Developing Fundamental Theory and Enabling Technologies for Parallel Operation of Inverters to Facilitate Large-scale Utilisation of Renewable Energy

UK Research Councils have set up a RCUK Energy Programme, investing more than £530 million in research and skills to pioneer a low carbon future. Energy is also a major application area funded by TSB. Several major global companies, including BP, Caterpillar, EDF Energy, E.On, Rolls-Royce and Shell, have joined their forces with the UK government to establish the Energy Technologies Institute, creating a potential £1billion investment fund for new energy technologies. The ongoing research programmes cover various aspects of energy from generation, transmission to end use, in order to create affordable, reliable and sustainable energy for heat, power and transport. Increasing the share of renewable energy, e.g. wind, solar, marine and biomass, and improving energy efficiency are the two most important ultimate goals for all energy-related programmes.

The renewable energy needs to be connected to the grid, preferably, via inverters in order for them to take part in the grid regulation, in particular, for large-scale renewable installations. However, the capacity of individual power inverters is limited and multiple inverters are needed to be operated in parallel to achieve the power capacity needed. For a 5GW offshore wind power site, 1000 of 5MW inverters are needed. How to make sure that the inverters will share the load proportionally/evenly is a challenge. It should not be assumed that inverters could be connected in parallel automatically. Without proper mechanisms in place, circulating currents may appear and some inverters may be overloaded, which may cause damage. The system may even become unstable and lead to unwanted behaviours. The parallel operation of inverters has been a major problem in industry that prevents the large-scale utilisation of renewable energy sources. This is a simple problem which has not been solved properly for many years. The conventional droop control strategy is a promising technology but the sharing accuracy cannot be guaranteed. Very recently, the PI has revealed that the conventional droop control scheme and its variants do not possess a mechanism to make sure that the sharing accuracy is robust against numerical computational errors, parameter drifts and component mismatches. A robust droop controller is then proposed, which is able to maintain accurate sharing of real power and reactive power at the same time and also to maintain good voltage regulation when the inverters are of the same type. The problem is still unsolved when the inverters are different.

The major aims of the project are to develop fundamental understanding about parallel-operated inverters and to develop enabling contorl technologies to facilitate the large-scale utilisation of renewable energy and distributed generation. The ultimate goals of the project are to develop universal control strategies that allow the parallel operation of inverters with different types of output impedances and to develop a fundamental theory to guarantee the stable operation of power systems with parallel-operated inverters.

The proposed research addresses a challenging fundamental problem being faced by the renewable energy industry for many years. The solution of the problem will allow inverters of different types to be operated in parallel, directly or in close proximity, and hence this will considerably facilitate the large-scale utilisation of renewable energy, making a significant impact to sustainability and contributions to a low carbon future. Because the technologies developed can be applied to different applications, e.g. wind, solar and wave energy, the return of the investment is very high. The technologies will facilitate the mass production of inverters, which will help strengthen the manufacturing capability of the UK.

The technologies developed can also be applied to other areas where inverters are needed. For example, emergency lighting, uninterruptible power supplies, high-voltage DC (HVDC) transmission, static synchronous compensator (STATCOM), high-power motor drives, active power filters (APF) etc. This considerably widens the impact of the proposed research, which is not just limited to the large-scale utilisation of renewable energy.

An Industrial Advisory Committee (IAC) has been set up to provide valuable inputs from industry. The IAC will be chaired by the Chief Technology Officer in Electrical Systems of Rolls-Royce Plc and the current IAC members include the UK Power Systems Operator (National Grid), four major international suppliers and market players (Rolls-Royce, Alstom, Yokogawa and Texas Instruments), two UK SMEs (Turbo Power Systems and Power Systems Warehouse), and one non-profit organization Midlands Energy Consortium who will provide support on disseminating the research via regular MEC events. More industrial partners will be taken on board when necessary. The project will provide the partners direct access to the latest enabling technologies.

The PI and his team will work closely with industry, which will considerably enhance the quality of the research because the most challenging problems are from industry. The proposed research tackles fundamental problems faced by the industry, which will stretch the team to investigate the problems in depth and establish a fundamental understanding before solutions can be found. As a result, the research outcomes will be very significant. It will also enrich the experience and improve the skills of the PDRA, the technician and the PhD student involved in the project who is funded by the department.

The invention of inverters with capacitive output impedances (C-inverters) is of significant impact as this fills a gap in the theory and application. This may lead to some further important developments.

The way to look at the power delivery to a current source instead of a voltage source opens up a new direction, which may lead to solving some fundamental problems.

Treating harmonic voltages and currents at individual frequencies will be able to considerably improve the performance of the inverters.

The stability of systems with parallel-operated inverters is crucial for the large-scale utilisation of renewable energy. Hence, solving this problem is of theoretical and practical significance.