IEEE Power & Energy Magazine - January/February 2015 - 79

dc grid was undertaken, as well as the interconnection of
the physical dc grid to a simulated ac grid using a realtime simulation facility. The mock-up is the first of its
kind, with 15-km cables and dc protection (including dc
breakers); this equipment was used to validate innovative
dc fault detection principles (main differential protection
and backup protection) with realistic fault propagation
conditions, as presented in a live demonstration.
The short-term analysis was backed up with simulations
verifying that the step-by-step development of a small dc
grid starting from radial connections is quite feasible. The
topologies considered were either treelike or meshed. for
each, dc power flow could be mastered via autonomous dc
voltage droop controls with possible predefined power-sharing policies and without any requirement for communication
(depending on the grid layout). in addition, risk-based strategies to preventively shift dc power injections on the ac network were illustrated.
furthermore, dedicated controls were designed to provide
various ancillary services (voltage support, inertia emulation,
frequency support, and power oscillation damping), as well as
fault ride-through (frT) capability during severe disturbances.

Demo 4: Management of Offshore Wind Power
in Extremely High-Wind Situations
during the fourth TwenTies demonstration project, experience from very tough weather conditions in western denmark during the autumn of 2012 proved that if offshore wind
turbines are equipped with new high-wind ride-through control, it is possible to run them longer during periods with high
wind speeds, which means there is less risk of turbine shutdown and thus less risk to the stability of the power system.
in addition, total output during these periods is higher
from turbines equipped with the new high-wind ride-through
control than from those with the old control algorithm,
which would abruptly shut down the wind farm when the
wind speed exceeded 25 m/s. Measurements from horns
rev ii during stormy weather proved that the wind turbines
equipped with the new high-wind ride-through control software could stay in operation in wind speeds of up to 32 m/s.
The new high-wind ride-through controllers, developed by
siemens, led to less abrupt changes in production for the
wind farm as a whole.
in the autumn of 2012, several storms hit western denmark. during these storms, power production at horns rev ii
hardly went down at all. simulations show that if the old control algorithm had been in operation, the wind farm would
have shut down completely. The extra power production with
the new system was considerable. Moreover, if wind speeds
are high enough to cause the power produced to decrease,
the drop in output happens much more gradually than would
have been the case with the old system. This is a huge advantage for balancing the electricity system.
The power system in western denmark is operated as
a single area that needs to be in balance. Problems occur
january/february 2015

only rarely thanks to advanced operating systems. everyone
responsible for balancing in the power system in denmark
is obliged to update detailed production and consumption
schedules every five minutes. The detailed schedules provide
energinet.dk with the opportunity to avoid larger imbalances
by manually activating regulating power.
because the nordic power markets function extremely
well, almost all power production in these countries is balanced. and thanks to the norwegian hydropower system and
the hVdc connection to norway, today a very large share of
the wind power variability in western denmark is balanced.
with the new high-wind ride-through controller installed in
offshore wind parks, it is possible to integrate more offshore
wind into the system without jeopardizing system security at
times of high winds.
The goal of doubling the share of wind power in final electricity consumption in denmark (from approximately 25% to
50%) within seven years along with increased growth in wind
power capacity across europe will significantly challenge the
power system. advanced controllers have been considered
to effectively balance power variations between the nordic
region and western denmark so that power system balance
restoration is possible in case of unforeseen large variations
in (offshore) wind power generation.
This demonstration project has proven that it is possible to
find a solution to these future challenges with large amounts
of offshore wind power production. at the horns rev ii
wind farm, the new high-wind ride-through control has made
a big difference. not only has energy output increased, but
the implications of the technology for lowering risks to the
security of the overall power system, such as instability and
blackouts, are very positive.

Demo 5: Network-Enhanced Flexibility
The variability of wind generation creates quicker and more
significant changes in the flows of electricity through the
transmission network than generation using conventional
fuels. grid reinforcements are needed to connect and integrate more wind by managing variations from wind output
while retaining the same level of security of supply. furthermore, the upgrading and installation of new overhead lines
and underground cables takes time. More flexible solutions
are needed to cope with these variations in a timely and costeffective way.
Partners in the fifth TwenTies demonstration project showed that network flexibility enables more power to
be transported to where it is needed without compromising
operational security. The demonstration has shown how a
more accurate monitoring and enhancement of grid control
lets TsOs plan and operate the networks more accurately.
dynamic line rating (dlr) devices like ampacimon
vibration monitors (which measure temperature) can be
installed quickly on overhead lines to provide accurate
measurements of the capacity of the transmission system in
real time. This capacity depends on wind conditions and is
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - January/February 2015