String inverters without DC fuses pose a safety trap

1. Introduction

As an overcurrent protection device, the fuse is connected in series in the circuit. It can cut off the fault circuit in time when a short circuit fault occurs in the system to ensure the safety of the system. Therefore, it is widely used in various electrical systems, and the DC side protection of photovoltaic systems is no exception. The DC side of the photovoltaic system generally consists of multiple strings in parallel. Whether it is a centralized or string inverter solution, the use of fuses as DC side overcurrent protection has become a standard solution in the industry. In recent years, some manufacturers have proposed that for string inverters with only two strings in parallel in each MPPT, the DC side does not need fuse protection. Through in-depth analysis of the system structure and research on IEC standards, the author found that if there is no overcurrent protection device such as a fuse in the system, there will be huge safety hazards, and it does not meet the IEC62548 "Photovoltaic Array Design Requirements".

2. Risk analysis of two series-parallel systems without fuse protection

2.1 Equivalent circuit of two series-parallel systems

The equivalent circuit of the two-group series-parallel system is shown in Figure 1 (a). The system consists of multiple MPPT input circuits. Each MPPT is connected to two strings. The photovoltaic strings are connected in parallel after passing through the Boost circuit. The front-stage Boost circuit is generally connected in parallel with a bypass element. The purpose is to bypass the Boost circuit when the voltage rises to improve the efficiency of the system.

Figure 1 Equivalent circuit of two series-parallel systems

When the Boost circuit is working, the Boost circuit can be equivalent to an inductor and a diode in the series loop, as shown in Figure 1 (b). When the Boost circuit is not working, the current will flow through the bypass element, as shown in Figure 1 (c). When only analyzing the reverse current blocking characteristics of the circuit, whether the Boost circuit is working or not, the circuit can be simplified to be equivalent to the equivalent circuit shown in Figure 1 (d), that is, the photovoltaic string and the inverter DC bus are equivalent to a diode connection.

Regarding the inverter system shown in Figure 1, some manufacturers have proposed that since there are only two strings in each MPPT, even if one of the strings is short-circuited, the short-circuit current only flows between the two strings, and the energy of the other strings and the DC side is reversely blocked by the diode, which will not damage the components, so the DC side does not need fuse protection. However, through analysis, the author found that if there is no overcurrent protection device such as a fuse in this system, there will be huge safety hazards and it does not meet the requirements of IEC62548.

2.2 Risk 1: When arcing occurs at the short circuit point, the fire hazard increases

For a 1000V photovoltaic DC system, when the DC side is short-circuited due to insulation damage, loose joints, etc., if there is no fuse to cut off the fault circuit in time, arcing will continue to occur. When the arc burns freely in the air, the current can reach more than 200A, increasing the risk of fire and may even exceed the tolerance of components and cables, causing damage to components and cables.