IEEE Power & Energy Magazine - May/June 2019 - 64

figure 3. A photo of a 765-kV double-circuit line in Korea. (Photo courtesy of KEPCO.)

equipment in response to a contingency. however, this proj-
ect also utilizes an overload capability for the hVdc system
in coordination with controls of flexible alternating current
transmission system (fACts) to minimize generation rejec-
tion. Critical contingencies accelerate generators in the east,
but they remain stable when hVdc transfers west are sud-
denly ramped up, which provides a cost-effective solution for
these rare system events. ramping the hVdc up also relieves
potential thermal overloads of other transmission facilities.
the plan still requires sps actions to reconfigure the ac trans-
mission system, reject some generation, and provide reactive
power support.

Combined LCC-HVdc
and FACTS Feasibility Study
the study of this hVdc project began in 2013 with a con-
ventional feasibility study to estimate its size and termi-
nal locations. the short circuit ratio (scr) and effective scr
(escr) provide critical indices used to compare the strength
of an ac system with acceptable hVdc expansion options.
the short circuit levels and scr were examined for the most
severe transmission outages for base case conditions with
up to two generators normally out of service.
Analysis using a transient stability analysis (tsA) tool
evaluated numerous credible operating scenarios to identify
the optimal connection points for the hVdc stations. five
locations on the 765-kV transmission lines were identified
and simulated as the worst-case three-phase bus faults in
the studies. however, kepCo engineers performed tsA of
all credible contingencies throughout the system and par-
ticularly at the hVdc terminals. All results for a given plan
must be stable to fulfill the intended role of the hVdc that
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ieee power & energy magazine

supports continuous power delivery, even if there is a three-
phase fault on the double-circuit 765-kV transmission lines.
however, this lCC-hVdc project needs more than the
conventional approach normally used for planning hVdc
facilities. to meet the high degree of accuracy required for
studying the complex power system in the republic of korea,
additional modeling, tools, studies, and analyses for this spe-
cific project were performed as follows:
✔ a maximum power curve analysis with stAtCom
(mpCws) for examining the initial overloading ca-
pability of hVdc based on different system strengths
✔ an equal area criterion (eAC) analysis for estimating
the initial overloading amount of hVdc
✔ a feasibility study of a proposed tCsC with over-
load capability
✔ a feasibility study of a stAtCom to support the hVdc
✔ a tsA using an advanced hVdc model
✔ an analysis to determine the hVdc ramping rates for
automatic operation and scheduling changes
✔ an electromagnetic transient (emt) simulation analy-
sis, including detailed models of the hVdc and fACts.
Analyses based on the eAC with hVdc and mpCws turn
out to be useful for analyzing the initial required overload-
ing capability of the hVdc before refining the amounts using
tsA analysis. Both analyses are different from conventional
approaches because they can provide initial parameters for the
detailed study. the eAC analysis with hVdc can provide an
initial overloading amount for the hVdc, which makes it pos-
sible to manage the deceleration of generators near the rec-
tifier side of the hVdc. this analysis can estimate the level
of overloading amount that secures enough stability margin
so that the number of generators required to trip by sps can
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