Ultra-efficient grid-tie inverter technology

A grid-tie inverter (GTI) is an essential sub-system when feeding power from a renewable energy source into the grid. A GTI is a power electronic circuit that takes unregulated power, normally in dc form, produced by the source and converts it into ac form at the correct frequency and magnitude. When converting power from, for example, a small domestic photo-voltaic (PV) solar panel array, the inverter is normally a single-phase type and is typically rated at 1kW. Desirable features in a GTI are:

i. High efficiency over a wide load range to maximize the return on investment (ROI);

ii. Low cost-per-watt, again to maximize ROI. Compared with power electronic circuitry operating at similar voltages, such as computer power supplies, the cost-per-watt is high;

iii. High reliability. Again, compared with other power electronic circuitry, reliability is poor.

Given the probability density function of the available power from a renewable energy source, the GTI's part-load efficiency is important. Whilst efficiency is highly desirable to maximize the ROI from a solar panel installation, a corollary is low inverter losses. This implies low internal temperature rises. Limiting the temperature rises of the power semiconductor devices is essential if high reliability is to be attained. Forced-cooling is not normally feasible in GTIs due to the cooling fan's limited reliability and lifetime. Furthermore, the parasitic energy load of the forced cooling system itself is not insignificant. Limiting the amount of power dissipation at source is therefore important.

Recent years have seen significant improvements in the switching and conduction capabilities of power semiconductors. Silicon carbide (SiC) and other technologies are seen as potent alternatives to the established silicon (Si) IGBT in high efficiency power conversion. Due to the relatively high costs of the new devices their initial application is foreseen in areas where ultra-high efficiency and/or high power to weight are a paramount requirement.

The objective of this application will be to investigate different inverter topologies, power semiconductor devices and passive component designs with the objective of attaining very high conversion efficiencies over the wide range of operating conditions encountered in real world applications. The specific research outcomes will be:

i. A methodology for identifying and evaluating the optimal inverter topology and device combinations for ultra high efficiency power conversion;

ii. New high fidelity models for the selected switching devices within the chosen circuit implementations, capable of estimating the loss over the full operating range of powers and conditions;

iii. Improved calorimetric test methods and practices for measuring the loss within ultra-high efficiency power conversion systems, at a component and system level.

The proposed research will culminate in a fully characterised prototype inverter operating from a 400V to 450V dc rail and supplying up to 1kW into a 230V mains supply with an efficiency of over 99.2% over a load range from 20% to 100%.

The research is timely due to current concerns over global warming and the need for alternative low carbon energy sources. Academic research into GTIs in the UK has tended to focus on issues such as grid synchronization algorithms, maximum efficiency point tracking (MEPT) techniques, control and power quality. In contrast, relatively little has been done on the topic of efficiency in the UK and this will be the main theme of the proposed research. The prior art here has mainly originated in Germany and the USA. The proposed research will help to identify the UK with this theme. Although the primary intended objective is in GTIs for PV installations, the technology will have applications in many other areas, for example, in machine-drive inverters and electric vehicle drives.

Seminars:
Seminars on solar power are held frequently in the UK. However, inverter hardware features comparatively infrequently in these events. The applicant will use these opportunities to disseminate the research to the academic and industrial communities. Within the last year the EEMG has attended two European Centre for Power Electronics (ECPE) workshops on power electronics and a workshop on SiC devices. The applicant would intend to continue our representation at these events by attending at least one similar workshop. These events represent a good opportunity to present and discuss our work with some of the most prominent researchers in the field, for example, academics from ETH Zurich and Virginia Tech.

Conference presentations:
It is intended to submit at least two conference publications. These are likely to include the IET Power Electronics, Machines and Drives Conference (PEMD) and the IEEE Energy Conversion Congress and Exposition (ECCE).

Journal publications:
It is intended to submit at least one journal paper. Target journals are likely to include the IEEE Transactions on Power Electronics, the IEEE Transactions on Industrial Electronics and the IET Power Electronics journal. Also, papers presented at PEMD and ECCE may be short-listed for further peer-review for consideration by the IEEE Transactions on Industry Applications.

Government and industry funded community organizations:
There will be opportunities to communicate the project outcomes to the wider community through the applicant's research group's membership of the ECPE, the National Microelectronics Institution (NMI), and research council funded initiatives. The research group is a member of the core management team defining the vision and delivery strategy of £18M EPSRC funding for a UK Virtual Centre in Power Electronics, and a partner on the £3M EPSRC funded Vehicle Electrical System Integration (VESI) project, which is exploring novel integrated power conversion technologies for low carbon vehicles. These programmes involve the UK's leading academic institutes and many key industry partners.

Exploitation:
The proposed research will improve the University's capabilities in ultra high efficiency power conversion and demonstrate the potential of exploiting emerging new power semiconductors in the grid-tied inverter (GTI). Although the research focus is to develop high-efficiency GTI technology, the outcomes are also expected to have applications in other commercial low carbon energy sectors, for example, in the on-board battery chargers used in electric vehicles. In addressing the challenges of ultra-high efficiency the developed capability will inform and de-risk developments in these other sectors, such as automotive EV/HEV drive trains and aircraft power systems. The knowledge gained will feed into pre-competitive research activities, such as that being proposed within the EPSRC Virtual Centre in Power Electronics.

Commercial exploitation:
The applicant has strong links with companies directly involved with the design and manufacture of power electronic converters used in renewable energy converters. The wider activities of the research group in aerospace and automotive power conversion will provide opportunities for exploitation in other sectors. Additional collaborative links exist with semiconductor device fabricators through the University's cross-faculty Bristol Power Electronics Innovation Centre.

Undergraduate teaching:
Bristol is a research-led university and the research activities of staff inform their undergraduate teaching. The proposed research will inevitably provide spin-off activities that will provide subject-matter for final-year undergraduate projects. Graduates will therefore benefit from having exposure to power electronics in the context of renewable energy applications. There is a UK skills shortage in the area of power electronics and the research will contribute to redressing this.