Introduction to Solar PV Systems – Part I

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In the previous article, we took an in-depth look at solar PV modules. In this and the next few articles, article I will give you an introduction to solar PV systems.

Solar PV systems can be broadly classified into three types: 1) On-grid systems 2) Off-grid systems and 3) Hybrid systems. In this article, I will talk about on-grid systems.

on-grid system

Figure 20.1: A typical on-grid system


On-Grid Systems

As their name suggests, on-grid systems are systems that are “on the grid” or connected to the grid. On-grid systems are also called grid-tie systems or grid-tied systems or grid-paralleled systems.

The figure above shows a typical on-grid system. It has three main components:

1. Solar PV modules: The solar PV modules generate DC electricity, as explained in the previous two articles: Introduction to Solar PV and An In-Depth Look at Solar PV.

2. On-grid inverter: The on-grid inverter, also called grid-tie inverter or grid-tied inverter, converts the DC electricity generated by solar PV modules into AC. Before feeding it into the grid, it “synchronises” it with the grid. In other words, it ensures that the AC signal that it generates is “in-phase” with the grid. Yet another way of saying this is that the AC signal it generates is at the same point in the cycle as the AC signal in the grid. (Refer to the description of alternating current in the article War of Currents.)

3. Meters: There are three meters in the system:

       a. Sell-back meter: The sell-back meter, also called the “export” meter, measures the amount of energy generated by the on-grid inverter and fed or “exported” to the grid. This is always less than the amount of electricity generated by the PV modules by an amount equal to the various losses in the system: PV module mismatch losses, resistive losses in the cables, inversion losses in the inverter.

      b. Purchase meter: The purchase meter, also called the “import” meter, measures the amount of energy purchased or “imported” from the grid.

     c. Net meter: The net meter measures the net energy imported from the grid by calculating the difference of the imported and exported energy. If the net energy is positive, it means that the consumer consumed or imported more than he generated or exported, and therefore has to pay money to the distribution company as per the applicable rate. If the net energy is negative, it means that the consumer generated or exported more than he consumed or imported, and therefore the distribution company has to pay him as per the agreed rate for excess generation.

Although they are not visible in the figure, some other components called Balance of System components, or BoS components in short, are also part of the system. They are:

1. Mounting structures: These are used to mount the solar PV modules on the roof or the ground, depending upon whether the system is a rooftop or ground-mounted system.

2. AJBs: AJBs (short for Array Junction Boxes), also called SCBs (short for String Combiner Boxes) or PV Combiner Boxes, combine the output of several strings into one as their name suggests. This is required if the number of PV “strings” (i.e. a string of PV modules connected in series) is more than the number of inputs that the grid-tie inverter has. AJBs are available in various configurations: 2:1, 4:1, 8:1, or 16:1, and in general, they can be made in any configuration that one desires. Besides combining the output of multiple strings, they can, and in most cases do, perform one other important function: string monitoring. Before combining the output of several strings, they monitor the current in that string. Doing this is important to figure out where the problem lies in case the system is not generating as much it should. If the current in one string is less than those in other strings, you know that there is a problem in that string; in all probability, one of the PV modules in that string is malfunctioning or is shaded.

3. Cables: Cables are required to connect the strings of PV modules to the AJBs and the output of the AJBs to the grid-tie inverter; the cables to connect the PV modules to each other are already provided at the back of the modules.

4. Earthing: The earthing system is needed to “ground” the system, which literally means connecting the system ground to the actual ground.

5. SPDs: SPDs, short for Surge Protection Devices, are required to protect the system from surges which typically arise due to lightning. It is very important to have a good SPD; we want the system to last a long time, i.e. 25 years, and in that long a timespan, it can experience surges on multiple occasions and will get damaged without the proper protection.

Do on-grid systems work when the grid is unavailable?

The answer to this question is: no. When the grid is unavailable, or in other words when there is a power outage, on-grid systems cannot work. This comes as an absolute shock (pun intended ) to someone who is new to solar PV technology, and the question that follows this revelation is, “When there is a power outage is exactly when I need the system to work the most. If it doesn’t do that, what’s the bloody point?”

While I accept the point wholeheartedly, on-grid systems simply can’t do that. And the reason is very simple: on-grid systems need a “reference” to work properly, and it is the grid which provides the reference. Therefore, when the grid is unavailable, they don’t get the reference and therefore cannot work. That’s just the plain simple truth, whether you like it or not. If the power outage happens to be around noon when the PV modules generate the maximum amount of electricity, you will lose the best hours on that day. Therefore, on-grid systems should be installed in areas where the grid availability is high, thus ensuring that you can get the most out of your solar PV on-grid system.

If you have a DG (Diesel Generator) set backup, which is the case in many industries, it is a different situation though. When the power outage happens, the DG set will come online, either automatically or when someone starts it manually. When that happens, the on-grid system will again get a reference and will start working again because it doesn’t care where it gets the reference from, the grid or the DG set; for it, a reference is a reference, and as long as it gets it, it works.

To that, you might say, “This is great! It will reduce the diesel consumption and will therefore be greatly beneficial to the environment, not to mention the savings to the owner.” While that statement is true in most cases, it may not result in as much savings as you might expect. Why? Because the efficiency of a DG set starts reducing when it is operated at sub-optimal loads, and the savings calculations have to be done carefully on a case-by-case basis to avoid getting rude shocks later. More on this later in the series.

Sustainably yours,

Prashant Karhade.
Writer, Publisher, Entrepreneur

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