A solar inverter or PV inverter is a type of electrical converter that converts the direct variable current (DC) output of a photovoltaic (PV) solar panel into an alternating utility frequency (AC) that can be fed into a commercial electrical grid or used by a local, off-grid electrical grid. It is a vital system balance (BOS)–component in a photovoltaic network, allowing the use of ordinary AC-powered devices. Solar power inverters have special functions tailored for use with photovoltaic arrays including optimum monitoring of the power point and protection against insulation.
Solar inverters may be classified into three broad types:
- Stand-alone inverters, used in shielded systems where the inverter draws its DC energy from photovoltaic array charged batteries. Many stand-alone inverters also integrate integral battery chargers, where appropriate, to replenish the battery from an AC source. They do not usually communicate with the power grid in any way, and as such are not expected to provide anti-insulation protection.
- Grid-tie inverters, which process matches a sine wave supplied by the utility. For safety purposes, grid-tie inverters are designed to automatically shut down upon loss of power supply. Throughout utility outages, they don’t have backup power.
- Battery backup inverters are special inverters designed to extract battery energy, control battery charging through an onboard converter, and export excess energy to the power grid. Such inverters are capable of supplying AC energy during a service outage to selected loads and require anti-islanding protection.
- Intelligent hybrid inverters manage photovoltaic collection, battery storage and power grid all of which are directly connected to the device. Such modern all-in-one systems are typically highly flexible and can be used for grid-tie, stand-alone or backup applications, but their primary purpose with storage is self-consumption.
Maximum power point tracking.
Solar inverters use full power point tracking (MPPT) to get as much power out of the PV array as possible. Solar cells have a complex relationship between solar irradiation, temperature, and total resistance, resulting in a non-linear performance efficiency called the I-V curve. The MPPT device is intended to measure the output of the cells and to establish a resistance (load) in order to achieve full power for any given environmental conditions.
The fill factor, more commonly known by its abbreviation FF, is a parameter that determines the maximum power from a solar cell in conjunction with the panel’s open circuit voltage (Voc) and short circuit current (Isc). Fill factor is known as the ratio of the solar cell to the Voc and Isc product with the maximum power.
Three key types of MPPT algorithms exist: perturbation and observation, gradual conductance, and constant voltage. The first two methods are also referred to as methods of hill-climbing; they depend on the power curve plotted against voltage rising to the left of the maximum power point, and dropping to the right.
As of 2014, conversion efficiency exceeded more than 98 percent for state-of-the-art solar converters. Though string inverters are used in residential to medium-sized commercial PV systems, the wide commercial and utility market is served by central inverters. Market share for central and string inverters is roughly 50% and 48% respectively, leaving less than 2% to micro-inverters.
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