Transformers are critical components in solar-energy production and distribution. Historically, transformers have “stepped-up” or “stepped-down” energy from non-renewable sources. There are different types of solar transformers including distribution, station, sub-station, pad mounted and grounding. All transformers have specialized needs that impact costs. Mike Dickinson of Pacific Crest Transformers explained these special needs.
First off, Dickinson says solarpower applications experience steadystate loading during inverter operation. When the sun’s out, there’s a dampened reaction process and more constant loading on the transformer.
Also, fault ride through has not been defined for photovoltaic systems. Dickinson says this may be because it’s easier to turn solar systems on and off quickly, or because regulatory requirements have not caught up with the young technology. This may change in the future, but so far he hasn’t seen solar systems with such a requirement.
As for harmonics, he says the solar inverter’s typical harmonic content is below 1%, which has almost no impact on the system. The lower harmonic profile is because there are no generators and switching and protective controls such as those found on wind turbines. Solar transformers do require step-up duty. Yet, the inverter converts DC input from the PV array to AC voltage for the transformer in a smooth transition with no overvoltage from unloaded circuit. Because solar transformers operate at a steady voltage, with the rated voltage controlled by inverters, voltage and load fluctuations are considerably lower than in wind turbines. Solar systems also operate close to their rated loads.
Solar-power systems also have special design issues. Dickinson explains that because the largest inverter size is about 500 kilovoltampere (kVA), designers are building 1,000 kVA transformers by placing two inverter connected windings in one box. The transformer must have separate windings to accept completely separate inputs. Design issues also stem from running cables long distances to convert from DC to AC.
Restrictions on inverter size also limit the size of solar systems. Increasing the size by adding more inverters into one transformer box is extremely difficult, says Dickinson. With the required box size and running cabling to convert DC to AC, things get complex.
The key to solar transformers is to understand the variables in every system. Transformers need to customize to work with each particular system. Dickinson notes that inverter technology has been slow to advance, and it remains to be seen whether this comparative disadvantage will be a fatal flaw in the advancement of solar technology to the same level as wind farms.
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