Research On The Identification And Responsibility Quantification Of Unbalance Sources In Distribution System

In power system, three-phase unbalance is an age-old problem, with the development of modern electric power industry, the three-phase unbalance problems widely exist in the whole electrical environment. All kinds of unbalance disturbance sources inject the non-positive sequence components of voltage and current into the system. It may affects both sides of the supply and utilization and produces a variety of hazards, so the three-phase unbalance has become one of the most common concern of power quality. For this problem, access provisions are formulated by the electric power department and relevant organizations to limit the accessing of the three-phase unbalance sources, but this conflict with the deepening of the electricity market reform seriously. For the better control of the unbalance in the grid, academic circles put forward the "rewards and punishment scheme" of power quality. The premise of smooth implementation of this scheme is identification and responsibility quantification of corresponding power quality disturbance sources accurately.After scheme was put forward, research workers at home and abroad made a lot of beneficial exploration in the power quality problems such as the disturbance sources identification in harmonic, voltage sag, etc, however the research results about disturbance source identification in three-phase unbalance is too rare. In recent years, the research results about three-phase unbalance in domestic and foreign are mainly concentrated in the unbalance causes, influences, relevant standards, definition and management measures, etc. In the actual system, the three-phase unbalance problem at the point common coupling (PCC) tends to be a result both of the upstream unbalance sources and downstream unbalance sources and the downstream unbalance may consist of multiple unbalance source feeders. In order to avoid power quality disputes and to control the unbalance in the power grid effectively, identification and responsibility quantification of corresponding unbalance sources are needed, objectively. This could lay the theoretical foundation for the establishment of a fair and effective rewards and punishment mechanism to restrict and control unbalance pollution, and is expected to contribute to the progress to the user side power quality intellectualization in the intelligent power grid in our country.The paper carries on the thorough discussion on identification and responsibility quantification of three-phase unbalance sources. First, the shortcomings of the existing literature about the quantification of unbalance contribution are summarized, and developing an unbalance quantification method which requires only voltage and current measured data of PCC identification method is confirmed. Second, the study of this paper is divided into three stages:the main unbalance disturbance source identification in the upstream and downstream system; the main unbalance disturbance source responsibility quantification in the upstream and downstream system; the unbalance disturbance source responsibility quantification in the multiple unbalance sources system. The circuit models of corresponding stages are established in this paper, and the corresponding identification and responsibility factors are established based on the model. The main research contents of these three stagesare as follows:(1) Accurate localization for the main unbalance disturbance sources of the upstream side and the downstream side. The circuit relationships of the upstream and downstream system are studied and the equations and unbalance curves which could represent two extreme cases of only downstream side contains unbalance sources and only upstream side contains unbalance sources are obtained. According to the relationship between the normal unbalance case and the two extreme cases, unbalance contribution factors based on unbalance curve ranging could be established, so the main unbalance source in the unbalance system could be confirmed.(2) Responsibility quantification for the main unbalance disturbance sources of the upstream side and the downstream side. For establish the unbalance equivalent circuit, the unbalance sources in the upstream side are proposed to be equivalent as a Thevenin voltage source, while the unbalance sources in the downstream side are equivalent as an unbalance current source. Based on the measured data of voltage and current at PCC, two kinds of "non-intrusive" negative sequence equivalent circuit parameter estimation method are proposed, and then a corresponding sampling data filtering method are proposed. Based on the circuit superposition theorem, the unbalance responsibility factors which could quantify the corresponding responsibility to PCC of upstream and downstream unbalance sources are established.(3) Responsibility quantification for the multiple unbalance disturbance sources system. The equivalent unbalance source circuit could be derived based on the circuit relationship and it could be development into the multiple unbalance sources field, and the corresponding unbalance responsibility factors could be established respectively. According to the relationship of equivalent circuit, the upstream and downstream unbalance responsibility factors could be simplified, and put forward that using "actual current" instead of "theory of current" in the calculation of the multiple unbalance source responsibility factors, so the algorithm of multiple unbalance sources responsibility factors could be simplified.Finally, this paper obtained a set of complete system, which used for disturbance sources localization and responsibility quantification in the upstream and downstream main unbalance source system and the multiple unbalance source system. Method only requires the measured voltage and current data at the PCC, and do not need to know the internal parameters of the system and loads, so it is suitable for promotion in the practical engineering application.