Since 2023, various inverter manufacturers have successively released inverters with DC protection switches. Different experts in the industry have discussed the protection capability and standard compliance of DC-side switches, and there have been differing opinions. Several media outlets have also reported on this matter.

The main focus of the debate is on the system design where “the number of series strings connected to a single MPPT exceeds 2” without adding protective devices such as fuses or circuit breakers. Can safety be achieved solely through isolation switches?

Today, such products have been applied in actual power stations. How effective is their actual protection capability? The author conducted reverse connection and reverse current testing on-site to verify the system’s safety performance.

  1. String Reverse Connection Test Verification

In actual construction, installation, and maintenance processes, inverters often encounter situations like reverse connections, incorrect connections, and short circuits, which can pose significant safety risks. Therefore, the equipment’s system protection capability is crucial.

The selected equipment connects 5 series of photovoltaic modules to each MPPT. The on-site current of a single string is nearly 20A. One string of the equipment was chosen for positive and negative reverse connection testing. After the reverse connection, this string forms a circuit with the other 4 strings, causing a reverse current of 70A to flow into the other 4 strings.

Dc Isolator Switches4

String Reverse Connection Test Circuit Diagram

 

After the first test trigger, the isolation switch activated the protection but tripped after 240ms, which differs from the manufacturer’s claimed 15ms cutoff capability. In the second test, the isolation switch did not trip, rendering the protection ineffective. The continuous reverse current led to a rapid increase in the temperature of the components in the corresponding string, with one diode temperature exceeding 150°C, causing it to explode on the spot. Such a fault poses a significant risk of fire.

 

微信截图 20231103172007

 

  1. String Reverse Current Test Verification

Most power stations are located in complex environments, with factors such as uneven sunlight, shading, module aging, and more. These conditions can result in voltage differences among strings, leading to reverse current. Without effective protection, this can create a safety risk for the system.

Therefore, one string of the inverter was selected for reverse current testing, maintaining a certain voltage difference with the other 4 strings in the same MPPT. This voltage difference caused reverse current to flow into the faulty string, reaching 30A. Three rounds of testing were conducted under these conditions.

 

Dc Isolator Switches2String Reverse Current Test Circuit Diagram

 

After the first two tests triggered, the isolation switch activated the protection but tripped after 250ms, which is significantly different from the manufacturer’s claimed 15ms cutoff capability. In the third test, the isolation switch did not trip, and the protection failed. The temperature of the corresponding components and circuit rapidly increased, with one component rising by 49°C within a minute (reaching 76.8°C). If it is not disconnected in a timely manner, the temperature will continue to rise, potentially causing the component to catch fire and be destroyed.

The above experiments demonstrate that when a single MPPT connects to more than 2 series strings, the isolation switch cannot effectively protect the system, leading to component damage, fire, or even greater risks.

In summary, isolation switches can be used as redundant protection to enhance system safety, but they cannot replace the overcurrent protection devices specified by standards, such as fuses and circuit breakers. In systems where a single MPPT connects to more than 2 series strings, it is imperative to provide overcurrent protection devices that comply with the standard requirements. Safety concerns cannot be overlooked.