Solar panels convert solar radiation into DC electricity. To convert DC into AC energy (as required for homes), we need a converter known as an inverter. The inverter converts the DC energy into AC energy that can be fed to the utility grid or home-based or commercial loads.
Inverters with their primary conversion function also help in tracking the energy supply and storing the excess energy in the battery.
For grid connection, we need to synchronize the frequency of the grid and inverter power to match the frequency for smooth operation.
Inverters are generally classified as a conventional transformer-based inverter or transformerless inverter.
Transformer-based Inverter :
- Conventional inverters are built with a transformer that synchronizes the voltage of the grid and the load connected.
- The generated DC is converted into AC with the help of an oscillator and metal oxide semiconductor field-effect transistor (MOSFET), this AC power is matched with the frequency of the grid/load with the help of a transformer.
- But these traditional inverters suffer from transformer losses, thus decreasing inverter efficiency. Also, the weight of such inverters is high thus leading to an increase in transportation and installation expenses. To overcome these losses transformerless inverters are gaining popularity.
Transformer-less Inverter :
- Presently, the transformer-less inverter is acquiring ubiquity. As no extra transformer is required it decreases the extra expense, weight, and size of the inverter. This sort of inverter is more productive (efficiency is practically 97%) as the misfortunes caused by the transformers are overcome.
- These types of inverters use a computerized multi-step process, it converts the DC output of solar panel into high-frequency DC using boost converter and is converted into AC as per the grid/load requirement using an H-Bridge converter and oscillator.
- The DC boost converter replaces the traditional step-up transformer,thus reducing the losses and space requirement.
Solar Inverter with Maximum Power Point Tracker (MPPT) :
The solar module has a limited DC supply and it varies throughout the day depending on the solar radiation, to extract maximum possible power a technique known as maximum power point tracking is used within an inverter.
In simple terms, MPPT is a DC to DC converter. The MPPT circuit in the inverter constantly detects the array DC voltage and current and adjusts the setting to extract maximum power and drive the solar inverter at maximum possible efficiency. The maximum power is the product of the maximum voltage and current.
If multiple solar panels are mounted across the roof area in different directions multiple MPPTs can be needed because the amount of solar radiation falling on each string will vary.
MPPT inverter is classified as a single and dual inverter. Dual MPPT inverter proves to be advantageous as it can connect two or more strings without any need of a combiner box and can efficiently handle different tilt angle, azimuth angle, voltage and power.
Types of Solar Inverter :
Solar inverters depending on the connection are broadly divided into three categories as follows:
Off-Grid Solar Inverter :
- The off-grid solar inverter also known as a standalone inverter draws DC power from a battery source that is charged using solar arrays or other sources like a diesel generator, wind, or hydro turbine.
- These inverters do not have any contact with grids or utility systems and hence can operate even if grid failure takes place, they are usually preferred in remote or rural areas where connecting to the grid is a costly or impractical task.
- They are pocket friendly and easy to install
Grid Connected Solar Inverter :
- The grid connected solar inverters are the most widely recognized sort of inverters for residential and commercial areas in a metropolitan territory that has access to the utility grid.
- These inverters are used for solar as well as hydro and wind energy systems connected to the utility grid.
- With a grid connected inverter, the frequency and phase of the grid and inverter output should be synchronized. Modern grid type inverters have a fixed unity power factor i.e. voltage and current ratio is one.
- This inverter comes with an additional advantage of earning for the surplus energy generation, one can sell the surplus power to the utility system and get paid for the same.
- They are easy to install and cost-effective.
Grid inverters are classified as follows:
- String Inverter
- Central Inverter
1.String inverter: String inverters are in high demand due to their feasible cost and durable technology. In this multiple strings are connected to build a single string inverter. It comes with a drawback that if a single string gets damaged or comes under the shadow it may lead to the performance degradation of an entire inverter.
2. Micro-inverter: As the name indicates this type of inverter is mostly used where space is the factor over cost (they are more costly than string type inverter), or in case of uneven shading in small solar installations. These inverters are placed on every solar panel, thus independently converting DC to AC supply. In this, if any panel is shaded or damaged, the performance of the other panels is not affected.
3. Central inverter: Central inverter is similar to string inverter with a high-capacity range, needing only 1 large inverter for the entire plant/section of the plant. They are mostly used for large utility-scale applications.
Hybrid Solar Inverter :
- Hybrid inverters are also known as multi-mode inverters. It is the combination of both grid and off-grid systems, these inverters can provide a continuous power supply even if the grid connection fails.
- They offer a lot of flexibility for power delivery by choosing between solar power, battery backup and grid connectivity.
- Installing hybrid inverters with solar panel systems will cut the need for a separate battery, it can function as both battery and as an inverter.
- With hybrid inverters, we can monitor the electricity consumption and battery performance under one plane.
- Hybrid inverters are mostly used in areas with frequent power cuts, faults, and load shedding and also in places with low feed-in-tariff rates.
Selection Criteria of Solar Inverter :
The lifespan of inverters can be affected by the parameters such as maintenance, heat, humidity, and performance of the solar system. As an inverter plays an important part in the solar energy system one needs to choose it carefully. The output of the inverter depends on the following criteria:
- Rated output power
- Maximum photovoltaic(PV) input power
- Operating temperature
- Frequency output
- Maximum open circuit voltage
Advantage of Solar Inverter :
- Easy to maintain
- Reliable as compared to other generators
- Pollution free – Solar inverter as it operates on clean energy i.e solar radiations it helps in reducing greenhouse gas emissions
Disadvantage of Solar Inverter :
- Depends on solar intensity
- Expensive as compared to the generators
- Requires more space for installation
- Battery backup is required to operate at night or during cloudy weather
- How does heat affect the solar inverter?
Well, too much heat can affect the solar inverters and can degrade their performance and life span, but these semiconductors are quite sturdy and can tolerate high heat without breaking down up to a certain prescribed temperature limit.
- What is the difference between a normal and solar inverter?
The solar and normal inverter is similar, but the solar inverter comes with an additional solar charge controller and switching circuit. It consists of an additional terminal that helps in connecting to the solar panels as per the rating criteria, the output from the solar panel during a sunny day is used for charging the battery that helps in reducing the electricity cost.
- What is a smart solar/intelligent solar inverter?
The smart/intelligent solar inverters, like traditional inverters, convert the generated solar DC power into AC power. The smart solar inverter is an intelligent power electronic device with bidirectional communications capability, digital architecture and robust software infrastructure. In simple terms, these inverters are capable of monitoring the energy data and sending the message quickly to the users to take corrective actions remotely. These inverters are gaining popularity in the residential, commercial and utility sector as they can efficiently cut the system cost as well as improve operation and maintenance of the system.
- What are the MNRE guidelines for solar inverters?
The Ministry of New and Renewable Energy(MNRE) according to the 2020 guidelines has laid down standards covering safety, efficiency, environmental, and islanding prevention measures tests for utility-interconnected photovoltaic inverters. To ensure that the inverter is manufactured according to the prescribed guidelines, the following test must be carried out and a report must be attached with each inverter, the test includes: Protection against abnormal voltage, protection against abnormal frequency and response to utility recovery.