|A DNV KEMA ultra high-voltage transmission test field in China
Credit: DNV Kema
head, New Energy Technologies & Director Strategic Research Unit
principal consultant, Smart & Super Grids
director ,Smart Energy
The dynamics of the energy flows through transmission and distribution power grids is rapidly growing to unprecedented levels. Several factors responsible for this encroach on each other and make their presence felt. Global demand for electricity will go up by more than 70 percent by 2035 and electricity from sustainable energy will increase from 20 to 31 percent, according to the World Energy Outlook 2012 of the International Energy Agency.
In some parts of the world present policy and regulatory schemes cause wind turbines, photovoltaic panels, hydroelectric power plants and geothermal installations to move towards a favourable economic position relative to conventional forms of generating electricity. This applies to large-scale as well as small-scale renewable sources. However, these decentralised energy sources are often subject to seasonal, daily and hourly fluctuations that have a big effect on the balancing and stability of the electricity grid. Therefore, as a key primary trend, we will need to focus on how to happily marry renewable and fossil-based energy supplies in the future.
A second trend is the increase in cross-border transmission. Almost all the electricity generated 10-15 years ago stayed in the country of origin, now 15-20 per cent of energy in Europe is imported. And this is increasing. The longer the transportation distance of electricity, the higher the voltage must be to limit loss of energy. How well is the present alternate current (AC) high-voltage grid capable of meeting this? Dealing with the increase in cross-border and long-distance transmission is a second task that the system planners and operators must face.
At the same time, energy customers are showing a tendency to generate more electricity themselves by means of photovoltaic panels installed on the rooftops of houses and offices, or micro-combined heat and power units. Moreover, energy customers will also consume more electricity with their electric cars and heat pumps.
Invisible energy flows
In this dynamic situation, distribution-system operators (DSO) have to manage their grids locally. This is also necessary to prevent a further increase in the fluctuations in the bulk power grids. For, contrary to the situation before, transmission-system operators (TSOs) no longer “see” everything that happens locally. It is estimated that, at present, 10-15 per cent of the energy flows are already invisible to a TSO. The question is: how do TSOs get a grip on a situation they have no influence on? With the increase in uncontrollable sustainable energy sources, the number of controllable conventional power stations will naturally decrease as older plants become uneconomical to operate. The synchronous generators of the conventional plants that AC connected to the grid will be replaced by generators connected to the grid by means of power electronic converters.
Such generators do not contribute to the inertia (rotating mass) of the grid. This inertia can be visualized as an enormous flywheel with 3,000 rotations per minute (a frequency of 50 Hz), which acts as the grid frequency’s stabiliser. At a frequency of 49.8 Hz, the full immediately available reserve power (primary reserve) is deployed. If the frequency falls below 49.2 Hz, automatic protective devices spring into action and switch off load. The decrease in the inertia in relation to the total installed generation capacity will cause the frequency of the European interconnected high-voltage network to make larger excursions in response to changes in the system. Reserves will be called on more often and will be more prolonged, which will increase the risk for depletion and lead to load shedding, or even a blackout.
It is clear that the developments take place so fast that system operators have trouble keeping up. However, they have little choice. When we plunge into the problems, a number of issues are of importance.
First: maintaining the balance between demand and generation, and thereby the frequency. Balancing is a real-time process done on a system-wide (eg EU) scale in the total connected system. As a result of previously mentioned developments, there is pressure on the tuning of supply to demand. This is complicated further by the fact that the changing flows – including cross-border flows – require sufficient transport capacity on the connections. It is illustrative that TSOs have to intervene increasingly often in operations.
Capacity management in the distribution system is also high on the agenda. When all heat pumps in a neighbourhood are switched on simultaneously, electricity consumption peaks and the local grid is overloaded. Overloads also occur when too much is generated.
|Traditional high-voltage transformer arriving by boat at DNV KEMA
Credit: DNV Kema
Just as crucial is voltage control. The presence of voltage is essential for electricity to be transported over the AC grid. This requires reactive power that reverses the polarity of the required electromagnetic field 50 times per second. No reactive power means no voltage, and without voltage there is no transmission. A short-circuit requires a great deal of reactive power to prevent the voltage collapsing in the rest of the grid, and to give the protective system time to react.
Keeping the grid energised requires reactive power, and therefore reactive current. The higher the voltage and the longer the distance, the more reactive power is required (see Figures 1 and 2). At some point the entire current-carrying capacity is needed for the reactive current, and the maximum distance is reached. Undersea transmission connections that link wind parks with the mainland need significant amounts of reactive power, limiting the distance between the wind farm and the substation. Although the need for reactive currents of overhead high-voltage lines is much lower, the distance is limited too. It is for a good reason that pioneering country China has built only a small number of HVAC connections over a distance of 1,000 km.
|Figure 1: In 2012, the break-even point is typically in the 500 km to 800 km range for overhead lines, and 60 km to 100 km for cables.|
|Figure 2: In DC transmission, 2% line loss per 1000 km is a common design figure, as opposed to AC power, which has four to 6% line loss per 1000 km, though achieving that level of power loss would be expensive. As transmission distances increase from 500 km to 1000 km to 2000 km and higher, this loss will add up more and more. Losing 2% of transmitted power, while certainly a loss, is acceptable for most utilities. However, losing 5-6% over 3000 km with DC transmission, 10-15% with AC transmission, begins to tip the economic balance.|
What is more, there are increasingly fewer operational machines that supply the grid with reactive power automatically. Wind turbines and photovoltaic panels connected to the grid by power electronic converters, for instance, do not, in principle, supply reactive power (settings of the electronics control), and may affect the voltage quality.
Another issue is that voltage control has to take place locally. Reactive power must be fed into the grid close to where it is needed, because the transport of reactive power results in large drops in voltage. This affects both the distribution and the transmission grids.
|DNV KEMA testing laboratories
Credit: DNV Kema
Distribution and transmission system operators are therefore faced with the question of which investments should be made to control the technical and economic risks. And suppliers, manufacturers, (local) authorities, property developers and end-users also need to be sure of where their opportunities lie. For the energy system of the future, we need to think small on the one hand and big on the other. System operators need to have more means of control and regulation, since at present the distribution grid lacks this intelligence, and the AC transmission grid can only be controlled to a certain extent.
At the distribution side, smart grids as part of a smart and controllable energy system are on the up. The distribution network is more and more equipped with real-time intelligent monitoring, advanced metering infrastructure, control and storage possibilities – from the operating centres to the meters. The point of no return is in the past; smart grids are showing growth rates of up to 30 per cent a year.
With smart grids, the capacity management problem can be dealt with. The grids enable peaks to be flattened, better use to be made of the existing capacity, and more fluctuating sources can be fed in, which makes it possible to avoid heavy loads on the grid. It is crucial that demand follows supply, rather than vice versa. A market mechanism is needed to encourage customers to deal with their energy consumption in different ways. By creating a solid demand response system and a market by means of incentives, supply and demand can be fine-tuned.
Operators of smart distribution networks have to find a solution for voltage control as well. Because sustainable energy sources are just as responsible for a stable grid, operators demand that the voltage quality be kept up, and also make requirements on the so-called fault ride-through: sustainable generators must keep supplying reactive power for a certain amount of time when short-circuiting occurs. With power electronics in smart grids, this can be controlled more accurately.
To face up to the challenges on transmission level, mega electricity grids – “super grids” – are being developed to transport large amounts of energy over long distances. Super grids contribute to a better capacity management because they continue to integrate regions, energy markets and regulatory regimes, and therefore create a more balanced spread of a higher demand and a larger supply of controllable and fluctuating sources.
A solution for the reactive power issue is to switch to direct current so that the EM field has to be built up only once. Moreover, direct current is more easily controlled this way. The conversion from the present alternating-voltage transport grid to a direct voltage high-voltage (HVDC) grid is fairly simple, without the need for new line routes with accompanying licence procedures. Moreover, HVDC connections can transport 40-200 per cent more energy.
The drawback is that complex and costly converter stations are required to convert AC to DC and vice versa. Building a meshed DC grid also continues to face a number of problems.
Therefore, it is best to start with the introduction of DC in a few smartly chosen places in the AC grid, and make a hybrid grid this way. Smart grids and super grids belong together. As smart systems are installed across the distribution grid, the ability for the TSOs to see into what is happening at the distribution grids increases. Power can be rerouted from traditional generation sources or even from another distribution grid to balance the load. Long-distance transmission, along with smarter grid technologies, is making this sort of load balancing much more practical.
While smart grids make long distance transmission more economical and efficient, the benefits are reciprocal; in many ways, long-distance transmission makes other smart grid projects easier to implement. The integration of renewable energy, for example, becomes much more feasible, as long-distance transmission lines enable access to many remote power sources. Long-distance transmission also allows greater diversity in energy markets, as trading and variable pricing models become more and more prevalent. By increasing the number of available energy markets, power prices could be reduced as a result of added competition. If the challenges are addressed and an economic balance can be found, long-distance transmission will be a critical step to adding flexibility, reliability and choice to the smart grid.
However, super grids and smart grids raise new technical and economical questions that still need to be addressed. The major challenge lies in realising super grids and smart grids while existing grids are still operational. Research and innovations are required to make the right choices and understand the implications of the transition towards a reliable and sustainable system. A research and innovation team for smart grids and super grids was recently set up at DNV KEMA Energy & Sustainability in Arnhem. This international unit works with knowledge institutions and businesses to fill the gap between long-term and short-term solutions.
On the transmission level, it is necessary to acquire a deep insight into reliability risks due to increasing grid dynamics. At present, no one can make a reliable quantitative statement on the grid’s stability. This is urgently needed to make predictions on the occurrence of a blackout, especially for the transmission grid. Developing such a risk estimator is a first task for the research and innovation team.
Super grids allow better opportunities for control and steering. They also contribute to the stability of the transmission grid. However, what about the interoperability of the control systems for HVDC transmission? After all, to avoid increased reliability risks, they should not be hindering each other. At DNV KEMA we can play a third-party role to validate the interoperability of the systems.
Reliability is also still questioned. A super grid cannot be reconstructed on a table. Major investments have to be made to test the separate super grid components. Recently, DNV KEMA announced that it was to invest €70 million euros in an HVDC laboratory. Since it is impossible to reconstruct the complete system, models of a super grid are needed. To build validated simulators, measurements have to be done in the field that provides the input for the laboratories. Consequently, the field becomes part of the test system.
Smart grid tools and models
The transition to smart grids on distribution level changes the entire playing field. New parties enter the market for which energy is not a core activity – such as end users, local authorities, project developers and IT manufacturers. A smooth introduction without compromising on reliability requires collaboration between all parties. They need each other’s knowledge to provide added value. Not only technicians for grid integration and data management, but also (socio-) economists who can make the business case and translate it to the market and the end-user.
For smart grids, the absence of tools and simulation models is a shortcoming. Good models are a must to be able to address the uncertainties of existing and new parties. On one hand, DNV KEMA’s research and innovation team will focus on technical models, such as micro-grid models and dynamic load-flow calculations, which used to be needed once only, but are now required permanently. On the other hand, the team develops market models such as user profiles and scenario development to obtain insights into the size of the smart grid market, manufactures risks and appropriate incentive mechanisms.
The design of an advanced demand-response system also has priority. How large are the fluctuations? Can demand response flatten peaks, and does it allow more sustainable suppliers and consumers? How much can you save with demand response? How can it help prevent overloading of the grid?
Data management is another issue. How do you process and analyse the bulk of data supplied by sensors in the network into meaningful information, such as for smart asset management? The question of who has access to what data also needs to be addressed. With smart grids, privacy and cyber security are of major importance. It is essential to minimise the risk of hackers gaining access to confidential information and operating equipment.
To guarantee the reliability, voltage control and interoperability of smart grids, it is important to test the components and the entire system. For this, the researchers work closely with DNV KEMA’s Smart Grid Interoperability Laboratory and the Flex Power Grid Laboratory.
Theo Bosma is head of the New Energy Technologies & Director Strategic Research Unit, Peter Vaessen is principal consultant, Smart & Super Grids and Frits Verheij is director, Smart Energy. For more information on DNV KEMA Energy & Sustainability, visit www.dnvkema.com.
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