A PLL-Free Grid-Forming Control With Decoupled Functionalities for High-Power Transmission System Applications

Taoufik Qoria,Xavier Guillaud,Antoine Bruyere,Ebrahim Rokrok,Bruno Francois

doi:10.1109/access.2020.3034149

Abstract

This paper proposes a grid-forming control strategy with a generic implementation approach to meet the transmission system requirements asking for the massive integration of power electronic devices into the power systems. In this context, several grid-forming controls have been proposed in the literature either with or without a Phase-Locked Loop (PLL). The PLL-based techniques allow decoupling the different control functionalities (i.e. inertia emulation, frequency support, active power setpoint tracking in steady-state) while the PLL-free schemes, which aim to avoid the PLL drawbacks, create a compulsory coupling between the control functionalities. The proposed grid-forming control in this paper is able to decouple the control functionalities without any dedicated PLL, which makes it more advantageous compared to what have been already proposed in the literature. Since the power converters are exposed to the small and large grid events, the presented control has been tested in both situations. For the small grid events, a simplified small-signal model is developed to assess the active power and frequency dynamics. In case of large grid events, a current limitation algorithm is included to the control in order to protect the power converter. To deal with the transient stability issues linked to the current limitation and enhance the converter performance during the post-fault, a method based on adaptive inertia constant is proposed. To validate the overall approach, time-domain simulations (in Matlab-Simulink) and experimentations are performed.

Highlights

T HE beginning of the renewable energy story had a moderate influence on the power electronic converters market
A major share of the electronic converters are controlled under the presumption that they are connected to a strong AC voltage with a given magnitude and frequency in such a way that the converter can inject a controlled power
The outer frequency droop control for the Voltage Source Converter (VSC) depicted by the green block in Fig. 17 is an optional functionality that can be added in order to allow the power converter participating to the frequency support in steady-state

Summary

INTRODUCTION

T HE beginning of the renewable energy story had a moderate influence on the power electronic converters market. A major share of the electronic converters are controlled under the presumption that they are connected to a strong AC voltage with a given magnitude and frequency in such a way that the converter can inject a controlled power This control strategy of the converter is well-known as the grid-following control. To the best of our knowledge, until now, no control technique that allows decoupling the control functionalities without any dedicated PLL has been proposed This decoupling is very important since the owner of the power electronic device can decide if the converter participates to primary frequency support or not. The contributions of the paper are listed as follow: 1) A new grid-forming control scheme with the ability to decouple different control functionalities without a dedicated PLL. The grid is modeled by a Thevenin equivalent composed of a voltage source veabc , an inductance Lg and a resistance Rg

ACTIVE POWER CONTROL WITH THE MODULATED VOLTAGE ANGLE

ACTIVE POWER DYNAMICS

RSG ωe Governor

RESPONSE TO LARGE EVENTS AND TRANSIENT STABILITY

EXPERIMENTAL VALIDATION

CONCLUSION