Condition Monitoring Of High Voltage Machines By The Analysis of Electrical Partial Discharges 22 April 04 Dino Kouadria PhD MSc BEng CEng MIEE MIEEE

CONTENT What is Partial Discharge (PD) Analysis. Stator winding Common Faults. Basics Of Why & How To Measure PD. Case Studies. Open Discussions.

What is Partial Discharges Analysis It is an electrical condition monitoring technique. Used to assess the condition of hv insulation Wider applications static items i.E. Switchgear, cables transformers, etc. Dynamic items i.E. Electrical machines

PDs Analysis On HV Windings Apply to HV motors & generators (>5kv) Online & off line tests. Predominantly on-line tests - no shut down & tests under normal stress condition.

PDs Analysis On HV Windings Assess the current & development in condition of the windings Maintenance planning based on machines’ health. Elimination of unwanted/unexpected shutdowns. Assistance in targeting maintenance effort effectively. Leave Maintain Replace

Why Do HV Stators Fail? Failures due to windings’ insulation failure caused by the combined electrical, thermal, environmental and mechanical stress factors. Most common causes in modern insulation are mechanically induced defects i.E. Loose wedges, erosion due to bar vibration etc. - Short time degradation process. Discharge between phases due to cracks / low spacing Contamination of end windings. Others: internal voids, insulation ageing, delamination. Electrical partial discharges

Electrical partial discharges. So what are partial discharges? Regardless of the type of defects, they all result in the generation of Electrical partial discharges. So what are partial discharges?

What is a partial discharge Local breakdown that short circuits part of the Insulation. Vs = Applied voltage Cavity Vs Formation of corrosive ions, acids, nitrogen oxides, ozone. Detected using of chemical sensors for detection. Emission of light, sound & electromagnetic waves. Detected using ultrasonic devices, antennae, cameras.. Injection of high frequency current pulses. Detected using electrical partial discharges measurement (pd).

Partial Discharges Basic Theory - + Patterns & location of pulses within AC cycle Is the key in defects identification

Importance Of Monitoring PDs Different defects produce different PD patterns. Complex PD patterns are produced when multiple defects are present. Failures are due to localized defects. PD pattern development over time represents developing degradation processes. Provide an early warning of potential problems

Partial discharge monitoring over a machine life cycle Discharge levels Repair/action required Curing Process Fabrication Defects Contamination Mechanical failure 3 – 6 months ?? Years Time

North sea oil industry – mainly Rogowski coils PD Detection Two main type of permanently installed sensors. Capacitive couplers first developed in north america. Main users power industry. Rogowski coils first developed for the north sea oil industry. North sea oil industry – mainly Rogowski coils Ease of installation: no disconnection of supply cables, less installation time. Atex certified

Rogowski Coils: Typical Installation

Partial Discharge Analysis/Display: Typical Cycle

Partial Discharge Analysis/Display: Total Discharge Activity

Partial Discharge Analysis/Display: Pulse Height Analysis

CASE STUDIES

Case studies 1 Contamination on old pump motor Case studies 1 Contamination on old pump motor 11KV-3MW 2 New motor manufacturing problem 13.8KV-12MW 3 Coil Movement old motor 6.6KV-9MW 4 Plastics plant old motor high PD 11KV-2.5MW

Total Discharge Activity 3MW Pump Motor Total Discharge Activity

Typical Detected Faults Damage due to movement and contamination 3MW Pump Motor Typical Detected Faults Damage due to movement and contamination White power deposits clear signs of discharge activity

13.8 kV 12.7 MW Gas Compressor

0ver 70% of wedges loose or missing 13.8 kV 12.7 MW Gas Compressor 0ver 70% of wedges loose or missing

Typical Detected Faults Damage due to wedge movement 13.8 kV 12.7 MW Gas Compressor Typical Detected Faults Damage due to wedge movement

9MW Gas Compression Motor Banded Discharge is modulated which is due to coil movement .....

9MW Gas Compression Motor

9MW Gas Compression Motor Good Coil Air Duct Impressions

9MW Gas Compression Motor Damaged Coil Air Ducts (no abrasion) Lamination groves due to movement

9MW Gas Compression Motor Heavily Damaged Coil Due to impacting on slot bottom

2420KW 11kV ASEA synchronous compressor motor

Outcome of A PD Analysis Survey Detect any potential insulation problems. The contribution of the different problems. Location of problems within the winding. Development in windings condition (through regular monitoring) Early detection allows scheduling of remedial work/repairs. “Provide the maintenance engineer with the information to make The most cost effective decisions on planned maintenance”

Partial Discharge Analysis Not an exact Science. Does not predict when plant is likely to fail. BUT will highlight which machines are most likely to fail. Does not detect Inter-turn Faults More room for improvement in defect recognition. Future development in insulation/motor designs.

Condition Monitoring Of High Voltage Machines By The Analysis of Electrical Partial Discharges 22 April 04 Dino Kouadria PhD MSc BEng CEng MIEE MIEEE