# Comparative Analysis of Current Control Techniques to Support Virtual Inertia Applications +
## 1 Introduction  
The global demand for green energy has seen a rapid increase in the past few years due to the decrease in generation costs coupled with a variety of incentives for clean energy production and usage [1]. Consequently, renewable energy sources (RES), such as photovoltaic (PV) and wind energy, constitute a major share of the generation in modern power systems. These RES units have a commonality in terms of unpredictable power output and the fact that they are integrated with the power system through power electronic converters [2]. The traditional power system, which was dominated by large synchronous generators, is now evolving towards an inverter-dominated power system, which decouples the mechanical inertia from the grid [3]. Typically, these RES units are operated as non-dispatchable current sources and provide no inertial response, which leads to dynamic stability issues in the power system. In many cases, this limits the RES penetration levels in the power system [2].  
Virtual inertia emulation through appropriate control of power electronic converters has been widely proposed as a possible solution to counteract these issues. Virtual inertia is a combination of control algorithms, energy storage systems (ESS), and power electronics that emulates the inertia of aconventional power system [4]. Inertia can be emulated through use of ESS or by operating the RES units below their maximum power points, providing a reserve for the inertial response [5]. A number of algorithms has been proposed for this in the literature under different names such as virtual synchronous machines [6], virtual synchronous generators [7], synchronverters [8], and synchronous power controllers [9]. In most cases, a \"current feedback loop\" is employed to inject/absorb the inertial power through these systems. Because this power comes in short bursts based on frequency changes, a well-designed \"current feedback loop\" with good transient response and low steady-state error is required for a functional implementation.  
This paper presents the design and a comparative study of different current control techniques that can be employed for the design of the \"current feedback loop\". A comparison of some of the basic current control techniques, including proportional-integral (PI) control, proportional-resonant (PR) control, and hysteresis control was previously presented in [10]. This paper expands on our work in [10] by providing a more in-depth analysis of these techniques along with a discussion on two more advanced control approaches. The advancements in the computational capabilities of modern digital signal processors (DSPs) and field-programmable gate arrays (FPGAs) allows the implementation of these sophisticated control algorithms, which optimize the operation of the \"current feedback loop\". Furthermore, the maturity in wide bandgap semiconductor technology means that the power electronics switches can be operated at higher switching frequencies with fewer switching losses [11]. In this regard, the paper incorporates (i) a model predictive control (MPC), which generates optimized switching signals based on a current model [12], and (ii) a method of using an adaptive dynamic programming (ADP) controller to optimize the current control through an online learning-based approach [13; 14]. These techniques are compared through MATLAB/Simulink simulations, and conclusions are drawn for the selection of the appropriate current control technique for virtual inertia emulation.  
The paper is organized as follows: Section 2 introduces the concept of virtual inertia and describes the basic theoretical aspects of different current control techniques. The design procedure for each of the discussed current control approaches is presented in Section 3. Section 4 analyzes the simulation results and performs a comparative study, and Section 5 presents the conclusions.