Assessment of DPS installed in photovoltaic inverters

DPS protect solar inverters, preventing damage to equipment

7 minute(s) of reading

Solar inverters use semiconductors in their manufacture and, therefore, are vulnerable to direct and indirect atmospheric discharges. To protect them against the effects of lightning, it is necessary to use the MPS (Surge Protection Measures) described in the technical standard ABNT NBR 5419-1:20151. Among the MPS is the installation of a coordinated DPS system (Surge Protection Devices), which in relation to the inverter can be installed outside, very close to it, or inside the equipment itself. DPS protect electronic installations, including their equipment, against voltage or current surges caused by atmospheric discharges or switching. When installed on solar inverters (Figure 1), the DPS (Figure 2) have the sole function of protecting them, thus preventing damage to the inverter.

Figura 1: Inversor Solar. Fonte: Proauto — Figure 1: Solar Inverter. Source: Proauto

Figura 2: DPS para circuitos em corrente contínua. Fonte: DEHN — Figure 2: DPS for direct current circuits. Source: DEHN

For a DPS to act efficiently, it must be installed in the correct location, according to the concept of ZPR (Lightning Protection Zones) described in the technical standard ABNT NBR5419-4:20152. In relation to solar inverters, the interior of this equipment could be classified as ZPR 1, 2 or 3, depending on where the inverter is located, requiring the protection of a class II or I+II DPS, always installed, externally or internally, at the exact point where the cables enter the inverter. The option for solar inverters that already incorporate DPS must be carefully analyzed by photovoltaic system designers because it influences the reliability, cost and maintainability not only of the inverter, but of the entire system. Precisely because it is an apparently simple decision, with two possibilities, DPS incorporated or not, some aspects of this decision must be considered during the design phase. There are some basic parameters for specifying a DPS, such as its surge current and its protection level. To guarantee the integrity of the inverter, the protection level is the most important factor, as it is what limits the voltage at the equipment terminals, preventing it from reaching values that it cannot support. In order for the protection to be as effective as possible, it is recommended that the protection level (UP) of the DPS3 for inverter protection is lower than its overvoltage withstandability (Uw)4, preventing deviations in UP or Uw from compromising the protection. It is important that the SFV designer knows these parameters because it is always up to him to specify their nominal values, even if it is to validate the DPS that is incorporated into the inverter. If the DPS incorporated into the inverter is not ideal, according to the designer, it will not be so simple to install another one outside as a safety measure. Parallelism between varistors (normally the essence of a class II or I+II DPS) is somewhat complex, since two DPS that use only varistors, even identical models from a single manufacturer, will start to act from different, although close, voltage values, which will imply that the first to switch to low impedance will prevent the second from acting. In this way, it is possible that one of the DPS will always have to be changed, while the other will never come into operation, almost as if it did not exist. If there are two DPS from different manufacturers, this possibility will be even greater. For this reason, if there is an inadequate DPS inside a solar inverter, it will be more interesting to replace it than to try to compensate for its low performance by installing another DPS in the junction box. Another important aspect in inverters that already have DPS incorporated is the periodic need to replace these components. Many suppliers use products from manufacturers that do not operate in Brazil and, therefore, it becomes very difficult to acquire some modules for replacement (Figure 3). As bases and modules are not interchangeable between DPS from different manufacturers, it will be necessary to exchange the entire DPS, something that will mean an additional cost, often not evaluated in the photovoltaic system design. In this case, the problem can be avoided by only accepting solar inverters whose DPS come from manufacturers operating in Brazil, directly or through distributors. In addition to the benefit of easy module replacement, it will be possible to obtain technical support from these manufacturers if the useful life of their DPS is very low or they are unable to protect the inverters against surge currents or transient overvoltages.

Figura 3: A possibilidade de substituição dos módulos do DPS deve ser considerada quando da sua aquisição. Fonte. DEHN — Figure 3: The possibility of replacing DPS modules must be considered when purchasing them. Source. DEHN

SPDs that use only varistors have end of life and must be removed from the circuit when this happens. In the case of direct current power lines this requires special care. In DPS installed inside solar inverters, this precaution must be much more rigorous, since if the permanent current is not interrupted, the DPS may catch fire, completely destroying the solar inverter, causing much greater damage than would happen with an installed DPS. in the junction box. A positive aspect of DPS incorporated into solar inverters is the absence of connection errors, caused by excessive cable lengths between DPS, inverter and equipotential bar. With the DPS installed inside the inverter, it will be practically impossible for it and the equipment to be protected to have different potential references. The provision of DPS already installed inside solar inverters only makes sense if the inverter manufacturer installs all the DPS that will be needed, AC and DC power and signal lines. Due to the countless possibilities of DPS classes for each of their applications, it will be very difficult for inverter manufacturers to standardize a single set of DPS for their products that meets all their customers' needs. Based on what is presented here, we can conclude that the incorporation of DPS into solar inverters is only valid when the inverter manufacturer's option is endorsed by the photovoltaic system designer. By accepting inverters with low quality DPS, whose parameters are inadequate or manufactured by unknown companies in Brazil, you are creating a problem, especially in places with a high density of atmospheric discharges to the ground.

References

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Sergio Roberto Santos

Electrical engineer at Lambda Consultoria. Graduated from UFRJ (Federal University of Rio de Janeiro), with a specialization in economics and business (MBA) from UFSCAR (Federal University of São Carlos). He is currently a master's student in energy technology at the Institute of Energy and Environment at the University of São Paulo (IEE-USP).