Seasonal Patterns of Harmonic Disturbances

The "Dog Days" of summer, from late July to late August, are traditionally linked with heat and heavy air. The name comes from Sirius, the Dog Star, which rose with the sun in ancient Greece. While the term describes sultry weather, power grids also face their own Dog Days.

For utilities, this period is not only about peak demand from cooling. It also coincides with higher levels of harmonic disturbances. Harmonics are voltage or current components at integer multiples of the fundamental frequency (50 or 60 Hz). They distort the sinusoidal waveform, reduce efficiency, and shorten equipment life.

This article looks at how harmonics vary with the seasons, why summer is particularly challenging, and what utilities can do to mitigate the effects.

Understanding Harmonics

Harmonics arise from non‑linear loads. Instead of drawing current in a smooth sine wave, these devices pull current in pulses. Common sources are variable frequency drives (VFDs), inverters, LED lighting, and air conditioning units.

The impact of harmonics is measured by Total Harmonic Distortion (THD). It is defined as:

THD = \sqrt{\frac{\sum_{h=2}^{n} V_h^2}{V_1^2}} \times 100%

where V_h is the RMS voltage of the h‑th harmonic and V_1 is the fundamental.

Standards provide guidance for acceptable limits:

IEEE 519 recommends voltage THD below 5% at the point of common coupling (PCC).
IEC 61000‑2‑2 sets a limit of 8% THD for public low‑voltage networks.
IEC 61000‑2‑4 specifies 5–10% THD for industrial environments, depending on class.

Seasonal Patterns

Summer

During the Dog Days, harmonics increase due to:

Air conditioning units: Inverter‑based compressors generate odd‑order harmonics, especially the 3rd and 5th.
Variable speed drives: Used in HVAC and industry, they add 5th, 7th, and higher orders.
Solar PV inverters: At peak summer output, they inject switching harmonics, particularly in weak grids.
High temperatures: Transformers run closer to saturation, and cable impedance rises, both amplifying harmonic effects.

Case data from ERCOT (Texas) has shown localized THD rising from 2–3% in cooler months to 6–8% during heatwaves, driven by synchronized AC loads and inverter behaviour. These values are illustrative and vary by feeder and grid strength.

Winter

Winter harmonics are different:

Electric heating: Resistive heaters are linear, but they increase base load, reducing damping and making grids more sensitive to harmonics from other sources.
Lighting: LED lighting, especially older drivers, adds 3rd and 5th harmonics. Modern drivers often comply with IEC 61000‑3‑3, but large‑scale use can still create distortion.
Industrial loads: Arc furnaces and welding equipment contribute low‑order harmonics and interharmonics (non‑integer multiples).
Wind generation: Inverter‑based turbines can introduce subsynchronous components, particularly in weak grids.

Nordic utilities report localized THD spikes of 6–7% in winter evenings, linked to heating and industrial demand. Again, these are site‑specific peaks, not system‑wide averages.

Impacts on Utilities

Harmonics reduce transformer lifespan, increase cable losses, and cause nuisance tripping in protection systems. Motors suffer torque pulsations, and sensitive electronics can malfunction.

Economically, harmonics mean:

Higher maintenance and replacement costs.
Derating of transformers and cables.
Customer complaints about flicker and poor power quality.
Possible penalties under regulatory frameworks.

Mitigation Strategies

Monitoring
Deploy PQ meters with harmonics recording at substations and feeders. Seasonal audits help identify hotspots. Modern AMI systems and CLOU energy meters with harmonic recording functions allow utilities to pinpoint distortion sources down to feeder or customer level.

Filtering

Passive filters tuned to 5th or 7th harmonics.
Active filters for dynamic loads like data centres or EV charging hubs.
Hybrid solutions combining both.

Grid Operation

Staggering AC start‑ups to avoid resonance.
Enforcing inverter compliance with harmonic standards.
Using grid‑forming inverters in high‑renewable areas.

Customer Engagement

Tariffs that penalize poor power quality.
Incentives for harmonic‑friendly equipment.

Takeaway

The Dog Days of summer bring more than heat — they bring harmonic stress to the grid. Seasonal patterns matter: summer harmonics rise with cooling and solar inverters, while winter harmonics stem from heating and industry.

For utilities, the response is clear: monitor, filter, and plan. By treating harmonics as a seasonal risk, power providers can protect assets, improve efficiency, and maintain customer trust.

At CLOU, our advanced metering infrastructure (AMI) systems and energy meters with harmonic recording capabilities give utilities the tools to pinpoint when and where distortions occur. Continuous monitoring down to feeder or customer level allows operators to distinguish between local disturbances and system‑wide issues. With this insight, utilities can act faster, target mitigation more precisely, and reduce unnecessary equipment stress.

In short: harmonics don't have to be a hidden seasonal burden. With the right measurement and analysis tools, they become a manageable parameter in grid operation — and a step towards a more resilient, smarter network.