Closed-form analysis of adjustable-speed drive performance under input-voltage unbalance and sag conditions

Abstract

Abstract-Voltage unbalance or sag conditions generated by the line excitation can cause the input rectifier stage of an adjustable-speed drive (ASD) to enter single-phase rectifier operation. This degradation of the input power quality can have a significant negative impact on the induction-machine performance characteristics. This paper provides an approximate closed-form analysis of the impact of line-voltage sags and unbalance on the induction-machine phase voltages, currents, and torque pulsations for a general-purpose ASD consisting of a three-phase diode bridge rectifier, a dc link, and a pulsewidth modulation (PWM) inverter delivering constant volts-per-hertz excitation. Attention is focused on the impact of the dominant second harmonic of the line frequency, which appears in the dc link voltage during the sag/unbalance conditions, neglecting the impact of the other higher order harmonics. In addition to the closed-form analytical results that assume constant rotor speed, both simulation and experimental results are presented, which confirm the key analytical results, including the dominance of the second harmonic in the resulting torque pulsations. The analytical results can be used as a valuable design tool to rapidly evaluate the approximate impact of unbalance/sag conditions on ASD machine performance.Index Terms-Adjustable-speed drives (ASDs), input-voltage sag and unbalance, power quality, torque pulsation.

Operation of Active Front-End Rectifier in Electric Drive under Unbalanced Voltage Supply

“…The value of this current can be controlled by suitable switching of solid-state elements in the front-end stage. Suitable control of the front-end AC/DC converter can be employed in order to draw constant input power from the power network even at unbalanced voltage supply www.intechopen.com Lee et al, 2006;Cross et al, 1999;Song & Nam, 1999). The switching functions for the front-end AC/DC converter are generated so that a constant voltage across the DC bus is maintained.…”

An Improved Scheme for Voltage Sag Override in Direct Torque Controlled Induction Motor Drives

“…In addition to this, additional harmonics can appear at unsymmetrical input voltages that exist in the cases of B, C, and D voltage sag types. Oscillatory components commonly appear as a second and fourth harmonics of input angular frequency ω i , where amplitudes depend on voltage type and p dc [15], [12]. Therefore, with a reasonable accuracy, in case of input voltage disturbance, voltage v dc can be presented as follows:…”

“…Such sensitivity has been examined in numerous studies and research papers [8,[10][11][12][13], where the basic testing criteria was drive tripping (pass/fail). In the experimental study [8], adjustable speed drive (ASD) sensitivity on symmetrical and unsymmetrical voltage sags was confirmed, where the sag type, remaining voltage magnitude, and motor load primarily influenced the drive susceptibility.…”

“…Some voltage sags can provoke ASD performance degradation: symmetrical ones cause available torque reduction [10,11], while unsymmetrical ones predominantly increase DC-link voltage ripple. Unsymmetrical voltage sag effects on the torque and speed of ASDs were analytically examined in [12,13], taking into account a simple V/Hz control scheme. Reference [14] presents some experimental results referring to voltage sag influence and recommends decreasing the usual limits for under-voltage protection settings aimed to overcome ASD fault.…”

Effect of Doubly Fed Induction GeneratorTidal Current Turbines on Stability of a Distribution Grid under Unbalanced Voltage Conditions

“…All of them show that even a slight imbalance could have a clear influence on rotor losses and torque pulsations [3,22]. The effects of voltage imbalance on power converters, due to the ripples in the output currents and the DC-link voltage, are also taken into account and a series of strategies have been proposed to minimize these effects [23,24].…”