During the last years, electric power generation was based mainly on large centralized fossil fuelled power plants. However, nowadays and in the future, efforts are made to increase the share from renewable sources, as a result of increasing environmental concern. Indeed, in some networks areas, wind generators gradually start to replace conventional synchronous generators, while the European Union has set a “20 - 20 - 20” target plan, which aims to achieve a 20% reduction in green house gas emissions compared to 1990 levels, have 20% of the energy on the basis of consumption coming from renewable sources and a 20% increase in energy efficiency by 2020. Due to the increment of wind generator share, power system stability and reliability may be affected. This is attributed to the fact that the characteristics of wind turbines are different from the ones of conventional plants. Therefore different frequency, voltage and transient stability system performances arise which require different measures to be tackled. This report, underlines the importance of rotor angle stability studies in power systems with high share of power electronic interfaced generation units. These studies are related to the ability of the synchronous generators to remain in synchronism when subjected to large disturbances (e.g. faults). A supplementary damping control as a mitigation solution against rotor angle stability threats is studied in power systems dominated by full-scale power electronics wind generation units. This solution, as it is shown in this report, is cost-effective and is adopted for modification of the outer control of fully decoupled wind generators. Concerning related work reported in existing literature, several researchers have used the frequency deviation as input for power system stabilizer added to the grid side converter of a fully decoupled wind generator, whereas other researchers have used the terminal voltage. However, rotor angle remote signals for damping the Synchronous machines oscillations can also be used. Therefore, a supplementary damping control for fully decoupled wind generators, considering rotor angle deviations as input signal, is adopted in this report. The thesis report also provides a detailed overview of the adopted structure of supplementary damping control. Besides, a discussion, based on EMT and RMS simulations performed by using RSCAD and DIgSILENT PowerFactory tools respectively, is presented to address research gaps in existing literature, namely, identifying the most suitable wind generator locations for addition of supplementary damping control, the degree of improvement in damping performance due to the use of rotor angle deviation as input signal, the effectiveness of wind generator with the damping controller as affected by distance to reference generator, and the effectiveness of the controller when the distance between synchronous generator and wind generator is taken into account. Different indicators related to transient stability studies are used so as to evaluate the performance of the controllers, whereas optimization techniques are also utilized to find their appropriate settings. For the rotor angle stability studies, different power systems were studied. In particular, faults were examined in a modified IEEE 9 Bus benchmark system as well as the test system of the Great Britain (GB) transmission power system. The aforementioned systems are used to examine the performance of wind generators with damping controller in systems with different dynamic behavior. For each system, different scenarios were applied, by simulating different fault locations (modified IEEE 9 Bus system/GB system), or different wind share level (GB system), different time delays due to data latencies (GB system) and different input signals in the proposed damping controllers (GB system). The selected cases include critical operational scenarios, i.e. fault locations in lines with high pre-fault apparent power dispatch, faults in prone to instability synchronous machines’ terminals, high latency delays of 200ms. Faults with relatively high clearing time of 120ms are applied in the system, so the systems are examined close to their limits.