IEEE Power & Energy Magazine - September/October 2020 - 61

✔ parallelism to existing ac lines
✔ the length of the HVdc link.

The Impact of Reactive Power Support
From the Future Embedded HVdc
Systems in Germany
Germany has ambitious goals to increase the share of renewable power sources to 80% as part of the country's total
demand by 2050. The potential for large-scale renewable
power is mainly located in the north with abundant on- and
september/october 2020

High Impact
by Reactive Power
System Benefit of Reactive Power

The greatest benefit is achieved by installing one HVdc
converter in an area facing a deficit of reactive power supply.
The reactive power support from the HVdc will then boost the
transfer capacity of the legacy ac grid. The influence on reactive power support with respect to meshing, parallelism, and
distance is depicted in Figure 2, which provides two examples
of embedded HVdc (noted as ① and ②).
Embedded HVdc ① is a relatively short link, strictly in
parallel to an ac line in a fairly meshed surrounding ac system. This link could obviously boost the ac system transfer
capability by means of reactive power supply due to its short
length, the high degree of parallelism, and meshing. The
short length of the link allows each terminal to provide reactive power to the highly parallel and meshed ac grid from
each side and have influence over power transfer capacity.
Embedded HVdc ② is a long link between north and south,
as shown in Figure 2. On the one hand, this link can very
effectively boost the power transfer over the ac system
because it can reduce the loading of any of the parallel ac
lines between its two terminals, which may be the bottleneck
for north-to-south power transfer. In this respect, link ② is
more effective in active power control. On the other hand,
due to its positioning, link ② cannot effectively control the
voltage in the ac nodes far away from its terminals; the
expected benefit from reactive power is low, and the rating of
the converters might be better invested in increasing active
power transfer capabilities.
The diagram presented in Figure 3 provides an initial
assessment of the system benefit of reactive power. The
horizontal axis represents the configuration of the embedded HVdc system and the surrounding ac grid (e.g., including the degree of meshing of the ac grid and parallelism of
the HVdc link with respect to the ac grid and the length of
the corridor). The vertical axis illustrates the system benefit
of reactive power. Looking at the main factors introduced
to evaluate the benefit of reactive power support, it can be
concluded that each grid topology must be assessed individually. Reactive power is beneficial when the HVdc link
is integrated into a highly meshed ac grid with parallel lines
and a rather short distance, enabling converter stations to
support the legacy ac grid mutually from both sides of the
link (noted as ①). In contrast, a long-distance, nonparallel
link might not be capable of supporting the legacy ac grid,
even if the converter stations have extensive reactive power
ratings (noted as ②).

1
Limited Impact
by Reactive Power
2

Configuration
(Length, Meshing, and Parallelism)

figure 3. The benefit of reactive power support from
embedded VSC-HVdc depends on the configuration (the
HVdc link length, meshing of the surrounding ac grid, and
parallelism of the embedded HVdc link with regard to the
ac grid). The green circles represent ac nodes selected to
highlight the influence from reactive power flows (green
arrows) coming from the HVdc converter stations (red
squares). Active power flow over the HVdc link (red line) is
shown with red arrows. The thickness of the green arrows
represents the impact the reactive power flow can have on
the corresponding ac nodes.

offshore wind resources. However, because the large demand
centers are mainly located in the southern and western parts
of the country, a significant upgrade of the existing transmission grid infrastructure is required. The grid infrastructure
at former conventional central power plants, such as nuclear
and coal-fired plants, is planned to be the entry point(s)
for at least five north-to-south corridors using embedded
VSC-HVdc technology with a total transmission capability of
10 GW. The increased north-to-south power transfer capacity aims to reduce the redispatch cost caused by the relatively
fast-changing power generation landscape and the slower
pace of transmission grid expansion. Redispatch costs occur
when the dispatched active power of power plants needs to
be adjusted because the transmission grid risks congestion
and cannot transport the scheduled power from the generation to the demand centers. Reducing the redispatch energy
volume and cost is one of the top measures used to lower the
cost of electricity and avoid splitting Germany into different
price zones.
Studies looking at the future need for ancillary services
in changing power systems consider HVdc systems as the
potential provider of such services. Shutting down conventional sources of reactive power (which, in the German
case, are primarily nuclear and coal-fired power plants)
leads to a deficit of reactive power in specific geographic
areas. The capabilities of the HVdc converters could make
up for this deficiency.
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IEEE Power & Energy Magazine - September/October 2020

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