Your questions answered: Detecting and reducing harmonics, distortion and noise in power systems

Sam Fopma, PE, engineering manager at Interstates, addressed the requirement for power systems for industrial, manufacturing and other energy-intensive production facilities in an educational webcast on November 14, 2024, “Power Systems: How to detect and reduce harmonics, distortion and noise.”

In this article, Fopma addresses some of the questions that were not answered during the webcast.

Courtesy: Interstates

Question: How long should a power quality study measure power?

Fopma: Monitoring duration should be selected based on the needs of the evaluation.

• A point load reading will tell you the performance of the system at that moment, which is valuable for an initial indication of the level of harmonics and system load.
• Longer-term measurements are more useful to see how the system functions during normal operation and to validate the point load data.
• Sampling rates and storage capacity will impact the duration of monitoring. Fast sampling intervals will fill up the device memory more quickly. If a long-duration observation is desired, the sample rate and captured data points need to be selected carefully.
• A 30-day duration evaluation that includes load data has the benefit of complying with NEC 220.87(1).

Given the work it takes to hook up a power quality analyzer, it’s recommended to measure for a minimum of a few days to see the trends of harmonics, loading and other power measurements.

Question: How do I mitigate harmonics when the whole plant is already built and running?

Fopma: Though it is more challenging to get installed during full operation, a similar group of solutions can be considered for harmonic correction on operating plants. Adding line reactors to [variable frequency drives] VFDs and adding Active Harmonic Filtering to areas of specific concern can both be planned and executed in short downtime windows. Solutions such as adding isolation transformers or changing to higher-pulse drives are possible to execute but will usually take more downtime and space.

Question: We have previously used a UPS to HELP with power quality. Are there UPS’s that can help with voltage regulation?

Fopma: Yes, UPS units are a great way to improve power system resilience against several power quality issues. Because the input and outputs are non-linear, UPS units can contribute to harmonics; however, manufacturers commonly design their systems to limit the harmonic output and publish data on the THD% that can be expected on the input and output of the UPS. In many cases, the output of a UPS unit will be cleaner than the input power on a system with high harmonic content.

Question: How do I define the “Point of Common Coupling” in an Electrical System?

Fopma: As defined in the IEEE 519 Introduction, the Point of Common Coupling (PCC) is:

“usually taken as the point in the power system closest to the user where the system owner or operator could offer service to other users. Frequently for service to industrial users (i.e., manufacturing plants) via a dedicated service transformer, the PCC is at the HV side of the transformer”

Another good way to understand this point in the electrical system is to identify the PCC as where electrical system ownership transitions from the utility to the customer. In addition to clarifying the location related to harmonic evaluations, this can also help clarify the jurisdiction of the AHJ (Authority Having Jurisdiction) who would inspect the customer’s electrical system.

Question: Do VFDs ahead of motors cause harmonics in the branch circuit itself?

Fopma: Absolutely! The output of a traditional VFD is non-linear on both the line and load side. The harmonics on the line side can be managed through line reactors, DC link chokes, higher-pulse drives, or even active-front-end technologies. The harmonics on the load side can be managed with filters or load reactors but will typically also require extra resilience to be installed in the form of proper cable selection and the use of inverter-duty motors.

Question: How to find the harmonic information of different equipment? Manufacturers normally do not provide the harmonics data.

Fopma: The first line of defense is to ask the manufacturer to provide what information they do have available. For electrical components, such as VFDs or UPS units there should be published data available. For more complex systems with multiple non-linear sources, such as industrial control panels, it would be best to take measurements of a similar system or to approximate the system by modeling the significant non-linear components together.

For example, approximating the performance of a motor-starting panel with multiple VFDs and a power supply could be accomplished by obtaining the manufacturer’s data for the various VFDs and combining that with a generic power supply model in power system modeling software to approximate the performance of that unit.

Question: What is the recommended insulation (conductors) for VFD wiring instead of THHN?

Fopma: The challenge with THHN wiring on the motor branch circuit after a VFD is that it doesn’t carry the same resilience against higher voltages associated with harmonics and transients and is prone to premature failure. Thermoset insulation types such as XHHW provide a bit more resilience but won’t provide any noise protection so they should be paired with ferrous-type conduits. A quality VFD cable, properly installed, will provide both the benefit of additional insulation capacity and shielding to help reduce noise.

Question: True or False – Harmonics only matter at the utilization level and can be ignored at higher voltages.

Fopma: False. Harmonics can and will be translated from the secondary to the primary of the transformer and will impact the power system at the distribution level.

Question: What rule of thumb do you use to determine a “significant motor” that may benefit from higher pulse or AFE drives?

Fopma: “Significant motors” will need to be defined based on their relative size and impact to the power system.

• One or two motors that are contributing the majority of harmonics on a system would be a great situation to look into the cost benefit of higher pulse drives instead of other forms of harmonic mitigation.
• When addressing a power system with a significant quantity of small-to-medium size drives the analysis likely favors a centralized harmonic correction solution as opposed to the cost and space requirements of numerous higher pulse drives.

In most cases, the best decision will be made by comparing the relative cost, lead time, and space limitations for each option.

Question: How does the length of conductor between drive and motor impact harmonics?  What length starts to become more of an issue?

Fopma: The harmonics on the load side of a VFD include voltage spikes associated with Pulse-Width-Modulated (PWM) outputs. These voltage spikes can be compounded in long motor branch circuits where the length increases cable capacitance and the motor appears as a high impedance and causes a reflective wave phenomenon. In both cases, the waveform on the cable can include voltage spikes even higher than those on the output terminals of the drive.

This phenomenon should be checked for any motor branch circuit length over 100’ and mitigated where necessary through a combination of motor selection, cable selection, and the application of output filters on the drive. Many drive manufacturers also publish data indicating distances and applications when load reactors or other mitigation solutions are required.

Question: At what level of % harmonic content, in general, triggers plants to spend money to improve their power quality?

Fopma: Every plant has a unique operation requirement and power system design, but the guidelines in IEEE 519 do provide a helpful framework for gauging how far “out of bounds” a power system might be. In my experience, plants having systemic issues with harmonics are well outside the IEEE 519 guidelines at the individual utilization system. However, it bears remembering that even if the system is performing well as compared to IEEE 519, there can still be individual issues caused by harmonics and associated electromagnetic noise that need to be addressed from an install perspective. In my opinion, installation according to best practices should be done all the time, with additional active harmonic management added at the system level when appropriate.

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