Ian Culbert (deceased)
Iris Power-Qualitrol

Editor’s Note:  The following article was written by Ian Culbert, an engineer with Iris Power - Qualitrol in Mississauga, Ontario, Canada. It was submitted for publication by John Letal of Iris Power - Qualitrol and member of EASA’s Technical Services Committee. Sadly, Mr. Culbert passed away recently. We appreciate his contributions to the industry. 

Introduction
Due to their ever decreasing costs, variable frequency drives (VFDs) are becoming more popular for energy conservation and the reduction in inrush currents during motor starting. The most widely used type of drive today is a voltage-source, with pulse width modulated (VS-PWM) inverter, since it tends to be less expensive than other possible topologies.

In the past decade, medium and high voltage motors rated 2.4 kV to 13.8 kV fed by VS-PWM drives have become more common. Currently motors rated up to 100 MW are being designed. Motors supplied from such drives have seen premature stator winding failures since the voltage impulses from the drive can lead to rapid insulation system aging. In most cases, as the aging progresses, the partial discharge (PD) activity increases. Thus, there is a desire for on-line PD detection for such motors. 

Unfortunately measurements with conventional electrical PD detectors tend to be masked by 1000-3000 V impulses created by multi-stage VS-PWM drives. The rise time of the voltage impulses from the multi-stage VS-PWM drives tend to be 500 ns or longer. This article describes the technical issues in on-line PD detection on motors fed by VS-PWM drives, and gives an example of one system that detected the PD successfully.

Challenges in measuring on-line PD in VFD supplied motors 
With minor modifications, standard PD test methods can be used for variable speed motor stators using cycloconvertors, load commutated inverters (LCI) and current source PWM inverters. Conventional PD detectors can be used since these drives do not produce short-rise time voltage impulses of any significance. LCI drives do produce voltage impulses, but they use gate turn-off thyristor (GTO) or thyristor switching devices that have rise times significantly longer than 1 µs, which can be readily filtered out by PD detectors operating in the very high frequency (VHF) (30-300 MHz) range.

On the other hand, as can be seen from Figure 1, the output voltage waveform for a 3-level PWM voltage source drive with insulated-gate bipolar transistor (IGBT) or similar switching devices, produces short-rise time voltage pulses which can be in the order of 500 ns. Such impulses can produce pulse frequencies up to about 1 MHz.

One reason that PD measurement is difficult with VS-PWM drives is that the frequency content of the impulse voltages (which are about 10,000 times or 80 dB higher than PD pulses) significantly overlaps the frequency content of the PD. In conventional PD detection, the mV PD pulses are most commonly separated from the high voltage 50/60 Hz using a single pole high pass filter. With a PD detector operating in the VHF range, the high voltage 50/60 Hz is reduced to < 10 mV, while the magnitude of the PD (which in atmospheric air has a rise time of about 3 ns) is largely unaffected. This makes it easy to extract the PD from the residual power frequency voltage.

In contrast, a 500 ns rise time, 3000 V impulse will be reduced to only about 50 volts by a single pole high pass filter with a lower cutoff frequency of 30 MHz. Of course PD detectors operating in the high frequency (HF) (3-30 MHz) and low frequency (LF) (<3 MHz) range will have even higher residuals from the voltage impulses. Clearly additional electronic or passive filters or other means are needed to further suppress the impulse voltage by another 60 dB or so, in order to reduce the residual to less than the typical PD levels (using a VHF measurement method, significant PD has a magnitude of several hundred mV). 

On-line PD monitoring system for medium and high voltage VFD suppied motors 
To overcome the problem of separating the PD from the residual impulses from the VFD, a conventional VHF on-line PD detector system was used as a basis. It included standard high voltage fixed 50/60 Hz frequency PD sensors – 80 pF capacitors using a mica splitting dielectric. Tests had shown such sensors yielded high impulse flashover voltages, e.g., > 75 kV for a 16 kV capacitor. 

To separate the stator winding PD pulses from those imposed by the VFD, a supplemental multi-pole high pass filter was designed. This would take the 30-50 V residual impulse voltage to < 200 mV, which would reduce the 3 ns rise time PD pulses less than 3 dB. The nominal cutoff frequency chosen was 5 MHz and this filter would be installed between the 80 pF coupler terminations and the PD measuring instrument (see Figure 2).

Experience from several prototype installations revealed that a reliable fundamental frequency synchronization signal using the 80 pF PD coupling capacitors could not be obtained. So a capacitive voltage divider that was optimized for the frequency range 150 Hz and below was developed. The optimum placement of the voltage divider was at the inverter output (Figure 2), before voltage doubling of the impulses occurred due to impedance mismatches. However, further experience showed that the divider would work satisfactorily when installed at the motor terminals.

Case study
Prototype PD monitoring systems were installed on the stator windings of several 65 MW, 11 kV compressor motors. The waveform of the impulse voltage measured at the stator using a high voltage divider is shown in Figure 3. The installation of an 80 pF PD sensor is shown in Figure 2. A modified Iris Power TGA-B with an additional 5 MHz high pass filter at the TGA-B input terminals was used to record the PD signals. The fundamental frequency synchronization signal came from a heavily filtered capacitive voltage divider installed at the drive. The test results shown in Figure 3 confirmed that it was possible to separate PD from VFD voltage pulses.

Reference
G.C, Stone, I. Culbert, S.R. Campbell, “Progress in On-Line Measurements of PD in Motors Fed by Voltage Source PWM Drives,” Conference Proceedings of IEEE EIC, June 8-11, 2014, Sheraton Philadelphia Downtown Hotel, Pennsylvania, USA.

Categories: AC motors, Case studies, Drives/controls

Tags: Variable frequency drives Partial discharge (PD) testing

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