Researchers Achieve Higher Voltage PV With Inverter System

In photovoltaic (PV) systems, reducing cable size is essential for economic and environmental reasons. As PV installations scale to meet the growing demand for renewable energy, the quantity of cabling required multiplies. Thicker cables consume more copper, a material with significant cost and limited availability.

 

Installing PV panels. Image used courtesy of Oregon DOE

 

Efficient cable management through size reduction is a pivotal aspect of optimizing PV systems, ensuring they remain economically viable and sustainable. To this end, a group of researchers at the Fraunhofer Institute for Solar Energy Systems (ISE) recently developed an inverter system to enable significantly reduced cabling requirements in PV systems.

 

Higher Voltage, Smaller Cable

Reducing the cabling requirements is extremely important as PV systems scale up. To this end, a promising strategy is to increase the system voltages.

The principle behind this is rooted in the relationship between voltage (V), current (I), and power (P), as described by the electrical power formula P = V×I. When the voltage, V, is increased for a given power, the current required is reduced. Since the current carrying capacity of a cable is a determinant of its size, a lower current allows for the use of cables with a smaller cross-sectional area.

Smaller cables offer several advantages. First, they are less expensive because they use less material, which is particularly significant when considering precious resources like copper. The price of copper is subject to market fluctuations and has a notable impact on project costs. Reducing copper usage not only cuts costs but also eases the demand for this limited resource, aligning with sustainable resource management practices.

 

The modern power grid already employs a high-voltage power transmission scheme. Image used courtesy of Edison Tech Center

 

Second, reducing cable size has environmental benefits. The production of copper and other cable materials has an environmental impact, including energy consumption and greenhouse gas emissions. By using thinner cables, the environmental footprint of manufacturing, transporting, and disposing of these materials is reduced.

Lastly, high-voltage systems can transmit power more efficiently over long distances with reduced losses. This is because electrical power is also defined as P = I^2*R. Hence, at a higher voltage (i.e., lower current), the losses in power transmission are significantly reduced. This is impactful for renewable energy sources like solar and wind, which are often located far from consumption centers. 

 

Fraunhofer’s Solar Inverter Study

In a recent study by the Fraunhofer ISE, the researchers developed the world's first medium-voltage string inverter for large-scale PV power plants. Unlike conventional PV string inverters, which typically operate at lower output voltages of 400 VAC to 800 VAC, the solution from the study outputs voltage as high as 1,500 VAC @ 250 kVA.

 

Different cable cross sections for different voltages. Image used courtesy of Fraunhofer ISE

 

The team tackled the challenge by employing silicon carbide semiconductors, which possess a higher blocking voltage than traditional silicon semiconductors. The use of these advanced semiconductors was complemented by a novel cooling concept utilizing heat pipes, which enhanced the system's efficiency and reduced the need for aluminum in its construction. By stepping up the voltage to the medium-voltage range, the inverter reduces the current for a given power output. This reduction in current directly translates to a decrease in the required cable size, yielding substantial cost savings and resource conservation.

According to the team, a traditional 250 kVA string inverter would necessitate cables with a cross-section of 120 mm², but with the medium-voltage inverter, the cable cross-section is reduced to just 35 mm². This reduction could save approximately 700 kilograms of copper per kilometer of cable.

 

Far-Reaching Implications of a Medium-Voltage Grid

The study's success in feeding power into the medium-voltage grid is a testament to its practical viability. It paves the way for the next generation of large-scale PV power plants and sets a precedent for more resource-efficient energy system electrification. Importantly, the study's implications extend beyond PV systems. The medium-voltage inverter concept can be applied to wind turbines, electric mobility, and industrial applications, where similar benefits in terms of resource efficiency and cost savings can be realized.

The researchers are now seeking partnerships with solar farm developers and grid operators to field-test their new concept, which could transform how we harness and distribute renewable energy.