Many solar designers often ask me which are the key parameters based one which we can select the solar inverter. Here are some important parameters (Technical Aspects) that need to be compared before finalization of any make of solar grid tied inverters.
First of all as per site climate if the irradiation graph is parabolic curve (radiation vs. time) rather than Central/Monolithic, inverter shall be used in comparison with Modular type Inverters.
Basic Parameters (Protection) Provided By Inverter Manufacturer:
DC Overvoltage, AC Overvoltage, DC Fuses, AC Short Circuit, DC Short Circuit, Frequency Out of Voltage, Voltage out of range, DC inverse Polarity, Ground Fault, Negative Grounding(GFDI), Insulation monitoring, HVRT, LVRT, Anti Islanding, AC output Breaker.
These all are the basic protection provided by inverter manufacturers but out of above the following two, Insulation Monitoring Relay & AC Output Breaker Type need to be confirmed from our end.
1) Output Breaker Type: Some manufacturers provide MCCB in place of ACB. So do speak to your manufacturer and please confirm this point from manufacturer itself.
2) Insulation Monitoring Relay: Inverter is equipped with Insulation Monitoring Relay but please confirm the make of this relay. To save cost some manufacturers provide simple ABB etc. make relay in place of Bender relay which is more sensitive for insulation failure.
Apart from these, the below technical aspects are very important to freeze solar inverter for your upcoming solar projects:
1) Maximum Efficiency: Higher Efficiency will affect Higher generation
2) Auxiliary Consumption: Lower auxiliary consumption will result in a higher generation. This auxiliary consumption is divided into three sub categories which are:
The number of fans that are provided with inverter to cool inverter and the consumption of it. If VFD is with FAN then surely you will save some amount of generation.
When inverter is running in the daytime, the IGBT cards need some aux power, this is called Control Circuit Consumption. Some manufacturers directly take this consumption from the DC Bus of the inverter and this extra power is not included in provided max efficiency of the inverter. So this again must be verified from the manufacturer.
At the night time, inverter will be in sleep mode but will continue to draw some amount of power from grid. This is called Standby Consumption.
3) Output Voltage: Due to Higher output voltage you can save some amount of cable runs from inverter to transformer so as to reduce cost.
4) Fresh Air Requirement(m3/h): This is required as low as possible to avoid any extra filtration unit for cooling purpose.
5) MPPT Voltage Range: Window of MPPT voltage should be wide enough(550-950V). Many manufacturers provide 600-850V window. In this case at higher temperature (at noon) series voltage will be reduced to say 580V as voltage is indirectly proportional to temperature. To avoid losing a fare amount of generation, this parameter must be carefully looked into.
6) Temperature Derating : Check with inverter manufacturer the graph of temp vs. output KVA of inverter. This will help you understand when your output KVA will start derating and at which temp. Some manufacturers might say that the output of inverter is 990KVA. But from 25deg C onwards, the output KVA will derate to say 900KVA upto 50deg C.
7) Shut off temperature: At which temp inverter will shut off. This indicates the max temp rating upto which inverter will provide some amount of output in terms of KVA.
8) Power Factor (0.8 lead to 0.8 lag) & Reactive Power Compensation: Generally inverter is fixed at unity power factor but this shall be adjusted as per grid situations manually. It can also sometimes sense the grid and be automatically adjusted. This is a very important aspect. If inverter feeds reactive power to the grid, you will loose some amount of active power generated. Also if in the signed copy of the PPA, government authorities have specified the details of reactive power generated at the project, directly feeding to grid can cause some amount of penalty resulting in further losses. Also note as inverter is directly connected to the transformer it accordingly senses power factor at this level only. But at grid main meter power factor will be different in comparison with at trafo level because in between both (inverter trafo & main meter at grid), cable capacitance and trafo inductance is not included. For sensing power factor at main meter some manufacturers provide reactive power controllers which are connected directly in between inverter to main meter via RS485 cable to sense actual power factor of the plant for better reactive power compensation.
By Amit Upadhyay. Amit is a solar energy enthusiast and is currently Deputy Manager-Design & Engineering at Mahindra Susten.
Disclaimer: The views above are strictly those of the author and do not reflect those of any other organisation or individual or this publication.
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