Paper_Sealed Winding Conformance Testing and Recent Revisions to NEMA MG-1_2007

Vista previa del material en texto

1 
SEALED WINDING CONFORMANCE TESTING AND RECENT REVISIONS 
TO NEMA MG-1 
 
Copyright Material IEEE 
Paper No. PCIC-2007-14 
 
 Meredith K. W. Stranges Bharat Mistry Ramtin Omranipour 
 Member, IEEE Member, IEEE Member, IEEE 
 GE Consumer & Industrial GE Consumer & Industrial GE Consumer & Industrial 
 107 Park St. N. 107 Park St. N. 107 Park St. N. 
 Peterborough, ON K9J 7B5 Peterborough, ON K9J 7B5 Peterborough, ON K9J 7B5 
 Canada Canada Canada 
 meredith.stranges@ge.com bharat.mistry@ge.com ramtin.omranipour@ge.com 
 
 
Abstract - The sealed winding conformance test is often 
performed during the final acceptance test sequence for 
industrial motor stator insulation. The stator winding is first 
completely soaked with water, then receives a 10-minute 
insulation resistance (IR10) measurement, followed by a 60-
second high potential (hipot) test at 1.15 times rated line-to-
line voltage (VLL). A 1-minute insulation resistance (IR1) test 
is performed immediately after the hi-pot test. To define the 
test requirement, API 541 and 546 standards reference 
NEMA MG-1 20.18, which specifies an acceptance criterion 
for both IR measurements of (MΩ) ≥ VLL + 1, where [VLL + 1] 
is in kV. NEMA MG-1 20.18 refers to IEEE Std 43 for the 
insulation resistance test method and minimum insulation 
resistance. Until its most recent revision, IEEE Std 43 
recommended the minimum IR1 (MΩ) ≥ VLL + 1 (kV). 
However, the current version of IEEE Std 43 (2000) 
recommends IR1 ≥ 100 MΩ for all stator windings built after 
about 1970. Modern epoxy-mica insulation systems should 
be able to meet this value when fully clean and dry. The wet 
conditions encountered during sealed winding tests may 
produce a different response in the insulation system. The 
sealed winding test is outside the scope of IEEE 43. The next 
revision of NEMA MG-1 20.18 will clarify the sealed winding 
test acceptance level of IR (MΩ) ≥ VLL + 1 (kV). This paper 
investigates the effect of test solution conductivity on the IR 
measurement, provides the NEMA-approved clarification of 
the sealed winding test specification and suggests further 
clarifications to the sealed winding test procedure. 
 
Index Terms — Sealed winding test, insulation resistance, 
hipot, stator winding. 
 
I. INTRODUCTION 
 
A. Purpose of Sealed Winding Test 
 
The sealed winding conformance test verifies that a form-
wound stator can be energized in a very humid atmosphere 
without danger of winding failure. It uses insulation resistance 
(IR) to confirm that the complete insulation system, including 
the endwinding, is sealed. The principal benefit of the sealed 
winding conformance test is its ability to detect moisture-
sensitive defects in the winding of a machine, particularly in 
the connections. If the phase connections, pole jumpers, and 
circuit rings are not adequately sealed, test failure is likely. 
Other test regimens employ condition-based monitoring to 
evaluate insulation performance trends for a specific installed 
machine [1, 2], or to assure the reliability of system design [3, 
4, 5]. A high IR value alone does not prove the quality of the 
insulation, nor does it guarantee its long-term performance in 
operation. 
The sealed winding conformance test is customer-
specified and generally applied to open machines. Examples 
include open drip-proof (ODP), and weather-protected (WP) 
units operating in wet, dirty and chemically corrosive 
conditions. The sealed winding test can also prove a sealed 
system on totally enclosed machines equipped with water-air 
or air-air coolers (TEWAC or TEAAC). 
 
B. Test Procedure 
 
The sealed winding test uses IR and hipot testing as 
acceptance criteria. NEMA MG-1 [6] provides the sealed 
winding test procedure and IR acceptance criterion. IEEE Std 
43 [7] explains the theory and method for the IR test. 
All exposed areas of the stator winding must be completely 
soaked with water of specified surface tension. This can be 
done either by immersion of the entire stator, or by thoroughly 
spraying the stator while it is held in a test fixture; the 
methods are considered interchangeable. Stator windings 
that cannot be submerged must be sprayed for thirty minutes. 
NEMA MG-1 does not specify the wet test solution 
conductivity, although it provides a maximum surface tension. 
The soaked stator winding receives a 500-V dc IR test. The 
measurements are taken after applying the dc voltage for one 
minute (IR1), then every minute thereafter for a total duration 
of 10 minutes. The insulation resistance at 10 minutes (IR10) 
in MΩ must be at least VLL + 1 (kV). The winding is hipot 
tested at 1.15 times VLL. The IR1 taken following the hipot test 
must also meet the minimum value of VLL + 1 (kV). 
This paper presents data for stators tested using solutions 
of different conductivities. The clarification of the NEMA MG-1 
acceptance criterion is also discussed, along with some 
further ideas regarding the interpretation of the sealed 
winding test. 
 2 
II. SEALED WINDING TEST 
 
A. Effect of Solution Conductivity 
 
Three stator windings rated 13.8 kV were subjected to the 
sealed winding conformance test, using solutions of different 
conductivities. The intent was to determine the effect of 
varying the solution electrical conductivity on the insulation 
resistance values measured during the sealed winding test. 
The solutions chosen were deionized (DI) water, a mixture of 
DI and tap water, and tap water alone. A precipitation rate of 
5 ± 0.5 mm/min was used, according to the recommended 
value for the wet test parameter [8]. A surfactant was added 
to each solution to reduce the surface tension to less than the 
value specified in NEMA MG-1. Table 1 shows the tested 
properties of each solution. The solution conductivity for test 
B was recommended by [8]. 
Fig. 3 shows a stator prepared for its spray test. Shown are 
the connection end of the stator and the spray nozzles. Test 
solutions were applied in order of lowest to highest 
conductivity. The stator was not dried between the tests. 
Sufficient time was allowed between successive tests so that 
the absorption current levels would not be influenced by the 
prior test. Figs 4 to 6 show values of IR vs. time for the 
subject stators, corrected to 40°C. 
 
TABLE 1. 
PROPERTIES OF AQUEOUS SPRAY TEST SOLUTIONS 
Stator Test Solution 
Surface 
Tension 
(dyn/cm, 
at 25°C) 
Conductivity 
(mS/m, at 
20°C) 
A DI Water 22 3.0 
B DI + Tap Water 25 5.0 1 
C Tap Water 28 19.3 
A DI Water 30 1.0 
B DI + Tap Water 25 5.0 2 
C Tap Water 30 19.2 
A DI Water 30 2.2 
B DI + Tap Water 26 5.9 3 
C Tap Water 29 19.8 
 
 
Fig. 3: A 13.8 kV stator prepared for spray testing 
 
Fig. 4: Insulation resistance for stator 1 during the spray test, 
using solutions of different electrical conductivity 
Fig. 5: Insulation resistance for stator 2 during the spray test, 
using solutions of different electrical conductivity 
 
0
5
10
15
20
25
30
35
40
0 2 4 6 8 10 12
Time (minute)
IR
 (G
oh
m
s)
Test A Test B Test C
0
5
10
15
20
0 2 4 6 8 10 12
Time (minute)
IR
 (G
oh
m
s)
Test A Test B Test C
0
5
10
15
20
0 2 4 6 8 10 12
Time (minute)
IR
 (G
oh
m
s)
Test A Test B Test C
 3 
Fig. 6: Insulation resistance for stator 3 during the spray test, 
using solutions of different electrical conductivity 
III. DISCUSSION 
 
A. Spray-testing Experiments 
 
Insulation resistance is the quotient of the applied direct 
voltage across the insulation divided by the total resultant 
current at a given time. The total resultant current is the sum 
of four different currents: surface leakage, geometric 
capacitance, conductance, and absorption [7]. When the 
direct voltage is applied to the stator winding of rotating 
machines, the geometric capacitance current decays to zero 
within a few seconds and the IR increases rapidly. The 
increase continues at a lower rate as the absorption current 
decays. The conductance and surfaceleakage currents 
remain constant through the IR test. For modern epoxy 
insulation systems, the conductance current is very small. 
The value of the surface leakage current depends on the 
condition of the stator winding and the length of the leakage 
path. During the sealed winding test, the stator winding is 
soaked with the water; this can create high surface leakage 
current, resulting in low IR. This is especially true for stator 
windings with relatively small clearances and short 
endwindings. 
Insulation resistance depends on the design of the 
particular winding; end arm length, coil-to-coil and phase-to-
phase clearances, length of grading material and voltage 
rating all play a role. The voltage distribution over the 
endwinding of a dry stator varies depending on location along 
the coil end arms. During the sealed winding test, the 
distribution of moisture over the end winding and connections 
ensures that the entire winding is maintained at common 
ground potential while the dc- and ac test voltages are 
applied [9]. 
The results of the spray test using different solutions 
suggest that the solution conductivity did not significantly 
affect the IR. Values in the GΩ range indicate that the stator 
windings were sealed. Stators 1 & 3 had similar endwinding 
geometry and showed similar IR values. The endwinding 
geometry of stator 2 differed from that of the other two units, 
and it had less coils than the other units. It is curious that the 
measured IR values for the test with DI water were less than 
the other two tests. One possible explanation is the presence 
of minor contamination on the stators endwindings that was 
washed off during the first test. 
 
B. Test Standards 
 
The 2006 revision of NEMA MG-1 is vague, because it 
requires spray tested stators to have IR (MΩ) ≥ VLL + 1 (kV), 
and references IEEE Std 43 for IR acceptance criteria. The 
intent of the reference to IEEE 43 is simply to establish a test 
method, but its 2000 revision recommends IR1 ≥ 100 MΩ for 
all stators with modern epoxy-mica insulation systems. 
Previous versions of IEEE 43 recommended IR1 (MΩ) ≥ VLL + 
1 (kV). The reference to the capability of modern systems to 
meet a higher IR value does not consider the sealed winding 
test. It should be noted that IEEE 43 cautions the reader 
about the limited information offered by the IR test, and that 
the observed values can be affected by even a small degree 
of moisture. The unintentionally conflicting statement in 
NEMA MG-1 offering two distinct acceptance criteria creates 
a potential confusion for the reader. 
The clarification as approved by the committee for NEMA 
MG-1 says: 
“a) After the spray test: Using 500 V dc, take a 10-minute 
insulation resistance measurement, following the procedure 
as outlined in IEEE 43. The minimum insulation resistance in 
megohms shall be machine rated kilovolts plus 1. 
b) After the 60Hz, 1.15 times rated voltage hi-potential test: 
Using 500 V dc, take a 1-minute insulation resistance 
measurement, following the procedure as outlined in IEEE 
43. The minimum insulation resistance in megohms shall be 
machine rated kilovolts plus 1.” 
Although NEMA MG-1 calls for a 30-minute spraying time 
on the winding, it is not clear whether the initial IR 
measurement should be taken after the half-hour of spraying 
is complete, or if the ten minutes of IR measurements may 
begin after twenty minutes of spraying. NEMA MG-1 does not 
specify the aqueous solution conductivity. 
 
IV. CONCLUSIONS 
 
The sealed winding conformance test is a brief and 
superficial check of the condition of exposed areas of a stator 
winding. Measurement of insulation resistance and withstand 
of the accompanying hipot test used during the sealed 
winding test do not fully evaluate insulation system reliability 
(i.e., long-term life). Acceptance by a sealed winding test 
does not serve as a stand-alone quality measurement, but 
should be accompanied by other diagnostic tests to provide a 
more complete measure of insulation system acceptance. 
Although NEMA MG-1 clearly relates the sequence of 
events of the spray test, it is not clear if the IR testing or hipot 
should begin while the stator is being sprayed, or afterwards. 
Aqueous solution parameters and precipitation rates are also 
not provided. Test results from spraying three sealed 13.8 
kV-rated windings with different solutions suggest that the 
solution conductivity does not significantly affect the IR. 
There is no doubt that the details of the test procedure can 
certainly affect the test result. To ensure a reproducible test 
result, the method must be consistent. The authors believe 
that the value of the sealed winding test and its ease of 
interpretation would be enhanced by further clarification of 
the method, and by including solution properties, perhaps by 
reference to IEEE Std 4 [8]. Further work is required to 
determine the effect of solution electrical conductivity and 
precipitation rate. 
The NEMA committee has approved the clarification of 
minimum acceptable insulation resistance given in NEMA 
MG-1 for sealed winding tests. The acceptance value 
remains IR (MΩ) ≥ VLL + 1 (kV), and the document will 
continue to reference IEEE 43 for the insulation resistance 
test method. Unique machine specifications may request a 
higher IR value, but the industry standard for a soaked 
winding does not exceed IR (MΩ) = VLL + 1 (kV). 
 
V. REFERENCES 
 
[1] IEEE Std 1434 (2000), Trial-Use Guide to the 
Measurement of Partial Discharges in Rotating 
Machinery 
 4 
[2] IEEE Std 286 (2000) Recommended Practice for 
measurement of Power Factor Tip-Up of Electric 
Machinery Stator Coil Insulation 
[3] IEEE Std 1043 (1996), Recommended Practice for 
Voltage-Endurance Testing of Form-Wound Bars and 
Coils 
[4] IEC 60727, Parts 1 & 2, Evaluation of electrical 
endurance of electrical insulation systems (IEC Central 
Office, Geneva, Switzerland) 
[5] IEEE Std 429 (1994), Recommended Practice for 
Thermal Evaluation of Sealed Insulation Systems for 
AC Electric Machinery Employing Form-Wound 
Preinsulated Stator Coils for Machines Rated 6900 V 
and Below 
[6] NEMA MG 1-2006, Motors and Generators (National 
Electrical Manufacturers Association, Rosslyn, VA) 
[7] IEEE Std 43 (2000), Recommended Practice for 
Testing Insulation Resistance of Rotating Machinery 
[8] IEEE Std 4 (1995), IEEE Standard Techniques for High-
Voltage Testing 
[9] M. Costello, F. Cook, F. Heredos, “Winding Immersion 
Testing: “Are Our Fears All Wet?” (PCIC Conference 
Record, 13-15 Sept. 1993) pp. 167 - 170. 
 
VI. ACKNOWLEDGEMENTS 
 
The authors would like to thank R. Perks for stator testing 
and D. Messervey for his technical review of the paper. 
 
VII. VITA 
 
Meredith Stranges holds degrees in Chemistry and 
Metallurgical Engineering from Brock University and 
McMaster University, respectively. She joined General 
Electric in 1997, and in 2004 became the Lead Insulation 
Engineer for GE Peterborough. Meredith specializes in 
qualification of insulation systems and materials for large 
industrial motors, and she has co-authored several papers in 
conjunction with colleagues. A senior member of IEEE, she is 
active in the Dielectrics and Insulation Society and the 
Standards Association. Meredith participates on international 
standards working groups for the IEEE-SA and IEC TC 2 on 
Rotating Machines. She is registered as a Professional 
Engineer in the province of Ontario. 
Bharat Mistry received the B.E. degree in electrical 
engineering from South Gujarat University, Surat, India in 
1972. He immigrated to Canada in 1979 and began 
practicing as a Professional Engineer in Ontario. He has 
served industry in many areas including quality, maintenance 
and design. During the past 15 years, he has devoted his 
time to design and application of large rotating electrical 
machines for hazardous and non-hazardous locations, built 
to National and International standards.He has published 
many papers in “Electrical India”, and as an author and 
coauthor for IEEE/PCIC papers. He is a working group 
member for the development of NEMA MG-1 (global 
standard), IEEE 1349, and IEC TC 31/WG 27. 
Ramtin Omranipour graduated with a B.Sc. in Electrical 
Engineering from University of Science and Technology, 
Tehran, Iran, in 1991. Between 1991 and 1999, he worked as 
an Electrical Engineer for multiple organizations where he 
was in charge of maintenance and repair of electrical 
systems and equipment including high voltage transformers 
and rotating electric machines. In 2000, he began graduate 
studies at University of Waterloo, Ontario, Canada, where he 
conducted research on high voltage insulating materials 
mainly silicone-based products. In 2002, he joined the GE 
Large Motors & Generators Technology team at 
Peterborough, Ontario, Canada, as a technical designer. His 
area of interest is the design and development of large AC 
motors and generators, and development of insulation 
systems for large rotating machines.