Photovoltaic system security guardian - intelligent string breaking isolation switch

25 Dec,2023

With the continuous progress of photovoltaic technology, the DC side voltage of photovoltaic systems has been increased to 1500V, and 210 solar cells have also been widely used. However, these technological advancements also place higher demands on the electrical safety of photovoltaic systems. The increase in system voltage increases the risk of insulation breakdown in components, inverter wiring, and internal circuits, making it crucial to take timely and effective protective measures in the event of faults.

In order to be compatible with higher current components, inverter manufacturers have increased the string input current from 15A to 20A. The inverter manufacturer has optimized the internal design of MPPT to handle three or more string connections. In addition, DC switches with "intelligent string breaking" function have emerged to address the potential issue of string current backflow, providing strong guarantees for the safe operation of photovoltaic systems.

 

The difference between traditional isolation switches and intelligent DC switches

Firstly, traditional DC isolation switches can disconnect normally within the rated current, for example, a nominal 15A isolation switch can disconnect currents of 15A and below at the rated voltage. Although manufacturers may declare that isolation switches have a certain overload breaking capacity, they are usually unable to break short-circuit currents.

The biggest difference between circuit breakers and isolating switches is that circuit breakers have the ability to break short-circuit currents, and the short-circuit current during faults is often much greater than the rated current of the circuit breaker. Due to the fact that the short-circuit current on the DC side of photovoltaics is usually about 1.2 times the rated current, some isolation switches or load switches can also disconnect the short-circuit current on the DC side.

At present, the KSiG1 series DC isolation switch of Kingsi also has a certain capacity for overcurrent breaking. It can break overcurrent faults within the nominal short-circuit current range, effectively solving the problem of string current backflow. At the same time, the intelligent DC switch is combined with the DSP of the inverter to enable the tripping unit of the switch to accurately and quickly achieve functions such as overcurrent protection and short circuit protection.

The design standards for photovoltaic systems require that when the number of string input circuits under each MPPT is greater than or equal to 3, fuse protection should be installed on the DC side. However, the advantage of string inverters lies in their fuse free design, which reduces the maintenance workload of frequent fuse replacement on the DC side.

The design scheme adopts an intelligent DC switch, and each MPPT can input 3 sets of strings. However, in extreme fault situations, there may be a risk of backflow of 2-way series current to 1-way series current. At this point, the intelligent DC switch will pop open the DC switch through the shunt release, timely disconnecting the circuit to ensure rapid fault removal.

The shunt release is essentially a circuit breaker coil with a release. When a specified voltage is applied to the shunt trip coil, electromagnetic attraction and other actions are used to trip the DC switch actuator to open the circuit. Shunt release is commonly used in remote automatic power outage control.

The intelligent DC switch configured on the inverter can be commanded by the inverter DSP to trip and open the DC switch circuit, thereby disconnecting the DC switch circuit.

When using an inverter with shunt trip protection function, it is necessary to first ensure that the control circuit of the shunt coil receives control power in order to ensure the normal operation of the main circuit's trip protection function.

The application prospects of intelligent DC switches

With the increasing attention paid to the safety of photovoltaic DC side, safety functions such as AFCI and RSD are gradually receiving attention. The SSLD intelligent string breaking technology also plays an important role in ensuring safety. When a malfunction occurs, the intelligent DC switch can utilize its remote controllability and overall control logic. After AFCI or RSD action, the DSP sends a trip signal to automatically disconnect the DC isolation switch, forming a clear disconnection point to ensure the safety of maintenance personnel.

The application of intelligent DC switches also makes it possible for inverter equipment to achieve reliable "one click shutdown" in household scenarios. Through the design of DSP controlled shutdown, in emergency situations, the DC switch of the inverter can be quickly and accurately turned off through DSP signals, forming a reliable maintenance disconnection point.

The application of intelligent DC switches mainly solves the protection problem of current backflow. However, whether its remote release function can be applied to other distributed and household scenarios, forming a more reliable operation and maintenance guarantee, and improving the ability of users to handle faults in emergency situations, still requires the industry's application and verification of intelligent DC switches.