# An Observer-Based Current Sensor-Less Control Scheme for Grid-Following Converters
## 1 Introduction  
Grid-following (GFL) inverters are the fundamental elements of inverter-based distributed energy resources in the future, modern power systems. Currently, GFL inverters are increasingly adopted as the power conversion interface for grid integration of renewable energy resources [1]. In this regard, different studies are being conducted on current, power, and the DC-link voltage control of GFL inverters, with the focus on grid synchronization, especially in weak grid conditions with/without phase locked loop (PLL) [2, 3, 4, 5, 6, 7], the stability of the phase locked loop (PLL), faults right through the capability of th eGFL inverters and their operation under faulty and unbalanced grid conditions [8, 9, 10, 11], reliability enhancement and cost minimization with a reduction in the number of the required sensors in the control loops [12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]. The grid integration of a GFL inverter is mainly done through L- or LCL-type filters to limit the harmonic current injection. An L-type filter has a weak harmonic attenuation characteristic, compared to the LCL filters. However, control with an L-type filter is simple, which makes it the preferred option for the grid integration of the GFL inverters. Typically, in the control of the GFL inverters with an L-type filter, at least two AC voltages, two AC currents and one DC-link voltage sensors are required. On the other hand, an LCL filter has superior attenuation characteristics compared to L-type filters, but at the cost of a complexcontrol structure to damp its resonance characteristics. To add more damping to the system, further sensors are required in the control structure. In the case of sensor failures, the inverter usually loses its stability. Thus, proposing a control structure with a reduced number of the sensors is a key solution in improving the reliability of GFL inverters. In addition, the need to use a reduced number of sensors considerably decreases the cost and the complexity of the system. In this regard, different studies in the literature have tried to propose a control structure with a minimum number of the required sensors [12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]. Specifically, active damping and control of the GFL inverters with an LCL filter requires the use of multiple signals, which necessitates the use of a reduced number of sensors for the sake of cost and complexity reduction. In this respect, using state observer-based control schemes are presented in the existing studies in the literature [12, 13, 14, 15]. While the studies in [12, 13] use only grid-side current sensors and grid voltage, the presented methods in [14, 15] also eliminated the necessity for a grid voltage sensor. The conventional synchronization schemes of GFLs mostly rely on the use of measured grid-voltages. To remove the grid-voltage measurement from the control structure, virtual-flux observer-based [19] and grid voltage observer-based [16, 17, 18] methods are presented. However, current sensors are usually more expensive and their accuracy has more impact on the system-control performance. Therefore, the elimination of the grid current sensors has considerable effect on the cost reduction, reliability enhancement and fault tolerant capability under sensor failure. An observer-based robust current sensor-less control scheme is presented in [20] for the inverter-based multi-terminal direct current transmission system, which eliminates the current control loops and current measurements. In addition, reference [21] proposed a new control scheme for a double-fed induction generator, which substitutes the stator-current regulators for the rotor-current regulators, and eliminates the necessity for rotor-current measurement. A current sensor-less control scheme is presented in [22], for the control of a permanent magnet synchronous motor. Despite the different available studies, the presented methods are not applicable for GFL inverters.