Solar inverter


A solar inverter or photovoltaic inverter is a type of power inverter which converts the variable direct current output of a photovoltaic solar panel into a utility frequency alternating current 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 –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:
  1. 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.
  2. 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.
  3. 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 electricity from solar panels or batteries into alternating current 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.
  4. 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.
  5. 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 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 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.

Grid tied solar inverters

The key role of the grid-interactive or synchronous inverters or simply the grid-tie inverter 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 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.

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, 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.

Three-phase-inverter

A three-phase inverter is a type of solar microinverter specifically designed to supply three-phase electric power. In conventional microinverter designs that work with one-phase power, the energy from the panel must be stored during the period where the voltage is passing through zero, which it does twice per cycle. In a three-phase system, throughout the cycle, one of the three wires has a positive voltage, so the need for storage can be greatly reduced by transferring the output of the panel to different wires during each cycle. The reduction in energy storage significantly lowers the price and complexity of the converter hardware, as well as potentially increasing its expected lifetime.

Concept

Background

Conventional alternating current power is a sinusoidal voltage pattern that repeats over a defined period. That means that during a single cycle, the voltage passes through zero two times. In European systems, the voltage at the plug has a maximum of 230 V and cycles 50 times a second, meaning that there are 100 times a second where the voltage is zero, while North American-derived systems are 120 V 60 Hz, or 120 zero voltages a second.
Inexpensive inverters can convert DC power to AC by simply turning the DC side of the power on and off 120 times a second, inverting the voltage every other cycle. The result is a square-wave that is close enough to AC power for many devices. However, this sort of solution is not useful in the solar power case, where the goal is to convert as much of the power from the solar power into AC as possible. If one uses these inexpensive types of inverters, all of the power generated during the time that the DC side is turned off is simply lost, and this represents a significant amount of each cycle.
To address this, solar inverters use some form of energy storage to buffer the panel's power during those zero-crossing periods. When the voltage of the AC goes above the voltage in the storage, it is dumped into the output along with any energy being developed by the panel at that instant. In this way, the energy produced by the panel through the entire cycle is eventually sent into the output.
The problem with this approach is that the amount of energy storage needed when connected to a typical modern solar panel can only economically be provided through the use of electrolytic capacitors. These are relatively inexpensive but have well-known degradation modes that mean they have a life expectancy on the order of a decade. This has led to a great debate in the industry over whether or not microinverters are a good idea, because when these capacitors start to fail at the end of their expected life, replacing them will require the panels to be removed, often on the roof.

Three-phase

In comparison to normal household current on two wires, current on the delivery side of the power grid uses three wires and phases. At any given instant, the sum of those three is always positive. So while any given wire in a three-phase system undergoes zero-crossing events in exactly the same fashion as household current, the system as a whole does not; it simply fluctuates between the maximum and a slightly lower value.
A microinverter designed specifically for three-phase supply can eliminate much of the required storage by simply selecting which wire is closest to its own operating voltage at any given instant. A simple system could simply select the wire that is closest to the maximum voltage, switching to the next line when that begins to approach the maximum. In this case, the system only has to store the amount of energy from the peak to the minimum of the cycle as a whole, which is much smaller both in voltage difference and time.
This can be improved further by selecting the wire that is closest to its own DC voltage at any given instant, instead of switching from one to the other purely on a timer. At any given instant, two of the three wires will have a positive voltage and using the one closer to the DC side will take advantage of slight efficiency improvements in the conversion hardware.
The reduction, or outright elimination, of energy storage requirements simplifies the device and eliminates the one component that is expected to define its lifetime. Instead of a decade, a three-phase microinverter could be built to last for the lifetime of the panel. Such a device would also be less expensive and less complex, although at the cost of requiring each inverter to connect to all three lines, which possibly leads to more wiring.