IEEE Power & Energy Magazine - May/June 2019 - 29

that precontingency power transfers from a generation bus to
table 4. The capacity increase following
load buses would be restricted to a maximum of 880 MW
ac-to-dc line conversion.
to prevent an unacceptable overload of a nearby circuit for a
Total Maximum
given outage. The figure demonstrates how respecting this
Total Flow
System Operation
limitation results in the capacity of ac lines being underused.
Contribution (%)
Limits (MW)
a generic system-planning solution would be to add
AC-to-dc
Capacity
another ac line from bus 101 to bus 103, which could be a
Load 1
Load 2
All ac
Conversion Increase (%)
costly solution requiring additional land acquisition and
30
70
880
1,500
70
permitting. however, increased capacity could, instead, be
50
50
1,040
1,500
44
achieved by rebalancing the power flow in the existing lines. if,
70
30
786
1,500
91
for example, the line from bus 102 to 103 is converted to dc, this
would enable the operator to set the
power flow, which would result in a
70% increase in the system oper-
ating limit (up to 1,500 MW) with-
235 MW
out the need to increase the thermal
capacity of the ac transmission line.
This is illustrated in Figure 10.
229.7 kV
Table 4 quantifies the increase
in system operating limit (sol)
obtained for the ac-to-dc conver-
sion for different load distributions.
← 38% (188.2 MW)
a significant increase in sol can
be achieved without the need to
← 28% (141.4 MW)
← 38% (188.2 MW)
increase the thermal capacity of
Outaged Line
the ac transmission line.
240 MW
The preceding example dem-
← 99% (98.6 MW)
onstrates that ac-to-dc conversion
230 kV
229 kV
137.4 kV
138 kV
results in a more robust solution
when compared to an ac-only
Note:
system. other dc solutions (e.g.,
Line flows are indicated as follows: % loading (MW loading).
back-to-back converters) have
been considered for these types of figure 11. Sample system 2.
applications; however, the limiting
factor remains the thermal capac-
ity of the ac line. a phase-shifting
transformer could also be used, but
235 MW
it would not give the same level of
controllability as an hVdc line,
and it could cause stability issues
228.8 kV
because it increases the angle
between the two ac buses.
Figure 11 illustrates a second
example, where the limiting circuit
← 74% (365 MW)
would be located on the 138-kV
network, and the outage occurs at
← 100% (495 MW)
← 74% (365 MW)
a higher-voltage circuit nearby. in
Outaged Line
this scenario, if the 138-kV line is
570 MW
converted to dc, as shown in Figure
← 75 MW
12, a 50% thermal capacity increase
230 kV
and a 137% sol increase could be
138 kV
138 kV
Note:
achieved on the circuit; this once
230 kV
Line flows are indicated as follows: % loading (MW loading).
again demonstrates a scenario
where ac-to-dc conversion could be
a very advantageous solution.
figure 12. Sample system 2 following ac-to-dc line conversion.
may/june 2019

ieee power & energy magazine

29



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
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