Getting Electric Power from Here
Feb 1997 - Volkmar Dimpfl - Archives
What's behind the household electrical outlet? A
lot more than meets the eye. As network interconnections
multiply, management is becoming increasingly complex.
From Shanghai to New York, from Alaska to the Sahara,
power supply networks ensure that energy is supplied
all over the world, even under extreme conditions.
These networks are much more than a mere linking up
of lines. Most people only become aware of how complex
the entire system is when they read about large-scale
blackouts in the newspapers or when they themselves
are affected by a power failure.
Wherever you see cities glowing in the night or factories
working and electrified railways in operation, elements
of a network are involved. Professor Dusan Povh, President
of System Planning at Siemens Power Transmission and
Distribution Group, describes the structure of a power
supply network this way: "First of all, there are
the individual power stations, which can be very large
and complex units. After this come the transformers
where the voltage is stepped up. The power network
is used for exchanging power and transporting it to
load centers that, in Germany, are about 50 to 80
km away from power stations. Additional elements are
switching stations, for example, which configure and
protect the network."
A special feature of alternating current, which generates
alternating fields, is reactive power; it is needed
for field generation or release during field decay.
With the help of compensators, reactive power can
be produced or consumed as required. If a large amount
of power is transported over a long distance, however,
it must be controlled very quickly. Reactors, transformers,
capacitors, inductance levels, capacitance levels,
thyristors, and control system are combined to form
static compensators. "SVCs reduce costs," explains
Povh, "because they allow power lines to be utilized
to a greater extent and they help to stabilize the
A Job for Direct Current DC is suitable for long distances,
an application in which reactive power is irrelevant.
In the 1930s, the concept of HVDCT (high-voltage DC
transmission) was developed and put into practice
with mercury-arc valves. The breakthrough came with
semiconductor valves in the late '60s. The best known
HVDCT project in the '70s was the linking of Cahora
Bassa in Mozambique with South Africa (1400 km).
"In Europe, more and more of our power needs are being
met by remote hydroelectric generating plants," says
Povh. "The HVDCT links from Norway to the European
grid are an example." A second task of HVDCT is the
interconnection of non-synchronous power networks.
This applies to 50 and 60 Hz systems (South America,
Japan) as well as to networks with different frequency
constants. The UCPTE (Union pour la Coordination de
la Production et du Transport de l'Electricité) in
Europe works with a narrow frequency band whereas
the Ukraine, Russia, Romania and Bulgaria allow larger
fluctuations. NORDEL (Nordiskt Samarbete på Elkraftområde)
in Scandinavia also works non-synchronously with the
Improved Transmission of AC
As utilization of power supply networks increased,
even as resistance to new cables grew, a new system
was born called FACTS (Flexible AC Transmission Systems).
In deregulated systems, a utility company has to connect
its customers to the most economical producer. "This
alters load-flow requirements within power networks,"
explains Povh. "With FACTS units, these requirements
can be fulfilled in existing networks. Without FACTS,
power is not always transmitted along the prescribed
route and the natural load flow fails to comply with
contractual obligations. When French power is exported
to Italy, a part of it passes through Belgium, Switzerland
and Germany. When there are more transits, detours
are no longer acceptable. In addition, technology
now makes it possible to calculate load flows in advance
and to steer them along their routes." As a supplement
to shunt controllers such as SVCs or STATCOMs (Static
Synchronous Compensators), load-flow controllers connected
in series have been developed, such as thyristor-controlled
series compensators, thyristor-controlled phase shifters
and universal load-flow controllers. "Developments
are taking place very rapidly," explains Povh, "because
they combine existing techniques and do not require
any new technology. FACTS can be installed in a very
short time (one to two years), thus solving problems
quickly and bridging the period before a long-term
solution is required."
Power failures can be extremely costly. Some big ones
have recently occurred in the western U.S. where,
according to Povh, the network is highly extended.
In Povh's opinion, it is the most complex power supply
network in the world. He explains the blackouts this
way: "Line routes have to be watched carefully, otherwise
there can be short-circuits across trees, for example,
which is just what happened. Protective devices did
not function correctly, which caused parallel cables
to be disconnected as well." This initiated a domino
effect. One power station failed because it was no
longer able to feed in power, so there was power missing
from the network. Then, parts of the network disconnected
as planned. But because a lot of power comes from
the north, there was not enough power in Southern
California—which caused the blackout.
"In the event of a fault," explains Povh, "a power
supply network should be split up in a controlled
way and less important loads should be disconnected
in order to ensure operation of the most important
ones. A single fault must not be allowed to cripple
the entire network but, with complex power supply
networks, this is not easy to control." In Europe,
a large-scale blackout, according to Povh, is less
probable "because the exchange of power between parts
of the network is insufficient to make them dependent
on each other." But this could change with increased
deregulation. What has happened in England and Wales,
says Povh, is interesting. Here, new power stations
have been established in the north where North Sea
gas is available. The problem is that the power has
to be transported 200 to 400 km. Compensation systems
have therefore beed installed although demand has
A second point is the quality of the voltage; it should
be constant and flicker-free. This is important because
more and more electronic devices produce harmonics,
straining the power network. Filters are therefore
used. "Conventional filters," explains Prof. Povh,
"are based on inductance and capacitance. We are offering
a dynamic solution in the form of electronics that
feed in harmonics negatively and thus destroy them.
This area will develop as faults and demands increase."
That's important because a growing number of industries
rely on high-quality power. In the paper and textile
industries, for example, voltage fluctuations can
significantly affect product quality. Similarly, automotive
paint shops require unwavering voltage because paint
layers are so thin that the smallest interference
can leave a gap.
System planning must include environmental protection,
urban growth, and power system expansion. Since projects
take years to complete, it is essential to examine
how loads will develop and how to adapt the network
accordingly. Planning also ensures that the expected
load can be fed into the network. According to Povh,
"Our simulators ensure stable and fault-free operation."
He points out, for instance, that control systems
and networks can be simulated so accurately that dynamic
processes can be examined. "We have a program with
which thousands of nodes and tens of thousands of
lines can be emulated," he says. The entire UCPTE
network, he explains, can thus be simulated together
with neighboring power networks, as well as what would
happen if the networks were to be interconnected.
"For real-time investigations, we use analog and hybrid
systems. This allows us to test the behavior of devices
in the network, but it also allows us to teach our
people about commissioning," explains Povh.
There are also programs for load increases. These
programs take into account the temperature, time,
and other factors such as whether a football game
is being shown on television. The load is then calculated—a
step which, according to Povh, would be impossible
without the use of neuronal networks. "These networks,"
he says, "collect data, learn from it, and use it
to predict demand."
One World, One Network
Recently, Poland, Slovakia, the Czech Republic and
Hungary were linked synchronously with the UCPTE.
To reach this stage, these countries invested in frequency
control, then operated in parallel to the UCPTE. The
philosophy of the UCPTE is that everyone has separate
reserves, and interconnects to exchange power on a
short-term basis. At present, Morocco and other North
African countries are being connected with the UCPTE;
and it is entirely possible that the entire Mediterranean
basin will eventually be interconnected. Will there
ever be a global grid? According to Povh, "Trials
and experience show that power can be transmitted
over thousands of kilometers without any problem.
Parts of the global grid will certainly be built in
the near future. There are plans, for example, to
bring power from the hydro-electric resources behind
the Urals to Europe, and from the Congo to Egypt."
The planning and implementation of networks has changed
dramatically in recent decades. Conventional techniques
have been supplemented by power electronics and communications.
"These technologies are being mixed," says Povh, "and
as a result. our engineers are being called upon to
be much more than power experts. They also have to
know about other fields, especially information processing.
It's amazing how much has changed in the last thirty