IEEE Power & Energy Magazine - September/October 2020 - 62

The relative redispatch energy volume is defined as the
division of the annual redispatch energy calculated for each
variant and the annual redispatch energy of the base case.
The base case consists of the five HVdc links having a total
of 2-GW transmission capacity without any reactive power
capabilities and corresponding to a 100% converter rating.
The case study is the result of a full-year simulation of the
planned German 380-kV grid in 2030. Figure 5 depicts the
evolution of relative redispatch energy volume (vertical axis)
with respect to the converter rating (horizontal axis). On the
one hand, adding 3% extra converter capacity with reactive
power (Q corresponding to roughly 25% of pmax) would
improve the total redispatch volume by approximately 1.2%.
Beyond that point, the improvement rate lowers significantly,
meaning that additional reactive power does not lead to a significant reduction of redispatch volume.
On the other hand, for 8% extra converter capacity and
onward, a 50/50 share of additional reactive and active power
(labeled in the Figure 5 legend as "Equal Share of Active/
Reactive Power") leads to the largest redispatch volume
reduction. Intersection B of the "Maximizing Active Power"
(yellow) and "Maximizing Reactive Power" (blue) curves is
located at approximately 106% of the converter rating. The
graph shows that for converter ratings higher than 106%, the
active power extra capacity benefits the reduction of redispatch
volume more than it does for the reactive power extra capacity. Intersection A displays the same effect for the comparison of the variants "Equal Share of Active/Reactive Power"
and "Maximizing Reactive Power," where the first becomes
advantageous already at converter ratings greater than 104%.

Relative Redispatch Energy Volume W (%)

The planned embedded HVdc links in Germany cover a
long distance (300-700 km) in a highly meshed legacy ac grid
and are plugged into the former strong grid connection points
of nuclear power plants (see Figure 4). Consequently, converters will be integrated at locations likely to have a deficiency
in reactive power due to the shutdown of the nuclear plants.
These locations would therefore be vital for voltage control.
According to the initial assessment presented in Figure 3, this
configuration would result in a medium impact considering
the favorable integration in parallel to a highly meshed system
as well as the long distance of the HVdc links.
Once the desired HVdc corridor capacity is determined,
the final consideration is how much additional converter
capacity should be reserved for reactive and active power
or a combination of the two. A case study on a full-year
redispatch scenario for the projected German 380-kV grid
in 2030 was performed to answer this question for the three
variants: maximizing reactive power, maximizing combined
active/reactive power, and maximizing active power.

figure 4. The German 380-kV ac transmission grid (the
green-blue circles are ac substations, and the solid gray
lines are ac transmission overhead lines or cables) and
the planned embedded HVdc links (the red squares are
HVdc converter stations and the solid red lines are dc
transmission overhead lines or cables). Image courtesy of
OpenStreetMap (https://www.openstreetmap.org/). Power
grid data courtesy of Network Development Plan 2017-B
(https://www.netzentwicklungsplan.de/de).
62

ieee power & energy magazine

Maximizing Reactive Power
Equal Share of Active/Reactive Power
Maximizing Active Power

100
99.5
99
98.5
98
97.5
97
96.5
96
95.5
95
100

B
105
110
115
Converter Rating S (%)

120

figure 5. Case study results from the full-year simulation of the German 380-kV grid in 2030. The horizontal
axis depicts the HVdc converter rating compared to the
base case. The vertical axis shows the full-year redispatch
energy compared to the base case. Intersections A and B
highlight the converter ratings where one scenario becomes
advantageous over another.
september/october 2020

https://www.openstreetmap.org/ https://www.netzentwicklungsplan.de/de

IEEE Power & Energy Magazine - September/October 2020

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