Solar inverter

Internal view of a solar inverter.

A solar inverter or PV inverter is a critical component in a photovoltaic system. It converts the variable DC output of the solar panel into a utility frequency alternating 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.

A PV inverter installed in an attic

Classification

Simplified schematics of a grid-connected residential PV power system

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 (MPPT)

I-V curve for a solar cell, showing the maximum power point Pmax.

Main article: Maximum power point tracker

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

Main article: Islanding

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. 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.

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.^{[citation needed]}

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 long 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

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.

While there have historically 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; Since 2005, the NFPA's NEC allow 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 typical solar charge controller kit

See also: 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

^{[citation needed]}

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

• 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
• Vibrations

Inverter bridge failure

• Usage beyond its rated operating limit
• Overcurrent and overvoltage
• Other malfunctioning components
• Thermal shock
• Thermal overload
• Extremely cold operating temperature

Electro-mechanical wear

• Component stress
• Contamination at contacts
• Extreme temperature conditions

See also

• Grid tie inverter
• Inverter (electrical)
• Solar module
• Solar cell
• Battery charger
• Fill factor
• Power optimizer

External links

• List of Eligible Inverters per SB1 Guidelines (Go Solar California)
• Model based control of solar power inverter with battery backup Description and working VisSim source code diagram for digital control of solar power inverter.