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

30 mm 1.8 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

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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

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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

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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.

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Knife Test

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

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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%

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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

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Rated Flux Test

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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

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Magnetization curve

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Rated Flux Test on Turbo

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POWER

SOURCE

Power

Cables

Rated Flux Test on Hydro

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Rated Flux Test

~15C above ambient

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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

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

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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

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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.

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Measuring Fault Current with a Chattock Potentiometer

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Positioning the Chattock

Understanding Fault Magnitude

IQUAD 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

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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)

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…