Line Filtering for Solar Power Inverters

By Don Scansen

Contributed By Hearst Electronic Products


Photovoltaic energy harvesting comes in many forms, from tiny solar cells for back-up power on calculators, to rooftop solar panels, to utility-scale installations measured in megawatts.

For powering portable electronics or charging batteries, the direct current supplied by any solar cell is perfect. But any system intended to power household AC appliances or tie into the grid to sell electricity back to the power company depends upon a DC/AC converter or inverter.

Regardless of the inverter’s power rating, the fundamental operation of the device creates some difficulties for the grid and appliances that appear as loads.

The basic function of any inverter system is to switch the DC current supplied by the solar panel on and off in order to provide the fundamental power line frequency (50 or 60 Hz depending on the location). Sophisticated electronics incorporating microcontrollers improve the purity of the AC signal presented to the grid. However, there are many sources of noise introduced that may find their way into sensitive electronics, potentially reducing their lifetimes or causing spurious performance.

An inherent problem

Every kind of switch-mode power converter produces a broadband interference spectrum and harmonics as a by-product of its operation. At the AC output of the inverter, the broadband spectrum produced by the switch-mode power electronics can create a number of undesirable situations. The AC side could face noncompliance with electric utility codes and power-quality standards. For example, the AC signal can stray beyond utility company limits for harmonic content. The system also may fail compliance testing for electromagnetic compatibility (EMC). In extreme situations, other grid-tied customers may experience equipment failures.

Upstream of the inverter on the DC side, the effects are less obvious, but still quite serious. The DC side of the conversion faces premature aging of the solar panel due to superimposed high-frequency currents and leakage currents as well as electromagnetic interference (EMI) radiated by the panels, which could exceed regulatory limits. The function or efficiency of the solar panel can be impacted and its lifetime may suffer.

Off-grid PV inverters represent a good power source in remote areas without the availability of a power grid. Without a power grid to feed into, utility codes and power quality standards are minor issues. Still, since off-grid inverters supply power for numerous electronic/electrical products and because the line impedance of the grid is missing, the inverter output signal must be a pure sine wave, without harmonics or high-frequency components, which can only be achieved via appropriate filtering.

Filter solutions

Essentially, there are three different components that address the deleterious effects introduced by the switch-mode power inverter (Figure 1). To address the frequency interference on the DC side, a DC EMC filter should be employed. Again for the upper frequencies, an AC EMC filter is recommended but on the output AC side. To filter the low-frequency problems, a sine wave filter can be included. The sine wave filter typically allows decoupling of the inverter from the grid along with its prime directive of improving signal purity for utility requirements.

Schematic diagram of PV system

Figure 1: Schematic diagram of PV system incorporating line filtering (Courtesy of Schaffner).


Several products are available to specifically address the renewable energy market from Schaffner, a well-known manufacturer of power filters and components. The Schaffner FC2200 line of filters (Figure 2) is the most compact standard solution on the market. Filters are available over a very wide power range. All the FC2200 filters are specified for 1,200 V. For low-current applications, the FN2200-25-33 is rated for 25 A. Power loss is only 8 W while providing attenuation of at least 50 dB for frequencies from 50 kHz to 10 MHz as tested to CISPR specifications at 50 Ω. For higher power applications, FN2200-1500-99 is rated for 1,500 A. Attenuation performance is inversely related to the power specification as the series of curves for the entire FC2200 line-up show.

HF attenuation performance for FC2200

Figure 2: HF attenuation performance for FC2200 DC EMC filters (Courtesy of Schaffner).

The Schaffner DC filters are very compact (Figure 3). The 25 A FN2200-25-33 is housed in a 170 x 140 x 152.5 mm can. The dimensions do not include the connectors that protrude beyond the ends of the case. The higher-power devices, handling 1,500 A, offer a footprint of 200 x 300 mm.

FN2200-25-33 DC EMC filter

Figure 3: FN2200-25-33 DC EMC filter rated for 25 A (Courtesy of Schaffner).

For AC filtering, components are available in both single- and three-phase configurations. The Schaffner FN612 family is designed for general-purpose single-phase filtering. The FN612-6-06 is a 6 A capable device rated for 250 V and adds 750 µH of inductance to the system.

The FN3270 family provides filtering of all three phases in a single component. A wide range of power handling is available down to 10 A. At the heavy-duty end, the FN3270H-1000-99 is rated to handle 1,000 A. Power loss for this largest of filters is only 81 W.

Summary

Modern switch-mode inverter electronics offer excellent performance and efficiency while limiting the bill of materials for solar energy harvesting systems funneling power onto the grid. However, their fundamental operating principle can lead to broadband interference that can wreak havoc on sensitive electronics making use of the scavenged energy. A wide selection of filters is available for use in photovoltaic solar cell applications that provide improvement in system reliability and efficiency, reduction of conducted EMI into the power grid, fulfillment of international EMI/RFI regulations and more. As such, careful consideration should be given to the system design to include high-quality power filters such as those mentioned in this article. For more information use the links provided to access product information pages on the Digi-Key website.
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