| In our country,it’s a realistic question that the distribution of primary energy and productive forces is uneven.Hydraulic energy,coal resources and even renewable energies are mainly in western and northern areas.While electricity demand are mainly in our eastern and southern developed areas.This maldistribution of energy resource and electricity demand determined the energy transmission direction.Based on that,lots of "West-to-East Power Transmission" projects are constructed.It’s now a certain conclusion that High Voltage DC(HVDC)transmission project is one efficient way to realize the large-scale and long-range power transmission,which has great significance for optimized allocation of energy and resources in our country.Since the first HVDC project(the Gezhou Dam to Nanqiao HVDC project)was put into operation in 1989,there’re more than 30 HVDC projects in operation in China now.The DC voltage level rose from±500kV to the highest±1100kV.The total length and transmission capacity of the HVDC projects ranked first in the world now.However,it also brings some tough problems when HVDC occupies more and more dominant position in our power system,such as harmonic distribution and low-order resonance problems in DC side,too large occupied areas of reactive compensating devices in converter stations and increasingly shortage of line corridor sources.On that basis,research works are taken in the dissertation mainly on harmonic impedance characteristic,active filter system and multi-terminal structure of HVDC transmission projects.The specific work is as follows:1)For HVDC projects,it’s determined by studies of system harmonic characteristics that whether low-frequency block-filters or DC-filters are needed to be installed for precaution of low-order resonance or not.Firstly,based on two basic DC circuit "pole-pole"/"poles-earth" structures and calculation methods of line impedance per-unit,bipolar equivalent parameters considering overhead ground wires are obtained in detail.And,an engineering method of long-range cascaded π model is proposed and compared to distribution line model.Secondly,an optimal fitting method to approach original Carson integral results is proposed.With the equivalent wave impedance results of two DC circuit structures,the DC line harmonic impedance of different frequency is derived.Finally,depending on field-test DC line impedance results of the Lingzhou-Shaoxing±800kV UHVDC project,as well as original Carson integral calculation results,the correctness of distribute parameters obtained and the accuracy of optimal fitting method are further demonstrated.2)A novel AC filter system application architecture for LCC-HVDC converter station is proposed.By combining hybrid active power filters(APF)and the existing reactive compensation devices,harmonic suppression performance is greatly improved.At the same time,number of reactive compensation devices can be remarkably cut down so that AC yard area layout could be further optimized.Firstly,based on different types of APF,comparison work is done to finally decide adopting series resonance topology.Secondly,in-depth studies are carried out on APF’ s control strategy and HVDC’s impedance characteristic.Suitable control system architecture is confirmed to assume harmonic suppression as well as dynamic reactive power support.Finally,according to condition parameters of a practical project,the active filter system is designed and analyzed.Simulation is done on PSCAD/EMTDC to prove the superiority compared to traditional filter systems on filtering performance,reactive power providing ability and restriction on reactive compensation group number.3)Researches on engineering application architecture of MTDC system are presented.Advantages and weakness of several typical types of MTDC are analyzed.And based on the most applicable LCC parallel MTDC(P-MTDC)system,studies are done on main topology,including polarity-reversal switches to realize power reversion.Then P-MTDC main circuit calculation method is presented,which decoupled the main circuit calculation into two parts:DC network and converter equation calculation.The general admittance matrix of DC network is deduced,also a novel constant-voltage control strategy is proposed to simplify the control logic and reduce the control system complexity,by means of setting the nearest point of common coupling from rectifier station as the voltage control reference point.Finally,one practical study case is taken for example of certain 3-terminal DC project,rated DC parameters are calculated,simulation model of MTDC system is built on PSCAD/EMTDC,validity of the strategy is demonstrated by simulation. |
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