The paper presents a simple and direct "root based" algorithm with efficient data structure to determine the topological observability from the available measurement data set. The root based approach assigns measurements to the branches of the system graph and builds up a tree and determines the topological observability. If the system is unobservable from the available data set, it identifies observable islands with no additional computation and also shows the potential locations for additional meter placement or pseudo-measurements so that the system is observable. The algorithm needs no floating point computations and cannot fail due to numerical round-off problems. In implementing the root based algorithm, the proposed efficient data structure plays a very significant role in drastic reduction of time for observability test. The added advantage of the proposed data structure is that the same is useful for implementing any selected state estimation algorithm exploiting the sparsity of Ybus-matrix. The root based algorithm is tested on 6, 9 and IEEE 14, 30, 57 and 118 bus systems for given sets of measurement data and it could yield correct classification of observable and unobservable cases with reduced memory and execution time.
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