# PSO-Based Voltage Control Strategy for Loadability Enhancement in Smart Power Grids
## 1 Introduction  
Voltage instability has always been an important issue for grid engineers during the past few decades [1]. Nowadays, renewable generation, energy storage, demand response, and electric vehicles add complexities to power system operation. These factors present more challenges in maintaining voltage stability in a prevailingly open access environment.  
The grid components such as system loads, transmission networks, and generators may contribute to voltage instability [2]. When system voltage drops dramatically, it endangers the integrity of the system. Therefore, it is imperative that system operators be provided with good information regarding the system voltage stability [3; 4; 5]. Indeed, when voltage instability is detected (i.e., the computed value of voltage stability margin is less than or equal to the predefined security margin), appropriate remedial actions should be initiated to prevent the system from possible voltage collapse. For a bulk power system, however, just relying on grid operators to manually adjust many widely dispersed reactive power resources in real-time is an un-realistic and uneconomic way. Thus, the effectiveness and robustness of the control systems are always expected.  
To address this issue, the coordinated automatic voltage control (AVC) has been developed. It is organized as a hierarchical structure with three levels: the tertiary, secondary, and primary voltage control, in which tertiary is the highest level and primary is the lowest level. Each higher control level commands the voltage set point for the lower level. In recent years, many research studies have focused on the secondary level for the achievement of the stability and security operation in smartgrids [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. The secondary voltage control (SVC) method was first established in France [6], and has been applied in several countries, such as some European countries [6; 7; 8; 9; 10], Brazil [11], South Africa [12], US [13; 14], and China [15].  
On the other hand, the particle swarm optimization (PSO) technique, which is the family of the evolutionary computation algorithms, has attracted a lot of attention in many research areas [28; 29; 30]. Indeed, PSO is a heuristic optimization technique, inspired by the social behavior of bird flocks and fish schools. With flexibility and well-balanced mechanism, PSO has the abilities to enhance the global and local search [31]. Due to the simplicity of the concept and implementation, PSO has been effectively applied to various optimization problems in smart grids [32]. For example, PSO based methods were used to solve the problem regarding the optimization in the distribution networks [33; 34].  
This research is mainly devoted to the protection of any disturbances that may result in power grid voltage collapse. The main contribution is the development of a new voltage stability enhancement methodology for safe power grid operations. To this end, a PSO-based optimal voltage control methodology, which enables achieving reliable and efficient voltage profile/stability regulation in power grids, is proposed. The remainder of this paper is organized as follows: Section 2 gives a brief review of the related work. The basic concept of voltage stability is introduced in Section 3. In Section 4, the problem formulation of power gird Q-V control is stated. Section 5 proposes an optimal voltage control algorithm using particle swarm optimization. Test results are presented in Section 6. The paper finishes with some concluding remarks in Section 7.