop". Sponsored by the US Department of Energy, prepared by McNeil Technologies. eere.energy.gov. Retrieved 2011-06-10. 

External links[edit]