Environmental Science & Engineering - www.esemag.com - September 2005
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Eliminating harmonics improves productivity and reliability at wastewater treatment plant

Power electronic loads such as drives, ozone generators and UV filtration equipment have become abundant in the water and wastewater treatment industries due to their many benefits. But they have one major drawback in common: they might produce a problematic condition known as harmonics. The Corvallis, Oregon Water Reclamation Facility fell victim to harmonics, but found a solution in a power correction system from Schneider Electric.

The term harmonics is used to describe a component of a periodic voltage or current waveform having a frequency that is an integral multiple of the fundamental power line frequency, 60Hz in the U.S. This irregular waveform results because the waveform required by power electronic loads is quite different than the sinusoidal voltage delivered by the utility. This ‘non-linear’ current draw results in a distorted waveform.

High levels of harmonic distortion can stress the electrical network within a water treatment facility and on the servicing utility, causing problems for sensitive electronic equipment. The impact on a facility can be very costly, especially when processes are disrupted or shut down, reducing productivity, increasing repair and maintenance costs, and possibly spilling partially treated or untreated wastewater into nearby waterways.

Besides cleanup costs, which can run into the millions, in the US, Environmental Protection Agency imposed fines are extremely harsh for sewage spills, and the impact of such a spill on the health of local wildlife and people can be substantial.

Warning symptoms of problematic harmonic levels include overheating motors, drives and cables, thermal tripping of protective devices and logic faults of digital devices (CNC, PLC, computers) and generator faulting, all of which can result in process downtime.

These warning symptoms began at the Corvallis facility after a major upgrade that included four new influent pumps, new distribution equipment and a SCADA system to provide overall control, data collection and plant performance documentation. Variable frequency drives (VFD) were chosen as the most efficient means for controlling the two new pumps.

After completion of the equipment installation, commissioning progressed smoothly until the last of the new equipment, the SCADA system, was brought on line. The facility managers were immediately plagued with considerable downtime due to logic faults in the SCADA system, nuisance tripping of ground fault circuit breakers, frequent activation of zero sequence relays in the LV switchgear, and random and autonomous switching of the influent control gates.

The plant reverted to manual control but the faults – although less frequent – continued. Operators then noted that these problems seemed to increase when two or more drives were operating on a common MCC bus at one time.
  1. Ground fault trips occurred when the speed of the pumps was changed.
  2. Ground fault breakers tripped when the VFD was started or stopped.
  3. Influent pump gates switched when the speed was changed.
  4. Negative sequence relays activated when the speed was changed.
Facility managers hired an independent test service to analyze the problems. They measured harmonic voltage and current distortion, current and voltage transients during acceleration and deceleration of the pumps, control signals, zero sequence current during speed changes, and neutral to ground voltage in the breakers. The harmonic voltage and current distortion were measured at varying carrier frequencies to determine if such changes affected the harmonic conditions.

The tests indicated that:
  1. There was no measurable difference in the line harmonics when the carrier frequency was changed from 4.5 kHz to 1.5 kHz.
  2. The total harmonic current distortion [THD (I)] level measured at 89% output speed (approximately 95% load) of one VFD was 38.8% with the 5th order and 7th order having the highest amplitude. Note that each VFD had 3% impedance line reactors.
  3. The total harmonic voltage distortion [THD (V)] measured 3.8%.
  4. A high frequency noise, very close to that of the VFD carrier frequency, appeared on the 4-20 mA DC gate signal when the VFD was accelerated, decelerated, started or stopped. These noise impulses caused the gate to adjust position.
  5. The zero sequence (neutral) current and voltage (208/120Y V) were sensitive to changes in the carrier frequency. As the carrier frequency increased from 1.5kHz to 4.5kHz, the zero sequence current increased from 2mA to 400 mA. This corresponded to a dramatic increase in the incidence of breaker trips.
  6. Notably, a high frequency, consistently seven times that of the carrier frequency, existed in the zero sequence current waveform.
After several remedies were suggested and implemented, the problems occurred less frequently but were not entirely eliminated.

Fifteen months passed before a local electrical distributor working with the Corvallis facility contacted the power quality specialists at Schneider Electric’s plant in Salem, Oregon. Several members of the AccuSine Active Harmonic filter team met with representatives of the facility, toured the plant and reviewed test data.

The team noted several likely contributors to the problems. First, cables of VFD are broadcast antennae for high frequency noise that can radiate to other cables in the same vicinity. All cables act like receiving antennae for this high frequency noise and low voltage control wiring is highly susceptible. As a result, the low voltage signals are easily corrupted. To protect control wiring from this noise, it should never be installed running parallel to VFD power cabling even if each set of cables is in a separate metal conduit. In fact, the only time that VFD power cables should come close to control cables is when they cross at 90 degrees. Because of this, the team recommended that the power cables be installed in grounded metal conduit separate from the control wiring.

Second, a review of the harmonics test data for the facility revealed that THD (V) exceeded 5% when two VFD were operating on one bus, and the TDD (total demand distortion) exceeded IEEE 519-1992 recommended limits of 20% for this installation.

A THD (V) that exceeds 5% can affect many different types of equipment – often, electronic equipment – in an electrical network. The fact that the SCADA tripped occasionally suggested that high THD (V) was involved.

To reduce the levels of TDD within the guidelines of IEEE 519-1992 and THD (V) to less than 5%, the team recommended an AccuSine PCS active harmonic filter rated at 50 amperes.

After the AccuSine unit was installed and the power cables rerouted through grounded metal conduit, the performance and reliability of the plant electrical system were greatly improved. The TDD was measured in the 5-7% range for one VFD and 18- 21% range for two VFD. The THD (V) was less than 5% for all circumstances.

The damaging effects of harmonics should be considered during the design of a power electronic system. Proper installation techniques and harmonic mitigation equipment can eliminate problems before they occur. From an economic and performance standpoint, active filters are often the best choice for harmonic filtering when compared to such alternatives as passive filters or 18 pulse drives. With active filters, an engineer can review a group of loads easily and accurately in the design stage without having to perform any expensive harmonic studies. Standards compliance can be guaranteed and the potentially costly problems are avoided.

The Corvallis case shows how harmonics can disrupt water and wastewater treatment processes, driving up costs. This troubling power issue should be considered during the design stage of a project. Research your many options in harmonic mitigation equipment to find the most cost effective and reliable solution for your facility.

For more information please contact Eric Truesdale at etruesdale@bader-rutter.com

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