IEEE Power & Energy Magazine - May/June 2019 - 24

AC-to-dc conversion allows for a more efficient use of the existing
assets and rights-of-way and, under adequate work procedures,
can be performed with relatively short outages.

their performance for air pollution under dc voltages is
inferior compared to their performance under ac volt-
ages. To minimize outage times, insulator replacement
can be done before the conversion to dc, while the ac
line remains in operation.
✔ Locating the converter stations as close as possible to
the existing ac yards: With the widespread use of volt-
age source converter (Vsc) high-voltage dc (hVdc)
technology, it is easier to find a location closer to the
existing ac yard given that Vsc stations have a re-
duced footprint compared to their line commutated
converter (lcc) counterparts. however, this is not
always straightforward because standard half bridge
Vsc configurations generally require that a dc fault be
tripped on the ac side, thereby yielding long recovery
times, which are not necessarily compatible with the ac

ac Phase
a

ac Phase
b

ac Phase
c

network capabilities. several experimental dc breakers
are under development, but lcc-hVdc is still widely
considered for ac-to-dc conversion applications.

Engineering Considerations
although the conversion of an existing ac line to dc is an
attractive uprating solution, several technical aspects must
be evaluated to verify the feasibility of the conversion as
well as to select the most suitable implementation.

Pole Configuration
one of the challenges of ac-to-dc conversion of transmission
lines is that ac lines have three fully rated phases, which can
be composed of a bundle of conductors, whereas dc lines are
designed with two or three conductor configurations and, in
some cases, use a ground electrode for a neutral return path.
it follows that the mapping of the ac conductors to a dc sys-
tem must be studied on a case-by-case basis to determine the
optimal dc configuration, while avoiding modifications to
the existing structures as much as possible.
Single-Circuit ac Line to HVdc Monopole

dc
+ Pole
Conductor

dc
+ Pole
Conductor

dc
+ Pole
Conductor

figure 1. The converting of an ac single-circuit line to an
HVdc monopole.

ac Phase
a

ac Phase
b

ac Phase
c

although an hVdc monopole could effectively use the three
existing phases in a single-circuit line, the return current
would have to flow into a grounding electrode and through
the earth. Figure 1 illustrates this concept of converting the a,
b, and c phases shown at the top of the diagram to a common
positive dc voltage shown below the red arrow. implementa-
tion of the ground return involves extensive site testing and
permitting, and the continuous operation of a ground elec-
trode may not be allowed due to potential adverse effects in
the surrounding area and infrastructure, such as corrosion in
nearby pipelines. This defeats the initial purpose of increas-
ing asset performance with minimal environmental impact.
For this reason, in general, monopole configurations are not
suitable for ac-to-dc transmission line conversions.
Single-Circuit ac Line to Floating HVdc Monopole

dc + Pole

0
Emergency Return

dc - Pole

figure 2. The converting of an ac single-circuit line to
either a floating monopole or a bipole.
24

ieee power & energy magazine

another alternative is the so-called floating monopole, which
would be set up as shown in Figure 2. in this case, each con-
verter station would consist of a single bridge, with a ground
point at the center of the converter. in the event of a conduc-
tor fault, the third circuit may be used to keep the system in
operation. This is not, however, the optimal use of all avail-
able conductors. also, there is no redundancy in case of loss
of one of the converters, as opposed to a bipole configuration.
may/june 2019



IEEE Power & Energy Magazine - May/June 2019

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2019

Contents
IEEE Power & Energy Magazine - May/June 2019 - Cover1
IEEE Power & Energy Magazine - May/June 2019 - Cover2
IEEE Power & Energy Magazine - May/June 2019 - Contents
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