A Solar PV system needs several components other than the PV panels and Inverters or the machine that converts DC electricity to AC electricity are the second most important part of PV systems. Unless you are having DC appliances to run, you certainly need inverters to chnage the raw DC(Direct current) power generated by the solar panels to AC(Alternating currents) power that is mostly used in homes and offices all over the world.
With increasing interest in buying Solar PV Systems in India and all over the world many people are looking for good information about solar pV systems, its components, guides, installation procedures and all sorts of information regarding Solar PV modules. In this post i will cover basics of Solar Inverters , their various parameters and things to lookfor while buying a solar Inverter for your project.
While designing a Solar PV system our main target remains to bring the current and voltage produced at the PV panels to the desired level. Most of the time the current and voltage levels are different than what is generated at the PV panels. To acieve this we need power converters. Most of the appliances arround us use AC power while the PV modules produce DC power. Therefore conversion from DC power to AC power is required before we can use teh solar power. This conversion of DC power into AC power can be obtained using devices like DC to AC converters or Inverters.
Some losses take place in power converters while converting one form of power to another form of power and, therefore the efficiency of the power converters is always less than 100%.
Solar inverters are an important interface between the solar pV modules and the electrical load.Depending on whether batteries are used in the PV system or not, the solar inverters can be classified into following categories.
Off-grid or Stand alone Inverters : As the name suggest these type of inverters are not connected to the electrical grid system. These kind of inverters are mostly used in small scaled solar PV power plants designed for home or office buildings. This type of inverters are coupled with batteries to supply power during the no sunshine hours or night times.
Grid-connected or Grid Interactive Inverters : These are also called as string inverters. These are used in Grid connected PV power systems and there is no battery system involved here. The Grid, here, is used as the backup for electrical power. The grid interactive inverters have specially designed circuits to match the current and voltage of the power generated by the PV system to that of the Grid. Most of the Grid-tied inverters have MPPT or maximum power point tracker system to maximize the power generation from the system. These inverters also have inbuilt system to sense any cutoff of power generation from the modules and draw power from the grid when need arises. This kind of inverter is very easy to size because it is independent of the appliances you want to run on it. It completely depends on the size of the Solar PV System you are installing.
Grid-tied inverters with battery back up: These type of inverters have both the features of the above two inverters..
Inverter Specifications and features: There are certain parameters under which the inverters give maximum result. Knowing about these parameters and how these affect the inverter performance can help us selecting the best Inverter for our PV system and maximize the solar power output. The below mentioned parameters are generally provided by the manufacturer of the inverter on its technical specification sheet.
While buying a Solar Inverter here are a few basic things that you should look at:
- Battery Voltage: It is the voltage range, otherwise called as DC input window, that can be fed to the Inverter. So, we have to select appropriate battery according to the inverter or Inverter according to the battery available. This value is only needed if we are considering battery system for the PV power plant or you are considering an off-grid inverter system.
- DC Current : It is the maximum DC current from the solar panels or the battery system the Inverter can handle.
- Voltage Range: This is the range of voltage generated from teh solar panels, that can be fed to the inverter.
- Maximum Short circuit current: The maximum current from the solar panel, the inverter can handle.
- Continuous power output: It is the maximum power the inverter can produce continuously for the longest time at a certain temperature. Here, it is important to remember that this power output depends on the ambient temperature. With increase or decrease in temperature from the standard value, the power output of the inverter decreases.
- Output Voltage: It is the voltage at the output of the inverter. It can be 230 Volts or 415 Volts depending upon the design.
- Frequency: Inverters are designed for certain frequencies. For India and most of the other countries it is 50 Hz, while for US it is 60 Hz.
- Efficiency: You may install solar panels that produce more power, but all of it does not get converted to useful electricity. Some of it is lost during the conversion from DC to AC. So the more efficient the inverter is, more electricity will be converted to useful electricity. The efficiency of the Inverter is generally specified with respect a standard load as inverter efficiency varies along with load connected at the output of the inverter. Efficiency of an Inverter is always less than 100% as it consumes some energy while converting the DC power to AC power. So do buy an inverter with higher efficiency to get more electricity output from your solar panels. Go for solar inverters with effficiency higher than 94%(with galvanic isolation feature) or more than 97% (without galvanic isolation).
- No Load Power Consumption: It is defined as the power consumed by the inverter when the inverter is not connected to any electrical load at the output. This value is important for us as, higher value will decrease the efficiency of the inverter. It should be less than 1% of the rated power output.
- Surge Power: Inductive loads like motors draw 3 to 6 times higher current at the starting than the normal running condition. So, while selecting an Inverter we should check that it has s surge power value of 3 to 6 to meet the need.
- Output wave form: It is very important that you choose an inverter with the right waveform. Look for a Solar Inverter that has pure sine wave property. It means the AC current and the AC voltage produced have pure sine wave property. And they are considered as high quality AC power. Any deviation from teh Pure sine wave or sinusoidal property will introduce harmonics and degrade teh quality of AC power. This property is measued by a parameter called THD or Total Harmonics Distortion. The more the value of THD, the lower is the quality of AC power produced by the Inverter. For good quality Solar inverters the THD value is always less than 4%. Avoid going for a Square wave or modified sine wave inverter. A small saving in the money here may cost your appliances.
- MPPT Charge Controllers: Always go for inverters with inbuilt MPPT Charge controllers control the flow of charge from battery to Inverter and Inverter to battery. So they cut off the current flow during battery over charging and over-discharging. MPPT or Maximum Power point Tracking charge controller allows to have different voltages accross battery banks PV array.PWM or Pulse Width Modulation charge controllers or standard charge controllers
- Ambient Temperature Range: It is the temperature range at which the inverter gives maximum output.
- Grid frequency Synchronization range: It should be 3 Hz or more.
- Humidity: Its value should be higher than 87 and should be non-condensing
- Ingress Protection or IP rating : Ingress protection rating or IP rating is a rating that is given to a product which certifies its level of protection from dirt or humidity. You may see IP65, IP66, IP67, or some other IP number mentioned on the inverter. The higher the number, better is the protection. IP65 is the minimum standard for any product to protect from dust and rain.
- Power factor or PF : It should be greater than 0.9.
- Built in Data logging: A good inverter will have a software that can help you monitor the power generated by your PV system and also check the health of your PV system
- Anti Islanding feature: In case of grid connected inverters, the most important protection required is Anti-Islanding protection. This makes sure that in case of power failure of the grid, your solar PV system is not sending electricity generated by your system to the grid. This is to protect engineers and technicians working on the grid to fix it during power failures. Your grid connected system will not get approval to connect to the grid till your inverter has Anti- Islanding feature.
- Inverter Warranties : Based on my observation i have found that you can get up to 5 years warranty on a Grid Tied inverter, whereas off grid inverters are available only for 1-2 year warranty. Just remember inverters from reputed solar inverter manufacturing companies can save you a lot of headaches and troubles in future.
This post on inverter and its parameters will certainly help you in buyingsolar inverters for your next project. If you need any help in sizing inverters, need any solar related help or designing solar PV projects you can contact me.