ISBN 978-0-620-44584-9
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
RE-EVALUATION OF HIGH PHASE ORDER TRANSMISSION LINES
1
A. A. Beutel1*, A. C. Britten1 and T. Motloung1
Eskom Research & Innovation Centre, Sustainability & Innovation Department
Private Bag X40175, Cleveland, 2022, South Africa
*Email: BeutelAA@eskom.co.za
Abstract: The feasibility of high phase order (HPO) transmission in a South African context is reevaluated. HPO transmission involves transporting power using alternating current (AC), but with
more than the usual three phases. It is a possible alternative to conventional AC and high voltage
direct current (HVDC) transmission in this application. Much work has been performed by
different authors in the past, and such work is extensively referenced in this paper. Despite the
generally favourable light that is shone on HPO technology by the primary reference, a 1998
dissertation, HPO transmission has not been implemented in any part of the world. It was hence
felt that re-evaluating this technology would be worthwhile, especially since many hundreds of
kilometres of new transmission lines are planned for South Africa in the next few years. Several
topics are covered in this paper, including advantages and disadvantages of HPO transmission,
surge impedance loading, ground-level electric fields, corona-related effects, servitude size and
economics. From this the suitability of HPO transmission for different scenarios is evaluated and
conclusions regarding the feasibility of HPO transmission are drawn.
1.
transmission are presented, followed by a listing of
advantages and disadvantages of HPO. Next, the
performance of HPO transmission is evaluated in more
detail using criteria such as surge impedance loading
(SIL), ground-level electric fields, corona-related
effects, tower and servitude size and economics. Based
on this, the suitability of HPO transmission for
different scenarios is evaluated and conclusions
regarding the feasibility of HPO transmission are
drawn, taking technical, economic and social factors
into account.
INTRODUCTION
Since about 1994 South Africa has been on a drive to
electrify the homes of all South Africans. The
consequential additional load requirements, together
with an expanding local economy, have resulted in
additional generation capacity being needed. An
example of this is the Medupi Power Station which is
under construction in the Waterberg area of the
country, about 500 km North-West of the largest
concentration of load (in and around the city of
Johannesburg). Present estimates indicate that upwards
of 10 GW need to be transported from Medupi,
resulting in multiple transmission lines and hence large
cost. Traditional options for these transmission lines
include single-circuit high voltage alternating current
(HVAC) lines operating at voltages of 400 kV or 765
kV. Double-circuit lines of the same voltages could
also be considered. High voltage direct current
(HVDC) lines operating at 600 kV or 800 kV may also
be possible.
2.
HPO FUNDAMENTALS
Conventional HVAC power transmission employs
three phases. The rationale behind higher phase orders
is that the same servitude is filled with more
conductors, resulting in a greater power transferred in
that servitude [1]. The supply voltage used to describe
3-phase systems is usually the phase-to-phase voltage,
which is √3 times the phase-to-earth voltage. Higher
phase orders for transmission use more than three
phases, usually six or twelve. In a 6-phase system the
phase-to-phase voltage reduces to become equal to the
phase-to-earth voltage, and in a 12-phase system it
reduces further to approximately half the phase-toearth voltage [1]. Figure 1 illustrates this using phasors.
One way of arranging the conductors is in a circular
array, as shown in Figure 2. Note that the phase-toearth voltage is used for comparison between systems
with different phase orders, unless otherwise stated.
All of the above options have been implemented in
various parts of the world and the technologies (except
for 800 kV HVDC) are proven. However, changing
conditions have caused budgets to be more tightly
constrained than before and that the land area
(servitudes) taken up by transmission lines are reduced.
Additionally, social pressures, such as the appearance
of transmission lines, do exist and need to be taken into
account. The above, together with the large costs of
any transmission line, mean that the feasibility of all
possibilities should be studied.
For HPO, 3-phase AC is converted to HPO at the
sending end and the HPO is converted back again to 3phase AC at the load end [1]. Conversion to six phases
can be achieved by converting three phases to three
phases in phase with the original phases (e.g. phases A,
C and E in Figure 1) and another three phases 180° out
of phase with the original phases (e.g. phases B, D and
The purpose of this paper is to assess an alternative
technology to three-phase alternating current (AC) and
HVDC. This alternative is called high phase order
(HPO) and involves AC transmission using more than
the usual three phases. First, the fundamentals of HPO
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ISBN 978-0-620-44584-9
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
F in Figure 1). This can be achieved using specially
built transformers or by using two 3-phase
transformers [1].
3.
ADVANTAGES AND DISADVANTAGES
Bortnik [1] mentions the following as advantages of
HPO transmission over the 3-phase case:
• Reduced phase-to-phase spacing due to
reduced phase-to-phase voltage.
• More conductors in a servitude, leading to
greater power density. As an example from
elsewhere, it is reported that the surge
impedance loading (SIL) is 3275 MW (3phase), 4494 MW (6-phase) and 5485 MW
(12-phase) for systems having the same phaseto-earth voltage (462 kV), number of
conductors, right of way, air space and
thermal ratings [3].
• Line compaction, resulting in smaller poles
and narrower servitudes.
• Minimal current unbalance.
• Ability to perform single-pole switching.
• Reduced magnetic field rate-of-change with
lateral distance away from the line (it is this
aspect of the magnetic field, rather than the
magnitude, that is considered to be the most
damaging as it induces a voltage – and hence
current – into nearby objects).
• Minimization of corona-related effects such as
radio interference and audible noise.
• Reduced ground-level electric field levels.
• Compatibility with existing 3-phase systems.
• Increased system stability.
• Line
transposition
and
series
line
compensation could be discarded for 6-phase.
• Possibly more visually attractive.
Additionally, the following economic advantages over
3-phase transmission are mentioned [1]:
• Lower power losses.
• Lower line costs.
Bortnik [1] mentions the following disadvantages
compared to the 3-phase case:
• Higher cost of terminal equipment.
• Greater incidence of insulation flashover if a
shielding failure occurs.
• No significant improvement in overall
lightning and switching surge performance.
Figure 1: Phasor diagrams of voltages in 3-, 6- and 12phase systems (top and centre from [1], bottom adapted
from [2]).
4.
EVALUATION OF HPO TRANSMISSION
PERFORMANCE
HPO has not been implemented anywhere in the world
and the only practical information available is from test
results obtained on two test lines built in the USA.
4.1.
The first test line [4]
The line was 0.366 km long, operated at a phase-toearth voltage of 80 kV and had a circular conductor
arrangement with 1 m spacing between conductors. It
was built in the late 1970s and early 1980s. Figure 3
shows the 6-phase towers used.
Figure 2: Comparison of HPO and conventional tower
structures [2].
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ISBN 978-0-620-44584-9
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
predicted ground-level magnetic field for the converted
line was much lower than for the unconverted case and
it was found that live-line maintenance is possible on
HPO structures. Bortnik [1] reports two contrasting
opinions on the results gained from this study. Stewart
concluded that HPO technology is ready for
application and is economically justifiable for uprating
double-circuit 3-phase lines and for constrained
servitudes. However, if space is not an issue, HPO
transmission is not feasible. However, Guyker doubted
the overall value of this paticular study due to the
relatively short line length and that the simplified
transformer could not show value over simple 3-phase
uprating of the line.
4.3.
Computational studies were conducted on a 462 kV
phase-to-earth line with 3, 6 and 12 phase
configurations [2]. A constant total number of the
same conductor type arranged in a circular array was
used for this study. The thermal rating (17,900 MW)
was found to remain unchanged irrespective of the
configuration; however, the SIL increased with the
number of phases. For longer lines, a higher SIL results
in a greater stability margin. Calculated SIL values
from other references for a phase-to-earth voltage of
462 kV (phase-to-phase voltage of 800 kV for 3-phase)
are plotted in Figure 4. This shows a significant
increase in SIL with phase order. HPO power transfer
performance may be explained in other ways:
• For constant current and power transfer, the
phase-to-earth voltage halves for 6-phase
transmission and is reduced by a factor of four
for 12-phase transmission.
• For constant voltage and power transfer, the
current required halves for 6-phase
transmission and reduces by a factor of four
for 12-phase transmission.
Figure 3: Towers used on the first test line [4].
Performance advantages of HPO for this line over the
3-phase case were: a saving in space and servitude, a
reduction in fair weather radio noise of about 15 dB for
the 6-phase option, similar behaviour for wet conductor
audible noise and a reduction of approximately 83% in
corona power loss for the 6-phase case. Disadvantages
of HPO compared to 3-phase were found to be a larger
ground-level electric field (depending on which 3phase case HPO is being compared to), slightly higher
magnetic field and marginally worse lightning
performance.
Stewart et al [2] report that for a circular array of
conductors the SIL for 6 phases is about 1.8 times the
SIL for 3 phases; for 12 phases the SIL is about 2.5
times the 3-phase case. As a comparison, Eskom
calculations show that the SIL for a double-circuit 3phase line (765 kV) increases by a factor of
approximately 2.1 to 2.2 from a single-circuit line of
the same voltage. The above discussion shows that
HPO transmission can result in a significant
improvement in SIL.
Other outcomes were that it was shown that it is
possible to design, analyze, implement, put into service
and operate HPO lines; the line was visually attractive;
the hexagon shape was found to be the best
configuration and the compaction level is largely
determined by switching surges between the phases
that are separated by 60° from one another.
6,000
Ref [8]
5,000
SIL (MW)
4.2.
Surge impedance loading (SIL)
The first utility application – GoudeyOakdale line
This project was undertaken in 1988, and resulted in a
6-phase, 2.4 km long test line being put into service. It
operated at 93 kV phase-to-earth, and was converted
from a 115 kV double-circuit 3-phase line. Several
references report on this work, among them [5-7]. The
line worked as expected and the conversion to HPO
resulted in a 38% increase in power transfer. The
4,000
Ref [3]
3,000
2,000
1,000
0
0
3
6
9
12
Phase order
Figure 4: Surge impedance loading.
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4.4.
Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
Ground-level electric fields
Numerous studies have been undertaken in the
determination of ground-level electric fields using both
analytical techniques and field tests. Conflicting results
on the intensity of ground-level electric fields, and on
whether HPO results in reduction of these fields, are
contained in some of the surveyed papers. It is
suspected that this may be due to differing design
conditions. A conclusion that can be reached from this
is that care should be taken when comparing groundlevel electric fields of different cases, and that each
case should be treated on its own merits. Some figures
from the literature are:
• Stewart et al [2]: for the same phase-to-ground
voltage, ground-level electric fields are
significantly higher for 6-phase than for 3phase transmission, and higher for 12-phase
than 6-phase transmission. For 289 kV phaseto-earth voltage, the maximum ground-level
field was found to be less than 5 kV/m for 3phase, between 5 and 10 kV/m for 6-phase
and greater than 10 kV/m for 12-phase.
• Grant et al [8]: approximate maximum
ground-level fields were determined as 7.7
kV/m for 462 kV 6-phase and 12-phase.
• Stewart et al [9]: the observed ground level
electric field results at the Goudey-Oakdale
line were 1.4 kV/m, which is below the 1.6
kV/m limit set by New York State standards.
• Stewart et al [4]: approximate maximum
ground-level fields were found to be 7.3 kV/m
for 500 kV 3-phase, 6.1 kV/m for 199 kV 12phase and 3.3 kV/m for 133 kV 12-phase
transmission.
• Metwally [10]: maximum electric fields at
1 m above ground level are up to 12 kV/m for
619 kV 6-phase, 6 to 9 kV/m for 442 kV 6phase and 5 to 9 kV/m for double-circuit 765
kV 3-phase transmission. It is shown that
under-strung earth wires reduce the groundlevel fields.
•
•
•
clearances are reduced according to reduced
voltage for this case, the reduction is
approximately 6.2 dB.
Stewart et al [4]: maximum fair weather radio
noise reduces from approximately 47 dBA to
about 34 dBA when converting from doublecircuit 3-phase to 6-phase on the same
structure (80 kV phase-to-earth voltage).
Grant et al [8]: the maximum fair weather
value for 462 kV 6-phase is 51.8 dB.
Converting this to 12-phase with the same
phase-to-earth voltage but a larger structure, a
reduction of about 4 dB was found.
Metwally [10]: approximate maximum values
of 57 dB for 6-phase 619 kV and 43 dB for 6phase 442 kV were found.
Audible noise:
• Stewart et al [2]: for 80 kV, maximum heavy
rain audible noise is reduced from
approximately 48 dBA to about 43 dBA when
converting from double-circuit 3-phase to 6phase with reduced clearances due to reduced
voltages.
• Stewart et al [4]: for the same case as reported
under “Radio noise” above, a reduction from
57 dBA to 46 dBA was found.
• Grant et al [8]: the maximum wet conductor
value for 462 kV 6-phase is 56.3 dBA.
Converting this to 12-phase with the same
phase-to-earth voltage but a larger structure,
an increase of about 1 dB was found.
• Metwally [10]: approximate maximum values
of 55 dBA for 6-phase 619 kV and 41 dBA
for 6-phase 442 kV were found.
The reduction in radio noise levels mentioned above
make sense as the HPO conductor surface gradients are
lower than for the 3-phase case. Also, while the
reduced HPO audible noise levels are still being high,
the reduction is significant.
4.6.
Details of the above examples can be found in the
relevant references. These details are important to note
when comparing different cases and in judging the
feasibility of HPO transmission. Also, many of the
figures are calculated or simulated, rather than
measured.
Tower and servitude size
As with ground-level electric fields, corona-related
effects need to be determined for each case. The
examples below give some experience from the
literature. Once again, the details of each case may be
found in the respective references.
There are several reasons why tower size needs to be
minimised as far as possible while maintaining
required clearances and meeting other constraints:
• Structures that require more material are more
expensive.
• Taller structures are more susceptible to wind
damage.
• Towers that require more ground area increase
the cost of the land required and of vegetation
clearing costs.
• Larger structures can be less visually
attractive.
Radio noise:
• Stewart et al [2]: a reduction of approximately
8.6 dB in the maximum value is reported for a
conversion from double-circuit 3-phase to 6phase on the same structure (80 kV). If
Figure 5 shows a comparison between a 3-phase 765
kV phase-to-phase tower used by Eskom (above) and a
6-phase tower for 462 kV phase-to-earth (below [8]).
Typical SIL values are approximately 2,600 MW for
the 3-phase tower and 4,800 MW for the 6-phase
4.5.
Corona-related effects
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Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
ISBN 978-0-620-44584-9
tower. Figure 5 shows that the 6-phase structure is
significantly smaller and would therefore result in a
smaller servitude and hence lower land costs.
4.7.
Six-phase transmission is therefore not economically
viable for this case. However, no terminal equipment
was needed for the 3-phase case as both ends of the
line were already at 400 kV phase-to-phase. A
hypothetical line where terminal equipment would be
needed for both 3-phase and 6-phase cases was next
costed. The following breakeven distances, above
which the 6-phase line would be cheaper to build, were
determined:
• Land costs excluded: ± 226 km.
• Land costs included: ± 214 km.
Economics
The most relevant study for South Africa is a case
study performed by Bortnik [1]. This was performed
by choosing a 3-phase base case, designing a 6-phase
alternative with similar SIL, costing the two cases and
comparing their costs and calculating a breakeven
distance. The 3-phase base case chosen was the
(existing) 400 kV single-circuit Camden-Duvha line.
This line is 100 km long, has a servitude of 55 m and
16.4 m between outside conductors, uses a cross-rope
suspension tower and has an SIL of about 636 MVA.
The 6-phase alternative concept was designed for 173.2
kV, a 35 m servitude, a circular phase geometry of 3 m
radius and hence a 6 m separation between outer
conductors. This concept therefore results in significant
compaction. The results of costing these two
alternatives were (using 1998 currency values):
• 3-phase line cost: R579,000 per km.
• 6-phase line cost using light conductor:
R468,000 per km.
• Total 3-phase line cost: R57.9 million.
• Total 6-phase line cost: R108.5 million
(including terminal equipment)
Other cost studies have also been published in the
literature. Two separate sources have studied the
upgrade of existing double-circuit 3-phase lines to 6phase operation [11,12]. Bortnik [1] reports that the
authors of [11] found that HPO would be more
economical than 3-phase transmission for lines longer
than 22-42 km (terminal equipment costs were
included). However, a different conclusion is reported
in [12], where it was found that 6-phase transmission
would probably be more expensive for the particular
case studied in that reference. Bortnik [1] reports that
Stewart et al [13] also looked at breakeven distances
for extra high voltage (EHV) and ultra high voltage
(UHV) cases. Load levels were up to 6 GW. Six and
twelve phase cases with phase-to-earth voltages of 317
kV and 462 kV were compared to the 1,200 kV 3phase case. It was found that of the ten scenarios
studied, eight had breakeven distances of 56.3 km or
less and two would never see HPO being the cheaper
option.
± 40 m
4.8.
Discussion
The information presented above shows that HPO
transmission is a viable technology and that its use
could result in cheaper transmission lines. It is also
clear, however, that this does not apply to every case,
and hence each case must be treated separately. It is
especially important to note that several criticisms are
made of the US cost studies and test lines, which once
again emphasises that every case must be treated
separately. This is supported by the fact that there are
several situations where HPO was shown to the
cheaper than 3-phase and other situations where the
opposite was true.
± 50 m
± 23 m
It is also important to make realistic comparisons when
comparing higher phase orders to the 3-phase case, so
that comparisons made are valid. Three-phase uprating
was mentioned as a possible alternative to HPO – this
should be considered for cases where existing lines are
to be upgraded, rather than built new.
± 58 m
Although HPO has been demonstrated to have several
attractive properties, this technology has not been
applied. One possible reason for this could be
mechanical problems with conductors clashing under
short circuit conditions. Finally, there are some aspects
of HPO transmission where further information is
required, such as more information on SIL achievable,
ground-level electric fields and corona-related effects.
Figure 5: Comparison between 3-phase and 6-phase
self-supporting structures.
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Proceedings of the 16th International Symposium on High Voltage Engineering
c 2009 SAIEE, Innes House, Johannesburg
Copyright °
7.
SCENARIOS
[1] J. Bortnik, “Transmission line compaction using
high phase order transmission”, Dissertation for
Master of Science in Engineering, University of
the Witwatersrand, Johannesburg, South Africa,
1998.
[2] J. R. Stewart and D. D. Wilson, “High phase order
transmission – a feasibility analysis, Part I –
Steady state considerations”, IEEE Transactions on
Power Apparatus and Systems, Vol. PAS-97, No.
6, Nov/Dec 1978, pp. 2300-2307.
[3] S. N. Tiwari and A. S. Bin Saroor, "An
investigation into loadability characteristics of
EHV high phase order transmission lines", IEEE
Transactions on Power Systems, Aug. 1995,
Volume 10, Issue 3, pages 1264 – 1270.
[4] J. R. Stewart and I. S. Grant, “High phase order –
ready for application”, IEEE Transactions on
Power Apparatus and Systems, June 1982, Volume
PAS-101, Issue 6, pages 1757 - 1767.
[5] M. T. Brown, R. V. Rebbapragada, T. F. Dorazio,
J. R. Stewart, “Utility system demonstration of sixphase power transmission”, Proceedings of the
1991 IEEE Power
Engineering Society
Transmission and Distribution Conference, pp.
983-989.
[6] T.F. Dorazio, J. R. Stewart, D. D. Wilson, “Six
phase power transmission: first utility application”,
Cigre 600-01, Symposium Leningrad, 1991.
[7] R. V. Rebbapragada, M. T. Brown, T. F. Dorazio,
J. R. Stewart, “Design modifications and layout of
utility substations magnetic field reduction using
high phase order lines”, IEEE/PES Transmission
and Distribution Winter Meeting, New York, USA,
January 1992.
[8] I. S. Grant and J. R. Stewart, "Mechanical And
Electrical Characteristics Of EHV High Phase
Order Overhead Transmission", IEEE Transactions
on Power Apparatus and Systems, Nov. 1984,
Volume PAS-103, Issue 11, pages 3379 - 3385.
[9] J. R. Stewart, L. J. Oppel, G. C. Thomann, T. F.
Dorazio, M. T Brown, "Insulation coordination,
environmental and system analysis of existing
double circuit line reconfigured to six-phase
operation", IEEE Transactions on Power Delivery,
July 1992, Volume 7, Issue 3, pages 1628 - 1633.
[10] I. A. Metwally, “Electrostatic and environmental
analyses of high phase order transmission lines”,
Journal of Electric Power Systems Research
(EPSR), No. 61, 2002, pp. 149 - 159.
[11] E. Kallaur and J. R. Stewart, “Uprating without reconductoring – the potential of six phase”,
Canadian
Communications
and
Energy
Conference, IEEE 82 CH 1825-9, 1982, pg 120.
[12] W. C. Guyker and D. F. Shankle, “138-kV sixphase uprating of a 138-kV double circuit line”,
IEEE Transactions on Power Apparatus and
Systems, Vol. PAS-104, No. 9, Sept 1985.
[13] J. R. Stewart, S. Zelingher, G. I. Stilman, “HPO
line practical for limited R/W”, Transmission and
Distribution, Oct 1985, pp. 32-36.
Technical, economic and social factors should be taken
into account when evaluating different designs for a
new transmission line. There are cases where HPO
technology warrants further investigation, and other
cases where it should be discarded without significant
investigation. The following are scenarios where HPO
could be investigated further:
• Servitude width and/or tower size are major
constraints, e.g. the land is expensive or
environmentally sensitive, or there is
extensive farming or ecotourism activity in
the area.
• Sufficient time and resources for research and
development are available.
• Social pressures make this technology more
easily justifiable, e.g. the lines are found to
have lower visual impact.
• The proposed transmission line is of
significant length, i.e. longer than the
breakeven length, if such a length exists.
• The proposed line has very few or no “T-offs”
since the cost of terminal equipment for HPO
is relatively expensive [1].
The following are scenarios where HPO should not be
investigated further:
• Servitude width and/or tower size are not
major constraints, e.g. land is relatively cheap,
readily available and in an area not suitable
for farming or other development.
• Fast turn-around time is required in
transmission line development, making the
use of a new technology difficult.
• Social pressures make the use of a new
technology difficult.
• The line is shorter than the breakeven length
or no breakeven length exists for the particular
situation.
• The line has several “T-offs”, making HPO
not cost-effective [1].
The above lists show that a preliminary comparative
costing should be performed for each case where it is
thought that HPO could be viable, using as much
simplification as is reasonable and possible. Examples
of such studies have been referenced. However, HPO
could be discarded before this stage if time, resource or
social pressures dictate this.
6.
REFERENCES
CONCLUSIONS
This paper has shown that HPO is a viable technology
for electric power transmission and has several
advantages
over
conventional
3-phase
AC
transmission. However, each case should be considered
on its own merits. There is little operational experience
available on HPO. Therefore, implementation of HPO,
although having been shown to be possible, would
require significant research and development.
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