Natural Variations

By Richard P. Bingham | Sep 15, 2012




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On most distribution networks (except maybe in rural areas), the voltage levels typically reduce to a couple of percentage points from nominal when the sun rises, people wake up and they start using more electricity. Conversely, as the sun sets, the voltage creeps back up, and by 10 p.m. or so, it usually gets a couple of percentage points above nominal.

Figure 1 below shows such a pattern. As the summer heat builds, the voltage may continue to decline longer during the day and into the early evening due to air conditioning loads, but the pattern of the two plotted lines are basically inverted from each other. Considering Ohm’s and Kirchhoff’s Laws, it makes sense, since loads drawing more current will cause a larger voltage drop across the source impedance (distribution wires), leaving less voltage for the loads.

Conversely, when demand drops at night, there is less voltage drop in source impedance and more for loads. Of course, there are voltage regulators and power factor correction capacitors that are used to reduce the effect of this fluctuation and provide a smoother voltage regulation, but +/–2 percent from nominal isn’t unusual at all.

Throw some of the renewable or alternative-energy sources into the mix, and suddenly, those typical patterns aren’t so typical anymore. Renewable-power sources can work in two modes: one where loads are switched over to run off the renewables, and in the other, the loads are grid-connected, where the voltage waveform generated by the alternative source is synchronized in phase angle, frequency and amplitude to the grid’s alternating current (AC) voltage. The latter setup is similar to another utility generator powering the grid. Well, almost.

Let’s take solar panels for example. Solar panels convert photons to electrons in a direct current (DC) mode. Stack a bunch of panels in series, and you get more DC voltage potential. Parallel more rows of panels, and you get more current capacity. Since it is DC power and the grid is AC, an inverter or switching power circuitry is used to convert the DC into AC. The synchronization circuitry must ensure the phase/frequency/amplitude are basically the same before connecting the alternative source into the distribution system. If the alternative source is connected at the point of common coupling (PCC) between the utility and the facility, the current provided from the solar power system can actually put power into the grid if the load of the facility is lower than its present output. The word “present” is important because there is another variable in the equation: the sun.

The sun actually works to the advantage of the power requirements of most loads because, when the sun rises, so do most loads. So, the power is available when it’s needed most, making it potentially a great source if it weren’t for variables, such as clouds and temperature.

Figure 2 illustrates the effect on the current output from cloud cover, which can reduce the output by 50 percent or more. Temperature also works against the needs of some loads, such as heating, ventilating and air conditioning, since the output is also reduced as the temperature increases.

Variables aside, it is still a renewable source that can help cut carbon dioxide emissions and electric bills. At a lightly loaded facility with two solar panel arrays connected to the PCC, Figure 3 shows the voltage output at the PCC from a typical two-day period in May. Note that the daytime power consumption increases by two to three times the nighttime load, while the voltage is increasing to support this demand. With the alternative source so close, there is relatively little loss due to source impedance. And since the solar arrays produce more power during peak sunlight than the facilities use, the voltage also increases during the day, rather than typically decreasing. A couple of exceptions are noted, where reductions occurred between 11–12:30 and after 2 for the remainder of day 1 (clouds followed by scattered thunderstorms), as well as 1–3 p.m. on day two.

This increase in voltage during the winter actually reached levels that bordered on being too high, as the lower temperatures increased solar panel output and lower loads at the facility meant more power could be put into the grid. Line-to-neutral voltages were more than 132 volts, which is above the UL 1741 trip points of –12 and +10 percent of nameplate-rated voltage within 1 second to prevent possible damage to equipment. Most utilities require renewable-power sources that are grid-interconnected to meet IEEE 1547 Standard for Interconnecting Distributed Resources with Electric Power Systems and UL 1741 Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources.

Another possible concern with inverter-driven power sources that are interconnected is how they handle large inductive or capacitive loads being connected and disconnected, which this facility experienced. The synchronization circuit needs to be able to handle such conditions without getting the generated voltage waveform out of synch with the utility voltage waveform. Inverters, also by nature, are harmonic-generators, due to the switching nature of their operation. At this site, the voltage harmonic distortion would increase by 2 percent when the inverters were producing power.

The sun is shining on the growth of alternative-power sources, but it is best to learn how they operate and can misoperate before installing and/or maintaining such a system.

BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.

About The Author

BINGHAM, a contributing editor for power quality, can be reached at 908.499.5321.

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