India is one of the economically developing countries with growing populations, thus the demand for electrical energy is rapidly increasing here. To fulfill this growing electrical energy demand, fossil fuel energy, nuclear energy and renewable energy sources are utilized. The fossil fuels are limited in nature and produce greenhouse gases that are responsible for global warming. Moreover, their prices are continually increasing. The nuclear energy looks promising, however; it is not reliable from safety point of view. The renewable energy resources are freely available, pollution free and not emitting green houses gases. Thus, renewable energy based electricity generation systems are more reliable for safety of society, environment protection and competitive prices.
There are various kinds of renewable energy sources available in nature; solar energy is one of them. India is a tropical country and over India’s land; vast solar energy about 5,000 trillion kWh per year is incident. Therefore, the Govt. of India targets to achieve the ambitious goal of 100GW installed capacity from solar energy till the end of the year 2022. Out of this, till the end of November 2021, 48.55 GW of solar plant has been installed including large and small scale with ‘on grid’ as well as ‘off grid’ systems. To achieve the targeted goal, the Govt. of India has launched various schemes such as, Solar City Program, Pradhan Mantri Solar Panel Yojana, Grid Connected Solar Rooftop Scheme and so on. However, the growing population, limited land and electrical energy demands are major challenges. The small scale solar photovoltaic roof-top systems play an important role to cater the above issues.
The ‘on grid’ solar photovoltaic roof-top supply systems are integrated with three phase and single phase utility grid. At small scale level; solar photovoltaic rooftop systems are mostly integrated with single phase utility grid and applied for residential applications. According to the power conversion stage, these are classified into two categories, 1) Single stage and 2) Double stage as shown in Fig. 1. They have their own features and drawbacks compared to each other as given below,
- The single stage topologies are less complex than double stages.
- Power conversion efficiency of single stage topologies is higher than double stage.
- In single stage topologies, PV panels suffer from a double line frequency component that reduces the life of PV panels, but double stage topologies offer less effect on double line frequency component due to two power processing stages.
Further, it has been classified into two categories, isolated and non-isolated. In isolated topologies, galvanic isolation between PV panels and utility grid is provided by a transformer (High frequency or low frequency). Moreover, in non-isolated topologies galvanic isolation between them is unavailable. The isolated topologies improve the safety concern, however; its efficiency is less than non-isolated topologies. Various topologies of single stage and double stage conversion including isolated and non-isolated converters are given in Figs. 2 and 3 respectively.
Overview of non-isolated double stage solar PV rooftop supply system
Fig. 4 shows the boost converter based non-isolated double stage solar PV rooftop supply system. It is implemented in the laboratory for small scale rating of 640W and 110V 50Hz AC supply system. The prototype is developed using a solar simulator (as a Solar PV panel), boost converter, full bridge voltage source inverter (VSI), gate driver circuit, AC filters, sensor circuits and microcontroller. Detailed specifications of the prototype are given in Table 1.
Performance analysis
The performance of developed system is obtained in both steady state and dynamic conditions. It is presented in details in the next few paragraphs:
Steady state performance: Steady state performance of a developed system is obtained at constant solar irradiation i.e., 1000W/m2 with 109 W non-linear loads at Point of Common Coupling (PCC). Fig. 5 shows the performance at PV panel terminals. It shows that the PV panel delivers maximum PV power through a derived control algorithm implemented on the microcontroller. Under the similar conditions, grid side performance is recorded and depicted in Fig. 6. As the proliferation of non-linear loads in the grid system creates power quality issues, the non-linear loads are considered at PCC. It is noticed that the load demands power of 109W as shown in Fig. 6(a) and the load current is non-sinusoidal in nature and its Total Harmonic Distortions (THDs) are 27% as shown in Fig. 6(b). Under similar conditions, power quality of grid supply voltage is recorded and its THDs are 2.1% as shown in Fig.6(c). Fig. 6(d) shows the grid supply voltage and current injected to it. The additional generated power (i.e., 500W) is injected to the grid supply as shown in Fig. 6(e). The grid current THDs are 4.9% as shown in Fig. 6(f) i.e. under limits of international standard IEEE-519 and it is done due to proper control of VSI.
Dynamic performance: Dynamic performance is obtained under dynamic change in solar irradiation and load demands. It is noticed in terms of boost converter performance and grid side performance as shown in Figs. (7-9). Figs. (7&8) show the dynamic performance under dynamical change in solar irradiations from 300W/m2 to 500W/m2. Under increases in solar irradiation, current delivered from PV panels and power generation increase as shown in Fig. 7. Fig. 8 presents the grid side performance under same solar irradiation conditions with zero load demands. Under rise in solar irradiation current as well power injected to the grid also increases as shown Fig. 8.
Fig. 9 shows the grid side performance under dynamic change in load demands at constant solar irradiation of 1000W/m2. When load is connected at the PCC current injected to the grid reduces as the PV power is constant. As the connected load is non-linear in nature, its current shape is non-sinusoidal. Moreover, current injected to the grid is purely sinusoidal due to proper VSI control.
Conclusion
This article has given an overview of the state of the art topologies of single phase solar photovoltaic roof-top supply systems. It also presented performance validation of a developed prototype of boost converter-based double stage single phase solar PV roof-top system in the laboratory. Its performance is obtained satisfactory under different operating conditions.
Pemendra Kumar Pardhi is currently working for the Ph.D. degree in Electrical Engineering from Shri Govindram Seksaria Institute of Technology and Science Indore, India, PIN – 452 003.
Dr. Shailendra Kumar Sharma received the Ph.D. degree from the Indian Institute of Technology Delhi, New Delhi, India, in 2012. He was a Postdoctoral Research Fellow with the Department of Electrical Engineering, École de technologie supérieure (ÉTS), Université du Québec, Montr´eal, Canada, during January– December 2015. He was an Executive Engineer (Design and Commissioning) with M/s Dhar Textile Mills Ltd., Indore, during 1998–2002. He is an Associate Professor in the Electrical Engineering Department, Shri Govindram Seksaria Institute of Technology and Science Indore. He has two Indian Patents in his credit in the area of solar and wind energy conversion systems.
