| Previous power system flow calculation that ignores the actual existence of asymmetry in the power system and uses single-phase flow calculation model can only meet the requirement of low asymmetry three-phase power system in planning,design and operation analysis.The reason that transmission lines do not transposition or incomplete transposition and there are a lot of single-phase loads in distribution network make the degree of asymmetry of the structure and operation in distribution network is more serious than transmission network.Therefore power flow calculation based on symmetric assumption in distribution network has a certain non-adaptability.And with the development of smart grid and the gradual increment of distributed generators,electric vehicles,and energy storage device,the traditional single supply and fixed flow direction distribution network will be replaced by a multi-supplies and various flow direction one.These changes not only increase the complexity but also raise requirements in accuracy in the distribution network operation control.The traditional power flow calculation method can not meet the demand of the development of modern distribution network.Distribution network power flow calculation under the condition of asymmetry has aroused extensive attention.Many research results has been formed in this field.But the following problems still exist:(1)most studies based on the premise that three-phase load power is known,(2)it was generally supposed that three-phase power of source is equal and three-phase voltage of source is symmetrical,(3)hardly consider load static voltage characteristic,(4)PV nodes usually adopt forecast and modify algorithms.In fact,in addition to the neutral point grounding three-phase load,the load of single phase power value cannot be obtained before the flow calculation results is known;under the condition of asymmetry,the three-phase power of source is different and three-phase terminal voltage is also unbalance;due to the asymmetry,each phase voltage may be significantly deviated from normal,load voltage characteristic will have great influence on actual flow.PV node reactive power prediction which has certain blindness,increases the number of iterations.Aiming at the problems above,on the basis of study in depth,this paper puts forward the solving measures,establishes the corresponding models and algorithms.The main research content and innovation are as follows:(1)Considering the load static voltage characteristic and connection modes,creates four asymmetric load models(neutral grounded load,neutral non-grounded load,delta connection load,three-phase coupling load),which covers all possible load access ways and enhances the load processing capacity.(2)By introduction negative and zero sequence impedance,solves the problem of creating flow equations for three-phase coupling load and power nodes which includes PV and balance nodes,this make the processing for PV nodes is no longer a problem,and the calculation about three-phase powers and voltages of power source is possible.(3)Sequence component algorithm and phase-sequence component hybrid algorithm for asymmetric distribution network power flow calculation are studied.According to the characteristics of the flow equations,some equations use Gauss-Seidel and some equations use Newton-Raphson in an iteration,solves the problem of solution of flow equation.Based on the established models and algorithms in this paper,develops asymmetric distribution network power flow calculation program in the C++Builder XE7.The models and algorithms are verified through IEEE33 node example. |
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