Methods for Synchrophasor-Based Power SystemInstability Detection and HVDC Control
Sammanfattning: The recent availability Phasor Measurement Unit (PMU) functionalities in relay technology has opened up new opportunities for power system protection, allowing this microprocessor-based technology to be used beyond traditional protection purposes. This technology now also considers the computation and communication mechanisms that allow the transmission of synchronized phasor measurements. This makes possible the use of these features in protection applications. As a result, Wide-Area Monitoring, Protection and Control (WAMPAC) systems can expand by using protective relays built with synchrophasor capabilities which may facilitate many applications such as the synchronization distributed generation to large power grids or for the integration of renewable sources of energy.This thesis rationalizes the need of coordination between protective relays with synchrophasor capabilities and High Voltage Direct Current (HVDC) controls to steer power systems away from instability conditions. The concept of coordination involves the use of feasible communication mechanisms which can be exploited by protection systems to send out synchornized voltage and current phasors to a mechanism which determines preventive, corrective, and protective actions particularly by taking advantage of the availability of HVDCs. Coordination refers to the ability of the protective systems and HVDCs to cooperate and to harmonize their actions so that voltage instability can be avoided. Synchrophasor processing capabilities allow for the exploitation of phasor measurements while satisfying protective relaying requirements.The author addresses the challenge of mitigation of voltage instability through two sequential approaches. First, voltage sensitivities computed from synchrophasor data can be used for voltage stability monitoring and can be exploited for delivering wide-area early action signals. These signals can be used for activating controllable devices such as HVDCs which may also exploit phasor measurements for control. In order to provide reliable information, synchrophasor data must be pre-processed to extract only the useful features embedded in the measurements and to correct for errors. The methodology is derived by considering both positive-sequence simulations for methodology development purposes, and real phasor measurement data from a real-time (RT) hardware-in-the-loop (HIL) laboratory. The use of the RT-HIL laboratory allows to test the robustness of the developed approach in a more realistic environment, this will guarantee its performance for use in control rooms. The methodology has also been tested with real PMU data obtained from the Norwegian transmission system showing the validity and applicability of the method.The wide-area early action signals derived from the method are then used for voltage stability mitigation through HVDC control. The signals are used to trigger the operation of HVDCs or to change their control modes before they reach stringent operating conditions. In addition, synchrophasors are also exploited as feedback signals feeding supplementary stability controls. The proper selection of signals and activation of these special HVDC control is investigated.The second approach is used to ensure that HVDCs will operate securely when their transfer is pushed towards the maximum transferable power level. It is shown that Classical HVDCs are prone to voltage instability when operating on weak AC grids. To cope with this delicate operation scenarios, the Automatic Voltage Stabilizer (AVS) and Automatic Power Order Reduction (APOR) controls are implemented for HVDC control to cope with these undesired conditions. The implementation is carried out both in off-line and real-time simulation environments.
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