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

Lead Research Organisation: University of Sheffield
Department Name: Automatic Control and Systems Eng

Abstract

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 guarantee 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.

Planned Impact

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.

Publications

10 25 50
 
Description We have developed a series of enabling technologies to facilitate the large-scale utilisation of renewables, energy storage systems and EVs, including the harmonic droop controller and inverters with capacitive output impedances (C-inverters), a self-synchronised synchronverter that mimics virtual synchronous machines without a PLL, a parallel PI Voltage-H8 Current Controller to provide an independently controlled neutral point for distributed generation, a bounded Droop Controller for Parallel Operation of Inverters, a Single-phase Four-Switch Rectifier with Significantly Reduced Capacitance to improve power density of power converters.



G. C. Konstantopoulos, Q.-C. Zhong, B. Ren, and M. Krstic, "Stability analysis and fail-safe operation of inverters operated in parallel," International Journal of Control, vol. 88, no. 7, pp. 1410-1421, 2015.

Q.-C. Zhong and Y. Zeng, Universal Droop Control of Inverters with Different Types of Output Impedance, IEEE Access
Exploitation Route We expected some of these technologies to be taken forward by industry and the academia for further development.
Sectors Aerospace, Defence and Marine,Education,Energy,Transport

 
Description Distinguished Visiting Fellowship: Robust control of systems with delays and control applications in energy systems
Amount £6,000 (GBP)
Organisation Medical Research Council (MRC) 
Sector Academic/University
Country United Kingdom
Start 05/2013 
End 02/2014
 
Description Distinguished Lecture at the 26th Chinese Control and Decision Conference (CCDC), Changsha, China, May 31-June 2, 2014. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Chinese Control and Decision Conference.
Year(s) Of Engagement Activity 2014
 
Description IEEE Power Electronics Society, 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Jan. 2014 - Dec. 2015.
Year(s) Of Engagement Activity 2014
 
Description Keynote speech at 2014 IEEE International Energy Conference, Croatia, May 13-16, 2014. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact IEEE International Energy Conference.
Year(s) Of Engagement Activity 2014
 
Description Keynote speech at Real-Time 2014 Conference, Montreal, June 9-12, 2014. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Real-Time 2014 Conference.
Year(s) Of Engagement Activity 2014
 
Description Keynote speech at the 5th International Conference on Power Engineering, Energy and Electrical Drives, Riga, Latvia, May 11-13, 2015. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact 5th International Conference on Power Engineering, Energy and Electrical Drives, Riga, Latvia.
Year(s) Of Engagement Activity 2013
 
Description Plenary talk at Delta Power Electronics Forum, Suzhou, China, May 30, 2014. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Delta Power Electronics Forum.
Year(s) Of Engagement Activity 2014
 
Description Plenary talk at the 1st Indian Systems and Controls Conference, Chennai, Jan. 5-7, 2015. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact 1st Indian Systems and Controls Conference.
Year(s) Of Engagement Activity 2015
 
Description Plenary talk at the 29th Annual Conference of Chinese Universities on Electric Power Systems and Automation, Yichang, China, Nov. 7-10, 2013. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Annual Conference of Chinese Universities on Electric Power Systems and Automation.
Year(s) Of Engagement Activity 2013
 
Description Plenary talk at the 29th Annual Conference of Chinese Universities on Electric Power Systems and Automation, Yichang, China, Nov. 7-10, 2013. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Conference of Chinese Universities on Electric Power Systems and Automation.
Year(s) Of Engagement Activity 2013
 
Description Plenary talk at the 3rd Int. Conference on Advances in Control and Optimization of Dynamical Systems (ACODS), IIT Kanpur, India, March 13-15, 2014. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Conference on Advances in Control and Optimization of Dynamical Systems.
Year(s) Of Engagement Activity 2014
 
Description Plenary talk at the 4th IEEE Conference on Power Electronics for Distributed Generation Systems (PEDG) in Rogers, Arkansas, July 8 - 11, 2013. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact IEEE Conference on Power Electronics for Distributed Generation Systems.
Year(s) Of Engagement Activity 2013
 
Description Plenary talk at the 6th IEEE Annual Green Technologies Conference in Corpus Christi, Texas, April 3-4, 2014. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact IEEE Annual Green Technologies Conference.
Year(s) Of Engagement Activity 2014
 
Description Plenary talk at the 8th Symposium on Power Electronics and Electrical Drives (SPEED), Wuhan, China, April 11-13, 2014. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Symposium on Power Electronics and Electrical Drives.
Year(s) Of Engagement Activity 2014
 
Description Tutorial on Control of Power Inverters for the Smart Grid, at the 2014 American Control Conference, Portland, OR, June 4 - 6, 2014. 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact American Control Conference.

Received invitation to visit the Headquarters of Texas Instruments
Year(s) Of Engagement Activity 2014
 
Description UK representative, European Control Association (EUCA), August 2013 - 2015 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact European Control Association (EUCA) . Awarding Body - European Control Association (EUCA), Name of Scheme - EUCA
Year(s) Of Engagement Activity 2013,2014,2015
 
Description University Technology Partnership Board Member, Rolls-Royce Plc, 2014- 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Rolls-Royce Plc . Awarding Body - Rolls Royce plc, Name of Scheme - University Technology Partnership Board
Year(s) Of Engagement Activity 2014
 
Description Workshop on Control of Power Inverters for the Smart Grid at the 2013 American Control Conference, Washington, DC, June 16, 2013. 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Industry/Business
Results and Impact American Control Conference.
Year(s) Of Engagement Activity 2013