By Mike Dickinson, Pacific Crest Transformers
Solar conversion systems, which had been lagging behind large scale wind farms in the renewable energy sweepstakes, are now coming on strong. Interest in transformers to be used for solar installations has quadrupled in the past year, now clearly outstripping wind farm equipment. Key reasons for the popularity include the fact that solar systems offer more constant loading, far less harmonics, fewer problems associated with over-voltages caused by unloaded generators, and reduced voltage and load fluctuations. On the minus side, solar systems still have to contend with special issues surrounding the use of inverter technology, including effective size restrictions posed by existing limitations of inverter technology.
Photovoltaic (PV) systems are used most frequently in large scale solar installations. With PV, silicon dioxide crystals use energy from solar rays to generate DC current. This can be sent either to a battery for storage or to an inverter for conversion to AC voltage for use on the grid. A specially designed step-up transformer is needed to connect the solar inverter systems. It cannot be the type now used for a wind turbine, because the operating environment in the emerging PV solar conversion process is not the same, and the transformer needs to be very different. Designers should consider a variety of parameters, summarized in Table 1, when designing a transformer for use in a solar system.
It is important for designers to understand that renewable energy sources should not use a standard, off the shelf transformer, but neither can every renewable energy source use the same transformer. Understanding how the characteristics of solar power affect transformer requirements can have a huge effect on costs for an installation, as well as reliability.
|Table 1 – Design Parameters for Solar Transformers|
|Loading||Steady state loading when inverters are operating|
|Fault ride through||Not yet defined|
|Harmonics||Harmonics content is less than 1%|
|Step-up requirements||Step –up duty, but without over –voltages caused by unloaded generators|
|Voltage||Operation at rated voltage controlled by inverters|
|Loading issues||Operation at rated load|
|Special design issues||Design requires 2 separate inputs|
|Size issues||Size limited by inverter technology (currently at 1000 kVA)|
Loading – In contrast to wind-powered transformers, which experience variable loading due to wind gusts, solar power facilities experience a steady state loading when inverters are operating. When the sun comes out, there is a dampened reaction process and loading on the transformer is more constant.
Low voltage (LV) fault ride through – Fault ride through has largely not yet been defined for solar systems, and we have not yet seen solar power systems with this requirement. This might be either because it is easier to turn solar power systems on or off quickly than wind systems, or it might just be that the technology is so young that regulatory requirements have not yet caught up. We may see this changing in the future – or it might be overlooked by regulators. In either case, solar power transformers may address the issue of the low voltage fault ride through differently than those of wind farms.
Harmonics –The solar inverter system’s typical harmonic content is less than 1 percent, 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. This contrasts favorably with wind farms, which experience high levels of harmonics stemming from unbalanced loads from rotating equipment and electronic controls.
Generator step-up duty – Transformers can be stepped up or stepped down and must be appropriately designed to handle the very different and unique problems encountered because of the current inrush that each type will experience. With solar transformers, step-up duty is required, but without the problems associated with over-voltages caused by unloaded generators. The inverter converts DC input from the PV array and provides AC voltage to the transformer, giving a steady and smooth transition, with no over-voltage caused by unloaded circuits. By contrast, generator step-up duty is much more severe for wind transformers and must be specifically designed to meet those requirements.
Voltage – Solar transformers operate at a steady voltage, with the rated voltage controlled by inverters. Therefore, voltage and load fluctuations are considerably reduced. With wind energy, the transformer must be sized to operate at lower than the minimum voltage, because it must often operate as a function of wind speed, which fluctuates widely. However, it must provide the same amount of power at the lower voltage level, so it must be designed for that consideration.
Nominal loading average – Solar power systems typically operate very close to their rated loads. Wind transformers, on the other hand, operate across a very wide range of loads, because the wind may be blowing very hard – or not at all!
Special design issues – Solar power systems use inverters to convert DC to alternating current (AC). Since the largest practical inverter size, to date, is about 500 kilovolt-ampere (kVA), designers are building 1000 kVA transformers by placing two inverter connected windings in one box. In this case, the transformers have to have two separate windings to accept completely separate inputs, which is not something encountered in a wind farm. Design issues also stem from running cables long distances to convert from DC to AC.
Size of installation – The size of a solar system is limited by inverter technology, since inverters can currently only be built to about 500 kVA. This means that nearly all solar applications are using pairs of 500 kVA inverters to drive the transformer, producing about 1000 kVA. Increasing the size by adding more inverters into one transformer box is extremely difficult, due to complexities associated with the size of the box required and the practicalities of running cabling to convert from DC to AC. Inverter technology has been slow to advance, because it is an electronic technology. 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 in the renewable energy arena.
Solar power systems may see a far less severe duty cycle than a wind farm, but solar power has its share of special considerations that affect the transformer design. Paying heed to these special needs to ensure that the solar installation is cost effective and reliable will go a long way towards fulfilling solar energy’s growing place in the race towards renewable energy systems.