Professor Tim Green, FREng

My principal research focus at present is the control, stability and protection of electrical power grids when almost all resources (generators, energy storage and flexible demand) are inverter-based resources rather and synchronous machines. This change away from rotating electrical generators makes dramatic changes to the dynamics of the systems. Perturbations to the system (such as a sudden change in generator output) causes much faster changes to the angle, frequency or amplitude of the system voltages. These three types of system dynamics were previous quite separate which aided analysis of stability but now, for instance, frequency dynamics are much faster than before sand couple to angle dynamics. A further complicating factor is that inverters are largely defined by their control software rather than their physical properties. Thus the dynamic model of an inverter is generally commercially confidential rather than openly known. This means there is an emphasis on developing grey-box models (somewhat more open than black-box models) and data-driven models, perhaps based on measure frequency spectrum data.

I also pursue the use of power electronics to create flexibility and enhance functionality in existing power networks. Although power electronics is a relatively expensive technology compared with traditional transformers and cables, it can provide facilitate much better use of existing assets and thus make a good case for investment.

I have previously worked extensively on technologies for High Voltage DC (HVDC) connections from very large windfarms in the North Sea and to and from other European countries. We can foresee a set of subsea connections around the British coast and these cables will need to be operated on DC.  My work focused on how to optimise the design of very large DC to AC power converters to interface these links. This work has been supported by EPSRC and Alstom Grid (now GE Grid Solutions). 

Power electronics also has a role in the final distribution from the local substation into homes and offices. This well-established network will soon be under stress from greater power flows as we connect more electric vehicle charging points, more heat-pumps and more roof top-top photovoltaic panels. Here power electronics can play a vital role in realising greater power flow capacity from existing assets by managing voltage control better. This avoids the expense and disruption of digging up pavements and roads to replace cables. I have worked on using power electronics to create fast-acting tap-change transformers and also to create "soft open points" that provide controlled meshing of radial networks. This work was been in conjunction with MR and UK Power Networks with support from EPSRC.

Recent Ph.D. Completions

Y. Zhu, Ph.D., September 2022, “Impedance Model Analysis and Measurement for Power System Stability”

C. Collins, PhD., October 2021, “Multilevel Converter Topologies for MVDC”

Y. Li, PhD., December 2020, “Modelling and Control of Power Electronic Converters in Power Systems”

Pérez Olvera, Ph.D., December 2020, “Active Network Management for Improved Flexibility in Distribution Networks”

Thiago R. F. de Mendonća, Ph.D., December 2019, “A Distributed Control and Coordination Strategy Based on Voltage Sensitivity for Network Management”

Xin Xiang, Ph.D., March 2019, “The Modular Multilevel DC Converters for MVDC and HVDC Applications”

J. Wylie, Ph.D., January 2019, “Reliability Analysis of Modular Multi-level Converters for High and Medium Voltage Applications”

G.P. Chaffey, Ph.D., January 2017, “The Impact of Fault Blocking Converters on HVDC Protection”

P. Judge, Ph.D., November 2016, “Power Converter Design for HVDC”

C. Sheridan, Ph.D., January 2016, “Assessment of HVDC Technologies for an Offshore MTDC Grid”

C.E. Spallarossa, Ph.D., September 2015, “Frequency and Voltage Stability of Mixed AC and DC Systems”

T. Lüth, Ph.D., December2014, “DC/DC Converters for High Voltage Direct Current Transmission”

M. Collins, Ph.D., December 2013, “Multi-Agent System Control and Coordination of Distributed Energy Resources in Electrical Power Networks”

N. Bottrell, Ph.D., October 2013, “Small-signal Analysis of Active Loads and Large-signal Analysis of Faults in Inverter Interfaced Microgrid Applications”

M.M.C. Merlin, Ph.D., January 2012, “Hybrid Multi-Level HVDC Converter and Multi-Terminal DC Networks”

J.M. Bloemink, Ph.D., December 2012, “Distribution Level Power Electronics: Soft Open Points

Y. Pipelzadeh, Ph.D., July 2012, “Coordination of Damping Control in Transmission Networks with HVDC links”