MAINTENANCE PRACTICES • Types of Maintenance: • PREVENTIVE MAINTENANCE • CORRECTIVE / Breakdown MAINTENANCE • MAINTENANCE CO-ORDINATION • Condition based maintenance • OVER HAULING • FAILURE ANALYSIS
CONDITION MONITORING / CONDITION BASED MAINTENANCE • CONDITION MONITORING: • TAN DELTA AND CAPACITANCE • DISSOLVED GAS ANALYSIS • THERMOVISION SCANNING • VIBRATION MEASUREMENT • DCRM OF CIRCUIT BREAKERS • LEAKAGE CURRENT OF LAs • DEW POINT OF SF6 GAS/AIR
MAINTENANCE SCHEDULES PURPOSE OF PERIODIC MAINTENANCE -- TO ENSURE TROUBLE FREE OPERATION OF THE SUBSTATION EQUIPMENT AND ASSOCIATED LINES PREVENTIVE MAINTENANCE SCHEDULES --- DAILY --- WEEKLY --- MONTHLY --- QUARTERLY --- HALF YEARLY --- ANNUAL --- 2YEARLY / 3YEARLY / 4YEARLY --- S O S
Different Substation Equipment • Current Transformers (CTs) • Capacitive Voltage Transformers (CVTs) • Lightning Arrestors • Isolators , Earth Switches & Busbars • Circuit Breakers • Transformers • Reactors
Current Transformers • Monthly • Oil level in view glass. • Expansion of bellows • Yearly • Tan delta & Capacitance measurement(0.007,0.007 – 0.01half yearly, above 0.010 replace) • Thermovision scanning of tank and top dome. • Oil leakage from secondary terminals • 2Yearly • Checking of N2 pressure(0.2 to 0.5kg/cm2) • SOS • IR value, ratio test(+-3%, +-1%), magnetisation, secondary resistance, DGA(initially one month after testing)
Capacitive voltage transformers • Monthly – Oil leaks • Fortnightly-1.2 to 2V / Monthly-0.8 to 1.2V / 3Monthly-0.5 to 0.8V, (-0.8 to -4)-15days, <(-4) replace • Monitoring of CVT secondary Voltages at yard MB. Frequency depends on difference of secondary voltages.(6monthly) • Yearly – HF earthing, thermovision scanningof stacks and EMU • 4 Yearly – Capacitance & tan delta • +-2% 3yearly, 2 – 3% yearly, above 6% replacement 0.002 to 0.003 yearly, >0.003-alarm • SOS- BDV
Lightning Arrestors • Yearly • Testing of the discharge counters • Measurement of THRC or third harmonic resistive component • Limit 150µA for gapless LAs(ZnO) and 350µA for gapped LAs. 150-350 – IR value stackwise(>1000Mohms)
Lightning Arrestors • In case the THRC values are outside specified limits, we have to go for additional tests. • Capacitance, tan delta and IR value measurement are to be carried out to ascertain healthiness of the LA. • Isolators and ES • Yearly • Thermovision scanning • 4 Yearly • Contact resistance measurement <300
Circuit Breakers • Yearly • Functional checks: • Compressor start / stop pressures. • SF6 gas alarm & lockout. • Various lockouts & pressures. • AR lockout, closing lockout, Tripping lockout / operation lockout.
Circuit Breakers • Yearly contd.. • Timings: • Trip, Close, PIR, Duty cycle, Pole disc. • Trip time 25ms max. • Close time 150ms max. • Between poles +/-3.3ms • Between breaks +/-2.5ms • PIR time 8-12ms (12-16ms for BHEL)
Circuit Breakers • 2 Yearly • SCRM (<150µΩ) • DCRM and Travel measurement (CO –300ms) • 4 Yearly • Capacitance & Tan δ measurement of grading capacitors. • Dew point measurement(<-30degC)
Static Contact Resistance The CB is kept in closed condition. Current is injected in CB contacts and millivolt drop is measured only once. The calculated contact resistance value reflects the contact resistance of main contact Dynamic Contact Resistance The CB undergoes C-O operation. The current is injected in CB contacts and continuous samples of contact resistance are taken. Resistance Variation of Contact System During contact insertion is plotted on time scale. Measuring Dynamically… The Concept
CB Interrupter Voltage Pickups Current Injection The Static contact Resistance measurement R=V/I Millivolt Trolley
Incoming Flange Outgoing Flange Main Contact, Moving Main Contact, Fixed Fixed contact Holder Arcing Contact, Moving Arcing contact, Fixed Moving contact Holder A Look at Contact Assembly…
Contact Resistance Main Contact Arcing Contact Event Break Make Time CB Contacts during Make and Break …
Practical resistance values… • Main contacts: 40 to 80 micro-ohms • Arcing contacts: 125 to 600 micro-ohms • Flange to flange joints: 100 to 1000 micro-ohms
Observations • Moving Arcing contact was found de-shaped and had expanded • One Counter Head Screw of Moving Main Contact assembly was found loose • PTFE Nozzle of moving contact assembly found cracked. • Scratch marks were also seen on Main Moving Contact
Maintenance Schedules for Transformers & Reactors • Monthly • Half Yearly • Yearly • 2 Yearly • 4 Yearly
Details of Testing • Monthly • Checking oil leaks from bushings, main tank, conservator. • Checking of fans and pumps. • 6Monthly • Testing of oil from main tank for BDV and DGA.(>50kV)
Details of Testing • Yearly • Testing of Bucholz relay of Main tank and conservator for actual operation. • Testing of Pressure Relief Device. • Testing of OLTC operation from local and remote. • BDV of OLTC oil.
Details of Testing • Checking auto starting of fans and pumps by actual simulation of WTI. • 2 Yearly • Tan delta & capacitance of bushings. • Tanδ <0.007 at 20degC • Tanδ at TdegC = Tanδ at 20degC x K Where K=0.8 at 10degC …. 3.0 at 70degC • Capacitance can vary +/-5%
Details of Testing • Calibration of WTI and OTI using temperature calibration bath. • Vibration measurement for reactors. • Average 60microns, 200microns Peak to Peak • 4 Yearly • IR value of windings, PI and DA ratio. • IR value should be 1000MΩ with 5kV/10k Megger.
Details of Testing • PI=IR value 10min/1min • 1.25 to 2 --- Good • DAR=IR value 1min/15sec • >1.5 • Tan delta and capacitance of windings. • 5 Yearly • Winding resistance at all taps. • R75=Rt(273+75)/(273+t)
SOS / Breakdown Tests • Short circuit test • Frequency response analysis • Testing of Bushing CTs • Magnetizing current test • Turns ratio test • Oil filtration and de-gassing, dryout.
Sweep Frequency Response Analysis is known as SFRA The SFRA method is used to check the eventual change in the internal geometry of the active part of the transformer : displacements or deformations. A tool to investigate mechanical integrity: transformers, reactors.
Normal Response Three phases respond differently at low frequencies Three normal open circuit responses
Shorted Turn One phase has clear inductive roll off associated with short circuit measurement After an incident - one year later...
Over Hauling • Over Hauling: • Proposed by STATIONS and recommended O&M for approval of HQ. • Major overhauls taken up in coordination with manufacturers. • Taken up on need basis, analysis of conditioning monitoring results. • Age of equipment also considered.
INVESTIGATION OF MAJOR FAILURE -- • INVESTIGATION OF MAJOR FAILURE -- COMMITTEE CONSISTING OF MEMBERS FROM ENGG, O&M, Quality, O&M & MANUFACTURER • MEDIUM & MINOR -- REGIONAL LEVEL. • GUIDE LINES FOR INVESTIGATION : • VISUAL INSPECTION • CONSEQUENTIAL DAMAGES • HISTORY OF THE EQUIPMENT