Diesel Alternators and Thyristor Rectifiers - Potentially Unstable Partners

Over the past 12 years Telecom Australia has introduced thyristor phase-controlled rectifiers in new and existing installations, but recently, a spate of instability problems has occurred between these rectifiers and diesel-alternator plant. The problem is complex and numerous theories have been tendered as a solution. Traditional methods of specifying rectifiers and generating sets would seem inadequate in this regard. Specifying a limit of alternator harmonic distortion on a linear load achieves very little when loads are predominantly phase-controlled rectifiers. A number of areas are being investigated, including the role played by rectifier harmonic filters and power factor correction components, alternator capacity verses percentage non-linear load, notching, separately-excited versus self-excited alternators and the deleterious affect of other essential loads such as fluorescent lighting. This paper discusses the above investigations with a view to developing new specifications to improve the compatibility of rectifier and alternator plant.