Stress test with Megavolt

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When we draw elec­tric­i­ty from the sock­et at home, it is elec­tric­i­ty that changes polar­i­ty – in oth­er words, alter­nat­ing cur­rent,” says Dr. Ralf Pietsch, Head of Sci­ence and Doc­u­men­ta­tion at HIGHVOLT. Over long stretch­es of land and sea, how­ev­er, the elec­tric­i­ty flows as direct cur­rent and is only con­vert­ed to alter­nat­ing cur­rent by con­vert­er sta­tions near pop­u­la­tion cen­ters. Pietsch explains, “Dur­ing these switch­ing process­es, how­ev­er, faults can cause the volt­age on the DC cables to dou­ble from 525 kilo­volts to more than one mega­volt for a few microsec­onds to mil­lisec­onds, or to even change polar­i­ty.”

This so-called tem­po­rary over­volt­age (TOV for short) is pure stress for DC cables and can dam­age their insu­la­tion. It can also cause space-charge phe­nom­e­na inside the cable, which can lead to local field surges in the cable when the polar­i­ty of the volt­age changes, there­by dam­ag­ing the cable. If the cable fails, near­by elec­tri­cal equip­ment or even peo­ple are then at risk.

Manufacturer want stress

To ensure that noth­ing dan­ger­ous hap­pens in the event of an over­volt­age or total fail­ure of the cable, more and more cable man­u­fac­tur­ers are test­ing their DC cables with the new TOV type test. “This test does not yet have IEC stan­dards but is already described and rec­om­mend­ed in the CIGRE brochure as an option­al test method,” Pietsch tells us. The test is car­ried out at the end of a large test series as the ulti­mate stress test. For about 100 mil­lisec­onds, a mega­volt is applied to the cable in a con­trolled man­ner. Only when the cable has sur­vived this can it be deliv­ered to the net­work oper­a­tor.

How­ev­er, TOV test­ing is very com­plex. After all, the test set­up fills the area and height of an entire test hall. Ralf Pietsch adds: “In addi­tion, the indi­vid­ual test com­po­nents must also with­stand the over­volt­age – and not just once like the cable to be test­ed, but with each sub­se­quent test. This requires spe­cial mate­ri­als, bush­ings and insu­la­tion.”

The test itself con­sists of three sub-tests to check all pos­si­ble types of tem­po­rary over­volt­age in cir­cuits. In all cas­es, a DC high volt­age of 525 kilo­volts is applied via a DC volt­age source to an end ter­mi­na­tion that is con­nect­ed to the test cable. In addi­tion, a volt­age divider is also con­nect­ed to the end ter­mi­na­tion, via which the tem­po­rary over­volt­age can be mea­sured. A sec­ond DC volt­age source is used in par­al­lel to charge a 16-meter-high capac­i­tor tow­er (a mod­i­fied pulse volt­age gen­er­a­tor) to over one mega­volt.

Con­nect­ed to this is a sphere gap with two oppos­ing spheres made of a tung­sten-cop­per alloy. “This par­tic­u­lar alloy is impor­tant because on the spheres, the accu­mu­lat­ed over­volt­age of the capac­i­tor turns into an extreme­ly hot arc, and nor­mal cop­per would sim­ply melt in the process,” explains Pietsch. “The over­volt­age then pass­es through an inte­grat­ed resis­tor to the end ter­mi­na­tion and ulti­mate­ly into the cable.”

This is how the TOV test is structured

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In the first part of the test, the over­volt­age is slow­ly dis­si­pat­ed via the ground. In the sec­ond part of the test, how­ev­er, the volt­age drops abrupt­ly. For this pur­pose, anoth­er sphere gap is con­nect­ed between the 525-kilo­volt DC source and the earth, which abrupt­ly removes the over­volt­age from the cable. In the last sub-test, the over­volt­age also changes polar­i­ty at a high fre­quen­cy.

“We are see­ing more and more net­work oper­a­tors request­ing TOV test­ing.”

Dr. Ralf Pietsch, Head of Sci­ence and Doc­u­men­ta­tion at HIGHVOLT

Know-how and components

“Since the TOV test is rel­a­tive­ly new, it is not yet manda­to­ry for cable man­u­fac­tur­ers,” says Ralf Pietsch. “But we see that more and more net­work oper­a­tors are demand­ing this stress test.” HIGHVOLT has been involved here from the very begin­ning and has already been able to sup­ply com­po­nents or the com­plete test set­up for cable man­u­fac­tur­ers such as Prys­mi­an, Nex­ans or inde­pen­dent test insti­tutes such as FGH Mannheim

“With these orders, we have been able to gain quite a bit of expe­ri­ence in recent years in order to improve and stan­dard­ize the test­ing even fur­ther,” Pietsch con­tin­ues. “Fur­ther­more, I am sure that TOV type test­ing will become manda­to­ry in the near future. After all, grid ope­rators want to play it safe with cables before they lay them in the ground.” In the con­text of the ener­gy tran­si­tion and the expan­sion of the Ger­man pow­er grid with under­ground cables, it can there­fore be assumed that the demand for TOV test­ing will increase sig­nif­i­cant­ly. It’s good that HIGHVOLT already has the exper­tise and all the com­po­nents to pro­vide such test­ing.

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YOUR CONTACT

Do you have ques­tions about the TOV-test?
Ralf Pietsch is there for you:
 R.Pietsch@highvolt.com