the 50 hertz electricity highways how overhead lines work

The 50 Hertz electricity highways How overhead lines work

Electricity is all around us and something we take for granted. It

flows from the place where it is generated to wherever it is used. All

domestic consumers need to power their electrical devices is a socket

on the wall. But how does the electricity get there ? Where is power

produced, and what stations does it have to pass through before it

arrives in our homes ?

Energy is transported from the point of generation to the con-

sumer via power lines and substations. The ‘electricity high-

ways’ among power lines are the high-voltage transmission

networks. These are the cornerstone for a secure energy sup-

ply, for a functioning electricity market in Germany and across

Europe, and for the integration of renewable energy sources.

The German Renewable Energy Act ( EEG ) encourages the

development of energy production from renewable sources

in Germany. The aim is to increase the share of green energy

in the electricity supply to at least 35 per cent by 2020. The

majority of renewable generators, especially wind turbines, are

located in the north-east lowlands. More and more wind farms

are also being built offshore.

Already today, more than 40 per cent of the wind energy prod-

uced in Germany is generated in the 50 Hertz control area in

north and east Germany, although the same regions consume

only 20 per cent of all electricity in the country. This means that

large amounts of electricity must be transported to the centres

of consumption, especially in the south and south-west. Due

to increasing demand, it is essential to develop and expand

networks.

But what exactly are transmission networks, how do they

work and what impact do they have on people and the

environment ?

50 Hertz The 50 Hertz electricity highways · 3

Our name says it all

Our brand name is also the grid frequency, and it is synonym-

ous with security as well as innovation and internationality.

At 50 Hertz, we ensure a reliable and efficient electric power

transmission network.

As an independent transmission system operator and member

of the international Elia Group, our mission is to maintain the

constant pulse of the European electricity supply – a grid

frequency of 50 hertz – together with our domestic and foreign

partners. Furthermore, we aim to drive the development of the

electricity market, to integrate renewable energy in a reliable

way and to expand our network accordingly.

Since 2010, the Belgian transmission system operator Elia and

the Australian infrastructure fund IFM have been shareholders

of 50 Hertz Transmission GmbH.

Our company

At 50 Hertz, around 800 committed and highly qualified em-

ployees work to ensure a power supply that is safe and reliable

at all times. Our network is one of the most modern in Europe

and has a total length of nearly 10,000 km. At a total of eight

locations, our team endeavours to provide a constant supply

of electricity – 24 hours a day, 365 days a year – for more

than 18 million people in the states of Berlin, Brandenburg,

Hamburg, Mecklenburg-Western Pomerania, Saxony, Saxony-

Anhalt and Thuringia.

Our ‘electricity highways’ bring power to the centres of con-

sumption, collecting all the energy from renewable sources

and crossing national borders to create an effective European

electricity market.

We are situated at the heart of the continent – a meeting point

for northern, eastern and central Europe as well as the world’s

largest export region for renewable electricity. Our subsidiary

50 Hertz Offshore GmbH is responsible for constructing and

operating power lines that connect wind turbines in the Baltic

Sea to the onshore electricity grid.

As an electricity system operator, 50 Hertz works in a trans-

parent and non-discriminatory manner, in accordance with

the independence and neutrality of the grids required by the

European Union ( EU ). As a natural monopoly, we are subject

to strict regulation by the Federal Network Agency, just like

gas and telecommunications network operators.

A transmission system operator in the service of society

50 Hertz

Elia ( 60% ) IFM ( 40% )

4 · 50 Hertz The 50 Hertz electricity highways

The 50 Hertz grid covers an area of 109,360 square kilo-

metres and has a length of approximately 9,980 kilometres.

This is roughly the distance between Berlin and Rio de

Janeiro. On land, electricity is transferred almost exclusively

by overhead lines, but 50 Hertz does also operate some

underground cables. Certain principles govern the selec-

tion of routes for new lines. These include the protection of

people, animals and the environment. Specifically, residen-

tial areas are avoided as much as possible, and a conserva-

tive approach prevents excessive damage to natural areas

and landscapes. Whenever it is possible and sensible to do

so, 50 Hertz bundles power lines with existing infrastructure

routes ( such as railways and motorways and existing lines ),

and it adapts the precise layout of the line depending on

the overall landscape.

Legend

Switching stations :

( in large part with transition to distribution system operators )

220 kV

380 kV

380 kV planned / under construction

380 / 220 kV

Other companies

Power lines :

Power line 380 kV

Power line planned / under construction 380 kV

Power line 220 kV

Operating voltage ( kV ) 110

Other companies 380 / 220 kV

HVDC link 400 kV

Offshore network connection 150 kV

Offshore network connection 150 kV planned / under construction

Network users :

Our customers are regional distribution system operators and power plants, pumped storage plants, wind farms and large industrial users connected to the transmission grid.

Conventional power plant ( lignite- or coal-fired, or gas turbine power plant ) under construction

Pumped storage plant

Wind power plant onshore / offshore

planned / under construction

Proposed offshore wind farm

The 50 Hertz transmission grid

Lower Saxony

Schleswig-Holstein

Mecklenburg-WesternPomerania

Brandenburg

Berlin

Saxony

Saxony-Anhalt

Hesse

Hamburg

Thuringia

Bavaria

PSEPoland

Energinet.dk Denmark

Denmark

TenneT

TenneT

C EPSCzech Republic

110

380+220

220

110

Güstrow

Gera

LeipzigHalle

Erfurt

Eisenach

Rostock

Schwerin

DresdenWeimar

Potsdam

Cottbus

Chemnitz

Zwickau

Jena

Magdeburg

Frankfurt (Oder)

Neu-brandenburg

Bright light bulb : high flow of electrons

Liquid with an excess of electrons

Liquid with an electron deficit

Noble metal ( e.g. copper )Ion bridge

Many electrons are flowing here

Base metal ( e.g. zinc )


Electric current is the term for the flow of small negatively charged elementary particles ( electrons ) in a certain direction.

This usually takes place in a conductor – a length of conduct-

ive material. However, lightning is proof that current can also

flow without a conductor. The best way to explain current is to

use the example of a battery.

Electrochemical processes in the battery cause a separation

of charge : the negative electrons are collected on the negative

side; on the other side, the positive pole, there are only ions

with a positive charge. The charged particles mutually attract

each other and repel particles with the same charge. This cre-

ates a flow of electrons from the negative pole to the positive

pole. The principle behind electrical current is that electrons

always strive to reach a neutral state.

The current ( I ) indicates how many free particles move simul-

taneously through a conductor such as a cable, and this is

measured in amperes ( A ).

The more electrons that flow through a conductor per second,

the greater the electrical current, and, therefore, the brighter

the light bulb connected to the circuit.

What exactly is electricity ?

CurrentSymbol : IUnit : A ( ampere )Formula : I = V ( voltage ) ÷ R ( resistance )


Voltage is the force or pressure that makes free electrons

move, and it is therefore a requirement for electric current.

It is measured in volts ( V ). The pressure is created by the dif-

ference in charge between the positive and negative poles.

An electric field describes a special case in which mechanical

forces are exerted on charge carriers because of the voltage.

The 50 Hertz transmission grid carries current with high

‘pressure’, which is to say with a peak voltage of 220 or

380 kilovolts. This makes it possible to transfer large amounts

of power across long distances. Before this power reaches

consumers and can be drawn from the wall socket, it has to

be transformed several times to a lower voltage.

Electricity needs voltage

VoltageSymbol : VUnit : V ( volt )Formula : V = I ( current ) × R ( resistance )

An insulator being connected in a substation. The arcing is a result of capacitive reactive currents ( see also page 23 ).

What does electricity look like ?

Electricity is invisible, even if it is often depicted in the form of a

yellow lightning bolt. Like the wind, which moves trees and drives

wind turbines, power cannot be seen other than by the effects it has.


The electric current produced by batteries, for example, is

called direct current ( DC ) because it does not change direction

over time. Virtually all electronic devices in the home, such as

radios, computers and kettles, need a DC power supply. Since

the mains supply is alternating current, these devices have a

built-in transformer with a rectifier to convert the current to DC.

In an alternating current ( AC ) circuit, the positive and nega-

tive poles switch roles at regular intervals. This means that the

charge carriers move in one direction and then back in the

other direction. In the German and European electricity grid,

this happens 100 times a second. The technical advantage of

a very high AC voltage is that less energy is lost in transmis-

sion. Furthermore, it is easier to convert different voltages – for

example, from the high voltage levels of transmission systems

to low voltages for the domestic grid.

Direct current from batteries, alternating current from the mains

t ( time )t ( time )

V ( voltage )V ( voltage )

Waveform of single-phase alternating current

Waveform of direct current

How fast is electricity ?

Electrons are slow in a live conductor. Their so-called drift velocity

depends on the material. In copper, for instance, they travel at a

speed of less than 0.5 metres an hour.

The key to the lightning-fast transport of electricity is therefore not

the speed at which electrons move, but the transfer of the electrical

impulse. This impulse is transmitted from electron to electron almost

without a delay at about two-thirds the speed of light, which means

it travels even faster than lightning. The same principle explains why

a tube full of balls will instantly release a ball at one end if another

ball is inserted at the other end – no matter how long the tube is and

even though the balls hardly move at all.


The mains supplies a special form of alternating current :

three-phase alternating current, sometimes shortened

to ‘three-phase’. This consists of three individual alter-

nating currents of the same frequency, each with a fixed

phase shift of 120°. The advantages compared to a

single-phase AC system are that it can be transformed

to almost any voltage level, and the material costs can

be halved with no loss in electric power.

120° 120° 120°

V1 V2 V3

t ( time )

V ( voltage )

Waveform of three-phase alternating current

A steel-reinforced aluminium conductor cable. The conductor is the most important physical component of a power line because this is where energy is transported.

Frequency

FrequencySymbol : f Unit : Hz ( hertz ) Formula : f = 1 ÷ T ( period )

Frequency ( f ) is a physical dimension of alternating current

and is measured in hertz ( Hz ). It indicates how many times the

current oscillates in a second. The European electricity grid is

an AC system with a frequency of 50 Hz ( in North America

it is 60 Hz ). This means that the current changes direction

100 times a second, completing 50 oscillations. The frequency

may vary between 49.8 and 50.2 Hz. Within this range, large

plants and small devices are able to function properly.

Our name therefore says it all. As a transmission system

operator, 50 Hertz is responsible for guaranteeing a stable

electricity grid and a reliable power supply around the clock.

At our Transmission Control Centre, which is located in

Neuenhagen bei Berlin, we constantly maintain a balance

between production and consumption and ensure there are

reserves in place to compensate

for deviations at any time.

49

.950.0 Hz

50.1

Consumer Producer

The frequency of 50 Hz represents a balance between electricity production

and consumption and is the basis for a stable grid.

What does 50 hertz sound like ?

The pitch of notes is also measured in hertz. An easy way to imagine

this is to think of the strings of a violin. These begin to vibrate when

the bow is drawn across them and the note can be heard.

With a good ear, you will be able to hear a very low hum when

standing below a 50 Hz power line: this note is three Gs below

middle C. The sound that can be heard in the vicinity of a substation

has a frequency of 100 Hz due to the way such installations operate.

This corresponds to a G one octave higher.

What time is it according to network time ?

The network time is often used as the basis for time displays in

electrical equipment. In Europe, the network time is based on the

standard mains frequency of 50 Hz. Here, one second corresponds

to precisely 50 oscillations of alternating current. Deviations in the

network time are caused by fluctuations in frequency. When the fre-

quency is less than 50 Hz, the 50 oscillations take longer. They are

faster when the frequency exceeds 50 Hz. This means that seconds

of network time can be longer or shorter than actual seconds

depending on the frequency. If the deviation between the network

time and International Atomic Time is more than 20 seconds, the

frequency of the network is corrected.


Electrical resistance ( R ) is an import-ant parameter in the design of power cables.

It indicates to what extent current is ‘slowed down’ when it

passes through a material. In other words, it describes how

much voltage is needed to transport energy through the

material. Resistance depends on the length and cross section

of the cable : the shorter the cable and the larger its cross

section, the less resistance is encountered by electrons. As a

result, power lines covering long distances need a high voltage

in order to transport large amounts of electricity.

Another factor is specific electrical resistance, a constant that

describes the inherent resistivity of a particular material.

The inverse of electrical resistance is electrical conductance,

which describes the ability of a material to conduct current.

It follows that there is also a material constant called specific

electrical conductance. For these reasons, certain materials

that are especially suitable are used as conductors in trans-

mission systems ( see section ‘The elements of a power line’,

from page 14 ).

Resistance and conductance

Resistance Symbol : RUnit : Ω ( ohm )Formula : R = V ( voltage ) ÷ I ( current )

Low resistance

High resistance


The path of electricityThe four levels of the German power grid

The German electricity grid consists of four voltage levels.

The top level consists of the national transmission systems,

the so-called electricity highways that represent the backbone

of the energy infrastructure. They carry large amounts of elec-

tricity from major renewable and conventional generators to

regional distribution systems – at 220 or 380 kilovolts ( kV ) at

the highest level and with little energy loss over long distances.

Furthermore, the transmission systems with interconnectors

link the German electricity grid with those in neighbouring

countries and thereby enable energy to be traded across

borders in Europe.

The second level is covered by the distribution systems of

regional electricity supply companies. These provide power

at a voltage of 60 kV or 110 kV ( high voltage ) in urban areas

and supply most of the industrial sector.

High-voltage transmission system

usually 220 or 380 kilovolts ( 50 Hertz )

High-voltage distribution system

usually 60 or 110 kilovolts

Substation

Medium-sized conventional power plants (coal and gas)

Medium-sized hydroelectric and pumped storage plants

Medium-sized renewable-energy plants (e. g. onshore wind farms and large solar plants)

Energy-intensive companies

Consumers

Power generation

Consumers

Power generation

Commercial companies Industrial companies Small towns

Small decentral power plants (combined heat and power plants)

Small businesses Households

Small renewable-energy plants (e. g. onshore wind farms, domestic roof systems)

Substation

Small conventional power plants (gas)

Small hydroelectric and pumped storage plants

Consumers

Power generation

Small renewable-energy plants (e. g. onshore wind farms, solar parks and roof systems, biomass)

City

Substation

Large conventional power plants (coal and gas)

Large hydroelectric and pumped storage plants

Large renewable-energy plants (e. g. onshore and offshore wind farms)

Very energy-intensive industry (steelworks)

Consumers

Power generation

Interconnectors to neighbouring countries


Transmission system operator : 50 Hertz is responsible for the operation, maintenance, planning and development of the 220 and 380 kV transmission system in the north and east of Germany and for connecting the wind farms in the Baltic Sea. Our network is what connects energy producers and consumers. Distribution system operator : A distribution system operator is a company that operates electricity networks in the low-, medium- and high-voltage range ( up to and including 110 kV in Germany ).

The lowest voltage level ( low voltage with less than 1 kV ;

usually 230 or 400 volts ) is used for small-scale distribution.

Households, small industrial companies, commercial prop-

erties and office blocks are connected to the low-voltage grid.

The various voltage levels are connected to each other by sub-

stations. This is where the voltage is increased or decreased.

The third level is made up of local networks ( medium voltage

with less than 110 kV; usually 3, 6, 10, 15, 20 or 30 kV ), and

these supply power to industrial and commercial operations.

The electricity is distributed to local transformer stations or

directly to larger facilities such as hospitals and factories.

Medium-voltage distribution system

3 to 30 kilovolts

Low-voltage distribution system

usually 230 or 400 volts

Substation

Medium-sized conventional power plants (coal and gas)

Medium-sized hydroelectric and pumped storage plants

Medium-sized renewable-energy plants (e. g. onshore wind farms and large solar plants)

Energy-intensive companies

Consumers

Power generation

Consumers

Power generation

Commercial companies Industrial companies Small towns

Small decentral power plants (combined heat and power plants)

Small businesses Households

Small renewable-energy plants (e. g. onshore wind farms, domestic roof systems)

Substation

Small conventional power plants (gas)

Small hydroelectric and pumped storage plants

Consumers

Power generation

Small renewable-energy plants (e. g. onshore wind farms, solar parks and roof systems, biomass)

City

Substation

Large conventional power plants (coal and gas)

Large hydroelectric and pumped storage plants

Large renewable-energy plants (e. g. onshore and offshore wind farms)

Very energy-intensive industry (steelworks)

Consumers

Power generation

Interconnectors to neighbouring countries


Overhead lines consist of steel lattice transmission towers on a base, one or more earth cables and live power cables, which are mounted to the cross arms of the tower with insulators.

They have a service life of around 80 to 100 years. In the

50 Hertz high-voltage transmission system, overhead lines are

generally designed to accommodate two power systems each.

In order to connect sections of line, however, some lines also

have four or six systems, in which case the system voltage

levels may be different. For a safe and uninterrupted power

supply, the load of each system should not exceed 70 per cent

in normal conditions. In the event of system failure, it is not

permitted to operate another system beyond its maximum load.

This so-called n-1 principle ( ‘n minus one’ ) governs system

security at 50 Hertz, and it is carefully applied to our entire elec-

trical system.

Conductor cables

The cables are the most important physical component of a

power line because this is where energy is transported. The

material used depends on its electrical properties ( such as its

Power transmission on land Elements of an overhead line

Cross arm

Earth cable

Insulator string5 m

Tower heightapprox. 50 – 60 m

Conductor slack span

13 m

Minimum ground clearance

8.5 m

16 mLine section width approx. 72 m

Tower base approx. 50 – 170 m2

12 m 8 m

Maximum conductor cable swing

Safety area

Earth peak

Traverse

Conductor cable

Insulator

Installation of high-temperature cables on the 50 Hertz section

between Redwitz and Remptendorf in May 2012 ( right and next page ).

Typical dimensions of a 380-kilovolt transmission line ( two systems )


In 2012, an important connection in the 50 Hertz control area

between Thuringia and Bavaria was upgraded to a high-tem-

perature conductor in a move that significantly boosted trans-

mission capacity.

Earth cables

Overhead lines with a voltage of 50 kV or more are fitted with

earth cables. These do not carry current, but they are still elec-

trically conductive cables. Their main purpose is to protect the

conductor from direct lightning strikes. In certain conditions,

a lightning strike could cause a power line to be shut down.

Earth cables are therefore connected to the top of the pylons.

The number of earth cables is mainly determined by the

arrangement of conductor cables. Generally speaking, one

or two cables are fitted. Modern earth cables often contain

a high-speed fibre-optic cable for data transmission.

specific electrical resistance ) and mechanical properties ( such

as strength ). When electric power was first transmitted in the

19th century, copper was mainly used as a conductor, but it

was later replaced by aluminium, which weighs less and is

more cost-effective. Another of the many advantages of alu-

minium is that it forms a dense oxide layer when it comes into

contact with oxygen in the air, and this protects the metal from

further corrosion, even in harsh weather conditions. In order

to increase the mechanical strength of conductors, steel-

reinforced aluminium conductor cable is used in the 50 Hertz

control area.

At higher voltages, individual cables are replaced with bundled

conductors to increase transmission capacity by enlarging the

overall cross section and to keep the noise level low. These

conductors consist of two or more individual cables that are

kept parallel with spacers.

Additionally, innovative high-temperature conductors are in-

creasingly being used to strengthen networks. Whereas stand-

ard conductors may reach a maximum operating temperature

of 80°C, the limit for high-temperature cables is 210°C. This

makes it possible to increase transmission capacity by up to

50 per cent – without increasing the diameter of the conductor.

Why do cables get hot ?

When electrical energy flows through a conductor, it creates

heat. The smaller the cross section of the cable, the more

individual electrons collide into each other. This friction produces

the so-called Joule effect. However, the precise heat of the cable

also depends on other factors such as the air temperature, wind

conditions and the sun.

Transmission towers

Overhead line towers, also known as electricity pylons, are

structures used to suspend overhead power lines. They are

usually positioned 300 to 500 metres apart. Pylons that serve

only to carry power lines are known as suspension towers.

Anchor towers are used at angled sections, where it is

necessary to absorb the tensile forces of the conductor

cables. Here the insulators do not hang down but are angled

in the same direction as the cable. There are also special

towers that perform specific tasks : junction towers to branch

off cable and terminal towers to create a transition to sub-

stations or underground sections of cable.

Different pylons are used depending on the voltage of the

overhead line, the arrangement of the cables and the features

of the natural environment. The most widely used transmission

tower in the 50 Hertz control area is the two-level pylon, known

as the Danube pylon, followed by the single-level pylon and

the barrel pylon.

In some countries, such as Switzerland and Finland, some

towers have already been erected with a new design. 50 Hertz

is also involved in research into new pylon designs and is

examining their suitability for use.

The two-level pylon is a tower for two three-phase circuits

with the conductor cables for each arranged in a triangular

shape. There are two conductor bundles per circuit on the

lower cross arm and one on the upper cross arm. Two-level

pylons are the most common design of high-voltage transmis-

sion towers in Germany for two circuits because they combine

good characteristics with respect to pylon height, construction

costs and route width.

Height : 40 – 70 m

Shape : Triangular

arrangement of cables

A barrel pylon is a design for overhead power lines with three

cross arms. The three cables of each circuit are arranged

among themselves. The widths of the three levels create a

barrel-shaped profile. Barrel pylons require a narrow track

width, but they are taller than comparable two-level masts.

On single-level pylons, all the conductor cables are arranged

horizontally. This arrangement results in a low tower height,

which goes hand in hand with a greater track width. It is used

when the pylons should not be too high – for example in areas

with a high level of bird migration.

Height : 20 – 55 m

Shape : Cables arranged horizontally

Special features : The tallest models are often used to

bridge over forests.

Height : 40 – 80 m

Shape : Cables arranged

vertically

Special features :

Used when passing

through forests to

minimise track width.

How many transmission towers are in the 50Hertz control area?

In the area covered by 50Hertz, there are around 13,900 pylons

made of steel.


Insulators

The operational safety and reliability of overhead lines is

largely a question of isolating the conductor. Due to their low

conductivity, insulators prevent current from flowing through

the cable mountings into the earthed pylons. They are main-

ly used outdoors and are therefore subject to various environ-

mental factors such as rainfall, temperature variations and dirt

deposits. Because of the electrical and mechanical demands

on insulators, three alternative materials have become estab-

lished around the world : porcelain, glass and certain plastics.

In its control area, 50 Hertz mainly uses long-rod insulators

made of porcelain; these are especially suitable for a system

voltage of 110 kV or more. For very high voltages, chains of

several insulators are even employed. The advantages of long-

rod insulators are their reliable electrical and mechanical prop-

erties and low maintenance requirements. For several years,

composite insulators with a fibreglass core for mechanical

strength and a silicone shell for insulation have grown increas-

ingly popular. These insulators are longer and weigh less.

Metal cap

Dry zone

Portland cement

Porcelain

Transformers are also equipped with a variety of insulators.


1 insulator = 110 kV

2 insulators = 220 kV

3 insulators = 380 kV

You can usually

identify the voltage

level of a power line by

the number of insula-

tors on the pylon.

Why do insulators often look like

chimney cakes ?

Weather conditions can have a significant

impact on electrical insulation. Dirt and mois-

ture can cause a conductive film to form on

the surface, which compromises the function

of the insulator. The result may be leakage

current along the surface, which in turn could

cause transmission loss or a short circuit.

To counter this, the design of the insulator

increases the leakage path with a ribbed

shape. The conical roof-shaped umbrellas

provide protection from rain and dirt deposits.


Wherever there is a current, there are also electric and

magnetic fields. Electric fields are caused by voltage, whereas

magnetic fields are caused by current. The strength of the

fields surrounding a power line depends on :

– the level of the voltage and current

– the distance from the main line and the overall height of the

conductor cables

– the arrangement and spacing of the cables to one another

– other circuits on the transmission tower

The field strength decreases as the distance from the conduct-

or cables increases. The strongest field is found at the mid-

point between pylons, where the cables are closest to the

ground. The distance here may never be less than 8.5 metres,

even when the cables are under a heavy load, for example due

to environmental influences.

From time to time, overhead lines are suspected of being

harmful to health because they constantly emit electric and

magnetic fields. However, studies have not identified a link be-

tween electric and magnetic fields and any damage to health.

To rule out any possible negative impact on people’s health,

precise limits were set out in the 26th Ordinance Implementing

the Federal Emission Control Act. The following maximum

What are electric and magnetic fields ?


values apply to new and existing systems for buildings and

land that are not solely intended for the temporary presence

of people :

– electric field strength : 5 kV / m

– magnetic flux density : 100 μT ( microtesla )

50 Hertz complies with all the statutory regulations and in most

cases is well below the required limits. Compared to other

countries, the German limits are particularly strict and may not

even be exceeded under maximum load.

The fields are strongest where the cables are closest to the ground, which is generally at the midpoint between pylons.

Do electric and magnetic fields only exist around overhead

power lines ?

No, the main source of such fields in everyday life is not overhead

lines, but electrical equipment and electrical installations in the

home. Due to the low operating voltage of 230 V in households, the

electric field is not very strong. However, the magnetic flux density of

devices can be much higher. When a hairdryer is held around three

inches away from the body, for example, the magnetic flux density

can be as high as 2,000 μT.


The transmission of electrical energy via overhead power

lines may generate noise under certain weather conditions.

The causes of noise are :

– electrical discharges that ionise the air, fragmenting air

molecules and creating so-called corona effects, which are

perceived as a crackle and hum

– wind noise and vibrations on the conductor cables and the

edges of the steel tower beams at wind speeds of 15 metres

per second ( wind force 7 )

The wind noise is a whistling tone in the latticed frame of the

pylons. The mandatory guidelines are included in the Technical

Instructions on Noise Abatement issued by the German

government and differ depending on the type of area. In purely

residential areas, for instance, the guideline is 50 dB( A ) during

the day and 35 dB( A ) at night.

Transmission system operators are required to adhere to the

noise limits. We are required to demonstrate compliance with

these guidelines in expert reports compiled by independent

bodies such as engineering companies. Certain technical

measures can be implemented to minimise noise emissions.

For example, corona effects can be reduced by certain shapes

of insulator.

Noise emissions by overhead power lines

How loud is 35 dB( A ) and 50 dB( A ) ?

Decibels ( A ) are a measure of the sound pressure level to determine

noise levels. The addition of ‘( A )’ indicates that the sound fre-

quencies in question have been assessed differently, taking into

account human perception. In other words, more attention has been

paid to mid-range frequencies. A person with healthy ears has a

hearing threshold as low as 0 dB( A ). When noise levels exceed

120 dB( A ), the strain on the ears becomes unbearable. A noise level

of 35 dB( A ) can, for example, be compared to the ticking of an alarm

clock. The volume of a regular conversation or a quiet radio would

be around 50 dB( A ).


In Germany, around 6 per cent of all electrical energy produced

is lost in the power grid across all voltage levels. 50 Hertz

minimises such losses on its overhead lines by employing lines

with a maximum voltage of 380 kV as a matter of preference,

among other measures. This is because the higher the voltage,

the less power will be lost.

There is inevitably some loss when energy is transmitted in an

electrical system. Transmission loss describes the difference

between the amount of electric power generated in the power

station and the amount of electricity used. This is mainly due

to ohmic resistance, which results in heat loss ( ohmic losses ).

Another source of losses are corona discharges, which can

be heard as crackling and seen as luminous effects on over-

head lines. These electrical discharges occur when a non-

conductive gas or liquid surrounding the electrical conductor

is ionised. Further transmission loss takes place when voltage

is converted.

Transmission loss

In substations, too, transmission losses are constantly monitored.

What is reactive power ?

Reactive power is a form of energy that is pushed back and forth

in the conductor in sync with the frequency of the AC voltage. It is

necessary to establish the electric field for the current, for example.

However, in contrast to active power ( actual power ), reactive power

cannot be used directly by consumers.


Overhead lines and underground cables mainly differ in terms

of their design, their impact on the environment, the security of

supply and the cost of constructing the cable system.

Overhead lines are directly exposed to weather conditions,

which can lead to frequent network failure. The likelihood of

a problem is lower for underground cables because they are

concealed in the ground. However, this makes them more

difficult to access, which means repairs take longer and are

more complicated and costly. Overhead lines can be used for

around 80 years. Based on current knowledge, it has been

estimated that modern plastic cables will remain durable for

40 years. The construction costs for underground cables are

four to ten times higher than for overhead power lines. When

cables are laid in a tunnel, the costs can be as much as

25 times greater.

When laying underground cable, all of the soil must be

replaced. The cable trays have to be kept free from deeply

rooted plants, and the ground may not be developed in any

other way – indefinitely. Furthermore, underground cables give

off heat. This affects soil moisture and can lead to the drainage

or desiccation of bogs, among other things. There is therefore

a considerable impact on flora and fauna, and it also becomes

difficult to cultivate the land. It is also necessary to install coup-

ling structures along the cable path at intervals of 500 to

700 metres when laying underground cables.

With overhead power lines, on the other hand, it is still possible

to build in the area with certain restrictions, and the land can

be used for agricultural purposes. Detailed investigation is

required on a case-by-case basis when determining whether

underground cabling is suitable for certain sections of a new

line, such as in the vicinity of residential areas.

Different technologiesOverhead lines or underground cables ?

A tunnel for the 380 kV diagonal connection in Berlin – the tracks for the maintenance railway can be seen at the top.

Is an underground cable like this an alternative to overhead power lines in rural areas ?


At the end of the 19th century, people already knew that

energy could be transmitted as direct current ( high-voltage

direct current transmission, abbreviated as HVDC ). Thomas

Edison, the inventor of the light bulb, preferred DC transmis-

sion technology in the early days of electrification in the United

States. However, AC has the distinct advantage that trans-

formers can be used to convert the voltage level, which allows

current to be transported at high voltage levels over long

distances. For this reason, AC became established around

the world as the accepted standard for power transmission

systems.

HVDC technology experienced a renaissance when it became

possible to convert AC into DC without significant loss. Con-

verter stations are needed at both ends of the power line for

this process, in which AC from the transmission system is

converted into DC and then back again. HVDC cables can

transport even larger amounts of electricity over long distances.

This is a crucial advantage given the growth of renewable

energy sources and the resulting increase in distance between

generators and the place of consumption : electricity can be

transferred in large quantities exactly to where it is used.

The only drawback is that the construction costs are higher

than for a comparable AC transmission system due to the

obligatory converter stations.

For more than ten years, 50 Hertz has been working with the

Danish system operator Energinet.dk to operate the ‘Kontek

line’, a 170 km HVDC connection ( 400 kV, 600 MW ) through

the Baltic Sea from Germany to Denmark.

The national grid development plan identified the need for four

major HVDC lines in Germany, which was then officially taken

up by the federal government as part of its requirements plan-

ning. One of these high-performance HVDC lines, the South-

East DC Passage, is scheduled to go live in a few years in

the south of the 50 Hertz control area. Leading into southern

Bavaria, it will transport power generated in the north of the

country to centres of consumption in the south.

High-voltage direct current transmission

A 50 Hertz power converter station for the ‘Kontek’ HVDC link in Bentwisch, near Rostock.


Overhead power lines sometimes run through forest areas

and carve aisles through them. From an ecological perspec-

tive, these corridors can be considered lifelines. As set out

in the Federal Nature Conservation Act, 50 Hertz is obliged

to offset any deforestation by planting new trees. When con-

structing new overhead power lines, an important consider-

ation is to minimise the effect on plant and animal life.

The cleared corridors give rise to new habitats, most of which

even receive active support. Over time, a wide range of new

biotopes and living spaces can develop here – many of which

are worth protecting – for insects, reptiles, birds and mammals.

The newly created and naturally developed habitats are also

home to rare plants that depend on extensive cultivation and

regular clearance, such as many of the native orchids.

In 2009, 50 Hertz carried out an EU-funded study entitled

‘Ecological Forest Aisle Management’ in cooperation with local

partners. The occasion behind this initiative was the planned

construction of the South-West Interconnector between Halle

( Saale ) and Schweinfurt. The results show how to minimise

future interventions in the landscape, thus preserving as much

valuable cultural and natural heritage as possible.

Electricity pylons provide birds with additional breeding oppor-

tunities. By fitting nesting aids on overhead pylons – boxes,

wicker baskets and steel bases for eyries – 50 Hertz is helping

to preserve the number of jackdaws, kestrels, hobby falcons,

bats and the ‘pylon-nesting’ osprey. In certain areas, earth

cables are made more visible for birds in flight with so-called

bird protection markings.

Using power linesCreating new habitats

Why are birds able to sit on conductor cables ?

When birds perch on an overhead power line, they sit on the earth

cable, which does not carry any energy. Theoretically, they could

also alight on a conductor cable – provided that they touch only one

of the cables. However, the electric field surrounding high-voltage

power lines prevents them from doing this.

Putting ecological forest aisle management into practice : the cultivated pilot corridor in Oberweis-senbach, Thuringia, following completion of the work. The result is an area for new habitats.

carbon-neutral printing using environmentally friendly inks

Contact details50 Hertz Transmission GmbH Eichenstrasse 3 A · 12435 Berlin Germany T + 49 ( 0 )30 5150-0 F + 49 ( 0 )30 5150-4477 info@50 hertz.com

Publishing informationPublished by : 50 Hertz Transmission GmbH Picture credits : Jan Pauls, Andreas Teich, 50 Hertz archive Concept and design : DreiDreizehn Werbeagentur GmbH, 313.de Printing : Kehrberg Druck Service

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