IEEE Power & Energy Magazine - September/October 2020 - 60

✔ Reactive power support can boost the capability of the

ac grid.
✔ Power oscillation damping controllers can free up some
capacity reserve margins in the ac grid by suppressing
poorly damped modes.
These capabilities augment the power transfer capacity increase well above the actual installed rating of the
HVdc system. Some ingredients of the (embedded) VSC-HVdc

Offshore Wind
Integration
With HVdc

Asynchronous Grids
HVdc Interconnector
Embedded
HVdc Links
Legacy ac Grid Substations
HVdc Converter Stations
Offshore Wind Installations

figure 1. An example of HVdc links in a power system.
The circles are substations of the legacy ac grid, the squares
are HVdc converter stations, and the lines are transmission
overhead lines or cables (red: HVdc; black: legacy ac grid).

North

capabilities do not directly increase the power transfer capability but are nevertheless important features:
✔ Black-start capability can help energize parts of the
grid with the assistance of remote generators, and HVdc
could support the resynchronization of a split synchronous grid during grid restoration.
✔ Fast power reversal can help with responding to system contingencies, thereby avoiding cascading failures in the power system.
✔ Fault ride through and controlled fast fault current
injection offer the converter the opportunity to stay
connected during grid disturbances while providing
continuous system support.
✔ Frequency response could be used when combined
with energy storage (embedded) or when it is from an
asynchronous grid (interconnector).
This article takes a closer look at the additional value
proposition of steady-state reactive power support from embedded VSC-HVdc. Grid codes do have steady-state reactive
power requirements for integrating HVdc links into the power
system depending on the voltage and power level (for an
example, refer to the European Commission regulation in
the "For Further Reading" section). On the one hand, extensive reactive power requirements increase the size and cost
of converter stations. On the other hand, too little reactive
power capability might lead to a local deficit of reactive
power (especially in the case when local traditional generation is replaced by remote renewable sources) and unfavorable conditions for the nearby legacy ac network. It is therefore important for the system operator to understand the
reactive power capabilities of HVdc converters and assess
their value for power system operation.
In this article, we focus on the VSC-HVdc steady-state
performance for reactive power. The inherent dynamic voltage
control capabilities of VSC-HVdc are important to ensure
grid stability during disturbances but are not within the scope
of the article.

Assessing the Benefits of Reactive Power
Support From Embedded HVdc

1
South

Legacy ac Grid Substations
HVdc Stations of Embedded Links

figure 2. Examples of embedded HVdc links. The circles
are substations of the legacy ac grid, the squares are HVdc
converter stations, and the lines are transmission overhead
lines or cables (red: embedded HVdc; black: legacy ac grid).
60

ieee power & energy magazine

To compensate for the charging current of ac lines (either
overhead or cables) and enable stable power transfer, reactive power compensation may be needed depending on the
length of the line. Embedded VSC-HVdc does not need any
reactive power to transmit active power over the HVdc line
but can inject or absorb reactive power independently at each
converter terminal connection to the ac grid. Reactive power
can therefore be used to boost the transfer capacity over the
adjacent ac grid. The main factors that influence boosting
efficiency are
✔ a local lack of reactive power near the HVdc terminals, e.g., due to the early retirement of fossil-based
power generation
✔ the degree of meshing along the HVdc link and near
the terminals
september/october 2020

IEEE Power & Energy Magazine - September/October 2020

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