Abstract
In three-phase inverters used in uninterruptible power supplies (UPSs), three-limb inductors and three-limb transformers are commonly used in consideration of cost and size. However, magnetic coupling exists between the three phases of the inverter, which can result in complex models. When instantaneous feedback control strategies are introduced to achieve high quality output waveforms, the transient analysis of the closed-loop inverters becomes difficult. In this paper, the phenomenon of magnetic coupling in three-phase inverters due to three-limb inductors and three-limb transformers is analyzed. A decoupled dynamic model is derived based on the instantaneous symmetrical components transformation, which comprises three decoupled equivalent circuits of instantaneous symmetrical components. Analyses based on this model indicate that magnetic coupling may have a significant impact on the performance of three-phase inverters under unbalanced load conditions and transient responses. For three-phase inverters in UPSs with Proportional-Integral-Differential (PID) closed-loop control strategies, the interactive influence between instantaneous closed-loop regulation and magnetic coupling is researched. Finally, a method of reliability analysis and PID controller design for inverters with magnetic coupling is derived. Simulation and experiment results validate the model and conclusions.
Highlights
In consideration of cost and size, the three-phase transformers and three-phase inductors of three-phase inverters used in uninterruptible power supplies (UPS) normally have magnet cores with a three-limb structure, known as a core-type structure [1,2,3]
For three-phase inverters as shown in Figure 1, combining the instantaneous symmetrical components (ISC) equivalent circuits of the three-limb inductor and three-limb transformer, the ISC equivalent circuits can be given as shown in Figure 5, where uPWM, uPWM, uPWM are the ISCs of uPWMA, u PWMB, u PWMC
For three-phase inverters used in UPSs, three-limb transformers and three-limb inductors are commonly used in their construction, which can bring about magnetic coupling
Summary
In consideration of cost and size, the three-phase transformers and three-phase inductors of three-phase inverters used in uninterruptible power supplies (UPS) normally have magnet cores with a three-limb structure, known as a core-type structure [1,2,3]. In inverters where transient responses are fast, the three-phase variables can no longer be treated as periodic waves They cannot be expressed by SCs. For inverters with a three-limb transformer and three-limb inductor, to obtain optimal instantaneous closed-loop control, magnetic coupling should be considered in the dynamic models so that transient performance can be properly considered. Because ISC-based dynamic models for three-limb transformers and three-limb inductors are simpler in formulation [26,27], it may be a useful tool when magnetic coupling is taken into account in transient responses of inverters. It has been seldom reported in the literature.
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