What is the smart DC switch that is as important as AFCI?

The voltage on the DC side of the solar energy system is increased to 1500V, and the promotion and application of 210 cells put forward higher requirements for the electrical safety of the entire photovoltaic system. After the system voltage is increased, it poses challenges to the insulation and safety of the system, and increases the risk of insulation breakdown of components, inverter wiring, and internal circuits.This requires protection measures to isolate faults in a timely and effective manner when corresponding faults occur.

In order to be compatible with components with increased current, inverter manufacturers increase the input current of the string from 15A to 20A.When solving the problem of 20A input current, the inverter manufacturer optimized the internal design of MPPT and extended the string access capability of MPPT to three or more.In the case of a fault, the string may have a problem of current backfeeding. To solve this problem, a DC switch with the function of “intelligent DC shutdown” has emerged as the times require.

01 The difference between traditional isolating switch and intelligent DC switch

First of all, the traditional DC isolating switch can break within the rated current, such as a nominal 15A, then it can break the current under the rated voltage of 15A and within.Although the manufacturer will mark the overload breaking capacity of the isolating switch, it usually cannot break the short-circuit current.

The biggest difference between an isolating switch and a circuit breaker is that the circuit breaker has the ability to break the short-circuit current, and the short-circuit current in the event of a fault is much greater than the rated current of the circuit breaker; Since the short-circuit current of the photovoltaic DC side is usually about 1.2 times the rated current, some isolating switches or load switches can also break the short-circuit current of the DC side.

At present, the smart DC switch used by the inverter, in addition to meeting the IEC60947-3 certification, also meets the overcurrent breaking capacity of a certain capacity, which can break the overcurrent fault within the nominal short-circuit current range, effectively It solves the problem of string current backfeeding. At the same time, the smart DC switch is combined with the DSP of the inverter, so that the trip unit of the switch can accurately and quickly realize functions such as overcurrent protection and short circuit protection.

Electrical schematic diagram of smart DC switch

02 The solar system design standard requires that when the number of input channels of the strings under each MPPT is ≥3, fuse protection must be configured on the DC side.The advantage of applying string inverters is the use of no-fuse design to reduce the operation and maintenance work of frequent replacement of fuses on the DC side. Inverters use intelligent DC switches instead of fuses. MPPT can input 3 groups of strings. Under extreme fault conditions, there will be a risk that the current of 2 groups of strings will flow back to 1 group of strings. At this time, the intelligent DC switch will pop open the DC switch through the shunt release and disconnect it in time. circuit to ensure rapid removal of faults.

Schematic diagram of MPPT string current backfeeding

The shunt release is essentially a tripping coil plus a tripping device, which applies a specified voltage to the shunt tripping coil, and through actions such as electromagnetic pull-in, the DC switch actuator is tripped to open the brake, and the shunt tripping It is often used in remote automatic power-off control.When the smart DC switch is configured on the GoodWe inverter, the DC switch can be tripped and opened through the inverter DSP to disconnect the DC switch circuit.

For inverters using the shunt trip protection function, it is first necessary to ensure that the control circuit of the shunt coil obtains control power before the trip protection function of the main circuit can be guaranteed.

03 Application prospect of intelligent DC switch

As the safety of the photovoltaic DC side is gradually getting more attention, safety functions such as AFCI and RSD have been mentioned more and more recently.Smart DC switch is equally important. When a fault occurs, the smart DC switch can effectively use the remote control and overall control logic of the smart switch. After the AFCI or RSD action, the DSP will send a trip signal to automatically trip the DC DC isolation switch. Form a clear break point to ensure the safety of maintenance personnel. When a DC switch breaks a large current, it will affect the electrical life of the switch. When using an intelligent DC switch, the breaking only consumes the mechanical life of the DC switch, which effectively protects the electrical life and arc extinguishing ability of the DC switch.

The application of intelligent DC switches also makes it possible to reliably “one-key shutdown” of inverter equipment in household scenarios；Secondly, through the design of DSP control shutdown, when an emergency occurs, the DC switch of the inverter can be quickly and accurately shut off through the DSP signal, forming a reliable maintenance disconnection point.

04 Summary

The application of intelligent DC switches mainly solves the protection problem of current backfeeding, but whether the function of remote tripping can be applied to other distributed and household scenarios to form a more reliable operation and maintenance guarantee and improve user safety in emergency situations. The ability to deal with faults still requires the application and verification of smart DC switches in the industry.