IEEE Power & Energy Magazine - May/June 2019 - 25

Given that the conversion process involves work on an
existing operational ac transmission line, it must focus on
solutions that will not involve long line outage durations.

Single-Circuit ac line to HVdc Bipole

Bipole configurations can be implemented from a single-
circuit line in different ways. For example, the conductor
arrangement shown in Figure 2 can be used. This setup will
have the added advantage of allowing monopole operation in
the event that one of the poles is lost. however, the downside
of this configuration remains that one set of bundled con-
ductors is underused.
it is, therefore, necessary to rearrange the conductors to
use them more effectively with a bipole configuration. in the
configuration shown in Figure 3, all of the conductors are
effectively used, but conductor or structural modifications
would most likely be needed given the change in bundle
size. Furthermore, the addition of a ground return would
be required to have the capability of unbalanced operation.
alternatively, the shield wires could be uprated to carry the
ground current, but this approach would require significant
transmission tower modifications.
The option shown in Figure 4 consists of having one
phase carry twice the current of the other two. To achieve
this, it would be necessary to add conductors or reconductor
this phase. This setup may also require a return path for the
unbalanced current, whether through the earth or an uprated
shield wire. The reconfiguration of conductors needs to be
evaluated depending on the tower type and its design char-
acteristics and, in some cases, may not be possible. This
conductor rearrangement also increases the required out-
age time for the line's ac-to-dc conversion. changes in the
tower structure, the cost of the additional conductors, and
the effective cost of the outage time may not make this solu-
tion an economically viable option.

ac Phase
a

ac Phase
b

dc + Pole

ac Phase
c

dc - Pole

figure 3. The converting of a single-circuit ac line to an
HVdc bipole.

ac Phase
a

dc
+ Pole
Conductor 1

ac Phase
b

ac Phase
c

dc - Pole

dc
+ Pole
Conductor 2

figure 4. A second option for converting a single-circuit
ac line to an HVdc bipole.

Single-Circuit ac line to HVdc Tripole

another alternative worth considering is the tripole con-
figuration, which consists of a bipole and a monopole, as
shown in Figure 5. The monopole would require antipar-
allel two-bridge converters capable of operation in both
polarities. at moderate power transfers, one pole carries
the positive current while the other two share the negative
current. as the power levels increase, the bidirectional
pole will alternate between sharing the current with
pole 1 or pole 2, depending on the system needs. This
arrangement ensures that the thermal capacity of all three
poles is fully used. Furthermore, this arrangement does
not require any conductor rearrangement and/or any addi-
tional return path.
may/june 2019

ac Phase
a

ac Phase
b

ac Phase
c

dc + Pole

dc ± Pole

dc - Pole

figure 5. The converting of a single-circuit ac line to an
HVdc tripole.
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IEEE Power & Energy Magazine - May/June 2019

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