1 Abstract-- The continuous increase on the penetration levels of Renewable Energy Sources (RES) in power systems has led to radical changes on the design, operation and control of the electrical network. This paper investigates the influence of these changes on the operation of a transmission network by developing a set of indices, spanning from power losses to G.H.G emissions reduction. These indices are attempting to quantify any impacts therefore providing a tool for assessing the RES penetration in transmission networks, mainly for isolated systems. These individual indices are assigned an analogous weight and are mingled to provide a single multi- objective index that performs a final evaluation. These indices are used to evaluate the impact of the integration of RES into the classic WSCC 3-machine, 9-bus transmission network. Index Terms—Energy policy, multi-objective assessment, power system planning, renewable energy sources. I. INTRODUCTION UROPEAN Union countries have a set of specific targets to promote the use of energy from Renewable Energy Source (RES) in accordance to the Directive 2009/28/EC of the European Parliament [1]. These National Action Plans (NAPs) consider and set targets for the final use of energy for heating and cooling, electricity generation and transportation. In particular, electricity generation is of great interest as it requires the liberalization of the electricity markets. The 16% of global final energy consumption comes from renewable sources during 2012, with 10% coming from traditional biomass, which is mainly used for heating, and 3.4% from hydroelectricity. New renewable sources (small hydro, modern biomass, wind, solar, geothermal, and bio- fuels) accounted for another 2.8% and are growing very rapidly [2]. The share of renewable sources in electricity generation is around 19%, with 16% of global electricity coming from hydroelectricity and 3% from new renewable sources [2]. Nevertheless, RES have not been a significant part of the energy mix for the vast majority of countries around the world, fact which has led governments to provide incentives to entities that are interested in investing in RES electricity generation, in most cases using wind and solar power. Consequently, it is of crucial importance to investigate how RES generation affects the network’s operational ability and which potential configurations could prove beneficial. Hence, a series of technical aspects must be considered by the planners in order to evaluate the pros and cons of such penetration. In particular the minimization of power losses has so far been the most important issue for the planners [3]-[4]. However, other grid related technical aspects have to be considered since they are significant as well. Such aspects are: voltage profile improvement, short- circuit level alteration and maximization of the network’s Available Transfer Capability (ATC), [5]-[6]. In addition to these, the greenhouse gas (GHG) emissions’ reduction is increasingly becoming more important as it reflects on the environmental side of the energy problem. Moreover, the system’s security is of great significance since access to reliable, cheap electricity relates to the quality of life of a society. Table I shows a brief summary of the relevant existing literature regarding indices used to evaluate the integration of RES. TABLE I INDEX-RELEVANT LITERATURE REFERENCES Reference Power Losses Voltag e ATC SCL Emission Reduction Spinning Reserve [3] Yes No No No No No [4] Yes Yes No No No No [5] Yes Yes Yes Yes No No [6] No Yes No Yes No No Present work Yes Yes Yes No * Yes Yes * : Due to software limitations There are several aspects to be considered in order to integrate RES into traditional networks. However, there are two parameters that have high impact on the integration of RES plants in the network: the selection of the size (rated capacity) and the installation’s location of such plants. This paper investigates these effects by developing a series of indices, spanning from power losses to GHG emissions’ reduction, which quantify this impact and provide a tool for assessing the RES penetration in transmission networks, mainly for isolated systems. The paper is organized as follows; Section II introduces the indices that are used and they are being thoroughly described. Section III presents the test network that is used in this paper together with the results obtained for each index evaluated. Finally, in Section IV a multi-objective assessment is carried out to investigate the overall impact of RES generation on the system’s performance. II. DESCRIPTION OF ASSESSMENT INDICES In this section the assessment indices are presented. Six individual indices are considered in this paper to evaluate the steady-state performance of the network, each one relating to a specific technical aspect. Table II tabulates the indices’ description and acronyms. TABLE II Indices to Assess the Integration of Renewable Energy Resources on Transmission Systems Alexandros Nikolaidis, Francisco M. Gonzalez-Longatt and C. A. Charalambous E