The Issue May Be Harmonics
Greg Pietras | February 1, 2024
Unfiltered harmonics can be the primary cause of asset failure.
By Howard W. Penrose, PhD, CMRP, CEM, MotorDoc
You meet IEEE 519 harmonic levels in your plant, at least at the point of common connection with the facility. Sure, there is about 2% THD (total harmonic distortion) in 3rd, 5th, and 7th, and even some harmonics below 1%, in voltage. While current harmonics are at a total of about 8% THD, they meet the TDD (total dynamic distortion) requirements. There was an additional increase in 3rd harmonic content with the addition of LED lighting in the facility and, while the electric bill has gone up slightly, you’ve been assured that you’re saving energy. The calculations told you so.
However, bearing fluting has been on the rise in your electric motors, even those not on drives, and your IT and maintenance staff are complaining about rising electronic equipment failures. Consultants, engineers, and salespeople have told you that’s to be expected with aging machines, though most of the failures are occurring within a few years of installation. So, it must all be normal.
But it’s not.
While electrical energy is invisible to the human eye, the effect on physical equipment is very present. Electrical energy is all around us and is even how we, as human beings, operate. Cause disruptions in that electrical energy and bad things can happen. It misbehaves in a human being and we get muscle twitches, heart issues, or worse. It misbehaves in a facility and the machinery gets sick.
One of the diseases that cause us trouble in the electrical-equipment world is harmonics. Whether they are present in the power side, grounds, or neutrals, the effects are the same, i.e., parts of the facility’s body begin to fail. It might be observed as an increase in lighting failures, motor heating and bearing fluting, and/or higher-than-usual transformer temperatures, accompanied by a tinny hum. Take a power meter, ESA (electrical signature analysis) device, or oscilloscope to the power, ground, or neutral conductors and things just don’t seem sinusoidal. Electrical equipment is designed to operate on sinusoids locally and in the bulk grid.
It was predicted by the U.S. Department of Energy through WSEO (Washington State Energy Office) in 1997 that harmonic content would have an energy impact of about 2% depending on facility type. This was before smart phones, let alone most portable communications, liberal use of PCs and laptops, and related devices. The internet had just made the transition to the World Wide Web, there was no cloud, data centers were located at the companies that needed the servers, backups went on tape drives, and variable-frequency drives had made the transition to pulse-width modulation (PWM).
Total energy dealing with data management was a fraction of a tenth of a percent of the electric energy generated worldwide. Lighting improvements were relatively limited and all other electrical reliability (power supply side) opportunities, including loose connections, made up another 2% energy-improvement opportunity.
Things have changed since the digital dark ages. To reduce energy consumption there has been a transition to non-linear loads through VFDs, electronics, communications, even exporting data and social media. Data centers (cloud) and cryptocurrency mining now make up more than 2% of the total electrical energy generated, with some reports showing it as high as 5% and projected to exceed 7% total electrical energy by 2030.
Harmonics result in additional losses through I2R, which are higher than at line frequency due to skin effect, loading of capacitors (including PF correction), heating, transformer, and motor-core resonance. A quick review of the harmonics present on the power side at the meter can provide an idea of the losses associated with even low levels of harmonic voltage and current. The losses in a three- phase system when you have voltage and current harmonics are calculated as shown below:
The equation represents the voltage and current for each harmonic (even and odd) for phase A being evaluated. Power factor (PF) is the Cos φ for each harmonic. You then total the watts for each phase to determine the total power lost.
On the secondary of a typical sub, as shown in figure 1, which is drawing 1052.4 kW at 0.99 PF from power factor correction capacitors, the total is 65.5 kW of loss due to harmonics only, which is 6.2% of the total demand.
At 8,760 hr./yr., this would represent 573,780 kWh lost, or 573.8 MWh. Using Midwestern U.S. EPA eGrid values (Emissions & Generation Resource Integrated Database (eGRID) | US EPA) for carbon (CO2) inventory of 0.475 MT/MWh and an electrical cost of $0.10/kWh, the cost related to harmonics is $57,380/yr. and 273 metric tonnes of CO2. This, of course, does not account for losses associated with equipment degradation or ground/neutral harmonics.
What about the output from a drive to an electric motor? In the 450-hp electric motor data shown in figure 2, the motor has no output filter between the drive and motor. The result at 241.8 kW load is 17.7 kW losses due to harmonics representing 7.3%, which would typically be ~2% in a filtered output. This represents a cost of $15,505/yr. and 74 metric tonnes of CO2/yr.
Corrective action for the above instances is proper harmonic filtering of nonlinear loads. This can be accomplished at each load or through active filtering systems. Detection can be made through power-quality metering or electrical-signature-analysis equipment that segregates fundamental voltages, currents, and power factor from harmonic content.
Howard W. Penrose, PhD, CMRP, is president of MotorDoc LLC, Lombard, IL (motordoc.com). He chairs the wind-power standards and government relations participation for American Clean Power (ACP/AWEA), is the USA representative to CIGRE for high voltage electric machinery, holds various IEEE standards positions, and is a past chair of SMRP. He holds certifications from SMRP as a Certified Maintenance and Reliability Professional and the Association of Energy Engineers (AEE) as a Certified Energy Manager. Reach him at firstname.lastname@example.org.