Dielectric Withstand (Hi-pot) test spec?

http://wiki.answers.com/Q/What_is_a_megger_test

I understand that in CE countries the "rule of thumb" is to test at 220V*2+1000V = 1500V (rounded up). It's the duration requirement I don't understand. I see both 1 minute and 1 second commonly mentioned.

Yes 1500 volt is correct with one minute, But where in mass volume its not possible to check 100% of products for 1 minute, people reduces time to one second. But for product validation it has to be for 1minute.

However in my personal experience, product passing 1 second can withstand for hrs and days, provided it is designed with a good factor of safety.

Thanks.

One of the related problems I'm having is sourcing a component for our product. If 1500V / 1min (or 1sec) is the general standard - why can't I find components rated above, say, 500V to 900V / minute?

what is that component?

A potentiometer.

Dear Guest

I don't know what we are talking about the object, let say we are talking for rotating electric machines, following is the HIPOT standard references.

Copyright Material IEEE Paper No. PCIC-2004-XX

Greg C. Stone IEEE Fellow Iris Power Engineering 1 Westside Drive, Unit 2 Toronto, Ontario M9C 1B2 Canada

III. IEEE 56 – MAINTENANCE AC HIPOT TEST

IEEE 56 is an extensive guide on various tests and inspections that can be performed on rotor and stator windings, as well as a review of the major repair methods.

The document saw its last major revision in 1977 , and is now the subject of a complete revision by a working group that is combining IEEE 56 with IEEE 432, so that one guide will cover all form wound motors and generators. The revised version of the standard will probably be published in 2004.

Although IEEE 56 discuses many tests, of relevance here is the maintenance AC hipot. A hipot test is a 'high potential' applied to the winding. In order to find gross flaws in the winding, the 'high potential' test voltage is normally higher than what the winding sees in service. The basic idea is that if the winding does not fail as a result of the high test voltage, the winding is not likely to fail anytime soon due to insulation aging when it is returned to service. If a winding fails the AC hipot test, then a repair or rewind is mandatory, since the groundwall insulation has been punctured.

The AC hipot is similar to the DC hipot (section IV), with the exception that power frequency (50 or 60 Hz) voltage is used. Sometimes 0.1 Hz AC is also employed, as described in IEEE 433. Both commissioning (acceptance) and maintenance AC hipot versions of the test are in use. This test is most commonly applied to form wound stator windings. The maintenance AC hipot is rarely used in North America.

A. Purpose and Theory

The purpose of this test is to determine if there are any major flaws in the groundwall insulation, before a winding enters service (commissioning or acceptance hipot test) or during service (maintenance hipot test). The principle is that if there is a major flaw in the insulation, a high enough voltage applied to the winding will cause insulation breakdown at the flaw. By IEC 60034 and NEMA MG1 standards, all new windings (original or rewound) are subjected to a successful hipot test prior to being accepted by the customer.

Of course the main problem with hipot testing (both AC and DC – see the next Section) is that the winding may fail. If failure does occur, then either:

1. The insulation that punctured must be replaced.

2. The coil with the puncture is removed from the circuit.

3. The coil or even the complete winding is replaced.

These are all expensive alternatives, and all involve a delay in placing the machine in service.

Since a hipot test can be destructive and delay a return to service, many people decide not to perform a maintenance AC hipot. The rationale is that the hipot test may cause a failure that would not occur for a long time in service, resulting in rewinding or significant repairs before they are really needed. This is true. However, the proponents of hipot testing argue that for many critical machines, an in-service failure (that could have been prevented if a hipot test was

done) can result in a greater disruption to plant output than a hipot failure.

For example, the in-service failure of a critical pump motor in a petroleum refinery can stop production for days or weeks, and cost as much as $1M per day. Also, an in-service fault can sometimes cause consequential damage such as stator core damage, a fire or coils being ejected from the slot, resulting in much higher repair costs.

Thus, whether an AC hipot is performed as a maintenance test depends on how critical the machine is to the plant, the availability of spares, and the philosophy of plant management to avoid unexpected plant shutdowns.

With the AC hipot, the voltage distribution across the thickness of the groundwall insulation is the same as the distribution in service since the applied voltage is AC, and capacitances determine the distribution. NEMA MG1 and IEC 60034 define the AC acceptance hipot level as

2E + 1 kV,

where E is the rated rms phase-to phase voltage of the stator. IEEE 56 recommends the AC maintenance hipot be 1.25 to 1.5E. and this is unlikely to change in the current revision. For example, if the guidelines in IEEE 56 are used, the AC hipot test voltage for a 4.1kV motor would be about 6kV rms.

The hipot test is applied between the copper conductor and the stator or rotor core. The AC hipot will age the insulation. In most cases, the hipot voltage is sufficiently high that significant partial discharge activity will occur. These partial discharges will tend to degrade the organic components in the groundwall, thus reducing life. However, calculations based on IEEE 930 indicate that insulation deterioration from a 1-minute AC hipot test at 1.5E is equivalent to about 235 hours or 10 days at normal operating voltage. Therefore, the life is not significantly reduced by a hipot test if the expected life is about 30 years.

B. Test Method

The key element in an AC hipot test is the AC transformer needed to energize the capacitance of the winding.

A 13.8 kV motor stator winding with a capacitance C of 1 μF, requires a charging current of 8 A at f = 60 Hz for a V = 1.5E maintenance hipot test .

A minimum transformer rating is over 150 kVA. This is a substantial transformer, and is definitely not very portable as compared to a DC hipot set.

The AC hipot set is also much more expensive than the DC supply. It is because of the size and expense of the AC hipot supply that an AC hipot is rarely performed as a maintenance test in North America.

C. Interpretation

A winding either passes or fails the AC hipot. There is no other diagnostic information provided. If the winding fails, as determined by the power supply circuit breaker tripping, then repairs, coil or winding replacement is required.

IV. IEEE 95 - DC Hipot Test

A. Purpose and Theory

IEEE 95-2002 describes the test methods and suggests tests voltages for the DC hipot test [3]. There are differences between a DC and an AC hipot test. Most of the description given for the AC hipot test described above is relevant for the DC hipot. Specifically, the DC hipot is a go-no go test that ensures that major insulation flaws which are likely to cause an in-service fault in the near future, can be detected in an offline test. The previous version of IEEE 95 was published in 1977. The major difference between the DC and AC test is the test voltage applied, and how the voltage distribution across the groundwall insulation. Both are linked. With DC voltage, the voltage dropped across insulation components within the groundwall and in the endwinding depends on the resistances (resistivity) of the components. Components with a lower resistance will have less voltage dropped across them. In contrast, the AC voltage dropped across each component in the groundwall or in the endwinding depends on the capacitance (dielectric constant) of each component. Thus, there tends to be a completely different electric stress distribution across the groundwall between AC and DC tests.

In older insulation systems, particularly asphaltic –mica systems, the differences between the AC and DC stress distributions were less pronounced because of the finite resis tivity in older groundwalls due to the absorption of moisture. However, with modern epoxy mica insulations, the resistivity is essentially infinite, thus the DC voltage may all be dropped across a very thin layer of insulation. Consequently, significant flaws may not result in puncture with a DC test that would be easily detected with an AC test because of the more uniform voltage distribution with AC stress.

For modern windings, the AC hipot test yields an electric stress distribution across the groundwall thickness that is the same as occurs during normal operation. Consequently, the AC hipot is more likely to find flaws that could result in an in-service stator failure if a phase-to-ground fault occurs in the power system, causing an over voltage in the unfaulted phases. For this reason, the AC hipot is superior to the DC hipot, especially with modern thermoset insulation systems.

In the 1950s there was considerable research on the relationship between AC and DC hipot tests, and specifically the ratio of the DC to AC hipot voltages [3, 7]. Eventually a consensus was reached that, under most conditions, the DC breakdown voltage is about 1.7 times higher than the AC rms breakdown voltage. This relationship has been standardized in IEEE 95. This research was based on older insulation systems, and unfortunately is largely irrelevant in modern insulation systems, since, as described above, the voltage distribution is completely different under AC and DC. There have, however, been a few studies of the relationship between AC and DC breakdown in modern groundwall insulation systems. One of the largest of these studies pointed out that the ratio of DC to AC breakdown voltage on average was 4.3 in epoxy mica insulation [8]. The 1.7 factor, then, no longer seems to be valid, but since the variability is so large, no replacement ratio has been proposed. Thus the 1.7 ratio is maintained in the latest version of IEEE 95.

B. Test Methods

There are several different methods for performing a DC hipot. Most are reviewed in IEEE Standard 95, and the 2002 version highlights a new variation of the DC hipot called the DC Ramp test. Some of the variations reduce the risk of a failure during the test, and some also give information of a diagnostic nature.

For all types of DC maintenance hipot test methods, the critical decision to be made is the maximum test voltage. For form wound stator windings, IEEE 95 gives guidance. It suggests that the maintenance hipot should be 75% of the acceptance hipot level. NEMA MG1 and IEC 60034 stipulate that the DC acceptance hipot be 1.7 times the AC hipot acceptance level of

2E+1 kV,

where E is the rated rms phase-to-phase voltage of the stator winding. After performing the arithmetic, it works out that the DC maintenance hipot level should be about 2E. That is, a 4.1 kV winding would be tested at about 8 kV, DC. This level was originally suggested since it approximates the highest likely over voltage in the motor that can occur if a phase-toground fault occurs in the powe r system. Consequently, a maintenance hipot just reproduces, in a controlled, off-line fashion, the over-voltage a stator can see in service. The idea here is that if the winding can survive this hipot, it is unlikely to fail in service due to a voltage surge created by a power system fault.

The DC hipot does not age the winding insulation since partial discharges occur very infrequently under DC voltage.Thus, if the winding passes the DC hipot, then the insulation has not been deteriorated in any way by the test. However, one should be aware that if the DC hipot test is done from the switchgear, and if the power cables have been soaked in water for years, then the DC hipot might age and even fail the power cables. This occurs because power cables rated 2300 V and above often fail by a mechanism called 'water treeing'. A DC potential accelerates water treeing. If the cables have always been kept dry, then DC hipot testing should pose no risk to the cables. There are several alternative DC hipot test methods.

1. Conventional DC Hipot

In the conventional maintenance DC hipot, a suitable high voltage DC power supply (available from many suppliers) is connected to the winding, either at the switchgear, or at the machine terminals. The DC voltage is quickly raised to the test voltage and held for either 1 minute or 5 minutes. After this time, the voltage is quickly lowered, and the winding is grounded. If the insulation is sound, there will be no high current surge, and the power supply circuit breakers will not trip. If the power supply breaker trips, then it is likely a puncture has occurred, since the insulation resistance will have instantaneously dropped to zero, which causes an 'infinite' current to flow (by Ohm's law), and the power supply can not deliver this 'infinite' current. Circuit breaker tripping is an indication that the winding has failed and winding repairs or replacement is required. The conventional test contains little diagnostic value, although one can measure the DC current after the 1- or 5-minute application of the test voltage. If one trends the leakage current over the years, then an increasing trend is an indication that contamination is occurring.

2. Step-Stress Hipot

A variation is to use the same supply as described previously, and gradually increase the voltage in either equal or unequal steps. For example, the DC voltage can be increased in 1 kV steps, with each voltage level being held for 1 minute before it is increased again. One then measures the DC current after the end of each step (since by this time the capacitive current will have dropped to zero), and plots it on a graph of current versus DC voltage. Ideally, the plot will be a line with a gentle upward curve. However, sometimes the current increases abruptly above a certain voltage. This may be a warning that the insulation is close to puncturing. If the tester acts rapidly, the test can be aborted (voltage turned off) before a complete puncture occurs. Experience shows that warning is likely if the flaw is in the endwinding, but little or no warning is given if the flaw is within the slot. By carefully applying this test, a hipot failure may be avoided.

However, if the voltage at which the current instability was detected is below operating voltage, there is a high risk in returning the winding to service without repairs.

3. DC Ramp Hipot

A third variation of the DC hipot is called the Ramp test. In this case, the DC voltage is smoothly and linearly increased at a constant rate, usually 1 or 2 kV/minute. Thus, there are no discrete steps in voltage or current. The current vs. voltage plot is automatically graphed and displayed. By increasing the voltage as a constant ramp, the capacitive current is a constant current which can be easily ignored, unlike in the stepped stress test. The primary advantage of the ramp test is that it is by far the most sensitive way to detect when a current instability is occurring, since the capacitive charging current is not changing with time.

Consequently, the ramp test is the method most likely to enable the user to avoid a puncture, and it may even enable detection of significant delamination A DC Ramp test unit is now commercially available.

C. Interpretation

Fundamentally, the DC hipot test is not a diagnostic test that gives a relative indicator of the insulation condition.

Rather it is a go-no go test, where the winding is in good condition if it passes, and in severely deteriorated condition if it fails. However, the DC current measured at the time of the test can give some qualitative indication of condition, much like the IR and PI tests. Specifically, if the current at any particular voltage increases continuously over the years, it is an indication that the insulation resistance is decreasing, and the winding is gradually getting wetter or becoming more contaminated. However, caution is needed when trending the current over time. The current is very dependent on winding temperature and atmospheric humidity. Thus, in most cases the trend is erratic, and impossible to interpret.

Most standards, national or iinternational, specify the hi-pot test voltage to be twice the rated plus 1000 v for one minute. Certification agencies perform this test as required in the course of their evaluation and testing of the equipment. That's in their lab.

However, they recognize that performing this test on a production line is not feasible, so they often state in their Certification Report that while every unit must be hi-pot tested, the manufacturer has a choice of twice rated plus 1000V for 1 minute, or to raise the voltage by 50% for 1 second.

The reason this info is hard to find is that it's not in any of the standards, but is a Certification Body option to assist manufacturing.

Grae