PARTIAL DISCHARGE MEASUREMENT ON ROTATING MACHINES Engr. IÑIGO V. ESCOPETE, JR. ITC Level 2 Certified Thermographer PHIL-NCB NDT-UT Level 2
Partial Discharge testing is a Condition Based Maintenance tool for motor and generator stator windings. It is the only on-line electrical test that correctly identifies and warns of pending stator winding failure mechanism such s loose windings, contamination, thermal deterioration, poor manufacture, problems due to load cycling, and more. The test can be applied to any stator winding 3kV to 26kV. CIGRE studies indicate that approximately 56% of all generator failures attributed to stator winding insulation
WHAT IS PARTIAL DISCHARGE? A partial discharge (PD) is a localized electrical discharge that only partially bridge the insulation between conductors.
PD occurs if electrical stress exceeds electrical breakdown point of gas Voltage builds across a gas-filled void.
Neglecting Partial Discharge (PD) activity can lead to further degradation of the insulation system, resulting in possible complete discharge. PD is measured in AMPLITUDE (mv, nc) and Repetition Rate (PPS)
Why Test for Partial Discharge? RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
Failure mechanism of Power Generators CIGRE Study Committee SC11,EG11.02,2002
Many different types of materials in a generator. These material heat and cool at different speed. Gaps will form between the different materials as the generator gets hot and cold. More fluctuating load = more heating/cooling = more gaps. More gaps = more PD
PD Related Failure RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
Thermal Cycling Effect RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
Slot Vibration Discharge RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
Slot Vibration Discharges RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
Bar to Bar PD RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
Contamination RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
Typical Rotating Machine PD Monitoring Layout
PD Detection Sensor for Motors and Generators Epoxy-Mica Capacitors(EMCs) 80 pf Directly connected to HV bus on the terminals 80pF creates a High Pass Filter with a 40 MHz cut-off frequency
Bus Coupler Installation RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
Bus Coupler Installation RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
Directional Time-of-Arrival (PD Detection)
Directional Time-of-Arrival (Noise Separation)
Pulse Shape Noise Discrimination
Periodic On-Line PD Testing RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
Benefits of On-Line Monitoring Monitor under normal machine operation all the time. Get maximum PD readings and prior warning of impending failure in time. Facilitate remote monitoring and reduce man-hours with periodic testing (using portable instrument) Capable o connection to plant control system.
Basic Interpretation of PD Testing Results Instrumentation provides: Pulse Magnitude Analysis Graph Pulse Phase Angle Analysis Graph Instrumentation also produces summary indicators: Qm for Peak PD magnitude and NQN for total PD activity.
Magnitude Analysis Graph Qm the peak magnitude in mv at 10-pulse per second Measure of void size: Qm Void NQN the representation of area beneath the curve Measure of total PD activity, total amount of deterioration monitored
PD Results: PD Trend RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
PD Results: Phase Resolved Partial Discharge
PD Results: PRPD Polar RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
PD Results: PRPD 3D RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
PD Results: Pulse Height Distribution
AC Phase Position of PD RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
PD Pulse Polarity RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
No Polarity Predominance RMEAC - REGISTERED MASTER ELECTRICIAN AFFAIRS COMMITTEE
Positive polarity predominance
Negative polarity predominance
Three Basic Method for Interpretation Compare graphs from identical machine (stator with highest PD is most deteriorated) Compare test result from a machine to a similar machines. Trend PD over time (2 times increase per year is significant rate of deterioration)
Trend PD Over Time Trending of Data Most powerful, reliable means of interpreting data. If Qm low and constant trend, winding in good condition. Collect data over the years, if Qm increases slowly, winding is stable. Trending can also determine if repairs effective.
Summary: Stator windings are subject to a variety of failure mechanism. These failure mechanism create problem areas. Partial Discharge occur in these problem ares and are symptoms of these failure mechanism. Stator windings can be monitored on-line through partial discharge testing.
Thank you for listening. Questions?