FIELD OF THE INVENTION
The present invention generally relates to the field of quality control test
parameters for impregnating epoxy resin and impregnation process used for
Global Vacuum Pressure Impregnation (VPI) of high voltage and high capacity
electrical rotating machines.
More particularly, the invention relates to a method for determining an
impregnability parameter of an impregnating resin-hardener mix, of viscosity and
reactivity exceeding the established norms, which eliminates the wastage of the
mix.
BACKGROUND OF THE INVENTION
Bisphenol-A epoxy resin and methyl-hexahydrophthalic anhydride system is
widely employed as an impregnant for the manufacture of large size electrical
generators and motors by VPI technology. The impregnant is a critical
component of the insulation in terms of cost of production and over-all reliability
of the electrical machines.
The VPI technology involves the use of a large quantity of the impregnating
resin-hardener mix in the range of around 60 tons for a number of impregnation
cycles/charges over a period of 1-2 years for large size turbogenerators. In order
to obtain a prolonged life. VPI resin is stored at temperature of 10-20°C under
dry N2 gas pressure of 1.5-2 bar or under vacuum less than 50 mbar. At the time
of impregnation of the electrical machines, this resin-hardener mix is heated to
impregnation temperature usually 60°C - 70°C and the quality of an
impregnation process is monitored through control of the following parameters in
order to obtain a void free insulation:
Viscosity: The VPI resin viscosity governs the rate and ability of impregnating
resin to penetrate the layers of insulation during VPI process. It is usually
measured at the impregnation temperature for example, 60°C or 70°C. The
viscosity of the fresh epoxy-anhydride VPI resin lies in the range of 18 to 30 cP
depending upon the temperature of measurement.
Reactivity: The VPI resin reactivity represents the rate of reaction of the epoxy
resin with anhydride hardener. It is determined by measuring the changes in
viscosity relative to initial value after 20 hours storage at 100°C. The value of
reactivity for the fresh VPI resin is limited to 3cP. The reactivity of the VPI resin
further increases when it comes in contact with the catalyst of the tape during
penetration into insulation. At higher reactivity, gelation of the resin may occur
during penetration before reaching the bottom layer of insulation, even if the
viscosity is within the permissible limit. Therefore, VPI resin reactivity is also to
be considered in addition to the resin viscosity for optimum control of
impregnation process.
The values of above mentioned parameters increase with the usage of the
impregnating resin for impregnation of electrical machines over a period of 1-2
years on account of its exposure to impregnation temperature, contamination,
heating/cooling cycles and catalyst contact during each impregnation process of
turbogenerator.
As per prior art practice, the impregnating resin system is allowed for use for
impregnation of electrical machines of 16.5 KV or more, only up to a viscosity of
40 cP and viscosity rise of 9 cP respectively measured at impregnation
temperature 60°C. These threshold values are maintained to avoid improper
impregnation, which however causes voids in the impregnated winding and thus

resulting in deteriorated tan delta and partial discharge (PD) values of the
electrical machines. After exceeding the above norms of viscosity and viscosity
rise, the whole quantity (60 metric tons) of the impregnating resin is rejected
and replaced with the fresh impregnating resin-hardener mix.
As per existing practice, the VPI epoxy resin having a viscosity and viscosity rise
measured at 60°C exceeding the norms of 40cP and 9cP respectively is not used
for further impregnation of the electrical machines.
The known parameters appear to be inefficient for assessing the impregnability
of the VPI resin. The present inventors during a series of experiments conducted
recognized that the prevalent norms and practice is oversimplification of the
process and that, there is a strong possibility of using the impregnating resin
beyond these norms if the inherent factors, controlling the quality of
impregnation are considered. As the quantity of the resin-hardener mix involved
is very large, without exploring the possibility of its further use and its early
rejection is totally uneconomical and effects bottom-line negatively.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method for determining an
impregnability parameter of a resin-hardener mix, of high viscosity and reactivity
compared to the establish norms, which eliminates the wastage of the mix.
Another object of the invention is to propose a method for determining an
impregnability parameter of a resin-hardener mix of high viscosity and reactivity,
which eliminates the wastage of the mix, which increases the productivity and
reduces the cost.
SUMMARY OF THE INVENTION
Accordingly, there is provided a method for determination of viscosity and
reactivity of a resin- hardener mix for improved impregnation of turbo-generator
in a vacuum pressure impregnation (VPI) process, the method comprising the
steps of measuring the viscosity (V) and reactivity (R) of a resin- hardener mix,
and concluding applicability of the resin- hardener mix, if V<40cp and R<<9 cp;
determining gel-time (t) at impregnation temperature if viscosity (V) and
reactivity (R) of the resin mix is more than 40cp and 9cp respectively; rejecting
the resin- hardener mix of gel-time (t) is less than 5-hours; determining cure
initiation temperature (T) if gel time of the resin- hardener mix is more than 5
hours; rejecting the hardener-resin mix if T<65°C (proposed impregnation
temperature); measure the variation of viscosity and reactivity if T>65°C, and
concluding the applicability of the resin- hardener mix, if V <40cp and R <9cp at
impregnation temperature range; and rejecting the resin- hardener mix, if
variation of viscosity and reactivity is more that 40co. and 9cp respectively.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows comparisons of tan delta values of U-phase of turbogenerators
impregnated in the VPI resin of viscosity and viscosity rise within and exceeding
the norms.
Figure 2 - shows comparisons of tan delta values of combined phases of
turbogenerators impregnated in the VPI resin of viscosity and viscosity rise within
and exceeding the norms.
Figure 3 - DSC curve indicating the start of cure temperature of VPI resin
containing 4phr zinc naphtenate catalyst.
Figure 4 - depicts a process flow chart for determination of impregnation
parameters of a resin mix for impregnation of a turbo generator according to the
invention.
DETAIL DESCRIPTION OF THE INVENTION
METHODOLOGY FOR ASSESSING THE IMPREGNABILITY OF VPI RESIN
During the impregnation process, the VPI resin comes in contact with the
catalyst of the insulating tape and also exposed to impregnation temperature 60
to 70°C. Accordingly, the inventive process determines the reactivity the
reactivity of the VPI resin in presence of the catalyst at the impregnation
temperature to asses its capacity of penetration into the insulation layers up to
the bottom layer. To simulate a testing process similar to real process, under
present invention, the change in the reactivity of VPI resin-hardener mix in
presence of the catalyst was studied. Hence, the reactivity of VPI resin which has
crossed viscosity of 40cP and viscosity rise of 9cP measured at 60°C in the
presence of catalyst and at the proposed impregnation temperature of 65±2°C
was determined by gel timer and Differential Scanning Calorimetery (DSC):
Determination of Gel-time
The gel time of the VPI resin-hardener mix plays a very important role in the
control of degree of penetration into the insulation of electrical machines. At
higher reactivity of VPI resin in presence of a catalyst of the Zinc Napthenate of
the insulating tape, and use of higher impregnation temperature, the VPI resin
may get gelled during impregnation before reaching the bottom layer of
insulation, even if the viscosity is within the permissible limit. To address this, the
gel time of the VPI resin in presence of the Zinc napthenate catalyst of the tape
at the proposed impregnation temperature of 65±2°C VPI resin was determined
and ensured that the gel time is higher than the duration of impregnation of 3 to
4 hours.
For determination of the gel time, a mix containing 4 parts of zinc napthenate
per 100 parts of VPI resin-hardener mix is prepared and data of gel time is
recorded. If the gel time so recorded is more than the duration of impregnation
time of 4 hours, it can be concluded that the VPI resin is capable of penetration
to the bottom layer of insulation.
Determination of temperature for initiating the cure reaction
The prior art is absolutely silent on a very important factor, i.e the determination
of cure initiation temperature. The cure initiation temperature should be
adequately higher than the impregnation temperature of 65 ±2 °C of the
generators that even if the impregnation process is delayed the resin-hardener
mix should not solidified.
The DSC (Differential Scanning Calorimeter) study of the VPI resin mix was
carried out for resin-hardener mix containing 4 phr (4 parts per 100 parts of VPI
resin) of Zinc Naphthenate to determine the initial point of cure reaction of the
resin-hardener mix in the presence of catalyst. This temperature should be
adequately higher than the impregnation temperature of 65 ±2 °C of generator.
Study of variation of viscosity and reactivity with temperature
As a precaution, in addition to the above test parameters, a further study on
change in viscosity and reactivity with temperature was made. The study reveals
that the viscosity and reactivity of the resin-hardener mix when exceeds 40 cP
and 9cP respectively, the viscosity measurements on the sample of VPI resin as
received and after its aging at 100°C for 20 hours with the variation in
temperature from 60 to 70°C during measurements should be carried-out. If the
viscosity and viscosity rise of the VPI resin up to impregnation temperature of 65
± 2 °C are below 40cP and 9cP respectively, the resin mix can be used
successfully at these temperatures.
A possibility was explored whether or not the resin-hardener mix can be heated
to a temperature higher than 60°C and the effect of higher temperature on its
reactivity. As the experimentation proved that the start of cure temperature as
determined by DSC (Differential Scanning Calorimetery) being about 90°C, the
mix can be heated above 60°C but below 90°C.
APPLICATION OF THE EXPERIMENTATION RESULTS
In accordance with the invention, the VPI resins having different viscosity and
viscosity rise more than the specified norms as indicated in the following table
were used successfully for impregnation of four Nos. of turbogenerators.
Viscosity and viscosity rise of different VPI resins used for impregnation of
Generators
EXPERIMENTS AND RESULTS
In accordance with the invention, the gel times and start of reaction
temperatures by DSC were determined for the above VPI resins having viscosity
and viscosity rise more than the specified norms. These results are shown in the
following table:
Gel time and start of reaction temperature by DSC of different VPI resins used
for impregnation of Generators
A typical DSC curve depicting the start of reaction obtained for one of the above
VPI resin is shown in Figur-3.
VERIFICATION OF THE IMPREGNABILITY OF VPI RESIN FOR
COMPLETE IMPREGNATION OF THE ELECTRICAL MACHINES
The performances of the above different impregnating VPI resins in respect of
their impregnability were assessed by conducting the following tests:
Impregnability test
A sample bar of 22 layers of Mica Fine glass tape to replicate the actual stator
bars was prepared. The ends of the bar were sealed with epoxy putty so that the
resin flows only perpendicular to the layers of the tape. This sample bar was tied
with the generator at the highest possible location i.e. at top most point of the
generator. After impregnation, individual layers were removed and examined for
their wetting with the resin. It was found that the resin has penetrated to the
last layers of insulation.
Quality of impregnated generators
The tan delta values obtained on the four Nos. of generators impregnated in the
VPI resin of respective viscosity and viscosity rise exceeding the norms are
shown in Table-1. The comparison of tan delta values of U-phase and combined
phases obtained for these generators have been made with those obtained for
the generators impregnated in the VPI resin of viscosity and viscosity rise within
the norms have been made in figs. 1 and 2. It may be observed from the graphs
that than delta values for the generators impregnated in the VPI resin exceeding
the norms of viscosity and velocity rise are better.
Partial discharge measurements
The partial discharge measurements carried out on the turbogenerators
impregnated in the VPI resin exceeding the norms of viscosity and viscosity rise,
indicate that the discharge inception voltage obtained is 15 kV which is higher
than the values of 8 to 10 kV obtained on the Turbogenerator impregnated in
the VPI resin having viscosity and viscosity rise within the norms.
BENEFITS
Besides quality improvements advantage, the proposed methodology helps in
deriving added benefit by using the VPI resin-hardener mix for about 20% more
number of impregnations.
WE CLAIM
1. A method for determination of viscosity and reactivity of a resin- hardener
mix for improved impregnation of turbo-generator in a vacuum pressure
impregnation (VPI) process, the method comprising the steps of:
- measuring the viscosity (V) and reactivity (R) of a resin- hardener mix,
and concluding applicability of the resin- hardener mix, if V<40cp and
R«9 cp;
- determining gel-time (t) at impregnation temperature if viscosity (V) and
reactivity (R) of the resin mix is more than 40cp and 9cp respectively;
- rejecting the resin- hardener mix of gel-time (t) is less than 5-hours;
- determining cure initiation temperature (T) if gel time of the resin-
hardener mix is more than 5 hours;
- rejecting the hardener-resin mix if T<65°C (proposed impregnation
temperature);
- measure the variation of viscosity and reactivity if T>65°C, and
concluding the applicability of the resin- hardener mix, if V <40cp and R
<9cp at impregnation temperature range; and
- rejecting the resin- hardener mix, if variation of viscosity and reactivity is
more that 40co. and 9cp respectively.
2. A method for determination of viscosity and reactivity of a resin- hardener
mix for improved impregnation of turbo-generator in a vacuum pressure
impregnation (VPI) process as substantially herein described and
illustrated with reference to the accompanying drawings.

The invention relates to a method for determination of viscosity and reactivity of
a resin- hardener mix for improved impregnation of turbo-generator in a vacuum
pressure impregnation (VPI) process, the method comprising the steps of
measuring the viscosity (V) and reactivity (R) at a resin mix, and concluding
applicability of the resin- hardener mix, if V<40cp, and R<<9 cp; determining
gel-time (t) at impregnation temperature if viscosity (V) and reactivity (R) of the
resin mix is more than 40cp and 9cp respectively; rejecting the resin- hardener
mix of gel-time (t) is less than 5-hours; determining cure initiation temperature
(T) if gel time of the resin- hardener mix is more than 5 hours; rejecting the
resin- hardener mix, if T<65°C; measure the variation of viscosity and reactivity
if T>65°C, and concluding the applicability of the resin- hardener mix, if V
<40cp, and R <9cp at impregnation temperature range; and rejecting the resin-
hardener mix, if variation of viscosity and reactivity is more that 40cp and 9cp
respectively.