At mains frequency, the frequency of the electrical power supply is designated by the AC voltage in a power supply. The grid frequency is in a power supply network uniformly and, except for minor technical control deviations from the nominal value, constant in time. The grid frequency is specified in Hertz.
Line frequencies and networks
In Europe, a mains frequency of 50 Hz is used for the general power grid, the so-called grid used. Some railways use deviating mains frequencies. Thus, for example, use the ÖBB, SBB and the German Railway for their traction power supply has a nominal frequency of 16.7 Hz was past the nominal rail network frequency 162/3 Hz, which corresponds to exactly one -third of the 50 Hz used in the grid. With the formerly used in rotary converters three phase synchronous machines, this frequency could be adhered to exactly. With the increased use of more efficient and maintenance poorer asynchronous converters the nominal orbital frequency was changed to 16.7 Hz from regular technical reasons.
In North America, we generally used a power supply with a mains frequency of 60 Hz for railways and industrial customers you will also find a mains frequency of 25 Hz before. The mains frequency can also be assigned a pitch, 50 Hz correspond almost a contra -G ( G ). The sound, which can be perceived, for example, from a local substation as humming, has because of the magnetostriction of the iron core twice the mains frequency, ie 100 Hz, and corresponds to one octave higher G.
The comparatively low, especially in the railway sector network frequencies resulting from the technological development of the first electric machines: the beginning of the 20th century you could build electrical machines greater power with these low frequencies. This is true since the mid-20th century, no technological limitation more and it would be possible for the railway sector higher frequencies - including 50 Hz - to set up, which would allow for easier power supply from the grid. Because of the large conversion effort, however, the introduced then low power frequencies are also maintained in the rail industry today.
In specific areas, such as in the electrical system of aircraft, higher line frequencies are common, for example, 400 Hz, because it can build smaller and lighter transformers and the line lengths are short.
The mains frequency and its deviation from the nominal value is a direct indicator of quality of the relation of the producers offered through such as power plants electric instantaneous power and the decrease in the electrical instantaneous power by consumers. Electrical energy can hardly be stored in interconnected networks, but only distributed between producers and consumers. The output power must, to the reactive power in AC, at any time facing an equally large power consumption.
If there are any deviations, which leads in AC systems to changes in the mains frequency: With a glut of electric power leads to an increase of the mains frequency, at a lower offer to a reduction. Normally, these deviations are in the Western European network is minimal and move below 0.2 Hz, the task of capacity control in interconnected networks is to compensate for the fluctuations and so to keep the system frequency at a constant nominal value. The smaller the power grid and the worse the line regulation works, the stronger fluctuations occur at the grid frequency.
If it can not be compensated by error to a massive imbalance between supply and demand of electric power, corresponding to strong grid frequency fluctuations are the result, as it is adjacent figure for the blackout in Europe in November 2006. Shown is the profile of the network frequency for a part of the West European power system: At the time of the failure, there was a massive under supply of electric power and thus to a lower frequency. In the same time frame it came in the Eastern part of the interconnected system to an oversupply and an increase in the power supply frequency. In the time domain the failure of the main power grid was automatically divided by protective devices in several autonomous segments, which worked asynchronously to each other. Through load shedding this Netzsegemente could be connected together gradually in line again.
Measures the power producers to the network management
Is the power removed more power than is fed via the generator, the loss of output from the rotational energy of the generator is removed, whereby these are slower, and the power supply frequency falls. When power is too low or too high extraction feed the frequency increases. In an interconnected chain of rectifier and inverter however, this effect does not occur to the same extent on or may be more easily controlled.
Primary control has the function of limiting the decrease of the grid frequency. As a faster proportional controller it feeds power into the grid, which is proportional to the frequency deviation from the nominal value.
Secondary and minute reserve
Secondary control is the task of the frequency due back to the setpoint value. It is designed as integral controller. Once the secondary control is active and with their performance reduces the frequency error, the primary regulation takes back power and thus free again for the next use. If it is foreseeable that the secondary control power longer have to remain active (eg forecast errors in consumption, power plant failure or wind forecast errors ), then the minute reserve is activated manually (even tertiary control ), whereby the power of the secondary control automatically goes back. Thus, the secondary control power is free for the next use.
The grid frequency is within the European network is very suitable for small deviations from the nominal frequency as a timer for synchronous clocks. Despite the small discrepancies, errors may range from a few seconds per day arise. To keep the time error small, the network time deviation is centrally recorded as the difference between Coordinated Universal Time and the determined based on the network time and frequency corrected by the control performance. This is also referred to as Quartärregelung. Exceeds the network time deviation / -20 seconds, then the nominal frequency for the frequency controller in pretriggered network time is reduced by 10 mHz to 49.990 Hz, increases in lagging network time by 10 mHz to 50.010 Hz. As a result, the network time slowly adjusts back to the Coordinated Universal Time. The network time thus represents a long-term very accurate time base with short-term fluctuations in the seconds range dar.
Criteria for selecting the mains frequency
The choice of the grid frequency is a compromise between various technical constraints. The determination was carried out in the early days of electrification, so the turn of the century between the 19th and 20th centuries. The relevant boundary conditions were therefore those that arose at that time. Here are some of them:
- In contrast to direct current can be reacted by AC transformers in voltage. This makes it possible that one performs relatively low and therefore relatively safe voltages to the end user, while one can use high voltages for the minimization of losses in transmission lines.
- Higher frequencies allow the use of smaller transformer cores. The transformers are thus less for the same performance, easier and cheaper.
- Higher frequencies produce larger losses in pipes due to the skin effect. Thus, the maximum economic thickness of a line is determined in practice.
- The mains frequency in a composite system must be common and synchronized.
- Higher frequencies correspond to shorter wavelengths. In spatially widely distributed composite systems, thus make more noticeable phase shifts, so synchronization is difficult.
- The grid 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 speed ( possible problems with centrifugal forces and / or storage ) or an increase in the number of poles ( greater technical complexity and therefore higher costs).
- A frequency conversion is expensive. One is for a converter. At the beginning of electrification stood as a converter only a coupling of motor and generator. Transformers are not able to convert the frequency. Today, this inverter and appropriate power electronics are used. In the area of energy supply and for coupling asynchronous power grids find the high-voltage direct current (HVDC ) and HVDC short coupling application.
- AC has, in contrast to DC a natural zero, which simplifies the switching large currents and provides for the deletion of an arc.
- AC as part of a three-phase system provides the possibility of generating a rotary field. For at least three phases are necessary.
- AC makes it possible to provide reactive power.
Since deviations from the correct line frequency often lead to problems, especially in interconnected networks or in the parallel connection of several power generators, it is of immense importance to monitor the mains frequency. Thus, measures to protect the network can be initiated in case of problems, for example, load shedding.
For measurement of the line frequency, there are several different types of instruments. Classically, and primarily for manual reading reed frequency meters are used. In larger networks, the network frequency is measured at several points automatically by means of digital instrumentation and electronic frequency meters and recorded the course.