Deepdive into virtual inertia
With the penetration of the RES in today’s power grid, the system lacks inertia provided by the conventional synchronous generators (SGs). Recently, SGs are being replaced by the power-electronic driven converters to connect the RES and, thus, RESs interface with the grid through those converters. In contrast to SGs, those converters do not have inherent inertia, for instance, PV systems do not have any mechanical energy and wind turbines are fully decoupled from the grid.
Inertia in general is the resistance of a moving system when subjected to a change of its state of motion. The inertia in power systems plays a crucial role in the frequency control due to the kinetic energy stored in the rotating masses of synchronous generators to compensate the imbalance represented by the difference between the output power and the load. Therefore, substituting inertia in the power grid is required. Recently, RWTH Aachen in cooperation with E.DIS developed two different approaches namely, Linear Quadratic Regulator (LQR) and Linear Quadratic Integral (LQI), to introduce virtual inertia control strategies in the power system by utilizing a Battery Energy System (BESS). These control strategies mimic the behavior of the SG by absorbing or releasing power from the BESS.
The approaches were validated by means of simulation and compared to the traditional open-loop Virtual Synchronous Machine (VSM) model. Nevertheless, the frequency control in the legacy power system is performed completely by the TSO. In contrast, introducing the virtual inertia require the participation of DSO to maintain the system stability in future with the presence of RES. Furthermore, the performance of the proposed concepts was evaluated in different case studies, namely:
- Tracking power reference
- Islanding of Friedland MV network
- Islanding of Hagedornstrasse LV network
The results of the simulation are illustrated in Figure 1 for the scenario of islanding the MV network. The upper subplot shows the change in the inverter output power following islanding, whereby all controllers succeed in increasing their power to cover the load. However, regarding the frequency regulation, the different controllers behave differently.
For in-depth details about the different frequency controls strategies refer to (1).
Moreover, an algorithm was developed for estimation of the virtual inertia and damping by the BESS converter. The performance of the inertia estimator was also evaluated within the framework of IElectrix project.
(1) Nouti, Diala, Martina Josevski, and Antonello Monti. "Optimal Control for Improved Damping of Virtual Synchronous Machines." 2020 IEEE PES Innovative Smart Grid Technologies Europe (ISGT-Europe). IEEE, 2020.
