Alternator Testing

Testing Of Alternator During Annual Overhauling Presented By “Kapil Dev Dubey & Manas Mohapatra & Gaurav Gaur ” Professional Circle Presentation

Overview

Tests During Maintenance - Electrical Tests of Stator - Electrical Tests of Rotor - Mechanical Test of Stator - Mechanical Test of Rotor Short Circuit and Open Circuit tests of Alternator Hydro Testing of Coolers Copyright © 2010 Jindal Steel & Power Ltd.

Need Of Testing

Large Alternators are most critical equipment for power utility. Unlike Transformers, Its not stationary hence subject to mechanical wear and tear also. High speed and vibration puts stator and rotor winding at risk and problems. Any failures needs long repair time and consequential lost revenue loss is very high. It must run trouble free between planned overhauling shutdown (Can be as long as 2 years). Copyright © 2010 Jindal Steel & Power Ltd.

Stator Electrical Tests

 Stator Winding Resistance  IR testing and Polarization Index  Tan- Delta Test  Natural Frequency Test  Absorption Coefficient  Dielectric Test (High Voltage Test)  Core Imperfection Test Copyright © 2010 Jindal Steel & Power Ltd.

Mechanical Tests

Stator : Core Tightness Core and Frame vibrations (At Normal Running) Air Leakage and soap Bubble Test Rotor : Vibration (At Normal Running) DP (Die Penetration) Test Nitrogen Leak Test Copyright © 2010 Jindal Steel & Power Ltd.

Rotor Electrical Tests

 Rotor Winding Resistance  Insulation Resistance  Polarization Index  RSO Test (Recurrent Surge Oscillation)  Rotor Earth Fault (REF)  Rated Flux Test  Dielectric Test (High Voltage Test) Copyright © 2010 Jindal Steel & Power Ltd.

Stator Winding Resistance  Very low resistance values to four significant digits.  All 3 phases resistances should be equal (maximum

of 0.5% deviation).  Compare with manufacturer's values and with previous tests.  Can detect shorted turns, incorrect connections and bad joints. Copyright © 2010 Jindal Steel & Power Ltd.

Stator Winding IR Test

Insulation resistance is a function of dryness.(

A moist

insulation has a lower resistance).

The resistance varies during IR Test. The ratio of 10 minute value to 1 minute value is the Polatisation Index. IR declines first and then increases before becoming steady. Along with cleaning and dry out process, PI Improves. Copyright © 2010 Jindal Steel & Power Ltd.

Polarizing Index

To perform IR Test of stator winding, NGT point should be disconnected along with Phase side link.  Conducted for 1 Min.  Min. value in Meg Ohms = Line Voltage in kV+1 With assembly (Gen. + GT + UAT + BUS DUCT), Initial

Meggaring is necessary for reference. At each disconnection and assemble of Units, There should be reference value of IR. Copyright © 2010 Jindal Steel & Power Ltd.

PI (Time Dependence)

Polarizing Index- Values Polarizing index - Accepting Values differs for different class of winding insulation.  Class A insulation: 1.5  Class B insulation: 2.0  Class F insulation: 2.0  PI should be carried out before dielectric (HV) test

as “Go-no-Go” test. If PI is low, HV test should not be done. Copyright © 2010 Jindal Steel & Power Ltd.

NFT For Stator winding

 Natural Frequency Test is performed on Phase Joints and Bus bars joints. Copyright © 2010 Jindal Steel & Power Ltd.

NFT For Stator winding

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Tan d Measurement

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Tan d Measurement Generator Stator winding Tan/Delta testing measurements.

Kit Used- M4000 Method Applied- GST Applied Voltage – 500V Phases R, Y, B and Neutral in shorted condition. Applied Volt ImA Watt Dissipation Factor Capacitance 500 V

218.34

5.492

5.046

1.4053µF

After cleaning the generator terminal bushing with Lectra cleaner, again same test procedure is followed. Applied Volt ImA& Power Ltd.Watt Copyright © 2010 Jindal Steel 500 V

218.94

5.547

Dissipation Factor Capacitance 5.062

1.4050µF

Dielectric Test (DC)

 Both AC & DC tests are in use.  Test voltage (dc) for new winding (2 × Line voltage + 1000) × 1.7

Test voltage for machines in service 125 to 150% of (Line voltage × 1.7) kV dc  Multiplying % will depend on age and general condition.  Test voltage to be applied gradually by automated test kit  If manual, ramp rate is 3% per minute. Copyright © 2010 Jindal Steel & Power Ltd.

Dielectric Test (DC) Leakage current for Healthy Winding

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Dielectric Test (DC) Leakage current for Defective Winding

Copyright © 2010 Jindal Steel & Power Ltd.

Dielectric Test (AC)  (2 × Line voltage + 1000) kV ac for new windings  125 to 150% of Line voltage kV ac  Dielectric ac test at power frequency  Very Low frequency ac at 0.1 Hz is the current

practice  A defective winding may fail with visual indication (next slide)  Winding may be retested after repairs Copyright © 2010 Jindal Steel & Power Ltd.

Partial Discharge Measurement Offline tests can be done using different methods: Capacitive coupling (entire winding) Stator slot coupler Electromagnetic/Corona probe (Localized Detection)

Stator Winding View

Epoxy putty is used with glue to overcome this yellow powder formation.

Stator Core Problem Any problems with stator core laminations may cause localized heating. Traditional method: By inducing rated flux value and checking for heating by infrared sensors.  Also called as Ring/Loop Flux method.  High powered source to induce large stator flux.  High manpower requirement

Stator Core Problem Current practice uses Electromagnetic Core Imperfection Detector (EL-CID) Test.  Requires Low excitation level (3 to 4 % of normal value)  Low Power source (Typically 220v, 10-15Amp AC)  4 to 7 turns of wire loop.  Detects abnormal behavior by the magnetic flux produced by circulating current in the lamination.  Measured by a sensor placed across two stator teeth (Chattock potentiometer)

EL-CID Test (Analog Method)

EL-CID Test (Digital Method)

Rotor Electrical Tests  Rotor Winding Resistance Measurement Similar as in the case of stator test. It can detect: Shorted turns Bad connections Wrong connections Open circuits

Dielectric Test (DC & AC). Similar to stator test.  Test voltage dc: 1500V to 10*Rated field voltage  Test voltage ac: 10*Rated field voltage at power frequency Copyright © 2010 Jindal Steel & Power Ltd.

Rotor Electrical Tests  Check for shorted field turns Abnormal vibrations may be a result of such short circuits : Recurrent surge oscillation (RSO) method using time domain reflectometry principle. By conducting an open circuit characteristic test By winding impedance measurement Search coil method (supplied as a part of the generator scheme)

Copyright © 2010 Jindal Steel & Power Ltd.

RSO Trace Indicate Multiple Shorted Turns

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OCC Test Indicate Problem

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Short Circuit Tests  Short circuit test is performed by putting a shorting link on

the alternator terminals and controlling the excitation to obtain rated current.  Currents in all 3 phases must be nearly equal.  Any internal problems such as open circuit of coils, bad joints etc. can be detected with this test.  Requires proper provision for shorting in such a way that measurement CTs in the alternator circuit can be used for the test. Copyright © 2010 Jindal Steel & Power Ltd.

Stator Mechanical TestCore Core Tightness :  Using knife insertion to check for any looseness (next slide shows a photograph)

Copyright © 2010 Jindal Steel & Power Ltd.

Stator Mechanical TestWinding Wedge Tightness Test:  Done by manual tapping.  Abnormal vibration/sound indicates looseness (next slide)  Comparison with other similar machines can easily pinpoint problems

Copyright © 2010 Jindal Steel & Power Ltd.

Stator water cooler Test Any leakage in hydrogen cooler is harmful for stator winding and core. Hydro test @ 6.0 Ksc is performed after cleaning of tubes.

Stator water cooler Test

Rotor Mechanical Test Non Destructive test Checking for cracks, porosity and inclusions by checks such as: Visual Radiographic Magnetic particle Liquid penetration Ultrasonic Eddy current Copyright © 2010 Jindal Steel & Power Ltd.

DP Test

Copyright © 2010 Jindal Steel & Power Ltd.  Die penetration test is carried out for any abnormality in bearings, Rotor End fans blades etc.

Ultrasonic Test

Copyright © 2010 Jindal Steel & Power Ltd.

Rotor Nitrogen Leak Test

 After Multi-contact pin O-Ring replacement work, Nitrogen Copyright © 2010 Jindal Steel & Power Ltd. leak test is performed @ 6.0 Ksc. And 4.0 Ksc Both.

Rotor Nitrogen Leak Test

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