Discrete time current regulation of grid connected converters with LCL filters
Creators: name: Parker, Stewart
Publisher: name: RMIT University
Year: 2024

description: Two important components of a grid connected power electronic converter are the line filter and the closed loop current regulator. Together they are largely responsible for system stability, power flow and power quality into the grid. The LCL filter is a smaller and cheaper line filter alternative because of its third order filtering capability. However the LCL filter has a resonance that must be appropriately damped using either passive or active techniques, generating more losses or adding complexity to the controller respectively.<br><br>It is now generally accepted that the PWM transport delay due to discrete/digital implementations is the main limiting factor for controller bandwidth in L filtered systems. However, despite the large body of literature for the LCL filter, there is still only limited consensus regarding the implications of PWM transport delay on the current regulator and active damping controller for this type of filter. This thesis applies discrete time models to these systems to overcome these perceived limitations and hence develop the optimal controllers. This knowledge is then used to enhance the current regulator to overcome further practical problems.<br><br>The first part of this thesis focuses on the development of discrete time current regulation for a grid connected inverter. The benefits of discrete time modelling and control for current regulation are demonstrated by using a discrete state feedback controller for an L filter system. A precise discrete time model of the LCL filter system is then developed to exactly identify the frequency region where active damping is mandatory, and the high frequency region where active damping is not required. The critical frequency, which separates these two regions, is identified as a fraction of the sampling frequency, demonstrating the controller's dependence on PWM transport delay. Controllers and gain selection methods are developed and verified for each region. A generalised approach for analysis of the LCL filtered system is then developed so that all forms can be evaluated on a precisely comparable basis. Using this generalised approach the particular advantages and disadvantages of each control method are readily identified.<br><br>The second part of this thesis looks at the impact of two practical issues for current regulation of LCL filtered grid connected converters. It firstly identifies that practical converters generally do not match their ideal output current quality expectations. The reasons for this distortion are explained and harmonic compensators are then proposed as an effective solution to overcome it. Secondly the implications of a virtual neutral common mode EMI filter on the current regulator are investigated. A virtual neutral filter design is proposed that utilises the primary LCL filter components. The active damping current regulator is then enhanced to avoid interference from the additional current path and to actively damp the common mode resonance.<br><br>All theoretical work is validated by extensive simulation and experimental results.