Transient Response Analysis and Design of Current-Controlled Grid-Tied Converters

Supervisor: Jesús Doval-Gandoy

Cosupervisor: Francisco D. Freijedo

Abstract

In the current scenario of the electric grid, high penetration of renewable energy sources, distributed power generation systems and power electronic converters coexist. In order to prevent destabilization, the system operators have defined international grid codes (GCs), which, in case of a fault, ask generation units to remain connected and support the grid during a certain time, by means of reactive current injection. Aside from the compliance of these and other ancillary services, installations must fulfill the specific demands of the end-customers.

Regardless of the generation unit nature, voltage source converters (VSCs) are the most common option as interface for grid connection in most of these applications. In order to guarantee the proper behavior of the system, together with the satisfaction of all the restrictions, a rigorous analysis and design of the control loops of such converters becomes crucial. This challenge should start from the innermost loops, which are usually current ones, since they establish the performance of the overall system, specially in terms of transient response. Therefore, the main objective of this dissertation is to provide a thorough study of the current control closed loop, oriented to an accurate tuning of the regulators.

In order to achieve such goal, a precise model of the current loop is needed. Linear low order time-averaged plant models are usually selected, since their use allows to apply the well-known linear analysis and design methods. Additionally, the plant model parameters should be accurately identified in each specific working conditions. Linked to this aspect are some of the contributions of this thesis. Firstly, a method to estimate the VSC equivalent loss resistance is developed. This parameter permits to reflect on the plant model the effects associated to the converter losses, which have a great impact on the current loop behavior, particularly when the design constraints are established in terms of transient response, as it is demonstrated. Even though the role of the converter equivalent loss resistance is commonly accepted, just a few works aimed at current loop analysis and design consider it and, at this point, the available information on how to obtain its value is scarce.

Aside from the variations caused by the working conditions in the resistance, the plant inductance may also present important uncertainties with respect to the value of the VSC L/LCL interface filter measured at rated conditions (e.g., owing to core saturation and current magnitude, to temperature, to minor internal faults or changes...). Besides, extra impedance may be present in the electric circuit due to the existence of additional elements, such as the coupling transformer, the grid impedance or long wires. Therefore, a method to identify both parameters in the previously referred plant model, namely the equivalent inductance and resistance, is presented. Such estimation technique is an improvement of the above-mentioned one presented in this thesis (aimed only at the resistance identification). Both proposals have their basis on model reference adaptive systems. Unlike the available techniques, these are specifically targeted at transient response optimization of grid-tied converters. The variables to be identified are iteratively estimated so as to minimize the error between the actual and the expected closed-loop current step response. Consequently, the proposed identification methods are directly oriented to the fulfillment of time-domain specifications. They can be applied either online or offline, during a pre-commissioning stage.

Once the plant parameters are identified, and after selecting the most suitable controller structure, the regulator has to be accurately designed. One common choice in grid-connected applications are resonant controllers, given their ability to regulate with zero steady-state error both sequences, while providing a good combination of simplicity and high performance. Regarding the integral gain of proportional-resonant (PR) controllers, although some qualitative guidelines are available for its tuning, they mostly do not consider the delay effect. Furthermore, they are not aimed at the optimization of the disturbance rejection transient response, which is crucial to fulfill the low-voltage ride-through requirement of GCs and, as proved in this work, more demanding than the reference tracking one. One of the contributions of this thesis is a methodology to assess and design both types of transient responses when PR current controllers are implemented. The developed methodology is based on the analysis of the error signal transfer function roots by means of root loci. The optimal gains are determined to attain fast and nonoscillating transient responses, i.e., to minimize the settling time. As demonstrated, such gains result from a tradeoff between optimizing the transients caused by reference changes and the transients due to alterations at the point of common coupling.

On the other hand, a study about the suitability of different resonant regulators in grid-tied applications in terms of settling time, overshoot and harmonic distortion is not available in the literature. In this dissertation, an in-depth comparison between vector proportional-integral (VPI) and PR controllers in these terms is provided. Since the VPI controller tuning oriented to those aims has not been approached yet either, firstly, a rigorous analysis and design is conducted by applying to this regulator the methodology described above, which was originally designed for the PR controller.

Contributions of this dissertation have been published in four JCR-indexed journal papers and presented at three international conferences.

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