Solar inverter

A solar inverter or photovoltaic (PV) inverter is a type of power inverter which converts the variable direct current (DC) output of a photovoltaic 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 balance of system (BOS)–component in a photovoltaic system, allowing the use of ordinary AC-powered equipment. Solar power inverters have special functions adapted for use with photovoltaic arrays, including maximum power point tracking and anti-islanding protection.

Classification

Solar inverters may be classified into four broad types:

Stand-alone inverters, used in stand-alone power 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. Off-grid inverters, also known as stand-alone inverters, are designed for use in power systems that operate independently of the utility grid. These inverters convert direct current (DC) electricity from solar panels or batteries into alternating current (AC) for use in homes, cabins, or remote areas without access to grid power. They typically rely on battery storage systems, which are charged by photovoltaic arrays, and are capable of powering AC loads directly. Off-grid inverters do not require anti-islanding protection, as they are not connected to the grid. Battery backup inverters are special inverters that 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. Intelligent hybrid inverters manage photovoltaic array, battery storage and utility grid, which are all coupled directly to the unit. These modern all-in-one systems are usually highly versatile and can be used for grid-tie, stand-alone or backup applications but their primary function is self-consumption with the use of storage.

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 (Voc) and short circuit current (Isc) 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.

Grid tied solar inverters

The key role of the grid-interactive or synchronous inverters or simply the grid-tie inverter (GTI) is to synchronize the phase, voltage, and frequency of the power line with that of the grid. Solar grid-tie inverters are designed to quickly disconnect from the grid if the utility grid goes down. In the United States, for example, 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 the AC side. Since 2005, the NFPA's NEC allows transformer-less (or non-galvanically isolated) inverters. The VDE 0126-1-1 and IEC 6210 have also 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 ensure 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, frequency and phase of the grid. When a grid is not detected, grid-tie inverters will not produce power to avoid islanding, which can cause safety issues. In addition, advanced solar inverters now have online diagnostics that can be used to detect damaged photovoltaic modules.

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 the speed of the pumps in order to save the pump motor from damage. Solar pumping inverters usually have multiple ports to allow the input of DC current generated by PV arrays, one port to allow the output of AC voltage, and a further port for input from a water-level sensor.

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