TRADE-OFF BETWEEN STRING MONITORING SETUP COST AND ELECTRICITY YIELD ECONOMICS
Most of the solar PV system installed in last three years in India have used central inverters and monitor the plant parameters at inverter level which is typically 500 kW, 750 kW or 1 MW capacity. In a typical 5 MW plant there are about 10, 500 kW inverters connected to the solar field and plant parameters are monitored at each inverter (this configuration is assumed) and parameters at individual inverter level can be compared and the underperforming inverter can be identified. Subsequently, the operation staff visit the related array field and detects the fault at string level through a physical visit. However, there is an output uncertainty in parameters recorded at central inverter level, which is about 5 %. This means, if any array connected to a typical inverter has fault may remain un-noticed while O&M staff monitoring at the central level.
A typical string is off 5 kW capacity. Which represent 1% of the 500 kW inverter, hence multiple string failure can go un-noticed until the field technicians physically visit those arrays and check the array parameters with the help of tong tester. Many times the string failure could go un-notices for few month until and unless there is defined scheduled inspection of each string by the O&M team.
In order to understand the quantum of losses consider that a plant has two strings which are faulty and having average annual yield of (two strings) 5 kW* 2= 10 kW. The average annual yield from these two string is about 16000 kWh (1600 kWh/ kW). If we consider that the faulty strings remain unnoticed for a quarter (3 months), losses could be of the order of 40000 INR (Considering the electricity price at 10 kWh).
There are about 1000 string in a typical 5 MW solar plant and it is quite possible that about 2% of the string may get faulty at any point of time in a year. This shows that plant has overall 2 % reduced output as compared to a plant monitored at string level. We have already demonstrate in our early article that 1 % loss reduction in 5 MW plant can save 20-25 lac in a year. Typical cost of string monitoring system in a solar plant is about two rupees per watt. Which means in a typical 5 MW plant if you want to string monitoring system you need to just have an incremental furniture of 10 Lac which can be recovered through your saving released through quick recovery and detection of strings in first year itself. In countries like USA japan, most of the solar plant have installed string monitoring system. However, in Indian context many investor just ignore installing the string monitoring system and is not even discussed with the EPC as a part of their scope of work.
While market is moving to central inverter to the string inverter many investor have realized the benefits associated with string monitoring and incremental cost of string inverter as a part of their strategic decision at string level.
Firstgreen team is highly professional and specialized in solar plant performance audit.
Posted By: Gaurav K. Sharma & Dr. Sanjay Vashishtha