# A Low-Cost Evaluation Tool for Synchronization Methods in Three-Phase Power Systems
## 1 Introduction  
The integration of renewable energy sources (RESs) such as photovoltaic and wind power in electric power systems is an alternative that can lead to reductions in green-house gas emissions and, at the same time, to reduced energy costs [1, 2, 3]. Moreover, the implementation of RESs such as distributed energy resources (DERs) in facilities near electricity consumption, or in customer facilities, reduces transmission losses, grid congestion, and maintenance costs [4] and can achieve a more reliable electrical grid [5]. One drawback of RESs is the power fluctuations due to climatic factors, such as changes in irradiance and wind speed. As a solution, RESs can be implemented together with energy storage systems (ESSs), achieving (i) smooth short-term energy fluctuations and (ii) improved backup power capabilities [6]. The RES and ESS group uses power converters based on power electronics devices, allowing for energy control, injection, or storage. The implementation of power converters requires the connection of power devices between the energy generator, the storage system, and the electric grid. Due to the requirements of their control systems, the design and implementation of digital synchronization methods are a relevant step for adequate electric power injection into the grid. In addition, the synchronization methods are used in other applications, such as the control systems of electric generators and the energy measurement systems of the electric grid [7]. The synchronization methods allow for the estimation of several values of the grid voltage, such as (i) the phase, (ii) the frequency, and (iii) the amplitude of the positive sequence component [8]. The three-phase synchronization methods can be classified as open-loop [9; 10; 11; 12; 13] and closed-loop [14; 15; 16] (Figure 1). One of the most documented closed-loop methods is the phase-locked loop (PLL) [17; 18]. The PLL-based methods include (i) synchronous reference frame PLL (SRF-PLL), (ii) moving average filter PLL (MAF-PLL) [19], (iii) adaptive cascaded delayed signal cancellation PLL (CDSC-PLL) [20], and (iv) dual second-order generalized integrator PLL (DSOGI-PLL) with a low-pass filter (LPF) [21]. In the development stage of a control strategy for power converters, the implementation of a synchronization method needs to be experimentally validated before it can be used in the electrical power system. In addition, the validation stage allows for the evaluation of the algorithm's performance under different scenarios or grid voltage disturbances, such as (i) sag, (ii) swell, (iii) harmonics, (iv) frequency variations, and (iv) unbalance between phases. Previous works have used different tools to validate the synchronization algorithms experimentally. These tools can be classified into (i) emulated digital processing systems (EDPSs) and (ii) acquisition from the electrical system (AES) (Figure 2). Works that use EDPSs can emulate electrical signals using computational simulations. In [22], the authors simulated the power grid voltages and implemented the synchronization algorithms using MATLAB/Simulink. Their work analyzed the synchronization capability of the algorithms in distributed generation systems during balanced and unbalanced conditions by comparing the overshoot (OS) of the frequency step changes. In [23], the synchronization algorithms were implemented externally using an electronic board to acquire the simulated signals and to execute the algorithms, a concept known as a processor in the loop (PIL). With the above, the authors evaluated the performance of a photovoltaic inverter connected to the grid where the electrical system and the photovoltaic inverter were simulated in MATLAB/Simulink, and the synchronization systems were implemented on the STM32F407-based board. In [24], several algorithms were evaluated by connecting two electronic devices based on micro-computerized systems: one for the emulation of the  
Figure 1: Synchronization methods for three-phase systems.