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 AC-powered equipment. Solar inverters have special functions adapted for use with photovoltaic arrays, including maximum power point tracking and anti-islanding protection.
Solar inverters may be classified into three broad types:
- 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
Solar inverters use maximum power point tracking (MPPT) to get the maximum possible power from the PV array. 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.
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.
There are three main types of MPPT algorithms: perturb-and-observe, incremental conductance and constant voltage. 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.
Solar micro-inverter is an inverter designed to operate with a single PV module. The micro-inverter converts the direct current output from each panel in to alternating current. Its design allows parallel connection of multiple, independent units in a modular way.
Micro-inverter advantages reside mostly on single panel power optimization, independent operation of each panel, plug-and play installation, improved installation and fire safety, minimized costs with system design and stock minimization.
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.
Grid tied solar inverters
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.
Historically, there have 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. 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
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
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.
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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:
- Electrolytic materials age faster than polycarbonate and other dry dielectric materials
- Voltage stress
- Continuous operation under maximum voltage conditions
- Frequent short-term voltage transients
- Current stress
- High current increases the internal temperature
- Thermal stress on component terminals
- Improper Charge and discharge rates
- Not operating in ambient temperatures
- Mechanical stress
Inverter bridge failure
- Usage beyond its rated operating limit
- Overcurrent and overvoltage
- Other malfunctioning components
- Thermal shock
- Thermal overload
- Extremely cold operating temperature
- Component stress
- Contamination at contacts
- Extreme temperature conditions
- Ultrasonic vibration originating in (magnetic cores of) inductive components
- Grid tie inverter
- Inverter (electrical)
- Solar module
- Solar cell
- Battery charger
- Fill factor
- Power optimizer
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- Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques
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- Photovoltaics: Design and Installation Manual. Newsociety Publishers. 2004. p. 80.
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