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  Jun 5, 2021   

A Simple Guide to Grid-Tied Solar Energy Systems

The rising electricity bill is the most common concern of home and business owners. 

But do you know, you can reduce your electricity bill by more than 50%?

The world is techno-driven, we cannot even imagine a few hours without electricity, but this grid dependency is just raising our electricity bills, especially in summers.

Well, yes you can avail of this deal by just making a switch from conventional grid connection to solar grid connection without compromising your comfort and contributing towards a greener environment.

In this article, we’ve defined a grid-tied system in detail. By the end of this article, we hope to give you a detailed understanding of a grid-connected photovoltaic system.

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What is a grid-connected solar energy system?

 A grid-connected or grid-tied solar energy system is connected to the local utility grid. In simple terms, one can get benefits of both solar energy as well as grid connection. 

In a grid-tied solar system, the flow of electricity is bidirectional, i.e., it can import and export the energy to and from the grid. This means that we can sell and buy electricity to and from the grid.
This makes people with grid-connected systems PROSUMERS – they are both producers and consumers.

Grid-connected solar energy system consists of three main components:

  1. Solar Panel
  2. Inverter
  3. Utility Grid

How does a grid-tied solar system work?

When solar radiation is incident on the solar panels, with the help of the photovoltaic principle it generates direct current (DC). The generated direct current is converted into alternating current(most of our day to day appliances and the grid-connected runs on AC supply) with the help of an inverter which is further supplied to various AC loads connected. 

The surplus generated energy is sent to the grid. The electricity sold to the utility is measured using a  bidirectional meter (net meter), and the prosumer gets paid for this exported amount of energy.

When a solar panel during cloudy weather or at night is not able to produce the required amount of energy then the required power is imported from the utility grid and one needs to pay for the imported energy. 

Net metering is added with the consent and approval from the local grid connection, so don’t worry!! The import and export energy will be recorded and added to your electricity bill at the end of your billing cycle.

Grid-tied inverters play an important role in the solar energy system, as without this one cannot supply energy to the AC loads or to the grid.

Grid tied inverter: How it works?

The inverter converts the DC energy from the solar panel into AC energy and further supplies it to the grid. 

The grid-tied inverter comes with a function of monitoring the voltage, frequency and phase of the source side. To supply power from the inverter one needs to take care that both grid and inverter supply power with the same phase and also the power generated by the inverter is at a higher voltage level. 

Components of grid-tied inverter:


  1. Maximum power point tracker (MPPT): We know that the solar intensity varies throughout the day hence to extract the maximum possible DC power from the solar panel, MPPT is used.
  2. DC and AC protection: The inverter comes with an inbuilt DC and AC protection unit that ensure protection against overvoltage, current and other faults occurring in the system. An AC and DC disconnect switches are integrated with the inverter.
  3. DC to AC converter: It helps in converting the direct current generated from the solar panel into the usable alternating current.
  4. Isolator: An isolator is used to isolate the supply from the inverter to the grid in case of any failure
  5. Grid inverter can be transformer-less or with a transformer attached to synchronize the inverter output voltage with respect to the connected grid voltage. The external transformer is connected between the inverter and grid to ensure synchronization between them.

Key protection function of a grid-tied inverter:

The grid-tied inverter performs the following functions:

  1. Input overvoltage protection: When the input DC side voltage is higher than the permissible grid-inverter voltage, the inverter sends the signal and stops functioning for a while (approx. 0.1s).
  2. Input overcurrent protection: The solar panel output of each string is connected to the inverter input, if the input DC is higher than the permissible inverter current then an alarming signal is given by an inverter. Inverter stops the intake of the current till it reaches a normal acceptable value.
  3. Output overvoltage protection: The grid-tied inverter on its output side monitors the overvoltage/over frequency i.e., it monitors the abnormal condition of the power grid and sets a warning signal and stops functioning till normal grid voltage/frequency is achieved.
  4. Output current protection: It is equipped at the AC output side of the inverter that monitors and sends an alarming signal when the output AC voltage is higher than the grid or when the grid undergoes any fault. In the case of a short circuit, the inverter must take action within 0.5 sec of the fault occurrence and must start normally once the fault is overcome.
  5. AC / DC surge protection: To protect the system against lightning and surges, solar grid inverters should have a surge protection function.
  6. Anti-islanding protection: The grid-connected inverter must have an islanding protection function, generally inverters follow active or passive protection schemes to detect the presence of any fault in the grid.  Islanding protection is a type of protection for generators such as grid-connected solar, wind or diesel which ensures that the generator disconnects the electrical supply to the grid in case of any fault/blackout.
  7. Over-temperature protection: This is one of the common causes of failure, the inverter must consist of an over-temperature protection scheme that will give a warning signal if the inner ambient or component temperature increases than the required normal temperature.
  8. Automatic recovery: Grid-tied inverters with a function of automatic recovery can lead to a resumption of power supply once the grid starts functioning normally.

Types of grid-connected inverter:

The grid-connected inverter, in general, is classified into the following types:

1.String Inverter:

  • In a string type inverter, a bunch of solar modules/panels are wired together in series to form a single string inverter, the basic function of each string is to convert the generated direct current from the solar panels into alternating current. 
  • It is best suited for small residential or commercial consumers with a low solar budget typically below 1MW power. In general, these solar inverters are in the range of 1 KW – 100 KW
  • String inverters as compared to micro inverters come with the limitation, in this even if a single panel in that string gets damaged or comes under the shadow it may affect the performance of that entire particular string.

2. Central Inverter:

  • A central inverter is similar to a 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.
  • Typically, in the range of more than 1MW solar plants, they are used for large commercial or utility solar farms.
  • They are mounted in a protective sheet hence highly reliable and less prone to harsh environmental damages.
  • Easy to install but high replacement cost.
  • Performance is adversely affected even if one string comes under the shaded zone or fails to operate.

3. 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, even if one panel is shaded or damaged, the performance of the other panels is not affected. 
  • Micro-inverter gives benefits of monitoring individual solar panels and makes it a good choice for installation in areas with varying solar energy.
  • They come with a typical capacity in the range of 200-400W but they offer 27% more efficiency as compared to string inverters under low shaded zones.
  • They are easy to expand in the future, but when any one of the inverters fails then maintenance is a difficult task.

Advantage of a grid-tied inverter:

  1. Less Expensive
  2. Easy installation
  3. Net metering: Grid-tied systems offer you an advantage of earning from your surplus amount of electricity generated, one can sell the extra power as well as can take a loan from the connected local DISCOM station
  4. No limitation on the number of loads to be connected
  5. Battery backup is not required.

Disadvantage of a grid-tied inverter:

  1. In case of grid failure, the inverter supply gets halted. 
  2. Solely reliable on-grid connection, no battery backup

Grid tied inverter: Selection criteria:

Before selecting an inverter for your grid system, one should be aware of the following basic conditions:

  1. The inverter output power must be in the range of 0.9/0.95 of the peak output power of the solar panel.
  2. The maximum and minimum output voltage of the solar array should be checked with the permissible input DC voltage of the inverter
  3. The operating voltage of the solar array must be within the MPPT voltage range.

FAQ:

Which is better – String or Central inverter?

One always gets confused between these two, string inverter is the popular choice for rooftop or most captive solar projects. When it comes to very large solar power plant projects of few MW, a central inverter proves to be a good alternative.

What one should check before buying a solar inverter?

Before purchasing a grid-tied inverter make sure it is national standards council certified. This ensures that all the safety guidelines are followed by the manufacturer.

How will I know that my grid-tied solar energy system is generating power?

Well, you don’t need to go and check, a back-and-forth meter also known as a net meter which is installed between the distribution panel and the grid will help in analyzing the difference between the amount of power generated and used. It moves in the forward direction when the energy generated is less than the energy used and reverse direction when the condition is opposite.

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