Technical Discussion on Ensuring the Safe Collection and Transmission of Currents from Multiple Photovoltaic Strings to Inverters in Photovoltaic Power Systems

Introduction

Photovoltaic (PV) power systems, as a green and renewable energy solution, have been widely adopted globally. The core components of such systems are photovoltaic panels, which convert solar energy into direct current (DC) electricity through the photovoltaic effect. In large-scale PV systems, multiple photovoltaic strings need to gather the generated current and transmit it to inverters, where DC is converted into alternating current (AC) for grid use. Ensuring that these currents are safely and efficiently collected and transmitted to inverters is critical for maintaining system stability. This article discusses the technical methods and optimization measures to ensure the safe collection and transmission of currents from PV strings.

Challenges in Current Collection and Transmission

In a PV power system, a photovoltaic string consists of several PV modules connected in series. These strings feed the DC power into a combiner box, where the currents are collected and then transmitted to an inverter. However, given the large number of strings, the process of current collection and transmission faces several challenges, including current imbalance, performance differences between modules, cable transmission losses, and the risk of current overload. If these issues are not properly addressed, they could lead to reduced system efficiency, equipment damage, or even safety hazards.

Role and Optimization of DC Combiner Boxes

The DC combiner box is a key component in the current collection process. Its main function is to combine the currents from multiple PV strings and then transmit them via one or more cables to the inverter. To ensure safety in the current collection process, the combiner box must be equipped with various electrical protection devices, such as anti-reverse diodes, overcurrent protection, and lightning protection devices.

Anti-reverse diodes ensure that the current flows only in the correct direction, preventing current backflow between strings and protecting the PV modules from damage. Overcurrent protection devices, such as fuses or circuit breakers, automatically disconnect the circuit when the current exceeds a set limit, protecting the equipment from damage. Additionally, surge protection devices are needed to safeguard the system from voltage spikes caused by lightning strikes or grid faults.

The design of the combiner box must also consider heat dissipation and protective measures. Adequate ventilation or cooling systems should be in place to prevent overheating, which could impact the equipment's lifespan. The enclosure should have a protection rating of at least IP65 to prevent dust and water ingress, ensuring safe and reliable operation.

Cable Selection and Layout

Cables play a critical role in the transmission of currents within PV systems. The choice of cables and their layout can significantly affect transmission safety and system efficiency. First, the cable cross-section must be selected according to the current magnitude and transmission distance. Higher currents require thicker cables to prevent overheating or damage. For longer distances, thicker cables help reduce energy losses and improve transmission efficiency.

The choice of cable material is equally important. Common cable materials in PV systems include copper and aluminum. Copper cables offer superior conductivity and mechanical strength, making them suitable for situations where reliability is a priority, although they come at a higher cost. Aluminum cables, on the other hand, are more economical but have lower conductivity and mechanical performance. Therefore, the choice of cable material should be based on the specific application and budget.

In addition, the cable layout must be carefully planned. Cables should not be exposed directly to extreme weather conditions. In hot or cold climates, proper protective measures, such as burying cables or placing them in protective conduits, should be taken to minimize environmental impact and prolong cable life.

Inverter Selection and Optimization

The inverter is one of the core components of a PV power system, responsible for converting DC from the PV strings into AC. In large-scale PV systems, inverters often handle the current collection from multiple strings, making their selection and configuration crucial.

Modern inverters usually feature multi-string input designs, allowing for the simultaneous connection of several PV strings. This design enables independent control of each string, preventing the entire system from being affected by a fault in one string. Additionally, multi-string inverters are equipped with multiple Maximum Power Point Tracking (MPPT) modules, ensuring that each string operates at its optimal point, thereby maximizing overall system efficiency.

Inverters should also be equipped with robust overload protection to manage current fluctuations. By continuously monitoring current levels, the inverter can automatically adjust its operating state, preventing damage or reduced efficiency caused by excessive current.

Balancing and Matching Currents between PV Strings

Current imbalances between PV strings are a common issue that can affect the safety of current collection and transmission. When currents are not evenly distributed, some strings may be overloaded, while others may not operate at full capacity. To address this problem, the following technical solutions can be employed:

First, when designing PV strings, it is recommended to use modules of the same specification and model to ensure consistent electrical characteristics between strings, thereby reducing current differences. Additionally, in specific situations, current balancing devices can be used to equalize the output of different strings, ensuring that all strings operate within safe current limits.

By properly matching PV strings, current imbalances can be minimized, leading to improved efficiency and stability in the overall PV system.

Monitoring and Maintenance of PV Systems

To ensure the long-term safety of current collection and transmission, PV systems must be equipped with comprehensive monitoring and maintenance mechanisms. Modern PV plants typically use intelligent monitoring platforms to track key parameters such as current, power, and temperature for each string in real-time. If a string behaves abnormally, the monitoring system can trigger alarms, allowing maintenance personnel to quickly identify and resolve issues.

Moreover, data analysis and predictive maintenance play an essential role in enhancing system safety. By analyzing historical operational data, maintenance personnel can predict potential equipment failures and take preventive measures in advance, thus avoiding system downtime or equipment damage.

Conclusion

Ensuring the safe collection and transmission of currents from multiple PV strings to inverters is crucial for the safe and stable operation of a PV power system. By optimizing the configuration of DC combiner boxes, selecting appropriate cables and inverters, balancing string currents, and implementing comprehensive monitoring and maintenance systems, it is possible to enhance system efficiency and ensure long-term operational safety. As technology continues to advance, the techniques for current collection and transmission in PV systems will become more refined, providing a more reliable and secure foundation for photovoltaic power generation.