Solar Inverter Step-Up Transformers Need New Design Approach

A word to the wise – don’t rely on using wind power transformers

By Mike Dickinson, Pacific Crest Transformers

It is often argued that “there is nothing new under the sun,” but when it comes to solar power inverter step-up transformers, something new is required – a new design approach.

Large scale wind farms are becoming a common site in the 21st century global economy. We are bombarded with images of windmills in all forms of print, broadcast, and electronic media while wind farms dot our rural landscape from coast to coast. Solar conversion systems, while lagging behind the established “green” technologies, are gaining acceptance in the emerging renewable energy marketplace. Most frequently used in large scale installations are photovoltaic (PV) systems, in which solar rays are used by a silicon dioxide crystal to generate current to a battery,

This brings a new challenge to designers, engineers and planners who must find ways to move evolving solar technology to the same plane as the maturing wind power technology’s contribution to the national power grid.

At first glance, it might seem as if the typical wind turbine step-up transformer provides the same functionality as the step-up transformers connected to solar inverter systems and so therefore could be used for solar systems. However, the severe duty that defines the typical wind turbine step-up transformer’s operating environment is not the same as the emerging photovoltaic solar conversion process, and the transformer needs to be very different.

It is important for designers to understand that renewable energy sources cannot use a standard, off the shelf transformer, but neither can every renewable energy source use the same transformer. Understanding how differences between the two energy sources affects the transformer requirements can have a huge effect on costs for an installation, as well as reliability. Designers should consider a variety of parameters, summarized in Table 1, when designing a transformer for use in a wind power versus a solar system.

Table 1 – Comparison of Wind and Solar Transformer Transformers
Parameter Wind Power Solar Conversion
Loading Variable loading due to wind gusts Steady state loading when inverters are operating
Fault ride through Low voltage (LV) fault ride through required Not yet defined
Harmonics Harmonics resulting from rotating equipment and electronic controls Harmonics content is less than 1%
Step-up requirements Generator step-up duty Step –up duty, but without over –voltages caused by unloaded generators
Voltage Sized to operate at lower than nominal minimum voltage Operation at rated voltage controlled by inverters
Loading issues 35% nominal loading annual average Operation at rated load
Special design issues Design requires 2 separate inputs
Size issues Sizes increasing as generators increase in output Size limited by inverter technology (currently at 1000 kVA)

Loading – Wind powered transformers experience variable loading due to wind gusts, so a wind power transformer is designed to be able to handle this lack of steady load, where it may be expected to power up and down all the time. Wind shadows in a wind farm will reduce the flow to some turbines, while others may be spinning very fast. Transformers therefore see a constantly variable load. By contrast, 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 Wind farms are difficult to stop and start quickly. They are usually required by local regulations to stay online even during difficult conditions when they may sustain faults. The amount of time they must stay online is determined by those who run the grid, who want to avoid a voltage sag in their system. The transformer may see severe duty during the fault, including severe mechanical and electrical forces, and if it cannot sustain them, the transformer may fail. In order to keep the transformer online, an LV fault ride through is required.

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 – Wind farms experience high levels of harmonics stemming from unbalanced loads from rotating equipment and electronic controls. The extra load is not part of the design load. 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.

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. Generator step-up duty is severe for wind transformers and must be specifically designed to meet those requirements. A standard transformer will not hold up to that kind of duty. 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.

Voltage – The wind transformer is 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. Solar transformers operate at a steady voltage, with the rated voltage controlled by inverters. Therefore, voltage and load fluctuations are considerably reduced.

Nominal loading average – Wind transformers operate across a very wide range of loads, because the wind may be blowing very hard – or not at all! In fact the average nominal loading is about 35 percent. This presents a unique design problem, because the wind transformer has to be designed to operate between widely divergent ranges. Solar power systems typically operate very close to their rated loads.

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 – Wind farm sizes are increasing as generators increase their output and wind farms mature. As opportunities to install wind farms in optimum locations and positions is reduced, it will become more and more important to get as much power out of each installation as possible. We will likely see taller towers with bigger generators. Recently the average wind installation is almost double the size of the original wind installations. We are seeing larger and larger transformers being used in these applications, ranging from 1500 to 5000 kVA. Established designs will be need to be changed to meet changes in such variables as the size of the tower, generator, grid, and transformer.

By contrast, the size of 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.

The duty cycle seen in wind farms may be more severe than that of solar power systems, but solar power has its share of special considerations that affect the transformer design. Pay heed to these special needs to ensure that the solar installation is cost effective and reliable.