ABB’s view of available options for securing the European power grid is set out in this document.
Changes in the power industry
Since the early 1990s, there have been two fundamental changes in the power supply industry. The first is the liberalization of electricity markets, which has happened across the world. It has led to the separation of power generation (including wholesale and retail) and transmission and distribution (T&D). This “unbundling” of the energy business has reduced the coordination between power generators and transmission system operators.
The second important change is a reduction in the use of fossil fuels, through improvements in the efficiency of conventional generation and supply systems, and by increasing use of renewable energy sources. More combined heat and power plants have been built, and wind farms are growing rapidly.
Over the last 15 years, wind power has led growth in the renewable energy sector. This is because of the abundance of primary energy and improvements in technology that have made harvesting wind power more competitive. The use of renewable energy sources has also become a central theme in government energy policies around the world and significant investment subsidies are available in some regions.
At the end of 2006, the capacity of the world’s wind farms was around 74,000 MW. In Germany, which has the world’s highest installed capacity, wind power can provide up to 20,000 MW of power (figure 1) — an impressive figure considering the peak load for the entire country in the winter of 2005/2006 was almost 77,000 MW.
figure 1 : Growth of wind power in Germany
(source: Bundesverband Windenergie e. V.)
Consequences of increased renewable generation
Wind power is characterized by its dependency on local wind conditions, which are rather unpredictable, and the fact that many of the best locations for wind farms are remote, far from the centers of demand. This is entirely different from the set up in conventional power systems, which grew up around a network of extremely reliable thermal power stations, near to their customers. This avoided the long-distance bulk transmission of electricity for which the system was never designed.
To cope with erratic renewable power supplies from far-flung locations, the capacity and flexibility of the established transmission network must be increased. Losses incurred by long-distance transmission must be minimized and periods of calm weather must be compensated by reliable back-up supplies. Renewable energy must be at least as reliable as conventional supplies.
Back-up power for renewable energy supplies can be provided by energy storage facilities, load management or conventional power stations. Since the technology required to store energy in sufficient quantities remains prohibitively expensive, load management and the use of conventional power stations are the only options.
In conventional power stations, this means adapting operating procedures to allow the rapid start up of reserve capacity to meet peaks in demand, while maintaining the plant’s own efficiency.
Figure. 2 : situation in the UCTE grid during disturbance of November 4, 2006 (source: UCTE)
The impact of independent generating units
The European blackouts of November 2006 demonstrated the impact of independent generating units on an interconnected power system: Following the planned shut-down of one high-voltage line in Germany, a number of other lines tripped unexpectedly and the European grid was split into three separate areas, with severe power imbalances in each area (see fig. 2). As a result, blackouts of between 20 and 90 minutes were experienced in more than 15 million households and there was widespread disruption of transport and communications systems. One of the major factors contributing to the severity of the disruptions was a lack of coordination between European transmission system operators.
After the initial disturbance, wind power in northern Germany was automatically cut from the grid, without consulting the system operators. In this particular case, cutting the power did stabilize the system (the wind farms happened to be in an area of over production), but such a move could just as easily have made things worse.
Restoration of power supplies was delayed by poor communication between the generating companies and the operators responsible for each part of the grid: Operators did not have access to real-time data from power units connected to the network.
Clearly, grid management systems need to be changed and equipment needs to be updated. To quote a report on the blackouts by UCTE (the Union for the coordination of transmission of electricity);
“The growing market activities and the fast and successful development of regional intermittent energy generation with low predictability (wind power) led to [a] significant increase [in] cross-continental power flows. Even though the grid was originally developed for mutual assistance, it has now become the platform for shifting ever increasing power volumes across the continent. Against this background, grid operation has become much more challenging.“
New options for the transmission grids
The new demands of renewable energy and power trading mean that interconnected transmission grids can no longer rely on a regional balance of supply and demand. There are several possible approaches to rectify the situation.
Some of the challenges could be met by building new power lines, but this approach tends to run into problems with local planning authorities and the low public acceptance of high-voltage power lines in residential and environmentally sensitive areas. A preferable alternative would be to make better use of the existing lines.
To this end, Flexible Alternating Current Transmission (FACTS) devices, such as Static VAR Compensators (SVC) and series capacitors, could be installed. These improve voltage stability and allow more power to flow on existing lines. They also defend the grid against “system swings” and other disturbances.
The need to enhance bulk transmission capacity, prompted by the concentration of wind power in coastal regions, far from demand centres, can be met either by reinforcing the existing grid, or by constructing an overlay network dedicated exclusively to bulk transmissions. Bearing in mind that the existing grid was designed for a completely different purpose, i.e. pooling reserve rather than long-distance transmission, the construction of an overlay grid, or at least some dedicated backup transmission lines, might be the better solution. Such a grid could either be an alternating current (AC) system, with raised voltage levels, or a high-voltage direct current (HVDC) system. The latter offers higher transmission capacity per corridor and does not consume reactive power. If used in conjunction with IGBT (Insulated Gate Bipolar Transistor)- based voltage source converters (VSCs), an HVDC system can also offer reactive power support to local distribution networks.
The VSC-HVDC system is an attractive solution for regions in which wind farms are concentrated, i.e. remote areas at the periphery of the grid. ABB’s VSC-HVDC system is called HVDC Light©. It has been available since 1997 and can now achieve power ratings of 1,100 MW.
The ability of HVDC Light® to generate reactive power and to start up from zero output (i.e. blackout conditions) makes the technology particularly suitable for the connection of off-shore wind parks to on-shore transmission grids. In many cases makes sense for the HVDC links to extend inland, beyond coastal regions, to make high-performance connections with the grid, effectively realizing the concept of an overlay grid.
More transparency for systems operators
As grids transmit more and more energy from renewable sources, they must adapt not only their transmission structures, but also their response to greater and faster changes in load flow caused by intermittent generation. Effective grid management will be achieved only by improving data collection and communication procedures, providing operators with up-to-date information on the status of grid components. New wide-area monitoring systems can provide such a service, using time-synchronized phasor measuring units (PMUs) at strategic positions in the network (fig. 3). The units capture real-time data on the status of the grid, allowing operators to identify and deal with faults before they develop into wide-spread disturbances.
fig. 3 : Wide area monitoring system PSGuard (green) with synchronized phasor measurement units (PMU) (blue).
During the 2006 blackouts, system operators were unaware that the grid had split into three asynchronous sub-systems and restoration of the grid was delayed. The use of a wide-area monitoring system and improved communications would have avoided such a situationlead to much better information and thus faster system restoration. As the volume of long-distance transmissions across the grid continues to increase, such disturbances will become more frequent. The effective collection and communication of system-critical data to system operators is essential for the future stability of power supplies and is mentioned explicitly in the UCTE’s report on the 2006 blackouts.
Challenges for thermal power stations
As we have seen, increasing use of erratic primary energy sources such as wind places additional demands on a power system’s control equipment. These demands also effect reserve generation capacity (i.e. capacity that provides short term power to compensate frequency disturbances/unplanned outages and load fluctuations). This is of particular relevance in systems that rely on coal-fired plants for their reserves. Starting up such generators in response to surge in demand can be expensive and reduce the overall efficiency of the plant. However, the application of modern control technology to optimize the process can provide considerable savings. ABB’s model-based systems, MODAN and MODAKOND, can be used to fine tune the operation of turbines and boilers in steam power plants, leading to softer start up and shut down procedures, which reduce wear and tear on equipment, and contribute percentage savings in the auxiliary power needs of the plant. In throttled operating mode – vital for providing spinning reserves – absolute efficiency gains of 0.48 % have been demonstrated.
As the use of unpredictable wind power grows, reserve capacity becomes increasingly important. The modernization of existing plants to improve the reliability and efficiency of their reserve capacity may be a technical necessary to maintain the security of supply, but it is also a business opportunity for the plant operator – in a liberalized energy market, reliable reserve capacity is a commercial commodity. For example, in one case, the coordinated modernization of the turbine, boiler and unit control boosted the plant’s maximum power adaptation rate (i.e., the rate at which a plant can raise its power output) from 2 MW/minute to 50 MW/minute, and the control accuracy from +/-5 percent to +/-0.5 percent. The modernization also allowed this power station to provide primary and secondary reserve control.
Power systems of the future will be comprise more and more renewable energy, distributed, independent generation capacity and, in many cases, significant volumes of power trading. This will result in new requirements for the conventional generation sub-systems and the transmission grids because of the regional dependency on availability of primary energy, unpredictable primary energy sources, limited information on the behavior of distributed generation systems and the impact of energy trading. For the system operators, the most important change will be the predictability of the system. In the past, the only outside variable the operators had to contend with was erratic consumer demand (load), everything else could be predicted. This led to the evolution of a system organized entirely around the load. In future operators will have to consider many more variables outside their control. They will need to balance the system to provide the secure, reliable and efficient power supply that modern industrialized societies have come to expect and rely on.
Technologies already available to meet these challenges include modern control systems for power stations, solutions to enhance transmission capacity and improved data gathering and communication systems. The challenge of the coming years is therefore not the development of more technology, but the application of the available solutions into existing networks. A comprehensive understanding of existing power systems will be needed to implement the right solution in the right places at the right time.