FIELD OF INVENTION
The present invention relates to a core flux test set up with alternate primary winding
system and process for carrying out core flux testing for all types of turbo generators.
BACKGROUND OF THE INVENTION
A turbo generator is the combination of a turbine directly connected to an electric
generator for the generation of electric power. It consists of a stator with three phase winding
for electromagnetic induction and a rotor with rotating filed. A mutual flux developed across
the air gap between the rotor and stator causes the interaction necessary to produce an EMF.
As the magnetic flux developed by the DC field poles in rotor crosses the air gap of the stator
windings, a sinusoidal voltage is developed at the generator output terminals.
In lower ratings stator and rotor windings are air cooled or hydrogen cooled. In upper
ratings, stator core and rotor are hydrogen cooled while stator windings are water cooled.
Turbo generator comprises of three main components - stator, rotor and stator core. A
TG stator has three phase winding in two layers laid down in stator core slots and the stator
core is in turn welded to stator frame which is rigid fabricated cylindrical frame and withstands
weight of core & winding, forces & torques during operation.
A TG rotor is a high strength alloy steel single forging prepared by vacuum cast steel to
withstand the forces and torques while running at 3000 rpm. It contains slots for housing field
windings to provide rotating magnetic flux necessary for electromagnetic induction.
The stator core is made up of stacked thin ETS sheets with a low loss index with
insulated varnished on both the sides. Vent segments are present at designed intervals for

flow of cooling gas. To make the core a monolithic mass, the stampings are tightened with
long steel bolts passing through the stampings and core press ring at both the ends. The
stator core can be made inside the stator frame, as is the case in higher ratings, or outside the
frame and later fitted into it.
The purpose of the stator core is two ways:
1. Support the winding
2. Carries the flux
To minimize the hysteresis losses, ETS sheets used are 4% Silicon Alloyed COLD
ROLLED NON-GRAIN ORIENTED SHEETS (CRNGO). To reduce the eddy current losses, the
core is buildup of 0.5mm thickness laminations, which are insulated from each other. The
sheets are insulated by CLASS-B type of varnish. The insulation used is ALKYD PHENOLIC
VARNISH dried at suitable temperature. The lamination sheets are passed through a conveyor,
which has an arrangement to sprinkle the varnish, and a coat of varnish is obtained. The
sheets are dried by a series of heaters at a temperature of around 300°-400°C. Two coatings
of varnish are provided in the above manner. The thickness of varnish should be 8-10 microns
when measured by a mini tester. Each lamination should be dried for around 90sec at constant
speed.
Generators are subjected to high electrical, thermal and mechanical stresses over the
course of their service life. Continuous extreme thermo-mechanical stresses and thermal cycles
have a particularly severe impact on the stator core. Over time, this can lead to deficiencies in
the core insulation and shorting of the ETS sheet stampings. This results in increased eddy
currents and local hot spots. During core flux test on Stator core, the stator core is energized
through primary winding (also called excitation cable) at 80-100% of rated flux density for 45
minute. Throughout the test temperature rise is monitored either by RTDs or by Infra-red

camera OR both. Abnormally increase in localized temperature is termed as hot spot. Such hot
spots are very dangerous for the life of a stator core as they gradually spread and keep on
degrading the core insulation and may cause failure of a winding bar or an earth fault in
generator. It may also result in increase of stator operating temperature. In both the cases it
causes total shutdown of the generator and thus power plants. Hence it is essential to ensure
that inter laminar core insulation remains intact to prevent core burning and damage to bar
insulation.
To detect the presence of hot spots and prevent such shutdowns, following three tests
are conducted on stator core for assessing its condition.
• Electromagnetic Core Imperfection Detection (EL-CID)
Excitation at a low energy level with 4% of rated induction, corresponding to an
induced longitudinal voltage of approximately 5 V/m.
• 50-hertz core flux testing
High-flux ring test at high energy level of up to 85% of rated induction. This
corresponds to a magnetic induction of ~1.3 T or approximately 100 V/m of induced
longitudinal voltage.
• 500-hertz core flux testing
A test method that works by exciting the object under test at a higher frequency
(500Hz) and induces voltage levels comparable with those of a high-flux ring test - roughly
100 V/m.
In last two methods, site conditions are simulated for stator core with 80% to up to
100% flux density and hot spots are detected with resistance temperature detectors (RTD) or

infra-red cameras. A 400 sq.mm cable was being used as primary carrying up to 1500 A
current for production of flux density.
Infrared Equipment (camera) is non-contact type temperature indicator works on the
principle of infra-red radiation and used for measuring temperature of components /
assemblies from remote / inaccessible location.
Infra-red radiation is electromagnetic radiation whose wavelengths are greater than
those of visible light but shorter than those of microwaves. All objects, whatever their
temperature, emit infrared radiation. The intensity emitted depends upon the temperature and
emissivity. If the emissivity can be known or can be estimated, then the temperature of the
object can be determined.
Resistance temperature detectors (RTD) are contact type temperature sensors, which
work on principles for resistance offered by metallic element and its variation based on
temperature. Metal which provide predictable, stable and linear resistance gradient are
generally used for this purpose like platinum, nickel, copper etc. They (element) operate as a
positive temperature coefficient device when an excitation voltage is applied to convert
changes in temperature to voltage signal by the measurement of resistance.
A test setup is being developed for 500-hertz core flux testing which can produce
excellent results.
The fundamental purpose of Stator core is to provide an easy path for flux in order to
facilitate flux linkage. The core is built up from thin laminations (of high permeable material)
to limit eddy-current losses in the core iron from the alternating flux induced in the core during
operation. Each lamination is insulated on both sides with varnish. The purpose of the

interlaminar insulation is to confine any induced eddy currents to a path along the same
lamination where it is induced, without bridging into neighbouring laminations.
The Core Flux test used to check the integrity of the insulation between the laminations
in the stator core.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a core flux test set up with
alternate primary winding system and process for carrying out core flux testing for all types of
turbo generators, which is capable of testing stator cores through core flux test employing 14
turns of 95 sq.mm cable in place of single turn of 400 sq.mm. cable used as primary in prior
Art.
Another object of the invention is to propose a core flux test set up with alternate
primary winding system and process for carrying out core flux testing for all types of turbo
generators, which is able to make the testing procedure easier, safer than before involving
less manpower, less time and eliminates the use of cranes.
A further object of the invention is to propose a core flux test set up with alternate
primary winding system and process for carrying out core flux testing for all types of turbo
generators, which is capable of improving productivity and achieve huge economical benefit.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 : Connection diagram for conducting core flux test of a stator core.

Fig. 2A : Shows photographs of MG SET used to generate 0-500V regulated supply for
conducting core flux test by old method Figure.
Fig. 2B: Shows photographs of MG SET used to generate 0-500V regulated supply for
conducting core flux test by old method.
Fig. 3: Shows photographs of alternate primary winding system developed.
Fig. 4A: Shows photographs of alternate primary winding system in use during core
flux test.
Fig. 4B: Shows photographs of Alternate primary winding system in use during core
flux test.
Fig. 4C: Shows photographs of alternate primary winding system in use during core
flux test.
Fig. 5: Shows photographs of RTD mounting for core temperature measurement.
Fig. 6: Shows photographs of RTD data logger for checking core temperature
measurement.
Fig. 7: Shows photographs of Power Meter Analyzer for measuring electrical
Parameters.
Fig. 8: Shows photographs of detection of hot spots in stator core by Infra-red
equipment through core flux test.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
A generic scheme for performing core flux testing is as per fig.1 which is being detailed
below:
Referring to figure 1:
Stator Frame (1): It is a rigid fabricated cylindrical frame that withstands weight of
core & winding, forces & torques during operation.
Stator Core (2): It is made up of punched segments of thin sheets of ETS core
punching/stampings coated by insulating varnish. It carries the winding laid in its slots and
helps in uniform distribution of magnetic flux. It acts as a transformer core during core flux
test.
Magnetizing Winding (3): Also called primary winding, it is wound around the core
to produce magnetic flux and simulate site conditions. Magnetizing winding is formed by laying
14 individual turns of cable around the core and then inter connecting them in series through
a connection panel. It thus acts as a single cable wound around the core 14 times.
Measuring Winding (4): Also called as secondary winding, it is a single turn of
winding wound around the core perpendicular to the primary winding in open circuit condition.
When stator core is magnetized through primary winding, emf is induced across the open
secondary winding.
6.6 KV Supply (5): This is 6.6 KV direct supply given from the grid to magnetize the
magnetizing winding and hence produce magnetic flux in the stator core. Current in the

magnetizing winding is adjusted such that 80% of flux density is achieved as compared with
site conditions.
Voltage measuring device for Primary voltage (6): This device is connected
across the primary supply at supply end to measure any variations in 6.6 KV supply.
Current measuring device for Primary current (7): This device is connected in
the primary winding to measure the current flowing through the primary winding.
Voltage measuring device for Secondary voltage (8): This device is connected
across the secondary winding to measure the induced voltage in the measuring winding.
Primary current and secondary voltage are measured through a power analyzer
instrument in core flux test. Power Analyzer have the facilities to measure the
Secondary/Control winding voltage, measurement of current, measurement of frequency and
measurement of power in watts together.
Earlier core flux test was conducted using a single turn of 400 sq.mm 1200V grade
cable acting as a primary winding. Due to its large dimensions and weight, crane was required
for its transportation and laying in the stator core. Also it used to take a long time for it. The
connection panel for the cable input supply was underground, which made it very risky, and
its connections were very time consuming.
The new system includes: 1) Two 100sq.mm supply cables for applying 6.6 KV direct
supply from the grid to the magnetizing system. 2) 14 turns of 100sq.mm cables to be laid in
the stator core constituting the magnetizing primary winding. 3) A box to keep all the cables
and having a connection panel to connect the 14 turns in series to form a winding. 4) A

secondary winding constituted by a single turn of 2.5 sq.mm cable to be laid perpendicular to
magnetizing winding in core.
The new alternate system to perform this test is described below.
♦ In earlier supply system, 0-500V regulated supply was used from the MG set.
But in new system 6.6 KV direct supply is available from the grid directly and core flux test can
be performed easily at most places.
♦ The input current flowing in the primary winding with earlier cable was in the
range of 1500Amps while with the new method, it has decreased up to 150Amps. This has
made the test relatively easier and safer to conduct and also increased the security of task
performers and job likewise.
♦ Many number of equipment like wattmeter, voltmeter, ammeter and frequency
meter were required for measuring the input supply parameters and output secondary voltage
in earlier system of 0-500V regulated supply. But now in the new supply system of 6.6 KV, all
measurements can be done by a single power meter analyzer only.
♦ Instead of the single turn of earlier primary cable, 13 or 14 turns of 100 sq.mm
cable are used for the primary winding. This has made its laying in the core much easier in
lesser time and also eliminated the use of cranes.
♦ The connection panel is also on the ground level and its connections are much
easier and risk free.

Process of Conducting Core Flux test with new system
1. Stator body shall be earthed solidly.
2. Magnetizing primary winding consisting of predetermined number of turns (usually 14
turns), to be wound around the stator through the bore of the core as shown in the figure 1.
3. Magnetizing primary winding to be connected to 6.6 KV supply (150Amp) through oil
(insulated) circuit breaker (OCB).
4. IR of the primary winding cable is measured with respect to stator core and physical
healthiness of the cable is checked.
5. Dry rubber sheets and Textolite sheets should be laid below the magnetizing cable to
avoid any damage to cable during laying and subsequent adjustment.
6. Ohmic resistance of the magnetizing cable is measured along with the ambient
temperature.
7. A single coil measuring (control) winding or secondary winding to be laid around the
stator, preferably at right angle to the magnetizing winding laid as per figure. This is for
achieving maximum flux linkage and measured secondary voltage as close to actual secondary
voltage.
8. All the measuring instruments are to be connected.

9. 13 no. RTD's to be installed at the teeth portion of the core and in the slot bottom,
uniformly distributed along the core length periphery. 3 nos. RTD'S to be installed equal space
peripherally in 4 nos. equidistance planes longitudinally. All RTD's are connected to the
temperature recorder for measurement of the core temperature during core flux test. The
location of all the RTD's to be recorded in the log sheet.
10. Check all the connections and instrument readiness.
11. Safety cord to be corded around the core for no entry of the people during testing.
12. To start the test, 6.6 KV supply given to primary winding and following parameters to be
recorded.
a. Exciting voltage V1 (Primary voltage)
b. Measuring (control) voltage V2 (Secondary voltage)
c. Exciting supply frequency
d. Exciting current I (Primary current)
13. The test shall be carried out for duration of 45 minutes. The measurement and
recording of temperature read by RTD's and other p arameters is to be done at 10 minutes
interval.
14. Infrared equipment should also be used during testing to detect the presence of hot
spots.

15. After 45 minutes duration, the supply is switched off and detection of presence of any
hot spot to be done by hand touching or Infrared equipment. While using infrared camera,
complete stator core bore surface and also the slot depth to be scanned.
16. Record the highest temperature and hot spots locations (if any).
17. Flux density to be calculated as per formula Bc = V2 x T1 / 4.44 f Q T2 Tesla.
V2= control / measuring winding voltage.
T1 = no. of turns of magnetizing winding.
T2 = no. of turn control winding.
f = measured frequency. Bc = flux density.
Q = Cross-section area of the stator core.
18. Maximum temperature rise (Δt max) at the end of test should not exceed 25°C in any
of the RTDs.
19. The difference between maximum and minimum temperature rise δt in any two
adjacent RTD/ Thermocouple should be less than 10°C at the end of test.

BENEFITS ACCRURED
1. Financial Benefits:
A. Cost saving by reducing manhours for core flux test

B) COST SAVING BY ELIMINATING USE OF OVERHEAD CRANES

Overhead Crane is not required for the laying of core flux cable by new method. Hence the
cost of crane is saved by the new method.
COST SAVINGS (B) = 64000 Rs.
C) COST SAVING BY ELIMINATING MG SET FOR 500V, 1500A INPUT SUPPLY


Cost saving by saving electricity cost per set = 45,000 Rs.
MG Set is not required for the new method. Hence the total cost of MG Set is saved by the
new method.
COST SAVING (C) = 60000 + 45000 = 1,05,000Rs.
D) COST SAVING BY SAVING MAINTENANCE COST

COST SAVING (D) = 3,07,200 – 1,15,200 = 1,92,000Rs.
E) COST SAVING OF THE EARLIER CORE FLUX CABLE
Cost saving by salvaging the scrap cable 400 sq.mm, special PVC heavy duty single core
unarmored copper power cable of 100 meter length = 90, 000 Rs.
COST SAVING (E) = 90,000 Rs.

♦ Total cost of saving by conducting core flux test by new method = A+B+C+D+E =
52,800 + 64,000 + 1,05,000 + 1,92, 000 + 90,000 = 5,03,800 Rs.
♦ If on average there are for example 3 THW TG Stators per year, the total saving
through conducting Core flux test per year by new method by implementing this project = 3 x
5,03,800 Rs. = Total Saving = 15,11,400 Rs.
2. PRODUCTIVITY
In earlier method it took 80 man hours to complete a core flux test including its
preparation but introduced method takes only 14hrs for the same work. This has resulted in
increase of productivity by 82.5%.
Similarly, earlier the crane was taking 32 man hours but after introduction of new
method use of crane is eliminated.

WE CLAIM
1. A core flux test set up with alternate primary winding system and process for carrying
out core flux testing for all types of turbo generators, the said core flux test set up comprising
and characterized in that;
two 100 sq.mm supply cables for applying 6.6 KV direct supply from the grid to the
magnetizing system;
fourteen turns of 100 sq.mm cables for laying in the stator core for constituting a
primary magnetizing winding;
a box for keeping all the cables and having a connection panel for connecting the
fourteen turns in a series to form a winding; wherein
the secondary winding is constituted by a single turn of 2.5 sq.mm cable for lying
perpendicular to magnetizing primary winding in the core.
2. A process for core flux testing with alternate primary winding system for all types of
turbo generators comprising;
making the stator body earthed solidly;
winding 14 turns of 100 sq.mm cables in the stator core to form primary winding;
magnetizing said primary winding;
disposing two 100 sq.mm cables to supply 6.6 KV and 150 Amp. supply from the grid
through insulated oil circuit breaker to the magnetizing primary winding;

measuring internal resistance (IR) if the primary winding cable with respect to stator
core;
checking the physical condition of the cable;
lying dry rubber sheets and textolite sheets below the magnetizing cable to avoid any
damage to cable during laying and subsequent adjustment;
measuring ohmic resistance of the magnetizing cable and the ambient temperature;
lying a single turn of 2.5 sq.mm cable perpendicular to primary magnetizing winding in
the core to constitute a secondary winding to achieve maximum flux linkage and measured
secondary voltage as close to actual secondary voltage;
connecting all the measuring instruments;
installing thirteen numbers RTD’s at the teeth portion of the core in the slot bottom
uniformly distributing along the core length periphery;
installing three numbers RTD’s to equally spaced peripherally in four numbers
equidistance planes longitudinally;
connecting all RTD’s to the temperature recorder to measure the core temperature
during core flux test;
recording the location of all the RTD’s in the log sheet;
checking the connections and instrument readiness;
disposing safety cord to be corded around the core for no entry of the people during
testing; wherein,

the core flux test is started by giving a 6.6 KV supply to primary winding to record the
parameters such as exciting voltage/primary voltage V1, measuring (control)/secondary
voltage (V2), exciting supply frequency and exciting/primary current I, when the test is carried
out for a period of 45 minutes and the measurement and recording of temperature read by
RTD's and other parameters are done at 10 minutes interval, wherein, an Infrared equipment
is disposed during testing to detect the presence of 'hot spots' when after 45 minutes
duration, the supply is switched off and presence of any 'hot spots' is detected by hand
touching or infrared equipment while an infrared camera is disposed to scan complete stator
core bore surface and slot depth when the highest temperature and hot spots locations are
recorded when flux density is calculated based on a formula,
Bc=V2xTl/4.44 f.Q.T2 Tesla,
Where,
V2=control/measuring winding voltage.
T1=no. of turns of magnetizing primary winding.
T2=no. of turn control/secondary winding.
f= measured frequency
Bc=flux density
Q=cross section area of the stator core.