Improving energy efficiency is essential to ensure that photovoltaic power generation is economically viable. Using a larger battery string to increase the DC operating voltage can reduce I2R loss and save deployment costs, but it poses a challenge to the design of the auxiliary power supply for monitoring and control circuits.
Develop the solar market
Although government subsidies for photovoltaic power generation fluctuate from time to time, the installed capacity is constantly increasing. Based on the 178 GW in 2014, it is estimated that the global installed capacity will reach 540 GW in 2019. Europe has the largest share and is expected to reach 158 GW in 2019, but other countries and regions are growing faster. For example, China and the United States are expected to increase their installed capacity by four and three times during the same period. The successful solar energy industry is also very beneficial from an economic point of view. In 2014, the industry directly employed approximately 55 million people.
For photovoltaic power generation to achieve these forecasts and grow further, the cost per watt must continue to decrease. One obstacle is that the efficiency of the solar panels themselves is generally low. Today, the working efficiency of the most efficient single crystal cell is about 25%, which is close to the theoretical maximum of the technology.
Increase working voltage to save energy
Obviously, every joule obtained from the sun’s rays is precious. From the DC output of the solar module to the AC feed delivered to the grid, energy-saving management is very important in order to minimize the loss of each part of the system (Figure 1). Connecting multiple modules in series to generate a high-voltage DC output helps reduce current, thereby reducing the I2R loss between the photovoltaic array and the inverter. It is common for grid-connected systems to operate at 1000 VDC. A typical system consists of 22 modules connected in series to form a battery string. Each module contains 90 batteries to produce an output voltage of approximately 45 V. Such a battery string can generate 5.5 kW of peak power, and 2727 battery strings can be combined to obtain an installed capacity of 15 MW.
By increasing the number of modules in each string to increase the output voltage to 1500 VDC, the maximum current entering each combiner can be further reduced to 66.6% of the corresponding value of 1000 VDC. The resistive cable loss is even lower, only 44.4% of the previous value. This provides system designers with greater flexibility to improve energy efficiency and reduce installation costs by reducing cable size and specifying smaller connectors. In addition, fewer battery strings are required to achieve a given output power, thereby reducing the number of combiner boxes required. Assuming that each combiner box handles 20 battery strings, 15 MW of installed capacity will only require 94 combiner boxes, and 137 combiner boxes at 1000 VDC, which is a 31% reduction in comparison. GTM research has calculated that designing a 10 MW power plant operating at 1500 VDC can reduce deployment costs by approximately US$400,000 compared with a 1000 VDC system
1500 V design challenge
These potential cost savings and efficiency improvements are certainly attractive, but the insulation of the entire system must be upgraded, and the combiner box and inverter must be able to work at higher voltages. Fortunately, there are already suitable inverters on the market, some of which are based on the latest wide band gap semiconductor technology and are more efficient than silicon-based alternatives.
However, another important aspect of 1500 VDC system design is that these photovoltaic combiners and inverters need to obtain their own low-voltage power supply from the 1500 VDC line to supply power to the monitoring and control circuits. It is difficult to find a small DC-DC converter on the market that can meet the demand: not only must it provide a wide enough input voltage range to operate at 1500 VDC, but it must also be able to handle large voltage drops-the minimum output voltage of the battery string can reach 200 VDC. It requires an input range of at least 7.5:1, which is not a common specification.
Shows the power architecture of the solar combiner unit, which contains a wide-input DC-DC converter that provides a 24 VDC output, which is used to power the communication and processing/detection modules through additional isolated and non-isolated converters. This high-voltage DC-DC main converter requires comprehensively enhanced safety isolation, usually specified as 4000 VAC
Security considerations
The applicable standard for safety is IEC 62109-1 “Safety of Power Converters Used in Photovoltaic Power Generation Systems”, which is related to systems up to 1500 VDC. Part 1 of the standard specifies general requirements, and Part 2 specifies specific requirements for inverters. The scope of IEC 62109-1 covers design and construction methods to ensure protection against electric shock, mechanical hazards, high temperature, fire, chemical hazards and other potential hazards.
The standard also includes a reference to IEC 60664 “Insulation coordination of equipment in low-voltage systems”. Particularly related to DC-DC converters is the requirement to conduct tests to verify the absence of partial discharges; partial discharges may occur when the micro-holes in the insulator are broken down under high voltage, resulting in degradation of device performance or even complete failure. The test is closely related to the 1500 VDC working voltage and requires a special structure for the isolation barrier of the DC-DC converter.
The insulation requirements of IEC 62109-1 depend on the system voltage, device overvoltage (OV) category, and environmental pollution degree (PD). OV category II is used for photovoltaic panel circuits in systems with 1500 VDC busbars, with a minimum impulse withstand voltage of 6000 V. For the grid-connected inverter stage, OV III should be used, with an impulse withstand voltage of 8000 V.
As an industrial-grade application with certain environmental protection requirements, the device is subject to PD 2, which only allows non-conductive pollution and occasional condensation. IEC 62109-1 contains many other specifications that must be considered.
In addition, the United States applies the UL 1741 standard, which covers more general applications of “distributed energy resources”, including requirements for “converters and controllers.”
New auxiliary power topology
These standards set specific performance requirements for auxiliary DC-DC converters operating in this environment. For standard flyback or forward converter topologies, the ultra-wide input range and fairly high maximum input voltage are extremely challenging. When the pulse width is changed to adjust the output, extremely high internal peak voltage and current may be generated. In this case, a more complex topology is required to limit the stress on the components.
Protection is also very important, the purpose is to ensure that the converter can continue to operate in the case of frequent “power down”, because when the lighting level is low or the panel is in the dark, the input will drop below the minimum value. Various fault conditions that may occur in remote facilities, such as overload, short circuit, or overvoltage, must be prevented to avoid damage to the converter. The converter must also be able to withstand high operating temperatures, because photovoltaic systems are often placed in sunny places in order to maximize the energy harvesting potential. It is also important to meet the insulation level specified by the agency.
Considering the combined impact of all these challenges, designing a 1500 VDC wide input DC-DC converter for photovoltaic applications is not an easy task.
CUI recently launched the AE series of DC-DC converters, which are suitable for photovoltaic applications with an operating voltage of 1500 VDC (Figure 4). This series is designed to handle the 200 to 1500 VDC input range required by solar auxiliary power, and provides 5, 10, 15 or 40 W rated power. The output voltage options are 5, 9, 12, 15, or 24 VDC. These converters comply with the EN 62109-1 standard (European version of IEC 62109-1), provide 4000 VAC isolation, and have a rated working height of up to 5000 meters. Some models also meet the UL 1741 standard. The converter provides options such as sealed plate mounting, base mounting or DIN rail form, and can operate at temperatures up to 70°C without derating.
Plug-in auxiliary power supply for 1500 VDC photovoltaic system
When designing industrial photovoltaic power generation systems for GW-level facilities, maximizing energy conversion efficiency is the most important goal. Increasing the output voltage of the solar array to 1500 VDC can support this goal, but requires comprehensive control and monitoring to achieve optimal performance. The auxiliary power supply used to maintain these functions must meet reliability and safety standards, while being able to operate with a wide input voltage range of 200 VDC to 1500 VDC. CUI’s latest generation of DC-DC converters are designed to overcome these challenges and provide a direct plug-in solution for photovoltaic system designers and integrators. To learn more about the CUI AE series, please visit Digi-Key’s DC-DC converter product highlight page for renewable energy applications.
Post time: Aug-16-2021
