VLF stands for Very Low Frequency. It is a type of high-voltage AC testing used to check the integrity of medium and high-voltage power cables, generators, and transformers. The frequency used in VLF testing is typically between 0.01 Hz and 0.1 Hz, much lower than the standard 50 or 60 Hz power frequency.

In VLF testing, a high AC voltage is applied at a very low frequency to the insulation system. Because of the low frequency, the reactive power demand is significantly reduced, allowing for the testing of long cables with smaller and more portable equipment.

VLF tests help identify insulation deterioration, moisture ingress, partial discharge activity, and other defects before they lead to costly failures in service.

Hipot stands for “High Potential”. It is a dielectric withstand test used to verify the insulation strength between electrical conductors and ground. The test applies a high voltage (AC or DC) across the insulation for a specific duration to see if it can withstand the voltage without breaking down.

Hipot testing is commonly used for quality control during manufacturing and maintenance inspections. It ensures that insulation can handle normal operating voltages and temporary overvoltage without failure.

Hipot tests can be performed using either AC or DC voltage, depending on the equipment type and insulation characteristics. AC Hipot tests are dynamic and simulate actual service conditions, while DC Hipot tests are simpler and commonly used for factory tests.

Hipot testing involves applying a high voltage between the conductors and insulation barrier (usually ground). The idea is to ensure that the insulation can withstand voltages higher than its normal operating level.

For example, a 480 V cable might be tested at 2,000 V DC or AC to check for insulation integrity. The current flow is measured during the test - if it remains below the acceptable limit and no breakdown occurs, the cable passes.

In AC Hipot tests, the polarity alternates just like in service conditions, providing a more realistic assessment. However, because AC Hipot tests require higher current capacity, the equipment tends to be larger and more expensive.

DC Hipot tests, on the other hand, use a constant polarity voltage. These tests are easier to conduct and require smaller equipment, but they can sometimes cause dielectric charging effects in aged insulation.

When comparing VLF test vs Hipot Test, the best method depends on the condition and type of cable being tested.

For new cables, both VLF and Hipot can verify insulation quality. However, VLF provides a more realistic simulation of AC operating stress, making it preferable for modern polymeric cables (like XLPE).

For aged cables, VLF is much safer. DC Hipot testing can cause polarization and charge buildup in older insulation, potentially leading to failure during or after testing. VLF testing applies stress similar to actual service without overstressing the insulation, helping detect weak spots before they fail in the field.

In short, VLF test is the modern standard for field acceptance and maintenance testing of medium- and high-voltage cables, while Hipot test remains valuable for manufacturing and short-term insulation verification.

At a substation power plant,  there are many types of electrical gear that require some form of high voltage testing, whether it’s Withstand/Proof testing, Insulation Resistance measurement, Hipot leakage current testing, Tan Delta/Power Factor and Partial Discharge measurement, and other tests that do not require high voltage AC or DC. Some of the mentioned tests require AC high voltage, some use DC voltage and some can use either. Most HV testing of electrical substation apparatus should be done with AC voltage, although there are several tests where DC voltage is suitable. Users should check with their vendors’ maintenance manuals and established standards for the recommended testing methods.

AC Withstand Acceptance Test is usually performed on all new installations and a similar maintenance test in later years. Apply the test voltage for the required length of time. The load either holds the voltage and is passed or it flashes over and is failed: a go/no-go test. The test voltage is defined by the IEEE, IEC or other standards followed. The test current is a function of the capacitance of the load, necessary to know to properly size the kVA of the hipot needed.

If you are Acceptance testing dozens of long 35 kVac cables, maybe will also need to locate cable and accessory faults, and maybe even need to perform AC high voltage dielectric testing to all the new substation apparatus installed, Wuhan Friendslink is capable of making your testing job go smoothly and easily.

The most common test is the VLF AC Withstand test. This is a pass/fail over voltage stress test performed to ensure that the cable insulation, attached accessories, and the terminations are all sound with no damage from installation errors or factory material defects. Since the whole system is new, it can be assumed that all components are healthy except for those that were not installed correctly or damaged during installation. If there is a defective splice or cut in the cable, the test voltage level and the test duration are designed to cause the defect to fail during the test, while not adversely affecting the rest of the healthy cable installation.

The IEEE 400.2-2012 standard for VLF and Tan Delta Cable testing is referenced. This standard dictates that for Acceptance testing cable rated for 35 kVac, a peak sinusoidal VLF voltage of 62 kVac be applied for 30-60 minutes. Based on the known electrical tree growth rate under these VLF voltage conditions, any defect severe enough to be driven into a partial discharge condition will have enough time to grow and penetrate through the insulation, causing a failure.