IMPROVED SYNCHRONVERTERS WITH BOUNDED FREQUENCY AND VOLTAGE FOR SMART GRID INTEGRATION

Abstract

Synchronverters are grid-friendly inverters that mimic conventional synchronous generators and play an important role in integrating different types of renewable energy sources, electric vehicles, energy storage systems, etc., to the smart grid. In this paper, an improved synchronverter is proposed to make sure that its frequency and voltage always stay within given ranges, while maintaining the function of the original synchronverter. Furthermore, the stability region characterised by the system parameters is analytically obtained, which guarantees that the improved synchronverter is always stable and converges to a unique equilibrium as long as the power exchanged at the terminal is kept within this area. Extensive OPAL-RT real-time simulation results are presented for the improved and the original self-synchronised synchronverters connected to a stiff grid and for the case when two improved synchronverters are connected to the same bus with one operating as a weak grid, to verify the theoretical development.

Existing System

Droop control with an inherent virtual inertia that can be adjusted according to the power system requirements is used.

Proposed System

In the proposed system, an improved version of the synchronverter connected to the grid with an LCL filter is proposed. The nonlinear model of the system is firstly derived using the Kronreduced network approach. Then, both the frequency loop and the field-excitation current loop are implemented by using a bounded controller, inspired by the bounded integral controller recently proposed in. The improved synchronverter approximates the behaviour of the original synchronverter under normal operation (near the rated value) and guarantees given bounds for both the frequency and the voltage independently from each other, without the need of additional saturation units that will complicate the proof of stability. Hence, depending on the grid voltage and the parameters of the synchronverter, the area where a unique equilibrium exists is obtained and the convergence to the equilibrium is proven for the given voltage and frequency bounds. According to the analysis, the stability of the self-synchronised synchronverter, where the synchronisation unit is no longer required, is proven as well. This may shed new light on establishing the stability of next-generation smart grids, which are dominated by power-electronic converters. A preliminary version of the proposed approach was presented in, where the bounded control structure was only implemented in the field-excitation current loop of the original synchronverter and the stability in the sense of boundedness was shown. This paper extends the method to maintain given bounds for both the field-excitation current and the frequency, which leads to a specific bound of the synchronverter voltage that is analytically calculated, and guarantees the asymptotic stability of the closed-loop system and the uniqueness of a desired equilibrium point based on non-linear dynamic modelling. Additionally, small variations of the grid voltage and frequency are also considered such as in the case of a weak grid. It should be noted that the improved version of the synchronverter presented in this paper and the stability analysis does not obsolete the existing methods; in contrary it can be combined with some of them, e.g. the alternating inertia, to further enhance the dynamic performance.

Architecture

Two synchronverters connected to the same bus