DC lightning protector
The threat of lightning to equipment in photovoltaic power generation systems mainly comes from the surface covered by the entire power generation system, the conditions of the distributed power supply site, nearby high metal structures (such as signal towers, etc.), and the level of local lightning activity. The heating surface of photovoltaic systems is often in isolated and exposed places, making lightning an important risk factor.
Large-scale distributed power sources use a huge number of photovoltaic cell modules, covering an area of more than ten square kilometers, and are installed outdoors, so they are extremely vulnerable to thunder and lightning. The lightning discharge and the switching operation of the upstream power system will cause inductive or capacitive coupling voltages. These surge voltages may damage the photovoltaic cell components and the inverter. Once an indirect lightning strike occurs, the photovoltaic cell modules, electronic components and semiconductor components inside the inverter are likely to be damaged.
In order to prevent the photovoltaic power generation DC power system from damage to the equipment due to lightning overvoltage or operating overvoltage, DC lightning protectors are installed in the photovoltaic DC combiner box between the positive pole and the ground, the negative pole and the ground, and between the positive and negative poles. According to the CGC/GF002 “Technical Specification for Photovoltaic Combiner Box” standard, its specifications should meet the following requirements:
(1) Maximum continuous working voltage (Uc): Uc>1.3Uoc (STC).
(2) Maximum amplification current (Imax): Imax (8/20) ≥ 40 kA; Nominal discharge current (In): In(8/20)>20 kA.
(3) Voltage protection level (Up): Up is the test value under the nominal discharge current In, The specific application requirements are shown in Figure 1.
(4) The lightning protection device should have the functions of disconnector and fault indication. Among them, STC (Standard Test Conditions) is the standard test condition. That is, the temperature of the photovoltaic cell is 25°C, the irradiance of the light source is 1 000 W/m2, and it has AM 1.5 (AM is Air Mass, which is defined as the ratio of the actual distance of light through the atmosphere to the vertical thickness of the atmosphere.
AM1.5 means that the actual distance of light passing through the atmosphere is 1.5 times the vertical thickness of the atmosphere) solar spectral irradiance distribution. Uoc(STC) is the open circuit voltage of the photovoltaic cell under standard test conditions.
The monitoring module is a dedicated device for the smart photovoltaic DC combiner box. The intelligent combiner box detects the operation of the system through the monitoring module, and transmits the detected data to the remote central control room through the configured dedicated communication port. Through the analysis and display of the monitoring system, when an abnormal situation occurs in the photovoltaic cell array, professionals will deal with it in time according to the specific information to quickly troubleshoot the fault. In order to facilitate users to grasp the working status of the photovoltaic array in real time, the monitoring module needs to have the characteristics of short sampling period, high conversion accuracy, low energy consumption, stable and reliable performance, etc.
When selecting a photovoltaic DC combiner box in a specific project, it is necessary to connect a certain number of photovoltaic cell modules with the same specifications in series to form a photovoltaic cell string according to the DC voltage range input by the inverter, and then connect several photovoltaic cells in series to the DC combiner box, which greatly simplifies the wiring process of the system. At the same time, it can also protect the inverter to a certain extent, effectively improving the reliability and practicability of the photovoltaic system, making it convenient for users to accurately grasp the working conditions of the photovoltaic power generation system in time, and ensuring that the photovoltaic power generation system exerts its maximum efficiency.