06-10 sr101_72dpi

8/12/2019 06-10 sr101_72dpi

http://slidepdf.com/reader/full/06-10-sr10172dpi 1/5

6 ABB review special report

MAX CLAESSENS – History is marked by a series of great inventions that haveswept across society, acting as stepping stones in the emergence of the modernworld. Most people would agree that re, the wheel, modern transportation andcommunication systems, culminating with the Internet all have a place in this list.Maybe less obvious but equally pivotal is the large-scale transmission and

delivery of electrical energy over long distances. This breakthrough that wouldnot have been possible without the transformer. This article takes a brief tour ofthe history and technology behind the transformer and looks at the different waysin which ABB has advanced and applied it.

ABB is the world’s largest transformermanufacturer and service provider

A world intransformation

8/12/2019 06-10 sr101_72dpi


7 A world in t ransf ormat ion

Power transform-ers were the mainreason that thethree-phase ACtransmission sys-

tem could estab-lish itself as themain T&D tech-nology around130 years ago.

dreds of volts. In the early 1880s, forexample, the “Edison Illuminating Com-pany” supplied 59 customers in LowerManhattan with electricity at 110 V DC.But the energy demand of the fastgrowing cities and industrial centerscalled for an increase in power trans-mission capability.

The small steam- and hydro generatorswere no longer sufficient and larger

power plants were erected more remote-ly from the cities. Voltage levels hadto be increased tokeep nominal cur-rents on the powerlines moderate andreduce losses andvoltage drops. Thiswas the time ofthe birth of a newcomponent: thepower transformer.

In a transformer, two coils are arrangedconcentrically so that the magnetic fieldgenerated by the current in one coilinduces a voltage in the other. This phys-

A round 130 years ago a techni-cal revolution took place thatwas to be a vital step in thedevelopment of modern so-

ciety. That revolution was the commer-cial generation, transmission and usageof electrical energy. Nobody today canimagine a world without electricity. How-ever, this article will start by taking thereader back to the early days when pio-neers like Thomas Edison and George

Westinghouse – and their ideas – werecompeting for the transmission systemof the future: Should it be DC or shouldit be AC?

Very early electrical installations werelocal: The sites of generation and con-sumption were at most a handful of kilo-meters apart: Direct connections fromthe steam- or hydro generators to theconsumers were in the range of hun-

ical principle can only be applied in ACsystems, as only a time-varying mag-netic field is able to induce a voltage. Byusing a different number of winding turnsin the two coils, a higher or lower voltagecan be obtained. The ability to transformfrom one voltage level to another onewas the main reason for the break-through of AC three-phase transmissionand distribution systems. These AC sys-tems operate at a frequency high enough

that human short perception does notsee the time variation (“flickering”) and

low enough that switching equipmentcan be operated safely. The best com-promise was the well-known 50 or 60 Hzof the today’s mains supplies.

Title picture Transforme rs are a vit al l ink in the power transmis-sion and distribution chain.

The power transmissionbreakthrough would not havebeen possible without thetransformer.

8/12/2019 06-10 sr101_72dpi



overvoltage impulse of a lightning strike.New coil designs mitigated these reso-nance effects.

Transformers are the main current-limit-ing element in case of short-circuit fail-ures in the transmission system. The so-called stray reactance, which representsthe magnetic flux outside of the mag-netic core limits the increase in current insuch an event. If high currents flow

through the coils uncontrolled, mechani-cal forces try to press the coils apart,and may cause damage if the construc-tion is not sufficiently robust.

Due to the resistance and inductance ofthe power lines themselves, the voltagelevel may vary depending on load condi-tions. This means that less voltage“arrives ” at the receiving end of a powerline when the load is high. To keep thevoltage level within an acceptable range,

power transformers usually include anon-load tap changer to vary the numberof active winding turns of coil by switch-ing between different taps. In mediumvoltage (MV) distribution, this is usuallydone ofine: This means the tap changersare adjusted once before the transformeris energized and then remain xed.

The increasing importance in recentdecades of UHV (ultra-high voltage) DCtransmission lines for high power trans-

mission over very long distances (greaterthan 1000 km) has made it necessary todevelop UHV-DC converter transformers,which are a huge challenge especially for

Transformers need an “amp lifier” for themagnetic field so that the number ofwinding turns can be kept low. This “am-plifier” is the so-called magnetic core.It consists of ferromagnetic iron, whichcontains microscopic elementary mag-nets that align to the transformer’s mag-netic field as a compass needle aligns tothe Earth’s magnetic field. The iron coreis made of many thin ferromagnetic steelsheets that are electrically insulated

against each other and stacked. Thisreduces classical eddy losses. The useof special alloys and manufacturingmethods enables a minimum needede nergy to change polarity of the elemen-tary magnets.

This basic physical princip le of trans-formers is still the same today as it was130 years ago, but energy density, effi-ciency, costs, weight and dimensionshave drastically improved. This can be

compared to the history of cars and theinternal combustion engine: Here too thebasic principle has remained unchangedin 100 years, but technical progress hastransformed the scope of possibilitiesalmost beyond recognition. During thefirst decades of electrification, the mainfocus in transformer research and devel-opment was to increase power capacity(the power that can be transmitted byone unit). Furthermore, more and moreeffects concerning voltage transients

became known that could endanger thetransformer’s insulation. These includeresonance effects in the coils that can betriggered by fast excitations such as the

Power transformertechnology madetremendous prog-ress during the last130 years.

1a The world’s largest transformer in 1942 (220 kV / 120 MVA) Värtan substa tion Stockholm, de livered by ASEA

1b The world’s rst 800 kV UHVDC power transformer for the 2,000 km Xiangjiaba-Shanghai tr ansmission link, delivered by ABB in 2008

1 Transformer development

8/12/2019 06-10 sr101_72dpi


9 A world in t ransf ormat ion

usually have a forced internal convectionflow of air to ensure sufficient cooling ofthe transformer core.

AMDT (amorphous metal distributiontrans formers) is an upcoming technologythat reduces losses inside the magneticcore. Although the amorphous materialsare still more expensive than standardgrain oriented steel, their application canbe justified depending on how theselosses are capitalized over the lifetime ofthe transformer.

Power transformers

When the transmitted power exceedsaround 10 MVA, special designs arerequired to cope with the mechanicalforces of short circuit currents, higherinsulation levels and increased coolingrequirements. For these ratings, liquid-filled transformers are usually used. Theinsulation between the windings be-comes more and more demanding athigher voltages. Furthermore, resonanceeffects inside the winding itself have tobe considered to avoid insulation failures

during highly dynamic impulse stressessuch as lightning strikes which mayreach amplitudes of one to two thou-sands kilovolt with a 1 µs rise time.

the electric insulation system. The 800 kVUHV-DC Xiangjiaba-Shanghai line ➔ 1 ,for example, has a capacity of up to

7200 MVA, which is roughly comparableto the consumption of Switzer land.

Distribution transformersOn the distribution level (transmittedpower up to 10 MVA) there are two maincategories of transformers ➔ 2 : Liquidfilled (using mineral oil or replacementfluids such as synthetic or natural esters)and dry type. The liquid filled transform-ers are the most compact and cost effi-cient solution, whereas dry type trans-

formers are preferred in environmentswhere fire safety is of special importancesuch as, for example, underground sub-stations, mining sites, marine and someindustrial applications ➔ 3, 4.

Standard versions of distribution trans-formers are cooled passively as the heatgenerated by losses is transported awayfrom the core by natural convection ofthe insulation medium. In the case ofliquid filled products, this heat is then

transported through the tank walls bythermal conduction and removed by thenatural or forced convection of air. Drytransformers in closed environments

Dry-type trans-

formers are thepreferred technol-ogy for applica-tions in which firesafety is of specialimportance.

2a Liquid filled

2b Dry-type

3 Dubai’s 868 m high Burj Khalifa building isequipped with 78 ABB dry-type transformers

2 Distribution transformers come in twomain categories

4 Single-phase pole-mounted transformers,for small power classes up to 167 kVA

5 One example of an extreme application isthis 600 MVA / 230 kV phase shi fter

8/12/2019 06-10 sr101_72dpi



Transformers with power ratings above

some ten MVA are a key element in thesupply of large regions or industrial areas.

As a rule of thumb, it can be consideredthat one person has an average electricalpower demand of 1 kVA, which means,that a 400 MVA transformer transfers thepower needed by 400,000, the equivalentof a medium sized city. Such transformershave to comply with special requirementson safety and reliability and also have toprovide a very high efciency and lowsound level. In recent decades, high volt-

age DC lines have also become increas-ingly important, especially in large coun-tries such as China where they connectindustrial centers to the remote regionswhere the electricity is generated. ABBnow offers standard solutions for DC con-verter transformers for up to ± 800 kV DC.

A transformers located directly next to apower plant is called GSU (GeneratorStep-up Unit). A GSU transforms the elec-tric power from the medium voltage of the

generators to the high voltage transmis-sion level.

To balance power ow between parallelpower lines, phase shifters can be used.

These are transformers (usually with a 1:1translation ratio) that adapt and control thephase angles of voltage and current tooptimize the power transmission capacityof the lines. Phase shifters exist up to apower rating of 1,500 MVA ➔ 5.

Today transformation efciencies of up to99.85 percent are achievable by usingspecial magnetic steel qualities and opti-mized designs. The heat losses, even at

these high efciencies, are still signicant:For the 400 MVA unit mentioned above, forexample, it would be still around 600 kWunder full load conditions. The cooling sys-tem thus remains a challenge. Additionally,the weight and size of such devices has tobe dimensioned carefully since there are

limitations in the maximum transportationpossibilities in the different countries ➔ 6.

Traction and special transformersRailway vehicles use a special type oftransformer that must be highly compact,reliable and robust. Operating frequenciesvary (according to countries and systems)from 16.7 Hz to 60 Hz with power classesof up to 10 MVA. To permit trains to crossborders between countries, traction trans-formers must be compatible with the dif-

ferent frequencies and power systems. ABB offers optimized solutions for all thesedifferent railway applications, stretching upto high speed trains with their challengingneeds ➔ 7.

Moreover, ABB makes a variety of furthertransformers for special applications, forexample for subsea electrication or foroperating variable speed drives.

7 ABB is supplying the traction transformerfor the new “Velaro D” high speed train

Max Claessens

ABB Power P roduct s, Transformers

Zurich Switzerland

[email protected]

6 ABB is the world’s largest transformer manufacturer and service provider, capable ofdelivering high-quality durable products and services all over the world

ABB t ransformers are found wherever elect ricity is genera ted, transported and consumed – in powerplants and substations skyscrapers and shopping malls, ships and oil platforms, locomotives andrailway lines, wind parks and solar fields, water and wastewater plants. The world’s tallest building,the 828 meters high Burj Khalifa in Dubai is equipped with 78 dry-type ABB transformers to ensurepower reliability ➔ 3. The nearby Dubai Founta in, which is i lluminated by 6.600 li ghts and shoots water150 meters into the air, is also equipped with ABB transformers.

For almost 120 years, ABB has produced commercial transformers and continued to enhance themby developing new technologies and materials that raise efficiency, reliability and sustainability to anew level.

ABB ha s set the world records for t he mos t powerful many times – from the world’s fir st tr ansfor mersof 400 kV and 800 kV in the 1950s and 1960s, to the most powerful UHVDC tr ansformers for the800 kV, 2000 km Xiangjiaba-Shanghai transmi ssion link in China ➔ 2. By developing new high-perfor-mance materials and using fire-resistant insulation liquids, ABB has improved the efficiency, safety andenvironmental friend liness of transformers. The new eco-friendly product lines can achieve energysavings of 40 – 50 percent.

For almost 120years, ABB hasproduced commer-cial transformers.

ABB has set theworld record forpower many times.

Documents

06-10 sr101_72dpi