Inter-area minimisation of reactive power flow for voltage improvement in large electric grids

Traditionally, Electric grids are designed to transport the bulk power generated at conventional power plants to load centres. Electricity regulators are tightening the grid codes to improve the performance and efficiency of electric grids. The coordinated planning of reactive power management for voltage profile regulation is an essential aspect of the seamless operation and control of electric grids. The adverse effects of seasonal variation in load profiles and the de-commitment of generating units based on their viability magnify the voltage regulation challenge in the large electric grid. These operational scenarios create low-voltage pockets in the distribution network, and there is a high draw of reactive power from the upstream transmission system. The power transfer capabilities of inter-regional tie lines decrease, and subsequently, this leads to overloading of power equipment and increases active loss in the utility grid. In this research article, reactive power management is formulated as a nonlinear optimisation problem. The objective function is minimised as the sum of reactive power flow over inter-intra-regional tie lines in a large electric grid. The optimisation problem is solved with a set of constraints imposed by placing the local reactive power support devices at critical locations. Selection of critical locations is identified through a new hybrid voltage-grid strength sensitivity index. The proposed index utilises the grid strength in addition to the voltage sensitivity of bus selection of buses for the installation of reactive power support devices. The performance of the proposed algorithm has been tested on real data from the Northern region of the Indian grid, which includes seven power transmission utilities with over 9000 buses. The simulation results show that the proposed reactive power optimisation, based on the hybrid voltage-grid strength sensitivity index, reduces reactive power import from 1592 to 383 MVAr by injecting 9421.8 MVAr at 33 kV at only 14.1% of the highly sensitive buses identified through this index. It is observed that injecting compensation devices at 33 kV buses improves the average bus voltage to nearly 0.98. The reactive power in the inter-intra tie lines decreases by 76%, and active power losses are reduced by 7.99%. In conclusion, the proposed algorithm can serve as a guiding tool for planning reactive power management in large-scale grid utilities, aiding voltage improvement with the optimal installation of compensating devices at a minimal number of 33 kV buses.