In this blog, we will cover the common types of Grid-Tied or Grid Connected Solar Inverters used in roof-top Solar Power Plants: String Inverters, SolarEdge Optimizer System, and Enphase Micro-inverter System.
Solar Power Plants that use only utility grid as a complementary source of power are called grid-tied or grid-connected systems. In a grid-tied system whenever there is a shortfall in power production from solar due to external conditions, the grid power will compensate the shortfall. Similarly, if the power production by a solar PV system installed in a facility, is more than that of the consumption in the facility, based on inverter setting the excess power is
Either pumped back to the grid by the inverter as export
Or the inverter will limit the power production from Solar PV so that there is no export of Power
Those who needs an introduction to different types of Solar Power Plant can read our blog on the same here.
Key Functions of a Simplified Solar Inverter
For the sake of simplicity, A Solar Inverters function can be simplified into two parts:
DC Power Extraction: Extracting DC Power from the Solar Panels efficiently is one of the key functions of a Solar Inverter. A circuitry with a Maximum Power Point Tracking (MPPTs) algorithm is responsible for this function. A Solar Inverter can have multiple MPPTs depending on it's design.
Inversion of Power from DC to AC: The extracted DC power needs to be converted to AC at a suitable voltage and frequency acceptable by the Grid. Most of the inverters will have the functionality to configure a grid-profile as per the standards set by the power utility in the region.
Please note that a few key functions such as data monitoring and safety features are not covered, as we want to focus on the above two functions to classify the different Grid connected Solar Inverters.
String Inverter
In a string inverter system, the DC Power Extraction Unit (MPPT) and the Inversion unit is bundled in a single box. In such systems solar modules are connected in series using DC
cables to form DC strings of high DC voltage (up to 1000 V / 1500 V). The strings are then connected in parallel to the MPPT of the Inverter. In a string Inverter, a single MPPT typically manages a large number of modules.
Based on site conditions, due to multiple factors such as shading, soiling, moving clouds, difference in module surface temperature, and irradiance, the production among Solar modules in a power plant will vary. Since the modules are connected in series, and current in a DC string is constant, a string inverter is not very efficient in handling mismatch in solar modules production. The string inverters are not considered safe compared to other inverter architectuers due to the presence of high voltage DC. High voltage DC is a fire hazard, if the system is not of good quality or is not properly installed or maintained.
The life of a string inverter is typically between 7 to 10 years. In a string inverter any inverter failure will cause a down time for the entire power plant. Most of the string Inverters provides cloud based monitoring application for total Power production.
Despite, it's disadvantages String Inverters are widely deployed in developing markets such as India due to their cost effectiveness.
Microinverters
Microinverters as the name indicate are miniaturized inverters. Enphase pioneered the Microinverter technology in 2006.
In micro-inverter architecture each Solar Module is connected to one Microinverter. The micro-inverters are then connected in parallel to form AC branches. Micro-inverters architecture is highly efficient, reliable and safe compared to traditional inverter architecture.
Since each solar module is equipped with a micro-inverter, the system can effectively handle any mismatch in solar power production among solar modules in a Solar Power Plant. ENPHASE micro-inverters offer a product warranty of 25 years. The micro inverter architecture is very safe as the system doesn't have presence of high voltage DC. An inverter failure in a micro-inverter system only impacts that Inverter and module, and while the rest of the system function without any problems. The system also offers high modularity as a single unit of micro and panel can be added to the system at a later stage.The only draw back of micro-Inverters is that the initial investment required for micro-inverters is relatively high compared to other architectures.
In micro-inverters, end customers can monitor individual solar modules power production. This ensures ease in maintenance and warranty claims.
Optimized String Inverter System
We cannot speak of optimized String Inverter system without mentioning SolarEdge. SolarEdge established in 2006, pioneered the optimizer Technology.
In the optimized string inverter system individual units called Optimizers responsible for DC Power extraction is installed under each Solar Module or pair of Solar Modules. The DC optimizers are connected in series to form DC Strings. Optimizers work at a constant string voltage, and maximize extraction of DC power from individual modules more or less independently. The optimized DC Power is fed to an Inversion unit for conversion to AC and synchronization with the grid.
DC optimizers managed to reduce the cost of the system while optimizing the impact of module mismatch losses to some extend. SolarEdge gives a product warranty of 25 years for Optimizers (Approximately 50% of Total Inverter Cost) and 12 years product Warranty for Inversion unit (the other 50% of Total Inverter Cost).SolarEdge inverters come with safety features such as DC Arc detection and Safe DC. The Safe DC feature reduce voltage in a string to less than 60 V once, the Inversion unit is turned off.
In case of SolarEdge an optimizer failure will only impact a single module or a pair of modules (if we use the optimizer which can be wired to two modules). But an Inversion unit’s failure will impact the whole system. SolarEdge Inverters have a solar module or a pair of module level monitoring of system performance.
At Cares Renewables, we are focused on delivering quality solar power plants to our customers and partners. Please leave a comment, if you have any queries on this blog. Visit https://www.caresrenewables.com/savegen to view a few of our case studies.