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Test Date: 06/12/12 Table - 4.1.1(b) Frequency: 49.89 Hz

Speed Voltage (V) Current (A) Impedance ( ) 0 200 28.12 7.112

500 200.1 28.29 7.073 1200 199.81 28.39 6.798 *Sudden Increasing in Current, Voltage is put to 170 V

2000 170.57 28.60 5.963 2300 170.46 28.74 5.963 2400 170.52 28.76 5.929 2500 170.54 28.77 5.927 2600 170.55 28.77 5.928 2700 170.59 28.80 5.923 2800 170.59 28.82 5.919 2900 170.57 28.89 5.904 3000 170.53 28.92 5.890

The graphs showing Impedance characteristics are as below:

Analysis: Non Linearity of the impedance characteristics indicate that there is some abnormality in the rotor winding insulation system.

4.1.2 Recurrent Surge Oscillograph (Digital RSO) Test RSO test is performed to detect faults in rotor windings. The electrical faults in generator rotors fall into two main categories, faults from the winding to the rotor body (earth faults) and faults between parts of the winding (inter-turn winding faults). The existence of the faults will frequently display excessive mechanical vibration and cause serious concern. An RSO test identifies the type of fault and pinpoints the position of the fault.

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Fig - Rotor TDR Measurement System

Principle:

Rotor Reflectometer uses a technique known as time domain Reflectometry. Application of this technique in testing rotors is known as the RSO (Recurrent Surge Oscillograph). The method involves applying a 12V DC voltage between one end of the rotor winding and the rotor body. The transmitted wave at the far end of the winding and the reflected wave at the input end of the winding are monitored using two oscilloscope channels. If the voltage step is applied from each end of the rotor winding alternately, then two oscilloscope traces will be obtained on the oscilloscope screen. A healthy rotor winding will have two identical traces, which are superimposed. A rotor with a fault will have different traces and the positions of the fault may be deduced by scaling in the time domain.

Circuit Diagram: RW = Rotor Winding

SECS = Synchronous Electric Changeover Switch R1 = Input matching Resistor R2 = Output matching Resistor PG = Pulse Generator CH1 = Oscilloscope Channel 1 CH2 = Oscilloscope Channel 2 RB = Rotor Body

Reference: CDL, UK Test Measurement:

Test was conducted on the rotor winding for inter-turn & earth faults in the winding insulation. The test waveforms were recorded and are as shown below:

At Static Condition

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At 3000 RPM Condition

Equipment Used: 1) Rotor Reflectometer; Make: CDL, UK, S.No: 159 2) Digital Oscilloscope; Make: Fluke, U.S.A. Sr. No: DM 6680375

Analysis:

Non - super imposition of input end waveforms reveals inter-turn shorts in the rotor winding insulation system. 5.0 CONCLUSION

Overall the electrical tests on Generator rotor indicates that there are some inter-turn shorts in the rotor winding insulation system.

6.0 RECOMMENDATIONS

It is recommended to repair the rotor winding during next shutdown/overhauling.

It is recommended to repeat the present and all other electrical diagnostic tests during the next major overhaul to estimate the rate of insulation deterioration and for trend analysis.

7.0 REFERENCES

IEEE Standard 62.2 - 2004: Recommended practice for Diagnostic Field testing of Electrical Power apparatus Electrical machinery.