Photovoltaic systems are constantly being improved to improve their efficiency and financial viability. One trend is towards larger battery strings that convert higher DC voltages to AC voltages for grid use. The result is cost savings, but monitoring and control auxiliary power supplies must accept these higher voltages as input.
Photovoltaic (PV) power generation systems have struggled to justify themselves on a per-watt basis, and have been hampered by the initial inefficiency of the panels themselves. At present, single crystal cells with an efficiency of about 25% will lead the market, and the theoretical maximum will not be much higher. Therefore, developers are constantly striving to squeeze every last drop out of the system, minimizing connection losses and the process of converting panel DC to grid AC. One way is to connect panels in series to provide high voltage power with lower current and consequent “I2R” losses. For example, a grid connected system typically has 22 panel units with battery strings connected to 1000V producing 5.5kW per string. Then 2727 threads could be combined for a 15 MW plant.
However, if the number of panels in the string is increased to supply 1500 V to the combiner, then at the same 15 MW power, the current will drop to 66.6% of the 1000 V value due to “I2″ and a resistive cable. losses drop to 44.4% at In I2R. This benefit means higher system efficiency or lower installation costs for smaller cables and connectors. Since fewer strings are required to reach 15 MW, the number of merging units is reduced by 31%, for example, assuming each unit handles 20 strings, the number of merging units is reduced by 94 instead of 137. Of course, the wiring, connector and costs associated with the combiner for maintenance is also lower. GTM Research’s analysis of system cost per watt for 1000V and 1500V systems in a 10MW power plant shows potential deployment savings of $400,000, as shown in Figure 2.
Switching to higher circuit voltages certainly looks like an advantage, but there are also potential disadvantages: for higher operating voltages, it is necessary to upgrade the insulation of all lines, as well as junction boxes and grid inverters. However, this is not necessarily a problem as inverter technology typically operates at high voltages in other areas such as traction. The latest wide bandgap semiconductor technology is still applicable at 1500V, further improving conversion efficiency.
However, there is one area in the system that needs attention: photovoltaic combiners and inverters require low voltage isolated 1500V line power for monitoring and control, and small DC converters operating at these levels are rare. The lower voltage side is also important because the supply voltage can drop to 200V under certain conditions, so the converter must provide an input range of at least 7.5:1, which again is not a common specification. On fig. Figure 3 shows a typical solar combiner block illustrating the power architecture: 200 to 1500 Vdc input DC/DC converter and 24 Vdc output for additional isolated and non-isolated power for communication and processor/sensor power. The converter provides power. Primary HVDC converters require fully reinforced protective insulation, typically rated at 4000 VAC.
The standard related to the safety of photovoltaic systems is IEC 62109-1 “Safety of power converters used in photovoltaic systems”. Part 1 specifies the general requirements and Part 2 specifies the specific requirements for inverters. For systems up to 1500 V, the standard describes the design and manufacturing methods necessary to provide protection against common hazards such as electric shock, mechanical shock, temperature, fire, chemical attack, etc. Of particular importance for DC/DC converters is the reference to IEC 60664 “Insulation coordination of equipment in low voltage systems”. Unlike some of the earlier standards, IEC 60664 includes requirements for operation above 2000 meters and a partial discharge test that is very relevant to the operating voltage of 1500 VDC. Partial discharge is the progressive destruction of pores in an insulator under high voltage, leading to degradation and, ultimately, to complete failure. It is required that no partial discharges occur during testing and that a specially designed isolation barrier be provided in the DC/DC converter. As with all safety standards, insulation requirements depend on the mains voltage, the overvoltage category of the installation (OV) and the degree of environmental pollution (PD). For photovoltaic systems with a 1500 V DC bus, OV II is used in PV circuits with a minimum impulse withstand capability of 6000 V. While OV III is used in a grid-connected inverter stage that requires an considered industrial with a certain environmental protection, PD 2 is universally applicable, allowing only non-conductive pollution with occasional condensation. Designing to standards such as IEC 62109-1 is not an easy task and there are many more factors to consider than mentioned.
Another standard relevant to the US PV market is UL 1741, which applies to more general “distributed power” applications but includes requirements for “converters and controllers”.
Auxiliary DC converters operating in this environment must have certain characteristics. Very wide input ranges are difficult to achieve with standard flyback or forward converter topologies, especially at high maximum input voltages. By changing the pulse width to regulate the output, the internal peak voltage and current can be very large, requiring a more complex topology to limit the peak voltage. Protection is also of key importance: the converter must frequently “shut down” when the input voltage drops below a minimum value under various lighting conditions. The transmitter must not be damaged by this or any other fault that may occur in typical remote installations such as overloads, short circuits and overvoltages. Environmental conditions can also be harsh; you want your PV system to be in direct sunlight, so the temperature in the control cabinet can be high. Designing a DC/DC converter for photovoltaic applications is not an easy task, as the agency’s insulation ratings present another challenge.
Fortunately, there are ready-made solutions to these design problems. DC/DC converters such as the CUI AE series are designed for 1500 V DC photovoltaic systems. current to withstand high operating voltages and meet reliability and safety standards. These “set it and forget it” solutions ensure reliable operation in renewable energy systems.
I keep reading that only 600V boxes should be used in residential systems. I’m designing a 15kW to 20kW system and using a 1500V box seems to help. I know there are other security issues, but is it allowed or not recommended to use them in residential areas? Will it be cheap or not? Thanks for your article.
Hey Dan! In Canada, the electrical code states that the maximum voltage allowed for residential use is 600 V DC. I think it’s the same in the US – NEC and CEC are very well coordinated.
Hi, I have a 15.7 kW solar PV system with a 2-400V DC circuit in the house and a 1-400V DC circuit in the garage. So staying below 600V DC should be easy.
Post time: May-24-2023