Study On The Influence And Control Strategy Of Weak AC Tie-Line And Sending Terminal Area Caused By DC Fault

The current power grid is in a transition period of "strong DC and weak AC".The coupling between AC and DC is getting closer and closer,so does the sending end and the receiving end coupling.The impact on power grid caused by DC fault has been transferred from a local to a global level.Power transfer crossing regional and sending transient over-voltage problems have become increasingly prominent.Study on the influence of weak AC tie-line and sending terminal area caused by DC fault can reveal the power fluctuation mechanism of tie-line,deepen the understanding of original theory and improve tie-line power fluctuation theory.A design scheme of coordinated control system in regional power network for multi-DC faults is proposed in this paper.There are also control measures of suppressing transient overvoltage.These measures can effectively reduce risk of tie-line separation,improve DC transmission capacity and ensure the safe and stable operation of large power grid.The conclusions also have practical significance in Engineering.The impact on weak AC tie-line caused by DC fault is the first one to study.It's about large amplitude power fluctuation or even disconnection problem of tie-line caused by large-scale power transfer across region.By deducing time-domain equation of tie-line power based on two-machine equivalent system,the maximum power oscillation equation is modified considering the impact of electromagnetic mutation.Prony analysis method is applied to the oscillation mode recognition of tie-line power fluctuation.Taking a number of UHV DCs in a certain power grid as typical research scene,weak tie-line power fluctuation mechanism is analyzed through decomposing the tie-line power variation into inter-area oscillation mode component and inter-provincial oscillation mode component.Based on theoretical derivation and simulation analysis,the peak amplitude of the large-mode oscillation mode component is affected by the DC fault location,while The inter-province oscillation mode component has an influence on the phase of the power variation.Secondly,the design of regional power grid system protection scheme has been proposed to optimize the current control measures.This scheme can not only guarantee the stability of power system after a single DC faults,but also after multiple DC faults.It has good adaptability to multiple DCs fault scenarios.What's more,PSD-BPA program is used to verify the function of this control system.In this paper,the influence of DC fault on the weak sending terminal is also studied,mainly about the transient overvoltage problem.At the beginning of DC operation,supporting power construction and DC production are not synchronized.Operation effective short circuit ratio is used to measure the strength of sending grid.Compared with the transient voltage in the form of the following fault,the maximum voltage peak is found caused by DC commutation failure.The fault forms include DC unipolar locking,commutation failure and DC line recovery success or failure.Some comprehensive measures to reduce the transient voltage to a reasonable level have been proposed,such as speed up the production progress of power supply,equip with dynamic reactive power compensation device and limit DC power.