The Research On Faulty Model Building And Protection Principle For New Converter Transformer

Converter transformers are important technical equipments. Traditional HVDC set filters in net side to eliminate harmonic, but this filtering method couldn't solve the problem that harmonic and reactive components bring negative effects to converter transformers. In order to solve problem radically, coupling-compensation and harmonic-shielding converter transformer, that is the new converter transformer, and its corresponding inducing filter technology are put forward. It is a patent, and its application has been funded by the key science & technology project of the Tenth Five Year Plan(05GK 1002-1) and of the Eleventh Five Year Plan of Hunan Province(06GK1003-1) and National Natural Science Foundation of China(50907018).In order to apply the patent into projects, there are lots of academic analyses and scientific test work need to be finished. The reliability and usability of converter transformers are very important to the whole HVDC system, and the safety of converter transformers operation influences the continuity, credibility and stability of whole HVDC system. So, this thesis surrounds faulty model building and protection principle for new converter transformer to develop following work:The wiring scheme, structure type and operating principle are studied in detail. The structure and function of each part of one platform for research of new technology in HVDC are analyzed, and the parameters of the new converter transformer and corresponding filters are introduced. Based on the platform, the HVDC based on new converter transformer and traditional HVDC are contrasted by experiments in the aspects of eliminating harmonic, saving energy and lowering costs, decreasing vibration and noises, and improving phase conversion, the results of which show the HVDC based on the new converter transformer has obvious advantage in the above aspects, which indicate that principle of the new converter transformer is correct and has good applied foreground.Compared with the traditional converter transformer, the new converter transformer has unique connection mode. But the exiting simulation soft wares don't directly provide the simulation model for the new converter transformer with special winding wiring mode. Based on the coupling concept, this thesis regard the new converter transformer as mutual coupling multi-coils, and then the models of mutual coupling multi-coils are established, based on which, according to the special winding wiring mode of the new converter transformer, with a finite element simulation software, multi-coil coupling module of MATLAB was used to design the normal and internal faulty simulation model of the new converter transformer separately, and a nonlinear inductance branch was designed to simulate the characteristic of saturated core of the new converter transformer. So the universal simulation models which could simulate normal operation, external and internal faulty and inrush current of the new converter transformer have been designed.The new converter transformer has special winding wiring mode, so it is essential to study appropriate mathematical methods to derive normal and internal faulty mathematical models, calculate arbitrary external and internal faults of the new converter transformer. Sparse tabular method and additive process in electric network analysis have different advantages in the aspect of establishing network equations, but how to use sparse tabular method or additive process to build the equations of electric network containing coupled branches and form new equations according to the changes of nodes or branches are not introduced in any article at present. this thesis regarded the new converter transformer as a electric network containing coupled branches, according to winding wiring mode and corresponding coupled circuit of which, this thesis respectively adopts sparse tabular and additive process to directly build the equations of electric network containing coupled branches, and the normal condition's basic mathematical model of the new converter transformer is established. Considering that models can be selected flexibly in practical projects, through evolution of the basic mathematical model, the node evolution model including the earth impedance of the neutral point, the branch evolution model including the filters connected with the secondary corner delta winding's tap are established respectively. Based on normal mathematical model, according to the change of nodes or branches, sparse tabular method and additive process are used respectively not only to build the interior fault condition's mathematical models but also calculate all kinds of inner and external faults of the new converter transformer.Based on the study of principle of differential protection for ordinary transformer, the principles of differential protection for new converter transformer are researched. Based on the analysis on the special windings wiring mode and turns ration relation of the new converter transformer, according to the filters connected or not connected to the new converter transformer, the relation between bilateral currents of the new converter transformer were separately gotten, and based on which, two differential protection wiring schemes were separately put forward, and corresponding simulation and microcomputer differential protection wiring schemes were separately put forward too. The action characteristic of two differential protection wiring schemes were analyzed and compared according to all kinds of operation condition of the new converter transformer, and according to the results, the differential protection wiring scheme which is better has been selected.Mechanism of inrush current of the new converter transformer are studied, and the ultimate reasons why inrush current maybe cause the differential protection scheme act incorrectly has been analyzed from the angle of basic principle of the circuit. Considering the shortage of differential protection scheme in distinguishing inrush current of the new converter transformer, the principle of a new microcomputer protection scheme based on the transformer model which casts the method of differential protection and utilize synthetically the information of voltages and currents of a transformer is studied. The protection scheme based on the transformer model was proved to be also suitable for the new converter transformer by mathematic illation. Based on this precondition, this thesis applied the protection scheme based on the transformer model to the protection scheme design for single-phase and three-phase new converter transformers. Considering the use of difference instead of differential possibly maybe cause prodigious error and malfunction when microcomputer protection devices run, the new criterion was instituted by applying the trapezoidal integration to the old action equations.