Photovoltaic systems are continually evolving to improve their efficiency and financial viability. One trend is to move to larger strings of cells giving higher dc voltages to be converted to ac voltage for the grid. Cost savings result, but auxiliary power supplies for monitoring and control need to accept these higher voltages as inputs.
The PV Market
Despite the waxing and waning of government support for photovoltaic (PV) power generation systems, growth is still strong with installed global capacity increasing from 178 GW in 2014 to a projected 540 GW in 2019, according to a report by Solar Power Europe. Europe leads the way with an expected increase to 158 GW in 2019, but other countries are experiencing much higher growth rates. China, for example, expects a four-fold increase over the same period and the United States a three-fold increase. The solar industry also has a significant economic impact, with around 55 million people directly employed in the sector globally in 2014.
PV has always fought to justify itself in terms of $/watt of generated power, and is hampered by the initial low efficiency of the panels themselves. Currently, levels of monocrystalline cells at around 25% efficiency would be market-leading, and theoretical maximum values are not much higher. Designers therefore continually strive to squeeze the last drop of energy from systems by minimizing losses in connections and the conversion process from panel dc output to ac for the grid. One way to do this is to connect panels in series so that power is processed at high voltages where currents and consequent I2r losses are lower. For example, grid-connected systems typically have blocks of 22 panels with cells connected in strings to give 1000 V producing 5.5 kW per string; 2,727 strings might then be combined for a 15 MW installation. (Figure 1).
If, however, panel numbers in strings are increased to deliver 1500 V to the combiners, for the same 15 MW of power, current drops to 66.6% of the 1000 V value and resistive cable losses to 44.4% because of the I2 in I2R. This gain means higher system efficiency or lower cost of installation with smaller cables and connectors. Because there are fewer strings to achieve 15 MW, there are 31% fewer combiner boxes; for example, 94 compared with 137, assuming each box handles 20 strings. Of course, associated combiner cabling, connector, and maintenance costs are lower, too. GTM research has analyzed the system cost per watt comparing 1000 V and 1500 V systems in a 10-MW plant showing a potential deployment savings of $400,000. (Figure 2).
It certainly looks advantageous to move to higher string voltages, but there are some potential downsides: insulation all down the line must be uprated for the higher operating voltage, as do the combiner boxes and grid inverters. This is not necessarily an issue, though, as inverter technology commonly operates at high voltage in other areas such as traction. The latest techniques that make use of wide bandgap semiconductor devices are still applicable at 1500 V, further improving conversion efficiency.
There is an area in the system that requires attention. PV combiners and inverters need low-voltage isolated power for monitoring and control derived from the 1500 V line, and small dc-dc converters that operate at these levels are not common. The lower-voltage end is important, too, as the supply could dip to 200 V under particular conditions, so the converter needs to provide at least a 7.5:1 input range — again, not a common specification. Figure 3 shows a typical solar combiner unit illustrating the power architecture: a 200–1500 Vdc input dc-dc converter with a 24 Vdc output feeding additional isolated and non-isolated converters for communications and processor/sensor power. The main high-voltage dc-dc converter needs fully reinforced safety isolation and would typically be specified as 4000 Vac-rated.
The standard that relates to the safety of PV systems is IEC 62109-1 Safety of Power Converters for use in Photo-voltaic Power Systems. Part 1 specifies general requirements and Part 2 defines specific requirements for inverters. The standard is relevant to systems up to 1500 V, and its scope outlines the design and construction methods required to ensure protection against common hazards such as electrical shock, mechanical, temperature, fire, chemical, and more. Of particular relevance to dc-dc converters is the reference to IEC 60664, Insulation Coordination for Equipment within Low-Voltage Systems. Unlike some older standards, IEC 60664 does include requirements for operating at altitudes above 2,000 m, and partial discharge testing, which is very relevant to 1500 Vdc operating voltages. Partial discharge is the gradual breakdown of microvoids in insulation at high voltage, leading to degradation and eventually complete failure. Partial discharge is required to be absent during tests and necessitates particular construction of the isolation barrier in a dc-dc converter.
As in all safety standards, insulation requirements depend on the system voltage, installation over-voltage (OV) category, and pollution degree (PD) of the environment. For PV systems with a 1500 Vdc bus, OV II is used for the PV panel circuits with minimum impulse withstand of 6000 V, whereas OV III is used for the grid-connected inverter stage and requires 8000 V impulse withstand. As the installations are considered industrial-grade with some environmental protection, PD 2 is generally applicable, which allows only non-conductive pollution with occasional condensation. Designing to meet standards such as IEC 62109-1 is not trivial, with many more considerations required than mentioned. Another standard relevant to the US PV market is UL 1741, which is for the more general application of Distributed Energy Resources, but includes requirements for converters and controllers.
Auxiliary dc-dc converters working in this environment must have specific performance. The very wide input range is difficult to achieve with standard flyback or forward converter topologies, especially with the high maximum input voltages. With variation in pulse width to regulate the output, internal peak voltages and currents can be extreme, necessitating a more complex topology that limits peak stress.
Protection is key as well — the converters need to operate with frequent “brown-outs” as the input dips below the minimum under different illumination conditions. The converter must not be damaged by this or other fault conditions that might be seen in a typical remote installation such as overloads, short circuits, and over-voltages. Environmental conditions are tough as well — you really want your PV system to be in full sun, so temperatures in control cabinets are likely to be high. With the agency-specified isolation ratings presenting another challenge, dc-dc converter design for PV applications is not a minor task.
Fortunately, there are ready-made solutions available on the market to address these design challenges. A range of dc-dc converters specifically designed for 1500 Vdc photovoltaic systems is available that includes the required 200-1500 V input with variants available at 5, 10, 15, and 40 W ratings. Outputs available are 5, 9, 12, 15, or 24 V, depending on series. The parts are approved to EN-62109-1, the European version of IEC 62109-1, with 4000 Vac isolation up to 5,000 m altitude; some also meet UL 1741. The parts are encapsulated in a choice of board-mount, chassis-mount, or DIN-rail formats with an operating temperature up to 70 °C with no derating.
PV systems are all about efficient energy conversion up to the GW level, but require comprehensive control and monitoring to get the best performance. Auxiliary power supplies are not an insignificant part of the system and need special characteristics to withstand the high operating voltages while complying with reliability and safety standards.
This article was written by David Carroll, Director of Product Management at CUI Inc., Tualatin, OR. For more information, Click Here.