Mathematical Modeling and Current-Oriented Control of Double-Inverter-Fed Wound Rotor IM Emulated as Two Virtual Cage Rotor IMs

Stator- and rotor-side voltage source inverters (VSIs) (VSI-S and VSI-R) of a double-inverter-fed wound rotor induction machine (DI-WRIM) drive individually feed 1-p.u. power each, to extract 2-p.u. power at a 2-p.u. speed. This feature of the DI-WRIM drive is explored, in this article, to model this drive as two virtual cage rotor induction machines (CRIMs). The VSI-S connected to the stator is emulated as one VSI-fed CRIM with the rotor short circuited, while the VSI-R connected to the rotor is emulated as another VSI-fed CRIM with the stator short circuited. The mathematical modeling of the DI-WRIM drive as two virtual CRIMs, in the synchronous reference frame, is presented by considering the angle between the stator and rotor currents (γ) as a medium of coupling between the two virtual CRIMs. The proposed mathematical model of the DI-WRIM is used to demonstrate the performance of the drive in a current-oriented control (COC) technique. In the COC technique, the VSI-S is controlled in the stator current-oriented frame and the VSI-R is controlled in the rotor current-oriented frame. The proposed model and technique is tested on a 4-hp WRIM drive and the experimental results are also presented to demonstrate the satisfactory performance of the drive.

I. Introduction

Wound rotor or slip ring induction machine (WRIM) drives are popular dual electrical port systems that are known for their increased capability of power conversion and degree of freedom (DOF) [1]–[4]. One of the renowned dual electrical port configurations of the WRIM is the double-inverter-fed WRIM (DI-WRIM) drive [1]–[3]. In this topology, the stator and rotor terminals are connected to voltage source inverters (VSIs), VSI-S (stator VSI) and VSI-R (rotor VSI), with a common dc link. Owing to VSI-S and VSI-R, the WRIM can now be controlled from both the stator and rotor sides leading to increase in DOF. Furthermore, the stator and rotor sides can now individually feed 1-p.u. power such that the 1-p.u. WRIM can now deliver 2-p.u. power at a 2-p.u. speed and nominal torque [1]–[4]. On the other hand, instead of the 1-p.u. WRIM, a 0.5-p.u. WRIM can be used to extract 1-p.u. power at a 1-p.u. speed. When compared to its conventional counterpart, the single electrical port configuration with stator side connected to the grid and VSI-R, the DI-WRIM topology can be operated in all the four quadrants. Also, since the drive is not directly connected to the grid, the DI-WRIM is immune to grid-side disturbances [4]–[7]. Thus, the DI-WRIM drive is widely used in a high power drive [1], [4]–[7] and propulsion applications [8]–[10]. Moreover, in the recent times, the topology of the DI-WRIM is also been explored as an alternative to DFIG-DC topology [3], [12]–[14]. In the DFIG-DC topology, the stator side is connected to a 1-p.u. diode bridge rectifier and this connection introduces lower order harmonics into drive. These lower order harmonics can be mitigated by connecting another partially rated converter on the stator side.