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
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
1. 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.
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
(C) : 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,. Then power transferred is given by
Thus from the above equations (2) & (3) it is clear that 6 phase option not only includes less expenditure but also improves the power transfer capability to 1.732 times i.e., 73.2 % more power can be transferred. On the other hand 230KV, upgraded option not only involves more expenditure due to the change in tower design and increased conductor spacings, but also improves the power transfer capability by 1.67 times only from eq. (2). Thus 6 phase line will have more power transfer capability at reduced cost.
(ii) Surge impedance loading (SIL) :
It is the power delivered by a transmission line to a purely resistance load equal in value to the surge impedance (Zs) of the line.
(iii) Thermal loading :
Thermal loading follows a straight line relationship with phase order. Thus if thermal loading is the criterion for circuit rating, the capacity increase is proportional to number of phases. Thus 6 phase systems will have more thermal loading which is an advantage.
(iv) Current unbalance :
Six phase circuits & currents are better balanced (due to less angle i.e., 60o between any two phases) compared with the 3 phase circuits with the same conductor configuration. Sometimes transposition may be unnecessary [Transposition is normally done to reduce the unbalance between the voltages and currents independently].
(v) Electric fields :
The electric field which is maximum at the conductor surface decreases with phase order whereas the ground electric field will be more since line voltage is equal to the phase voltage for enhanced power. For the same power transfer since the line voltage can be reduced the electric field between the two conductors will be less.
(vi) Radio & Audible noise :
It is proved that 6 phase transmission system will have less radio & audible noise compared to the 3 phase double circuit line.
(vii) Switching surges :
The phase to ground switching surges are approximately same when 3 phase double circuit line is converted into 6 phase line for the same power transfer. But rate of rise of recovery voltage across the breaker terminals during normal opening is found to be less for 6 phase due to less line voltage for the same power transfer.
(viii) Lightning performance :
Owing to the reduced conductor spacing & reduced dimensions of 6 phase line the number of lightning strokes to the line are reduced by 20% when compared with the 3 phase counter parts for the same power transfer.
(ix) Terminal Insulation Level :
Terminal insulation level will be slightly higher for six phase systems for enhanced power supply.
(x) Reliability aspect :
6 phase line is said to have more reliability due to the following reasons.
1. More power demand can be met at the load point since power transfer capability is more in 6 phase systems.
2. Voltage regulation and efficiency are better due to less inductive reactance of the 6 phase line for the same power transfer.
3. It is proved that six phase line is more stable for both symmetrical & unsymmetrical faults.
CONSTRUCTION OF A
6-phase TRANSMISSION LINE:
6-phase transmission lines can provide the same power capacity with a lower phase to phase voltage and smaller, more compact towers as compared to standard double circuit 3-phase line. The geometry of 6-phase compact towers may also aid in the reduction of magnetic fields.
Six-phase transmission is accomplished by installing transformers at either end of a double circuit transmission line which shifts one of the three-phase line. This results in a 6-phase line with each phase 60 degrees apart. Also the phase to ground voltage equals the phase to phase voltage (see above Figure)
Operating in this manner offers the following advantages
Ø For the same power flow capability, the phase to phase voltage is reduced by √3 times, allowing the towers to be built smaller and more compact.
Ø For the same phase to phase voltage the power flow capability could be increased by 73%.
Ø Magnetic fields can be reduced.
Ø Harmonics may occur
Ø No technical knowledge over the operation of six phase transmission system
Six phase transmission technology provides a technique for reducing physical space requirements for transmission lines while providing a means for increasing power transfer capacity of a new or existing transmission corridor. The cost for constructing a new six-phase line versus a three phase line of the same voltage level, is not excessive, particularly if physical constraints exist. In India, the 6-phase transmission system is not existing because there is no technical knowledge over the operation of the 6-phase systems.