en Distribution Systems Automation Distribution Systems Automation Research Paper Research Paper <span style="font-size:10.0pt"><span style="font-family:&quot;Times New Roman&quot;,&quot;serif&quot;"><span style="letter-spacing:.05pt">The increasing penetration of distributed generation (DG) significantly complicates Distribution System State Estimation (DSSE) by introducing stochasticity and uncertainty. This paper proposes a novel DSSE framework that unlike conventional methods simultaneously estimates the system state, load demands, and DGs output power through a unified constrained optimization model. The model is efficiently solved using the Whale Optimization Algorithm (WOA), whose unique balance of exploration and exploitation enables robust solution search in complex, active distribution networks. Simulation studies on standard IEEE 37-bus and 69-bus test systems reveal that the proposed WOA-based approach achieves outstanding accuracy. For the 37-bus system, WOA attains a Maximum Individual Relative Error (MIRE) of 1.15% and a Maximum Individual Absolute Error (MIAE) of 2.303 on load estimation. On the larger 69-bus system, the method further reduces these errors yielding a MIRE of 0.886% and a MIAE of 1.12 for load, and 0.73% and 1.058 for DG power estimation, respectively. Across all experiments, WOA consistently outperforms leading metaheuristics including ABC, PSO, and GA highlighting its superior accuracy, scalability, and robustness for real-world DSSE challenges.</span></span></span> State estimation, Active distribution system, Distributed generation, Smart meter, Whale optimization algorithm 4180 4180 http://ijeee.iust.ac.ir/browse.php?a_code=A-10-6159-1&slc_lang=en&sid=1 Mojtaba Ajoudani mojtaba.ajoudani@iau.ac.ir 1800319475328460017303 1800319475328460017303 Yes Department of Electrical Engineering, BG.C., Islamic Azad University, Bandargaz, Iran. Seyed Reza Mosayyebi sr.mosayyebi@iau.ac.ir 1800319475328460017304 1800319475328460017304 No Department of Electrical Engineering, BG.C., Islamic Azad University, Bandargaz, Iran. Ramazan Teimouri Yansari ra.teimouri@iau.ac.ir 1800319475328460017305 1800319475328460017305 No Department of Computer Engineering, BG.C., Islamic Azad University, Bandargaz, Iran.