Total current harmonic distortion analyses of industrial UPS´s with SCR input converter working in redundant mode

Providing safety energy to the vital consumers is done in nearly all the cassis trough industrial, large power UPS´s (uninterruptible power supplies) working on-line. Self failure protection is usually realized by working in parallel redundant mode. The parallel working of multiple identical units with a total power being higher that the requested power with at least the equivalent power of one unit is known as “N + 1” redundancy mode. Currently this redundancy solution is mostly applied at small power. This paper will attempt to show the applicability of this method to large power and to find the appropriate constrains to the method when applied to large power units. While operating in parallel redundant mode the UPS´s are only partially loaded. Full loading is only achieved in the case of failure of one of the units. Due to this, the total harmonic current (THDi) as well as the power factor are very poor during normal operation. We will attempt to determine which configuration is both the safest and most efficient. The key factors will be considered: degree of safety, THDi of the original grid and price. This study can be applied to each consumers or partially loaded power supplies with the same upper stream configuration. We will begin by defining the mathematical expressions to describe the redundancy operations and then presenting digital simulations and the experimental results regarding THDi.