Generator Users Group Annual
Conference 2015
Core testing, low and high flux, tap
Mladen Sasic, IRIS Power
Stator Cores
Cores provide low reluctance paths for
working magnetic fluxes
Support stator winding, together with stator
wedges!
Cores and wedges must be capable of
withstanding operating forces: mechanical
and magnetic
Core provides primary heat removal from
indirect cooled stator winding
Westinghouse Core
GE Core
What is in the slot?
Requirement for Wedge Testing
Wedges are installed to hold coils firmly in place and
eliminate vibration
Vibration results in wear and erosion of insulation
Over time, this can result in electrical failure
Loose or improperly installed wedges have been identified as
a major contributor to this problem
All manufacturers agree on the need for well installed and
maintained wedges
Various wedge types
Top Ripple Spring
1.8 mm
30 mm
Thickness: 0.9 mm, length and width to fit the slot/wedge
Normal compression in 75-90% range
Side Ripple Spring
Stator Bar Slot Vibration Control
10
Typical tests
Visual inspection (end wedge/side filler
migration)
Evidence of greasing, dusting
Displacement measurements
Ripple Spring Compression measurement
Tap tests (manual or electronic)
Typical Tools...
Typical Problems...
TIGHT !!!
????
LOOSE...
Typical Electronic Tools...
RTI Idea
Measured (raw) values are compared to user selected
references for tight and loose
RTI (Relative Tightness Index, number from 0-100) is
displayed as a result of comparison between measured
values and references
Different calibration references will produce different RTI
RTIs are not saved in measurement file
RTI Summary
Tap test conclusion
A lot of uncertainty with any method
Personal feel often considered to be more
accurate than electronic methods
There is no unit for tightness and
there is no agreement on tight and loose
Introduction of on-line methods may be
helpful
Stator Core Testing
Mechanical and Electrical tests:
Core tightness test
Core vibration test
Through Bolts Insulation
Core loss test
Rated flux test
Low flux tests
18
Core Tightness Testing
Visual inspection
Suspected loose areas can be confirmed by a Knife
Test.
This involves trying to insert a knife with a 0.25 mm
(10 thou) thick blade into the core bore (stator) or OD
(rotor).
If the knife penetrates more than 5.0 mm (0.2 ins)
then the core is loose.
EDF Crabe
Bump test, 15-20 slots tested on hydro core, more air
in core= lower the acoustic wave speed.
19
Knife Test
20
When you test the core
What do you actually measure?
Core loss test: W/kg
LOOP test: temperature
Low power core test: mA or W
and what can affect your result?
Core loss and LOOP test: quality of material,
test time, induction level
Low power tests: quality and uniformity of
material, induction level
Magnetic permeability and core loss variation
may be detected with low power test but not
with LOOP test
Core Loss Test
Core is excited and power absorbed measured
by a wattmeter
Results are expressed as loss per mass of core
Should not exceed about 6-10W/kg
Increase from previous test should not be
more than 5%
23
Rated Flux Test
Purpose and Theory
The induced flux will generate excessive heating in
the areas of core where degraded core insulation
exists
Heat is generated by eddy currents flowing between
lamination due to insulation degradation
Excitation winding power supply system should be
fitted with a voltage adjustment device, ammeter to
obtain the correct ampere-turns to produced the
required flux
No agreement on excitation levels, test duration and
acceptance criteria
24
Rated Flux Test
25
Rated Flux Test
Two methods to calculate flux test level, i.e.
turn voltage:
-Winding diagram
-Size of the core
See IEEE 56 or IEEE 432
26
Magnetization curve
27
Rated Flux Test on Turbo
28
Rated Flux Test on Hydro
Power
Cables
POWER
SOURCE
29
Rated Flux Test
~15C above ambient
30
Rated Flux Concerns
High Voltage, Current and Magnetic Field
Fixed voltage supply
Localized core burning
General core overheating
Temperature attenuation
Labour intensive
Uncertain power requirements
Different flux patterns compared to normal
operation
31
Low power core testing
Recommended test level for low power tests is
in range of 2-10 % of nominal flux.
For 4% level, it is close to 5V/m of core length
for two pole turbo generators, but
It is NOT 5V/m for hydro and motor cores!
To achieve 4%, about 10 At/m (vs. 100-1000 in
LOOP) of core circumference is required, or
100-600 At.
Different instruments exist
EL CID Test
Purpose and Theory
EL CID is the abbreviation for Electromagnetic Core
Imperfection Detector
Works on the principle that:
If a low flux around 4% of rated flux is induced in back
of core currents flow through defective core insulation
Current is measured with Chattock Coil
Chattock Coil gives voltage output proportional to
fault current (IQUAD) and current (IPHASE) produced by
the flux induced flux
33
EL CID principle
Insulation breakdown causes fault currents to be set up as
illustrated.
These fault currents create hot spots which can cause further
deterioration to the core. If left unchecked, this can lead to
damage to the stator winding and the machine as a whole.
34
Measuring Fault Current with a
Chattock Potentiometer
35
Positioning the Chattock
36
Understanding Fault Magnitude
IQUAD 100mA at 4%
equates to 5-10C
on LOOP Test
Data Display Normal Traces
Core Visualisation
Advantages of EL CID
Low Excitation Power - 4%
No Risk of Further core damage
Fast, Portable - Easy to Setup
Low Manpower Requirements
Significant Reduction in Safety Hazards
Instant Interpretation of Test Results
Ability to Re-Test During Maintenance Cycle
Can be done with rotor in place
Disadvantages of EL CID
Requires competent trained test technician and experience to
interpret data (also with LOOP Test)
Difficult to detect small faults at the joints in hydro-generator
cores
Correlation to Ring Flux Test not perfect
Faults on cores with insulated key-bars difficult to detect (also
with LOOP Test)
Does not create the same flux pattern as in operation (also
with LOOP test)
41
What is an acceptable result?
Core Loss: 6-10 W/kg
Loop test: 5-10 K at 100% of nominal flux
Low power tests: Less than 100 mA of quad
current at 4% excitation level or 15 W
dissipation at 3-10 %
Conclusions
Evaluation of the condition of a core is a major
technical challenge - C. Maughan
Visual inspection is very important
Both, high and low flux tests, have limitations
Core problems are not that frequent, but
