Waste-to-energy (WTE) generation can contribute to renewable-energy utilization while introducing voltage-regulation challenges when connected to transmission networks. This study develops a grid-code-constrained transmission interconnection planning framework for a 40 MW WTE plant connected to the South Sulawesi 150 kV network. The WTE plant is modeled as a PV generator with fixed active-power output and an adjustable voltage setpoint. The analysis considers a specified steady-state generation and load condition, while time-varying operating scenarios and N−1 contingencies are outside the present scope. Distributed reactive-power support is represented as controllable capacitive injections at selected buses. Newton–Raphson AC power-flow analysis shows that the pre-optimization WTE-interconnected condition has a minimum voltage of 0.9489 pu at Bus 26, below the required 0.95–1.05 pu range. To identify effective support locations, a 10 MVAr test injection is sequentially applied to all 43 non-slack buses, followed by hierarchical ranking based on improvement in the minimum voltage and reduction in the system voltage-deviation index. The nine highest-ranked buses are retained as candidate locations for joint optimization of distributed reactive-power injections and the WTE generator voltage setpoint. Harris Hawks Optimization (HHO) is evaluated over 30 independent runs using 40 search agents and 150 iterations. All HHO runs achieve grid-code-compliant solutions, with a minimum voltage of approximately 0.9603 pu. The best HHO solution reduces the voltage-deviation index from 0.021852 to 0.010596, corresponding to a 51.51% reduction, with 110.99 MVAr of total distributed reactive support and an optimized WTE voltage setpoint of 1.0075 pu. Additional 30-run benchmarks using Particle Swarm Optimization and Grey Wolf Optimization also achieve 100% grid-code feasibility, with PSO providing the lowest mean objective value and run-to-run variability. These results indicate that the principal voltage-security improvement arises from the sensitivity-guided, grid-code-constrained planning formulation rather than from HHO alone, while demonstrating HHO as a viable solution method for the investigated WTE transmission-interconnection problem.