Abstract
In this paper, a real-time robust closed-loop control scheme for controlling the velocity of a Direct Current (DC) motor in a compound connection is proposed. This scheme is based on the state-feedback linearization technique combined with a second-order sliding mode algorithm, named super-twisting, for stabilizing the system and achieving control goals. The control law is designed to track a periodic square reference signal, being one of the most severe tests applied to closed-loop systems. The DC motor drives a squirrel-cage induction generator which represents the load; this generator must work above the synchronous velocity to deliver the generated power towards the grid. A classical proportional-integral (PI) controller is designed for comparison purposes of the time-domain responses with the proposed second-order sliding mode (SOSM) super-twisting controller. This robust controller uses only a velocity sensor, as is the case of the PI controller, as the time derivative of the velocity tracking variable is estimated via a robust differentiator. Therefore, the measurements of field current and stator current, the signal from a load torque observer, and machine parameters are not necessary for the controller design. The validation and robustness test of the proposed controller is carried out experimentally in a laboratory, where the closed-loop system is subject to an external disturbance and a time-varying tracking signal. This test is performed in real time using a workbench consisting of a DC motor—Alternating Current (AC) generator group, a DC/AC electronic drive, and a dSPACE 1103 controller board.
Highlights
To emulate a wind system in a lab application, a motor–generator couplet for reproducing the turbine operation can be used
The Direct Current (DC) motor drove a wound-rotor induction generator, operated as squirrel cage which freely delivered the generated energy to the grid, when the DC motor velocity was controlled above the synchronous speed
A classical PI controller was designed as the starting point for velocity tracking of a periodic square reference signal; the state-feedback linearization technique combined with the nonlinear second-order sliding mode (SOSM) super-twisting algorithm was applied to achieve better velocity tracking performance and to reject external disturbances
Summary
To emulate a wind system in a lab application, a motor–generator couplet for reproducing the turbine operation can be used. An emulator of a scaled wind system, where a sliding surface is defined applying the block control linearization technique, combined with a super-twisting algorithm for controlling both the DC motor and a doubly-fed induction generator is exemplified in [7].
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