IEEE Power & Energy Magazine - May/June 2019 - 65

The second project consists of a modular multilevel
converter-based voltage source converter-HVdc connection
between the mainland and Jeju Island.

be within the reliability standard. the aggregation process
of generators is required to perform such analysis. mpCws
offers an expected overloading capability of the hVdc system
under different system strengths and provides an initial rat-
ing of the stAtCom required to maintain the voltage on the
inverter terminal in steady state. these procedures can pre-
vent iterative tasks when performing further studies.
the use of advanced hVdc stability models, such as the
CdC7t model in pss/e, provides the most accurate results
in planning studies. through detailed modeling of the dc
line and control algorithms, the model allows the simulation
of hVdc performance as close as possible to the simulation
results achieved with an emt program. in addition, the sta-
bility model includes a controlled dc ramp-up rate, which is
an important factor in determining the required overloading
capacity of the hVdc for properly decelerating the generators
after a contingency.
e-trAn simulation provides detailed and accurate emt
studies after converting network data and dynamic mod-
els of tsA to those of emt. emt analysis is required to
determine parameters, such as dc voltage and current sensor
time constants, the size of the dc smoothing reactor and line
parameters, voltage-dependent current order limit coordina-
tion, proportional integral controller values, and firing angle
and extinction angle ranges. the analysis can tune controls
and identify the abnormal operation of the hVdc converter
valves that would be observed in the field but not seen by
tsA simulation results. for example, commutation failures
can be observed in emt simulations but not in the tsA. An
examination of the emt simulation results can inform inter-
pretation of tsA results and improve tsA models.
A feasibility study showed that the installation of fACts
resulted in improved hVdc performance and achieved bet-
ter utilization of existing transmission assets. series capaci-
tors reduce line impedances that increase ac power flows,
but a tCsC can also dynamically modulate the amount of
series compensation and control line flows. the study exam-
ined a tCsC with dynamic overload capability to increase
ac power transfer during transient conditions and reduce
dependency on the hVdc overload capability. the analysis
determined the tCsC firing angle, compensation level, and
capacitor and inductor ratings. studies also ascertained the
dynamic-mode range of compensation and expected operat-
ing ranges for normal and dynamic modes to avoid unde-
sired resonances and enable estimates of the optimal firing
angles for the tCsC.
may/june 2019

installation of a stAtCom would instantaneously pro-
vide needed reactive support and was selected as the best
option for obtaining more stable lCC hVdc performance.
the location of the stAtCom electrically near the inver-
ter side of the hVdc system must be determined prior to
establishing its rating. proper reactive power support and
dynamic capability of the stAtCom improve hVdc per-
formance by avoiding commutation failures under severe
contingencies and maintaining acceptable ac and dc volt-
ages at the inverter terminal.

Expected Commissioning
figure 4 is a diagram for the planned power system in the
area of interest. the east side is the main generation area,
equipped with large-scale nuclear and coal plants; the west
side of the system is the main load center of the country that
includes seoul, the capital city of the republic of korea. the
project aims to stabilize the power system by coordinating
the controls of the hVdc system with the fACts, consistent
with planning criteria that allow for up to two generators to
trip offline upon occurrence of the most severe contingency:
a three-phase fault on the double-circuit 765-kV transmission
lines resulting in their loss. the addition of the hVdc sys-
tem, the stAtComs, and tCsCs replaces the original plan
of building new 765-kV transmission lines. After reviewing
various scenarios and strategies, two bipolar lCC-hVdc
lines with a total rating of 8 gw have been planned. two
tCsCs, each with a rating of 500 mvar, will support hVdc
performance as well as maximize the ac power transfer capa-
bility under N-2 contingency conditions.

Choice of Tool and Expectations
for the Detailed Studies
the tool choice for conducting the feasibility study is quite
important in this lCC-hVdc project because of the com-
plicated nature of the future power system. traditionally,
steady-state analysis and tsA use pss/e or a similar tran-
sient security assessment tool. these practices and tools are
industry accepted and appropriate for the simulation of large
power systems. these studies determine the optimal size
and location of the hVdc systems.
however, a tsA tool is not appropriate for investigating
dynamics requiring types of studies or simulations model-
ing a small time step used to analyze commutation failures,
valve control response, filter control, subsynchronous tor-
sional interaction, and negative sequence behaviors. emt
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