Inverter is a device which basically converts DC into AC. In the PV system, solar inverter are used to convert DC supply coming from Solar Panel into AC supply. Then this AC supply is used to run the load.
Based on the connection of PV system with grid, the Solar inverters are also divided into two types:
- Utility Interactive Inverter
- Stand Alone Inverter
1. UTILITY INTERACTIVE INVERTER
The inverters which is used in ON-Grid PV system, are known as utility interactive inverters. ON Grid PV system is a type of PV system which is connected with the grid. Utility interactive inverter is a most common used inverter in PV system. These type of inverter are also divided into two types:
- Grid Direct
- Battery Based
Grid Direct Inverter
The grid direct inverter is a type of utility interactive inverter which is directly connected with grid without any storage for backup. These inverter convert DC supply coming from PV array into AC and send it to grid directly. These type of inverter are not used to charge the batteries. The voltage range of grid direct inverter is between 150V to 600V.
Battery Based Inverter
The battery based inverter is an another type of utility interactive inverter. In these inverters, DC supply coming from PV array is used to charge the batteries and then DC is converted into AC to send it into grid. Batteries are used as a backup source of supply so in case of power cut from grid, the battery based inverter converts DC coming from batteries into AC, to operate the home appliances. Battery based inverter mostly used between 1kW to 6kW.
2. STANDALONE INVERTER
These type of inverter are used in Stand alone PV system which is not connected with the grid. These inverter convert DC coming from PV array into AC during day time to run the load. During night or when sunlight is not available, these inverter convert DC coming from batteries into AC. In standalone system, one another source of power is also used as backup. In most of the cases, Generator is used as a backup source. When the battery bank discharges, then generator is used to give power to load and also charges the battery.
The inverters have some basic standard features regardless of their size or technology. Some of the important standard features are:
1. Safety Feature
One of the most important safety feature in inverters is to detect when the grid is disconnected. The inverter has a monitoring equipment which detects when utility grid goes off and immediately turn off the inverter automatically. This feature is known as an “Anti-Islanding”. The term islanding is refers to a situation when utility grid goes off and PV system start giving supply to utility grid. This situation is a safety issue for the workers, who may working on the transmission line. That’s why this is the most important feature of inverters. This feature is only used in grid connected PV system. If the PV system is standalone, then there is no need of Anti- Islanding feature in inverters.
The another most important safety feature is Ground Fault Protection or GFP. Ground fault protection is basically a fire safety device. It prevents the faults from starting fire. GFP reduces the risk of fire and gives alert about the problem. In ground fault protection, one of the current carrying conductor is connected to the GFP circuit and circuit is closed by small amperage fuse.
2. Maximum Power Point Tracking
Maximum power point tracking also known as MPPT is used to deliver maximum amount of energy to battery bank from PV array. MPPT is a type of charge controller which regulates voltage and current coming from PV array according to battery requirement. Maximum power point tracking as the name suggests track the maximum value of voltage and current to obtain maximum power. MPPT maximize the charging ability of the batteries and reduce the PV array size requirement.
3. User Interface
User interface basically consist of display which gives information about the important parameters of PV system. The display gives the value of Voltage, Power output and Energy production. The modern inverters comes with a variety of ways to collect, store and display the system information data gathered by the inverter. The modern inverters has internet connectivity and user can visit website to see real time information about his system.
POWER OUTPUT SIZES
Based on the size and power output, Inverter are divided into three categories:
1. Micro Inverter
Micro inverter are connected with single module. These inverter are installed on each panel. Micro inverter are used for less then 250W. Because these inverter installed on each solar panel, they are able to cancel out the effect of shading. Micro Inverter are expensive then string inverter.
2. String Inverter
String inverter as the name suggests, connected with strings of solar panel rather than one single panel. The multiple strings are connected with one single string inverter. String inverter are best suited for solar panels, which are positioned at single plane. These inverter are less expensive than Micro inverter but like micro inverter, they are not able to cancel out the effect of shading. So these inverters are good for installation without any shading issues. The string inverter can be used between the range of 1kW to 15kW.
3. Central Inverter
Central inverters are similar to String inverter but it can support more strings of solar panel as compared to string inverter. These inverters are larger in size and best suited for larger installations only. The central inverter can be used for the range of 15kW to 1MW. Combiner box is used between inverter and strings of solar panel. The purpose of combiner box is to combine the output of various strings and give one single input to central inverter.