Research On Operational Characteristics Of New Converter Transformer In DC Transmission System

From the "War of Currents" in the late1880s to well over70years after, AC(Alternating Current) has been regarded as the natural choice for electrical powertransmission. However, HVDC (High Voltage Direct Current) is now emerging as apractical and economical alternative that is very efficient for transmitting largeamounts of power for long distance, and provides a good solution for a number ofreliability and stability issues associated with connecting sustainable energy schemesto the main power grid.China is now the largest developing country, the second-largest energy producerand customer in the world. Although China is a vast territory rich in energy resources,but distribution of the resources is quite unbalanced geographically. Coal is scatteredmainly in the north and the northwest, hydropower is found in the southwest, and oiland natural gas are placed in the eastern, central and western regions and along thecoast. Moreover, the consumers are mainly in the southeast coastal areas, where theeconomy is thriving. Such a great difference of location between the producers and theconsumers has led to the following stratagem of China power industry:“Interconnection in the whole country, transmission of electric power from West toEast and exchange electric power between South and North”. HVDC, with all itsadvantages for long distance transmission, plays a major role in the creation of China’snational grid.In DC (Direct Current) transmission system, the function of converter transformeris transferring electrical energy between the grid side and valve side, while thefunctions of AC filters are compensating reactive power and elimination of harmoniccurrents. Because the filters are always placed at the grid side of the transformer, theydon’t have any influence on the transformer. With or without filters, the operationalparameters of the transformer are not changed, a large amount of harmonics andreactive power are entirely through the converter transformer from valve side to gridside which lead to unfavorable effect and increase the burden on convertertransformer.Unlike the conventional converter transformer, the new converter transformerwith its filters can enhance the operational characteristics. The new convertertransformer with its filters is a special kind of phase-shifting transformer whose gridside windings are connected to power grid, the valve side windings are connected to the converter, and the common side windings are connected to the filters. Similar tothe conventional converter transformers of12-pulse converter with Y/Δ/Y windingconnection, the new converter transformer also has an upper and lower bridge whichcorresponds to Δ and Y respectively.In this thesis, a systematical analysis of the new converter transformer’sshort-circuit impedance, reactive power compensation, harmonic suppression, matrixmodel, and commutation characteristics, are performed. The theoretical results arecompared with the simulation and experiment. The achieved results are useful fortheoretical analysis, design and manufacture, and operation of the new convertertransformer. The fundamental research and the related innovative achievement can besummarized as follows:Firstly, the theory of the DC transmission is briefly presented, including type ofthe DC transmission, converter station, characteristics of6-pulse and12-pulseconverter, and the AC filer and capacitor bank.Secondly, based on the configuration of the conventional phase-shiftingtransformer, the winding connection diagrams of the new converter transformer arepresented along with its equivalent circuit and basic equations of voltage and current.Additionally, the single-phase short-circuit is studied which permit to determine theparameters of the theoretical equivalent circuit, and furthermore establish thesimulation model for the new converter transformer.Thirdly, for the sake of analysis of the short-circuit characteristic, themathematical equations of valve side and grid side short-circuit impedances arededuced for both upper and lower bridge transformers. The analytical results show thatthe filters have a little effect on the short-circuit impedances, the grid side and valveside are almost symmetrical, and the short-circuit impedances of upper and lowerbridge transformers are almost identical.Fourthly, the operational characteristics of the new converter transformer arestudied. The mathematical models which reflect the reactive power compensationcharacteristic and harmonic suppression characteristic are established. The analyticalresults show that the new converter transformer not only reduces the grid sideharmonic currents and the load harmonic voltages, but also decrease the grid sidecurrent and apparent power, improve the grid side power factor, and increase the valveside voltage.Fifthly, the mathematical matrix model which reflects the relationship of voltagesand currents between the grid side and valve side for the new converter transformers is presented. The analytical results show that the traditional mathematical deducing canbe fully replaced by the matrix model which enables the analysis of the characteristicsof the transformer very easily, quickly and correctly.Finally, aiming to analyze the commutation characteristic, the mathematicalmodels which reflect the commutation process are established. The analytical resultsshow that the new converter transformer can improve the commutation characteristicof the converter, with DC voltage being increased, and the commutation angle beingdecreased.