PV Micro Inverter

PV Micro Inverter

A micro-inverter transforms the direct current (AC) coming from a single photovoltaic panel into AC, or the direct voltage into alternating current (AC), so that micro-inverters mostly are specified for the 200 - 300 W power range. Several micro-inverters can be bundled together to feed the electricity of several solar panels into the AC network. Therefore a lower performance of a single panel - for example, through temporary shading - does not affect the overall output performance of the photovoltaic system so strongly as when using a large inverter, which is connected to a number of solar panels connected in series. Many of the technical challenges that occur with large inverters can be much easier to control with micro inverters: from cooling to the size and dimensions of components. Since the passive components are now a key factor determining the total cost of the inverter, the use of smaller capacitors and other passive components will also affect the total cost of the system. However, large inverters, which are composed of several micro inverters, are usually considerably more expensive than a corresponding larger inverter.

PV Inverter

The connections of several such panel series circuits terminate in larger PV inverters

A photovoltaic (PV) inverter transforms the direct current (AC) coming from a single photovoltaic panel into AC, or the direct voltage into alternating current (AC) or alternating voltage. The individual panels are connected in series so that a sufficiently high voltage to the extent of several hundred volts is delivered at the end of the series. The connections of several such panel series circuits terminate in larger PV inverters. The task of the PV inverter is then to convert the voltage of each individual series-connected branch into alternating current and to feed the electrical energy with the respective mains voltage (for PV installations on homes, for example, 230 V / 50 Hz) into the access network. Efficiency is a very important aspect in the development of inverter designs, because ultimately the energy in the PV panel that has been ‘recovered’ from the sunlight should be fed into the grid with the minimum amount of loss, in order to minimise heat and maximise the inputted amount of energy. Since the efficiency of the PV inverter varies with the currently changing instantaneous power value, it is necessary to put in place circuitry measures to ensure that the inverter is working whenever possible at the point when it has the highest efficiency. This is done using MPPT technology, whereby MPPT stands for Maximum Power Point Tracking. Due to the relatively high performance levels which PV inverters are working with, they need active cooling in the majority of cases, so the noise level generated by the cooling fan certainly plays a part in the choice of the place where the PV inverter is installed. 


PV Junction Box

A PV junction box is required for each PV module

This is typically attached to the rear of the PV module. Several PV modules are operated in series, so a diode (freewheeling or bypass type) must be connected in anti-parallel to each module. The freewheeling diode is connected across the terminals of each module so that it is reverse biased in normal operating conditions (where the module delivers current). If the module does not provide any current because of shading or defect, the photodiode, which is now reverse biased, would decommission a string that consists of several solar modules connected in series. If the voltage of the series-connected solar modules that are functioning and irradiated block the voltage of the non-irradiated solar modules, this might even lead to their destruction. Since the other cells continue to supply current, overheating occurs, which can lead to a module fire. The freewheeling diode prevents this, allowing the current to flow through the freewheeling diode. A string can therefore continue to deliver an electrical power supply, albeit smaller. The current PV modules typically have 60 solar cells, where they are connected in series in 3 groups of 20 solar cells. Therefore, several freewheeling diodes are located in the PV junction box, so that partial shading does not mean that the whole module is decommissioned/short-circuited.