Natural Resources - Network Stability

Lights out in New York, radio silence in Italy. Ever-expanding power networks worldwide seem more susceptible to disruptions than ever before. But there are ways to prevent a collapse.

The events that appeared on television sets around the world on August 14 and 15, 2003, remain seared in people’s memories. Thousands of New Yorkers heading home by foot on the city’s roads. Nothing was working. Subway trains were frozen in place on their tracks. Blackout. When night fell on the city, New York was wrapped in a darkness. But New York wasn’t the only place where the lights went out. An area stretching about 1,000 km—extending all the way to Michigan and Canada—also lost power for many hours. In western Ohio, it took two days to get some areas back on the network, and within a month blackouts struck in Sweden, London and Italy. Hardly anyone believes it was a coincidence.

A task force of the German Association for Electrical, Electronic & Information Technologies (VDE) determined that the malfunction of two power plants and an important transmission line, caused by a bush fire, led to a domino effect in the United States. When the main transmission line went out of service nearly 1,000 km from New York, the remaining lines became overloaded. One after another, they shut down. And the power plants connected to the electricity network via those lines shut down as well. Overall, about 100 power plants in the United States and Canada went off-line. Within minutes, hundreds of error messages were blinking in control centers, and technicians were overwhelmed because of a lack of intelligent error-analysis systems. "We’re a superpower with a third world electricity grid," said Bill Richardson, a former U.S. energy secretary and now Governor of New Mexico.

The Italian blackout also was caused by the failure of an important transmission line—this time in Switzerland—and by a resulting overload elsewhere in the system. "In the foreseeable future, Germany will not experience such a widespread situation," believes Edwin Lerch of the Siemens Power Transmission and Distribution (PTD) Group in Erlangen. "But the situation is getting more difficult in Germany and Central Europe as well." A member of the VDE task force, Lerch is certain that Europe’s interconnected grid is being subjected to increasing stress by its liberalized energy market and unrestricted trade in electricity. As a result, electric power has become a product that is distributed according to supply and demand. "But the power grid wasn’t designed for this type of trading," says Lerch.

About 50 years ago, a number of countries, including France, Austria and Germany, established the Union for the Co-ordination of Transmission of Electricity (UCTE). As is the case today, UCTE countries and regions were self-sufficient. That’s because the regional utilities had always been reliable sources of electricity. The original idea behind the UCTE was for the network to provide help if power plants in one region suffered outages. To accomplish this, it would have to be possible to activate a total of 3,000 extra megawatts (equal to the output of several large power plants) within 30 seconds. But over the years, the UCTE network has steadily been turning into an electricity highway. Following the liberalization of the European market, power began to be traded at special exchanges like a commodity, and that trade is increasingly being conducted internationally.

But the UCTE network wasn’t designed for such electricity trading. Italy has been importing more and more electricity since the mid-1990s, for instance, and the country has decided not to build new power plants or expand transmission lines. Generally purchased inexpensively at night from France, Poland or Germany, electricity is steered into pumped-storage power stations. As a result, the transmission lines running across Switzerland are often pushed to their performance limits. And that’s not supposed to happen under the UCTE regulations. The key rule here is "n minus 1": When one line goes out of service, the others must pick up the load. This rule was broken during the Italian blackout that occurred at the end of September 2003. After one line went out of service, the others became overloaded, and eventually all the lines to Italy went down.

Physical Limits. Prof. Hans-Jürgen Haubrich of the Institute for Power Systems and Power Economics at the Aachen University of Technology has concluded that "today’s active electricity trading has already pushed the UCTE network to the limit." In order to keep the network stable in the future, trading must be restricted or new transmission lines will have to be built, Haubrich adds. But new lines are very expensive, and Haubrich believes there is little chance they will be built—at least not in Germany. As a result, engineers are working feverishly to push the capacity of today’s lines to their physical limits. Dr. Georg Rosenbauer, an energy expert in strategic planning at Siemens Power Generation in Erlangen, thinks such work makes sense. Right now, the capacity of overland transmission lines is restricted by a potential thermal limit, which is reached only on a few hot days each year. During cold weather, the same lines can carry a heavier load. To help the lines carry more electricity safely, sensors would have to be installed along a power line to measure the line’s true temperature. With the help of smart software, more electricity could be transmitted along the same line. This would increase transmission capacity and pay off for the network operator in real earnings.

A Network of Rubber Bands. The stability of the European UCTE network has made some people envious. Regions with less stability would like to join the network, which poses entirely new challenges. Today, the UCTE area extends from Poland to Portugal, from Germany to Italy, and from Belgium through the Balkans to Greece. "That makes it precariously big," says Ronald Völzke, a power network specialist at Siemens PTD.

"You can think of the supply system as a network of rubber bands. A number of large units, the power plants, and many small units—in other words, the consumers—hang on the knots," explains Völzke. When one power plant suffers an outage, one unit is lost. But when a line suffers a failure, one of the rubber bands snaps. A closely linked network with densely packed power plants can quickly absorb such a blow. But a large network can experience slow, long-range shock waves that subside slowly.

"If 3,000 MW are lost in Portugal, the network begins to oscillate," Völzke explains. Within seconds, 3,000 MW are swinging back and forth between Portugal and Poland, the other end of the network. Experts say this phenomenon has been occurring more frequently since the UCTE was extended eastward at the beginning of the 1990s. And they predict that if countermeasures aren’t taken it will increase when new partner countries are added.

Ending network oscillations. To enable Russia to join the Central European system, the country could be linked to a high-voltage direct-current connection that would not be susceptible to oscillations. This is how distances of up to 3,000 km are bridged between power plants and consumers in countries such as the United States and China (see Pictures of the Future, Fall 2003, More Power to You!). Aachen University’s Prof. Haubrich expects the network-oscillation problem to become even more acute if the frequently discussed Mediterranean ring is indeed added to the UCTE. This area includes Gibraltar, Morocco, Libya and Algeria, as well as Turkey. But this expansion would also be quite attractive to Europe, because low-cost wind power could be imported from windy Morocco.

Despite these difficulties, expansion seems within reach because a second solution to network oscillation (in addition to the direct-current approach) is available. FACTS (Flexible AC Transmission Systems) can be used as active damping elements, and, if carefully regulated, could check network oscillations in seconds. FACTS are a combination of controllable semiconductor devices and conventional components that can be integrated into points in the interconnected system. With FACTS, a network operator can quickly and precisely regulate how much output to transmit or to allow to pass between points. "Only a few of these expensive electrical dampers are in use across Europe now," Völzke says. "But given the further expansion of networks, we expect demand to grow." In the U.S., FACTS have been used for decades to stabilize the network.

Power in Reverse. Electricity networks are also facing a different challenge from below: the reversal of power flows. Electricity generally flows from big power plants to the consumer through a network of smaller and smaller branches. But if many decentralized electricity producers—consider fuel cells, for instance—feed large amounts of electricity into the network at some point in the future, the power’s direction of flow could sometimes reverse, at least at low voltage levels. The problem is that the system’s safety devices—the fuses in the network—are not designed to handle such reverse flows.

If a problem occurs in one network section today, the malfunctioning segment shuts itself down according to a hierarchical principle. But if the electricity flows in the opposite direction, the protective system simply isn’t activated. "The widespread inclusion of power from decentralized producers will eventually require new protection concepts," says Siemens Power Generation’s Rosenbauer. He adds that fast communication combined with decentralized management will allow operators to meet this challenge. In the process, the fuses could be linked by means of data lines, which would enable them to communicate directly with one another to locate problems. A defect could then be quickly repaired or held in check, says Rosenbauer. Such a software solution might have prevented the collapse in the United States.

Following the U.S. blackout, recommendations were drawn up by the members of the North American Electric Reliability Council (NERC), who represent the electric industry and other interest groups. In addition to supporting improved training for employees, the council said communication should be optimized between power plants, transformer substations and network operators’ fault sites. Effective information systems also should be developed, to provide improved and clearer reports on the state of the network while assisting workers with decision-making during emergencies.

Tim Schröder