Inverters function is to transform a DC input into an AC output. That goes through a chain of sequences where the DC input is chopped in a square wave and later modified into a sine wave, cleared of harmonics and synchronized with the grid by amplifying the same harmonics which are to be found on the grid line at that very moment. The full process is called PWM - Pulse with modulation. The bellow table illustrates the wave types an invertor is generating at different moments when is reversing a DC source into an AC source.
THD = The total harmonic distortion:-
Modified sine wave:-
The output of a modified square wave or quasi square is similar to a square wave output except that the output goes to zero volts before switching positive or negative. This process it is simple and it comes at a low cost (~$0.10USD/Watt) In the same time it is compatible with most electronic devices we suppose to energize using this source, except sensitive or specialized equipment: laser printers, fluorescent lighting, audio equipment. Most AC motors will run using this type of power source albeit at a reduction in efficiency of approximately 20%.
Pure sine wave:-
A pure sine wave inverter produces a nearly perfect sine wave output (less than 3% total harmonic distortion) that is essentially same as utility-supplied grid sin wave power. Thus it becomes compatible with all AC electronic devices. This is the type of wave used in grid-tie inverter. It's design is more complex and will costs more per unit power. The electrical inverter is a high-power electronic oscillator. It is named Inverter because early mechanical AC to DC converters were made to work in reverse and were named "inverted" when they where used to convert DC to AC.
Inverters used in a PV Power Plant can be classified into three broad types:-
Stand-alone inverters:- used in isolated systems where the inverter draws its DC energy from batteries banks 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 are not required to have anti-islanding protection.
Grid tie inverters:- which match with a utility-supplied sine wave source, the grid source coming to the dwelling. 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. Some of the grid tie inverters are designed to work together with power maximizes at panel level. Those maximizes are monitoring each panel individual at MPPT level and are incorporating security features for the PV panels as well.
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. Off grid only inverters are capable of supplying AC energy to selected loads during a utility outage, and are required to have anti-islanding protection. Other inverters are designed to work grid tie and have a battery bank as a backup. In some extreme cases this inverters are incorporating a battery charger connected to the inverter MPPT circuit.
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 will produce a non-linear output known as I-V curve. It is the purpose of the MPPT system to sample the output of the solar panel and apply a resistance (load) to obtain maximum power for any given environmental conditions. 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).
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.
Galvanic Isolation between the AC and DC sections of the solar power plant:-
Grid tied inverters, the majority of the inverters sold on the market, are incorporating high frequency components as IGBTs or thyristors and some of the manufacturer are supplying the chopping bridges without a transformer. It is a transformerless AC power generation. That it means it is not physical separation between the DC side [strings and dc power optimizers] and the AC side [grid tied connected]. There are concerns about having transformerless electrical systems feed into the public utility grid because of 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. There for an additional transformer on the AC side will do just that, galvanic separate the DC from AC. This solution comes with an economical cost: copper and additional weight.
Inverter intercommunication and monitoring:-
Some models include a communications “link,” which allows interconnecting multiple inverters to provide synchronized higher wattage and voltage outputs. In this situation we will have a master inverter which monitors the well being of a chain of similar inverters.
High efficiency conversion:-
One of the major component to maximizing the power output of a solar installation is to utilize high efficiency conversion. Efficiency is the measure of power out of the inverter as a percentage of the power into the inverter. Thus, high efficiency PV inverters use less of power in the conversion process and supply more of the power for use.
The information posted herein has been compiled by Clean Energy Brands from OEM product data and reputable publications. All rights reserved!