High Efficiency Commercial PV Inverter

Traditionally, importance of efficiency is valued the highest for PV (Photovoltaic) inverters. The driving factors are the benefits on installation costs, logistics and real-estate (in case of commercial installations). Obviously, lots of recent research has been dedicated to the performance enhancement of PV inverters. However, the best European efficiency of commercially available inverters (above 100 kW) is 98.3 % and this figure is true only for a part of the actual PV voltage range. For most of the practical purposes, the efficiency specified goes down by about 1.5-2 %. Many of the topologies are still confined to research and not able to make their way to the market, due to practical considerations of manufacturers and customers (agency norms, EMI, reliability etc). Lots of research has been dedicated to develop novel topologies but, very little has been done towards practical innovative solutions.

This research aims at achieving an European efficiency above 98.5 % and at the same time keeping it independent of varying input PV voltages (operating point). Also, the solution must give the best compromise between initial system costs and efficiency. Another primary objective is to come out with a practical and sustainable solution, suitable to the industry. The work is concentrated on commercial PV inverters in the power range of 100-500 kW.

Economics of Photovoltaic System:

The pie chart gives an approximate distribution of costs of a commercial PV installation connected to a 400V grid. It can be seen that the inverter contributes to only 8% of the system costs.A 1% improvement in inverter efficiency implies 80$/kW lesser initial costs and additional benefits on logistics (land costs, etc.)

ηeuro = 0.03η5% + 0.06η10% + 0.13η20% + 0.1η30% + 0.48η50% + 0.2η100% 

Therefore, partial load efficiencies play an important role in selection of a topology.

Loss distribution:

Obtaining the loss distribution in various topologies is a must due to the following two reasons.It gives us an opportunity to identify the major loss producing areas, which can be targeted for improvements.Topologies with lower switching loss components will be suitable for higher switching frequencies. Higher switching frequency operation reduces cost. Lower switching loss component also imply an efficiency which is DC –link voltage independent. This characteristic is highly desirable due to the large range of PV voltages(due to their temperature dependent characteristic).

Cost:

The cost is undisputedly the most important parameter which influences the commercial success of any product. The idea here is, however, not directly to reduce costs, but to make the best compromise. Increase in costs must be weighed with respect to the payback period (due to better efficiency), decrease in system costs, Real estate costs and other logistics.Also, In some cases, the cost is reasonably reduced by the use of control strategies, specific to each topology.

Application specific requirements:

Some of the installations feed into a medium voltage (10kV) grid, while others feed into a low voltage (400V) grid. The solution, which suits one configuration, is not the best option for the other configuration. Therefore, there is a need to identify optimal solutions for both cases. Last but not the least, the solutions must confine to the various agency norms.

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