1 Introduction
In 2014, China's PV power station installed capacity of 10.6 GW, ranking first in the world. In 2015, the National Energy Administration plans to build a new capacity of 17.8 GW of photovoltaic power plants. It is expected that the cumulative installed capacity of photovoltaic power plants in China will be ranked first in the world during the year.
In the rapid development of photovoltaic power plants in China, there are still differences in the design schemes and equipment schemes for the configuration of DC fuses on the DC side of PV systems. This article will analyze this difference based on the principle of DC fuse protection.
2, DC fuse protection principle
Photovoltaic power station DC side According to the different configurations of the PV inverter, multiple sets of parallel strings are collected in parallel to the DC combiner box (centralized inverter solution) or string inverter (string inverter solution) DC bus.
When several photovoltaic strings are connected in parallel, such as a short-circuit fault in one string, the other strings on the DC bus and the power grid will provide a short-circuit current to the short-circuit point. The lack of appropriate protective measures will result in the destruction of the PV modules and the cables connected to them. At the same time, it may cause burning of attachments near the equipment. At present, there are many similar roof photovoltaic fire accidents in China. Therefore, it is necessary to install protective devices in the parallel circuits of each string to enhance the safety of photovoltaic power plants. At the same time, the standard IEC 62548 [1] also explicitly addresses the overcurrent protection of the DC side of photovoltaic power plants.
Based on this, in the photovoltaic power station design scheme, many DC fuses are series-connected in the parallel loops of each parallel group.
2.1 Application of fuses
The fuses connected in series in the circuit use the thermal melting characteristics of the metal to cut off the circuit when an overcurrent occurs in the device to ensure safe operation of the circuit. As a kind of over-current protection device, it is widely used in power systems, power electronics, telecommunications, rail transportation, photovoltaic power generation systems and other new energy, industrial automation, aerospace and military and other related industries.
The current photovoltaic industry standard solution is to use fuses for over-current protection. The fuse has become a commonly used protection device for photovoltaic power stations, and is widely used in conjunction boxes and inverters. The mainstream international inverter manufacturers also regard the fuse as the basic component of DC protection. At the same time, fuse manufacturers such as Bussman and Littelfuse also introduced photovoltaic-specific DC fuses.
2.2 Technical Requirements for Protection of Fuses in Photovoltaic Power Plants
According to the requirements of standard IEC 61730-2 [2]: PV modules must have the ability to withstand reverse current overload. Its provisions for withstand reverse over-current tests are as follows: If a PV module with a maximum fuse current rating of 15 A is used, a 2 hour test at 20.25 A (1.35 times) is required to require that the assembly is non-flammable, the yarn muslin that contacts the assembly, and white Thin paper should have no burning or focal spot, and the MST 17 test (wet leakage current test) should meet the same requirements as initially tested.
Therefore, the components of photovoltaic modules certified according to IEC 61730-2 should have the ability to withstand 1.35 times the reverse current for 2 hours.
In response to these requirements, measures are taken to enhance the safety of the PV modules and extend their life spans, and to protect the DC bus on the PV inverter (or DC bus box) side. Therefore, the technical requirement for the IEC60269-6 [3] standard for photovoltaic fuses (15A) is: “It needs to be blown within 1 hour with a current of 21.75A (1.45 times)â€. In the UL2579[4] standard, it is required that "the fuse needs to be blown within 1 hour with a current of 20.25A (1.35 times)."
Looking at the reverse current overload capability of the integrated component (IEC61730-2), the requirements of the UL standard are more reasonable, and the IEC 60269-6 standard will also amend its conflicting requirements: For rated currents of 32A or less (including 32A) The fuse should be blown within 1 h after passing 1.35 times the rated current. When mainstream fuse manufacturers, such as Bussmann and Littelfuse, responded quickly to the technology, they also updated their product parameters when they were designing photovoltaic fuses.
Therefore, when the fuse fuse current is not greater than the PV module can withstand the reverse current limit and the fuse time is not greater than the PV module can withstand the reverse current time, the fuse can achieve protection of the PV module.
2.3 State of Application of Fuse in Photovoltaic System
In order to avoid the equipment loss caused by the short circuit on the DC side. At present, mainstream technical solutions all add fuses on the DC side. At the same time, international mainstream inverter manufacturers also require additional fuses on the DC side. For example: SMA's ST12000/15000/20000/24000TL-US series inverters; Sungrow's SG60KTL/SG40KTL series inverters; Kaco's Powador 60.0TL3 inverters are all protected by DC fuses. Design. As for the currently existing two-channel parallel-connection device scheme in each MPPT, in accordance with the requirements of the above technical standards, it is still necessary to configure an overcurrent protection device such as a fuse to protect the device. The use of overcurrent protection devices such as fuses or the use of diodes only for reverse current protection all present potential safety hazards that may cause equipment to burn or even cause fire.
Obviously, the DC-side (or DC-linked box) bus-distribution fuses of PV inverters are the mainstream solutions for the DC side of PV power plants.
2.4 Possible problems with fuse protection
Similar to circuit breakers and other protective devices in various types of power engineering, the "breakpoints" are unavoidably increased due to the configuration of the fuses, but the current technical means can avoid the power loss caused by equipment damage.
1) For the string inverter, because it has its own string detection capability, it can complete the collection of current and voltage of each string; it can realize the rapid detection of fuses; it can replace equipments in time and reduce the string open time.
2) For the photovoltaic power plant consisting of centralized inverters, the current intelligent convergence box also has string detection capability. It also avoids the above problems.
in conclusion
To sum up, no matter from the protection principle of the fuse, the specification requirements and application status of the photovoltaic power plant. The configuration of the fuses on the DC configuration side of a photovoltaic power plant has its rationality and necessity.
1) Photovoltaic modules are the main equipment investment for photovoltaic power plants and should protect the safety of their equipment. At the same time, according to the requirements of the international standard, fuses should be configured to protect them.
2) Through comparative analysis of the rationality of the technical specifications, the selected DC fuse should meet the technical requirements for melting within 1 hour at 1.35 times the rated current.
3) Fuses are currently widely used in the DC side protection scheme of photovoltaic power plants and are the mainstream design solutions. The international mainstream inverter manufacturers follow the IEC's standard technical regulations above and configure fuses on the DC side as protection devices.
4) At present, whether the centralized scheme or the string-type scheme is adopted in the photovoltaic power station, the rapid detection of the fuse failure can be realized and replaced in time. Conversely, for systems without overcurrent protection components such as fuses, the loss caused by a short circuit caused by a short circuit may be catastrophic, especially in large-scale grid-connected photovoltaic power plants with building photovoltaics and few people watching.
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