DETAIL INSPECTION ON THE STATOR WITH ROTOR REMOVAL
Stator Inspections
After rotor complete removing, visual can accessing to inspect entire stator bore, found damaged on the end core in approximately 1 meter from Turbine end side and exciter end side, damaged core was caused by copper melted from the rotor and friction between them during rotor still moving when copper was melted.
A. Stator core as a general view. Taken from exciter end, typical damaged found also at turbine end, Deposit of copper melted was found at end core area and core found damaged at exciter end and turbine end areas at least, 50-90 cm from each end side
B. Frame and dampen bars
Dampen bars Exciter End Side
Dampen core bars and support on turbine end side and exciter end side were inspected by visual, found overheating, but no welding crack or bent on dampen pipe and welding supports
Dampen core bars at Exciter end
Dampen Core Bars at Turbine Side
Frame
Found overheating on the turbine end side cover but we do not found anomalies on the frame, such as crack, broken, melt, etc.
Front View taken from Turbine end
Shields
Outer shield and inner shield were inspected when both of parts was success removed, found some of inner fan blades was removed by third parties, see photos.
B. ELCID INSPECTION
PREPARATION
Stator core was planned for ELCID (Electromagnetic Imperfection Detection) inspections to identify core fault due to interlamination short causing by thermal stress, physical damaged, etc.
After rotor was successes removal from stator bore, inspection can accessing entire stator bore , found damaged on the stator core due to thermal stress and foreign materials (rotor copper bars and steel wedges melting) are dripping and friction to the stator core surface when rotor still moving / spin
Core preparation, all of foreign melting material that deposit on the stator core including short interlamination due to physical damaged has been removed from the core surface,
Clean-up of metal melting on the core stator surface was carried out step by step, to remove of hard metal melting , used disc grinding , continued by using tungsten pencil grinding and finishing using sharp thin steel plate to separating the short interlaminations on the surface and high pressure dry air was employed to blowing-out metal dust from entire stator bore
clean-up at the first step using disc grinding,
ELCID Preparation.
Prior ELCID test has been done, preparation should be carried out, such as :
1. Measure the stator core length, divided by each meter long
2. Make slot numbering, starting form ‘6 o’clock as slot number 1”
3. Prepare the ELCID turn wire, turning into stator bore and keep straight
4. Calibrate the ELCID scanner
5. Prepare the power supply 220 V AC, single phase, 10 A
6. Prepare tables to keep the ELCID instruments
7. Prepare man power to scanning each slot during test
ELCID TEST THEORY
The generator stator core is built from thousands of thin steel sheets (laminations). These laminations are coated with a thin layer of varnish for prevention of circulating induced currents also known as eddy currents, which are induced because of rotating magnetic flux produced by the rotor. Hence, it can be envisaged that any defects in the inter-laminar insulation causes fault currents to flow locally in core. These circulating currents can thus cause localized overheating and hot spots in the damaged areas and this may further damage the core. In extreme cases, sufficient heat is generated to cause melting of small parts of core and premature failure of the winding insulation. Thus these hot spots should be detected and corresponding repair works must be carried out before the condition worsens. Unlike the core ring flux test that had several disadvantages like running of high current/voltage, mechanical stresses etc, the digital ELCID test uses only a fraction (4%) of rated flux level to generate fault currents within the core body.
These currents are then sensed by a pick up coil. The digital ELCID test helps in assessing the condition of Generator/motor core and gives vital information in the development of trend analysis, for use in diagnostic and predictive maintenance.
It offers the following facilities:
• Identification of faults below the winding.
• Distinguishes between surface faults and deep faults.
• Fault location is pin-pointed accurately
Principle:
The circumferential magnetic field of the core is due to the excitation, plus that due to any fault currents present. The effect of the magnetic fields is to produce magnetic potential gradient on the core surface. This magnetic potential gradient is detected by specially wound coil known as Chattock coil, which provides an output proportional to the difference in the magnetic potential between its two ends.
The ELCID equipment tests a core for faults by exciting the core using a toroidal winding to produce a ring flux of only 4% of its normal level of excitation. A sensing head (Chattock coil) is then passed over the surface of the core to detect magnetically the presence of fault currents themselves rather than the heating effect they produce. The output of the Chattock coils is a dc voltage proportional to the fault current component in phase quadrature with the core excitation current. The signal is an analogous signal, which is converted into a digital signal and displayed or stored in PC.
Test
This Generator undergone a severe fault and entire unit tripped, after dismantling and removal of rotor it was found the stator core was filled with melted copper all over the places. the core repair and in the process removed the melted copper deposits and applied grinding method to remove the interlaminate shorting where laminations were found fused.
ELCID test was performed initially after the above repair and later further grinding was performed on areas where ELCID fault current was found to be extremely high and the ELCID test was repeated after the repair.
The complete analysis on the Stator core based on ELCID tests conducted, repair adopted and visual inspection is as presented below.
ELCID was carried out three times, after all of slots were complete scanned at first test, some of slot location was try to make a correction by using pencil grinding, to reducing the hot-spot, some of them was successfully removed but some of locations can not to removed.
Trial corrections and re test are purpose to identifying of core repair possibility, therefore correction was carried out by random location. Following are record summary for ELCID test
Example :
Slot No. 1
First Test : 740 mA, at 1.5 M from Exciter End Side
After correction : 102 mA
Top graph (1); before make a correction, Bottom graph (1A) : after make a correction
Slot No. 2
First Test : 844 mA, at 1.5 M from Exciter End Side
After correction : 148 mA
Slot No. 3
First Test : 166 mA, at 1.5 M from Exciter End Side
After correction : 91 mA
DATA SUMMARY FOR ELCID TEST
ELCID CORE VISUALISATION FIGURE
ELCID test was performed on the mentioned Generator to assess the stator core healthiness. Initial ELCID test was performed after removal of melted copper and applying grinding to remove all burrs and core laminations shorts at various locations.
Initial ELCID test indicated that the fault current is much above allowable value of 100 mAs at various locations as indicated in the ELCID graphs and table consisting of fault current values. The fault current values are on extremely higher in various locations and the highest value has gone till 844 milliAmps.
The few fault areas (location in slot no. 1, 2, 3, 15, 29, 35, 40, 42, and 43) with excess current as indicated in initial ELCID test were attended for further repair. The ELCID test was repeated after this repair in these areas and found that the fault current levels have come down (the values and graphs are as shown above in test results), but still the fault current values after repair are much higher than the allowable values of 100 mAs.
Also from the ELCID test results, we get the clear indication that lamination fusing of stator core is observed at various locations especially at exciter side (connection end) at a distance of 1.5 mts from exciter side slot exit in all 60 slots and at a distance of 1 mts from turbine end in all 60 slots, also there are various locations in the middle of the core where fault currents are high.
Based on the ELCID Test conducted, repair adopted and visual inspection on stator core, the following conclusions and recommendations had been drawn for further action.
From the above study conducted and extensive analysis of the ELCID test results, Visual observations and repair adopted, it is found that the stator core has been damaged extensively due to the deposits of melted copper. The occurrence of stator core laminations is found in large areas, especially at the exciter side end and turbine side end of the stator core, in almost all areas throughout the core circumference. Also the ELCID data indicated that the fault currents are enormously high indicating that the damage is severe.
From the above study, it is concluded that the stator core needs a complete replacement of core with the new one or we have also an option of complete restacking of the stator core.
Termination / HV Bushings
No crack or broken was found on the ceramic HV bushings, found rusty on the terminal cover
Typically HV bushings inside frame
C. Winding Inspections
Visual inspection was carried on the winding at endwinding areas and entire bore stator, almost of insulation was found overheating and found resin melting.
Endwinding exciter end side
Endwinding at Turbine End
Typically winding connection / outgoing leads insulation
Whole of insulation has discolorisation and too dry due to over heating
Typically endwinding insulation conditions at turbine end and exciter end
Stator Wedges
Whole of stator wedges at endwinding region +/- 50-90 cm from each end core were found burnt due to overheating.
Core compression bolts
Core compression bolts has been inspected by visual, no red oxide deposit on its head or no loose indicate was found.
D. Winding measurements
Generator was storage at open air area since December 2009, termination was exposure without any shield or roofing.
Based on first inspection on November 2009, when generator still on foundation, winding insulation resistance is indicate no solid grounded but found very low insulation on phase “T” and winding resistance still in balance and in tolerance range when comparing to the factory measurements, (see previous pages report).
Insulation Resistance
Following is the Insulation measurements by using 500 VDC and 5000 VDC dated : 30 June 2010
Phase R – Ground = 1.8 MΩ at 5000 VDC
Phase S – Ground = 0.1 MΩ at 500 VDC
Phase T – Ground = 0.2 MΩ at 500 VDC
Comparison to first inspection on November 18-19, 2009
Phase R – Ground = 2.24 GΩ at 5000 VDC
Phase S – Ground = 1.98 GΩ at 5000 VDC
Phase T – Ground = 0.6 MΩ at 5000 VDC
Winding DC Resistance
Winding resistance was measured at 300C, dated : 30 June 2010
Phase U – x = 1.32 mΩ
Phase V – y = 1.32 mΩ
Phase W – z = 1.33 mΩ
Comparison , inspection dated: 18 November 2009,
Phase U – x = 1.4 mΩ
Phase V – y = 1.4 mΩ
Phase W – z = 1.4 mΩ
E. Covers Re assembly
After inspection was complete done, all of mechanical parts including, inner shields, bearings, oil deflectors, H2 shield, excluding rotor, to be re assembled on dated 1-2 July, 2010. Cover shaft was covered by temporary cover plate, while the rotor to be kept at workshop.
After inspection was complete done, all of mechanical parts including, inner shields, bearings, oil deflectors, H2 shield, excluding rotor, to be re assembled on dated 1-2 July, 2010. Cover shaft was covered by temporary cover plate, while the rotor to be kept at workshop.
F. Summary
Based on these inspection here we can be summarized as the following items.
1.Visual Inspection
Frame : as visual inspections, no anomalies was found on the external frame
Dampen vibration bars and core support: found overheating on the bars dampen and support, but no welding crack was found
Core: found damaged at both TE / EE side endcore +/- 50 – 90 cm from each end due to copper melt and metal touching when rotor still moving, disc grinding and pencil grinding were involved to removed copper and metal melting before ELCID test, some of thot-spot was successfully removed but some of them still remain
HV bushings: no anomalies (crack or chips) were found at frame inside and outside
Stator winding: found overheating and resin melt at endwinding and connections end, almost of winding at the slot region was found overheating.
Stator wedges, almost of stator wedges at 1m long from end core at both side TE/EE were found burnt due to overheating
2. Measurements
Stator winding: based on the first inspection dated November 2009, no solid grounded was found and winding DC resistance still in factory measurements tolerance. Measurement has been re inspect on June 30, 2010, Winding DC resistance at each phase still in balance.
Insulation resistance phases S - to Ground, T – Ground, are found zero using 500 VDC, and over than 1 MΩ on phase R – to Ground with 5000 VDC.
ELCID, Stator core was inspected and analysis using ELCID, almost of stator core still remain hot-spot, even copper melted was removed from core surface, random correction was try to identified and analysis for repairing programs.
Hot spot was found only at surface area. No hotspot was found in the slot areas or back core side areas.
Significant hotspot was successfully removed and come down when re grinding were involved to made a correction.
Example:
Slot No. 1, first test ELCID was measured at 740 mA, after correction, hot spot was comedown to 102 mA and slot No.2 was come down from 148 mA from 844 mA.
G. CONCLUSIONS
1. Frame, supports and HV Bushings
- Stator frame, no anomalies was found
- Core vibration dampen bars, found overheating at top side close to the stator outgoing leads but no crack on the welding areas was found
- HV bushings, No anomalies (crack, chips) are found on these parts, rust were found on the top cover due to generator was storage at open air for long time since December 2009.
2. Stator winding, surge rings, connections lead, and stator wedges
- Insulation Resistance Stator winding for phase S and T were measured zero MΩ using 500 VDC and 1.6 MΩ at phase R by using 5000 VDC, no solid grounded, very low insulation resistance are causing moisture absorption in the winding, due to stator was storage at open air for long time without any heaters are energized during in storage
- Winding insulation as physical inspections, found brittle, brownish and too dry due to overheating especially insulation winding at top side close to HV bushing.
- Winding insulation at endcore region, whole of insulation was found delamination due to overheating and copper melti touching to this areas when rotor still moving.
- Connection leads. Found brownish color due to overheating
- Stator wedges, whole of stator wedges at end core areas (+/- 1 m) found burnt and all of wedges at middle area was found moved due to loose.
3. Stator Core
Whole of stator core at end region (1m) at both end side TE/ EE found damaged due to metal touching when rotor still moving, copper and steel wedge was found meletd and deposit at this areas, overheating was also found in the middle region.
ELCID test indicate that, almost of the stator region identified hot-spot, random correction and re –test was carried out, some of the hotspot was successfully removed but some of them still remain, especially at end region.
Stator core is still possible to be repaired for rewinding, after total re-stacked and replace of end core region at least 1m long at each end core TE and EE side by new core.
Siswanto
Specialist Engineer
Power Generation Services
Email:
sis_cahya@yahoo.com
siscahyabhuwana@gmail.com
Mobile phones:
+62 81 311422270
+62 81 398199500
Based on these inspection here we can be summarized as the following items.
1.Visual Inspection
Frame : as visual inspections, no anomalies was found on the external frame
Dampen vibration bars and core support: found overheating on the bars dampen and support, but no welding crack was found
Core: found damaged at both TE / EE side endcore +/- 50 – 90 cm from each end due to copper melt and metal touching when rotor still moving, disc grinding and pencil grinding were involved to removed copper and metal melting before ELCID test, some of thot-spot was successfully removed but some of them still remain
HV bushings: no anomalies (crack or chips) were found at frame inside and outside
Stator winding: found overheating and resin melt at endwinding and connections end, almost of winding at the slot region was found overheating.
Stator wedges, almost of stator wedges at 1m long from end core at both side TE/EE were found burnt due to overheating
2. Measurements
Stator winding: based on the first inspection dated November 2009, no solid grounded was found and winding DC resistance still in factory measurements tolerance. Measurement has been re inspect on June 30, 2010, Winding DC resistance at each phase still in balance.
Insulation resistance phases S - to Ground, T – Ground, are found zero using 500 VDC, and over than 1 MΩ on phase R – to Ground with 5000 VDC.
ELCID, Stator core was inspected and analysis using ELCID, almost of stator core still remain hot-spot, even copper melted was removed from core surface, random correction was try to identified and analysis for repairing programs.
Hot spot was found only at surface area. No hotspot was found in the slot areas or back core side areas.
Significant hotspot was successfully removed and come down when re grinding were involved to made a correction.
Example:
Slot No. 1, first test ELCID was measured at 740 mA, after correction, hot spot was comedown to 102 mA and slot No.2 was come down from 148 mA from 844 mA.
G. CONCLUSIONS
1. Frame, supports and HV Bushings
- Stator frame, no anomalies was found
- Core vibration dampen bars, found overheating at top side close to the stator outgoing leads but no crack on the welding areas was found
- HV bushings, No anomalies (crack, chips) are found on these parts, rust were found on the top cover due to generator was storage at open air for long time since December 2009.
2. Stator winding, surge rings, connections lead, and stator wedges
- Insulation Resistance Stator winding for phase S and T were measured zero MΩ using 500 VDC and 1.6 MΩ at phase R by using 5000 VDC, no solid grounded, very low insulation resistance are causing moisture absorption in the winding, due to stator was storage at open air for long time without any heaters are energized during in storage
- Winding insulation as physical inspections, found brittle, brownish and too dry due to overheating especially insulation winding at top side close to HV bushing.
- Winding insulation at endcore region, whole of insulation was found delamination due to overheating and copper melti touching to this areas when rotor still moving.
- Connection leads. Found brownish color due to overheating
- Stator wedges, whole of stator wedges at end core areas (+/- 1 m) found burnt and all of wedges at middle area was found moved due to loose.
3. Stator Core
Whole of stator core at end region (1m) at both end side TE/ EE found damaged due to metal touching when rotor still moving, copper and steel wedge was found meletd and deposit at this areas, overheating was also found in the middle region.
ELCID test indicate that, almost of the stator region identified hot-spot, random correction and re –test was carried out, some of the hotspot was successfully removed but some of them still remain, especially at end region.
Stator core is still possible to be repaired for rewinding, after total re-stacked and replace of end core region at least 1m long at each end core TE and EE side by new core.
Siswanto
Specialist Engineer
Power Generation Services
Email:
sis_cahya@yahoo.com
siscahyabhuwana@gmail.com
Mobile phones:
+62 81 311422270
+62 81 398199500
