Can the #disconnector ensure #safety #protection?

The safety of #photovoltaic #power plants has attracted much attention under the #carbon net zero target. Recently, #inverter manufacturers, #certification institutions, and #switch device manufacturers have proposed to add electric #disconnectors on the #DC side of the #inverter in order to quickly cut off the connection between the #photovoltaic #module and the #inverter and improve the safety of the #photovoltaic power plant. A number of media released related news: "The safety of the #power plant must be paid attention to! Huawei, Sungrow Power Supply Co., Ltd. and China General Certification Center promote #Intelligent Breaking Technology."

#Inverter manufacturers add #Intelligent Breaking Technology on the #DC side

The so-called technology is to add an electric #disconnector on the DC side of the #photovoltaic inverter to judge the working state of the system by detecting the current, #voltage and other signals of the #inverter in real time according to the publicity #materials of the #inverter manufacturers. The #inverter can automatically control the #disconnector to break the fault loop when the system fails.

Presently, there are two mainstream inverters using intelligent breaking technology in the market. The 196kW inverter adopts the design of 4-5 strings connecting to 1 MPPT (see figure 1), and the other 320kW inverter adopts the design of 2 strings connecting to 1 MPPT (see figure 2).

A few years ago, people in the industry had an in-depth discussion on whether the #photovoltaic system needs to add fuses to protect the #safety of the system. Presently, can #intelligent breaking technology really improve system #security? Does it meet the requirements of #IEC and other relevant standards?

IEC standard has detailed requirements for DC side protection of photovoltaic system

Under what circumstances does it need overcurrent protection?

#IEC62548 6.5.3: standard has detailed requirements for #DC side protection of #photovoltaic system

What are the overcurrent protection devices?

#IEC 60364-7-712 712.533: The DC side overcurrent protection device of #photovoltaic system must be a #photovoltaic fuse for IEC60269-6 or a circuit breaker for IEC60947-2.

As a consequence, in terms of safety, there is a great system risk in using #disconnectors instead of fuses or circuit breakers as protection devices. In the specific system design, for the 2 strings #paralleled system (figure 3), there is no need to install over-current protection devices, and adding a breakable disconnector on the DC side improves the safety to a certain extent according to the standard. When a series short circuit or reverse connection occurs, even if the disconnector cannot be disconnected, the maximum short-circuit current or reverse current in the system is only 1 times the component short-circuit current. As a result, there is no risk of fire.

However, if the number of #parallel is more than 2 strings, for example, 4- or 5-strings are connected to 1 MPPT in parallel (figure 4), when the disconnector cannot be opened for various reasons, the remaining 4 string current will flow to the short-circuit point if one of the circuits is short-circuited. In other words, the fault circuit will bear 4 times short-circuit current, and related components such as cables will be damaged or even fire due to continuous overcurrent. The serious reverse current (4*Isc) will cause more serious faults such as overheating and fire of the reversed #photovoltaic module especially when the reverse connection occurs in the system due to improper construction.

#renewableenergy #solar

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