MECHANICAL INTEGRITY CHECKING OF GENERATOR STATOR OVERHANG BY NATURAL FREQUENCY MEASUREMENT

Transcription

1 MECHANICAL INTEGRITY CHECKING OF GENERATOR STATOR OVERHANG BY NATURAL FREQUENCY MEASUREMENT R.K.Bhatnagar, A.K.Mukherjee, National Thermal Power Corp., India INTRODUCTION Reliable operation of generator is highly dependent on the electrical and mechanical integrity of stator winding. Failure of generator stator conductor due to electrical or mechanical problems will lead to long force outage of the unit resulting into huge revenue loss. As per our experience, most of the epoxy mica insulated generator stator conductor failure occurs due to loss of mechanical integrity of either overhang conductor or core looseness. This paper describes our experience of ensuring mechanical integrity of epoxy mica insulated stator conductor at the overhang region. DESCRIPTION Singrauli Super Thermal power station is the first power station build by NTPC. It has installed capacity of 2000MW comprising of 5X200MW units and 2X500MW units. The Generators of 200MW units are designed by M/S Electrosila, Russia and manufactured by BHEL. The stator winding of generators are directly cooled by de-mineralized water passing through hollow conductors of the stator winding. Stator core and rotor are cooled by pressurized hydrogen gas. The rating of the generator is 235MVA, 15.75KV, 0.85lag, 3-phase, double star connection and 3000rpm. The generators are having 60 slots with 2 nos. stator windings per slot. Each stator winding conductor is made of both hollow and solid conductors. DM water passes through hollow conductors for conductor cooling. DM water pressure is maintained less than casing hydrogen pressure to prevent entry of water into the generator in case of hollow conductor leakage. For detecting hydrogen gas leakage through stator conductor, gas trap device has been provided. The stator winding conductors are insulated by F class epoxy based mica insulation. Overhang conductors of stator winding are supported on stator casing by fiber glass ring and fiber glass blocks. Conductors at overhang are tied with the fiber glass ring by polyester cord. Near the exit point of the conductor from core, one epoxy putty ring was made to reduce overhang conductor vibration. Top and side ripple spring has been provided in the slot to further reduce conductor vibration. OVERVIEW OF GENERATOR STATOR Iris Rotating Machine Conference 1

2 OBSERVATION The unit-3 Generator of SSTPS was commissioned on 28 th March On 23/12/04 stator conductor leakage was observed in the generator. The important observations are as given below:- On 23/12/04 consumption of hydrogen increased to 6 cyl. in 24Hrs. from 3 cyl. Small micro bubbles was observed to be coming out from the gas trap On 29/12/04 drops of water was found seeping from expansion tank cover Dissolved oxygen in stator water was 150ppb. Stator water flushing carried out through gas trap and dissolved oxygen of stator water increased to 700ppb. On 30/12/04 stator water flushing was stopped and dissolved oxygen was again reduced to 150ppb. Hydrogen was detected in expansion tank also (7%) after expansion tank vacuum was killed. The appearance of hydrogen gas bubbles in gas trap and expansion tank along with increase of hydrogen consumption indicates stator winding hollow conductor leakage. The reduction of Dissolved Oxygen of stator water from normal value of 2100ppb to 150ppb also supports the above analysis Machine was shut down on 31/12/04 at 0757Hrs. Leakage was detected in the connecting bar between 31 Top bar to C4 (B Ph.) bushing TE C C4 EE C1 PH TE NE TOP BAR BOTTOM BAR B PHASE ELECTRICAL CONNECTIONS OF STATOR WINDING Iris Rotating Machine Conference 2

3 Leaking bar FAILED OVERHANG CONNECTING BAR After removing the insulation, crack was detected in the first bend of copper conductor L shaped Aluminum support bracket just below the defective bar at 5 O clock position supporting phase connector 55B-C5 was found broken Some blocks of supporting bracket at 11 O clock position was found loose Black paste was found at the slot exit in top and bottom part of overhang Paste like fretting product was found in the putty ring area and overhang support area PASTE OBSERVED ON OVERHANG Minor cracks were also observed in the overhang support brackets After replacement of defective bends, machine was taken into service. Iris Rotating Machine Conference 3

4 SECOND FAILURE:- Again in May-05, stator conductor leakage was detected. Machine was shut down in June-05. Leakage was observed in first bend of 11 Top to C6 connecting bar. In this occasion natural frequency testing of overhang conductor was carried out for the first time At 4 places natural frequency was observed to be around 100HZ. Strengthening of the overhang conductor was carried out to shift the natural frequency from 100HZ range. Defective bend was replaced Machine was taken into service and no problem was observed till date The machine was re-inspected 2006 and no problem was observed. ANALYSIS OF PROBLEM To detect the root cause of repeated failure fractography analysis of the cracked surface of copper conductor was carried out. The analysis indicates that the crack has developed due to fatigue. Fatigue has developed due to repetitive loading caused by conductor vibration at the overhang region. No sign of overheating or corrosion was observed. Failure of aluminum support bar also indicates presence of vibration in the overhang region. The fractography analysis of the fracture surface of the aluminum support indicates presence of beach mark caused by fatigue. The above observation indicates presence of high overhang conductor vibration. The presence of fretting product also indicates overhang conductor support looseness and subsequent vibration of the overhang conductor. During shut down of the machine in June-05, natural frequency testing of overhang conductors revels that at 4 places natural frequency was near 100HZ particularly at the places where failure has taken place. The matching of natural frequency with core vibration frequency i.e. 100HZ resulted in excessive conductor vibration due to resonance when the generator is in service. The presence of excessive overhang conductor vibration has caused repeated failure of overhang conductor. Overhang conductor of these types of generators are supported by fiberglass ring and blocks. Gaps between fiberglass ring and overhang conductors are filled by glass webbing soaked in epoxy resin. But when the support blocks are tightened, some micro gaps exist in tangential direction. Secondly epoxy resin shrinks when it is cured. Due to this property of resin, micro gaps are formed between conductors and its support after prolong operation. So when machine is in operation, due to presence of micro gaps between conductor and support, the support and conductor will not vibrate as a monolithic mass. This will cause further looseness of conductor and subsequent increase of amplitude of vibration. Conductor vibration will become severe when natural frequency of conductor approaches to 100HZ (core vibration frequency) and will result into failure. Iris Rotating Machine Conference 4

5 CONCLUSION Looseness of overhang conductor (where overhang conductor is supported by fiber glass blocks, rings and tied by lacing) will occur after prolong operation of the machine. By measuring natural frequency of overhang conductor during overhaul, this type of problem can be detected at a very early stage and corrective action can be taken to avoid premature failure of stator winding conductor. In NTPC we are carrying out measurement of natural frequency of epoxy mica insulated overhang conductor (where overhang conductor is supported by fiber glass blocks, rings and tied by lacing) of generators during generator overhaul and corrective actions were taken on the basis of natural frequency of conductor. Reference: 1. I. Culbert, et al. Handbook to Assess the Insulation Condition of Large Rotating Machines, EPRI Power Plant Reference Series, Volume 16, 1989 Iris Rotating Machine Conference 5