Making solar inverters more reliable than the sun

alan000345

Making solar inverters more reliable than the sun [Copy link]

 

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On hot summer days, I usually do what most people do, hiding in an air-conditioned room to escape the heat. Then, as the weather gets cooler, I open the windows to let in the natural breeze, and then I receive the summer electricity bill, and then I ask myself if the short-term comfort brought by the air conditioning is really worth it.

Air conditioners are only a pyrrhic victory in the fight against hot weather. As an engineer, I saw this as a problem that needed to be solved. My solution was simple: if you can't replace them outright, put their light and heat to good use. So, instead of using up a lot of expensive utility electricity, we could power our air conditioners with solar panels on our roofs. Fortunately, I'm not the first person to think of this, and with solar power now nearly on par with traditional energy sources, everyone can enjoy the benefits of solar energy.

While most of the attention has been paid to photovoltaic panels, other parts of the solar power ecosystem cannot be ignored, such as power electronics. But this is only one key aspect. Photovoltaic panels produce a DC voltage, but the power transmission and distribution system is in an AC state, so a power inverter is needed.

To achieve cost targets for solar power systems, the U.S. Department of Energy has proposed the following requirements as part of the Sunshot Initiative:

• Conversion efficiency greater than 98%

• Service life greater than 25 years

• Power density>100W/in3

• System cost <$ 0.10/W (utility); <$ 0.125/W (commercial); <$ 0.15/W (residential) (includes life cycle cost of power electronics, including initial capital cost and operation and maintenance (O&M) costs over its useful life)

The goals of the Sunshot Initiative are very ambitious, and meeting them requires not only core optimization but careful consideration of the individual parts. Careful design can lead to dramatic impacts on the isolation boundary. The inverter’s interface between low-voltage DC and dangerously high-voltage AC requires galvanic isolation, which can result in the power field-effect transistors (FETs) or insulated-gate bipolar transistors (IGBTs) being on the opposite side of the isolation barrier from the controller that generates the gate signal. Reinforced-isolated gate drivers, such as the Texas Instruments UCC21520, are a great choice because they combine multiple functions to be able to pass signals across the isolation boundary and convert logic gate signals to actual gate drive from several devices to one device. Reinforced-isolated gate drivers are an ideal solution for series inverters with maximum output power ranging from 500W to 10kW.

The UCC21520 improves on these integration benefits by delivering leading performance in propagation delay and delay matching between the high and low sides. Switching-related losses are reduced because it turns on faster and reduces the required dead time, i.e., the dead time when the higher-loss body diode is conducting. These parameters are also less dependent on VDD, so you can relax the voltage tolerance design margin for the rest of the system, as shown in the benchmark data in Figure 1. Figure 1 also highlights that the linear performance of the UCC21520 on VDD is much higher than that of the competition.

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Figure 1: Texas Instruments (TI) UCC21520 propagation rise/fall delays compared to VDD and competitors

Microinverters are a rapidly growing architecture that converts power from a single PV module to the AC grid, typically designed for maximum output power in the 180-300W range. By distributing the inversion process, solar arrays can fit on more complex rooftops and enable the installation of smaller arrays that would not normally reach the input voltage of the string inverter. Texas Instruments' basic isolated gate driver, the UCC21220, offers an alternative for solar applications where basic isolation is sufficient. It uses second-generation capacitive isolation technology to reduce costs by shrinking the chip size, which not only helps reduce PCB space and system costs, but also maintains leading performance in propagation delay and delay matching.

Another device that needs to cross the isolation boundary is the auxiliary power supply. Ensuring that the solar inverter can operate “intelligently” and continuously, regardless of the state of the AC utility/load or PV panel, requires an isolated power supply to provide bias power to the inverter. Since this step requires crossing the isolation boundary, it also requires components that cross it. Primary-side regulation (PSR), where the output is regulated by an auxiliary winding that is grounded relative to the primary-side controller rather than the optocoupler, is a good way to reduce components and cost. PSR has the added benefit of increasing life because it eliminates difficult failure points during surge voltages. Primary-side flyback controllers, such as the UCC28700, maximize the performance and efficiency of the control scheme by implementing advanced algorithms with minimal external circuitry. The UCC28910 extends these advantages by integrating 700V power FETs and controllers into a single device, further reducing the size of the bias supply.

Texas Instruments solutions can help make solar energy affordable and reliable enough to power air conditioning on hot days.

dcexpert

Can the solar panels on the roof meet the normal electricity needs of a family?

alan000345

dcexpert posted on 2019-7-4 10:15 Can the solar panels on the roof meet the normal electricity needs of a family?

It’s hard to say, it depends on the installed power and the number of family members, but solar energy is a big investment.

dcexpert

alan000345 posted on 2019-7-5 11:00 It’s hard to say. It depends on the installed power and the number of family members. But solar energy investment is quite large.

The cost is high and maintenance is troublesome.

alan000345

dcexpert posted on 2019-7-5 11:15 The cost is high and maintenance is troublesome.

Yes, the operation and maintenance costs are also quite high. Without government subsidies, it is really not easy to do this kind of thing.