IEEE Power & Energy Magazine - March/April 2016 - 112

(MMDcT) meeting all of the above requirements at ratings up to 1,000 Mw
show results identical to cIgrE dc interconnections studies in which the dcto-dc transformer was modeled mathematically only. Simulation in other
system contexts showed it to meet all
of the criteria cited above.Those simulations were then extended to large
wind farms wherein dc power (resulting from accommodation of asynchronous ac) remains as dc, is stepped
up to a 32-kV dc collector system by
MMDcTs within each nacelle, and fed
to collector busses where it is stepped
up further to a transmission voltage
of 320 kV for transmission to shore.
This eliminates the weight and space
requirements of magnetic transformers and benefits from lower-cost dc
cables, both from nacelles to the collector platform and from that platform
to shore. Figure 4 shows two popular

configurations: 1) a parallel grouping
of turbines and 2) a series configuration where the dc-to-dc voltage transformation ratio is necessarily Vo/n,
where Vo is the voltage output of each
nacelle and n is the number of dc-to-dc
transformers in series.
The importance of dc-to-dc transformation at all levels of the power
supply chain is reflected in the growing number of technical papers now
focused on simpler versions of traditional dc-ac-dc architecture, traditional
series/parallel capacitor switching, and
capacitor-based emulation of the ac autotransformer. comprehensive simulation work shows the MMDcT option
is capable of meeting all nine of the
characteristics cited earlier. Details of
the MMDcT were presented at the cIgrE Symposium on HVdc and Power
Electronics, 21-26 September 2015 in
Agra, India.

Conclusions
As dc's role expands within the generation, storage, transmission, and utilization spheres, the development of an efficient, easily adapted dc-to-dc transformer
has become among the industry's highest
priority development challenges. At least
one relatively simple design, using building blocks and operational technology
well tested in modern converter terminals, appears to meet all the requirements
for such a device with rating capability
needed for even the highest transmission
voltages. Transformers of that nature will
be important to the eventual success of
dc supergrids and microgrids and the extended integration of renewable energy
resources in the world's electrical energy
mix. Dc-to-dc transformers are close at
hand...and where the power industry
started seems also to be where it's going.
p&e

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