A Downtime Mystery Solved

Investigations into unexpected shutdowns can lead plant personnel to assign fault to specific components, but often, only a thorough engineering analysis reveals the true causes.

Such was the case at an automotive plant, where premature failure of pulse-width modified (PWM) variable-frequency drives (VFDs) serving supply- and exhaust-fan motors was interrupting the plant’s automated paint process. The plant has 70 VFDs—from 30 to 400 hp—in service at several 2,000-kVA low-voltage substations.

Shutdowns of the paint-house VFDs were not infrequent. After each event, maintenance technicians would perform field diagnostics and would find one or two blown fuses at the affected drive. They also tested—and condemned—one or more gate turn-off thyristors (GTOs) on the voltage-source inverters. GTOs and fuses were replaced, and drives were successfully restarted without further incident. Each event involved a different VFD. None of the drives had experienced more than one shutdown event.

A VFD Mystery

Was there a more involved problem? As suspected, testing showed that harmonic distortion at the low-voltage bus increased with increasing numbers of VFDs in operation. Further, measurements showed that voltage distortion at the main buses approached 10% total harmonic distortion when normal levels of drives were operated. This is below the THD level at which VFD problems are usually encountered.

Further testing showed that VFD currents measured at the drives were erratic and did not resemble the expected two-pulses-per-half-cycle signature characteristic of PWM drives. The erratic current signature was unusual for a PWM signature and signaled an anomaly. High-frequency harmonics, about 960 Hz, were found in line currents.

While initial engineering analysis started with the plant technicians’ reports on GTO failures, measurements and computer simulations did not demonstrate a power event that would cause the actual damage levels seen on the GTOs. Drive engineers determined that drive design prevented GTO failure from drawing enough current to blow an AC fuse and asserted that the GTO locations in the drives weren’t getting enough available fault current due to upstream circuit components such as diodes and DC bus. In fact, none of the empirical evidence could explain why GTOs would unexpectedly fail. And without a long-term solution, degradation and premature fuse failure would continue.

Test results showed that none of the five GTOs were damaged, so the GTOs had been unnecessarily removed and replaced. Were the drive fuses at fault? These fuses had indeed opened during the shutdown events. Could the drive fuses have been damaged during prolonged operation under erratic harmonic currents?

A search of published articles found no precedents for this conclusion. But engineers continued to believe that the substantial high frequency content and dramatic fluctuation in peak current magnitude of the drive currents subjected the fuse elements to abnormal stresses. The result might have been from accelerated aging.

That theory was supported by the fact that no fuse failures had occurred on a circuit where drive operation was stable.

A typical recommendation might have been the use of harmonic filters. However, one encounters a common dilemma with the use of these items with transformers with a high percentage of VFDs. Conventional shunt filters contain capacitance that increases displacement power factor on the applied bus. VFDs, however, typically operate at high-displacement power factor, while producing high levels of harmonic current. Many VFDs on a bus means high levels of capacitance that can result in leading displacement power factor and overvoltage.

Further, the engineers used the Alternative Transients Program (ATP) “shareware” software developed in Canada. ATP modeling revealed that harmonic filters wouldn’t resolve the erratic drive current phenomenon. Low system inductance was the major factor contributing to that erratic drive current. Harmonic filters would appreciably change system inductance. Low inductance allowed the DC filter capacitors inside the VFD to charge erratically, resulting in the non-characteristic AC current.

Case Solved

Instead, engineers suggested a combination of line reactors and phase-shifting isolation transformers.

Simulation identified optimum simulated-voltage and current distortion reduction. The best technical solution was to increase system inductance seen by the VFDs. While line reactors alone could provide this inductance, engineers modeled delta-wye transformers to assess additional benefits.

While harmonic current passes through line reactors and wye-wye or delta-delta transformers without appreciable phase shifting, delta-wye transformers have a different effect. Fifth- and seventh-harmonic components, which comprise a significant portion of VFD drive currents, are phase-shifted by 30 degrees of the fundamental by delta-wye transformers. The result of such a phase-shift is currents that are 180 degrees out-of-phase with fifth and seventh harmonics from non-phase-shifted drives.

Thus, the combination of line reactors and delta-wye transformers contributed to a significant cancellation of the aggregate fifth- and seventh-harmonic current contribution of all the drives.

Of the 66 drives in the paint house that were not already equipped with inductive isolation—four 400-hp drives had line reactors—only seven were equipped with delta-wye isolation transformers, while 50 received line reactors. Delta-wye transformers were reserved for VFDs with ratings of 200 hp and above, while 100 125-hp drives had open-style reactors installed in the existing drive enclosure.

Inductive isolation was not required on drives under 100 hp. Fuses that had not failed and had been replaced were changed due to suspected deterioration.

All equipment was installed and operating within a month. Erratic voltage and waveforms were corrected to clean sinusoidal voltage and double-hump current waveforms. Harmonic voltage distortion was reduced to less than 5%, and drive currents returned to their normal signature.

Additionally, the auto plant upgraded training of maintenance technicians to avoid the early confusion about the problem’s source. Procedures were put in place to improve in-place testing of the GTOs—and to require that any electronic devices suspected of damage be subjected to laboratory testing. No further fuse failures have occurred.

From Pure Power, Summer 2002

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