Solar panels are always equipped with an inverter because they’re one of the most important components of the system. Selecting an inverter can either make or break the efficiency of the system. So it is important to be equipped with the right knowledge on inverters that are used for solar PV systems. This article provides the basic know-how of everything related to solar inverters ranging from the types of inverters, their functions and features, safety issues related to inverters and even inverter sizing requirements.
What are Inverters?
A solar PV system generates direct current (DC) and the electricity is drawn from the grid is alternating current (AC) which has high voltage. If you require electricity from the grid that is backed up by the solar PV system, then we need a device that can convert current from DC to AC. This device is known as an inverter. An inverter is defined as a device that can convert current from DC to AC and multiple types of inverters can be used as per the variation in the solar PV system installation techniques that are employed which will be discussed further in this article.
An inverter can be equipped for solar PV systems that are either grid-tied or used in stand-alone systems. In either case, it is necessary to ensure that the inverter is placed in an area that is well ventilated. The weight of the inverter must also be taken into consideration before deciding how to mount it and the location access should be easy so that periodic checks for maintenance can be carried out smoothly.
If small inverters with less than 2kW of continuous power rating are used, then such inverters can simply be placed on shelves or desks because of its lightweight nature and ease of portability. Medium-sized inverters with a continuous power rating between 3kW and 10kW need to be mounted on the wall and should be securely fixed because they are heavier. Large-scale inverters which have ratings more than 10kW must be securely bolted to the floor because they are too large and heavy.
When inverters are operating especially close to their rated output, they generate a lot of heat. Hence, it is essential to install them in locations with good airflow to prevent over-heating and subsequently, damage to the inverters. Over-heating is the primary reason for solar PV systems to produce significantly lesser power than its rated output so when selecting an inverter for utility applications, you must check its operating temperature range and how to maintain the system within that range. One way to prevent such losses is to install them in ventilated areas with clearance around them. They should be installed away from the ceiling and shouldn’t be installed in sealed locations like a cupboard. It’s possible to install inverters outside as well in locations such as the garage or the outside wall of a particular building. There are also options to invest in ‘outdoor inverters’ but they’re difficult to source and are expensive. Outdoor inverters must also be properly sealed against moisture, dust and other pollutants that could potentially affect the operation of the equipment. They must also be rated according to the regulatory standards of that location. Certain inverters come with an external cooling fan that’s temperature-controlled or an external heat sink to dissipate extra heat that is produced during its operation. Also, if a particular inverter is producing power which is more than 500W, then the inverter tends to make a low continuous humming sound which may be amplified due to the noise resonating with the wall. This may be a disturbing environment to work or live in and in such cases; there are options to dampen the mounting that is between the inverter and the floor/wall. However, these are rare occurrences and are easily solvable.
All inverters must be grounded and are included with ground fault protection to prevent hazards related to electrical faults. If ground fault protection is not included with the inverter, then it is necessary to incorporate a ‘Residual Current Device (RCD)’ which are also known as ‘Ground Fault Interrupters’.
When investing in an inverter, 3 things must be taken into consideration:
- Battery Bank Voltage: Depending on the type of installation, there are different inverters with different voltage requirements. Small-scale inverters which operate only up to 3kW rated output, require 12-volt systems whereas larger inverters would need higher voltage values.
- Power Rating: The maximum continuous power that an inverter can supply to all system loads is known as the power rating. Generally, it is better to go for a higher power rating than what your system produces. Installers often recommend purchasing inverters that have power rating at least one-third higher than the total system size because inverters can get quite hot during its operation when it’s close to its maximum rated output for a long time. The maximum power rating can be calculated by adding all device wattages together that are switched on at any time of the day. A margin error must also be added to the final value because inverters will only operate till its maximum rated power and will not go beyond that. Certain inverters also have a peak power rating which accounts for additional loads that are being supported by the PV system for a short time. The peak power rating along with continuous power rating is often mentioned in the inverter specifications of many inverter types.
- Waveform: Waveforms are sine wave signals that relate to the quality of the inverter. Inverters that are low in cost will deliver a modified sine wave signal whereas expensive inverters provide a pure sine wave signal. Modified sine wave inverters are cheaper and have high rated peak power but it may not operate perfectly with some equipment such as power supplies used for laptops, TV’s, etc. If pure sine wave inverters are used, these limitations can be overcome and will provide electricity that is on par with the standard electricity supply for domestic applications supplied from the grid.
Types of Inverters
Depending on the type of application, different inverters are used which vary in its properties. A grid-tied inverter does not work in the same way as a stand-alone inverter. Grid-tied inverters should work in combination with the grid. These inverters work directly with the solar panels and switch off when panels don’t produce enough electricity. For stand-alone systems, the power generation is entirely independent of the grid and in such cases; inverter operates by connecting with the battery bank. If the battery bank is low on charge, then the inverter also switches off.
- Central Inverters: Conventionally, there is one central/master inverter that controls the solar PV system which either connects the solar array to the grid or connects a solar array to a battery pack in an off-grid system. It’s usually in the range of around 1000kW.
- Micro-inverters: In the recent past, a new type of inverter has been developed which is known as the micro-inverter. These inverters have lately gained popularity because of their all-round benefits. Initially, these inverters used to be expensive but now due to the increasing momentum in popularity, their prices are dropping to be comparable to any other high-voltage inverter systems. Micro-inverters are attached behind a PV module individually such that each module delivers high-voltage alternating current. So each module becomes an independent energy system. They’re usually used for grid-tied solar PV systems and are not found to be suitable with stand-alone systems which have a battery backup. These micro-inverters are bolted to the solar panel frame that also holds the panel in place. The AC power cables are connected to the solar panels individually which directly feeds electricity into the building. The major advantage of this system is that if any of the modules are under-performing due to shading or damage, the affected module will not affect the overall rated output of the solar array because each panel/module acts as an independent unit. Using these inverters also gives the flexibility in mounting panels on rooftops in multiple angles or pitches to possibly avoid shading altogether. Since no direct currents are flowing from module to module, hazards are minimized thereby making installation, operation and maintenance less of a herculean task.
- Grid-Tied Inverters: Another type of inverter system is called as a grid-tied inverter. They are also known as interactive inverters. They convert DC power to AC power and transform the voltage to the same value as that of the grid. These inverters produce only as much rated power as it can irrespective of the loads. An example is, if you purchase a 10kW grid-tied inverter system, the inverter can put out only 10kW. These inverters encompass pure sine wave signals which have to align with the grid waveforms. Such inverters also have an additional safety feature wherein if the grid shuts down, these inverters cut off power supply from the solar array. Also, since these inverters are directly connected to the solar panels, the solar panel’s input voltage can wildly fluctuate which can only be handled by a grid-tied inverter. A normal inverter won’t be able to handle such wild fluctuations and hence cannot be compared with grid-tied inverters. So when buying a grid-tied inverter, it is crucial to consider various parameters such as rated power, input voltage, power tracking, string support by the inverter, reporting, diagnostics, safety features, operating environment, installation, utility regulations and certifications.
- Micro-Parallel Inverters: Conventionally, solar installers had to choose either between string inverters or micro-inverters both of which have their advantages and disadvantages. So a new type of inverter has been developed which is known as micro-parallel inverters. They combine the ergonomic feature of string inverters with the remote control, data display and energy efficiency of micro-inverters. These new inverters have the feature of enabling one single inverter to control multiple low power-rated panels without maximum power point inefficiencies and shading losses. They also reduce the installation and maintenance time by 70% compared to other inverter technologies while tackling one of the major factors for inverter selection-cost. They can be individually fixed on the back of a panel or they can be racked directly on a roof. They can be used with stand-alone systems, grid-tied systems and even grid-tied systems with emergency backup [1-6].
The other lesser-known types of inverters are:
- String Inverters: They connect module strings to the inverters. So even if one string produces lower power output or even fails in the worst scenario, the total power obtained from the array is not affected, only that particular string’s power is lost.
- Non-Isolated Inverters: This is a type of functional grounded inverters which are very safe because they do not have a current-carrying conductor that has the same voltage as ground. It’s also called a ‘transformer less’ inverter.
- Battery Inverters: These inverters are connected to a battery and not to the PV system. They receive voltage from the battery and generate power and current as per the need.
Central Inverters Vs String Inverters – Which Is Better?
Another common confusion when considering inverter types is the difference between central inverters and string inverters. Both of these inverter types are relevant but they’re reliant on the project type for the implementation of these inverters. If the inverters are to be implemented for rooftop/residential systems, string inverters are better because they offer benefits such as reduced cabling losses, modularity, lower space requirements. However, if the application is for large/industrial systems, then central inverters can be considered because of its ability to centralize the power to one location. It reduces the need for many inverters making it easier for maintenance. Mostly, string inverters are found to be more efficient but cost more than central inverters. This is backed up by Huawei’s and Sungrow’s PV inverters. Huawei’s string inverter showed 0.8% increased performance ratio than their central inverters. Sungrow PV inverters also showed a similar trend [10-12].
Safety Issues with Inverters
The next aspect to consider is the safety issues. Some of the safety-related concerns are:
- Humming or buzzing noise from electronic equipment that is powered with the help of an inverter. This occurs when the sine waveform generated by the inverter does not match with the grid. This is an indication of a faulty inverter, low power from the grid or even a grounding issue.
- Unexpected power loss and tripping of circuits: This safety issue is often difficult to diagnose due to the combination of faults. The most common reasons are overheating of the inverter which leads to less power generation. This may lead to the shutting off of the inverter so that it can cool down. To solve this issue, the area where the inverter is located must be ventilated further. If the circuit tripping and power loss occurred to inability to handle high loads, then it means that a more powerful inverter has to be purchased to handle the load. If the tripping occurs due to weather-related issues such as very hot days or rainy days, there’s a chance that a grounding problem is present. In any case, it is important to periodically check for faults and conduct maintenance tests to ensure the smooth operation of inverters for an optimum electricity yield.
To ensure that the inverter works optimally, the following factors need to be accounted to correctly size your inverter:
- Solar array size: The inverter size should be similar to the solar PV arrays in your system. For example, if an 8kW solar PV system is being installed, then the inverter should also have a power rating of 8000W with a small margin error to account for unexpected performances.
- Factors that are site-specific such as azimuth and tilt of the panels: These factors can affect the solar radiation that is being absorbed by the panel which in turn will affect inverter sizing. Usually, a derating factor which accounts for all losses under practical test conditions is incorporated in inverter sizing calculations to determine the efficiency of the equipment.
- Location: The geography as to where the panels and the inverter are being installed also affects inverter sizing. Hot locations may not perform optimally as compared to colder locations for the same rated output mentioned in the specification datasheet. This is because the DC power produced will vary for a hot location compared to a cold location.
- Input voltage: The input voltage of the inverter should match with the solar array and battery bank for inverter sizing. For small inverter systems which have a size of around 100W, the input voltage can be 12V, 24V or 48V. Larger inverter systems will have larger input voltages.
- Wire Length: The wire length between the solar array and the battery bank can affect the inverter performance. If longer wires are used, then lower voltages should be produced by the inverter because current increases and voltage decreases with long wires. Similarly, if large voltages and low current is being produced, then shorter wires have to be used [1,13-15].
In essence, it is important to consider the role of an inverter in the functioning of a solar PV system. From the various types of inverters used for different applications, its features and operation to the safety issues and inverter sizing, this article has been able to detail a general idea on everything you needed to know about inverters. The need for knowing these components is because the market for renewable energy is constantly increasing and solar technology is swiftly gaining popularity because of its ability to produce free electricity. So because of the growing demand of solar photovoltaics, the function of inverters also started gaining momentum and before you decide to invest in solar technology equipped with inverters, it is vital to understand the basics of inverters and what’s considered during inverter sizing before committing to such a project because if research isn’t done right, you can lose a lot of money. Hence, we at The Solar Labs got you covered and you don’t have to worry about being inexperienced in the field of solar technology which is the investment of a lifetime!