IEEE Power & Energy Magazine - May/June 2019 - 27

Examining the available conversion options
illustrates the need for evaluating the configuration
selection on a case-by-case basis.

systems are lower than those in ac systems. as such, the air
clearances for dc are determined by examining the lightning
performance requirements. additionally, the air clearances
for dc systems require a higher margin for high altitudes.
at 2,000 m above sea level, the air clearances for an hVdc
system need to be increased by 25%, as compared to 15%
for a high-voltage ac system.
dc systems are also more susceptible to pollution because
the dc electric field surrounding the insulation attracts
more pollution particles. This issue can be overcome by
the use of composite insulators, which have better pollution
performance than glass or ceramic insulators for a given
string length.

Corona Effect
The losses attributed to the corona effect are similar for both
ac and dc systems in fair weather conditions. however, rain
and frost cause a large increase in corona losses for ac sys-
tems, whereas the corona losses for dc systems are not as
impacted by these factors. additionally, dc systems in monop-
olar operation can lead to a surface gradient on the earth wires
that exceeds their corona inception gradient. This can result
in high levels of corona between the pole conductor and earth
wire, which will have a detrimental impact on the corona per-
formance of the dc line. however, this effect does not occur in
bipolar operation.

Audible Noise
dc lines produce less audible noise compared to ac lines,
mainly because the negative pole's contribution is practi-
cally negligible, whereas for the ac lines, the contribution
from all three phases must be considered. This means that a
lower conductor bundle size can be used for a dc line to meet
the same noise requirements when compared to an ac line.
audible noise will be at its highest under dry conditions for
dc lines, and it will be at its highest under rain conditions
for ac lines due to the effect of space charges. dc corona
discharge generates space charges that reduce the electric
field intensity between raindrops and the conductor surface,
so rain causes a slight reduction in audible noise produced
by dc lines. ac corona discharge does not cause constant
space charges around the conductor, and the presence of
raindrops in the ac electric field leads to more stream dis-
charges, resulting in an increase of audible noise. This dif-
ference must be taken into account when selecting the dc
conductor bundle size.
may/june 2019

Voltage Selection
as previously discussed, the requirements for dc line insula-
tion are more stringent than for ac lines, so the length of the
existing ac insulators is a defining parameter to select the
voltage at which the dc line will operate. To illustrate this
point, this section presents typical calculations for the selec-
tion of the dc voltage when considering existing ac transmis-
sion lines operating at various voltage levels.
Table 1 shows the requirements for dc insulators from
cigre and the international electrotechnical commission
(iec). The last two columns show the selected design value
in millimeters per kilovolt and in inches per kilovolt.
Table 2 shows the selected dc voltage for each conver-
sion, which was calculated based on the existing ac insula-
tors' leakage distance and the light pollution requirements
for dc insulators. it should be noted that, if the calculations
were done for a system under heavy or extreme pollution, the
conversion to dc would not be as advantageous because the
maximum allowed dc voltage would be too low compared to
the ac voltage.

table 1. The minimum creepage distances
for different pollution levels.
Selected
Design
Value
(mm/kV)

Selected
Design
Value
(in/kV)

Zone
(Pollution)

IEC
(mm/kV)

CIGRE
B4-105
(mm/kV)

I Light

27

26-34

30

1.2

II Medium

34

32-44

38

1.5

III Heavy

43

56

56

2.2

IV Extreme

53

70

70

2.8

table 2. The dc voltage selection.
AC
Voltage
(kV)

Insulator
Length

Number of Leakage
Insulators Distance

Selected
dc Voltage
(kV)

138

3 ft, 4.2 in
(1.02 m)

Seven

10 ft, 2.6 in
(3.1 m)

100

230

6 ft, 8.5 in
(2.05 m)

14

20 ft, 5.3 in
(6.2 m)

200

345

10 ft, 6.5 in 22
(3.2 m)

32 ft, 1.4 in
(9.8 m)

320

500

13 ft, 4.9 in 28
(4.1 m)

40 ft, 10.5 in 400
(12.5 m)
ieee power & energy magazine

27



IEEE Power & Energy Magazine - May/June 2019

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Contents
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