Download Direct current is the future

TRANSMISSION AND DISTRIBUTION
Direct current is the future
by J B Woudstra and H Stokman, The Hague University of Applied Sciences
A world with direct current grids instead of alternating current grids. Is that possible? Sustainable generation of electrical energy is always
linked with DC voltage. Solar cell systems produces a DC voltage, wind turbines always convert the AC voltage with a variable frequency in
a DC voltage to convert it into an AC voltage with a constant frequency.
Sustainable energy is becoming more and
more important in the future. In northern
Europe large offshore wind turbine parks
are planned. Solar cell systems on roofs
of houses, thermal heat installations in
houses, small urban wind turbines will
become more and more popular. When
you look at the loads, then you see an
explosive numbers of appliances in
households, offices and industry that run
on low voltage direct current. It is clear that
a DC voltage grid is a logical next step.
The choice to use alternating current for
distribution of electricity was made in a
discussion between two famous scientists
Tesla and Edison around 1880. Tesla
emphasised the practical advantages
of alternating current. Transformers make
it possible to easily increase AC voltage
allowing electrical power to be transmitted
via cables over great distances with very
little losses. With DC voltage, this was not
easy to achieve and there were great
losses during transport. Tesla's practical
arguments were at least for the time being
the deciding factor. It is now however
possible to design DC – DC transformers
with a high efficiency to increase or
decrease the DC voltage. So it becomes
time to reconsider the decision for the use
of DC voltage instead of AC voltage.
Examples include halogen lighting, energy
saving lamps, telecommunication and
computer equipment and cordless/
rechargeable devices and tools. In the
future the electrical car will replace
combustion engined cars. To charge
the batteries during the night will be a
challenge.
Sustainable generation of electrical energy
is always linked with DC voltage. Solar cell
systems produces a DC voltage, wind
turbines always convert the AC voltage
with a variable frequency in a DC voltage
to convert it again in an AC voltage with
a constant frequency. Sustainable energy
is becoming more and more important in
the future. Here in northern Europe large
offshore wind turbine parks are planned.
Another trend in generating electrical
energy is a shift from central generation of
electrical energy in large non sustainable
power plants to more decentralisation in
small local sustainable power plants. The
energy flow was always vertical from large
power plants to the users, but is becoming
Fig. 1: Dutch electricity grid.
more horizontal from users (with a small
power plant) to other users. It is clear that
a DC voltage grid is a logical next step.
In the paper the transition from a vertical
oriented grid to a more horizontal oriented
grid will be explained, as well as from a
fossil oriented production of electrical
energy to a more or completely sustainable
electrical energy production, and from AC
grid to a DC grid.
Electricity now and in the future
In the Netherlands there are a small
number of large power plants (28) fired
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by coal, gas or nuclear heat power plants.
The total installed Electrical power is
25 264 MW and 119 818 GWh is produced.
At this moment around 8,5% is produced
sustainably and there is 15 583 GWh
imported and 12 808 GWh exported. The
import and export is with countries like
Belgium, Norway and Germany.
Most of the power plants are large industrial
sites located at strategic locations, nearby
the sea, rivers or lakes for cooling water,
and close to energy resources or supply
routes. These large power plants are
connected to the transmission network by
step-up transformers and are controlled
TRANSMISSION AND DISTRIBUTION
in order to take care of the voltage and
frequency stability of the power system; this
is what we call “centralised generation”.
Until now, the power system is for a greater
part supplied by this centralised generation
and we therefore say that system is
"vertically" operated, as illustrated in
Fig. 2. We can see from the system layout
that there is a "vertical" power flow in the
system. At the top, power is generated by
a (relatively small) number of large power
plants. Via the transmission and distribution
grid, power finds its way down to the
consumers connected at the lowest level.
Fig. 2: Vertically operated power system.
Nowadays, the trend is to integrate more
and more decentralised generation,
also called distributed or dispersed
generation, into the system, by means
of connecting small-scale generators
at the lower voltage levels. Examples
of decentralised generation units are
windmills, solar panels or combined heatpower units (producing steam for industrial
processes and electricity as a by-product).
When this trend continues, a large-scale
implementation of these decentralised
generation units will lead to a transition
from the current "vertically operated power
system", (Fig. 2), into a more "horizontally"
operated power system' in the future, as
shown in Fig. 3.
Because of the increasing amount of
decentralised generation units the most
uneconomical and/or aged power
plants are taken out of service, and this
leaves a power system with the bulk of
the consumption and the generation
connected to the distribution network so
that more or less "horizontal power flow"
through the system results.
Fig. 3: Horizontally operated power system.
Possible developments and/or
consequences of the transition from
the current "vertically operated power
system"into a future more "horizontally
operated power system" for the power
system are:


Fig. 4: Horizontally operated power system without non-sustainable power plants.
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When most of the larger power stations
have vanished, shown in Fig. 4, and
the ‘horizontal' power system is a fact,
the transmission/distribution network
has lost one of its main purposes,
namely to facilitate bulk transport of
electrical energy from the centralized
generators to the distribution networks.
Such a situation requires different
control and operating strategies in
order to keep the system operation
within safe margins. The voltage, for
example, will no longer be imposed by
the large (centralised) power stations,
and voltage stability of the system
becomes an issue.
Traditional, the distribution network
is a passive network, that depends
totally on the transmission network for
energy deliver y, frequency control
TRANSMISSION AND DISTRIBUTION
becomes two-way traffic. This has a
fundamental impact on the protection
of the system.

Fig. 5: Sustainable energy sources in Europe and North Africa.
Autonomous networks could be
developed. When the total amount
of power generated in a certain part
of the distribution network is sufficient
to supply the local loads, the network
could be operated autonomously, by
disconnecting it from the rest of the grid.
From an operational view, this gives a
system that can be controlled more
easily. However interconnection of
networks offers quite some advantages
too. An important one is that the
operation can be supported by others
if there is a problem. For example
with the local suppliers because
of unexpected loss of generation.
Therefore, such autonomous operating
system will have to be equipped with
a connection to the main transmission
grid, or to neighbouring autonomous
distribution systems, to safeguard the
supply in case of a sudden emergency
situation. This is not as easy as it looks:
both systems operate in practice at a
slightly different frequency and usually
have a different voltage angle at the
instant in time we would like to make
the interconnection. Another issue
is that both systems have their own
frequency control, and the question
arises: which system will act as the
master and which system will be the
slave? These problems can be avoided
by having a DC-link connecting the
systems; in this way, the systems have
the possibility for power exchange but
are frequency-wise and voltage-wise
uncoupled.
Sustainable energy links North Africa
to Europe
Fig. 6: Sun power plant in Spain.
and voltage regulation. In a future
"horizontally operated power system",
the power is not only consumed by
but also generated in the distribution
network. Therefore, the distribution
network needs to change into an
active and intelligent network, which
is able to control and regulate the
system parameters, without strong
support from the transmission network.
It needs to become a "smart grid".

In the "vertical power system", the
direction of the power flow is more
or less predictable: centralised
generation → transmission network →
distribution network → consumers. In
the ‘horizontally' power system, with its
active distribution network, the situation
is different: (centralised generation
→) transmission network distribution
network  consumers. The direction
of the power flow in the network is not
predictable any more: one-way traffic
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Sustainable energy will become ver y
important in the nearby future. Fossil fuels
are becoming very expensive and we can
better use it for other things then burning
it with a low efficiency. In the horizontally
operated power system the sustainable
sources are partly locally placed, solar
panels, and partly in large offshore wind
mill parks. But when you want to have a
stable sustainable electrical power network
in Europe it is necessary to connect all the
sustainable sources in Europe and maybe
North Africa, see Fig. 5.
In North Africa and south Europe it is
possible to build large sun-energy power
plants. An example is shown in Fig. 6.
Actually it is possible to use only sun-energy
to provide the world from electrical energy.
In Figure you can see what kind of surface
you need to realise this.
Contact J B Woudstra,
Hagur University of Applied Science,
[email protected] 