ICIT - IEEE 2018

From 02/19/2018 to 02/22/2018

Centre de Congrès - Cité Internationale Lyon - 50, quai Charles de Gaulle 69463 Lyon

Tutorial 11 Dr Francisco HUERTA SANCHEZ

Title: Advanced design and control system techniques for grid-connected voltage source converters with LCL filters

Voltage Source Converters (VSCs) are widely used to connect renewable energy sources and other electronic devices to electrical grids. These converters produce high‐frequency switching harmonics that must be filtered out to comply with power quality standards. Recently, LCL filters have gained popularity with respect to L filters. However, the design of these filters is not straightforward, especially in grid‐connected applications with relatively small switching frequencies. Moreover, these filters produce a resonance that commonly interacts with the VSC control system. Traditionally, the control system of LCL‐filtered converters has been addressed borrowing control techniques from L‐filtered converters. However, in this case additional hardware or control system modifications are required to damp the LCL filter resonance. Passive damping solutions are robust, but they produce additional losses so active damping solutions are preferred. However, additional measurements are required so the number of sensors and the complexity of the controller increases. The proposed tutorial will address recent advances in control systems for VSCs with LCL filters. On the one hand, the tutorial will explore recent active damping proposals to improve the control system robustness in weak grids. On the other hand, the use multivariable controllers is addressed. These controllers inherently damp the LCL filter resonance and make it possible to apply advanced design techniques to improve the system robustness. A detailed structure of the tutorial session is described in the following points: 1. Introduction to the LCL‐filtered converters: The use of LCL filters instead of L filters will be justified. The design of LCL filters will be summarized and the resonance problem will be highlighted. 2. Control of LCL‐filtered converters at a glance: The problems of extending the control of L‐filtered converters to the LCL‐filter case will be presented. This section will briefly describe passive damping approaches, single‐loop controllers with active damping methods, etc. 3. Power quality issues in LCL‐filtered converters: In this section, some remarks of power quality are given. In particular, problems of LCL‐filtered converters connected to weak and distorted grids. 4. New advances in active damping methods: First of all, the basics of active damping will be shown. It will be shown that active damping can be provided at the design stage if the design constraints allow it. However, when this is not possible, an all‐pass filter in series with the current controller is used with this purpose. Two alternatives to implement this filter are shown: a first‐ and a second‐order all‐pass filter. With this addition a classical PI controller can be easily designed to control the grid‐side current of the filter. It will be shown that this strategy provides large stability margins and fast transient responses. This alternative will be compared with other alternatives already present in the literature. 5. New advances in multivariable state‐feedback controllers: After presenting the discrete‐time state‐space modelling of LCL‐filtered converters, this section will be present the structure of the proposed multivariable state‐feedback controllers. A methodology of design that allow to include more complex control structures will be presented and the structure will be extended to the case with resonant controllers used in high distorted grids. The one‐step adjustment procedure will be introduced along with the use of some modern control techniques such as LQR and LMI that automatize the design process and provide robust criteria. Additionally, this section will present the design and use of state estimators in order to reduce the number of sensors. Finally, the tutorial will cover some issues related to the implementation of these controllers and it will provide examples of their use that prove their advantages and feasibility in today’s control platforms.