With requirements to deal with ageing infrastructure and to meet environmental targets, the installed capacity of converters, including converter-interfaced generation and back-to-back converters, is expected to experience a continued growth. In traditional power networks, synchronous generators provided the overwhelming majority of electrical power, with some induction generators providing the remaining electrical power.The dynamics of operation of both machines are well understood, including potential stability issues. With the introduction of converter-interfaced generation, power networks are becoming diverse systems, associated with which is an increased number of stability issues to consider.The majority of modern converters employ dq-axis current injection control (or vector current control) in which a phase-locked loop is used to synchronise with the voltage at the point of common coupling where the converter is connected with the network. In recent years, potential issues with performance of PLL when the AC network it is connected to is weak have been reported. Poor performance of PLL under such circumstances will lead to a cascade of errors in the control system, culminating in an unstable system.This thesis investigates the stability of the power system when large quantities of converter-based generation are present. The stability thresholds, termed as system tipping points in this work, are evaluated for both small and large disturbances. A time-domain analytical method to assess the tipping points, the tipping point search method, is introduced. This can be applied to any network model. Additionally, a network frequency perturbation tool, which can also be applied in time-domain model, is introduced to visualise the response of individual generators to network frequency fluctuation and predict potential interactions among the generators.Following the identification of the limitations of vector current control, alternative control strategies which emulate synchronous generators, some more so than others, are investigated. Based on these studies, recommendations for power system engineers are made to ensure that the transition from a generator-dominated network to a converter-dominated network is as smooth as possible.
|Date of Award||11 Sep 2018|
- University Of Strathclyde
|Sponsors||National Grid & University of Strathclyde|
|Supervisor||Adam Dysko (Supervisor) & Andrew Roscoe (Supervisor)|