Maintaining the frequency stability in future power systems is a concern for system operators due to the reduction of synchronously connected kinetic energy and the subsequent loss of traditional response schemes. New renewable energy sources will be required to provide frequency response ancillary services, and a suitable candidate for facilitating this is the voltage source converter (VSC). This thesis investigates frequency stability contributing factors, VSC frequency response improvements including any resulting detrimental impacts, and the impact VSC frequency response schemes have under various operating conditions. Traditionally, frequency stability studies have primarily focused on assessing the impact of the system inertia as the main factor dictating system frequency behaviour. This thesis expands this analysis to assess the role that additional factors including topology, power flow and inertia distributions play in frequency stability studies. This research highlights the influence the different factors have on frequency stability. Fast frequency response requires the availability of energy that can rapidly be injected into, or absorbed from, the power system. A potential solution is to use the stored energy within a VSC system. This thesis will evaluate the impact that this stored electrostatic energy could have for ancillary services. To improve the responsiveness of a VSC proportional frequency containment controller, the use of a discontinuous dead band implementation is presented. Proportional control is commonly proposed and this thesis exploits the fact that VSC devices can operate faster than traditional synchronous generators. To advance and provide a targeted response from the VSC, an innovative frequency controller is presented. This controller operates to provide the energy that is delayed from traditional synchronous generator primary response due to the mechanical and thermal delays. The controller provides the difference between an ideal system response, that has no mechanical or thermal delays, and the estimated actual system response. This controller provides a response that is analogous to an inertial response without using a derivative control element that can be problematic to realise in practice. The need for fast frequency response is essential following large disturbances where the response provides a large quantity of power in a short timeframe to contain the frequency. However, the detrimental impacts of using fast frequency response schemes following a small disturbance could lead to unwanted converter behaviour. The incorporation of a large capacity of fast frequency response under small disturbance behaviour is investigated. A feedback based scheme is incorporated into the frequency controller that allows the VSC to respond un-hindered and reduces the responsiveness if any oscillatory behaviour is detected. The deployments of four different VSC frequency control schemes are investigated under different operating situations that derive from the network factors analysis. The operation is analysed to inform the deployment and improve the frequency stability. Overall the research in thesis has contributed to a greater understanding of frequency stability and the role that VSC devices can play in improving the frequency stability.