Solar power Inverters

Grid Tied Inverters

Inverters work by taking the DC power from the source, such as an array of photovoltaic modules (typically 250 to 600 volts DC) or micro hydroelectric turbines, and inverting it to AC power so it can be fed into the grid. The inverter must also synchronize its frequency with that of the grid (e.g. 60 Hz) using a local oscillator and limit the voltage to no higher than the grid voltage. Typical modern GTI’s have a fixed unity power factor, which means its output voltage and current are perfectly lined up, and its phase angle is within 1 degree of the AC power grid. The inverter has an on board computer which will sense the current AC grid waveform, and output a voltage to correspond with the grid.
Grid-tie inverters are also 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 even use no transformer. Instead of converting DC 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. Transformerless inverters, which boast lighter weight and higher efficiencies than their counterparts with transformers, are popular in Europe. However, transformerless inverters have been slow to enter the US market. Until 2005, NEC code required all solar electric systems to be negative grounded, an electrical configuration that interferes with the operation of transformerless inverters. 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.
Most solar grid-interactive inverters on the market include a maximum power point tracker that enables the inverter to extract an optimal amount of power from the solar array by tracking the array’s maximum power point.


Micro Inverters

A solar photovoltaic micro-inverter is a device that converts direct current (DC) from a single solar module (panel) to alternating current (AC).
Unlike a central or string inverter that aggregates and converts the power generated by the entire array of solar modules, a micro-inverter converts the DC power from a single solar module to AC. The concept of panels delivering AC power has appeal for small-scale home project applications at lower voltage levels.

Disadvantages of Micro Inverters

Inverters are generally acknowledged to fail at a higher rate than other components in a photovoltaic system. The warranty of an inverter is typically 10 years, whereas the rest of the system can be warranted for up to 25 years. Thus inverters are most-often rated for shorter warranties than other components. Detractors of micro-inverter technology claim it is not advisable to distribute the least reliable component of the PV system to every module, as the failure rate of an inverter at the module level multiplies failure points. Another issue is that the Micro Inverters would be located on the roof and daily temperature cycling would be detrimental to the long term reliability of the inverter.