Torque Dynamics Enhancement of Railway Traction Drives Using Scalar Control

Traction systems for railway typically use rotor field-oriented control (RFOC) at low speeds, and scalar control at high speeds to overcome the deterioration of the current regulator performance in the overmodulation region. Well-known limitations of scalar control are the slow dynamic response due to the coupling between torque and flux, as well as the risk of overcurrents. While this is not a problem for normal operation, as fast torque variations are not required, there are specific operating conditions in which fast torque response of scalar control might be required. This would include adhesion control, torsional torque vibration mitigation and torque ripple cancellation for traction systems fed from ac catenaries without a 2F filter in the dc-link. This paper proposes a method to enhance the dynamic response of scalar control. The principles of the proposed method are derived from vector control concepts. While the method could be applied to any electric drive using scalar control, the discussion presented in this paper will be targeted towards high power railway traction drives.