A HEURISTIC MODEL FOR PLANNING OF SINGLE WIRE EARTH RETURN POWER DISTRIBUTION SYSTEMS Geofrey Bakkabulindi 1* , Mohammad R. Hesamzadeh 2 , Mikael Amelin 2 , Izael P. Da Silva 1 , Eriabu Lugujjo 1 1 Makererere University P. O. Box 7062, Kampala, Uganda. 2 Royal Institute of Technology Teknikringen 33, Stockholm, Sweden. 1* Email: gbakkabulindi@tech.mak.ac.ug ABSTRACT The planning of distribution networks with earth return is highly dependent on the ground’s electrical properties. This study incorporates a load flow algorithm for Single Wire Earth Return (SWER) networks into the planning of such systems. The earth’s variable conductive properties are modelled into the load flow algorithm and the model considers load growth over different time periods. It includes optimal conductor selection for the SWER system and can also be used to forecast when an initially selected conductor will need to be upgraded. The planning procedure is based on indices derived through an iterative heuristic process that aims to minimise losses and investment costs subject to load flow constraints. A case study in Uganda was used to test the model’s practical application. KEY WORDS Power distribution planning, Power flow analysis, Single Wire Earth Return, Rural electrification 1. Introduction Since the pioneering work on Single Wire Earth Return (SWER) by Lloyd Mandeno in 1925 [1], the technology has proven to be very cost effective in electrifying scattered rural areas. Countries like New Zealand, Australia, Brazil and South Africa, among others, have several thousand kilometres of SWER lines installed with several lines having been in operation for well over 25 years [2]. However, many developing countries especially in sub-Saharan Africa have yet to mainstream SWER into their distribution networks despite prevailing low electrification rates. The major challenges facing these countries are lack of awareness, insufficient capacity for the required technical analysis and implementation as well as inadequate framework within which to design and plan these low-cost networks [3]. Considerable research has been done on SWER systems [1 - 3, 5, 9 -12] as well as power distribution system planning [4, 14, 15]. However, the planning of SWER distribution systems based on earth return load flow constraints has not been widely covered. The general objective of the distribution planning is to minimise the capital investment and operation costs of distribution substations and feeders to create a network that meets the projected load growth reliably and securely. This is achieved only if the constraints associated with equipment capacities, voltage limits, technical losses, and radial configuration are met [4]. SWER distribution systems use the earth as current return path. As such, the planning of these networks largely depends on an area’s ground conductive properties which are, in turn, a function of soil type and humidity [5]. By using a heuristic approach, this paper presents a simple iterative procedure for planning SWER distribution systems. A dynamic planning model is used to consider the impact of load growth over several time periods on system performance. By using a load flow algorithm for earth return networks, optimal conductor selection is carried out for the initial case and the algorithm presents the possibility to determine when the initial conductor will need upgrade. The aim was to minimise the costs of distribution losses, initial installation costs for feeders and subsequent upgrades subject to load flow constraints. The model is applied to a case study in Uganda to test its performance. All mathematical model formulations were done using the General Algebraic Modelling System (GAMS). 2. System Model Formulation 2.1 SWER Distribution Line Model The SWER distribution line model was based on Carson’s line [6]. This model considers a single conductor parallel to the earth with unit length and carrying a current with return path through the ground. The earth return is considered to be a single conductor beneath the earth’s surface with 1 m geometric mean radius (GMR), uniform resistivity and infinite length [5, 6]. The geometric mean distance (GMD) between the overhead conductor and the earth return path is a function of the soil resistivity, ρ [5, 7]. The total impedance, Z aa , of the overhead line as a result of the earth presence was derived in [5] and is given by (1). The ground self impedance, z gg , and the mutual impedance, z ag , between the earth return and the phase conductor are given by (2) and (3) [5, 7].