Influence Of Novel Converter Transformer And Its Filter System On Commutation In DC Transmission

Although our country is vast in territory, the energy distribution and the load demand development is not very balanced. So the remote and high-capacity power transmission systems as well as the interconnected local power networks are imperative. The DC transmission system will certainly play a very vital role on the sustainable development of our country's power industry for its technical and economical superiority.DC transmission system based on the novel converter transformer and its filter system, for short, novel DC transmission system, has adopted a kind of brand new filtering plan, which is named inductive filtering technology. It is not only different from the traditional passive filter way but also different from the existing active filter way. It adds the characteristic harmonic filter into the middle tap place of valve-side winding in the novel converter transformer. So the majority of harmonic current is short-circuited through the filter leg while few part of it can pass through the net-side winding, which can be ignored. Both the theoretical analysis and the field tests in this article indicate that the filtering effect in the novel DC transmission system is much superior to that in the traditional system. The design allowance to its rated capacity can be reduced with considering the influence of harmonics, which is in favor of reducing the manufacturing cost of the converter transformer and the operation loss. What's more, the vibration velocity of novel converter transformer drops to about one third of traditional converter transformer and the novel DC transmission system has certain superiority in lowering the vibration velocity and noise in the converter transformer.Furthermore, how does the novel DC transmission system influence commutation? What potential advantages does the novel system have by contrast with the traditional one? The contents of the paper are as follows:(1) Traditional computation method of stable state parameter in DC transmission system is either complex, especially when the related filter considered, or too simple to avoid large calculation error. In view of this question, we carry on Fourier decomposition to phase current of valve-side of converter transformer completely, and then establish a detailed mathematical model to calculate the operation parameters of the DC transmission system. This model will be help of analysis and calculation in the case of the DC system with filter. And it is able to reflect precisely the changing law of various operation parameters in the stable state. The computation process is very simple too.(2) Through the thorough research, the coupling reactance of AC system and leakage reactance in the converter transformer can lead to commutation failure. Although both the novel DC transmission system and the traditional system can eliminate the bad effects of coupling reactance to inverter commutation, the former can greatly weaken the negative effects of transformer leakage reactance and improve the commutation characteristic while the later cannot avoid these negative effects. Furthermore, this paper finds out the intrinsic reasons, namely the commutation reactance has greatly reduces in the novel DC transmission system.(3) Through systematic study, when filter is not added to the novel DC transmission system, the computation method of commutation reactance is the same as the traditional method. Contrarily, the traditional method is no longer suitable.Therefore, we propose a new computation method and prove that the commutation reactance is related to the loop equivalent inductance of the fundamental and harmonic wave components of commutation current and these components'occupancy.(4) The commutation reactance of traditional converter transformer is basically identical with the short circuit reactance and mainly determined by the leakage reactance of the converter transformer. Thus, the leakage reactance should be large enough to limit the short-circuit current, but it should not too oversized. Otherwise, the reactive loss increases and commutation fails. Our research shows that the short-circuit reactance of the novel DC transmission system doesn't change obviously while the commutation reactance reduces.(5) The feasibility studies prove that it is difficult for the traditional DC transmission system to be applied to the weak AC system, but the novel system has a certain application prospect. The commutation voltage required in the novel system has been greatly reduced and the intensity of the AC system reduces as well.This paper analyzes the fundamental theory about the commutation characteristic in the novel DC transmission system. Results show that the novel system can better the commutation and probably be used to the weak AC system. It has great advantage and wide application prospect.