By Matt Valenti, Application Engineer

In Part 1, we looked at how to find the source of harmonics without sending a crew. In this post, we explore a bigger issue: how everyday Distributed Energy Resources (DERs) like solar inverters, EV chargers and LED lighting quietly increase harmonic distortion and reduce the lifespan of your transformers.

Harmonics from DERs are not just about power quality. They’re a reliability issue. These loads inject distorted waveforms into the grid that heat windings, stress insulation and degrade transformers over time. Without real-time visibility, damage often goes undetected until failure occurs.

How DERs Create Harmonics

Three common DER types that often introduce harmonics into distribution networks:

• Solar Inverters: Can inject high-frequency harmonics, especially during fluctuations in voltage or power factor.

• EV Chargers: Fast charging stations frequently produce wideband current distortion and imbalance between phases.

• LED Lighting: Although energy-efficient, these non-linear loads distort the current waveform and often cause cumulative harmonic buildup.

Individually, these may seem minor, but combined and multiplied across neighborhoods, they put severe stress on local transformers.

Why Transformers Suffer

Harmonics affect transformers in ways that are not always visible until it’s too late:

• Higher eddy current and hysteresis losses due to non-sinusoidal waveforms

• Accelerated thermal aging from phase imbalance or total harmonic distortion (THD)

• Increased neutral currents, especially with heavy 3rd harmonic content from lighting or variable loads

To understand the full effect, it’s important to monitor harmonics across each phase and in real time. 

Real-World Example: Hidden Harmonics in a Rural Subdivision

In a recent pilot with a rural electric cooperative, EdgeSensor devices were installed at a feeder serving a new subdivision with large homes and a nearby commercial farm, ample EV chargers in the area were attributed as likely to be causing harmonic distortion. However, what looked like a standard harmonics issue from DERs turned out to have a more subtle root cause.  

• Severe harmonic distortion occurred during fire pump operation at a local hunting lodge, despite very low load on the transformer overall

• The transformer was significantly oversized, which amplified the effects of any non-linear load

• Engineers described it as similar to “shouting into a large, empty tunnel” versus speaking through a snug channel; the lack of load prevented the windings from damping the harmonic energy

• This insight helped the utility understand that simply resizing the transformer could significantly reduce harmonic amplification and avoid future issues

This case highlights how even a well-intentioned design choice, like oversizing a transformer for DER-driven future growth or rare peak loads, can unintentionally worsen harmonic distortion.

Key Points

DERs are here to stay and so are their harmonics. Without transformer-level monitoring, utilities are forced to react rather than prevent. To stay ahead, utilities should:

• Monitor per-phase iTHD and vTHD continuously, not just spot-check

• Use harmonic trends to inform DER hosting capacity

• Set event-based alerts to trigger proactive maintenance or mitigation

Next in the series: why IEEE 519 compliance might not mean your grid is safe.