Collaborative Work of Surge Protectors, Circuit Breakers and Fuses in Photovoltaic Systems: Functional Analysis and Necessity Discussion

Introduction

With the rapid development of the global photovoltaic industry, the safety and stability of solar power generation systems have become the focus of industry attention. Photovoltaic systems are exposed to the outdoors for a long time and are vulnerable to threats such as lightning strikes, power grid fluctuations, and equipment failures, which may cause equipment damage or even fire. Surge protectors (SPDs), circuit breakers, and fuses are key protection devices that each perform their duties and cooperate with each other to ensure the safe operation of the system. This article will deeply analyze their functions, coordination mechanisms, and necessity to provide reference for industry users.

I.The "Invisible Killer" Facing Photovoltaic Systems

Photovoltaic power stations are like "steel warriors" working in the open air, constantly enduring various harsh tests.

1.1 Lightning strike issues: 

In particular, in the Middle East and Southeast Asia, a single thunderstorm season can paralyze systems that lack protection. 
1.2 Power grid fluctuations: 
In the Chilean project I was in charge of, several pieces of equipment were burned out due to a sudden increase in grid voltage. 

1.3 Short-circuit risk: 
Last year, a project in Germany experienced a short circuit due to aging cables, nearly causing a fire. 

These risks are no exaggeration. According to the International Photovoltaic Safety Alliance, over 60% of photovoltaic system failures are due to inadequate electrical protection.

II. Core Functions of Surge Protective Devices (SPD)

2.1 Working Principle
SPD diverts transient overvoltage to the ground through metal oxide varistors (MOV) or gas discharge tubes (GDT), limiting the voltage within a safe range. In photovoltaic systems, SPDs are typically installed at the following locations:
DC side (between the modules and the inverter): To protect against lightning-induced surges.
AC side (between the inverter and the grid): To suppress overvoltage from the grid side.

2.2 Key Parameters
Maximum continuous operating voltage (Uc): Must match the voltage level of the photovoltaic system (such as 1000V DC or 1500V DC).
Discharge current (In/Iimp): Reflects the ability to discharge lightning current, and photovoltaic systems typically require 20kA or above.
Voltage protection level (Up): Determines the residual voltage size and must be lower than the withstand voltage of the protected equipment.

2.3 Necessity
Prevent expensive equipment such as inverters and combiner boxes from being damaged by surges.
Comply with international standards (such as IEC 6164331, UL 1449) and acceptance requirements for photovoltaic power stations.

Ⅲ.Function and selection of circuit breakers and fuses

3.1 Circuit Breaker
Function:
•Overload Protection: When the current exceeds the set value (such as 1.3 times the rated current), the thermal trip mechanism operates.
•Short Circuit Protection: The electromagnetic trip mechanism cuts off the short-circuit current (such as 10kA) within milliseconds.

•Application Characteristics for Photovoltaic:
A dedicated DC circuit breaker (such as DC 1000V/1500V) needs to be selected.
The breaking capacity should match the system short-circuit current (typically ≥ 15kA).

3.2 Fuse
Function:
By melting the fuse element, it can quickly isolate the faulty circuit and protect the series-connected branch. 

Advantages:
The disconnection speed is faster (at the microsecond level), suitable for high short-circuit current scenarios.
It is small in size and suitable for current-carrying boxes with limited space.

3.3 Collaboration with SPD

SPD is responsible for voltage protection, while circuit breakers/fuse protectors are responsible for current protection.
When SPD fails due to surge breakdown, circuit breakers or fuse protectors can promptly cut off the faulty circuit to prevent fire.

Ⅳ. Case study of multi-level protection system

Take a 1MW photovoltaic power station as an example:
4.1 Protection on the DC side
Component series branches: Install fuses (such as 10A gPV type) for each series.
Entry of the combiner box: Install Type II SPD (Up ≤ 1.5kV) and DC circuit breaker (63A).

4.2 Protection on the AC side
Output end of the inverter: Configure Type 1+2 SPD (Iimp ≥ 12.5kA) and molded case circuit breaker (250A). 

4.3 Fault scenario simulation
When a lightning strike occurs: The SPD releases surge current and clamps the voltage below 2kV; if the SPD fails due to short circuit, the circuit breaker trips.
When there is a line short circuit: The fuse melts within 5ms to prevent the spread of thermal spot effect.

Ⅴ. Precautions for selection and installation

5.1 SPD Selection
For the DC side, a photovoltaic-specific SPD (such as PVSPD) should be selected to avoid the reverse current problem of ordinary AC SPD.
Temperature margin should be considered (Uc needs to leave a margin in high-temperature environments).

5.2 Circuit Breaker/Fuse Matching
The breaking capacity should be higher than the maximum short-circuit current of the system (such as the fault current of the string may reach 1.5kA).
The rated current of the fuse should be more than 1.56 times the component short-circuit current (Isc) (in accordance with NEC 690.8).

5.3 System Integration Suggestions
The length of the wire between SPD and circuit breaker should be ≤ 0.5m to reduce residual voltage.
Regular inspections of SPD status indicators should be conducted, and failed modules should be replaced in time.

Ⅵ. Industry Trends and Standard Updates
•High-voltage demand: With the widespread adoption of 1500V photovoltaic systems, the withstand voltage levels of SPDs and circuit breakers need to be synchronizedly enhanced.

•Intelligent monitoring: Intelligent SPDs integrating temperature sensors and wireless communication functions are gradually being applied to achieve remote fault early warning.

•Standard reinforcement: The new version of IEC 625482023 has imposed stricter coordination requirements on protection devices for photovoltaic systems.

Conclusion
In photovoltaic systems, surge protectors, circuit breakers and fuses constitute a complete "voltage-current" collaborative protection system. Correct selection and configuration of these components not only can extend the service life of equipment and reduce operation and maintenance costs, but also are essential conditions for ensuring the safe operation of power stations. With the development of technology, the integration and intelligence of these protection devices will further enhance the reliability of photovoltaic systems in the future.