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... LOOSE...???? TIGHT!!!
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
Purpose and Theory Rated Flux Test 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 ~15 C 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
Purpose and Theory EL CID Test 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 (I QUAD ) and current (I PHASE ) 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 I QUAD 100mA at 4% equates to 5-10 C 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