Main components of photovoltaic DC combiner box
Photovoltaic DC combiner box refers to a box that connects photovoltaic strings and is equipped with necessary protection devices to realize parallel connection between photovoltaic strings. Since the photovoltaic DC combiner box is arranged outdoors, the protection level is generally required to be above IP65. The input side of the PV DC combiner box generally uses a fuse as a protection device, and the output side uses a DC circuit breaker as a protection device, and a surge protection device is installed on the DC busbar.
1) Fuse
The rated voltage of the fuse should generally not be lower than 1500V/1000V (IEC). The rated current of the fuse should generally not be less than 1.56Isc and not greater than the maximum rated current of the fuse allowed by the component manufacturer. The reason for choosing 1.56Isc is according to the provisions of Article 690.8(A) of the "National Electrical Code" (NEC, 2011 edition). This regulation assumes that the maximum current of the module is 125% of Isc. The reason is that under certain cloud coverage conditions, the cloud is equivalent to a magnifying glass, and the light intensity can be 25% higher than the irradiance (1000W/m2) corresponding to the STC. The second is the National Electrical Code (NEC, 2011 edition) 690.8 (B), which requires that the fuse must be able to withstand 125% of the maximum current of the component. The end result is that the rated current of the circuit breaker is chosen as 125% of 125% of 1sc, which is 156% of 1sc.
2) DC circuit breaker
After several groups are connected in series and parallel, the DC circuit breaker is generally used as a protection device, and the output is passed through the DC circuit breaker. The DC circuit breaker generally adopts a 4P DC circuit breaker, and each two poles are connected in series as one pole to achieve a withstand voltage level of 1500V/1000V. The rated current of the DC circuit breaker is also selected according to 1.56Isc.
3) Surge protection device
The output end of the photovoltaic DC combiner box should be equipped with a lightning protection device, and both the positive and negative poles should have lightning protection functions. Specifications should meet the following requirements:
1. The maximum continuous working voltage Uc:Uc>1.3Uoc(STC).
2. Maximum discharge current Imax: Imax(8/20)≥40kA, nominal discharge current In:I(8/20)≥20kA.
3. Voltage protection level Up, Up is the nominal discharge current.
4. The surge protection device should have the function of disconnector and fault indication.
4) Monitoring device
In order to facilitate users to grasp the working conditions of photovoltaic strings in a timely and accurate manner and ensure that the solar photovoltaic power generation system exerts the maximum effect, the photovoltaic DC combiner box is generally equipped with a monitoring device. The monitoring device can monitor the current of each output circuit, the voltage of the DC busbar and the working state of the lightning arrester. The monitoring device generally recommends self-power supply to avoid the problem of large-scale and long-distance power supply. The monitoring device generally adopts the RS485 communication protocol. In order to prevent the overvoltage caused by lightning from the communication line from being introduced into the photovoltaic DC combiner box, it is required to install a signal surge protection device on the 485 interface of the photovoltaic DC combiner box, and the communication line should be Use armored shielding or laying through steel pipes.
