International Journal of Engineering Research and Development e-ISSN: 2278-067X, p-ISSN: 2278-800X, www.ijerd.com Volume 11, Issue 07 (July 2015), PP.42-55 42 A Generalized Multistage Economic Planning Model for Distribution System Containing DG Units Ahmed A Arram 1 , Mahmoud A. Farrag 2 , Mohamed A. El-Sayed 3* 1 Distribution Company MEW Cairo, Egypt 2 Electric power & Machines Dept. Cairo University, Egypt. 3 Electrical Dept. College of Engineering & Petroleum Kuwait University (* corresponding author) Abstract:- Distributed generation (DG) has gained a lot of attractions in the power sector due to its ability in power loss reduction, increased reliability, low investment cost, and most significantly, to exploit renewable energy resources like wind, photo-voltaic and micro-turbines, which produce power with minimum greenhouse- gas emissions. The installation of DG units into distribution system requires efficient expansion planning technique to minimize the investment and operation cost of the system.. In this paper, a new mixed integer nonlinear model for solving the multistage distribution system network planning problem including DG has been developed. The model is able to deal with different planning scenarios such as buying energy from a nearby electric distribution company through an intertie, upgrading substations, upgrading feeders or investing in DG units. The model takes into account the operational constraints of equipment capacities and voltage limits as well as the dynamic load growth. Finally, the developed mathematical mixed integer model was applied to minimize the planning cost of the studied distribution network including DG units. The implemented mixed integer nonlinear planning model is coded using LINGO V14 optimization software. Keywords:- Distributed Generation, Distribution System, Mixed Integer non-linear Model, Expansion planning, System Optimization. NOMENCLATURE LB Total number of system load buses. NCP(t) Number of cable paths existing at year t. Number of cable sizes considered at path i. Maximum number of added DG units at any load bus. Number of existing and proposed substation sites. Number of installed transformer units in the substation at site i. NTS Number of tie-line power steps. Horizon planning year (in years). Electricity market energy price at year t in ($/MWh). Capital cost of proposed transformer unit j at substation site at start of planning interval t. DG investment of unit j added at load bus at the start of year t (in $). DG operating cost of unit j added at load bus at the start of year t (in $/MWh). Intertie electricity market price of imported power for step j of tie-line number i in year t. Cost per unit length for cable size j when added at year t. Apparent load demand in (MVA) at bus i and year t. DR Discount rate. FC The capital cost of distribution cables (in $) FG The total capital cost of the DG units VG The total running cost of all DG units (in $). FS Total capital cost of substations (in $). VS Total variable running cost of substations in LE or $. Vtie Running cost of inter-tie in LE or $.