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The safety of photovoltaic DC side is becoming more and more important, can the isolation switch realize safety protection?

Apr 01, 2021

The safety of photovoltaic DC side is becoming more and more important, can the isolation switch realize safety protection?

1. The safety of photovoltaic power plants has attracted much attention


Under the 3060 target, photovoltaic power generation will transform from supplementary energy to main energy, and the safety of photovoltaic power plants has also attracted much attention. Recently, inverter manufacturers, certification agencies, and switching device manufacturers have all proposed to add electric isolation switches on the DC side of the inverter, in order to quickly cut off the connection between the photovoltaic module and the inverter when a fault occurs, and improve the safety of the photovoltaic power station. Many media have carried out related reports: "Power station safety must be paid attention to! Huawei, Sunshine, and Jianheng certification promote "intelligent breaking technology".


What is intelligent breaking technology? What impact does this high technology have on the safety of photovoltaic power plants? Can it replace traditional fuses and circuit breakers to achieve effective protection under fault conditions? The author took these questions and consulted relevant experts in the industry. And combined with the relevant IEC international standards for careful study.


2. Inverter manufacturers add "intelligent breaking technology" on the DC side


According to the publicity materials of the inverter manufacturer, the so-called intelligent breaking technology is to add an electric isolating switch on the DC side of the photovoltaic inverter, and the controller of the inverter detects the current and voltage of the string in real time to determine the working status of the system . When the system fails, the inverter can automatically control the isolating switch to break the fault circuit.


Inverters from two mainstream manufacturers currently on the market that use intelligent breaking technology, one of the 196kW inverter adopts 4-5 string input and one MPPT design, and the other 320kW inverter adopts two string input. The design of MPPT all the way.


In the past few years, industry professionals have conducted in-depth discussions on whether the photovoltaic system needs to add fuses to protect the system. Nowadays, can intelligent breaking technology really improve system safety? Does it meet the requirements of relevant standards such as IEC?


3. The IEC standard has detailed requirements for the protection of the DC side of the photovoltaic system


1) Under what circumstances need overcurrent protection?


IEC 62548 6.5.3: Whether the photovoltaic system needs to install overcurrent protection devices in each string is determined by the number of strings in parallel.


Note: Ns represents the number of strings in parallel, ISC-MOD represents the short-circuit current of photovoltaic modules, and IMOD-MAX-OCPR represents the maximum overcurrent protection rating of photovoltaic modules.


When the number of parallel strings exceeds 2 strings, an overcurrent protection device needs to be installed in each string. For the inverter, when the number of strings connected to each MPPT reaches 3 strings or more, it is necessary to add an overcurrent protection device to each string.


2) What over-current protection devices are there?


IEC 60364-7-712 712.533: The DC side overcurrent protection device of the photovoltaic system must be a photovoltaic special fuse that meets IEC 60269-6 or a circuit breaker that meets IEC 60947-2.



Through the interpretation of this standard, does it mean that isolating switches cannot actually replace fuses and circuit breakers as DC overcurrent protection devices?


In order to further clarify whether the intelligent isolation switch can be used as a protection device, the author analyzes the protection mechanism of the three devices as follows:


Fuse: When the current in the circuit exceeds the specified value for a period of time, the fuse will melt the melt with its own heat, thereby disconnecting the circuit and protecting key equipment.


Circuit breaker: The current circuit breakers commonly used in photovoltaic systems mainly include magnetic tripping and thermal tripping. The working principle of the magnetic trip type is: the magnetic field generated by the large current overcomes the reaction force spring, the release pulls the operating mechanism to move, and the switch instantaneously trips; the working principle of the thermal trip type is: the current becomes larger, the heat is increased, and the bimetal The sheet is deformed to a certain extent to push the mechanism.


Intelligent disconnecting switch: real-time detection of the current of each string through a high-precision DC sensor. When a fault is detected, the inverter automatically controls the DC disconnecting switch to disconnect the fault circuit.


Combining the protection mechanisms of the above three devices, it can be seen that the fuse and circuit breaker are passive protection, that is, when the system has a short circuit and other faults, the protection can be achieved without relying on the external circuit, and whether the isolating switch using intelligent breaking technology can reliably break Opening the fault circuit relies on external circuits such as detection and control. When the system loses power or the control circuit fails, there is a risk that the isolating switch will not open.


Therefore, in terms of safety, the use of isolating switches instead of fuses or circuit breakers as protective devices poses a greater system risk. In the specific system design, for a two-way string-parallel system, according to the standard, there is no need to install over-current protection devices. The addition of a disconnecting switch on the DC side improves the safety to a certain extent. When a string short circuit occurs or In reverse connection, even if the isolating switch cannot be disconnected, the maximum short-circuit current or reverse current in the system is only 1 times the component short-circuit current, and there will be no risk of continuous heating and fire.


However, if the number of parallel strings exceeds 2 strings, for example: 4 strings or 5 strings are connected in parallel to 1 channel MPPT, when the isolating switch cannot be disconnected due to various reasons, if one of the circuits is short-circuited, the remaining 4 strings The current will flow to the short-circuit point, that is, the fault circuit will withstand 4 times the short-circuit current, and related components such as cables will be damaged due to continuous overcurrent and even fire. Especially when the system is reversed due to improper construction, serious backflow current (4*Isc) will cause the reversed photovoltaic modules to overheat and catch fire.


V. Conclusion


From the above analysis, it can be seen that although the intelligent breaking technology of the isolating switch increases the safety of the system to a certain extent, it cannot replace the fuse and circuit breaker as a DC protection, especially when the number of parallel strings exceeds 2 strings. It is recommended that in addition to the isolating switch, each string still needs to add a fuse or a circuit breaker as an overcurrent protection device.