Optimasi Penetrasi PLTS Atap pada Jaringan Distribusi Tegangan Menengah dengan Pemasangan Battery Energy Storage System (BESS) Secara Terpusat sebagai Ancillary Services = Optimization of Rooftop Solar Photovoltaic Penetration on Medium Voltage Distribution Networks with the Installation of Centralized Battery Energy Storage System (BESS) as Ancillary Services