Power Quality in Water & Wastewater Treatment Plants

America’s more than 14,500 water treatment plants (WTP) and wastewater treatment plants (WWTP) rely upon properly designed electrical distribution systems to supply clean, stable power. In this article, we will address power quality (PQ) as a first step in recognizing opportunities to improve operational reliability.

WTPs and WWTPs are some of the largest consumers of electrical power. Approximately 4% of U.S. electricity usage is linked to water treatment, corresponding to 30.2 billion kWh annually. From a strictly monetary standpoint, it makes sense to adopt a strict PQ policy because it allows a WTP or WWTP to dramatically cut operational costs. From a sustainability perspective, PQ translates into a smaller carbon footprint, along with fewer incidents requiring the discharge of wastewater into streams and lakes. PQ is key for continuous, safe, cost-effective, and sustainable treatment services throughout a plant’s lifetime.

Power Quality Problems in WWTPs

PQ refers to power characteristics, or more specifically, the measured quality of current and voltage using criteria like frequency, symmetry, frequency, and magnitude. Desired characteristics are clean sinusoidal waveforms free of harmonics, sags, swells, unbalances, or fluctuations.

First, let’s discuss harmonic distortions. Variable speed drives (VFDs) have made WTP and WWTPs highly vulnerable to their damage. For instance, compressors are frequently paired with VFDs to maximize efficiency, yet VFDs create harmonic distortions on both their input and output side. Besides VFDs, other low- and high-power nonlinear switching loads, like computers, power controllers, servers, telecom equipment, and switched-mode power supplies, force conductors to carry currents at frequencies other than 60 hertz. Low harmonic frequencies (e.g. 180, 300, or 420 hertz) are caused by low-frequency distorted currents and phase-shifted currents. High harmonic frequencies (between 1 and 3 kHz) are caused by switching of high currents in high-power, nonlinear electronically switched loads.

This problem is growing progressively worse as the power levels of electronic switching power supplies continue to increase, and more high-power electronic switching-based equipment is added to electrical systems. Another harmonics phenomenon, line notching, is caused by the switching of current rectifiers in DC motor drives, motor starters and power supplies. Line notching can produce high frequency harmonics destructive to capacitor banks and electronics.

Besides harmonics, WTPs and WWTPs are highly vulnerable to voltage sags (undervoltage) brought about by large loads coming online. Sags last at least one half cycle but can endure for up to several cycles. In severe cases of more than a 15% sag, PLCs and PCs crash and other equipment may abruptly shut down. These same issues can occur because of a voltage swell. The opposite of a voltage sag, swells are caused by large loads going offline or by a lightning strike, resulting in a sudden massive increase in voltage. In cases of more than a 10% swell, sensitive components may be damaged, and system lock-up or failure can corrupt valuable operational data.

And then there is noise. When small motors are started they are not likely to cause any major problems for the equipment itself. However, switching noise occurs and produces rapid voltage variations whenever equipment is turned on or off using electrical contactors. This noise will affect the sensitive equipment controlling and monitoring processing equipment.

Yet another problem is a compromised grounding network. Common in aging electrical systems, a compromised grounding network will permit leakage current to travel paths not engineered to act as grounds (i.e., building structure, piping, sensor wiring) because the mechanical joints of conduit and rigid pipe have become less capable of maintaining low-impedance ground paths. Compromised plant grounding systems with high impedance that provide a “clearing house” for disturbance currents are incapable of supporting nonlinear switching loads. Such grounding systems cannot dissipate and expel harmful low and high frequency disturbance currents into the Earth ground, leaving them available to create high-differential transient voltages, which cause damage to machines.

The average WTP and WWTP is 25 years old. Older electrical systems can be compromised by thermally hardened electrical conductors, overloaded circuits, and loose connections in electrical panels. The EPA recommends that WTP and WWTP electrical systems be replaced every 15 to 20 years.

Mitigation Solutions

Eighty percent of electrical disturbances that cause poor power quality originate internally. Severe weather, utility fault clearing, power line accidents and other external network issues like grid switching or power-factor correction capacitors represent the remaining 20%. External disturbances generate spikes or interruptions that can instantly damage equipment. Worst of all, these incidents are completely unpredictable and beyond anyone’s control. Due to the numerous sources and types of power disturbances, maintaining PQ effectively requires a multi-tiered approach outlined below.

Corrosion

Aggressive chemicals and gases, combined with the constant presence of moisture, create a highly corrosive environment. Exposure to corrosive elements prematurely deteriorates unprotected devices, forcing frequent repairs and replacements. Given these circumstances, devices in treatment plants must be specifically engineered to resist corrosion, as well as extreme temperatures, humidity, shock, and vibration. 

Cascading Protection

Power quality throughout a treatment facility requires more than a single PQ device located at the service entrance. Depending on the size of the plant, proper protection may require dozens of devices strategically located throughout the electrical distribution system. Installing PQ devices at all levels of the electrical distribution system is known as cascading (or layering), or as IEEE (Institute of Electrical and Electronics Engineers) refers to it, Protection in Depth. The closer to the service entrance, the more robust the device should be rated.

Conclusion

Given the sensitivity of today’s processing equipment, WTP and WWTP managers must take the proper steps to ensure continuous, reliable power quality before malfunctions and delays occur. If malfunctions are already taking place, this should be taken as a clear indicator that poor power quality is in a plant’s electrical system. Additionally, plants with outdated electrical systems or those who are looking to upgrade, should consult with PQ experts so that the devices they incorporate offer the optimal protection. With improved mitigation devices, a plant’s profitability and performance will be protected for years to come.