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# lecture10 - EE 740 Professor Ali Keyhani Lecture#10 Power...

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EE 740 Professor Ali Keyhani Lecture #10 Power Flow Problems

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Power Flow Problem: 1) A balanced three-phase power system is assumed, and the transmission system is represented by its positive-phase sequence network of linear lumped series and shunt branch. 2) The generators are assumed to be three-phase balanced voltage sources and only the generator positive voltages are present. 3) The generators are shown as the constant P-Q models, i.e. as the injected powers into the system. Therefore, the internal impedance do not enter in the Y bus or Z bus matrix formulation. 4) The load on each bus is assumed to be a three-phase balanced load. The load models are constant P-Q models. 5) Each bus of the system is described by four parameters: P, Q, /V/, and θ . Two of the parameters are known and the other two are unknown. 6) Slack (or swing) bus is a fictitious concept, created and defined in the problem formulations. It arises because the system I 2 R losses are not known precisely in advance of the load flow calculation. G G 1 2 3 S Ground bus (0)
Power Flow Problem: Given: Schedule of generations, loads and the system model. (P G Sch , P L Sch , Q G Sch , Q L Sch , Y Bus ) Find: Bus voltages Vi ∠θ i, i=1,2… Power flow through transmission lines Active and reactive losses Load buses: Load buses are modeled as constant P and Q power consumption. θ Generator buses: two types are used: Constant P G and Q G model. Constant P G and V G model. P K , Q K given V K , K unknown variables to be calculated

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Constant P G and Q G model: θ Note: For power flow studies, the internal impedance of the generators is not included in the system model. Constant P G and V G model: P G ,Q G given V G , G unknown variables to be calculated P G given V G given Q G , O G unknown variables to be calculated
Load Flow Study as an Engineering Tool: Application-Planning: The main objective of the load flow study for planning is to find out whether or not a specific system design alternative produces bus voltages within acceptable limits. Or, simply an answer to problems like these: For expected loads and generators several years ahead. Does this line need to be built? To correct a given system voltage profile to within acceptable limits, how much reactive power supply is required? Application-operation:

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lecture10 - EE 740 Professor Ali Keyhani Lecture#10 Power...

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