Research On Harmonic Suppressing Measures Of Hybrid HVDC Asynchronous Interconnection System

With the construction of energy internet,the increasing scale of power grid brings about not only the benefits of better energy distribution and larger short-circuit capacity,but a]so the complexity of power grid and the risk of large blackout caused by complex cascading faults.Therefore,asynchronous interconnection has become an inevitable trend.Hybrid HVDC,which combines the flexible control characteristics of VSC-HVDC and the large transmission capacity of LCC-HVDC,is an economical and practical way for asynchronous interconnection.However,hybrid HVDC asynchronous interconnection system may lead to a series of harmonic problems such as power quality declining caused by AC harmonics and electromagnetic interference caused by DC harmonic,which requires harmonic suppressing measures.At present,there's a comprehensive understanding of harmonics in LCC-HVDC and VSC-HVDC,but no research has been done on the harmonic character-istics and harmonic suppression of hybrid HVDC asynchronous interconnection system.In this paper,the topology and operation principles of hybrid HVDC asynchronous interconnection system are introduced firstly.Secondly,harmonic generation mechanism,fundamental harmonic characteristics and harmonic interaction between convertors and AC system are analyzed.On this basis,the harmonic suppressinog measures of hybrid HVDC asynchronous interconnection system are studied from three aspects:AC harmonic suppression,DC harmonic suppression for LCC.and DC harmonic suppression for MMC.In order to suppress DC harmonics in MMC-HVDC transmission unit,a DC harmonic suppressing control strategy for MMC is proposed.Simulations show that this control strategy can accurately detect and suppress the highest harmonic content in DC current of an MMC,so as to reduce the DC harmonic pollution in MMC-HVDC.Finally,taking advantage of that LCC and MMC are connected in parallel on the same AC bus,a harmonic co-suppressing control strategy is proposed and simulated.When some of the AC filters are out of tune due to faluts or frequency offset,the MMC is used to assist in absorbing the LCC generated harmonics,so as to reduce the probability of LCC-HVDC emergency shutdown,improve the power quality and operational stability of hybrid HVDC asynchronous interconnection system.