Final Thoughts

When we talk about transmission lines with complex configuration we typically think of double circuit lines, three-terminal or hybrid lines, line-transformer or other configurations. More complex configurations are the result of mixing of the above.

 

One such example is NYSEG's six-phase line which was energized in July 1992 and was operated for three years. One of the main advantages of such High Phase Order (HPO) technology is that the six-phase line can carry up to 73 % more electric power than a conventional double circuit three-phase line on the same transmission right of- way.
An existing 115 kV double-circuit transmission line between Goudey and Oakdale substations was electrically reconfigured into a single circuit, six-phase transmission line by connecting it to the 115 kV system through two 161 kV/115 kV transformers - one with grounded wye/delta connection and the other with inverted grounded wye/delta connection at each end of the line.

The phase-to-phase voltage was 93 kV between adjacent phases (60 degrees apart), 161 kV between phases (120 degrees apart), and 186 kV between opposite phases (180 degrees apart).
This voltage was chosen as the maximum voltage at which the existing conductors could be operated without going into corona. Breakers were located on the six-phase line side of the transformers and had single pole control capability. Different aspects of the HPO transmission concept such as operation with one, two or three phases out of service and the application of existing relay systems designed for the protection of conventional three-phase transmission lines were demonstrated.

The protection system at each end of the six-phase line consisted of several separate three-phase multi-functional IEDs tied together into a six-phase protection system by a common six-phase programmable IED. The six phases of the line were divided in two three-phase groups, one of them (ace) connected to the wye/delta transformer, and the other (bdf) connected to the inverted wye/delta transformer.

With today's advanced IEC 61850 based technology, protecting such circuits is much easier and can be used as an argument to convert existing double circuit lines to six-phase lines in areas requiring increase in power transfer capabilities due to the penetration of DERs.

Relion advanced protection & control.
BeijingSifang June 2016