Grid frequency
With mains frequency is in a power network , the frequency of the electrical power supply by means of alternating voltage , respectively. The network frequency is uniform in a power supply network and, with the exception of minor control-related deviations from the nominal value, constant over time. The network frequency is given in Hertz .
Network frequencies and networks
See also electrification # grid frequency .
In Europe, Asia, Australia, most of Africa and parts of South America , a network frequency of 50 Hz is used for the general electricity network, in so-called interconnected networks . In North America, a network frequency of 60 Hz is used in the public electricity network. The differences are due to the historical development history of the first electricity networks in the 1880s and 1890s and have no technical reason today.
Some railways such as ÖBB , SBB and Deutsche Bahn use a nominal frequency of 16.7 Hz for their traction power supply . In the past, the nominal railway network frequency was 16 2 ⁄ 3 Hz, which corresponds to exactly one third of the 50 Hz used in the interconnected network. For historical reasons, some railways and also industrial customers in North America are supplied with a network frequency of 25 Hz. The comparatively low network frequencies result from the technological development of the first electrical machines: At the beginning of the 20th century, electrical machines with greater power could only be built with these low frequencies. However, due to the great effort involved in the conversion, the low network frequencies introduced at that time are still retained today.
In special areas, e.g. B. in the on- board network of aircraft, higher network frequencies are common, z. B. 400 Hz, since smaller and lighter transformers can be built and the line lengths are short.
A pitch can be assigned to the mains frequency , 50 Hz corresponds almost to a Contra-G (‚G). The sound, which can be heard as a hum from a local transformer station , for example , has twice the mains frequency, namely 100 Hz, due to the magnetostriction of the iron core , and corresponds to the G.
Quality indicator
The grid frequency and its deviation from the nominal value is a direct quality indicator of the relationship between the instantaneous electrical output offered by producers such as power plants and the decrease in the instantaneous electrical output by consumers. Electrical energy can hardly be stored in interconnected networks ; it can only be distributed between generator and consumer. With the exception of the reactive power in the case of alternating current, the output power must be offset by the same power consumption at all times.
If there are deviations, this leads to a change in the network frequency in AC voltage networks: If there is an oversupply of electrical power, the network frequency increases; if there is an undersupply, it decreases. Normally, these deviations are minimal in the Western European network and are below 0.2 Hz. A frequency deviation of 0.2 Hz corresponds to a power difference of approx. 3 GW in the European network , which also corresponds to the so-called reference failure from the Continental Europe Operation Handbook and corresponds approximately to the unplanned failure of two larger power plant blocks. The task of power control in interconnected networks is to compensate for fluctuations over time and thus to keep the network frequency at a constant nominal value. The smaller a power supply network and the worse the network regulation works, the greater the fluctuations in the network frequency.
If there is a massive imbalance between the supply and demand of electrical power due to errors that cannot be compensated for, the result is correspondingly strong fluctuations in the network frequency, as shown in the figure on the right for the power failure in Europe in November 2006 . The graph shows the course of the network frequency for part of the Western European network: At the time of the failure, there was a massive shortage of electrical power and thus an underfrequency . In the same time frame there was an oversupply and an increase in network frequency in the Eastern European part of the network. During the period of the failure, the network was automatically divided into several autonomous segments by protective devices, which worked asynchronously with one another. By load shedding these network segments could be gradually synchronized and then connected together again.
Measures taken by electricity producers for network management
Primary control
If more power is taken from the network than is fed in via the generators, the missing power is taken from the rotational energy of the generators, which makes them slower and the network frequency drops. If the power consumption is too low or the feed-in is too high, the frequency increases. In the case of an intermediate chain of rectifier and inverter , however, this effect does not occur to this extent or can be controlled more easily.
The primary control has the task of limiting the deviation of the grid frequency. With proportional control , it changes the generator power proportionally to the frequency deviation from the setpoint.
Secondary and minute reserve
The secondary control has the task of bringing the frequency back to the setpoint. It is designed as an integral controller. As soon as the secondary control becomes active and its power reduces the frequency error, the primary control takes power back and is thus free again for the next use. If it is foreseeable that the secondary control power would have to remain active for longer (e.g. forecast errors in consumption, power plant failure or wind forecast errors), then the minute reserve (also tertiary control) is activated manually, whereby the performance of the secondary control power is automatically reduced. This frees up the secondary control power again for the next use.
Quaternary regulation
The network frequency in the European network is suitable as a timer for synchronous clocks because of the small deviations from the nominal frequency . Despite the small deviations, errors of a few seconds per day can occur. In order to keep the time error low, the network time deviation is recorded centrally as the difference between the coordinated universal time and the time determined on the basis of the network frequency. If the network time deviation exceeds ± 20 seconds, the nominal frequency for the frequency controller is reduced by 10 mHz to 49.990 Hz if the network time is ahead, and increased by 10 mHz to 50.010 Hz if the network time is lagging. This is also known as quaternary regulation . As a result, the network time slowly adapts to the coordinated universal time . The network time thus represents a very precise long-term time base with short-term fluctuations in the seconds range.
In Europe, Swissgrid records the deviations on behalf of the UCTE electricity network and coordinates the corrections.
Malfunction 2018
Although the averaged frequency deviations in the European network due to the quaternary regulation were low for years and the deviations from the international atomic time remained in the range of a few ± 10 seconds, there was a longer lasting underfrequency in early 2018. The difference in synchronous clocks means that at the beginning of March 2018 they are lagging behind by up to six minutes. On March 3, 2018, the highest deviation was reached with −359 seconds.
The cause of the deviation in the network frequency was a dispute in the control area "Serbia, Montenegro and Macedonia" (SMM) between Kosovo and Serbia over control power . Kosovo did not produce enough electrical power and Serbia initially refused to fill the gap by increasing the use of power plants. On March 8, the Association of European Transmission System Operators (ENTSO-E) announced that the irregularities between the two parties had been resolved. Less than a month later on April 3, 2018, the deviations were resolved by the quaternary regulation .
Criteria for choosing the network frequency
The choice of the network frequency is a compromise between various technical boundary conditions. The definition was made in the early days of electrification , i.e. at the turn of the century between the 19th and 20th centuries. The relevant boundary conditions were therefore those that arose at that point in time. Here are some of them:
- In contrast to direct current, alternating current can be converted into voltage using transformers . This enables relatively low and thus relatively harmless voltages to be carried to the end user, while high voltages can be used to minimize losses in overhead lines .
- Higher frequencies allow smaller transformer cores to be used. The transformers become smaller, lighter and cheaper with the same power . (This fact is nowadays exploited by switching power supplies .)
- Higher frequencies generate greater losses in lines due to the skin effect . In practice, this defines the maximum economic thickness of a line.
- The network frequency in a network system must be the same and synchronized everywhere.
- Higher frequencies correspond to shorter wavelengths . In spatially widely distributed network systems , phase shifts are more noticeable, which makes synchronization more difficult.
- The mains frequency is directly related to the speed and number of poles of generators and motors . An increase in the frequency requires either an increase in the speed (possible problems with centrifugal or centripetal forces and / or bearings) or an increase in the number of poles (greater technical effort and thus higher costs).
- A frequency conversion is complex. At the beginning of electrification, the only converters available were couplings from the motor and generator. Transformers are not able to convert the frequency. Nowadays, suitable power electronics ( converters ) are used for this . In the field of energy supply and for coupling asynchronous power networks, high-voltage direct current transmission (HVDC) and HVDC short coupling are used.
- By its nature, alternating current, in contrast to direct current, regularly has a zero crossing , which simplifies the switching of large currents, since it provides opportunities for an arc to be extinguished.
- As part of a three-phase system, alternating current offers the possibility of generating a rotating field. At least two phases are necessary for this.
- Arc lamps flicker when they are supplied with alternating current below about 42 Hz.
Measurement
Since deviations from the correct network frequency often lead to problems, especially in interconnected networks or when several power generators are connected in parallel , it is of immense importance to monitor the network frequency. In the event of problems, measures to protect the network can be initiated, for example load shedding .
There are several different types of instruments for measuring the mains frequency. Tongue rate meters are traditionally used primarily for manual reading . In larger networks, the network frequency is automatically measured at several points using digital measurement technology and electronic frequency meters and the course is recorded.
Analysis in forensics
Due to time-related slight deviations from the ideal network frequency, this has become more important in forensics in recent years . Even over a short period of time, the irregularities show a unique pattern. Depending on the quality and coding of an audio or video recording, filters can be used to draw conclusions about the frequency deviations that occurred in the power grid at the time of recording.
A kind of digital watermark is created , which is linked to a database such as B. is performed by network operators or criminal investigation offices can be compared. In the best case, a statement or at least a limitation about the place and time of the recording is possible.
See also
literature
- CENELEC (publisher): EN 60196: 2009-07 IEC standard frequencies . Beuth publishing house.
Web links
- Methods at the State Criminal Police Office - On the trail of crime . Süddeutsche Zeitung (report on the use of network frequency measurements to investigate crime in Germany)
- Online measurement of the network frequency:
- www.netzfrequenzmessung.de - Current network frequency with a representation of the course over time
- www.netzfrequenz.info - Network frequency measurement with various displays of the current network frequency
- Wide area monitoring - display of current European grid frequencies and voltage angle differences as well as deviations on the Swissgrid website
Individual evidence
- ^ A b BG Lamme: The Technical Story of Frequencies . In: Transaction . tape 37 , part 1. IEEE, Jan 26, 1918, p. 65-89 .
- ↑ a b Gerhard Neidhöfer: The way to the standard frequency 50 Hz. How the standard frequency 50 Hz emerged from a jumble of period numbers . VDE-Verlag, 2008 ( [1] [PDF; 1.8 MB ] Bulletin SEV / AES 17).
- ↑ C. Linder: Changeover of the target frequency in the central traction current network from 16 2/3 Hz to 16.70 Hz . In: Electric Railways . Issue 12. Oldenbourg-Industrieverlag, 2002, ISSN 0013-5437 .
- ^ Continental Europe Operation Handbook, Policy P1: Load-Frequency Control and Performance. UCTE OH, 2009, accessed June 10, 2018 .
- ↑ UCTE (Ed.): Final Report on the disturbances of November 4, 2006 . ( entsoe.eu [PDF]).
- ↑ Grid frequency measurement and information on primary control power
- ↑ swissgrid: Measurement of the grid time on behalf of UCTE ( Memento of the original from January 11, 2012 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.
- ↑ Swissgrid on grid time deviation ( memento of the original dated August 31, 2011 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.
- ↑ Strong deviations in the mains frequency. In: netzfrequenz.info. February 28, 2018, accessed March 9, 2018 .
- ↑ Manuela Nyffenegger: Why your oven clock suddenly slows down - and maybe soon will. In: Neue Zürcher Zeitung . March 5, 2018, accessed March 9, 2018 .
- ↑ Deviation in network time exceeds 5 minutes. In: netzfrequenzmessung.de. March 3, 2018, accessed March 9, 2018 .
- ↑ Continuing frequency deviation in the Continental European Power System originating in Serbia / Kosovo. ENTSO-E , March 6, 2018, accessed on March 9, 2018 (English, press release).
- ↑ ENTSO-E announced the cause of the network time deviation. In: netzfrequenzmessung.de. March 6, 2018, accessed March 9, 2018 .
- ↑ Markus Grabitz: Why so many clocks are just slowing down. In: Der Tagesspiegel . March 8, 2018, accessed March 8, 2018 .
- ^ Deviations affecting frequency in Continental Europe have ceased; ENTSO-E working on step 2. (No longer available online.) ENTSO-E , March 8, 2018, archived from the original on March 9, 2018 ; accessed on March 9, 2018 (English, press release). 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.
- ↑ The network time has normalized. Retrieved June 10, 2018 .
- ^ Digital audio recording analysis: the Electric Network Frequency (ENF) Criterion. In: journals.equinoxpub.com. Retrieved January 23, 2016 .
- ↑ Methods at the State Criminal Police Office: On the trail of crime . In: Süddeutsche Zeitung . ISSN 0174-4917 ( sueddeutsche.de [accessed on January 23, 2016]).
