Measuring Partial Discharge On Operating Motors With VS-PWM

Drives

Howard Sedding, Greg Stone & Connor Chan

Iris Power – Qualitrol

hsedding@qualitrolcorp.com

Outline

• Effect of voltage surges from inverter fed drives (IFDs) on stator winding insulation systems

• Challenges with on-line PD measurement due to voltage source PWM drives

• PD measurement system based on capacitive sensors

• Field experience

Effect of Drives on Stator Insulation

• Well known PD problems with low voltage IFD motors largely corrected by better stator winding impregnation

• IFDs now in use for motors up to 13.2 kV and 65 MW

• IFDs of the voltage source PWM (VS-PWM) type can cause premature failure of the insulation

• Other types of drives do not cause stator insulation problems

Possible Effects of PWM-VS Converters on MV Insulation

Four possible aging processes:

1. Turn insulation deteriorating due to PD

2. Groundwall insulation heating

3. Groundwall insulation PD

4. PD suppression coating degradation

Only PD suppression coating aging has resulted in significant failures

Effect on Stress Control Coatings

• High frequency capacitive currents will tend to flow through groundwall capacitance direct to ground at the end of the stator cores – bypassing the relatively high resistance conventional stress relief coatings

• Highest power losses are at slot end, rather than end of silicon carbide coating

• May raise surface temperatures 40 C above ambient • Coatings and adjacent groundwall degrade due to

oxidation • May result in extreme surface PD activity and ozone Stress control deterioration seems to be the biggest

problem with MV IFD motors right now

PD Measurement Challenges in Medium Voltage Motors Fed by VS PWM Inverters

• Off-line and on-line electrical PD measurement well-established for 50/60 Hz

• PD testing during short risetime voltage surges from VS-PWM inverters more challenging since surge has similar frequency content as PD

• Need to measure PD in VHF (30-300 MHz) or UHF (300 – 3000 MHz) frequency ranges to separate PD from surge voltages

PD Sensors

1. microwave horn antenna – tends to be only sensitive to end winding PD

2. Microwave patch antenna – sensitive to all types of PD, but most of the power cables are shielded

3. capacitive couplers in VHF range with additional filtering – 80 pF couplers with 40 MHz lower cutoff

- Advantage of latter is that one can use the PD interpretation database established for 50/60 Hz machines

AC Voltage Phase Reference

• Also require a fundamental frequency voltage to synchronize PD with AC voltage

• Fundamental frequency can vary from 10 Hz to 120 Hz

• Due to switching noise on AC voltage, found it necessary to use a separate capacitive voltage divider with additional filtering

Typical Voltage Reference Signal

Output of 80pF Sensors and Voltage Divider on an Operating Motor

PD Measuring System

• Minimize inverter noise using multi-pole filters to strongly suppress signals below about 30 MHz and measure the rise time of each detected pulse, amongst other features, and digitally classify pulses with a rise time longer than 6 ns as noise

Typical Installation

PRPD Plot – 7.2 kV, 45 MW motor

PRPD Plot – 4.1 kV, 4000 hp motor

PRPD Plot – 3.3 kV, 3.1 MW motor

Conclusions

1. Short risetime voltage surges from voltage source PWM drives have caused premature deterioration and failure in the stress relief coatings in service

2. Voltage transients 100s of times PD magnitude so significant challenge to extract PD signals from surges

3. Practical measurements of on-line PD have been demonstrated on dozens of motors fed with VS-PWM inverters

4. Phase position of the PD with respect to the fundamental frequency is “odd”