Parallel UPS Control and Configuration

The Multimodule Uninterruptible Power Supply (UPS) has been accepted as the most reliable power source. For further reliability improvement a parallel-redundant configuration is frequently used. The first principle of redundancy requires the elimination of points common to all modules. The second requires the availability of the redundant logic, to remove the faulty unit off line without affecting the others. Parallel operation in multi-module system is well described by L.H. Walker (1), with some systems considerations by Goodwin (3). This paper is an extension of the above publications with emphasis on development of control schemes currently used in some parallel redundant configurations. Two load sharing loop configurations are discussed, along with the behaviour of the circulation current under different fault condition. Special attention is given to the selective trip funtion. It is proven that a load sharing loop can be used as a source of necessary information required to remove the faulty unit off line. The presented method can also be used for a selective trip for over or under-frequency conditions of two or multi-module systems. The discussed method does sot employ control hardware except for the load sharing loop which is triplicated for redundancy.