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MODERN TRENDS IN POWER TRANSMISSION
Introduction:
a. The concept of six phase transmission lines was introduced by Barns & Barthold during 1972. Six phase power system is a part of multiphase power system. Due to harmonics effect and various other reasons Six phase systems and six phase machines are not popular but six phase transmission lines are more popular due to its increased power transfer capability by times, maintaining the same conductor configuration, rights of way, better efficiency, better voltage regulation, greater stability and greater reliability
b. High Phase Order(HPO) transmission is the use of more than the conventional three phases to transmit power over a given transmission corridor. Table I illustrates the increased power that can be potentially be transmitted if an existing double circuit line is
2. converted to a 6-phase line.
This increase in power may be coupled with a decrease in electric and magnetic field strengths for certain configurations and reduction in radio and audible noise levels. For the HPO Demonstration Project, a 6-phase system was selected since it represents an optimum between the proportional increase in loading and the proportional increase in surge impedance obtained by increasing the number of phases with the increase in power transfer capability.
In 1978 the U.S. Department of energy sponsored a project for the construction and testing of experimental 6 and 12 phase lines. As a part of this project, new high phase order tower, insulators and insulator spacer designs were developed. Test lines have successfully shown practical and simple constructions for 6-phase transmission and proved all advantages predicted before.
Method of obtaining 6 supply from 3 supply:
Phase and line voltage relations :
Consider V line between Va and Vc then from fig 1.2.1
From equilateral triangle M N K
Since MN = KN = Vph and the line voltage
Vline = VL = Vph
6 - PHASE TRANSMISSION TOWER &TRANSFORMER CONNECTION
The 6-phase line was modeled as double circuit 138kV line that had been up rated to 138kV 6-phase to provide additional transmission capability. The construction of the
tower is shown in the above figure. The figure below that represents the connection of two transformer banks consisting of six single phase transformers that are used for the three- to six-phase conversion.
Feasibility of an interconnected 3-phase / 6-phase transmission systems
Basic feasibility of interconnected 3 phase / 6 phase transmission system has been reported for normal steady state operation, over voltages, insulation requirements, power flow etc., in several studies.
The studies reveal several interesting features of six phase transmission system and found to posses better characteristics than those of its 3-phase counter parts. The important findings of these studies are consolidated here for ready reference.
(i) Power transfer capability :
Consider 3 practical cases as follows:
(A) : 138 KV, 3 phase double circuit line configuration
(B) : 230 KV, 3ph transmission line configuration
© : 138 KV, 6 phase transmission line configuration
To convert the existing 3phase double circuit line to 6phase transmission line with little modification of the terminal equipment in the substations to provide 3phase / 6phase transformer etc., without changing the transmission towers, without additional rights of way etc,.