Industrial implementation of a back-to-back intertie using two standard drives-a retrofit solution to obtain a clean utility interface

Tighter power quality regulations stipulate improvement in utility interface of the existing and new industrial drives employed in a typical process plant. A generic variable speed drive draws power from the utility through a passive rectifier which is the primary source of harmonics that pollute the utility. Several topologies for passive and active filters have been studied extensively to mitigate these harmonics in the past few years. However, the filtering approach needs extensive system studies and is not a commercially viable option in the present day market. As an alternative, industrial drives with active rectifiers are also gaining popularity as a drive with harmonic-free utility interface. In addition to supporting the development cost of an entirely new product (which reflects in the higher premium for the end-customer), this active rectifier approach poses significant shortcomings in solving the problems of the drives that are already commissioned and are currently in use today. This paper presents a systematic methodology to couple two standard industrial drives which can be bought off the shelf to form a back-to-back intertie. The inverter side of this intertie drives the motor while the rectifier side ensures a harmonic-free interface with the utility. This arrangement can be used either as a retrofit solution for the pre-existing industrial drive at the cost of just one new standard drive, or it can be installed as a completely new system. This solution does not require any system studies for reliable operation and can be tacked on with least disturbance to the existing drive set-up. The proposed scheme is verified through extensive system simulations and experimental testing of a prototype in the laboratory.