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Steady-state security region (SSR) provides a region-wise approach to assess the steady-state security of power systems. Based on the SSR model, we introduce the concept of `steady-state security distance' (SSD) to provide exact `quantitative analysis' on the scale of security margins for system operators. Then, mathematical models of SSD are formulated. On this basis, implementation of SSD is discussed towards various uncertainties within the power system operation. As the calculation of SSD is in essence a large-scale non-linear optimisation process, it requires very long computation time, and thus could not be utilised in practical application. In order to enhance the efficiency of computation, a novel algorithm is proposed, which decomposes the complex optimisation process into two steps: active SSR boundary identification and partial constrained solution. The proposed algorithm remarkably cuts the scale of the optimisation model as well as the number of calculations, thus significantly reduces the computation time to meet requirements of assessment towards day ahead and hours ahead generation schedules. In the end, an IEEE-30 bus case and a practical case on a provincial power system are both studied to testify the effectiveness of the proposed model and algorithm.