When diving into the world of solar energy, the photovoltaic DC combiner box stands out as a pivotal component. Acting as the heart of the photovoltaic array, it’s the power source for the entire photovoltaic station. The components within this box play a crucial role in its efficiency and reliability. So, how do you make the right choices?

DC Combiner Box Components

Unraveling the Photovoltaic DC Combiner Box

 What is it?

A photovoltaic DC combiner box is essentially a protective casing that links various photovoltaic strings. It’s equipped with protective mechanisms, ensuring that these strings can connect in parallel. Given its outdoor placement, it’s designed to withstand the elements, typically boasting a protection level of IP65 or higher.

A Glimpse Inside

Imagine a box where on one side, you have fuses acting as guardians, and on the other, a DC circuit breaker ready to protect. Add a lightning protector to the mix, and you’ve got a basic idea of what’s inside this box.

Making the Right Choices for Components

 The Fuse Dilemma

In many places, there’s no clear standard for selecting fuses. However, a good reference point is the “National Electrical Code (NEC, 2011 edition)” section 690.35(B). For systems that aren’t grounded, which is common in many photovoltaic DC systems, fuses are placed on both poles of the string. Given that the short-circuit current in these systems is primarily from the photovoltaic modules and is relatively low, it’s essential to have fuses specifically designed for this purpose.

 Decoding Short-circuit Current

Let’s say the combiner box has multiple collection paths, denoted as ‘n’. If each path has a short-circuit current of Isc, and a short circuit happens in one string, the current that the fuse of that string experiences is (n-1)*Isc. So, in a scenario with 24 paths and one short circuit, the fuse would deal with a current of 23Isc.

 

DC cable short circuit diagram

DC cable short circuit diagram

 Fuse Specifications

The fuse’s rated voltage should ideally be 1500V/1000V or higher. Its rated current should be at least 1.56Isc but not more than the module manufacturer’s maximum allowed current. Why 1.56Isc? It’s based on the NEC’s assumption that the module’s maximum current is 125% of Isc. This is because, under certain conditions, the light intensity can surpass the standard by 25%. The fuse must handle 125% of this maximum current, leading to the 1.56Isc specification.

Navigating DC Circuit Breakers

Once strings are connected in parallel, they pass through a DC circuit breaker. Typically, a 4P DC circuit breaker is used, with two poles in series acting as one. This ensures a voltage resistance level of 1500V/1000V. The breaker’s rated current is also based on the 1.56Isc standard.

Lightning Protectors – A Must-have

Positioned at the combiner box’s output end, a lightning protector is essential for both poles. It should meet specific criteria, including a maximum continuous operating voltage and discharge current. Additionally, it should have a disconnector and indicate faults.

 The Role of Monitoring Devices

To maximize efficiency, a monitoring device is often installed in the combiner box. This device monitors various parameters, ensuring the system operates optimally. It’s usually self-powered and uses a 485 communication method. To safeguard against lightning-induced overvoltages, a signal lightning protector is installed on the combiner box’s 485 interface.

 Conclusion

Choosing the right components for a photovoltaic DC combiner box is crucial for the efficiency and reliability of the entire solar power system. By understanding the role and specifications of each component, you can ensure optimal performance and longevity.

FAQs

  1. What is the primary function of a photovoltaic DC combiner box?
    • It connects photovoltaic strings and ensures their parallel connection while providing necessary protection.
  2. Why is the fuse’s rated current based on 1.56Isc?
    • It’s derived from the NEC’s assumption about the module’s maximum current and the need for the fuse to handle 125% of this maximum current.
  3. What protection level should a photovoltaic DC combiner box have?
    • It should have a protection level of IP65 or higher due to its outdoor placement.
  4. Why is a lightning protector essential in a combiner box?
    • It provides protection against potential lightning-induced surges, ensuring the safety and functionality of the system.
  5. How does a monitoring device enhance the combiner box’s efficiency?
    • It tracks various parameters, ensuring the solar photovoltaic power system operates at maximum efficiency.