1 I. INTRODUCTION Power system stability is considered as one of the important aspect for safe and reliable operation of the system. The increasing demand of reliability and stability in the power system emphasizes the need of system studies. The operational parameters that may lead to instability situation in the power system are the voltage, frequency and rotor angle. Power system stability studies can be classified as steady-state and transient stability. Classification of stability depends on magnitude of disturbance(steady/transient state), on time duration(short/long term) and nature of the parameter(Voltage/frequency/rotor angle) gets affected subjected to any disturbance in operation[1]. The importance of stable operation has been depicted by major blackouts in the history of power system around the world. Power system smooth operation depends mainly on the operating characteristics of generator’s active and reactive power control while operating within the thermal limits[2, 3]. In steady-state stability, generator dynamics may change due to small perturbations in load change while in transient stability the impact on generator dynamics will be severe. Three phase fault creates most severe impact on the system as the generator accelerates at higher rate due to large imbalance in applied mechanical and counter electrical torque on it[2]. Both governor and exciter play important role for the stability of the system. Response from the governor is sluggish in nature due to mechanical operation while exciter response is much faster due to electromagnetic nature of transient[3]. The governor response in case of mentioned disturbances has not been discussed as it is beyond the scope of this paper. Excitation system of alternators can be classified as DC, AC and Static system. Static excitation systems can be further categorized as compound source rectifier system, potential source and compound controlled rectifier excitation system[4]. Alternators from the perspective of reactive power control can be operated in voltage, droop, power factor or reactive power control mode [5, 6]. In this paper, voltage control mode of the generator is set using built-in models of ST1A and ST2A in MATLAB with typical values. This paper comprises of two major sections of literature study and analytical results. The first section briefly discusses the exciter models used in this simulation while the later section presents the comparative results of both excitation models. Responses of the excitation systems under the operating conditions having normal load on generator, small perturbation in load and the occurrence of three phase fault have been discussed. ST1A and ST2A excitation models for alternators are among the standard models used in MATLAB. The basic idea and key limitations of these models have been summarized in this paper. In reference[7], detailed description of these models have been discussed. II. SECTION ONE: LITERATURE REVIEW A. ST1A Excitation System ST1A represents potential-source controlled rectifier system of excitation. In this model, excitation power is supplied through transformer from generator terminals which is then regulated by controlled rectifier.[1] The excitation voltage may either of positive or negative polarity depending upon the firing Comparative analysis of ST1A and ST2A excitation system models for voltage stability of alternator Shariq Shaikh 1 , S.Taha Ahmed 1* , Shiraz Khan 1 , M. Maaz Naseer 1 and M.A. Rehman 2 1 Department of Electrical Engineering, NED University of Engineering and Technology, Karachi, 75290, Pakistan 2 SIEMENS, Pakistan (taha-ahmed_93@hotmail.com) * Corresponding author Abstract: Excitation system governs the response of alternator regarding reactive power (Q) supply from it. Different excitation models have been developed in past years to predict the generator response for stability of power system operation. Static excitation systems are most widely used Q-control system due to merits over other systems. In this paper, we have done a comparative study to investigate the response of ST1A and ST2A excitation system models under different operating conditions. ST1A models the potential-source controlled-rectifier excitation system while ST2A compound-source rectifier excitation systems. Two areas four machine power system configuration is used for analysis. Simulation work is carried out on Simulink (MATLAB) and the responses of two exciters under different circumstances have been discussed. The key focus in this comparative study is on the response of exciter voltage and corresponding generator terminal voltage. The principle result of this simulation is that under bolted fault conditions in power system, ST2A performance is considerably better from the perspective of terminal voltage stability and rotor oscillations as compare to ST1A. Keywords: Reactive power control, ST1A & ST2A excitation systems, Voltage stability. 2017 2 nd International Electrical Engineering Conference (IEEC 2017) May 19 th -20 th , 2017 at IEP Centre, Karachi, Pakistan