Medium voltage network
Medium-voltage networks are part of the electricity network for the distribution of electrical energy on routes ranging from a few kilometers to 100 km in rural areas. They are usually operated with high voltages of 10 kV, 20 kV or 30 kV. A medium-voltage network is typically used to supply electrical energy to a region that comprises several localities or, in cities, a district. Furthermore, 15 kV with the special frequency of 16.7 Hz and 25 kV with 50 Hz are used as contact line voltage in the traction power supply of mainline railways.
In electrical power engineering, medium voltage is understood to mean a high voltage in the range from 1 kV up to and including 52 kV. The upper limit is not clearly defined. The term medium voltage is not standardized or not precisely defined within the limits. The 60 kV lines that still exist in Schleswig-Holstein are used as medium-voltage lines, while in France 63 kV lines are used as high-voltage lines.
In some areas, such as in the Münster administrative district, there are two medium-voltage levels with 30 kV and 10 kV. The former is used for coarse distribution and the latter for fine distribution.
Network construction
Feed
Medium-voltage networks of the regional distribution network operators are usually fed in transformer stations from the higher-level high-voltage network such as the 110 kV level (distribution network level) and are used to feed the regionally distributed transformer stations that supply the individual low-voltage networks to the end customers. Medium-voltage networks do not serve the supraregional exchange of electricity. Larger electricity customers, such as industrial companies, hospitals, but also larger swimming pools and larger radio towers, usually have their own medium-voltage connections with their own transformer station.
The capital needed to supply power transformers for energiser are in the range of 20 MVA to 60 MVA, concrete values depend greatly on the particular service provider. As a rule, these power transformers are also the last level on which load-dependent voltage fluctuations can be compensated for using a step switch . If necessary, electronic medium-voltage regulators can be used for large feed-in power from decentralized regenerative energy sources. This allows the voltage to be raised or lowered to a target value in individual sections far away from the central substation. An electronic medium voltage regulator consists of two inverters which are connected via a DC voltage intermediate circuit. Depending on the load condition, these voltage fluctuations regulate the load side.
Medium-voltage networks always have a switchgear in front of each feed-in and reference point . These can be designed as open air systems or in the form of a cabinet.
Power transformer for feeding a medium-voltage network.
Electronic medium voltage regulator for 20 kV
Single line diagram of an electronic medium voltage regulator
Encapsulated switchgear for 20 kV
Security of supply
To ensure security of supply, medium-voltage networks are generally operated with earth fault compensation , and in exceptional cases with an isolated star point if the area is not very large - this is typically the case with industrial networks . In addition, so-called break switches can also be used to increase the average security of supply for overhead lines . In an extensive medium-voltage network, this limits a long-term power outage to a smaller supply area.
Network topology
Medium voltage networks in the topology as a radial network or ring network carried out, especially in urban areas loops are common. Ring lines have the advantage that a line section can be switched off, for example as a result of cable interruptions or for maintenance work, without the supply to the subordinate low-voltage networks being interrupted. In addition, medium-voltage networks can be fed from several points, and smaller power plants such as wind turbines , biogas plants and large photovoltaic systems also feed into regional medium-voltage networks. The legal framework for this is specified in Germany in the medium-voltage directive .
Control signals
At the medium-voltage level, the so-called ripple control signals are sometimes fed in, with which various switching operations at the low-voltage level, such as tariff changes or switching the street lighting on and off , are triggered. In most cases, however, these signals are now transmitted using radio ripple control technology .
cables
Medium-voltage networks are usually designed as underground cables in densely built-up regions . Line sections that lead through forests are increasingly being laid as underground cables in order to avoid earth faults in the event of a wind break . In free rural regions, medium-voltage lines are designed as overhead lines , also for cost reasons . Medium-voltage overhead lines are installed on wooden, concrete or lattice masts. Their conductors can also be placed on masts that also carry circuits for higher voltages. There are also overhead lines designed for 110 kV, which will be operated with medium voltage until a possible later changeover. Medium-voltage overhead lines practically always use single conductors and usually only carry one circuit, even if there are lines with 2 (and rarely) even more circuits in many places. Earth wires are rare in pure medium-voltage overhead lines, as are aerial cables for data transmission. Medium-voltage overhead lines have many branch lines, which often have mast separators in order to be able to selectively remove this line from the network. Another typical element of medium voltage lines are mast transformers for supplying power to smaller consumers. In the case of overhead lines, methods such as automatic reclosing are also used in the medium-voltage network, since the cause of the fault, such as a branch that has fallen onto the overhead line, a lightning strike or the like, often disappears by itself, and thus the security of supply can be increased.
Underground cables for 10 kV, as Höchstädter cable referred
Medium-voltage overhead line mast for 20 kV, as is common in rural regions.
Feed converter for ripple control signals
Measuring transducer from 30 kV to 100 V / 50 Hz
Switch cell in open design for medium voltage
Mast separator for 20 kV in rural areas
.JPG)
Substations
Transformer stations in medium-voltage networks are usually housed in closed buildings, which in rural areas are often implemented as tower-like buildings with medium and low-voltage lines at the wall. Modern transformer stations, especially in cities, are housed in container-like prefabricated buildings. Medium-voltage substations can also be located in buildings. Another design of the substation is the transformer mounted on a mast, the so-called mast transformer. Substations for lines from 30 kV can also be designed as overhead line switchgear. This is the rule for 60 kV lines, as it is for 110 kV lines.
Country-specific variants
In rural areas in Australia or Canada in particular, large medium-voltage networks are implemented as a single-phase single-wire earth return system (SWER) for cost reasons . Only one conductor is laid on the high-voltage side and the grounding is used as an operational conductor.
Even the medium-voltage networks implemented as three-phase networks are often so spacious in these countries that an impermissibly high voltage drop occurs along the line. To compensate for this voltage drop, therefore spatially distributed autotransformers (autotransformers) connected in series in the pipe, shown on a wooden pole with three individual transformers for 14.4 kV as in the illustration in a Canadian outdoor apparatus. The voltage can be adjusted and increased by up to 10% depending on the specific load situation.
natural reserve
On December 31, 2012, Section 41 of the German Federal Nature Conservation Act came into force, stating that appropriate protective devices such as bird protection fittings must be attached to overhead lines of the medium-voltage network with a high risk for birds .
literature
- Rene Flosdorff, Günther Hilgarth: Electrical energy distribution . 9th edition. Teubner + Vieweg, 2005, ISBN 978-3-519-36424-5 .
swell
- ↑ a b Rene Flosdorff, Günther Hilgarth: Electrical energy distribution . 9th edition. Teubner + Vieweg, 2005, ISBN 978-3-519-36424-5 , chapter 1.
- ↑ Additional conditions for connection to the medium-voltage network of Schleswig-Holstein Netz AG ( Memento of the original from April 8, 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.
- ↑ E-ON Hanse AG's medium-voltage connection usage contract
- ↑ RWE and Westnetz install new switchgear for the substation
- ↑ Dortmund University of Applied Sciences, Department of Information and Electrical Engineering. RWE Rhein-Ruhr Netzservice GmbH (Ed.): Pause switching procedure in the medium-voltage distribution network . Issue 5, No. 109 . ew - magazine for the energy industry, 2010, p. 36-40 ( online [PDF]).
- ↑ Vereinigung Deutscher Elektrizitätswerke (Ed.): Self-generation systems on the medium-voltage network, guidelines for connection and parallel operation . VDEW - eV, 1998 ( svo-netz.de [PDF]). ( 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.
- ↑ Medium-voltage networks ( Memento of the original from September 8, 2011 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. and operating voltages at Salzburg Netz GmbH.
- ↑ §41 Bird protection on overhead power lines
