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.
Hipot AC Test
Re: Hipot AC Test
