Document Type : Research article

Author

• Mohammad Naser Hashemnia

Department of Electrical Engineering, Mashhad Branch, Islamic Azad University, Mashhad, Iran

Abstract

Modeling electric machines is crucial for analyzing their behavior and designing controllers. It is of the utmost importance to make use of a consistent equivalent circuit of the Doubly Fed Induction Machine (DFIM) that is applicable to a variety of operating modes. This is because it helps in the calculation of the machine's steady-state performance, converter ratings, and controller set-points. Traditional models of doubly fed induction machines employ the steady-state equivalent circuit of a wound-rotor induction machine with all rotor parameters referred to the stator through a frequency conversion. The present study investigates the validity of the traditional steady-state circuit model by taking into account the sequence change in rotor voltages and currents at super-synchronous speeds. The validity of phasor diagrams constructed using the traditional circuit is assessed, with a particular focus on super-synchronous operation in both motoring and generating modes. It has been demonstrated that the existing model is applicable to all rotor speeds (whether sub-synchronous or super-synchronous). However, caution should be exercised when utilizing expressions of rotor reactive power that involve dynamic dq and steady-state phasor models. Therefore, modified expressions are developed for rotor reactive power that are applicable regardless of the operating speed. The accuracy of the proposed method for different operating modes is confirmed by comprehensive simulation results developed with Matlab® Simulink. An investigation is also conducted into the sensitivity of rotor reactive power direction to parameter changes, and it is shown that machine parameter changes have a negligible effect on rotor reactive power direction.

Highlights

• Proposing A new method to account for the change in phase sequence of rotor variables at super-synchronous speeds, affecting several traditional relationships
• A deep insight into the negative sequence phenomenon in super-synchronous mode of operation
• A unified modeling framework applicable to all operating speeds is proposed.
• Modifying some steady-state and dynamic relationships to take super-synchronous speed into account, while resorting to the well-established equivalent circuit

Keywords

• Doubly Fed Induction Machine (DFIM)
• Dynamic Model
• Phasor Diagram
• Reactive Power
• Super-synchronous Speed

Main Subjects

• Electric Machines