Mobile tester for medium voltage

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A tiny grain of sand is enough to plunge an entire city into dark­ness. If even the small­est par­ti­cles of dirt get into the con­nec­tion sleeves when lay­ing pow­er cables, a vio­lent flashover can occur under oper­at­ing volt­age. Not only do parts of the ener­gy sup­ply sys­tem col­lapse, but cable sec­tions and con­nec­tion sleeves are also destroyed and must be replaced. But how can this be pre­vent­ed? Defects in the insu­la­tion and con­nec­tion sleeves are not vis­i­ble to the naked eye. Over the last three decades, the res­o­nance test­ing method has estab­lished itself as a reli­able and proven prin­ci­ple for inspect­ing cables for faults before com­mis­sion­ing, but also dur­ing rou­tine checks or after repairs.

HIGHVOLT first devel­oped a mobile sys­tem for on-site test­ing back in the 1990s, there­by estab­lish­ing the method. The pro­ce­dure involves apply­ing a test volt­age and, at the same time, per­form­ing a par­tial dis­charge mea­sure­ment. This serves to detect dis­charges in the cable that could lead to a break­down. In this way grid oper­a­tors can elim­i­nate faults before the cable is con­nect­ed to the grid. Until now, the process has only been used at high and extra-high volt­age lev­els, but that is about to change. HIGHVOLT has devel­oped a new mobile test­ing sys­tem that is suit­able for both medi­um volt­age and the low­er high-volt­age lev­el. Michael Hensel, Senior Sales Engi­neer at HIGHVOLT, explains: “The spe­cial fea­ture of our sys­tem is that it cov­ers a very wide range with a test volt­age of up to 75 kilo­volts.”

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Compact cable tester  

The res­o­nance test­ing sys­tem for medi­um-volt­age cables is designed for a test volt­age of up to 75,000 volts and fits on small­er trucks and trail­ers. 

Medium voltage under pressure

In medi­um volt­age, the so-called very low fre­quen­cy (VLF) test has been the stan­dard method for test­ing cables. As the name sug­gests, these sys­tems oper­ate with a very-low-fre­quen­cy alter­nat­ing volt­age of just 0.1 hertz. The advan­tage is that this method does not require high test cur­rents, which means that the test equip­ment can be small in size. But the method also has its pit­falls, as Hensel explains: “For test­ing, a test fre­quen­cy is used that does not cor­re­spond to real con­di­tions at all. Depend­ing on the coun­try, the mains fre­quen­cy is 50 or 60 hertz, so test­ing at 0.1 hertz means there is a risk that faults will not be detect­ed at all.”

Since the insu­lat­ing medi­um behaves dif­fer­ent­ly depend­ing on the fre­quen­cy, it is also pos­si­ble that prob­lems may be indi­cat­ed at a low fre­quen­cy that would nev­er occur in nor­mal mains oper­a­tion. “There­fore, a sys­tem that comes as close as pos­si­ble to real con­di­tions is more suit­able. Our test sys­tems, which have been in use world­wide in high and extra high-volt­age appli­ca­tions for 25 years, have proven that they are capa­ble of doing this,” says Hensel. Reli­able fault detec­tion is also becom­ing increas­ing­ly impor­tant in medi­um volt­age, in par­tic­u­lar because the expan­sion of renew­able ener­gies places heavy demands on medi­um-volt­age cables. How­ev­er, the test sys­tems that HIGHVOLT builds for high and extra-high-volt­age are too large for these cas­es, because medi­um-volt­age cables are usu­al­ly locat­ed in places that are dif­fi­cult to access with a large truck.

Small reactor, big performance

The engi­neers at HIGHVOLT there­fore redesigned the sys­tem so that it does not exceed a total weight of 3.5 tons and can thus be trans­port­ed by a small truck or trail­er. The most impor­tant lever for achiev­ing this is the res­o­nance reac­tor, which pro­vides the test volt­age. This is no easy task, as the reac­tor must remain ther­mal­ly and mechan­i­cal­ly sta­ble through­out the entire one-hour test process with­out defor­ma­tion or over­heat­ing.

“We have the exper­tise to imple­ment such com­pact designs–not every­one can do that,” says Hensel. The reac­tor that HIGHVOLT has designed for the sys­tem is not only com­pact but also flashover-proof. This means that, unlike for larg­er sys­tems, no addi­tion­al pro­tec­tive mea­sures are nec­es­sary. One cus­tomer from the US and anoth­er from Asia are already using the new test sys­tem. “So far, we haven’t had any feedback–that’s a good sign,” smiles Hensel.