Getting Electric Power from Here to There

Feb 1997 - Volkmar Dimpfl - Archives - Seimens

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 power system."

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 UCPTE.

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."

Reliable Power
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 hardly changed.

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
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."

Interdisciplinary Work
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 years."