How it works: overhead lines 2 How it works: overhead lines

In our society, electricity is undeniable. At home, we simply need a wall socket and we are free to power or charge our electric appliances. However, to make sure that electricity flows without interruption, it must be transported from the point of generation to the point of consumption. How does the current arrive there? What points of contact does it pass through before eventually arriving at the home?

Energy is transported from the generator to the consumer via power lines and switchgear. 50Hertz operates the overhead lines at its highest voltage level. These “electricity highways” are the backbone of the energy supply. They enable a reliably functioning national and European electricity market and ensure that ever more renewable energy is integrated into the power system.

By 2030, energy from renewable sources should cover 65 percent of the German consumption of electricity. Germany’s federal government agreed to this in 2018. At the time, over 56 percent of the electricity in the 50Hertz grid area was already generated from renewable sources. Most of these can be found in the north-east of Germany, for the most part wind turbines. Wind farms are also rising in the Baltic Sea. We connect them to the 50Hertz power grid. To safely and reliably transport the generated electricity to the consumption centres, we are modelling our technical installations and systems to successfully deliver these transmission needs. How it works: overhead lines 3

Transmission system operator in service of society For a successful energy transition

50Hertz operates the transmission grid in the north- Our company east of Germany and we efficiently maintain our grid, expand it to meet the demands and ensure that we At 50Hertz, over 1,100 employees at ten sites ensure maintain the electrical balance between feed-in and that electricity flows around the clock in Berlin, electricity consumption in our grid area. We involve the Brandenburg, Hamburg, Mecklenburg-Western public at an early stage if and when the situation arises Pomerania, Saxony, Saxony-Anhalt, and Thuringia. that we need to extend or upgrade our lines. We supply power to around 18 million people. Our grid is one of the most modern in Europe and has an As a transmission system operator, we provide a electrical circuit length of about 10,200 kilometres, reliable infrastructure that flexibly responds to fluctua- or the distance between Berlin and Rio de Janeiro. tions. To this end, we are in constant exchange with our partners, in Germany and abroad. As part of the We are located in the heart of the continent and European interconnected grid, the 50Hertz transmis- are the central hub between northern, eastern and sion system is directly connected to our neighbouring central Europe. Furthermore, we are the largest countries such as Poland, the Czech Republic, Den- export region worldwide for renewably generated mark and (in the future) Sweden. electricity. We are responsible for the construction and operation of lines to connect wind farms at sea to the power grid onshore. Elia ( 80 % ) KfW ( 20 % ) By openly discussing the challenge of grid develop- ment and offering a reliable infrastructure that 50Hertz flexibly responds to the fluctuations, we ensure a successful energy transition in a sustainable world.

The shareholders of 50Hertz are the Belgian system operator Elia (80 %) as well as the KfW banking group (20 %), owned by the Federal Republic and the federat- ed states. As a European transmission system operator, 50Hertz is part of the Elia Group and a member of the European association ENTSO-E. 4 How it works: overhead lines

Key

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30 kV 220 kV Transformation 30/220 kV Transformation 30/50 kV Planned/under construction ther companies 0 perating voltage in kV

ine 30 kV ine planned/under construction 30 kV ine 220 kV HVC connection 00 kV HVC/C connection 300/320/525 kV planned/under construction ther companies 30/220 kV HVC/BacktoBackconverter 30/50 kV planned/under construction HVC/converter 00 kV HVC/converter planned/ 300/320/525 kV under construction 50Hertz transmission grid ffshore grid connection 50/220 kV ffshore grid connection 50/220 kV Our grid covers an impressive distance of around 10,000 planned/under construction kilometres across the entire eastern part of Germany with Berlin and Hamburg. On land, electricity is mainly transported ystem users: ur customers are regional distriution system via overhead lines; however, 50Hertz is operating more and operators as well as power plants, pumped storage plants, wind farms and maor industries connected more underground cables as well. Legally imposed principles to the transmission system. apply when selecting the routes for new lines. This includes Conventional power plant the protection of people, wildlife and the environment. This Pumpedstorage plant means that residential areas must be circumvented as much nterconnectors as possible and that nature conservation areas and landscapes nshore/offshore wind farm are occupied only sparingly. 50Hertz bundles lines with existing nshore wind farm planned/under construction routes (for example with railways, motorways and existing lines) ffshore wind farm planned/under construction whenever this is possible and worthwhile and adjusts them to the landscape by means of an appropriate line layout. New construction mostly along existing route

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What exactly is electricity?

Electric current is the name for the flow of small, negatively charged elementary particles (electrons) in a specific direction.

This usually happens within a ‘conductor’ of conductive material. Lightning, however, shows us that power can also flow without a conductor. Power flow is most easily explained using the example of a battery. Current Electrochemical processes in the battery separate the Symbol: I charge: the negatively charged electrons congregate Unit: A (ampere) at the negative pole, while on the other side, the Formula: I = U (voltage) / R (resistance) positive pole, only positively charged electrons are found. The charged particles are attracted to each

Bright light bulbs – other if the charge is different and repel each other if many electrons are moving the charge is the same. This creates an electron flow from the negative to the positive pole. The principle on which electric current is based, is the property of the electrons to always strive for a neutral state.

There is a large electron flow here The current (I) indicates how many free particles move through a conductor, such as the cable, at the Base metal Noble metal Ion bridge same time, and is measured in ampere (A). (e. g. zinc) (e. g. copper)

The more electrons per second flow through the conductor, the stronger the electric current and the brighter the light bulb connected to the electrical circuit. However, the voltage remains the same.

Flow with Flow with a an excess of shortage of electrons electrons How it works: overhead lines 7

Electricity needs voltage

The electric voltage is the driving force What does electricity look like? allowing free electrons to move and Electricity is invisible, despite the fact that it is often portrayed therefore the precondition for the flow in the form of a yellow lightning bolt. Like the wind, which makes trees sway and drives wind turbines, electricity cannot of electricity. It is measured in volt (V). be seen but is only identifiable through its results. This force is the result of the difference in charges between the positive and negative pole. The special state in which electrical voltage creates mechanical forces on the charge carrier is called the electric field.

Electricity flows through the 50Hertz transmission system with high power, mainly at an extra high voltage of 220 to 380 kilovolts. As a result, it is possible to transport large quantities of energy across wide distances. Before this elec- tricity can be brought to the consumer, in other words, our wall sockets, it must be transformed to a lower voltage several times over.

Voltage Symbol: U Unit: V (volt) Formula: U = I (current) × R (resistance) 8 How it works: overhead lines

Direct current and alternating current – a tense relationship

U (direction of electricity) The electric current generated for example by batteries is known as direct current, as its direction does not alter over time. Most electronic home appliances, such t (time) as the radio, computer or LED lights require direct Curve shape for direct current current as their power supply. Because the electricity that comes out of the wall socket is alternating current, these appliances have built-in switched-mode power supplies or transformers that convert the electricity.

In the case of alternating current, the positive and U (direction of electricity) negative poles periodically exchange their polarity. The charge carriers therefore alternate in direction. In the German and European electrical grids, this t (time) happens 100 times per second or at 50 hertz (see page 10). Technically speaking, alternating current offers the advantage that it can be transformed between Curve shape for single-phase different voltage levels at ease, for example from extra alternating current high voltage in transmission grids to low voltage in households.

U (direction of electricity) 120° 120° 120°

U1 U2 U3

t (time)

Curve shape for three-phase alternating current (three-phase current) How it works: overhead lines 9

A special form of alternating current flows through power grids: three-phase alternating current (or just three-phase current). It consists of three individual alternating currents of the same frequency, which have a fixed phase shift of 120° to one another. Com- pared to direct current, the advantage is, for instance, that it can be transformed to almost any voltage and that the need for materials diminishes when using three-phase current for transmission compared to alternating current.

How fast is electricity? Electrons in a live conductor are slow. Their so-called drift speed depends on the material; for copper wire, for example, their speed is less than 0.5 metre per hour. The key to swift transmis- sion of electricity is not the speed of electron movement, but the transmission of the trigger pulse. This pulse is transmitted almost instantaneously from electron to electron at a speed of about two thirds that of light speed, and is, therefore, faster than a bolt of lightning. The basic principle is the same as that of a drainpipe filled with balls, where one ball is added to the top of the pipe and another ball simultaneously falls out on the other end, regardless of the pipe’s length and even though the balls hardly move. 10 How it works: overhead lines

Frequency

The frequency (f) is a physical factor of alternating current and is measured in hertz (Hz). It indicates how many fluctuations the electricity makes per second. The European electricity grid is an alternating current grid with a frequency of 50 Hz, in North America this is 60 Hz. This means that the electricity changes direction 100 times per second and therefore com- pletes 50 oscillation cycles per second. The admissible fluctuation in the interconnected grid ranges from 49.8 to 50.2 Hz. Within this range, large installations and small devices function flawlessly. How does 50 hertz sound like? Pitch is also measured in hertz. Take for instance the Hence the name of our company. As a transmission strings of a violin. If one draws the bow across the strings, they begin to vibrate and the sound can be heard. system operator, 50Hertz is responsible for maintain- If you have good ears, you can hear a very low tone ing a stable electricity system and guaranteeing a underneath a 50 Hz power line: contra G. The tone that secure supply around the clock. In our Control Centre can be observed near a substation has a frequency of 100 (TCC) in Neuenhagen, near Berlin, we ensure that Hz because of the operating mode of the substation. This corresponds to a G that is one octave higher. the balance between generation and consumption is always maintained and that reserves are kept at the ready to compensate deviations should the need arise, and as such can observe the 50 Hz frequency.

Frequency Symbol: f Unit: Hz (Hertz) Consumers Producers Formula: f = 1 / T (period) 50 Hz . 50.

The frequency of 50 Hz stands for the balance between the generation and consumption of electricity and the basis of a stable power system. How it works: overhead lines 11

Resistance and conductivity

The electrical resistance R is a fundamental factor when designing power lines.

The electrical resistance indicates how strongly the What time is it in grid time? electricity is “slowed down” as it passes through a The grid time is often used as the basis for the time certain material. In other words: it describes how much display on electrical appliances. In Europe, the grid time is based on the standard grid frequency of 50 Hz. A second voltage is needed to transport the energy through corresponds to exactly 50 oscillations of the alternating this material. The electrical resistance depends on the current. Deviations in the grid time are caused by length and cross-section of the line: the shorter the frequency fluctuations. If the frequency is less than 50 Hz, line and the larger the cross-section, the lesser the the 50 oscillations require a bit more time. At more than 50 Hz, they happen faster. The grid seconds therefore last obstruction of the electrons. Lines that cover large a shorter or longer period, depending on the frequency. If distances, therefore, require a high voltage to be able the deviation between the grid time and the universal time to transport large quantities of energy. of the atomic clocks amounts to more than 20 seconds, the frequency in the grid is adjusted accordingly.

Another variable is the specific resistance of a material, a constant that indicates the fixed value of its electrical resistance. High resistance

The contrary of the electrical resistance is the electrical conductivity, which indicates the ability of a substance to conduct an electrical current. Accordingly, there is also a material constant called “specific electrical conductivity”. It is for this reason that for the transmis- sion grids, certain particularly suitable materials are used as conductors (see section “The components of an overhead line”, page 14 and following). Low resistance

Resistance Symbol: R Unit: Ω (Ohm) Formula: R = U (voltage) / I (current) 12 How it works: overhead lines

The path of electricity The four levels of the German electrical grid

Extra high voltage grid: High voltage distribution grid: usually 220 or 380 kilovolts (50Hertz) usually 110 kilovolts

Electricity generation Electricity generation

Large renewable Large hydro-electric Large Medium renewable Medium hydro- Medium energy installations and pumped storage conventional power energy installations (e.g. electric and pumped conventional (e.g. onshore and power stations stations (coal, gas) onshore wind turbines, storage power power stations offshore wind farms) large-scale photovoltaic stations (e.g. coal, gas) installations)

Cross-border connections Substation Substation with neighbouring states (so-called interconnectors) Power consumption Power consumption Very energy-intensive Energy-intensive Cities industry (e.g. steelworks) industry

The extra high voltage transmission grid transports The high voltage distribution grid of the regional large quantities of electricity from the large renewable system operators brings the electricity to conurba- and conventional producers to the regions across tions and supplies the majority of the industrial large distances and with very little loss at the 220 sector with a high voltage of 110 kilovolts. or 380 kilovolts (kV) extra high voltage level. What’s more, they connect the German electrical grid with those of the neighbouring countries and enable the transnational exchange of electricity within Europe. How it works: overhead lines 13

Medium voltage distribution grid: Low voltage distribution grid: 3 to 30 kilovolts usually 230 or 400 volts

Electricity generation Electricity generation

Smaller renewable Small hydro-electric Small conventional Small renewable energy Small decentralised energy installations and pumped power stations installations (e.g. onshore power stations (e.g. onshore wind storage power (e.g. gas) wind turbines, photovoltaic (e.g. CHP plant) turbines, photovoltaic stations household rooftop arrays and rooftop installations) installations, biomass)

Substation Power consumption Power consumption

Commercial company Industrial company Small town Trades Households Small town

The medium voltage distribution grid (medium The low voltage distribution grid (less than 1 kilovolt, voltage, usually 3, 6, 10, 15, 20, 30 kilovolts) supplies usually 230 or 400 volts) is intended for local distribu- the industrial and commercial sectors. The electricity tion. Connected to the low voltage grid are private is distributed to the regional transformer substations households, smaller industrial companies, trades or directly to larger facilities, such as hospitals or and administrations. factories. The different voltage levels are connected by substa- tions. Here, the voltage is raised or lowered. 14 How it works: overhead lines

Electricity transmission on land Components of an overhead line

As a rule, overhead lines are composed of steel lattice towers built on top of a foundation, one or more earth wires as well as the live conductors, which are attached to the tower cross-arms (transverse girders) via insulators.

The overhead lines of the 50Hertz high voltage Earth wire Tower height grid are usually constructed with two electrical approx. 50 0 m circuits. In order to bundle electricity lines, Earth wire peak overhead lines with 4 or 6 systems are also sometimes constructed: the electrical circuits can therefore have different voltage levels and frequencies. To achieve a secure and uninter- Crossarm nsulator string ruptible power supply, no electrical circuit can Clearance be operated at a higher load than the maximum admissible load in case of an outage on another nsulator string 5 m Conductor electrical circuit. This so-called (n-1) principle (pronounce “n minus one” principle) is the acknowledged state of the art and therefore, the Conductor sag principle on which grid and system security at 3 m

50Hertz and throughout Europe is based. ax. conductor sway The conductors Tower inimum foundation ground clearance approx. 2 m They are the most important component of 50 0 m2 a power line, as they transport the energy. approx. 2 m corridor width m 2 m 5 m The material used depends on the electrical properties (e.g. the electircal conductivity) Standard measurements of a and mechanical properties (e.g. strength). 380 kilovolt overhead line When electricity was first transmitted in the (two systems) 19th century, copper was the main conductive material used. However, it was later replaced by How it works: overhead lines 15

aluminium and steel-reinforced aluminium conduc- tors, considering that aluminium weighs less and is cheaper. As a result of the contact between alumini- Why does the conductor heat up? um and the oxygen in the air, it also creates a tight When electricity flows through a conductor, this creates oxide layer that protects the metal against further heat. The smaller the conductor cross-section, the more corrosion, even in rough weather conditions. often the individual electrons collide with each other. This friction produces the so-called resistance-heating effect also known as Joule heat. How hot the conductor becomes At higher voltages, the individual conductors are also depends on other factors, such as the outdoor replaced by bundled conductors to increase the temperature, the wind and the solar irradiation. transmission capacity by employing a larger cross- section and to minimise noise pollution. These consist of two or more individual conductors For the long-term reinforcement of the grids, existing (so-called bundled conductors) and run in parallel lines are more and more often refitted with innovative with the aid of spacers. high-temperature conductors (HT or HTLS). Whereas traditional aluminium/steel conductors cannot ex- The earth wires ceed a maximum operating temperature of 80 °C, the limit of HT/HTLS conductors can be up to 210 °C, thus Earth wires are earthed and conductive wires that are enabling an increase in transmission capacity while used on overhead lines with a voltage of 50 kilovolts at the same time maintaining the same conductor and more. They are fitted to the tower tops. Their diameter. Already in 2012, an important connection in main purpose is to protect the conductors against di- the 50Hertz grid area from Thuringia to Bavaria was rect lightning strikes. A lightning strike can, in certain retrofitted with a high-temperature low sag (HTLS) conditions, cause the line to be disconnected. conductor, greatly and permanently increasing its capacity, nonetheless staying well within legal limits. Grid optimisation and grid reinforcement For bottlenecks that only occur temporarily, weather- The need for transmission in the 50Hertz grid is increa- dependent overhead line operation (WAFB) is an sing, among other things due to the integration of rene- adequate instrument to increase the transmission wable energy. In order to guarantee the transmission of capacity. Depending on the relevant environmental electricity, we are therefore taking measures to optimise conditions (e.g. temperature), WAFB determines and reinforce and even enable a higher grid load of the temporarily transportable electricity and uses it the existing grid, even before developing it further. The in real-time grid operation. For example, when the focus lies on operating facilities that would in theory be weather is colder and the conductor can therefore chronically overloaded in case of a failure: the so-called be cooled off more easily, more electricity can be congested lines or structural bottlenecks. transported than at higher temperatures. 16 How it works: overhead lines

The towers

Transmission line towers are the supporting structures to hang the conductors of an over- head line. Generally, they are placed at intervals of 300 to 500 metres in the case of 380 kV lines. If the transmission line tower is only used as a support, it is called an intermediate support. At points where the line changes its angle, angle supports are used that must withstand the tractive forces of the conductors. Here, the insulators are not directed downwards but aligned with the conductors. There also exist special tower types for specific purposes: junc- tion towers to branch off a circuit and terminal supports for the transition to a substation or underground cable. Height: 55-65 m on average Depending on the voltage level, the conductor Shape: Triangular conductor arrangement configuration and the natural environment in which the transmission line towers are located, different types are required. The tower most The Danube or two-level pylon is frequently used in the 50Hertz grid area is the a tower for two three-phase current conventional two-level pylon (so-called “Danu- circuits. The conductors here are be” tower), followed by the single-level pylon always arranged in a triangle. On and the pylon with the vertical double arrange- the lower cross-arm, there are two ment (“barrel type”). conductor bundles per circuit, and one on the upper cross-arm. In some countries, such as Switzerland or The two-level pylon design is used Finland, several towers with new designs have most often for high-voltage towers already been erected. 50Hertz is also currently with two electrical circuits, as they researching new tower designs. One example is combine good properties regar- the development of the CompactLine. ding tower height, construction costs and corridor width. How it works: overhead lines 17

Height: 66-76 m on average Height: 37-47 m on average Shape: conductors arranged one Height: 32-36 m on average Shape: horizontal conductor underneath the other Shape: horizontal conductor arrangement Particularity: used when the corridor arrangement Particularity: two earth wires must crosses forests to keep the corridor Particularity: the route width is reduced always Run above the conductors. width as narrow as possible. thanks to the more compact design.

In the case of a single-level pylon, A barrel-type pylon or pylon compactLine is the new technical all conductors are arranged on with vertical double arrangement development of a 380 kV overhead the same level. The single-level is a design for overhead lines line. It was developed between arrangement makes for a lower with three cross-arms. The three 2013 and 2017 in the scope of a construction height of the towers conductors of a circuit are arran- research and development project but also for a wider corridor. It is ged vertically. The widths of the under 50Hertz’s lead. The over- used when the towers are not three levels create a barrel-shaped head line is characterised by solid authorised to be too high, for profile. Barrel-type pylons need a towers with innovative suspension instance in bird migration areas less wide corridor, but are higher of the live conductors; the const- than the similar Danube pylons. ruction height and route width are reduced compared to traditional overhead lines. 18 How it works: overhead lines How it works: overhead lines 19

The insulators

The operational safety of a line relies heavily on the Metal cap insulation of the electrical conductors. Because Bonding (Portland cement) of their low conductivity, insulators prevent the power flow from entering the earthed towers via the conductor fittings. They are primarily used outdoors and are therefore subject to different environmental influences (e.g. precipitation, temperature fluctua- Dry zone tions, accumulation of dirt). Due to the electrical and mechanical strain on an insulator, three alternative materials have prevailed worldwide: porcelain, glass and certain types of plastic. e. g. porcelain as insulating material In its grid area, 50Hertz mainly uses porcelain long- rod insulators, which are particularly suitable for a grid voltage of 110 kilovolts and upwards. In the case of extra high voltage lines, strings of several insulators are used. The advantages of long-rod insulators are their high electrical and mechanical reliability as well as their low maintenance. For some years, the num- ber of composite insulators (plastic insulators) with a fibreglass core for mechanical strength and a silicone sheath for insulation has been on the rise. This type of insulator is longer and weighs less than a comparable Why is an insulator often shaped porcelain insulator. like a wall plug? Weather influences can considerably degrade the dielectric strength. Owing to pollution and moisture, a conductive layer can form on the surface, thereby adversely affecting the insulator’s function. This creates a so-called leakage current across the surface, which in turn causes a transmission loss and in the worst-case scenario, can lead to a short-circuit. In order to avoid this, the creep- age distance is increased by means of the ribbed shape. The roof-like shields offer protection against rain and dirt accumulations. 20 How it works: overhead lines

What are electric and magnetic fields?

Definition

Wherever power flows, there are electric and mag- netic fields. Electric fields are the result of voltage, while magnetic fields are created by the current. The strength of the fields underneath an overhead line depends on several factors:

— the voltage level and the intensity of the current — the distance to the corridor and between the conductors and the ground — the configuration and the distance of the conductors to each other — other electrical circuits on the tower

In the case of overhead lines, the electric and mag- netic fields diminish quickly as the distance increases. They are at their strongest directly underneath the conductor bundle, where the conductors are closest to the ground (at the middle of the span). This dis- tance cannot be less than 7.8 metres at a voltage of 380 kV, also in case of high strain, for instance as a result of environmental influences.

Occasionally, overhead lines are suspected of being detrimental to health because of the normally occurring electric and magnetic fields. Studies, however, have found no correlation between electric and magnetic fields and the deterioration of health. The fields are strongest where the How it works: overhead lines 21 conductors have the lowest distance to the ground, so mainly midway between the towers.

The limit values

In order to exclude a negative impact on people’s 50Hertz not only complies with the legally prescribed health, the German legislation laid down exact limit limit values; in general, it does not even come close to values or precautionary values in the 26th Federal them. Compared to international limit values, Ger- Emission Protection Ordinance. In buildings and on many’s are very stringent and cannot be exceeded, properties where people reside longer than temporar- even at maximum load. ily, the following limit values apply to new and existing installations: Can electric and magnetic fields only be For alternating current lines… found near overhead lines? — … for the electric field strength: 5 kV / m No. The main sources of such fields in everyday life are — ... for the magnetic flux density: 100 μT (microtesla) not overhead lines, but electrical appliances within the own home. The electric field strength within homes is low because of the low operating voltage of 230 volts. For direct current lines... The magnetic flux density of the appliances, however, can — ... for the electric field strength: no limit value be much higher. In the case of a hairdryer, held about 3 defined centimetres from the body, the magnetic flux density can reach a value of up to 2000 µT. — ... for the magnetic flux density: 500 μT 22 How it works: overhead lines

Noise at overhead lines

­When electrical energy is transported via overhead lines, this can result in noise that is generated in certain weather conditions. Causes of noise are:

­— electrical discharges that cause ionisation of the air (fragmentation of air molecules), the so-called corona effect, can be distinguished as crackling and humming. These corona noises are generated directly at the conductors.

­— Wind noises and vibrations at the conductors and steel girder sides of the towers as of approx. 15 metres per second (wind speed 7).

The wind noises can be compared to whistling noises inside the frame of the lattice towers. The recom- mended values are delivered by the Technical Instruc- tions on Noise Abatement of the Federal Republic of Germany and differ depending on the type of area. For a pure residential area, for example, the recom- mended value is 50 dB (A) during the day and 35 dB (A) at night. How loud are 35 and 50 dB (A)? Decibel (A) is a measurement of the sound pressure The noise abatement reference values are binding for level. The added (A) indicates that the different sound frequencies are rated differently depending on personal transmission system operators. Expert opinions, perception. In other words, the middle frequencies are carried out by independent experts at 50Hertz’s taken greater account of. For a human with healthy ears, request, must confirm compliance with them. the hearing threshold is already 0 dB (A). If values over 120 Technical measures can minimise noise emissions. dB (A) are reached, the noise pollution is unbearably loud. A noise level of 35 dB (A) can be compared with a whisper. Corona effects, for instance, are mitigated by the 50 dB (A) is the normal conversation level or a radio playing shape of the insulators. quietly in the background. How it works: overhead lines 23

Transmission losses

When transporting energy in electrical systems, In Germany, about 6 percent of the electrical power energy losses occur. This refers to the difference provided is lost in the power grid across all voltage between the generated electrical power in the power levels. The larger part is lost at the distribution level. plant and the consumed electrical power. They are 50Hertz keeps the transmission losses on its over- mainly caused by the ohmic conductor resistance head lines as low as possible, for instance if we favour and in that case, occur in the form of heat losses the use of lines with an extra high voltage of 380 (ohmic losses). kilovolts. After all, the higher the voltage, the lower the loss of power. Corona discharges, which are noticeable by crackling and luminous effects on overhead lines, also lead to losses. These electric discharges are caused by the ionisation of an electrically non-conductive gas or a fluid surrounding an electricity conductor. When electricity is transformed from one voltage level to another, power is also lost. 6 % of the electricity supplied via the overall power grid is lost.

What is reactive power? Reactive power is energy that moves back and forth through the line with the frequency of the alternating current. It is needed to create the electric field required for the power to flow. Nevertheless, reactive power can, contrary to active power (actual electrical output), not be used directly by the consumer. 24 How it works: overhead lines

Cable undergrounding An alternative for the overhead line?

Alternating current and direct current Tunnel of the 380 kilovolt diagonal power link in Berlin - at the top you can see a rail for the maintenance track and two ducts for accompanying cables (FOC). As a rule, alternating current is transported at the extra high voltage level through overhead lines. However, especially near residential areas or bird sanctuaries, residents and environmental associations Facts about underground cable technology always request an alternative: the underground cable. To gain more experience with underground cable How many underground cables have to be laid? technology, a total of five legally stipulated pilot For the underground cabling of a powerful 380 kV projects are currently well-defined with the possibility three-phase current line with two circuits (polyphase of using underground cables in the future. alternating current), up to twelve underground cables (six cables per system) are needed. A similar direct cur­- For direct current transmission across wide distances rent line only requires about four underground cables. at the extra high voltage level, the underground The construction site and the necessary route width for cable, on the contrary, takes legal precedence since a direct current cable are therefore smaller than for a late 2015. The different assessment of underground similar alternating current cable. Consequently, the im- ­ cables for alternating current and direct current trans- pact on the soil and environment is also much smaller. mission is based on the different technical properties of both technologies. How does this relate to the costs? Underground cables are more expensive. The con- struction of an underground cable connection is about four to six times more expensive than a similar overhead line. As fewer cables are required and mainly the underground cables themselves are the main cost driver, direct current technology is prefer- able. The open trench method is standard. If laid in a tunnel, like the one underneath Berlin, the costs can even rise to 25 times that amount. What’s more, the manufacturers indicate a life span of about 40 years, while overhead lines can operate for up to 80 years. How it works: overhead lines 25

What happens in case of damage? What does the law say? Underground cables are resistant to storms and For three-phase (alternating) current connec- lightning. Here, rather, the couplings between indi- tions at the extra high voltage level, the Federal Requirements Plan Act stipulates five pilot vidual cable lengths (joints) are potential weak points. projects, in the scope of which underground If an accident occurs on an underground cable, the cables can be tested. damage must be located, the soil excavated and the The underground cable principle for direct cable section in question ought to be repaired or current connections at the extra high voltage level is found in the Federal Requirements Plan replaced. This can take several weeks. Damage to an Act (BBPlG). Nevertheless, the act also stipu- overhead line can more often than not fixed within a lates that partial sections can be executed as couple of days. overhead lines in strictly defined exceptions.

Which losses occur during transmission? In the case of three-phase current transmission or alternating current, reactive power must be compen- sated at regular intervals (see also page 27). This requires additional technical installations. For direct Laying of underground cables using the open current transmission, however, no reactive power trench method. Afterwards, the trench is filled up with earth again. should be compensated as the physics are different.

What effect does the heat have on the soil? Heat is generated whenever underground cables are operated. This heat spreads through the soil. How much of it eventually reaches the root bed, i.e. at a depth of around 20 centimetres, depends on the thermal conductivity of the soils and the humidity of the ground.

Which restrictions apply to the route? The area above the underground cables is entered in the land register as a buffer strip. Here, no deep- rooting trees can be allowed to grow. On top of this, no buildings can be constructed on top of the line, as there is also a magnetic field above the cable. 26 How it works: overhead lines

High voltage direct current (HVDC) transmission

The triumph of alternating current HVDC: used worldwide

When the age of electricity had just begun, it was still Today, HVDC lines are operated worldwide and many unclear how the electricity should be transported: as new ones are being built. Extensively long HVDC direct or as alternating current (also called three- overhead lines are for instance being constructed in phase current)? Thomas Edison, the inventor of the China, India or Brazil, with voltages of 600 to 80 kV. light bulb, favoured direct current in the USA. For plastic-insulated underground and submarine cables, a voltage of 320 kV has established itself. The In the end, however, alternating current became the current generation of underground cables has been worldwide standard for electricity transmission, as designed for 525 kV. this technology offered decisive advantages: for example, it is relatively simple to change the voltage level using transformers, which is important for the transmission of electricity across large distances. HVDC at 50Hertz Furthermore, it is easier to switch alternating current Since the mid-1990s, 50Hertz and Danish lines on and off. system operator Energinet.dk have been operating the Kontek cable, a 170-kilometre- long HVDC link (400 kilovolts, 600 megawatts) Direct current makes a comeback running through the Baltic Sea from Germany to Denmark. Direct current experienced a rebirth when it became The Federal Network Agency already deter- mined the need for three large HVDC links possible to transform alternating current to direct within Germany in 2012. This was legally current with little loss. Since then, direct current endorsed by the Federal Parliament. In the technology has made a worldwide comeback and is current Grid Development Plan for 2030 (2019), being used more and more often for long-distance the Federal Network Agency also acknowledged the need for the connections. connections. Because the voltage used remains high, One of these powerful HVDC links is the as in the case of alternating current, the term used is SuedOstLink. During the next few years, this high voltage direct current or HVDC for short. should transport energy generated in the windy north-east from Saxony-Anhalt to Bavaria. Another HVDC link, about 300 kilometres in length, is the Hansa PowerBridge, which will connect Mecklenburg-Western Pomerania with the transmission system of south Sweden in a few years’ time. How it works: overhead lines 27

Direct current technology offers advantages compared to alternating current:

— using direct current produces fewer losses than when using alternating current. In the case of longer direct current connections, this compen- sates for the transformation losses of the converter stations. — Contrary to alternating current, no installations need to be installed along the line to compensate reactive power. This makes the technology the preferred choice for long lines across the seabed. — With the same conductor cross-section and the same voltage, direct current can be used to transport more energy than alternating current. This is especially important for underground cables so that materials and earthworks can be economised on. — The converter stations that are needed for trans- formation make it possible to determine the exact quantity and the direction of the electricity: this is an important control instrument in times of However, there are limits as well: fluctuating feed-in from renewable energy sources. — With direct current, the grids of two countries that — An HVDC link requires expensive converter installa- have a different frequency can still be intercon- tions. These transform direct current into the nected. alternating current traditionally used in the grid, and vice versa. — At present, HVDC lines can only be operated as point-to-point connections. Today, a meshed grid, as used for alternating current, is still a technological impossibility. — Regarding the technical aspect, switching direct current at the extra high voltage level is considerably more problematic than for alternating current, and not yet finely tuned. — The longer the HVDC link, the more joints are required, increasing the vulnerability of the line. Bverskov 28 How it works: overhead lines weden

Tolstru rde

riegers Flak Schedule of interconnectors of the Denmark 50Hertz grid with other countries Baltic 2

Baltic 1

chleswigHolstein Bentwisch

strow Hamburg Mecklenburg Western Pomerania

ranik ierraden

Lower axony randenburg Poland

erlin

isenhttenstadt

axonyAnhalt

Key

Planning stage Under construction axony Operational Hagenwerder 50Hertz substation/switchgear Thuringia Rhrsdorf Non-50Hertz substation/switchgear Hesse Phase shifting transformers Cech epublic HVDC/converter

HVDC/Back-to-Back-converter Hradec avaria How it works: overhead lines 29

International interconnectors Interconnectors

The single European electricity market is coming together more and more. International interconnectors link the transmission systems of individual countries together. Interconnectors are part of a sustainable development of the European grid infrastructure. International cooperation is necessary to implement the energy transition in Germany as well as the EU’s plans to create a single integrated European electricity market with the intention of optimising grid development.

Towards a European grid

An interconnector is a power line connecting the Interconnectors at 50Hertz electricity grids of two countries. These interconnec- 50Hertz is a strong partner at the centre of the tors enable cross-border trade, increase the security European grid. The transmission system of 50Hertz is electrically connected to Poland via of supply and make it possible to take on a large share two international lines, to the Czech Republic of electricity from renewable sources. The national via one line and via two more lines with transmission grids and the interconnectors that link Denmark. A second interconnector with them together form the European interconnected grid. Denmark, integrating offshore wind farms, is about to be commissioned. A lot of preparation work is also being performed for the construc- tion of a new interconnector with Sweden, the Hansa PowerBridge. 30 How it works: overhead lines

Route usage Ecological aisle management

Parts of overhead lines run through forested areas In 2009, 50Hertz conducted an EU-supported study and cross them through aisles. From an ecological on the ecological aisle management in cooperation point of view, these aisles can be considered “life- with local partners. The reason for carrying out this lines”. As specified in the Federal Act for the Protec- study was the planned construction of the South- tion of Nature, 50Hertz is obliged to compensate for West Interconnector between Halle (Saale) and any deforestation with replanting. When constructing Schweinfurt. The findings revealed how future new overhead lines, 50Hertz makes a point of affect- interventions in the landscape can be minimised and ing flora and fauna as little as possible. how this can help preserve valuable cultural assets and natural resources to their optimum level. New habitats emerge in the aisles, often with active support. Here, the most varied and often sensitive biotopes and habitats can develop for insects, reptiles, Bird protection birds and mammals. Even rare plants, such as many of Transmission towers offer additional breeding the native orchids, can be found in these newly created possibilities for birds. By installing nesting aids on the towers (nesting boxes, wicker baskets or and emerged habitats. They depend on extensive steel nesting bases), 50Hertz supports the cultivation and regular clearance of the aisle. populations of daws, kestrels, hobbies and bats as well as the osprey, who prefer to nest on towers. In certain areas, the conductors are made more visible for birds by using so-called bird protection markers, so that the birds can more easily detect them from the air. How it works: overhead lines 31

It is often unavoidable for overhead lines to run through a forested area. The area is then crossed by the aisle in which the line is constructed and opera- ted. Its width and its course are important for the safe distance to the soil and the woody plants to the right and left of the line. Nothing may endanger the transmission of electricity. In order to mitigate the optical impact of the aisle and to permit new habi- tats to develop, 50Hertz implements ecological aisle management (EAM).

Underneath and in the vicinity of overhead lines, Implementation of ecological aisle management: new biotopes can settle within the aisle. Plants that the ecologically managed pilot aisle near Oberweißbach in Thuringia after the end of the works. New habitats emerge. cannot grow on the soil of a spruce forest, like the Thuringian forest, as a direct result of the shortage of light, can now flourish. The new settlements in turn attract animals that settle in this habitat. The objecti- At this time, 50Hertz does not only implement EAM ve of EAM is to create a biologically as well as visually for newly constructed overhead lines; existing aisles diverse aisle. are also cultivated in consultation with the landow- ners and nature conservation authorities. In Altlands- A partially overgrown aisle also does not form a berg, for example, a wild herb meadow will be created barrier for forest dwellers that want to go from one for many wild bee species. This will offer the endan- side of the route to the other: no territories are cut off. gered insects a welcoming habitat. The existing route is very maintenance-intensive because of fast- growing woody plants and invasive species, which spread quickly and in large numbers, and is not very attractive for indigenous forest nor meadow dwellers. In order to drive back the current growth, the route is planted with crops over the course of three years in cooperation with a local farmer. Thereafter, plants will be sown that offer the ideal habitat for many insects, such as wild bees.

More information about ecological aisle management can be found at www.schneisenmanagement.de About 50Hertz Imprint

50Hertz operates the electricity transmission system 50Hertz Transmission GmbH in the north and east of Germany, which it expands Heidestrasse 2 10557 Berlin as needed for the energy transition. Our extra high T +49 30 5150 0 voltage grid has an electrical circuit length of about F +49 30 5150 4477 10,200 kilometres, or the distance between Berlin and [email protected] Rio de Janeiro. Concept 50Hertz Communications & Public Affairs The 50Hertz control area covers the Länder of Bran- denburg, Mecklenburg-Western Pomerania, Saxony, Design Heimrich & Hannot GmbH Saxony-Anhalt and Thuringia, as well as the city states of Berlin and Hamburg. Within these regions, 50Hertz Picture credits ensures grid and system stability; about 1,100 emplo- Jan Pauls, Christian Brehm, Jens Regiment, visavis Filmproduktion GmbH, ewg3D – iStockphoto.com yees ensure that about 18 million people are supplied with electricity around the clock. Printing Das Druckteam Berlin 50Hertz is a forerunner in the field of secure integra- Links of interest tion of renewable energy: in our grid area, more than 50Hertz: 50hertz.com half of the electricity consumed is already generated BNetzA/Grid development: netzausbau.de from renewable sources, and the trend is rising. The Offshore Grid Development Plan (O-GDP) Grid Development Plan (GDP: netzentwicklungsplan.de shareholders of 50Hertz are the Belgian transmission BESTGRID: bestgrid.eu system operator Elia (80 percent) and the Kreditan- Renewables Grid Initiative (RGI): renewables-grid.eu stalt für Wiederaufbau (20 percent). As a European Federal Office for Radiation Protection (BfS): bfs.de transmission system operator, 50Hertz is part of the Information platform of the German transmission system operators: netztransparenz.de European Network of Elia Group and a member of the European Network of Transmission System Operators for Electricity Transmission System Operators for Electricity ENTSO-E. (ENTSO-E): entsoe.eu

Version of February 2020