Solar Struggles - Part II

Here at Megger, we love a good two-part blog series. Sometimes, when we’re feeling extra fancy, we even throw in a three-part edition - a trilogy, if you will. Thus, we are back at it again this week with all things solar, specifically power quality issues. Does it get more thrilling than that? No.

As always, here’s your friendly reminder to go read last week’s blog, if you haven’t already done so; otherwise, you’re at risk of being quite confused during today’s read. Don’t say we didn’t warn you!

Remind me about power quality issues, please.

Of course. When you install solar panels, you are tapping into the electric grid, which is a dynamic system. As new systems are attached to the grid, power quality problems arise for both the solar site and the utility, at large.

As we mentioned last week, inverters are used to convert a panel’s DC output to useful AC power. The output of a solar panel is not the most stable of things, since it is relying on the sunshine for input. If you’ve ever planned an outdoor event, you know that relying on the weather is about as unstable as it gets – no matter how often you stalk that weather forecast beforehand. Hence, unreliable, inconsistent AC voltage can (and will) lead to various power quality issues.

What power quality issues?

Yes, we were getting to that. Today, we are going to talk about the following problems…

• High Voltage
• Unbalance
• Transients
• Harmonics
• Power Reversals

Ready?

High Voltage

When a solar panel system is installed, high voltages are possible. Depending on the power conditioning equipment at work, this may be more or less of a problem. By the way power conditioning equipment (or voltage regulators) keep your electrical equipment in harmony by maintaining a proper level of voltage. Regardless, high voltages can still be destructive, particularly for motors and lighting. In motors, high voltage can cause excessive heat, which can lead to premature motor failure – not good. Same goes for lighting. In both incandescent and florescent lights, high voltage can force lights to burn brighter, which releases excessive heat, thus shortening a lightbulb’s (already short) lifespan.  

Unbalance

Unbalance will cause issues for the utility, rather than the solar site, and it all depends on what phases the solar array is connected to. For example, if your solar system is connected to a single phase, rather than a 3-phase, unbalance is possible. The problem gets exponentially worse as more single-phase systems are connected to the grid. Unbalance is particularly risky for 3-phase motors, as a motor will experience current unbalance that is 6-10x higher the voltage unbalance that caused it. For all of our math-savvy experts, a voltage unbalance of just 2% would translate to a current unbalance of up to 20% in a motor. At this rate, a motor is at risk of suffering from excessive heat, again, which can cause the coils to burn out, as well.  

Transients

Solar panels are constantly (and rather instantaneously) reacting to changes in solar radiation input, as you can imagine with changes in weather, temperature, and timing. However, solar panels have much better sensitivity than we do, as humans. For example, it might look like a clear, sunny day outside, but a solar panel’s output is still rapidly changing. Why is that? Well, even pollutants – blind to all of us humans – can cause a rapid change in output. Thus, without proper voltage conditioning equipment, as previously mentioned, high speed transients are possible (and potentially destructive). Fortunately, most modern equipment is armed with filters that eliminate transients. Unfortunately, if they are constantly hit with transients, these filters can fail, so they aren’t a full-proof solution in the least. When they fail, electronic devices in your home or office, such as televisions, microwave ovens, and computers can fail, as well. Nobody wants that.

Harmonics

In order to keep our reader’s engaged, we try to keep things relatively simple on the blog. Regrettably, harmonics are anything, but simple. We’ll try our best though. So anyways, we’ve spoken about inverters multiple times. These devices convert direct current (DC) to alternating current (AC), so we can easily put it to use in our homes and offices. Inverters like to operate at relatively high frequencies to maximize their efficiency, which is great except, the higher the frequency, the higher order harmonics it creates. Well, these harmonics can generate eddy currents in wires, which for all intents and purposes, can generate heat in the coils of transformers and motors. The higher the frequency of the harmonic, the greater the eddy current and the worse the heating problem is. If things get out of hand and the magnitude of harmonics is out of control, premature equipment failure is possible. When it rains, it pours, right?

Power Reversals

Last, but certainly not least – power reversals. The standard distribution power grid was designed, so that that power would always flow from the source to the load, A to B, if you will. Now, thanks to solar panels and the distributed generation, power can flow from B to A or A to B, depending on whether your solar system is stealing power from the grid or sending power to the grid. Thus, in these circumstances, power can switch direction or reverse. Utility companies will try to compensate by adjusting the tap settings on their transformers. Unfortunately, tap changers are mechanical, so they have a limited lifespan. If power flow is changing direction multiple times each day, and a tap changer can only withstand around a couple thousand cycles, you’re going to be switching out your tap changers far too often. Thanks to the wear-and-tear of power reversals, tap changers are at risk of premature failure and excessive replacement levels.

So, we gave you a list of all the bad things that can happen with solar panels. Now what?

There must be a silver lining.

Of course. You’ll just need a power quality analyzer. An advanced PQ analyzer can be used to identify power wiring configuration, measure unbalance, recognize voltage changes and other events, record transients and harmonics, in addition to a host of other tests - depending on the instrument in-hand.

Interested in learning more? We have an application note for that. If you’re more interested in learning how-to program a PQ analyzer for various issues associated with solar panels, click here. Or maybe, you want to better understand and analyze your solar PQ data; if so, click here. There’s also the option to just read both if you are a true, solar savant.