As a protective component in the DC side configuration of photovoltaic power stations, the design of DC fuses varies across different schemes. This article first describes the protective principle and application scenarios of fuses, combined with the technical requirements of various standards, and proposes parameter configuration requirements for photovoltaic DC fuses. It then discusses the issues and advantages of current international mainstream design and equipment schemes. The conclusion is that the configuration of DC fuses plays a crucial and indispensable role in ensuring the safe and reliable operation of photovoltaic power stations.
Protective Principle of DC FusesDepending on the photovoltaic inverter scheme, the DC side of a photovoltaic power station connects multiple string arrays in parallel to the DC combiner box (central inverter scheme) or to the DC busbar of a string inverter (string inverter scheme). When several photovoltaic strings are paralleled, if one string experiences a short circuit, other strings and the grid will supply short-circuit current to the fault point. Without appropriate protective measures, this can lead to the burning of photovoltaic components and associated cables, and potentially cause fires. Numerous rooftop photovoltaic fire incidents have occurred domestically, thus necessitating protective devices in each string’s parallel circuit to enhance safety. Additionally, standard IEC62548[1] specifies requirements for overcurrent protection on the DC side of photovoltaic power stations. Therefore, photovoltaic power station design schemes typically include DC fuses in each paralleled string circuit.
Application of Fuses, integrated into circuits, utilize the thermal melting properties of metals to interrupt circuits in overcurrent situations, thereby ensuring circuit safety. As overcurrent protection devices, fuses are widely used in power systems, power electronics, telecommunications, rail transportation, photovoltaic power systems, and other new energy, industrial automation, aerospace, and military-related industries. The current standard solution in the photovoltaic industry is the use of fuses for overcurrent protection. Fuses have become a commonly applied protective device in photovoltaic power stations, extensively used in combiner boxes and inverters. International mainstream inverter manufacturers also incorporate fuses as basic components for DC protection.
Technical Requirements for Fuse Protection in Photovoltaic Power Stations According to standard IEC61730-2[2], photovoltaic components must withstand reverse current overloads. The requirements for reverse overcurrent testing are as follows: For photovoltaic components using a maximum fuse rating of 15A, a 2-hour test under 20.25A (1.35 times) is conducted, requiring that the components should not burn, and the cotton yarn and white thin paper in contact with the components should show no burning or scorching, along with meeting the MST17 test (wet leakage current test) requirements as in the initial test. Hence, photovoltaic components certified according to IEC61730-2 should be capable of withstanding 1.35 times the reverse current for 2 hours. In response to these requirements, to enhance the safety and extend the lifespan of photovoltaic components, protective measures should be taken on the DC busbar side of the photovoltaic inverter (or DC combiner box). Therefore, standard IEC60269-6[3] sets the technical requirement for photovoltaic fuses (15A) as: “Should fuse within one hour under a current of 21.75A (1.45 times).” Whereas, in standard UL2579[4], the requirement is “Should fuse within one hour under a current of 20.25A (1.35 times).” Considering the reverse current overload capacity of components (IEC61730-2), the requirements of the UL standard are more reasonable, and the IEC60269-6 standard is also revised to address its contradictions: for fuses rated below 32A (inclusive), the requirement is to fuse within 1 hour at 1.35 times the rated current. Meanwhile, mainstream fuse manufacturers like Bussmann and Littelfuse, in designing fuses specifically for photovoltaic applications, have updated their product parameter requirements to address these contradictions.
Therefore, when the fuse’s fusing current does not exceed the photovoltaic component’s tolerable reverse current limit, and the fusing time does not exceed the component’s tolerable reverse current duration, the fuse can effectively protect the photovoltaic components.
Current State of Fuse Application in Photovoltaic Systems To prevent equipment loss caused by DC side short circuits, the current mainstream technical solution involves adding fuses to the DC side. Also, international mainstream inverter equipment manufacturers require the addition of fuses on the DC side. For example, inverters like SMA’s ST12000/15000/20000/24000TL-US series, Sungrow’s SG60KTL/SG40KTL series, and Kaco’s Powador60.0TL3 all employ a design scheme with DC fuse protection. For current devices with two string circuits in parallel per MPPT, according to the aforementioned technical standards, overcurrent protection devices such as fuses are still required to protect the equipment. The use of diodes alone for reverse current protection or the absence of overcurrent protective devices like fuses poses safety risks, potentially leading to equipment damage or even fires.
Evidently, the inclusion offusesin the DC busbar of photovoltaic inverters (or DC combiner boxes) is the mainstream design scheme for the DC side of photovoltaic power stations.
Potential Issues with Fuse Protection Similar to circuit breakers and other protective devices in various electrical engineering projects, the inclusion of fuses inevitably adds “breakpoints.” However, current technological means can avoid losses due to equipment damage.
1. Photovoltaic components, as the primary equipment investment in photovoltaic power stations, should be safeguarded. Additionally, in accordance with international standard requirements, fuses should be deployed for protection.
2. Through comparative analysis of technical specifications, the selected DC fuses should meet the technical requirement of fusing within one hour at 1.35 times the rated current.
3. Fuses are widely used in photovoltaic power stations as a mainstream DC side protection scheme. International mainstream inverter manufacturers all comply with the aforementioned IEC standard technical regulations, including fuses as protective devices on the DC side.
4.Currently, in photovoltaic power stations, whether centralized or string schemes, rapid detection and replacement of faulty fuses are achievable. Conversely, systems without fuses or other overcurrent protection components can suffer catastrophic losses from a single short-circuit incident, especially in building-integrated photovoltaics and large, unmanned grid-connected photovoltaic power stations.”