Keywords: regenerative braking energy, neutral section, Newton-Raphson load flow, bilateral co-phase system, power angle, slack bus, Indian Railways.

This paper takes the example of Indian Railways to show efficient utilization of regenerative braking energy(RBE) in bilateral co-phase systems. Indian Railways traction supply comes from the public utility grid. With the increase in three-phase locomotives there is an enhanced feedback of RBE to the grid especially when there is no available locomotive to absorb this RBE in the same feeding power sector. This often leads to impact on the utility grid (due to the harmonics in RBE) and presently there is no provision to account for the energy flowing back to the grid in the energy bill. Thus it becomes imperative to efficiently utilize this RBE in the railway network itself. In our work we model 25kV, 1-ø, AC traction system of Indian Railways and demonstrate the energy saving achieved by modifying the existing traction power system to bilateral co-phase system. We also demonstrate the importance of immediate switching over to 3-ø, regenerating locomotives by showing an energy saving of 1.93%. We show an additional energy saving of 0.28% by adopting bilateral co-phase system. We further show the dependence of energy saving on power angle(difference of voltage angles) between adjacent traction sub-station(TSS). Trains are power loads and determining voltages and currents drawn by trains is therefore a non-linear problem. In order to model our approach, we employ conventional iterative Newton-Raphson(NR) load flow method(due to non-linearity) to evaluate voltages, currents and energy saving. This approach also takes care of line losses, due to line impedance, and is accurate. The implementation is complex as compared to traditional power systems because of the presence of moving train loads. This leads to time-varying inter-bus impedance at every instant. The TSS crossing points also dynamically change the inter-bus configuration as TSS is treated as a fixed slack bus. Further, in the proposed bilateral supply system the two TSS are part of a single power network and NR algorithm is modified to take care of two distributed slack buses. The results we obtain show that a net energy saving of at-least 2.21% is achievable and prove the efficacy of our approach.

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