Solar Inverters

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

Classification :

    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 (MPPT) :

Maximum power point tracking is a technique that solar inverters use 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 apply a resistance (load) to obtain maximum power for any given environmental conditions.[citation needed] Essentially, this defines the current that the inverter should draw from the PV in order to get the maximum possible power (since power equals voltage times current).

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, because they depend on the fact that on the left side of the MPP, the curve is rising (dP/dV > 0) while on the right side of the MPP the curve is falling (dP/dV < 0).

Anti-islanding protection :

Normally, grid-tied inverters will shut off if they do not detect the presence of the utility grid.[citation needed] If, however, there are load circuits in the electrical system that happen to resonate at the frequency of the utility grid, the inverter may be fooled into thinking that the grid is still active even after it had been shut down. This is called islanding.

An inverter designed for grid-tie operation will have anti-islanding protection built in; it will inject small pulses that are slightly out of phase with the AC electrical system in order to cancel any stray resonances that may be present when the grid shuts down.

Since 1999, the standard for anti-islanding protection in the United States has been UL 1741, harmonized with IEEE 1547.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.

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.

Types of anti-islanding protection:

There are two types of anti-islanding control techniques:

Passive:

The voltage and/or the frequency change during the grid failure is measured and a positive feedback loop is employed to push the voltage and /or the frequency further away from its nominal value. Frequency or voltage may not change if the load matches very well with the inverter output or the load has a very high quality factor (reactive to real power ratio). So there exists some Non Detection Zone (NDZ). 

Active:
This method employs injecting some error in frequency or voltage. When grid fails, the error accumulates and pushes the voltage and/or frequency beyond the acceptable range.

 

Solar micro-inverters:

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).

Main article: Solar micro-inverter

Solar micro-inverters convert direct current (DC) from a single solar panel to alternating current (AC). The electric power from several micro-inverters is combined and sent to the consuming devices. The key feature of a micro-inverter is not its small size or power rating, but its one-to-one control over a single panel and its mounting on the panel or near it which allows it to isolate and tune the output of that panel.[citation needed] This also makes expanding the solar power system much easier since additional panels with microinverters can be added to the existing system without as much system change as in upgrading a conventional system with solar inverters.

Microinverters produce grid-matching power directly at the back of the panel. Arrays of panels are connected in parallel to each other and fed to the grid. This has the major advantage that a single failing panel or inverter will not take the entire string offline. Combined with the lower power and heat loads, and improved MTBF, it is suggested that overall array reliability of a microinverter-based system will be significantly greater than a string-inverter based one. Additionally, when faults occur, they are identifiable to a single point, as opposed to an entire string. This not only makes fault isolation easier, but unmasks minor problems that might never become visible otherwise - a single underperforming panel may not affect a short string's output enough to be noticed.

Microinverters have become common where array sizes are small and maximizing performance from every panel is a concern. In these cases the differential in price-per-watt is minimized due to the small number of panels, and has little effect on overall system cost. The improvement in energy collection given a fixed size array can offset this difference in cost. For this reason, microinverters have been most successful in the residential market, where the limited space for panels constraints the array size, and shading from nearby trees or other homes is often an issue. Micro-inverter manufacturers list many installations, some as small as a single panel and the majority under 50.

Another prime reason for the popularity of microinverters is the expandability. A solar array can start with as little as 1 panel & gradually have additional added to it with no maximum size. This is important as a standard string inverter will have to be replaced if the array grows outside of its capabilities.

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.

The issue at stake currently is that there are concerns about having transformerless electrical systems feed into the public utility grid since the lack of galvanic isolation between the DC and AC circuits could allow the passage of dangerous DC faults to be transmitted to the AC side.

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.

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.

 

Inverter failure:

Solar inverters may fail due to transients from the grid or the PV panel, component aging and operation beyond the designed limits.