Static Optimization Of Under-frequency Load Shedding And Transient Under-voltage Load Shedding Strategies For AC/DC Power Systems

Flexible HVDC transmission technology is widely applied in grid interconnectionand wind power generation. It is necessary to study the characteristics of hybrid AC/DCtransmission systems to ensure secure and economic operation of the integrated systems.Based on static optimization and dynamic simulation, the dissertation studies the staticoptimization of under frequency load shedding and transient under voltage loadshedding.Firstly, based on reduced model of MTDC systems, the existing integrated solutionis simplified by combining the variables, which has higher calculation efficiency andthe same accuracy. Then according to the performance sequence of frequency controlfacilities, nonlinear optimization method is applied to quantify the load shedding andrecovery in two stages. The method is easy and fast, beneficial to frequency recovery.Secondly, the relationship of the system frequency and field voltage is quantified todescribe the effect of frequency drop on the var output of the synchronous generators,yielding the frequency-related extended load flow algorithm, which helps to exactlydetermine change of bus voltage and var output at the generator buses when the systemis suffered from active power deficiency and frequency drop.Finally, based on the integrated dynamic model of the hybrid AC/DC systems, theunder voltage load shedding strategy considering the transient voltage security andVSC overload protection is proposed at the AC side of the inverter. When the constantvoltage control object is violated by the load surge, the control strategy is convertedfrom PV to PQ mode. With the expense of small voltage drop, the VSC outage isavoided. When the VSC is suffered from severe load surge, the fast under-voltage loadshedding helps to maintain the continuous power supply and transient voltage security.