Characteristic numbers of Primary Control for a small control area of inter­connected power systems, such as Estonian Power System, become a major consideration in perturbation’s islanding (isolated operating), or in initiating black start situation. Against a background of independence of the trans­mission system and electricity production towards the recently opened electricity market in Estonia arise the actuality of reliability in the context of the quality of supply (frequency and voltage) and successful parallel operation. Characteristic numbers of Primary Control, such as the network power frequency characteristic, governor droop and load response influence to the frequency regulation process in case of significant frequency deviation. Because of solely dependence on Primary Control executed by IPS central regulator, the limited number of frequency affecting events can be obtained for the reasonable figures. The controlled system separation test is the only opportunity for Estonian Power System that investigates the capability of frequency regulation. Primary Control relates to the supply and load responses that stabilize frequency whenever there is a change in load-resource balance. Based on the methodology suggested by ENTSO-E and local measurements, the study state behavior of Primary Control of the isolated Estonian Power System is described in the case study. The paper presents the characteristic numbers of Primary Control in the isolated system under the system contingencies such as the artificially created failure produced by network or switch-offs of individual generating blocks and switch-offs of the HDVC link.