Case study: Clean Power Delivers Clean Water

Power quality, Utilities

Download PDF: Case Study: Clean Power Delivers Clean Water

Measuring tools: Fluke 434 Power Quality Analyzer

Features: Power factor, harmonics

This story features a regional wastewater treatment plant on the shores of Puget Sound, Washington. An award-winning facility serving six cities, the plant processes an average of 21.1 million gallons of wastewater daily, with a daily capacity of up to 28.7 million gallons.

Most water treatment facilities use the same basic processes, but this one has been continuously updated since it opened in 1984, including high-intensity UV light and soon, a new Class "A" biosolids drying facility.

In this particular case, maintenance electrical specialist Mark Newport was in the process of installing new electronic power factor correction units at multiple motor control centers (MCCs) to improve power quality, protect equipment and reduce costs. However, since corrective measures can often introduce new power quality problems into the system, he used a handheld Fluke 434 threephase Power Quality Analyzer to make sure their efforts were successful.

System Details

The treatment plant relies on hundreds of motors, some as large as 500 hp. A SCADA control system is interfaced to the MCCs, as well as valves, actuators, sensors, and power monitors. It also allows instant visibility into the system at numerous stations.

If the plant went down for any reason, such as a regional power outage, sewage would continue to flow through the plant and though the sedimentation tanks, but it would be discharged into Puget Sound without full treatment.

To prevent this from ever happening, the plant electrical system has several levels of redundancy. It's served by dual utility feeds from two substations. Generally the load is shared between the two feeds, but either feed can handle the whole facility through a tiebreaker system. A 1.25 MW generator on an automatic transfer switch can take over within 8 seconds, if both utility feeds fail. Small, single-phase uninterrupted power supplies (UPS) maintain the elements of the redundant control system while the generator starts and synchs.

Power Correction

The power correction systems were being installed primarily to correct power factor. The same conductors were used to feed both adjustable-speed motor drives (ASDs) and across the line motor starters. The ASDs are non-linear loads and draw harmonic currents, especially at the fifth harmonic. Power factor capacitors present a lower impedance to higher frequencies, than to the 60 Hz fundamental. So a system with harmonic currents can cause overcurrent in correction capacitors. Maintenance staff at the plant realized this and decided to use a semiconductor-based device instead.

The device they connected to each MCC continuously monitors a circuit's power factor. It generates leading reactive power (VAR's) to counteract the inductive, lagging VAR's from the motors. It can respond instantly to load changes. In addition, it looks for any harmonic current and generates an inverted signal to cancel it. So, to any upstream devices it appears as if the MCC is a resistive load.

Before and After

The power correction system lived up to expectations. Six screens from the power quality analyzer tell the whole story.

The change in current is a real testimony to the impact of power factor correction. By correcting the power factor the rms current drawn by the MCC was reduced by 27 %. You can see what happens in the power measurements. The power conditioner uses a relatively small amount of power to run. But it counteracts the reactive power, and reduces the apparent power (kVA's) dramatically. You can see the power relationships in the vector diagrams below.

Prior to the installation, the upstream distribution equipment had to carry a lot of reactive current and harmonic current that was sloshing around without doing any real work. Now the system carries only hard-working current to the MCC's.

The power corrector cleaned up the harmonic content, too. Most of the non-linear load was from large motor drives. The 5th harmonic was a big contributor to the overall distortion. Both the amount of the 5th harmonic current and the current THD were cut by more than 50 %.

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