Rosa Unit-2 AVR Sensitivity Features of Automatic Voltage Regulator The control of excitation power to the generator is the primary function of AVR. The measure of reactive power demand in the Grid is indicated by MVAR. When MVAR is high on +ve side, the inductive demand is more and vice versa. The AVR takes the feedback of Generator terminal voltage and controls the excitation current to Generator Rotor. The control functions in AVR are designed to respond quickly to transient disturbances and the protection functions will protect the generator from OVER/UNDER excitation beyond its capability. Each AVR is having 2 redundant channels; working and standby. Whenever there is any problem in working channel, the control transfers to standby channel.
Problem Brief : 1. Each AVR is having 2 redundant channels; working and standby. Whenever there is any problem in working channel, the control transfers to standby channel. 2. During voltage fluctuation, the grid requires more reactive power to be contributed by generators to stabilize the voltage. Field over voltage tripping protects the generator from crossing its allowed operation limit while providing support to the grid. 3. During minor voltage fluctuations in the grid, Unit 2 AVR channel tends to change more frequently compared to other Units. 4. These change-overs are detected by function called dv/dt. During the change-overs, there is chance of unit tripping if stand by channel does not take over the control properly. 5. It was observed that Unit 2 was unable to supply the required reactive power as compared to unit 1 & getting tripped on under excitation protection. 6. Unit 1&2 comparison data is given in below table : UNIT # 1 UNIT # 2 Dates Before Disturbance (MVAR) After Disturbance (MVAR) Before Disturbance (MVAR) After Disturbance (MVAR) 30/04/14 + 97 +162 +93-82 11/09/15 +129 +180 +117-210 22/09/15 + 51 +404 +53-6 16/02/16 +38 79 + 66 Did not respond Unit 1 survived the disturbance Unit 2 tripped on all four occasions
Unit 2 Trip History : Period No of trips Reason Action taken 2010-11 10 Grid disturbance due to Insufficient transmission network 2011-12 7 Low Magnetization of Permanent magnet Commissioning of 400 KV lines Increasing the magnetic power and Cable modification With above changes the system became stable and for next two years there were no trippings due to AVR. Again from 2014 onwards trips due to AVR reappeared
Root Cause : The intervention of dv/dt function of AVR was not allowing the AVR to respond quickly during voltage fluctuation in grid and channel change over is happening. It was suspected that Field Over voltage protection is malfunctioning and tripping the Generator before AVR takes any action. Actions Taken dv/dt function sensitivity is decreased from 50 ms to 100 ms to allow AVR to respond more effectively during grid voltage fluctuations. Crow bar protection card has been replaced on 16 th Feb during similar incident. Present Status After changing Dv/Dt setting & Crowbar card replacement, Unit 2 has sustained 2 voltage fluctuations in the grid & supplied the required reactive power.
Rosa Unit-4 Turbine Bearing Failures Salient Points : Unit has been in commercial operation since 1st April 2012. HIP shaft has bend of 0.12 mm at intermediate gland region between HP & IP turbine. HP & IP inner cylinder has ovality of 0.8 mm. Inter-stage seal radial clearances are not symmetric and more than design values. Turbine has experienced Brg-1 failure problem three times till date.
Major Outage Statistics Unit 4 Failure FY 16-17 25 17th Jul 2016-12th Aug 2016 Bearing 1 Failure (3rd time) FY 15-16 28 20th Jul 2015-16th Aug 2015 Bearing 1 Failure (2nd time) FY 14-15 22 26th Jan 2015-16th Feb 2015 Turbine Lube oil moisture high FY 14-15 20 7th Oct 2014-26th Oct 2014 Bearing 1 Failure (1st time) FY 13-14 48 9th Dec 2013-25th Jan 2014 Turbine Gland steam Pressure High (HIP Cylinder Inspection) FY 12-13 32 7th May 2012-7th Jun 2012 Turbine gland steam pressure high (HIP outer gland Inspection) 0 10 20 30 40 50 60 70 No of Days
2 nd Outage (9 th Dec 2013-25 th Jan 2014) Purpose of Outage: To attend the problem of HIP turbine gland pressure high. Agency involved : SEC ( OEM )
3 rd Outage (7 th Oct 2014-27 th Oct 2014) Purpose of Outage : To attend Bearing-1 failure Agency involved : SEC ( OEM ) 5 th Outage (20 th July 2015-16 th Aug 2015) Purpose of Outage : To attend Bearing-1 failure Agency involved : SEC ( OEM )
6 th Outage (19 th July 2016-12 th Aug 2016) Rosa Power Supply Company Limited (RPSCL)
Event Occurrence : On 19th July 15 at 21:47 hours, Unit#4 was tripped due to Turbine Bearing-1 metal temperature high during rolling. Due to low load demand Unit#4 was under reserve shutdown and on 19th July 16 after getting clearance from SLDC, Boiler lit up at 17:37 hrs and turbine latched at 21:35 hrs. After achieving rolling parameters Turbine rolling started. While ramping up turbine speed, bearing-1 metal temperature and vibration values observed rising abruptly high and more than allowable value. Subsequent to above abnormal observation of Turbine Bearing 1, return oil line filter at main oil tank was inspected and observed traces of Babbitt metal trapped in strainer, indicating damage to the bearing. Following abnormal observations in total were noticed: Turbine bearing no-1 metal temperature went very high during rolling. Turbine bearing no-1 vibration went very high during turbine rolling. Rotor eccentricity increased and vent out of range. Zero speed sensors went out of range. Based on the above observations, it was required to inspect the HIP Turbine Front Journal Bearing-1, 2, 3, 4 and other associated subassembly parts to assess the quantum of damage and necessary rectifications. ROTOR JOURNAL WITH PEEL-OFF BABBIT
Observations & Recommendation : While the dismantling activity was in progress, following observations were found and rectification carried out based on the defect noticed, after discussion with SEC/OEM and SIEMENS turbine expert. Bearing No-1(HP-IP GOV END BRG): After removal & dismantling, top half pads (02 no s) had no peeling or rubbing marks observed. Pads of bottom half (02 no s) bearings had severe rubbing marks & peeling off babbit metal. It is advised to replace all 4 pads of bearing. The pads were replaced with new spares after machining at M/s Press Tools Ghaziabad. The bearing clearances were maintained as per design tolerance value. Bearing No-2(HP-IP GNN END BRG): After removal & dismantling, the top half pads and bottom half pads were observed in good condition. As per the experts recommendations the required UT and DPT checks, Spherical washer contact surface area were checked for >80% contact and found ok. Bearing No-3 (LPT GOV END BRG) : After dismantling the bearing, bearing bottom pads were checked. While doing the NDT tests, surface crack was observed in Dye Penetrate test, while 1.5 mm depth of crack was measured with Ultrasonic testing in bearing bottom left pad. Based on this observation, it is recommended to replace bearing assembly along with spare pads (bottom left and bottom right). MOP seal rings: MOP seal rings clearances observed on higher side then design value. Hence, it is recommended to replace the seal rings with new spare and adjust the clearances to design value. Action Taken: Based on the initial observations and recommendations of expert following action were taken: Bearing No-2(HP-IP GEN END BRG): Bearing-2 washer contact surface corrected to near 80% as per OEM recommendations. Also UT test done for the pads and found OK. U.T. TEST OF BRG PADS
Bearing No-1(HP-IP GOV END BRG): After removal and dismantling bearing no-1, the bearing along with spare/new pads were sent to Ghaziabad on 30 th Aug 2016 to get the pads machined as per the design criterion. The spare(reconditioned) pads were machined at Ghaziabad (Workshop: M/s Presstool) under supervision of SIEMENS expert as per required dimensions. After the machining job was completed, the bearing (along with machined pads) arrived at Rosa site. The following checks were carried out to decide on further course of action: Bearing Clearance Checking Bearing -1 Centre Line Checking After the recommended checks were completed, corrections were done & the bearing was assembled at site as per standard maintenance procedures under guidance by M/s SIEMENS experts.
UT & DPT test of Bearing-1 & Bearing-2 : Non-destructive tests (UT test &DPT test) were carried out by RPSCL Quality Engineers to further ensure that the pads contact area is free from any surface crack or weak bonding. The same was confirmed and were found as per standards. Bearing No-3 (LPT GOV END BRG) : Complete bearing-3 assembly replaced with new spare as per recommendations. Rotor Slope Inspection: Rotor slope inspected at jornal-1, 2, 3, 4 positions and corrected near to first erection & HIP overhaul values. To adjust the rotor slope, following adjustments were carried out in bearings position: o o Bearing-3 assembly was lowered 0.05 from its previous positions. Bearing-1 was lowered by downsizing the adjustable washers under bottom pads by 0.17 mm from both left and right sides. The bore geometry as per Siemens expert advice maintained by adjusting further the top side pads and bearing oil clearance. MOP Seal Rings replacement: MOP seal rings were found excessive clearance more than allowable values same were replaced with new spare and clearances corrected to design value. Root Cause for Bearing Failure: To investigate the reason for bearing failure, the vibration behaviour & bearing metal temperature of the machine at load and also during shut down were studied from the records and also compared with other machines. No abnormality was found in Bearing No-1 and Bearing No-3 in loaded condition or during shutdown. The sudden rise of bearing metal temperature on 19 th July 2016 which led to failure of no.1 bearing may be attributed to accidental breakage of oil film at low speed during coasting down & accidentally more dynamic loading on bearing no 3. Following points have been checked: Puncture / connection failure of flexible oil hoses : Puncture or looseness of oil lines may lead to insufficient /reduced supply oil to bearing resulting in bearing damage & failure. The same was checked visually & also with high pressure air. No damage in oil pipelines was observed Failure of Lube Oil Pumps :- Failure of these pumps may lead to oil starvation to the bearings and subsequently may damage the bearing Babbitt material. But on checking it was found that the lube oil pumps were developing normal oil pressure and temperature.
Oil line choking due to Foreign material inclusion Checked with pressurised air and through Boroscope: No chocking observed. Mal functioning of Bearing tilting pads & Dynamic loading of bearing Bearing Spherical washers & packers were lapped with new spare and colour matched to achieve required contact surface. Bearing loading were corrected by adjusting the bearing elevation and journal slopes near to erection and HIP overhaul values. Remedial Actions to be taken for Permanent Cure Permanent Solution S No Description Remarks 1 Re-fining of casing to address the ovality. Possibility of re-finning at site to be explored 2 Replacement of shaft seals, casing seals and other miscellaneous components Availability of required material at site to be checked. 3 Complete overhauling of HIP & LP turbine. Overhauling time schedule based on UPPCL / Grid requirement to be fixed