Need to overcome solar inverter challenges and issues in India
According to Dr. J. N. Roy, vice president (R&D/Engineering), Solar Semiconductor Pvt Ltd, while Indian made inverters could be mostly installed indoors, the foreign ones could be deployed both indoors and outdoors. In terms of durability, Indian inverters are expected to last at least 20 years, while foreign inverters had already proved that they can last this distance!
These limitations or challenges facing Indian made inverters were highlighted by Dr. Roy during a workshop on solar inverters, organized by the India Semiconductor Association (ISA) on 23 July, 2010, in Kolkata, India.
In terms of control, Indian inverters did not support wireless features as yet, while it is already available on foreign inverters. In terms of design and support, while Indian inverters were generally custom made, the foreign inverters were standardized and had simulation support. As for efficiency, while Indian inverters had efficiency less than 94 percent, their foreign counterparts boast efficiency higher than 97 percent — a telling differentiator.
In terms of battery voltage, Indian made inverters were available supporting 24V, 48V, 96V, 120V and 240V, respectively. Foreign made inverters support 24V and 48V as fixed. They are light and compact, as compared to large and heavy for locally made inverters. While Indian inverters are generally EC and TUV approved, the foreign inverters are EC and TUV approved and UL listed, respectively.
Dr. Roy also highlighted the challenges faced by power control units (PCUs) in India and touched upon the desired features of inverters.
Roy said that inverters today have a mean time between failure (MTBF) of less than 10 years. The computed MTBF above 100 years has already been achieved in recent modular solar inverters. Incidentally, solar PV panels easily meet the 25 years life requirement.
He provided case studies, such as off-grid DC couple system, off-grid AC couple system, and off-grid pump systems. In the off-grid pump systems, there were no batteries/grid as buffer. Power consumption can be matched with power generation by controlling speed of the pump. He advised making power generation as close as possible to maximum power.
Dr. Roy provided several examples of inverters. While multi string inverter and central inverter were examples of classification based on installations, solar inverters with and without transformer were examples of classification based on topology.
He touched upon challenges faced by PCUs. First, parallel operation of PCUs needs synchronizing. There should be some communication between the PCUs. Controlling should be done based on input from each PCU. The control should be based on inputs from each battery bank unit of the PCUs.
Next, load management control was necessary, while a master-slave mechanism should also be available. Control should take care of the optimal and safe operation of the PCUs. Failure of one or more of the PCUs should not affect parallel operation of others. He stressed upon the requirement and necessity of intelligent control.
Desired features of inverters
Dr. Roy also outlined the desirable features of inverters. They should be flexible in order to be easily installed indoor as well as outdoor. The inverter should be reliable — with a life span of over 25 years. Durability is yet another important factor. There should preferably be a warranty for a period of five years and an AMC for a period of 20 years.
The control should be impeccable. The inverters would be required to be compatible with a broad palette of the most diverse components. Finally, support — there should be design/simulation support.
Highlighting the limitations, Dr. Roy noted that the Indian supplier mainly manufactures the PCU. The PCU combines the inverter and battery charging unit. Next, the Indian PCU unit is custom made and not standardized to feed the customer requirements. Finally, engineering support has been minimal up until now.
All of these challenges and issues need to be addressed at the earliest, given that lot is at stake in order to make the Jawaharlal Nehru National Solar Mission a huge success in India.