8th Power Electronics, Drive Systems & Technologies Conference (PEDSTC 2017) 14-16 Feb. 2017, Ferdowsi University of Mashhad, Mashhad, Iran Tracking error minimization in multi-loop control of UPS inverters using the reference frame transformation Reza Razi, Mohammad Monfared Department of Electrical Engineering, Faculty of engineering Ferdowsi University of Mashhad Mashhad, Iran reza.razi@alumni.um.ac.ir, m.monfared@um.ac.ir Abstract-The main problem in most existing control methods for uninterruptible power supply (UPS) inverters is their excessive complexity. This problem is successfully solved in multi-loop control methods by simple proportional controllers. Nevertheless, using the simple proportional controllers instead of conventional controllers, such as a proportional-resonant (PR) or a proportional-integral (PI) controller, leads to several disadvantages such as the steady state and transient errors. This paper investigates the multi-loop control scheme by proportional controllers using a reference frame transformation for single phase UPS inverters. The proposed control method uses two nested proportional controllers in the synchronous reference frame. In fact, the steady state error of the UPS inverter in response to the ramp function changes to the steady state error in response to the step function using the reference frame transformation. Excellent tracking performance, no need to any derivative or integrator, fast dynamic response and simplicity of design and implementation are some advantages of the proposed method. The feasibility of the proposed control method is confrmed through simulations in MATLAB/SIMULINK. Kewords-Reference frame transformation; Tracking error; Multi-loop control; Single-phase UPS inverter. I. INTRODUCTION Recent developments in distibuted generation (DG) show the high potential of these resources as a good replacement for conventional power supply systems, especially for electrifcation of remote areas or sensitive loads. Generally, an interface is required for distributed generation sources to convert generated electicity into AC power with desired voltage and fequency. For this purose, of-grid power electronic converters, which are called UPS inverters, are used to adjust the outut energy of DGs. The UPS inverters act as a contolled voltage source, unlike the grid connected inverters [1-2]. Many contol schemes are provided for contol of UPS inverters [3-4]. The repetitive contollers, which are based on the interal model principle, in particular, are used in dealing with periodic signals. However, slow dynamics, need for large 978-1-5090-5766-5/17/$3l.00 ©2017 IEEE 311 Alireza Hadizadeh Department of Electrical and Computer Engineering, Faculty of engineering University of Tehran Tehran, Iran hadizadehI7@ut.ac.ir memory space and poor performance in the presence of non periodic signals are some disadvantages of this method [5-7]. Also, the non-linear contollers have some good features such as fast dynamics, but these techniques have some drawbacks such as complexity, sensitivity to parameter variations and steady-state errors [8-10]. Various methods have been proposed for multi-loop contollers [11-13], which typically use a combination of load voltage, load curent and capacitive or inductive current of LC flter sensors. Major disadvantages associated with multi-loop contol schemes are the complexity of the interal and exteral loops parameters design and need many sensors for measuring of feedback variables. Therefore, in [14 and 15], the complexity of the multi-loop contol method was reduced without using proportional-integral (PI) and proportional resonant (PR) contollers. However, an undeniable tacking error was observed in steady state conditions in [14] and in transient mode in [15]. This paper investigates a new multi-loop conol scheme by simple proportional contollers using a reference fame tansformation for single-phase UPS inverters. In this method, each of the feedback variables, afer creating a virtual phase and transform it fom the stationary reference fame to the synchronous reference fame, individually is contolled. Also, to increase reliability and reduce the cost and size, the Kalman flter algorithm is used to estimate the outut voltage. Finally, the performance of the proposed conol method and comparison with other methods is done by simulating in MA TLAB / SIMULINK. II. SYSTEM MODELING Block diagram of the power stage of the single-phase UPS inverter with output LC flter is shown in Fig. 1. Desired values of the circuit parameters are shown in Table 1. According to Fig. 1, the equations, describing the dynamics of UPS inverter, can be extracted.