Paper_Experience With High Potential Testing Hydro Generator Multi Turn Stator Coils Using 60 Hz AC, DC and VLF (0 1 HZ)_2003

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Experience With High Potential Testing Hydro Generator Multi Turn Stator 
Coils Using 60 Hz ac, dc, and VLF (0.1 Hz) 
Stefan0 Bomben 
Ontario Power Generation 
14000 Niagara Parkway. 
Niagara-on-the-Lake, ON CANADA LOS ILO 
stefano.bomben@opg.com 
Howard Sedding John DiPaul Rick Glowacki 
Kinectrics Inc. 
800 Kipling Ave. 
Toronto, ON CANADA M8Z 6C4 
Absfract: There are many different methods of employing a high 
potential test on a stator winding. Three such methods that this 
paper will explore with reference to one another are the ac (50-60 
Hz), de, and very low frequency (VLF) (0.1 Hz). Some users 
choose the ac high potential test knowing that this test best 
simulates the voltage stress on the winding while in service. 
Other users prefer the de high potential test largely due to ease in 
performing the test. However, the dc voltage does not stress the 
stator coils the same way as when they are in service and may 
result in overly pessimistic results due to the influence of surface 
contaminants in the end windings. Finally, the VLF test, due to 
recent advances in technology, is becoming more practical for use 
in field conditions. However, the present standard governing the 
test is almost 40 years old and there is significant interest in what 
VLF voltage level best correlates with the ac and de high potential 
tests. This paper reports preliminary test results on three 
generator windings that were destructively tested using the ac, dc, 
and VLF methods as part of an ongoing eNort to provide a 
database upon which to set the appropriate VLF hipot level for 
modern synthetic resin-based stator insulation systems. 
INTRODUCTION 
Throughout the electrical industry, whether a user or 
manufacturer, high potential testing of rotating machines is 
necessary to ensure the machine is fit for service as well as 
peace of mind. Thermal and partial discharge (PD) 
breakdown, combined with the mechanical stresses during 
operation, are common ways to degrade insulation. There are 
many different types of high potential tests that can be used as 
3 fmai check to detehine if the insulation system is fit to 
raium to service. Theie high potential tests include the ac, dc, 
aid very low frequency (VLF) 0.1 Hz [I]. Some users, 
depending on the size'of the rotating machine, will use an ac 
high potential test, knowing that this test best simulates the 
voltage stress on the winding while in service and the results 
?!e easily interpreted. The transformer required for this test 
can be quite large and bulky and is limiting in terms of where 
it can be moved around to test from place to place. The dc test 
is very popular due to the compact size of the test unit, and the 
ease and speed at which a dc high potential test can be done. 
Further, recent work implies that the test may be capable of 
providing diagnostic information- when done under controlled 
conditions [2]. The compact size offers portability from 
location to location and the electrical hookup is quite simple. 
Unfortunately, the dc test mainly measures the conducting 
breakdown strength of the insulation. Recent advances have 
rendered VLF test sets which are very compact, easy to 
transport from place to place and affordable. The VLF test 
also has the advantage of stressing the insulation similar to that 
in service, with the results reflecting the three.main insulation 
breakdoddegradation categories, which include electrical 
conduction, thermal, and PD breakdown. However, a key 
issue is what level should a hipot performed under VLF 
conditions be set at? 
BACKGROUND 
The IEEE 433 working group on the Recommended Practice 
for Insulation Testing of AC Electric Machinery with High 
Voltage at Very Low Frequency [3] is currently rewriting the 
document. Hence, test results from testing generator windings 
using the three methods of high potential testing are being 
considered by the working group. It was decided in the 
working group that more corroboration between the ac, dc, and 
VLF voltage values was necessary in order to assist in 
fmalizing the document. 
As part of ongoing maintenance, a number of large hydro 
generators at Ontario Power Generation have been rewound. 
Prior to the rewind, it was decided to use some of these 
machines as subject windings and test them to failure using the 
0-7803-7935-7/03/$17.00 02003 IEEE 435 
mailto:stefano.bomben@opg.com
ac, dc, and VLF high potential test methods. Before the high 
potential tests were performed, a series of tests that included a 
polarization index, dissipation factor, capacitance, discharge 
inception, and discharge extinction voltage were done for 
information purposes and will not he discussed in this paper. 
All the windings that were tested were wye connected and 
hence the neutral connection was removed in order to achieve 
phase separation. Within the phases, phase leg segregation 
was also done. Due to the reactive limitation of the ac and 
VLF test sets. the configuration of the windme or sub-sections 
0.1 Hz. Individual coils on one circuit of tho: winding WCI 
isolated and every third coil was subject to one of the abo: 
high voltage tests. The results of the testing on Machine A a 
summarized in Table 2. 
- I 
Table 2: Summary of Hipot Tests on Machine A of the winding had to he changed. This allowed for proper 
testing of the coils and a more controlled test atmosphere. The 
test configurations will be specified in the test results section 
where, BD =breakdown; FO = flashover; ws =withstand 
Inspection of Table 2 shows that out of 22 hipots only soli below for each machine tested. 
Insulating blankets were used to prevent inadvertent flashovers 
to grounded parts of the generator. An insulating blanket was 
inserted where the cut was made in the phase leg in order to 
isolate the tested and non tested portion of the phase leg. The 
other two phases were grounded during the test period. This 
way, the coils were being tested phase to phase and phase to 
ground, which is a typical method of testing a winding before 
returning it to service. Each one of the tests was concluded 
once a failure or flashover occurred or the maximum voltage 
of the test set was reached and no failure occurred. 
TEST RESULTS 
insulation breakdown occurred under the ac hipot. Ti 
breakdown voltage was 4 kV ac rms and occurred on a LC 
that was one up from the neutral, thus failure in-service wou! 
he of low probability even with a hipot level as low as 4 k\ 
All of the other ac hipots resulted in flashovers ranging fro, 
20 to 46 kV ac rms. Flashovers under dc test conditions tor 
place in the range from 80 to 120 kV dc with a withstix 
voltage of 125 kV dc recorded on one coil. No breakdow 
resulted from application of the VLF power supply; tf 
majority of test objects withstanding 54 kV peak, the limit c 
the test set for this particular ioad. 
Removing the one ac hipot failure from consideration, tk 
remaining 21 coils in this circuit satisfy the various factcl 
The nominal ratings of the machines subject to the test level standard hipot requirements set out in the relevant IEb: 
program are outlined in Table 1 below. documents. Interestingly, the factory test levels WCI 
surpassed for the majority of the coils tested in this aged ;.P 
heavily contaminated winding. 
Testing of Machine B was accomplished using the same ?e 
equipment described above. However, in this case, due t 
constraints imposed by the rewind schedule, the winding w= 
Table 1: Nominal machine ratings 
Previous experience with breakdown studies in a field 
environment [4] has demonstrated the difficulty in minimizing 
flashovers and verifying solid insulation breakdown. 
Consequently, in the Tables helow that summarize the test 
results, breakdown is defined as an event in which two 
conditions are satisfied. 
1. 
2. 
A flashover is not observed. 
Subsequent attempts to re-energize the test object result in 
lower breakdown voltages or failure to raise voltage 
configured as followsfor the purpose of the tests. The dc sr 
VLF tests were performed on entire circuits consisting of 1 
coils each. Due to limitations in the ac high voltage poii. 
supply, only the top six coils from the line end in three cirwii 
were subject to the ac hipot. 
Hipot Test Number Number 
Objects BD I FOP 
nr I I r n 
I " I " I J 
Table 3: Summary of Hipot Tests on Machine B 
High voltage tests on Machine A were accomplished using a 
transformer and a 60 kV peak VLF power supply operating at 
The test data contained in Table 3 show that dc and ac h ip 
hipots, failures were recorded at 24 and 3 1 kV dc, significauii 
125 kV' dc 'Owe' a 6o kv' 2A resonant failures were recorded on this winding. Itegarding the r' 
436 
d o w the level required of new windings, 50 kVdc for a 13.8 
~V ac rms voltage class. The ac hipot breakdowns occurred at 
’7 and 30 kV ac rms, the former failure being just helow the 
’9 kVac rms level specified for this voltage class. Only 
:.ithstand voltages, in the range of 33 - 34 kV peak, were 
scorded using the VLF test set. Unfortunately, the current 
imitation on the power supply did not permit testing at 
dtages approaching the IEEE 433 limits, i.e., 47 kV peak. 
*. third stator winding, Machine C, was also subject to ac, dc 
:?d VLF hipot testing. The high voltage tests were conducted 
311 coil groups consisting of three complete coils. This type of 
e$! object was considered to provide an opportunity to 
d o r m hipots on the maximum number of coils possible in 
he time permitted while considering the limitations of the 
Bower supplies available. The ac hipots in this case were 
:xomplished using a 30 kV, 2A power supply due to the 
evailability of the 60 kV supply during the narrow time 
:,indow permitted by the rewind schedule. The results are 
-0ntained in Table 4. 
Test Number I Objects I BD 
than the factory levels. The majority of the test results 
show that, despite the aged condition of the windings, 
almost 90% of the objects tested withstood levels 
significantly above the factory or IEEE requirements. 
No failures attributable to the application of the VLF 
hipot were recorded. 
Examination of the ac and VLF hipot results shows that, 
where comparable, i.e., machines A and C, the 1.63 factor? 
suggested in a IEEE 433 - 1974 is not obviously invalid. 
Inspection of the ac and dc hipot results shows that, where 
comparable, the standard 1.7 factor published in the 
literature is appropriate. 
3. 
4. 
5. 
The above results and conclusions should he considered as 
very preliminary and further work consisting of similar 
controlled breakdown studies on aged, or even new, stator 
windings is required to answer the key question. Is the 1.63E 
value for a VLF hipot test still valid for modem synthetic 
insulation systems? 
REFERENCES Number Number 
FO WS 
DC 1 6 
AC I 6 - 
VLF 
1 5 0 
.6 0 0 
0 0 6 
I . Ward, B.H., J.P. Steiner, “Rotating Machine Insulation 
Evaluation At Very Low Frequencies”, IEEE Panel Session on 
I - 
:iiiistand voltages of 30 kV, this level still exceeds the (2E+1) 
-V value for new windings. The VLF hipots were limited to 
-0 kV peak by the load; however, the values achieved were in 
xordance with the 1.63 factor described in the current 
zision of IEEE 433. 
;ONCLUSIONS 
series of hipot tests employing ac, dc and VLF excitation 
.:hemes have been carried out on three hydraulic generator 
:?!or windings prior to rewind. These tests are part of an 
mgoing program to aid definition of the appropriate factor to 
nie to provide some means of comparability of conventional 
lower 6equency ac hipot testing with VLF hipots. The 
d m i n a r y results ofthe study show that, 
. The majority of the hipots, whether ac, dc or VLF, result 
in flashover or withstands, not solid insulation breakdown. 
The lowest breakdown voltage, regardless of excitation 
type, recorded was 4 kV ac rms. Five other breakdowns 
were recorded, of which three occurred at voltages lower 
3. IEEE Recommended Practice for Insulation Testing of AC 
Electric Machinery with High Voltage at Very Low Frequency - 
1974. 
G.C. Stone, H.G. Sedding, B.A. Lloyd, and B.K. Gupta, ”The 
ability of diagnostic tests to estimate the remaining life of stator 
insulation”, IEEE Trans. Energy Conversion, EC-3, December 
1988. 
4. 
BIOGRAPHY 
Stefann Bnmben is Sr. Engineer Hydro Generators at Ontario 
Power Generation (formerly Ontario Hydro). He received his 
Bachelors of Applied Science in Electrical Engineering 60m 
the University of Windsor in 1991. Upon graduation he took 
employment with Ontario Hydro at the Sir Adam Beck 
generating and one year later went to work at the head ofice 
of Ontario Hydro for the Hydraulic Plant Equipment-Electrical 
Department. For the past 12 years he has been working on the 
maintenance, repair, and overhauls of hydro generators, 
switchgear, and transformers. Stefan0 currently works at the 
Electricity Production Central Office of Ontario Power 
437 
Generation in Niagara in the Electrical & Protection & 
Controls group responsible for the condition monitoring and 
maintenance of hydro generators, transformers, switchgear and 
protection systems. Stefan0 is a registered Professional 
Engineer in the province of Ontario and an active member of 
the IEEE.