String versus Central inverter – which one is better choice?
Most of the solar projects in india are designed with central inverter except few. Such as 5MW Aditya Birla Group in Balmer Rajashthan has SMA string inverter and 6MW plant in Andhra Pradesh by arhyama solar using string inverters. The standard warranty of these inverters is 5 to 10 years. However many inverter manufacturer offer extended warranty also considering string inverter is a costlier proposition as compared to a central inverter, however an apple to apple comparison can only be made consideration of not only cost per watt of string versus central, but also cost reduction of DC cables and other associated benefits such as reduced down time in case of string inverter .In case a string inverter fails the electricity lost will be associate with single array causing loss of 0.1 to 0.12 % depending on inverter capacity .However in case of central inverter if a 500KW central inverter fails it may cause loss of 10% energy in atypical 5MW plant.
A central inverter takes input from number of arrays and operate at single MPP. Hence the inverter MPP(maximum power point) is governed by the arrays which are having partial shading ,mismatch losses , modules with tolerances which may lead to reduce output in case of central inverter. However this can be reduced by selection of string inverter as different strings have different MPP so that the output is maximized.
Above figures shows performance analysis with string and central inverter of a typical 1MW plant with 280W module with tolerance of 2% in PVsyst .in case of central inveretr the power loss due to module mismatch,shading and module different MPP has effect of 2.1% loss compared to string inverter. Installation of central inverter is costlier as compared to string inverter as it requires a separate concrete foundation and an inverter room. However string inverter do not require any foundation can be mounted at the array structure itself which is great advantage .
Each string inverter has a separate voltage and current monitoring device which makes the system monitoring easier as compared to central inverter . In case of string inverter it is possible to monitor the overall plant performance at string level and the losses can be minimized without any additional expenditure in sophisticated monitoring system. Many times inverters are available in single phase and three phase power output can be achieved by bringing multiple strings together .
It is important to weigh the advantages and disadvantages string versus central not only from typical hardware installation point of view but also from the impact on generation from the plant.
Dr sanjay vashishtha & Rishikesh Muthyal
image courtesy: PVsyst ,ABB