AN INVESTIGATION OF AN ENERGY DIVERTING CONVERTER FOR HVDC APPLICATIONS

Wind power generation in Europe has experienced an unprecedented expansion
fuelled by a very favourable regulatory framework promoted to fight climate
change. It is currently the second largest power generation source accounting
for 17% of the total energy mix and in 2016 it covered an impressive 10.4% of the
total energy demand. With faster wind speeds and better availability, offshore
wind farm developments have also experienced a surge in recent years. There are
12.7 GW of cumulative installed capacity with the hot spot located in the North
Sea.
The grid integration of offshore wind farms has evolved to meet the
requirements of recent projects, much larger in power capacity and located farther
offshore. High voltage direct current (HVDC) connections using state of the art
multilevel voltage source converters are now the industry standard for distant
wind farms, with transmission capacities of up to 1 GW. The scale of the projects
and frequent grid weakness at the onshore locations challenge transmission system
operators which need to ensure the entire grid stability. Grid codes have evolved
to regulate such interconnections, with a set of well specified requirements which
need to be fulfilled. One such requirement is the fault ride-through capability,
which defines the need for the HVDC interconnector to remain connected during
onshore grid faults.
A Dynamic Braking System (DBS) is a power electronics device that provides
fault ride-through capability to the HVDC interconnector by absorbing the excess
energy injected to the link for the duration of the fault. This energy is commonly
dissipated in a resistive element. In this way the DC over-voltage is avoided and
the operation of the connected wind farms is kept undisturbed. There is a lack
of knowledge in the design and implementation of such devices. Therefore four
concepts put forward by industry and other researchers are studied in this work.
The rating of the different components in each circuit is investigated as the basis
for the comparison.
Taking into account the modular structure of AC/DC converters in HVDC
stations it makes commercial sense to reuse the same modules as building blocks
for the DBS. With modular structures, a good balancing of the total energy stored
in the converter and its distribution among the different modules is one of the
key elements. Modular DBS circuits can synthesize multilevel voltage waveforms,
allowing for advanced power modulation strategies. Two novel strategies are
developed in the thesis and an accurate mathematical modelling is performed to
ensure that the energy balance conditions are met for all points of operation.
An overall control strategy for each of the four circuits is also developed and
presented in the thesis.
A good coordination of the protective actions of the DBS and the main
HVDC converters is important to ensure that no negative interactions occur. An
operation strategy based on over-voltage thresholds is developed in the thesis.
Accurate simulation models of the HVDC link integrating the DBS and controls
are also implemented to give the required degree of confidence in the overall
system behaviour. These are finally validated by a laboratory scaled-down test
platform, where the control actions and the different converters are implemented
in real hardware, and the correct coordination of all the elements during a fault
event is experimentally tested.
The main drawback of the DBS solution usually highlighted in literature is its
cost. The option of adding some extra functionality to better justify the economic
investment is explored in this thesis, resulting on a multifunctional circuit named
Energy Diverting Converter (EDC). Two proposals including active filtering and
HVDC tapping are developed in this thesis, for which two patent applications
have been filed.