Solar Inverters

Introduction


In a solar system, the inverter is considered to be the most important part. It is a device that converts direct current (DC) which is generated by the solar panels to alternating current (AC) which is needed to power the electronic devices in the households. Inverters are just one example of a class of devices called power electronics that regulate the flow of electrical power. In this article we will look into how an inverter works, the inverter efficiency, what are the different kinds of inverters available.

How a solar inverter works


Fundamentally, an inverter accomplishes the DC-to-AC conversion by switching the direction of a DC input back and forth very rapidly. As a result, a DC input becomes an AC output. In addition, filters and other electronics can be used to produce a voltage that varies as a clean, repeating sine wave that can be injected into the power grid. Although the inverter’s main job is to convert DC power produced by the solar array into usable AC power, its role is only expanding. Inverters enable monitoring so installers and owners can see how a system is performing. Inverters can also provide diagnostic information to help O&M crews identify and fix system issues. These important components are increasingly taking on decision-making and control functions to help improve grid stability and efficiency.

Figure 1: Electrical inverters

Efficiency of inverters


A key element in stable power conversion is the efficiency of the inverter. The efficiency of the inverter can be said to be good when there is no significant difference between the input power and the output power. However, owing to the nature of the device, power losses are inevitable. The efficiency of an inverter indicates how much DC power is converted to AC power. Some of the power can be lost as heat, and also some stand-by power is consumed for keeping the inverter in powered mode. Efficiency generally ranges from 95 - 98%. So, for example, if your inverter is 98% efficient, only 2 % of the electricity generated will be lost. Efficiency is an important aspect when it comes to inverters.

Figure 2: Efficiency curve of an inverter available in the market at different

The efficiency curve as seen in Figure 2 tells you in detail how an inverter will perform with your solar array at different output levels. This, in combination with the knowledge of how well your solar panel array performs throughout the day, will enable you to most accurately understand your inverter’s performance. Understanding your inverter’s efficiency curve will be especially relevant in case if you have oversized your inverter with the solar array output. Let us look at the different kinds of inverters available.

Types of Inverters


String Inverter

String inverters have been there a long time in the market and it is one of the oldest available options to date. These kinds of inverters are connected to multiple strings of solar panels. This kind of inverter is capable of handling multiple strings of solar panels. For commercial and residential applications, string inverters are typically rated between 1 to 100 kW. Peak efficiency is usually in the order of 96% or 98%. The size of the string inverter in kilowatts (kW) and the wattage of the solar panels you use will determine how many panels you can string to one inverter without wasting energy. String inverters are effective and affordable solutions for small-scale solar systems. When the panels have little to no shading, string inverters are best suited for these kinds of systems.

Figure 3: Array of solar panels connected to string inverters

Micro inverter

Microinverters or solar micro inverters in contrast to string inverts are connected to a single solar module to convert DC to AC. One of the main advantages over the conventional inverter is small amounts of shading, debris, or even a complete module failure, do not disproportionately reduce the output of the entire array. Each microinverters have maximum power point trackers in it (MPPT) which optimizes the power output of the solar panels. Monitoring and maintenance are easier as many microinverter producers provide apps or websites to monitor the power output of their units. Disadvantages include still a relatively new technology and there are only a small number of manufacturers. More expensive than string inverters and potentially only useful in situations where shading is a major issue.
Figure 4: Released in 1993, Sunmaster 130S was the first microinverter

Central Inverter

Central inverters are high-capacity inverters designed to be used with large power station-sized solar systems like in a solar farm. It is like a large string inverter designed to handle more power and offer efficiencies/economies of scale. Central inverters are not used for residential solar systems as the name suggests all the module strings are merged to this one single inverter. Some of the advantages include low cost, more reliability as they are placed in protective environments and can produce much higher power. The main disadvantages include that since central inverters have a high rating of DC Voltage, it possesses a high-risk factor. It is also not easy to replace.

Figure 5: Central Inverter in a solar farm

Generally, grid-tied inverters are designed for outdoor use, although most off-grid inverters are not weatherproof and are generally mounted indoors, close to the battery bank. As a rule, inverters designed for outdoor use may be installed either outdoors or indoors, however indoor inverters can only be installed indoors. The great majority of grid-tied or string inverters available today are designed for outdoor installation. All major manufacturers have their products protected from wind, rain, snow, or additional protection against corrosion dust, etc. When choosing the site for installing an inverter, it is essential that the environmental conditions specified by the manufacturer be maintained. There are both temperature and humidity requirements for these devices. The ideal installation site for inverters is cool, dry, dust-free, and indoors. However, there are a growing number of applications for which this is impractical or undesirable. With an appropriate system design, an outdoor inverter installation can be a reasonable and successful option.

AFSHIN BAKHTIARI

AE Solar Senior Engineer



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