T5: Advanced Digital Current Regulation Strategies for Grid Connected Inverters

2:00PM - 3:50PM
Monday, June 3rd, 2019

Grahame Holmes
RMIT University, Australia



Current regulation plays a key role in power electronic conversion systems. The central concept is straightforward - compare a measured current against a defined reference, and minimise the error between these two quantities by adjusting the switching of the associated power electronic converter. However, while apparently simple in principle, achieving this goal in practice for AC inverter systems has proved to be very challenging. Over the last decade, such issues have become especially important for grid connected inverters, where the challenges of low per-unit (p.u.) filter inductances, unbalanced grid voltages with significant harmonics, a preference for direct digital control, and stability concerns with the inverter's associated phase locked loop subsystem, create unique and particular challenges for current regulation.

This tutorial will present the current state-of-the-art for digital current regulation of grid connected AC converter systems. It will begin by showing how PWM transport and sampling delays are the primary constraints for linear regulators. Strategies to overcome these constraints will then be explored, including back EMF compensation, PR resonant control and its equivalent synchronous d-q frame implementation. An analytical approach to calculate the maximum gains for these strategies will be developed, verified by simulation and matching experimental results. Next, these concepts will be extended to the particular difficulties of current regulation for grid connected inverters. Issues considered will include gain determination when a low p.u. inductive-capacitive-inductive (LCL) filter is used, formal strategies to design a current regulator in the sampled z-domain transform space, how to best manage modulation saturation, and the influence of common mode EMI filtering on the current regulator performance. The tutorial will then explore how to manage grid harmonics, unbalanced grid voltages and high impedance grid networks, by using cascaded PR regulators for each voltage harmonic, and positive/negative sequence current regulators operating in both the synchronous and stationary reference frames. Finally, the tutorial will present some recent advances to maintain stable current regulation for a stand-alone microgrid when stability issues associated with the inverter's phase locked loop are taken into account. All theoretical material presented will be supported by detailed matching simulations and experimental confirmation.



Professor Holmes graduated from the University of Melbourne in 1974, and has a Masters degree in power systems engineering, and a PhD in PWM theory for power converters. For 26 years he was an academic at Monash University, working in the area of Power Electronics, where he established the Power Electronics Research Group to support graduate students and research engineers working together on a mixture of theoretical and practical R&D projects. The interests of the group include fundamental modulation theory, current regulators for drive systems and PWM rectifiers, active filter systems for quality of supply improvement, resonant converters, current source inverters for drive systems, and multilevel converters. In 2010, Professor Holmes took up the position of Innovation Professor - Smart Energy at RMIT University. This position has allowed him to expand his research activities also into applications of power electronics, particularly in the area of Smart Grids and Smart Energy technologies.

Professor Holmes has a strong commitment and interest in the control and operation of electrical power converters. He has made a significant contribution to the understanding of PWM theory through his publications and has developed close ties with the international research community in the area. He has published over 230 papers at international conferences and in professional journals, and has published a major reference book on Pulse Width Modulation of Power Converters which is now recognised as a seminal work in the area. He is a Fellow of the IEEE and is an active member of the IEEE Power Electronics Society and reviews papers for all major IEEE transactions in his area.