Utility power quality can impact the performance and longevity of pumping systems, especially in remote regions. Poor power quality from factors such as harmonic distortion, voltage imbalances and undervoltage can reduce pump life span and efficiency. This article will explain how voltage sags, swells and imbalances can affect the reliability of pump systems, cause malfunctions and result in unexpected downtime that could interrupt treatment operations.
It will also explain how active front end (AFE) ultra-low harmonic (ULH) variable frequency drives (VFDs) help combat the effects of these voltage dips and other power quality problems. Additionally, it will discuss the typical VFD topologies and how they are impacted by power quality.
Poor Power Quality Factors
VFDs and other power conversion equipment are nonlinear loads, which generate harmonic distortion. The effects of harmonic distortion can include overheating of transformers and motors, erratic operation or failure of sensitive electronic equipment, breakers randomly tripping, generators becoming unstable and fuses blowing. Harmonics can cause electrical systems to trip offline or fail for no apparent reason, further impacting pumps, aeration, chemical dosing and instrumentation.
The resulting downtime decreases productivity and reduces utility network efficiency and reliability. In addition, what happens if utility power is interrupted? Emergency generators must come online to power the facility while waiting for utility supply to return. Emergency or backup generators are typically sized to keep capital cost down, and as a result, they sometimes struggle to provide enough stable power to carry the load demand and maintain operations when harmonics are present.
Another sign of poor power quality is undervoltage and unbalanced voltage, when the voltage from the utility to the municipality is more than +/- 5% of the nominal line voltage and 3% phase-to-phase voltage imbalance. This event can be for a short duration or may be continuous. This may occur due to numerous issues, including unbalanced incoming utility supply, unequal transformer taps settings, unbalanced single phase loads or possibly an open delta on the transformer. It may also be possible that environmental factors like lightning strikes and other weather-related issues may contribute to undervoltage and unbalanced voltage.
When an undervoltage or unbalanced voltage situation occurs, the motors and pump systems draw more current than normal, causing overheating in the electric motor windings and leading to premature failure. If the motor overload protection in the windings does not trip, this will cause excessive damage to the windings and potential catastrophic failure of the motor and pumping system. In addition to the damage to the motor, VFDs can also trip on overcurrent or direct current (DC) bus overvoltage faults, causing the pumps and process control to experience interruptions.
To understand why a standard 6-pulse VFD will trip due to undervoltage and voltage unbalance, we need to look at the relationship between voltage and current in the motor equation.
Motor horsepower =
(V x I x motor efficiency) / 746
When the required horsepower (hp) is constant and the supply voltage (V) decreases due to a voltage sag, the current (I) must go up to supply the required load horsepower. Therefore, the VFD must supply more current to the motor, which can cause the VFD to trip offline. Typically, the VFD will trip on an output overcurrent fault or DC bus undervoltage trip when the supply voltage decreases.
In addition to the power quality issues described, municipalities also endure the effects of harmonics. Harmonics are current and voltage distortions that impact the power source in the form of additional current draw and result in unwanted heat. They can be caused by nonlinear power loads in a facility. Some examples are single-phase digital power supplies, LED lighting and standard 6-pulse VFDs.
Harmonics cause many power quality and equipment problems. Transformers may overheat, circuit breakers can trip, fuses can blow, electronic equipment can be damaged and instrumentation can give false readings. Additionally, harmonics can cause communication equipment to experience interference and generators to become unstable or overload and trip. Motors and pumping systems may also overheat and fail.
Active Front End Ultra-Low Harmonic VFDs Help Combat Power Quality Problems
Using VFDs on pumping systems is typical to allow the pump to change speeds to maintain a flow rate or pressure set point. Energy savings is achieved when slowing the motor down and running at under the rated base speed. Selecting the proper VFD will mitigate power quality issues, including harmonics, that may otherwise negatively impact equipment.
There are multiple VFD options, including standard 6-pulse drives and 18-pulse system. Both use diode bridges to convert alternating current (AC) power to DC power, which causes harmonics. The DC bus is charged by the peak voltage of the AC input source.
Therefore, in undervoltage conditions, the DC bus is never fully charged and cannot supply full voltage to the motor. Harmonics are created when AC power is converted to DC and is injected back to the AC source (utility or generator). Harmonics can be mitigated with filters or phase-shifting transformers but never eliminated.
However, AFE drives are designed differently than 6-pulse VFDs, with technology that solves unseen power quality problems. AFE, or ULH, VFDs combat the effects of voltage dips and other power quality problems. When connected to main power, AFE VFDs produce negligible network harmonic distortion while existing legacy 6-pulse designs always generate more.
The AFE designs use a combination of an inductor-capacitor-inductor (LCL) filter and transistors instead of a diode-bridge front end, solving undervoltage and unbalanced voltage. Input insulated-gate bipolar transistors (IGBTs) actively track input power and charge the DC bus with current rather than voltage. The DC bus voltage can be programmed in the VFD to maintain a constant setpoint (650 VDC). In undervoltage conditions, the IGBTs conduct current for longer and keep the DC bus at setpoint.
As for harmonic distortion reflected to the AC Line, input IGBTs actively track input power and charge the DC bus without notching the voltage waveform. This does not create harmonic distortion. The LCL input filter attenuates (traps) most high frequency harmonics reflected to the power line.
The result is only 3% total harmonic distortion (THD)—well below IEEE-519 standards of 5%. ULH drives incorporate AFE technology to achieve low distortion and thereby prevent potential issues
with harmonics.
Utility Power Quality & Municipal Treatment Facilities
Remote regions that are located at the far end of the utility service are generally more susceptible to unseen power quality issues like voltage sags. Treatment facilities in remote regions must operate 24 hours a day, seven days a week and 365 days a year to meet consumer demand. If a facility experiences transient power brownouts or voltage imbalances, water supply or treatment quality can be adversely affected. Also, normal operations can be interrupted.
Unfortunately, this can result in an unhappy, unhealthy community and a tarnished public image. Applying AFE ULH drives can remedy unforeseen power quality issues for remote pump systems.
Power quality issues can cause havoc on pumping systems and municipal water treatment facilities. Choosing the correct VFD technology will help eliminate downtime for the motors and pumps systems. While this article covered utility power quality, the next article will address frequently encountered power quality issues and how to mitigate them when operating VFDs on emergency generators.