Coordination of protection system and VSC-HVDC to mitigate cascading failures

University dissertation from Stockholm : KTH

Abstract: The rapid development of the global economics has made power systems allover the world become large-scale interconnected grids. This increases the capabilityof power grids to transfer power over the long distance to serve the desiredpower demand with the minimum cost of operation. Unfortunately, it alsoenables the propagation of local failures into global networks. In other words,if a blackout happens in a power system, the size and the damage may significantlyincrease.One of the main ways in which blackouts become widespread is cascadingfailures. This type of failure originates after a critical component of the systemhas been removed fromthe service by protective relaying. As a consequence, theload handled by the failed component needs to be redistributed which mightcause an overloading on other components in the system.On the other hand, the high power electronics controllable devices suchas Voltage Source Converters-based High Voltage Direct Current (VSC-HVDC)transmission are recently developed. These electronics devices have the potentialadvantages such as the ability to independently control active and reactivepower, and maintain voltage to be at acceptable level. Therefore, they are consideredto be the promising devices that with an appropriately designed controlstrategy, they can substantially improve the performance and reliability of thepower system.This thesis presents the possibility to consider protection system status inthe control of VSC-HVDC link. A great deal of this research is development ofcoordination between this power electronic device and protection system which normally are considered separately. The derivation of protection system has been selected to determine the operation of VSC-HVDC. The methodology isbased on utilizing the signal created from a logical evaluation of relay and simplificationsof certain parameters. By introducing information from the relays tothe VSC-HVDC link via Central Control Unit (CCU), the modulation of transmitted power is devised in order to reduce the risk of system-wide failures. In turn,this means an avoided blackout.Furthermore, this thesis also includes the preliminary suggestion to selectthe location of VSC-HVDC. The methodology is based on predicting voltage instabilityusing voltage stability indices and related parameterswhich are derivedby using Singular ValueDecomposition method. The solutions indicate an effectivelocation for applying corrective action such as load shedding. This optimallocation is selected to reinforce the control strategy of VSC-HVDC in order toprevent cascading failures in the more encompassing systems.