DC/AC Hipot, insulation test

What is the difference between and AC and DC hipot? Is DC hipot be considered as insulation resistance test as well?
We are installing a 36kV line (direct buried) and it is being tested by 10kV DC insulation test. Now the other team is requiring HIPOT test (which we dont have any info whether it is DC or AC), which from our experience is only being used with bare coppers and conductors without insulation.

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Advantages of DC hipot test compare to AC test

One of the advantages of using a DC test voltage is that the leakage current trip can be set to a much lower value than that of an AC test voltage. This would allow a manufacturer to filter those products that have marginal insulation, which would have been passed by an AC tester.

When using a dc hipot tester, the capacitors in the circuit could be highly charged and, therefore, a safe-discharge device or setup is needed. However, it is a good practice to always ensure that a product is discharged, regardless of the test voltage or its nature, before it is handled.

It applies the voltage gradually. By monitoring the current flow as voltages increase, an operator can detect a potential insulation breakdown before it occurs. A minor disadvantage of the dc hipot tester is that because dc test voltages are more difficult to generate, the cost of a dc tester may be slightly higher than that of an AC tester.

The main advantage of the DC test is DC Voltage does not produce harmful discharge as readily occur in AC. It can be applied at higher levels without risk or injuring good insulation. This higher potential can literally “sweep-out” far more local defects.

The simple series circuit path of a local defect is more easily carbonized or reduced in resistance by the dc leakage current than by ac, and the lower the fault path resistance becomes, the more the leakage current increased, thus producing a “snow balling” effect which leads to the small visible dielectric puncture usually observed. Since the dc is free of capacitive division, it is more effective in picking out mechanical damage as well as inclusions or areas in the dielectric which have lower resistance.

Carrying out High Voltage or High Potential (Hi-Pot) DC testing on XLPE insulated cables has a tendency to 'polarize' the insulation. It can also lead to capacitive space charges within the tiny anomalies in the insulation. These space charges will eventually form water trees which will seek a path to earth. The ideal test would be a frequency (50Hz/60Hz) test but the test equipment is size and weight restrictive, especially on long lengths of cable because of the charging current required. A VLF (Very Low Frequency) test is something of a compromise, a true sine wave (ac) test but low frequency. Therefore none of the destructive qualities of a dc test.

Difference Between AC and DC Hi Pot

DC hipot testing measures insulation resistance by applying a high DC voltage to cables and measuring leakage current. This can damage old cables during testing. AC hipot testing applies a high AC voltage and measures a higher leakage current due to cables acting as capacitors, with AC current flowing more easily through capacitors than DC. While AC hipot requires less voltage, more power is needed due to low capacitive reactance at 60 Hz. DC hipot testing is preferred over AC hipot testing for cables since using DC avoids issues with cable capacitance affecting insulation resistance measurements.

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Difference Between AC Hipot Test and DC Hipot Test

High Point represents high potential and is a term used for a specific class of test equipment for electrical insulation.

Hipot testers are the reverse of the continuity tester- it conducts the discontinuity test checking no current flow condition between two points. Hipot tests are conducted to find crushed, or leakage in the insulation, and the Hipot tester is used for the ac dc hipot tester.

Let’s discuss the differences between DC and AC hipot. 

AC Hipot Tester

With an ac hipot tester, a long ramp time is usually not needed. The AC testing also has the benefits of checking both polarities of voltage and not requiring to discharge of the DUT after completing the testing. This testing also has its fair share of disadvantages – the test must consider the effects of both real and active currents. When an AC voltage is applied, the flowing current equals the voltage divided by the impedance.

But the impedance is complex as it contains both resistive and capacitive components.

The two components are out of phase with each other and combine in a complex manner to form the total current. As the magnitudes of these compounds can be different, the leakage current of a product with large amounts of capacitance can, with some testers, increase significantly without getting detected by the test.

Dc Hipot Tester

A DC hipot test applies a voltage with gradual steps, commonly known as ramping. It pauses after every increase to allow the capacitance of the DUT to absorb a charge and stabilise. The time needed for the charging current to decay after every step is known as the stabilisation time. The current flowing post the stabilisation time has passed represents the leakage current via insulation.

In case the voltage steps are too large, the sharp rise in charging current when the step gets applied might exceed the high current limit, causing the test to fail prematurely. It is important to carefully match the magnitude and timing of the steps to the characteristics of the DUT.

As a DC hipot test charges the capacitance of a DUT, the charge itself proves to be a hazard to testing personnel. It must be removed after the test is over.

Usually, the hipot tester will automatically discharge the DUT for the same period the test voltage was applied.

Conclusion

The DC hipot tester will use a higher voltage than the AC hipot tester, and the DC hipot test will use the DC voltage, whereas the Ac hipot test uses AC voltage at 50/60 Hz. DC hipot tester provides a more accurate reading of the leakage current as it only reads real current. AC hipot tester, on the other hand, measures total current and does not provide accurate leakage current measurement.

The Difference of AC Hipot and DC Hipot  

There are several hipot tests done to check the effectiveness of insulators on electrical devices. Let’s discuss the differences between DC and AC hipot.

DC hipot

measures the insulation resistance of cables by applying a high voltage on the test device. The leakage current is measured and computed to get the insulation resistance. The drawback of this test is that it can damage old cables during the test. This procedure is only advisable to be conducted on new cables. The voltage applied on the test cable is several times higher than AC hipot. This is why DC hipot test is more prone to flash overs and corona discharges.

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AC hipot test is done by applying a high AC voltage on the test conductor and measuring the leakage current. This test gives a higher leakage current reading because a cable can act as a capacitor. AC current flows easily on a capacitor than DC. The test voltage applied on the test conductor is lower compared to DC but more power is required because at 60 hertz, the capacitive reactance is low.

AC DC Hipot Test

Such a test applies a voltage to the DUT that is much higher than normal operating voltage; typically V AC plus twice the normal operating voltage. For a household appliance designed to operate at 120 or 240V AC, the test voltage is usually about to V AC.

A DC hipot test can usually be substituted for an AC hipot test. The best voltage for a DC hipot is normally higher than the AC test voltage by a factor of 1.414. A product that would be tested at V AC would be tested at V DC.

For double-insulated products, the required test voltages may be much higher, such as VAC or even VAC for a 120 VAC power tool. The voltage is applied between the operating circuits and the chassis or ground – the parts of a product that a consumer might touch or come in contact with.

Refer to Figure 5 for typical AC hipot test setup. The setup for a DC hipot test would be identical.

The purpose of the test is to make sure consumers do not receive an electrical shock when they use the product. This typically is caused by a breakdown of the electrical insulation. The test also detects possible defects in design and workmanship that cause components and conductors to be too closely spaced. The danger is that air gaps between conductors or circuit components may become clogged with dust, dirt, and other contaminants over time in typical user environments. If the design spacing is inadequate, a shock hazard can occur after a period of use. By subjecting the product to a very high voltage, the hipot test overstresses the product to the point that arcing may occur if the spacing is too close. If the product passes the hipot test, it is very unlikely to cause an electrical shock in normal use.

Withstanding a very high voltage means that a large margin of protection exists for the consumer. Regulatory agencies usually require a stringent hipot test as a product “type test” before releasing the product for sale to the public and another less demanding test to be used on the production line. As a rule, testing laboratories consider the hipot test to be the most important safeguard for the consumer. They may accept “design” or “type” tests for other types of tests, but always require hipot tests for 100% of the units in a production line.

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Advantages of DC hipot test compare to AC test

One of the advantages of using a DC test voltage is that the leakage current trip can be set to a much lower value than that of an AC test voltage. This would allow a manufacturer to filter those products that have marginal insulation, which would have been passed by an AC tester.

When using a dc hipot tester, the capacitors in the circuit could be highly charged and, therefore, a safe-discharge device or setup is needed. However, it is a good practice to always ensure that a product is discharged, regardless of the test voltage or its nature, before it is handled.

It applies the voltage gradually. By monitoring the current flow as voltages increase, an operator can detect a potential insulation breakdown before it occurs. A minor disadvantage of the dc hipot tester is that because dc test voltages are more difficult to generate, the cost of a dc tester may be slightly higher than that of an AC tester.

The main advantage of the DC test is DC Voltage does not produce harmful discharge as readily occur in AC. It can be applied at higher levels without risk or injuring good insulation. This higher potential can literally “sweep-out” far more local defects.

The simple series circuit path of a local defect is more easily carbonized or reduced in resistance by the dc leakage current than by ac, and the lower the fault path resistance becomes, the more the leakage current increased, thus producing a “snow balling” effect which leads to the small visible dielectric puncture usually observed. Since the dc is free of capacitive division, it is more effective in picking out mechanical damage as well as inclusions or areas in the dielectric which have lower resistance.

Carrying out High Voltage or High Potential (Hi-Pot) DC testing on XLPE insulated cables has a tendency to 'polarize' the insulation. It can also lead to capacitive space charges within the tiny anomalies in the insulation. These space charges will eventually form water trees which will seek a path to earth. The ideal test would be a frequency (50Hz/60Hz) test but the test equipment is size and weight restrictive, especially on long lengths of cable because of the charging current required. A VLF (Very Low Frequency) test is something of a compromise, a true sine wave (ac) test but low frequency. Therefore none of the destructive qualities of a dc test.

AC Hipot vs. DC Hipot Test: A Detailed Comparison

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This article explores the differences between AC and DC hipot tests, delving into their principles, advantages, and disadvantages. We’ll compare these essential tests used to evaluate electrical insulation.

Introduction to Hipot Testing

“Hipot,” short for High Potential, refers to a category of test equipment used to assess the integrity of electrical insulation. Unlike continuity testers, which verify a connection between two points, hipot testers perform the opposite: they verify the absence of a current path between two points. These tests are crucial for identifying insulation defects like crushes or leaks.

Image of a hipot tester

A typical Hipot tester, similar to this one, can perform both AC and DC tests.

AC Hipot Test

• Procedure: An AC hipot test applies a high-voltage AC signal, typically or volts at 60 Hz, to the insulation under test. The leakage current is carefully monitored.
• Failure Indication: If the leakage current exceeds a predefined threshold, the hipot tester indicates a failure.
• Features: Good AC hipot testers usually have adjustable ramp-up and ramp-down rates for the voltage, as well as an application time setting.

DC Hipot Test

• Procedure: A DC hipot test involves applying a high DC voltage to the insulation and measuring the resulting insulation resistance.
• Leakage Current Measurement: The leakage current is measured to determine the insulation resistance value.
• Applications: DC hipot tests are frequently used on new cables.
• Voltage Levels: The DC voltage applied in a hipot test is often higher compared to AC hipot testing.

Key Differences: AC Hipot vs. DC Hipot

Here’s a summary of the key distinctions between AC and DC hipot tests:

• Voltage Level: DC hipot tests utilize higher voltage levels than AC hipot tests.
• Voltage Type: DC hipot tests use DC voltage, while AC hipot tests use AC voltage at 50/60 Hz.
• Current Capacity: DC hipot testers generally have a much lower output current capacity compared to AC hipot testers.
• Leakage Current Accuracy: DC hipot testers provide a more accurate reading of leakage current because they measure only real current flow. AC hipot testers measure total current, which includes both real and capacitive currents, leading to less accurate leakage current measurements.
• Polarity Testing: AC hipot testers perform tests in both positive and negative voltage polarities. DC hipot testers do not perform polarity-based testing, as they charge the insulation in only one polarity.

In essence, both AC and DC hipot tests are valuable tools for ensuring the integrity of electrical insulation. The choice of which test to use depends on the specific application and the type of insulation being tested.

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