PROTECTION OF SIX PHASE TRANSMISSION SYSTEM
( Abstract of the Ph.D Thesis of Dr.G Chandra Sekhar under the Supervision of Dr.P.S.Subramanyam )
ABSTRACT
Multi phase system plays an important role in the transfer of Bulk power and Six Phase System is the most feasible solution as it can increase the power transfer capability by 1.732 times within the existing corridor and right-of-way and with the same phase voltage. Six phase transmission has other benefits also like greater reliability, higher efficiency, better regulation and transfer of Power with a few lines out also without affecting the Stability of the System etc.
Conversion of the existing Three Phase System completely into a Six Phase System is costly and cumbersome. There is a possibility of converting existing double circuit three phase lines into six phase lines with all the benefits of the Six Phase System, for Bulk Power Transmission.Hence an integrated Three Phase–Six Phase System can be formed with the existing three phase system without much difficulty, instead of going for a complete Six Phase system. Six Phase lines can be considered as alternative to HVDC lines. HVDC requires additional equipment like convertors and invertors with a lot of control circuitry, which can be avoided in the case of Six Phase System. The same Three Phase–Six Phase Transformers used in HVDC can be used in the case of Six Phase Transmission Line also.Six Phase System has some limitations in fault analysis and designing the protection scheme. A Good Protection Scheme is necessary for implementation of Integrated Three Phase-Six Phase Systems.
In the present work considerable amount of research had been done towards the Protection of Six Phase Transmission line using a New Logic and Simulation Based Method of comparing the Current or Voltage Wave Forms with the corresponding healthy wave Forms, a few cycles just before the occurrence of the fault. In earlier studies of fault analysis on six phase system it had been observed that in certain cases of six phase faults involving Phases of the opposing two Three Phase Sub Systems and ground, the zero sequence currents were found to be absent in spite of they being ground faults. Hence it may be noted that if zero sequence currents are used to relay ground faults the protection scheme fails. Except for symmetrical faults all other faults do possess negative sequence currents, detection of which can be used for protection against Unsymmetrical Faults. Bearing this in mind, interest had been taken on the development of Negative Sequence Detection for Protection Scheme against Unsymmetrical Faults.
The Highlight of the work is that the present Current or Voltage wave forms are being compared with the previous history of the corresponding wave forms during a few cycles continuously just before the occurrence of a fault so that when fault occurs the faulted current or voltage wave form is compared with the corresponding previous healthy wave form.
In view of the nature of the envisaged protection system it was possible to develop a single protective relay to cater all the needs of protection including backup protection, instead of going for different types of relays for different requirements of protection. Thus a logic and simulation based Unified protective relay can be developed that can be useful for an Integrated Three Phase- Six Phase System.
*********