Field of invention :
The present invention generally relates to monitoring of dielectric parameters
during curing and particularly to a device and method for continuous on-line
monitoring of dielectric parameters during curing of totally impregnated
stators of rotating electrical machines.
Background and prior art:
Insulation plays a vital role in the performance and life expectancy of any
electrical equipment, specially when they are meant for high voltage
applications. The latest and most preferred mode of insulation for stators of
large electrical machines is by means of thermo-setting resins. The stator
winding is dipped in low viscosity resin under specific temperature/ pressure
cycle to achieve total impregnation of the winding in the resin. The
impregnated winding is heated and then allowed to cool in a controlled
fashion in an oven, where the stator is made to rotate slowly so that uniform
distribution of the resin is achieved.

The controlled heating allows the resin to set. The process is called curing of
resin. Proper curing is very important for the resin to achieve its maximum
insulating property as well to make it resistant to mechanical shocks and
vibration. Quality of curing therefore directly affects the performance and life
of the machine.
To ensure proper curing, the dielectric parameters, namely tan 5 and
capacitance values should be measured during the entire curing process. In
the prior art, there was no device or method for measuring these values
continuously. The impregnated stator, called "job" or "object", is rotated in
the oven during curing. This posed great difficulty in the measurement of
these parameters in the prior art. The job had to be stopped for
measurements to be undertaken. These stoppages made the curing process
longer and distribution of resin also became unpredictable.
Presently multi-channel dielectric parameters monitoring systems are mostly
restricted to using hardware based signal processing and that too for
capacitance measurement only. Commonly available such systems are for use
with small samples which can be electrically floated (ungrounded object).
In some of the prior art systems for monitoring of impregnation process, only
current through insulation/ dielectric of job is monitored which is only indirect
and approximate indication of dielectric properties.
In case of curing the heavy object (impregnated stator winding whose
dielectric parameters are to be measured) is placed on heavy rollers of the

wagon so that stator core can be rotated during curing in the oven at high
temperature. As such the stator core is not isolated from the earth. This
poses problem in measuring dielectric parameters with the object earthed.
Further, since impregnated stator core is initially rotated for even distribution
of resin in the winding this also poses problem in making electrical contacts
between signal cables from moving stator core/winding and fixed cables from
control room.
The main drawbacks/disadvantages of the prior art monitoring systems are:
1. Systems are for use with jobs/objects which can be electrically
floated and cannot be used with object grounded condition.
2. Since impregnated stator core remains in rotation, poses
problem in making electrical contacts between signal cables
from moving part to stationary parts.
3. In some of the prior art systems, for monitoring of
impregnation/curing process, only current through
insulation/dielectric of job is monitored which is only indirect
and approximate indication of dielectric properties.
4. Following features are not available in prior art systems
- Alarm/warning level setting hardware
- Alarm/warnings signal detection & generation hardware
card
- Alarm/warning Status Display hardware for all channels

- LED indication of Alarm/Warning status, Relay Drivers &
Relays.
5. They use extensive electronics hardware circuits for processing
of signal (output from sensors/ electrodes across stator winding
in stator core),
6. Decreased reliability because of large number of electronics
components, circuits and interconnections
7. Higher cost
The present invention seeks to ameliorate these drawbacks existing in the
prior art.
Objects of the invention:
An object of the invention is to provide simple, compact and cost effective
device and method for direct on line continuous monitoring of dielectric
parameters during curing of total impregnated stator winding under rotation
in the oven.
Another object of the invention is to make curing data available throughout
curing to enable the operator to take corrective measures at any time.

Yet another objective of the invention is to achieve better curing in shorter
time.
Description of the invention:
The invention provides an input circuit, interface for connections and using
them to continuously process information regarding changes in the dielectric
parameters and their trend as the process of curing progresses. Since the
dielectric parameters are continuously monitored it is possible to determine
exact point of completion of curing process and to take corrective measures
in curing regime as deemed necessary. To date this is the only device to
ascertain the condition of the state of cure of winding of the generators
providing a window to the actual process of curing of the winding of the
generators. Further, data generated using this system can be used for
optimization of total process as such.
Thus, this system can lead to increase in productivity and overall
improvement in quality.
On the basis of many years of experience in the field of dielectric parameters
monitoring and analysis, the applicant has developed an apparatus and
method for On-line Continuous Dielectric Parameters Monitoring during curing
of totally impregnated stators. This system consists of inherent sensor
electrodes, earthed job inside oven (under mechanical rotation), slip ring

assembly for taking out electrical connections from rotating job, low-noise
signal cables, multi-channel signal conditioner, multiplexed data acquisition
interface, DAS cards, computer, digital signal processing software, operating
system software etc.
The invention will now be described in details with the help of following
accompanying drawings.
Brief description of the accompanying drawings :
Fig. 1 is the signal flow diagram of the apparatus according to the
invention.
Fig. 2 shows the schematic representing the basic circuit for monitoring
dielectric parameters.
Fig. 3 shows the block diagram of the apparatus for on-line dielectric
parameters monitoring according
Fig. 4 shows a typical plot of trend of dielectric parameters during curing
according to the invention.
According to the method of the invention, electrodes and transducers are
placed / configured at predetermined locations within the stator before it
is subjected to total impregnation in a resin chamber (not shown). The
impregnated stator, called "object", is then transported to an oven (3.1)
where it is rotatably installed and provided with slow rotating motion while
in the curing progresses. The electrodes of the transducers embedded

within the impregnated stator are connected to special slip-ring and brush
gear assembly to enable the generated signals to be taken out for analysis
as shown in Fig. 1 . Special low- noise signal cables one employed for the
connection.
The sensor (2.3) is connected in series with the winding insulation of the
object under test. Excitation signal is fed to the circuit from a signal
generator (2.1). The voltage drop across the sensing series resistor
gives the signal proportional to current flowing in the insulation of the
machine which is captured by the data acquisition system DAS (2.2)
Both the signals corresponding to current in the winding insulation and
excitation signal are fed to the multi channel DAS (2.2) and their
signal waveforms are captured almost simultaneously and stored.
These waveforms are processed to obtain their magnitudes and the
phase difference between these two signals and various other
parameters are calculated using this information (1.1). Measuring points/
connection of sensor electrodes and routing of cable are selected so as to
allow for safe and reliable measurement.
Exemplary setup depicting location of job (generator stator core) &
instrument layout, overall block diagram is shown in Figure 3. This is a
fully automated process for on-line and continuous monitoring and does

not require presence of operators. Also this gives on-line data in a
graphical form during curing and does not involve any post processing.
This data is processed and obtained in graphical form as dielectric
Parameters- capacitance and dissipation factor v/s time. A typical
sample
graphical plot is placed at Figure 4.
The hardware and an exhaustive digital signal processing software based
on statistical signal analysis technique and extensive digital filtering has
been developed for on-line monitoring of dielectric parameters. This
software is menu driven, interactive and highly flexible in nature.
All the above i.e. data acquisition, digital signal processing, display
storage, analog output and alarms signal processing are achieved using
onboard software and hardware in the PC which is a normal desk top PC
with common configuration having ISA/PCI interface slots and interrupt
driven DMA memory access as against using extensive separate
hardware like signal processing card , additional PCB boards for signal
conditioning etc. and sometimes even using additional PCs.
According to the invention (Circuit schematic shown in Figure 2), the
object (insulation /dielectric of winding) is provided with the excitation
from electrically floating excitation source or signal generator (2.1).

Simultaneously this excitation signal from signal generator is
connected to primary of the isolation transformer (2.4). Reference
signal corresponding to the excitation signal is tapped from secondary
of the transformer. One end of the secondary of this transformer (2.4)
is connected to earth. Signal corresponding to leakage current flowing
across the insulation of stator winding is sensed through sensor series
resistor (2.2), connected between LV end of excitation source and
earthed end of job and is fed to channel 2. Thus this circuit enables
tapping of both reference signal as well as current signal with respect
to earth, while retaining phase relationship between them which
otherwise would not have been possible.
Both these signals without any signal conditioning are fed to the
computer for digital signal processing through DAS interface
computer. This DAS interface translates analog signals into digital
ones for computer processing.
Further, as mentioned above, immediately after taking out of the
impregnated stator winding from impregnation tank it is placed on
rollers of wagon and wagon is moved in to the oven . To avoid uneven
resin flow in the winding stator is kept in slow speed rotation. So for
transmission of all electrical signals, from rotating stator winding in
the oven to stationary measuring circuit outside the oven, connections
are made using special brush gear and slip ring assembly.

Functions of various bocks depicted are briefly explained below in
the block diagram of the system in Figure 1.
Block-Multiplexing Input Signal. Sampling A/D Interface circuit:
Here high speed sequential sampling (predetermined rate) of analog
signal meeting required criteria of sampling speed is achieved. This is
carried out simultaneously for all channels in the form useable for
direct digital signal processing.
Block-Output signal conditioning, D/A converts, Interface circuit:
Integrated PC based system processes the signals to provide standard
analog signal outputs (voltage or current) proportional to magnitude
of capacitance & tan 5 to be connected to central digital data
acquisition/ processing system and it does not require any additional
hardware for alarm signal processing and converting indicated /
measured data into suitable standard analog output signal.
Block for Tabular display of data and Block for Graphical Trend
display & Plotting:
Provision is made for display of trend of processed data in a specific
format as shown in trend plot attache. (Fig.4) and for accessing the
details of the same.
Integration of complete system (consisting of input signal cables,
conditioning amplifiers, signal processing and display system, relay

circuits, analog output circuits, plotter ,power supply in a single
location stand-alone system makes it easy to handle.
The apparatus in accordance with the invention also has the
following features which were not available in the prior art.
- Alarm/warning level setting
- Alarm/warning signal detection & generation
-Alarm/warning status display
-LED indication of Alarm/Warning status, relay drivers & relays
The alarm/warning levels can be set as desired for each
operation, as these values are dependent on the quality of
resin, size of the job etc.
Block diagram (1.0) in Figure 1, partially depicts the On-line
Continuous Dielectric Parameters Monitoring Device (3.0)
according to the present invention. Here in input signal
conditioning block input circuit connection, excitation and
measuring signal conditioning is involved. The dielectric of
winding is provided with the excitation from unearthed
excitation source (signal generator) (2.1). Parallelly
this excitation signal is connected to primary of the isolation

transformer (2.4). Reference signal (2.5) corresponding to
this excitation signal is tapped from secondary of this
transformer. One end of the secondary of this transformer is
connected to earth. Signal corresponding to leakage current
flowing across the insulation of stator winding is sensed
through sensor (series resistor) (2.3) connected between LV
end of excitation source and earthed end of job and is fed to
channel no.2. Thus this circuit enables tapping of both
reference signal as well as current signal with respect to
earth, which otherwise would not have been possible.
Further as stator is required to be kept in slow speed
rotation, for transmission of all electrical signals, from
rotating stator winding in the oven to stationary measuring
circuit outside the oven, provision is made using special
brush gear and slip ring assembly. This ensures
seamless signal transmission to measuring circuit.
The detailed description of the invention discloses only an
exemplary and preferred embodiment of the invention.
This description however is deemed to cover any other
embodiment of the invention.

WE CLAIM:
1. A device for on-line continuous monitoring of dielectric parameters during
curing process of totally impregnated stators of electrical motors of a
curing oven comprising :
- an electrically floating excitation source signal generator for
generating and feeding excitation signals to a sensing circuit,
- the sensing series resistor connected in series with the insulation of
the impregnated stator;
- a multi-channel data acquisition system (DAS) capturing the
voltage drop across the sensing circuit which represents the current
flowing in the insulation of the stator;
- the multi-channel data acquisition system being fed with both the
signals corresponding to current in the winding insulation and the
excitation signal, the generated signal waveforms being captured
and stored;
- a computer apparatus with processor and memory operably
connected to the multi-channel DAS for processing the captured
waveforms to obtain their magnitude and the phase difference
between these two signals and associated data for the curing
process; and

- a display device interfaced to the computer apparatus to graphically
exhibit dielectric parameters such as capacitance, and dissipation
factor vs time.
2. Device as claimed in claim 1, wherein the stator under test is rotated
during curing, when electrical signals are carried to and from said job
through special slipring and brush gears.
3. Device as claimed in claim 1, wherein monitoring and plotting of dielectric
parameters are done continuously and automatically without any operator.
4. Device as claimed in claim 1, wherein generates alarm or warming signals
based upon the levels set for these signals.

ABSTRACT

TITLE : "A DEVICE FOR ON-LINE CONTINUOUS MONITORING OF
DIELECTRIC PARAMETERS DURING CURING PROCESS OF TOTALLY
IMPREGNATED STATORS OF ELECTRICAL MOTORS IN A CURING OVEN"
The invention relates to a device for on-line continuous monitoring of dielectric
parameters during curing process of totally impregnated stators of electrical
motors of a curing oven comprising an electrically floating excitation source
signal generator for generating and feeding excitation signals to a sensing circuit,
the sensing series resistor connected in series with the insulation of the
impregnated stator; a multi-channel data acquisition system (DAS) capturing the
voltage drop across the sensing circuit which represents the current flowing in
the insulation of the stator; the multi-channel data acquisition system being fed
with both the signals corresponding to current in the winding insulation and the
excitation signal, the generated signal waveforms being captured and stored; a
computer apparatus with processor and memory operably connected to the
multi-channel DAS for processing the captured waveforms to obtain their
magnitude and the phase difference between these two signals and associated
data for the curing process; and a display device interfaced to the computer
apparatus to graphically exhibit dielectric parameters such as capacitance, and
dissipation factor vs time.