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What are the points to keep in mind while selecting a grid connected inverter?

Things to check before finalizing the grid connected inverter for your system:

Warranty:

Compare the warranties offered with solar inverters. Most of the manufacturers offers 10 years of warranty with annual maintenance charges .

Also, check whether the manufacturer offers onsite services, years of establishment, no. of inverters sold & response time in case of technical faults.

Peak Efficiency

This is the specific ratio of how much energy is consumed and how much energy is output. Like other appliances, a certain amount of energy is lost in the conversion of electricity from DC to AC. Most inverters operate at 95% & above efficiency, and therefore resulting in greater amounts of energy to the grid.

Power Maximization (MPPT)

MPPT stands for Maximum Power Point Tracker. Its function is to maximize the energy available from the connected solar module arrays at any time during its operation.

The voltage at which PV module can produce maximum power is called ‘maximum power point’ (or peak power voltage). Maximum power varies with solar radiation, ambient temperature and solar cell temperature.

The major function of MPPT is to extract the maximum available power from PV module by making them operate at the most efficient voltage.

Data logger & Online monitoring 

Now-a-days inverters have data-logging feature enabling you to download information to a computer, or transmit it over Bluetooth or your Wi Fi network. With online monitoring you can check the real-time generation or periodic information about the performance of your system on your PC, smartphone etc.

Solar inverter

A solar inverter is a device capable to convert direct current (DC) into alternating current (AC) electricity. Inverters are one of the most important components of solar power systems since solar panels generate DC electricity and most devices used in homes and offices operate on AC voltage.

                     Solar Inverter

Inverter technologies have advanced significantly, such that in addition to converting DC to AC, they provide a number of other capabilities and services to ensure that the inverter can operate at an optimal performance level, such as auto-wakeup, Maximum Power Point Tracking (MPPT), over voltage & over current handling, remote monitoring, monitoring weather parameters (humidity, wind, velocity, insolation level, temperature), etc.

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[Publisher: EnergyX: Sustainable Energy: Design A Renewable Future]

How does a grid connected inverter work?

The grid connected inverter takes the direct current and, in simplified terms, runs it through a transformer. The inverter runs the DC through two or more transistors that are rapidly turned on and off and feeding two different sides of the transformer to convert DC into AC.

The grid connected inverters require reference frequency to synchronize with the grid (e.g. 50 or 60 Hz) using a local oscillator and limit the voltage to no higher than the grid voltage.

The power produced is fed into your house/ office electrical panel and provides power for all needs.  The excess power automatically flows out to the grid, spinning meter backwards.

Grid-tie inverters are also designed to quickly disconnect from the grid if the utility grid goes down to prevent the energy it transfers to the grid from harming any line workers who are fixing the line issues. This feature is known as Anti-Islanding.

 

What is the difference between a normal inverter and grid connected inverter?

Normal inverter

A normal inverter system or a UPS basically have batteries to store energy from the grid when the supply is available. When the power is cut off, the inverter takes the DC power from the batteries and converts it into AC used by the appliances. There is an automatic control in the system that intellects if the grid is not supplying power and switches the UPS or inverter into battery mode.

Grid connected inverter

A grid tied solar inverter, also converts DC power (produced from Solar Panel) to AC. But additionally they have built-in MPPT (Maximum Power Point Tracking) feature. As the voltage produced by Solar Panel varies depending on the temperature and obtainability of sun, MPPT allows solar inverters to draw maximum power from the solar panels. The solar inverter used for grid-connected system also comprises an anti-islanding functionality which basically means that the inverter is designed to shut down if the grid goes down, so that you won’t be inadvertently energizing the grid if a someone is working on it

What are different kinds of inverters used in solar power plants?

Broadly, the solar inverters can be categorized in the following types:

Grid Connected Inverters

Grid connected inverters are simply inverters which are connected to the grid. These inverters produce a sine wave that is congruent with the AC waveform produced by the grid.

They allow you to send surplus solar electricity, your system has generated back into the grid.

Grid connected inverters have Anti-islanding feature which allows the inverter to realize the sudden change in system frequency, voltage, rate change of frequency, increase in kW beyond normal levels, or a change in kVAR and shut down the inverter. When the inverter shuts down your system is no longer producing electricity for the safety of the people who will be working on the grid to repair it.

Grid connected inverters are sub categorized as follows:

Central Inverter:

These inverters are typically used for large scale projects (usually above 250kW). They are huge, have their own rooms with exhaust, etc. They are  are cheaper than string inverters & requires low maintenance due to lesser terminations on AC side.

String Inverter:

This is the most commonly used solar inverter for homes and business consumers. They are much lighter and resistant to harsh atmospheric
conditions. String inverters are based on modularization, each PV string goes through an inverter, with maximum power tracking peak at the DC terminal, and grid-tied at the AC side.

Micro Inverter:

Micro-inverters are small sized inverters attached to individual solar panels. They use power point tracking,  which means essentially if one solar panel is shaded from the sun, the rest of the panels are not impacted. This makes micro inverters somewhat more reliable but the cost is higher than other types.

[Source: Youtube]

[Publisher: Solar Power World]

Off – Grid Inverters

Off-grid inverters are used for stand-alone solar power systems i.e. the solar plant is not connected to the electricity grid and therefore requires battery storage.

Off-grid inverter sends power to the appliances in your property & any excess power to your battery bank.  When your solar system is not generating power (night time or cloudy days), your appliances will draw power from the batteries.

Unlike grid connected inverters, they do not have the ability to export excess solar electricity into the grid.

Hybrid Inverters

These inverters has the capability to charge batteries, draw energy from a battery like an off-grid inverter, as well as  export excess energy to the utility grid, like a grid connected inverter.

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[Publisher: Admin CER]

What is the loading ratio of inverters?

Loading Ratio or DC to AC ratio of an inverter is the ratio of Rated Array Power (DC kWp) to Rated Inverter Output (AC VA). This can be expressed as a percentage value.

For example, if array output of 55kWp DC were connected to a 50kW inverter, the Loading Ratio would be:
(55kWp / 50kW) x 100= 110%.

One can find the Rated Array Power (DC kWp) & Rated Inverter Output (AC VA) of an inverter on the datasheet provided by the inverter manufacturer.

Where should the inverter be placed in grid connected solar power system?

The Inverter used in the PV solar system is either to be mounted on the wall or placed on the rooftop depending upon the site features and space availability. Following points should be considered while placing the inverters:

  • Inverters should be easily accessible
  • Away from direct sunlight
  • In an area with at least ½ foot free space on all sides of it
Where can I view instantaneous generation of my solar rooftop plant?

Generation of the system can simply be viewed on the display unit of the inverter or through local & remote monitoring which can be performed by most common connection techniques like RS232 for local monitoring, RS485 and powerline for inverter interconnection. For wireless connection Bluetooth, GSM & Wi-Fi are most common used.

An inverter measures parameters like:

  • Immediate DC voltage and current readings from the PV array 
  • Immediate AC voltage and frequency readings 
  • Output power (W) 
  • Lifetime energy produced 
  • Switch-on/off time for the day 
  • Up-time & Down-time for the day 
  • Cumulative hours of operation (h)