The advancement of Distributed Generation (DG) technologies have caused great impact to power system operation. Inappropriate installation of DGs may lead to over-compensation or under-compensation situation. Thus, a reliable optimization is urgent to avoid any unwanted effect. This paper analyses the installation impact of different types-multi-DGs determined using a pre-developed hybrid optimization technique termed as Immunized-Brainstorm-Evolutionary Programming (IBSEP). It is imperative to study the effect of multi-DGs installation such that a relevant utility can make a correct decision, whether its installation is worth or vice versa. Rigorous study has been conducted in terms of identifying the optimal location and sizing, installed on transmission system for voltage control involving different DG types. Comprehensive results are embedded in this paper to demonstrate the effect of multi-DGs installation in transmission system which in turns beneficial to the utility. Effect of optimal multi-DG siting and sizing in transmission system… (Sharifah A. Shaaya) 647 ability to deliver or absorb reactive power [13,14]. These studies found that different type of DG will affect power network performance differently.Inventing robust and reliable optimization technique to determine optimal DG location and size is currently trending in the researchers community, aiming at alleviating setbacks in the existing optimization techniques [15][16][17][18][19][20]. Apart from finding higher probability of optimum solution towards meeting the objective functions, new optimization algorithms are developed to reduce computational burden in the classical optimization techniques [21,22] Brainstorm Optimization (BSO) is a newly developed algorithm that made its appearance in 2011. An algorithm that mimics the collective behavior of human has caught attention of many researcherst. Though it is proven reliable in solving science and engineering problems, it does has high computational burden due to its K-means clustering technique as well as easily trapped in the local maxima [23][24][25][26]. In this paper, an optimization technique that embed Artificial Immune System (AIS) optimization technique and BSO into the frame of Evolutionary Programming (EP) algorithm is used to determine optimal locations and sizes of multiple DGs, of different power delivering capabilities, in power transmission system for voltage control. The hybrid technique is termed as Immunized-Brainstorm-Evolutionary Programming (IBSEP).