RESEARCH PAPER Optimal Placement and Sizing of Distributed Generation Using Wale Optimization Algorithm Considering Voltage Stability and Voltage Profile Improvement, Power Loss and Investment Cost Reducing Meisam Yahyazadeh 1 • Hojat Rezaeeye 1 Received: 7 January 2018 / Accepted: 20 June 2019 Ó Shiraz University 2019 Abstract This paper suggests a method for optimal locating and sizing problem of distributed generations (DGs) as a nonlinear and multi-objective optimization problem. The offered method creates a trade-off among static voltage stability index, power losses, voltage deviations and investment cost as important indexes in DG location and size selection. The research is carried out using a steady-state voltage stability index for evaluation of system security. The suggested optimization problem has been carried out as a hierarchical optimization problem. In the first step, Wale Optimization Algorithm method has been applied to attain the coordinated plan of quantity and assignment of DG. Also, a MATLAB program has been developed to assess the voltage stability index, respectively, each coordinated pattern during WOA search. Suggested MATLAB program is based on a nonlinear optimization problem which it efforts to evaluate voltage stability. The proposed method has been implemented on IEEE 30 bus test system, and the results show the proper performance and acceptable operation. Keywords Distributed generation  Wale optimization algorithm  Optimal placement and sizing 1 Introduction In recent years, the penetration of DG resources in distri- bution and transmission grids is increasing worldwide and the connection of distributed generation to the grid and their influence on voltage support, voltage stability, voltage flicker, protection, harmonic, energy loss reduction and release of system capacity are investigated (Papathanassion 2007). These points of view guide us to reach a proper DGs allocation method (as an important power generation sources) based on power systems voltage stability margin (Khattam and Salama 2004; El-Sadek 2002). Economic and environmental reasons have compelled the transmission systems to be worked near to their security limits. With the increased loading of existing power transmission systems, the problem of voltage stability has become as a main matter in modern power system planning and operation which refers to the ability of the system to withstand disturbances against any violation in system operating conditions (El-Sadek 2002; Taylor 1994). Many methods have been suggested to define the opti- mal locations of DG until yet. Dispersed generation (DG), unlike conventional generation, aims to generate part of required electrical energy on small scale (typically 1 KW– 50 MW) closer to the places of consumption and inter- changes the electrical power with the network. The DG, also termed as embedded generation or dispersed genera- tion or delocalized generation, has been defined as electric power source connected directly to the distribution network (Ng et al. 2000; Ackermann et al. 2001). DGs include synchronous generators, induction genera- tors, reciprocating engines, micro turbines, combustion gas turbines, fuel cells, solar photovoltaic, wind turbines and other small power sources. They are cost-effective and environmental friendly as they remove the need for expen- sive construction of distribution and transmission lines. Besides, they provide high power quality and more reliable energy than conventional generations. The number of DG & Meisam Yahyazadeh Yahyazadeh@vru.ac.ir 1 Faculty of Engineering, Vali-e-Asr University of Rafsanjan, Rafsanjan 7718897111, Iran 123 Iranian Journal of Science and Technology, Transactions of Electrical Engineering https://doi.org/10.1007/s40998-019-00224-4