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8/16/2019 Neta World Warren Industry Topics

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TESTING ROTATING MACHINERY POLARIZATION/DEPOLARIZATION CURRENT (P

IR/PI is a useful indicator of contaminationand moisture on the exposed insulationsurfaces of a winding, especially when thereare cracks or ssures in the insulation. Tesetests are easily done (seeNE A World Winter2011 and Spring 2012 issues). Since squirrelcage induction motor rotor windings are notinsulated, these tests are not appropriate.

Resistance testing is principally a pass/failcriterion and cannot be relied upon to predictthe condition of the main insulation except when the insulation has already faulted.Experience has shown that IR/PI is a usefulindicator of contamination and moisture onthe exposed insulation surfaces of a winding,

especially when there are cracks or ssures inthe insulation. However, as discussed in IEEE43, the IR/PI test does not seem to be sensitiveto many other stator winding insulationproblems such as: • Loose coils in the slot that lead to

insulation abrasion • Delamination of the insulation due

to operation at high temperature

• Separation of the copper from thegroundwall insulation due to load cycling

• Deterioration of the stress relief coatings • Partial discharge (PD) between coils in

different phases due to insufficientspacing in the endwindings

AC tests, such as partial discharge (described inNE A World Winter/Spring 2013) are effective innding these issues, but are cumbersome to do inan offline conguration due to the need of a largeac power supply; therefore, more sophisticateddc tests that some have proposed may detectmore kinds of problems than the simple IR/PItest. Included in the NE A WorldSpring 2012issue was a section about the dielectric response

analysis (DRA) or polarization/depolarizationcurrent-measurement (PDC). Tis measuresthe charging and discharging currents of the

winding insulation of stator or rotor winding.Reportedly, the results of the measurementprovide information about the condition ofmachine insulation (cleanliness, humidity,ageing, corrosion, resin decomposition, andsimilar characteristics).

CURRENT (PDC)BY VICKI WARREN, Iris Power LP

INDUSTRY TOPICS

Te insulation resistance (IR) and polarization index (PI) tests [IEEEStd. 43-2000, ANSI/NE A A S/M S], should be done prior toapplication of any high voltage tests or return to service to assurethat the winding is not wet or dirty enough to pose a risk of failurethat might be averted by a cleaning and drying-out procedure.

94 • SPRING 2014

TESTING ROTATING MACHINERY

/POLARIZATION


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NETA WORLD • 95TESTING ROTATING MACHINERY POLARIZATION/DEPOLARIZATION CURRENT (PDC)

INDUSTRY TOPICS

THEORY he new draft standard (IEEE Std. 43 draft

revision 2012) has an annex that deals with

additional information that can be obtainedby applying a stable dc voltage to a completestator winding or individual phases for1000 to 2000 seconds and recording thepolarizing current IP versus time. hevoltage is then removed and the dischargecurrent ID is monitored as a function oftime using a suitable discharge circuit.

When the voltage is removed, reverse currentflows and the molecules in the insulationbecome disorientated and the space chargedissipates. his discharge current IDhas two main components: a capacitivedischarge current component, which decaysnearly instantaneously, depending upon thedischarge resistance; and the absorptiondischarge current, which will decay froma high initial value to nearly zero with thesame characteristics as the initial chargingcurrent but with the opposite polarity.

Normally, neither the surface leakage IL northe conduction current IG affects the discharge

current. Differences in the IP and ID [Figure 1]may indicate winding lack of curing, moistureabsorption, surface contamination, damage tothe voltage stress coatings, or severe thermaldeterioration of the bulk of the insulation.

EXAMINATION OF THE PDC TESTSince the IEEE 43 standard suggests that thePDC test is valid for testing windings forthermal deterioration, tests were done on amotor stator rated 13.2 kV, 6000 HP known tobe thermally aged, but clean and dry. Both the

PDC test and the ac offline partial dischargetest were done, as the latter has proven to beeffective for evaluating insulation delamination.Te stator had an asphaltic mica insulationsystem and was several decades old. Te three

Figure 1: Charging and Discharging Curves

Figure 2: PDC plot for A-phase with a 10 kV charge cycle (other phases were identical). Te chargecurrent is the upper line and the discharge current is the bottom line. Te logarithmic vertical scale goes from 0.1 µA to 1000 µA and the horizontal axis ranges from 1 to 1000 seconds.


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phases in the stator could be isolated from eachother to facilitate testing of each phase.

Polarization/Depolarization Test For these tests, a PD ech DRA 3 was usedto record the polarizing and depolarizing dccurrents. It applies a positive dc voltage tothe test object at a selected voltage (usually5 or 10 kV for these experiments) and for aselected time (usually 1000 s – about 16.7minutes), while measuring the chargingcurrent. Te dc supply is then removedfrom the test object and the test object isgrounded. Te discharge current-to-groundis then measured for the same amount oftime. Software records these currents, invertsthe discharge current, and displays both the

charge and discharge currentsin the same plot against time, with the plottime origin starting from either the start ofthe charge cycle, or the start of the dischargecycle (Fig. 2). he difference in the chargeand absolute value of the discharge currentcan also be displayed. he instrument alsocalculates the IR and PI. All tests were doneat 20°C.

Partial Discharge Test Partial discharge tests were conducted usinga conventional IEC 60270 PD detector, aPD ech DeltaMaxx, working in the lowfrequency (


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NETA WORLD • 97

Conclusionable I shows a summary of the test results

for each phase of the winding. PDC plotsfor each phase are shown in Fig. 2. When allthree phases are super-imposed, all the chargecurrents and all the discharge currents overlapcompletely.

It is clear from able I that A phase has thehighest PD in both frequency ranges. Sincethe positive and negative PD is about thesame (within +/- 25%), then based on normalPD interpretation rules (see IEEE 1434or IEC 60034-27), one suspects the PD isdue to groundwall delamination, and whencompared to the instrument manufacturer’sdatabase would be ranked as Very High.

PDC has been proposed as an off-line tool

that can detect issues besides contaminationand moisture absorption, such as somethingthat can be used to conrm the diagnosis ofinsulation condition obtained from on-linePD testing, without having to use a large actransformer to energize the winding for anoff-line PD or tip-up test. Te comparisontests described here did not produce consistentresults. Clearly more tests are required todetermine its efficacy.

INDUSTRY TOPICS

TESTING ROTATING MACHINERY POLARIZATION/DEPOLARIZATION CURRENT (PDC)

Table 1: Summary of Diagnostic Results for the Motor Stator Winding

Test Configuration IR PI VHF PD (mV) LF PD (nC) DF (@2 kV) Tip-Up

(GΩ) Qm+ Qm- Qm (%) (%) A 5.1 3.4 1066 888 9.7 4.7 0.79

B 5.2 3.4 275 330 3.7 4.6 0.73

C 5.0 3.4 433 440 2.9 4.8 0.80

REFERENCES1. IEEE, Recommended Practice for esting Insulation Resistanceof Rotating Machinery , IEEE std. 43-2000

2. ANSI/NE A, Standard for Acceptance esting Specications for Electr ical Power Distribution Equipment and Systems , 2009edition

3. ANSI/NE A, Standard for Maintenance esting Specications for Electr ical Power Distribution Equipment and Systems, 2007

edition

4. Stone, G., and Sasic, M., Experience with DC Polarization-Depolarization Measurements on Stator Winding Insulation,Electrical Insulation Conference (EIC), Ottawa, Ontario,March 2014.

5. IEC 60034-27, “Off-line partial discharge measurementson the stator winding insulation of rotating electricalmachines”

Vicki Warren, Senior Product Engineer, Iris Power LP. Vicki is an electrical

engineer with extensive experience intesting and maintenance o motor and generator windings. Prior to joining Iris in1996, she worked or the U.S. Army Corpso Engineers or 13 years. While with theCorps, she was responsible or the testing

and maintenance o hydrogenerator windings, switchgear,trans ormers, protection and control devices, development oSCADA sofware, and the installation o local area networks. At Iris, Vicki has been involved in using partial dischargetesting to evaluate the condition o insulation systems used inmedium- to high-voltage rotating machines, switchgear andtrans ormers. Additionally, she has worked extensively in thedevelopment and design o new products used or conditionmonitoring o insulation systems, both periodical and continual.Vicki also actively participated in the development o multiple IEEE standards and guides and was Chair o the IEEE 43- 2000 Working Group.

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