Power plant management

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Power plant management describes the ecologically and economically sensible use of an existing power plant park as well as the planning of any new power plants that may be required.

In Germany, this process takes place with the EnWG 1998 through the economic optimization of power plant use on the basis of the costs incurred and the achievable prices in energy trading and on the EEX ( power plant use optimization ). Since then, the transmission system operators have been responsible for system security within the meaning of the Basics chapter , who run an auction for the procurement of control reserve and take control measures (e.g. network switching) on ​​the network side. A similar organization of the energy industry can now also be found in Europe and the USA.

In the case of a market-based organization of the energy industry, as it is prevalent in Europe and the USA today, market prices determine both the use of existing power plants and the planning of new power plants.

Basics

As much electrical energy as is needed by the consumers must be fed into an electrical distribution network . (See also control power (power grid) ). Smaller deviations lead to a change in the mains frequency and voltage , larger deviations can cause large-scale power outages .

Large amounts of electrical energy can only be stored through lossy conversion into other forms of energy. To cover peak loads , electrical energy is therefore converted into other forms of energy and temporarily stored in pumped storage power plants or compressed air storage power plants . Since this storage of electrical energy in large quantities is uneconomical and should only be used for peak demand, power plant capacities must be available at any time according to their dynamic properties in order to cover either this strongly fluctuating peak load or the uniform base load.

The main task of power plant management is to keep the feed-in and purchase of electrical energy in balance . Control measures are primarily carried out on the infeed side, but there are also limited, lesser options for load control.

To be able to guarantee balance, one must

• estimate in advance how much energy will be available at what point in time ,
• can react to unforeseen excess or under-consumption,
• be able to react to problems in the power grid , at the power plants and at the consumers .

For this you need

• a wide range of power plants that can perform different tasks and, ideally, work cost-effectively,
• Measurement and control equipment that the current state of the power grid to record controlling intervene and statistics can produce on the power consumption,
• regulated agreements with neighboring networks in order to be able to fall back on the reserves of these networks in an emergency.

Features of generation

Dynamic characteristics of thermal power plants

The power output of power plants cannot be changed at will. Depending on the design, certain limits must be observed.

• The output of lignite-fired power stations can be changed by around 3% of the nominal output per minute, that of hard coal-fired power stations by around 4%. The output can be changed between 40–60% and 100% depending on the design. The start-up times after standstill and the subsequent minimum operating time are each over two hours.
• Gas turbine power plants achieve rates of change of up to 20% of the nominal output per minute and are therefore particularly suitable for covering rapid load fluctuations. They are also characterized by very short start-up times of just a few minutes. The power can be changed between 20% and 100%. Therefore this type is very suitable for peak load power plants .
• A distinction must be made between nuclear power plants:
  • Modern pressurized water reactors achieve rates of change of up to 5% of the nominal output per minute. The power can be changed between 20% and 100%.
  • In the mostly older boiling water reactors , the minimum output is 60% of the nominal output, the rate of change is 4–6% per minute.
  • Above 80% of the nominal power, change rates of up to 10% of the nominal power per minute can be achieved with both reactor types.
  • Older nuclear power plants, especially those optimized for base load operation, such as the British AGR or the Russian VVER-440, have significantly less flexibility.
  • Even with the flexible German and French nuclear power plants with pressurized water reactors, change rates of more than 1.5% of the nominal output per minute are rarely run in normal operation to avoid excessive wear.

Power plant categories

Load coverage in the power grid

Power plants are divided into three categories based on their rate of change in output and their operating costs per kilowatt hour:

• Base load : Power plants that are operated as base load power plants provide energy cheaply or have a low rate of change in output. If possible, they are operated around the clock at almost full capacity. The output of base load power plants does not necessarily have to be easy to regulate.
  • Run-of-river power plants are preferably used to generate base loads. They enable very good power control with high load gradients, as there is no upstream process, as is the case with thermal power plants. This ability qualifies them as peak load power plants, but if they were to be throttled, energy would be wasted in the form of water flowing past them. The same applies to other power plants where the energy source is volatile. (Wind, geothermal and photovoltaic power plants)
  • Lignite power plants can be regulated by an average of around 3% per minute and must be operated with at least 40–50% of the maximum output. Coal power plants have high fixed costs (investment costs) and low marginal costs in operation.
  • German nuclear power plants can be regulated by 3.8–10% per minute, depending on the type and output range, and must be operated with at least 50–60% of the maximum output. When using the minimum load with condenser heat dissipation, the minimum power drops to 0%. In French pressurized water reactors, minimum outputs of less than 30% were achieved without opening a condenser. Nuclear power plants are also characterized by high fixed costs (investment costs) and very low marginal costs in operation.
  • Hard coal power plants can be regulated by an average of 4% per minute, the most modern units must be operated with at least 38% of the maximum output, older units have higher values. They are also used in the medium load range.
  • Geothermal power plants can also cover the base load thanks to the constantly available energy supply.
• Medium load : power plants, which are operated as medium load power plants, can be controlled over a wide power range, but the scheme operates with a certain inertia, with high load gradients are traversed by peak load power plants and a wide dynamic range is therefore not necessary. Medium-load power plants vary their output according to the daily curve according to a predetermined program, the so-called timetable . Hard coal power plants in particular are used as medium-load power plants. Lignite-fired power plants and nuclear power plants could also serve medium loads, but for economic reasons they hardly do so.
• Peak load : Power plants that are operated as peak load power plants must be able to follow every change in power in the network and therefore have a very high dynamic. Peak-load power plants are usually only used for a few hours per day, namely at the absolute consumption peaks and in the event of unplanned fluctuations in electricity consumption, especially when other power plants fail. Gas turbine power plants and pumped storage power plants are mainly used as peak load power plants because they can react extremely quickly. Gas turbine power plants have comparatively low investment costs and high marginal costs for generation, so that their operation is only worthwhile in expensive hours. Nuclear power plants that are operated with at least 80% of their nominal output could contribute to the peak load in the range of 80% to 100% of their output.

Operation schedule of the power plants

Load, solar and wind feed-in and residual load (red) in June 2016 in Germany (data ENTSO-E)

Many power plants can be controlled as required within the framework of the applicable technical restrictions. Unpredictable deviations result from failures and technical malfunctions as well as from the feed-in of solar and wind power plants. The feed-in of wind-dependent wind energy and solar- dependent photovoltaics is predicted for short and medium-term periods using forecasting systems (see e.g. wind power forecast and solar power forecast ). The task of the controllable power plant fleet is then to supply the residual load.

Large power plants can be expected to be controlled and fed in in line with the market through established market mechanisms or the dispatch of the affiliated company. This is not necessarily the case with small, decentralized generation. So-called virtual power plants are a solution here . These are couplings of many decentralized generating units, the flexibility of which is accessed via suitable communication infrastructures in order to also control these units based on the market. For example, combined heat and power plants can be switched on and off for a short time ( mini combined heat and power plants ).

Cold reserve

→ Main article : Cold reserve

Power plant preservations are carried out on power plants that are not used for an indefinite period of time. These power plants are also called cold reserves . These power plants cannot be kept economically ready for production, but they should not be dismantled either, but rather can be reactivated in a relatively short period of time in the event of an unexpected shortage. In some cases, the operators receive remuneration for this.

Features of the load

Predictable load behavior

Due to customers' habits such as food preparation at certain times, the use of electric lights, as well as production processes the industry , fluctuations result in power consumption, which are included in the statistics. These statistics are used to forecast the load .

The energy requirement does not only depend on the time of day, but also on the day of the week (working day / weekend), on vacations, public holidays, seasons, outside temperatures, wind strengths, bad weather, economic data, sales forecasts, etc. The more precisely you can determine the dependencies of electricity consumption, the more accurate Forecasts for the energy demand can be incorporated into the power plant management.

Unpredictable load deviations

The behavior of customers can differ significantly from the forecast at certain times (e.g. unusual weather conditions, major events) . The power plant management has to react to this at relatively short notice. It may be necessary to react immediately to failures of large consumers.

Load control

Load regulation is also achieved within certain limits by controlling the consumption, namely via:

• Ripple control systems : This enables consumers ( consumables ) to be switched on or off according to the requirements of the energy generation company. This is used for major industrial consumers such as aluminum and electric steel works . For a certain discount in the electricity price , the electricity supplier can reduce or increase the power consumption of these industries. This is also used for night storage heating . These can be loaded if there is no other consumer for the current power plant output. As a result, a certain base load can be generated in the power grid even at night, thus increasing the proportion of base load power plants such as nuclear power plants in the power plant fleet. According to the German Electrical Engineering Association, half of the potential for load shifting lies with energy-intensive companies and half with private households, trade and commerce and services. Load management can balance demand and significantly reduce the costs of the energy transition.
• Storage power plants : Pumped storage power plants, for example, can always switch to pumping mode when there is no consumer for the current power plant output in order to pump water into a storage basin located higher up. They are primarily operated at night in order to better utilize the base load power plants. They also serve to briefly support the frequency in the event of load or generation fluctuations. The very high costs of the systems and the connection to suitable geographical conditions are problematic.
• In the future it will be increasingly possible to control the demand of certain consumers (see research project E-Energy of the BMWi ). For example, night storage heaters can be remotely controlled via cellular networks and switched off when there is a peak in demand. In addition, it is hoped that electricity meters (e.g. with remote reading ) will influence the demand behavior of the electricity customers connected to them.

Importance of the network

Neighboring networks

In addition, parallel networks are also included in the management of the electricity network in order to obtain or supply base load, medium load or peak load electricity. In the event of a malfunction, neighboring networks can help to stabilize the frequency of the entire network by providing or consuming more power. In Europe, the UCTE is responsible for coordinating operations and expanding the European network.

Network control

The control of the network with load flow calculations , cross regulators , transformers and network connections has an important function in achieving and maintaining security of supply (see network control technology ). The aim is to avoid circular flows and to balance the load in the network. Under the catchphrase intelligent power grid (smart grid), infrastructure improvements (transformers, battery storage systems, cross-regulators) and control technology, also at the low voltage level, have recently been developed in order to better control feed-ins at the lowest voltage level.

Power plant management

Management of predictable loads

From the forecast of the electricity demand and the forecasts for the non-controllable feed-in (mainly photovoltaic and wind energy) for generation, the energy supply companies determine the daily rate for the electricity demand of the customers they supply. The electricity demand for the day is aggregated from these demand schedules. The supply curve, in turn, results from the bids of the power plant operators, which in turn are derived from the different marginal costs of the generating base, medium and peak load power plants. The market price, which results from supply and demand for each hour of the day, then controls which power plants are actually used.

Management of unpredictable fluctuations

If unforeseen fluctuations in electricity demand occur, an attempt is made to react to these fluctuations by regulating the power plant output (not renewable power plant output: wind and photovoltaics can only regulate down. Therefore, such power plants need so-called shadow power plants ). If the changes occur slowly compared to the forecast, the changes can be intercepted by adapting the “timetables” for the medium-load power plants. If the additional changes occur quickly, peak load power plants may have to step in in order to be able to react quickly to the changes.

If power plants fail, a high output has to be replaced in a very short time. Then fast-reacting types of power plants, such as pumped storage power plants, are activated. At the same time, increases in output are requested in gas-fired power plants that react more slowly and in medium-load power plants and, if necessary, an additional power plant is also started up from the so-called warm reserve. In parallel to increasing the output in the medium-load power plants and in the replacement power plant, the output in the peak-load power plants is reduced. The so-called control market exists for tendering and calling up such quickly available services .

If a major consumer fails, the control of the network must run the other way around: Shutting down the output of medium-load power plants. Since this does not work immediately, replacement consumers must be switched on quickly (e.g. pumped storage power plants) or any active peak load power plants must be shut down quickly. The substitute consumers can be switched off when the medium-load power plants have reduced their output. This control is also carried out via the control market.

Technical connections

Relationships in the turbine / generator system

The mechanical power that a power plant turbine has to provide in order to maintain a constant speed of the synchronous generator depends on the active electrical load of the consumers that are connected. The required torque and the speed of the turbine are proportional to the product of torque and speed (revolutions per second). ${\displaystyle M}$ ${\displaystyle n}$

${\displaystyle P=2\pi \cdot M\cdot n}$

The torque that a turbine with a generator set has to deliver depends on the current that is drawn from the generator, and thus on the electrical power that is drawn from the generator. As a result of the current consumption, an opposing torque is generated in the generator.

If the mechanical power supply in the turbine and electrical power extraction in the generator are in equilibrium, the torque of the turbine has the same magnitude as the “counter” torque generated by the generator. The turbo set (turbine and generator) runs at constant speed.

Overload

If additional power is drawn from the generator, the current in the generator increases. This in turn leads to an increased “counter” torque being generated by the generator. If this torque cannot be absorbed by a simultaneous increase in output on the turbine side, the entire mechanical generator / turbine system is braked due to the difference in torque. The difference between the mechanically provided power and the electrically extracted power is then extracted from the rotational energy of the mechanical generator / turbine system.

A new equilibrium is now established at a lower speed: The reduced mutual induction increases the current drawn from the network. If a higher torque is required with the same mechanical power, this can only be provided at a lower speed. This means that electrical overload in the network leads to underfrequency , if not, e.g. B. by more gas supply in the gas and steam turbine, whose performance is increased. The lower speed, in turn, means that a lower voltage is induced in the generator, so that this also reduces the power drawn on the electrical side.

Under load

In the opposite case, if less electrical power is consumed than is mechanically provided, the lower current on the intake side reduces the “counter” torque in the generator set and the generator / turbine system is accelerated. The difference in output is converted into additional rotational energy if the fuel or steam supply to the turbine is not reduced. Eventually a new equilibrium is established in which this lower torque is delivered at a higher speed. That is, electrical underload in the network leads to overfrequency, if not z. B. the flow of energy is throttled.

A higher speed also induces a higher voltage in the generator, which under certain circumstances causes an undesirable higher output for electrical consumers. For this reason, the mechanical power supplied to all types of turbine must be constantly regulated in accordance with the electrical load.

activities

The task of power plant management is to detect network overload or network underload in good time. The very precise measurement of the mains frequency is also used for this purpose. Even with minimal deviations of a few per thousand in the network frequency, measures are taken to compensate for the overload or underload discovered in the network. A deviation in the network frequency of more than 2% already triggers drastic measures to stabilize the network, such as B. Load shedding in power plants (with underload) or in the power grid (with overload). If the grid frequency deviates by more than 5% from the setpoint, the grid can no longer be operated in a stable manner; power plants switch off automatically to protect the systems.

literature

• Valentin Crastan : Electrical energy supply 1–3 . 3 vols., Berlin - Heidelberg 2012.
• René Flosdorff , Günther Hilgarth: Electrical energy distribution , Wiesbaden 2005, ISBN 3-519-36424-7 .
• Klaus Heuck / Klaus-Dieter Dettmann / Detlef Schulz: Electrical energy supply. Generation, transmission and electrical energy for study and practice , 8th revised and updated edition, Wiesbaden 2010, ISBN 978-3-8348-0736-6 .
• Wilfried Knies, Klaus Schierack: Electrical systems engineering. Power plants, networks, switchgear, protective devices , Munich 2012, ISBN 978-3-446-43357-1 .
• Panos Konstantin: Practical handbook energy industry. Energy conversion, transport and procurement in the liberalized market . Berlin - Heidelberg 2009, ISBN 978-3-540-78591-0 .

Web links

• Energy knowledge - power plants
• The power supply network from Udo Leuschner

Footnotes

1. ^ EnWG 1998. Retrieved on August 14, 2016 . 
2. ↑ regelleistung.net Internet platform for the allocation of control power. Retrieved August 14, 2016 . 
3. ↑ THE HARMONIZED ELECTRICITY MARKET ROLE MODEL. (PDF) (No longer available online.) Entso European Network of Transmission System Operators for Electricity, archived from the original on August 14, 2016 ; accessed on August 14, 2016 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  
4. ↑ Udo Leuschner: Electricity must be generated at the same moment in which it is needed. Retrieved August 26, 2016 . 
5. ↑ Flexibility of nuclear power plants ( Memento of the original from August 18, 2016 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. / Control energy . 
6. ↑ ^{a b c d e f g} [1] (PDF; 5.0 MB) Compatibility of renewable energies and nuclear energy in the generation portfolio.
7. ↑ ^{a b c d e} Archived copy ( memento of the original dated September 23, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. 
8. ↑ ^{a b c d e} http://www.et-energie-online.de/index.php?option=com_content&view=article&id=326:kernkraftwerke-und-erneuerbare-energien-die-maer-vom-systemkonflikt&catid=21: kernenergie & Itemid = 27  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.  
9. ↑ See page 8 f. ( Memento of the original from January 24th, 2009 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. 
10. ↑ Intelligent grids can drastically reduce electricity requirements , Spiegel Online, June 8, 2012
11. ↑ § 13 EnWG § 13 System responsibility of the operators of transmission networks. Retrieved August 30, 2016 . 
12. ^ Siemens: Control technology for smart grids. Retrieved August 30, 2016 .