More Power to London

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Dark­ness in front, dark­ness behind. Every few min­utes, a rum­bling tube train speeds along above his head. Then every­thing becomes qui­et again and every word dis­ap­pears into the dark­ness, just like the light from the work lamps.

Andreas Wein­lein from the cable man­u­fac­tur­er Süd­k­a­bel GmbH in Mannheim still can’t help shud­der­ing slight­ly when he recalls his time work­ing far below Lon­don. As Head of Sys­tem Engineer­ing, he was respon­si­ble for lay­ing and test­ing high-volt­age cables over a dis­tance of just under 32 kilo­me­ters. 200 kilo­me­ters of cabling were required in total, as there had to be enough for three phas­es as well as the con­nec­tions to the pri­ma­ry sub­sta­tions. The work was car­ried out in three con­struc­tion stages from 2010 to 2017. The cus­tomer: Nation­al Grid, the British trans­mis­sion sys­tem oper­a­tor.

Heading down under the Megacity

Lon­don is a rav­en­ous con­sumer of pow­er. The megac­i­ty already devours 20 per­cent of the total elec­tric­i­ty used in the Unit­ed King­dom. And each year it gets greed­i­er. Demand is ris­ing by three to five per­cent per year on aver­age – twice as much as in the rest of the coun­try. The city’s pow­er net­work is groan­ing under the strain and is start­ing to reach its capac­i­ty lim­its. Many of the cables below the city were laid 40 or 50 years ago and are based on the out­dat­ed tech­nol­o­gy of oil-paper insu­la­tion. “These days, state-of-the-art cables have insu­la­tion made from cross-linked poly­eth­yl­ene,” explains Wein­lein.

“Con­nect­ing high-volt­age cables is a com­plex process. They can­not be con­nect­ed to the pow­er net­work with­out exten­sive test­ing.” Andreas Wein­lein, Head of Sys­tem Engi­neer­ing at Süd­k­a­bel GmbH

These XLPE cables are safer, as they pre­vent oil get­ting into the ground­wa­ter, and they are also cheap­er to main­tain. In one of the largest infra­struc­ture projects Lon­don has seen for decades, Nation­al Grid there­fore decid­ed to replace the old 275 kV oil-filled cables with mod­ern XLPE cables which, at 400 kV, sig­nif­i­cant­ly increase the trans­mis­sion capac­i­ty.

The old oil-filled cables are locat­ed direct­ly below the road sur­face. Lay­ing new cables in the same place would have meant tear­ing up the roads, result­ing in nev­er-end­ing con­ges­tion through­out the already traf­fic-plagued city. Nation­al Grid there­fore decid­ed to head fur­ther down and dig tun­nels.

But the city is grow­ing under­ground as well, with net­works of shafts from the tube sys­tem, sew­er lines, and a war­ren of road and pedes­tri­an tun­nels. Nation­al Grid there­fore had to go even deep­er: 12 to 60 meters into the earth. This is where they drilled the cable tun­nels, which have a diam­e­ter of three to four meters and have been designed to accom­mo­date more cables in the future if required. There are shafts at reg­u­lar inter­vals which lead to day­light and con­nect the under­ground line to the pri­ma­ry sub­sta­tions.

Highly Accurate Assembly

How­ev­er, before the first meter of cable could dis­appear down into the depths of the city, the mate­r­i­al had to get to Lon­don first. Fol­low­ing comple­tion and final inspec­tion at the fac­to­ry in Mannheim, Süd­k­a­bel trans­port­ed 200 kilo­me­ters of cabling in total along the Rhine and over the North Sea to the British cap­i­tal. The cabling was trans­port­ed on 182 drums, each weigh­ing 40 met­ric tons. In­stalling the cables on site was the next chal­lenge. “Con­nect­ing high-volt­age cables of this nature is a com­plex process – they can­not sim­ply be assem­bled like a nor­mal exten­sion cable,” says Wein­lein.

Südkabel’s tech­ni­cians con­nect­ed the cables using spe­cial sleeves. As it is a three-phase sys­tem, they had to fit three sleeves at every sin­gle con­nec­tion point. The process took a week and a half for just one set. “Every mil­lime­ter counts. You won’t find this infor­ma­tion in any text­book – it requires a huge amount of expe­ri­ence.” The tech­ni­cians fol­lowed the con­struc­tion draw­ings to the let­ter and made sure that noth­ing was con­t­a­m­i­nat­ed – all while work­ing in a nar­row tube dozens of meters under the hus­tle and bus­tle of the city.

A sin­gle error dur­ing assem­bly could lead to a cat­a­stro­phe: “If there were an error and the line were con­nect­ed to the pow­er net­work, it would cause break­downs and large parts of Lon­don would go dark.” To make sure this doesn’t hap­pen, the cables are test­ed pri­or to com­mis­sion­ing so that any prob­lems can be rec­ti­fied before­hand. “After assem­bly we can’t see into the cable sys­tem; it’s not trans­par­ent. We can only detect errors by apply­ing a test volt­age and tak­ing a par­tial dis­charge mea­sure­ment at the same time,” says Wein­lein. The par­tial dis­charge mea­sure­ment detects dis­charges in the cable which could cause a sparkover in the equip­ment.

On-Site Testing

The glob­al mar­ket leader for such test sys­tems is HIGHVOLT in Dres­den. Since 2002, the com­pa­ny has been part of the Rein­hausen Group. HIGHVOLT is a pio­neer when it comes to test­ing XLPE cables in high-volt­age appli­ca­tions. The com­pa­ny pro­duced the world’s first mobile test sys­tem for 400 kV cables for a project in Berlin in the 1990s (see “Cable Unifica­tion” box). It was at this time that XLPE was tak­ing over as the pre­ferred insu­la­tion mate­r­i­al for high-volt­age appli­ca­tions. Pre­vi­ous­ly, the oil-filled or gas-filled cables that were used for under­ground instal­la­tion had been test­ed by test­ing insti­tutes using direct cur­rent. How­ev­er, it soon became clear that this method was not suit­able for XLPE cables.

Michael Hensel, Area Sales Man­ag­er at HIGHVOLT, explains: “There were cas­es where test­ing with DC volt­age would always pro­duce pos­i­tive results, but then we would see cable defects on a reg­u­lar basis dur­ing oper­a­tion”. This is because, as a high­ly insu­lat­ing mate­r­i­al, crosslinked poly­eth­yl­ene can hold charges for days. When the cable is then con­nect­ed to the pow­er net­work, the inter­nal and exter­nal fields are added togeth­er, caus­ing local­ized over­load­ing of the insu­la­tion.

A dif­fer­ent type of test­ing tech­nol­o­gy was there­fore required. The chal­lenge is that, as the test volt­age is high­er than the oper­at­ing volt­age, the test sys­tem needs to be designed so that pow­er can also be sup­plied on site. “We have to work with mobile gen­er­a­tors – we can’t sim­ply set a pow­er plant next to the test sys­tem,” explains Hensel. HIGHVOLT there­fore decid­ed to base its solu­tion on a sys­tem that was already in use for on-site test­ing of gasin­su­lat­ed switchgears: res­o­nance test­ing with AC volt­age and vari­able fre­quen­cy. “Our test sys­tems work in the range of 20 to 300 hertz, so rel­a­tive­ly small gen­er­a­tors are enough to sup­ply the pow­er we need,” says Hensel. To keep the test sys­tem mobile, the HIGHVOLT engi­neers designed it to fit on a truck trail­er.

If the test sys­tem reg­is­ters a flashover, it shuts down with­in microsec­onds. As only a small amount of ener­gy is intro­duced at the defect point via the res­o­nance effect, the dam­age is less dra­mat­ic and defects can be iden­ti­fied more eas­i­ly and repaired more quick­ly.

“Under oper­at­ing con­di­tions, the repair costs would be sig­nif­i­cant­ly high­er as the pow­er plant would just keep going.” 300 test trail­ers are now in use all over the world – in Amer­i­ca, Europe, Asia, and Aus­tralia. “There is def­i­nite­ly a trend toward plac­ing high­er and high­er volt­age lev­els under­ground. No one wants to have trans­mis­sion tow­ers right in front of their door, plus they take up a lot of space. And space is hard to find in large cities,” stress­es Hensel.

Royal Seal of Approval

Wein­lein con­firms this trend. “Süd­k­a­bel is car­ry­ing out cable projects like this in many major cities. In addi­tion to Lon­don, we have also recent­ly installed cables in Moscow.” The expert often per­forms the final inspec­tions togeth­er with the test­ing insti­tutes who pro­vide the HIGHVOLT sys­tems for the tests. “With these sorts of tests, there are so many things that need to be decid­ed on the spot, and you might need to replace a sleeve some­where as well. The par­tial dis­charge mea­sure­ment in par­tic­u­lar is very com­plex and it is incred­i­bly impor­tant to draw the right con­clu­sions from the val­ues.” In Lon­don, two test trail­ers were used as the longest sec­tion to be test­ed mea­sured over 12 kilo­me­ters. “In this case, the sys­tems are sim­ply con­nect­ed in par­al­lel.” In total, the com­mis­sion­ing tests took a week to com­plete.

The tun­nel was offi­cial­ly opened at the start of 2018 with a bit of roy­al assis­tance: Prince Charles and Camil­la pressed the start but­ton as the song “I’ve got the pow­er” played out. Thanks to the new cable sys­tem, the megacity’s appetite for pow­er can now be sat­is­fied for anoth­er forty years.