AC DC Hipot Testing

HVT-Family Hipots by KEP

Concerns Associated With DC Testing

DC hipot testing is believed to be harmless to new solid dielectric cables as their insulation is homogenous and allows for an even distribution of electric stress. Also, DC can be safely applied for the installation, acceptance, and maintenance testing of cables with laminated-type insulation.

The most common concerns over DC hipot testing are related to aged cables. Applied to the non-homogenous insulation of such cables, DC is likely to cause space charging at the weak points of the insulation, a very typical problem for extruded cables. In their turn, accumulated space charges can result in electrical treeing and eventual insulation breakdown.


Another limitation of DC withstand testing is the fact that the test polarity cannot be changed during a single test, and no partial discharge can be initiated. Yet, without the partial discharge analysis some of the severe cable insulation defects may be overlooked.

Nevertheless, DC withstand testing is effectively applied to detect insulation errors to do with cable accessories or environmentally-affected interfacial and surface leakage issues.

DC Testing Parameters: Cable Temperature, Test Voltage & Duration

For a DC hipot test to provide accurate results, the cable or cable system under test should be at ambient temperature. This means that if the cable temperature is increased due to having been subjected to substantial load, some time should be allowed for the cable to cool down.

The DC hipot test should be started with test voltage that is up to 80 per cent higher than the cable’s rated ac rms phase-to-phase voltage. It should then be raised, either continuously or in steps, and brought to the maximum test voltage in the time interval of 10 to 60 seconds (this interval may have to be increased for longer cable systems under test); the maximum test voltage should then be maintained for 15 minutes.

Field Test Voltages for Shielded MV Power Cables

System voltage, kV rms, phase-to-phase	System BIL, kV crest	Acceptance test, kV dc, phase-to-ground	Maintenance test, kV dc, phase-to-ground
5	75	28	23
8	95	36	29
15	110	56	46
25	150	75	61
28	170	85	68
35	200	100	75

DC Testing Method

DC withstand testing is performed offline, on a disconnected cable or cable system under test. In case of multiconductor cables, each conductor is tested individually. The other conductors and shields should be grounded. The test lead of the hipot tester is connected to the first conductor under test, and the initial test voltage is supplied and gradually raised to the maximum level.

If the test voltage is increased continuously, it should be done at an even rate. If the test voltage is increased in steps, at least five steps should be made; at each step the technician should wait for the current level to stabilize, with current readings taken at the end of each step, 2 minutes after reaching the maximum test voltage, and at the end of testing.

In most cases steady or decreasing current readings received when fixed voltage was applied indicate that the insulation of the object under test is in an acceptable condition. 

A reliable indicator of the insulation quality is the resistance of one of the tree circuit conductors compared to the resistance of the other two conductors.  If the ratio of insulation resistance exceeds three to one for cables over 1000 m, it is a sign of the insulation quality deterioration. 

Advantages and Disadvantages of DC Withstand Testing

The benefits of the DC dielectric breakdown test include the following:

• The lower output capacity of a DC test system compared to an AC test instrument makes DC testing safer for the technician;
• The DC withstand test is much safer for the technician than AC testing on highly capacitive objects under test;
• DC withstand testing is used to detect insulation errors to do with cable accessories or environmentally-affected interfacial and surface leakage issues;
• DC hipot testers accurately display the amount of true leakage current of the cable or cable system under test.

Yet, DC withstand testing has a number of drawbacks:

• The cable or cable system under test has to be discharged after testing;
• DC withstand testing is potentially destructive for the insulation of service age cables;
• Some serious insulation errors cannot be revealed with DC dielectric breakdown testing.

So, DC hipot testing is still widely applied for the acceptance testing of newly-installed cables. Yet, when it comes to maintenance testing done in the field, VLF hipots are more commonly used.

AC versus DC Testing

AC withstand testing is typically used by cable producers when performing the installation testing of new cables. The AC hipot test is a Pass/Fail or Go/No-Go test, during which the technician raises the test voltage to a certain maximum value to check whether the object being tested can withstand the applied voltage, and therefore passes the AC test, or fails it. AC hipot testing is widely used to see whether the equipment complies with the applicable standard. 

In contrast, DC testing provides more information on the cable or cable system under test, giving the ability to measure leakage current and calculate the insulation resistance.

Another difference is the size, and therefore cost, of AC testers and DC testers, with the former being substantially larger and more expensive than their DC analogues. This is explained by the fact that AC testers supply a much larger charging current than DC testers do.

The strong advantages of DC over AC testing are that DC testers are smaller, more affordable, safer due to supplying less current, and give information on real leakage current. On the negative side, with DC testers there is a need to ramp up the test voltage and to discharge the object under test after testing.

The benefits of AC versus DC testing are that AC testing does not require ramp voltage and the object being tested does not need discharging. However, due to their size AC test sets are usually not practical for field testing; besides, the high current they supply present a safety hazard for the operator.

AC DC Hipot Testers by KEP

AC DC Hipot Tester HVTS-70/50

Understanding that today’s cable testing conditions require flexibility in terms of hipot testing method choice, KEP offers the portable test systems AC DC Hipot Tester HVT-70/50 with oil-insulated high voltage unit and AC DC Hipot Tester HVTS-70/50 with SF-6 insulated high voltage unit.

KEP’s high voltage test systems HVT-70/50 and HVTS-70/50 perform DC high potential testing of power cables (IEC 60502-2) up to 70 kV, power cables accessories (IEC 61442) as well as AC high potential testing, up to 50 kV at 50 Hz, of switchgear, reclosers, dielectric insulators, highvoltage dischargers (arresters), busbars and other dielectric materials with relatively low electric capacitance.

The main features of the HVT-70/50 and HVTS-70/50 AC DC hipot testers include:

• AC (up to 50 kV) and DC (up to 70 kV) modes
• Compact and portable design
• Exceptionally high safety level due to the new safety interlock and two-operator mode features
• Internal memory
• Auto and manual modes
• Wireless test data exchange with a PC via optional Bluetooth

Alexei Tiatiushkin

Marketing manager

KharkovEnergoPribor Ltd.

marketing@keppowertesting.uk

http://www.kep.ua

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