Music Legend Joni Mitchell “looked at life from both sides now” in the late 60s. In the power quality world, we have a similar perspective. When determining the cause of PQ problems we have two sides to consider: the electricity producer and the consumer. 

Forty years ago, studies showed that most problems originated within a facility. Improper construction of the electrical infrastructure, large electrical loads starting up, the proliferation of nonlinear loads (especially when they replaced linear ones) and lack of adequate electrical equipment maintenance were and still are leading causes of power quality problems. 

On the flip side, the electric utilities weren’t immune from blame, as faulted power lines, failed electrical equipment and misoperation of protective equipment affected numerous customers. Overall, the statistics back then showed that the source was more likely downstream or load side than upstream or electric provider side.

Problems with DERs

There has been significant information published lately suggesting this pattern may be changing. More incidents of power quality problems are reported to be originating from distributed energy resources (DERs) such as wind turbines or solar panels. More specifically, renewable energy sources have been attributed to being the cause or aggravating the problem by the direct injection of nonsteady-power, as publications call it. Some publications state that soon, more than 80% of AC power will be processed through these sources, which adds to the fear factor for the future supply quality. An unbiased look into the facts is the first step to getting a better handle on this potential problem.

According to the U.S. Energy Information Administration’s “U.S. Energy Facts Explained,” the nation’s primary energy consumption by source in 2022 was 13% from renewable sources. Within that category, wind was at 29%, followed by hydroelectric at 18%, then solar 14%. But biomass—which includes renewable resources such as wood—led at 37%. Biomass generates electricity in a similar manner as coal, natural gas and petroleum, and it shares the same prime mover method by converting heat from burning to generate steam to turn turbines. The rotational motion is used to spin an electric generator, producing relatively steady and basically pure sine wave voltage, due in part to the mechanical inertia of the system.

Solar and wind’s voltage production

This is quite different from how solar and wind generators produce voltage. Solar panels generate DC voltage. To convert that to AC voltage at the primary grid frequency (50 or 60 Hz) requires electronic components that operate similarly to the nonlinear loads that were the source of PQ problems, such as harmonic currents. Wind turbines are rotational prime movers, but they don’t turn at a constant, reliable speed.

The AC voltage of varying frequency is first converted to DC, then back to AC at the primary grid frequency. These electronic-based generators must have their outputs precisely synced to the fairly constant grid frequency and phase angle. Their source impedance and ability to absorb negative volt-ampere reactive is different from the rotational-based electricity generators. That sounds like a basis for those waving the trouble-ahead flag. But some simple math shows that the fear is somewhat exaggerated. Solar and wind is 32%, and when multiplied by 13% of total power from renewables, equates to approximately 4% of the total power sources coming from such generating sources. That’s quite a bit away from 80%.

Getting back to some real PQ problems, the following are a few that have been documented in peer-reviewed technical journals.

Solar

• Increased voltage harmonic distortion over the IEEE 519 levels recommended, especially with a higher number of paralleled panels 
• Noise on landline phone systems traced to a large solar farm
• Voltage imbalance, especially with single-phase rooftop units
• Voltage stability issues with cloud transients
• System protection power reversal challenges where customers can be a load or a source of power flowing into the grid

Wind

Wind turbines are directly affected by changing wind conditions as blades pass the towers, causing uneven power production that can propagate through the system, resulting in voltage variations that cause flicker.

Significant levels of harmonics result from using frequency converters, thyristor controllers and capacitors, with cases of resonance, causing even larger levels.

Some designs have a poor power factor due to reactive power, which can be an issue, and managing reactive power is essential for maintaining grid stability.

Manufacturers continue to improve their product’s output, with testing and compliance to the 2018 revision of IEEE 1547 Clause 7. This revision updates and expands the power quality requirements from prior versions, including limitations on the DER contributions of current distortion, voltage fluctuations, DC current injection and temporary overvoltage.

The PQ realm has always been and will continue to include electricity generators and consumers. We just need to keep our eyes and minds open to the changing technologies.

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