The Growing Importance of Photovoltaic Safety

With the ambitious “3060 target,” photovoltaic (PV) power generation is transitioning from a supplementary to a primary energy source. Consequently, the safety of PV power plants is receiving increased attention. Recently, inverter manufacturers, certification bodies, and switchgear manufacturers have advocated for the addition of motorized isolating switches on the DC side of inverters. This measure aims to quickly disconnect PV modules from the inverter in the event of a fault, thereby enhancing the safety of PV power plants. Multiple media outlets have reported on this development, emphasizing the importance of station safety and promoting “intelligent disconnect technology.”

Isolating Switches

 Inverter Manufacturers’ Adoption of “Intelligent Disconnect Technology”

According to public information from inverter manufacturers, intelligent disconnect technology involves adding motorized isolating switches to the DC side of PV inverters. These switches, controlled by the inverter’s controller, monitor the current, voltage, and other signals of the PV strings in real-time to assess the system’s operating status. In the event of a fault, the inverter can automatically control the isolating switch to disconnect the faulty circuit.

Currently, two leading manufacturers in the market use intelligent disconnect technology in their inverters. One 196kW inverter design employs 4-5 string inputs with one Maximum Power Point Tracking (MPPT) channel, while another 320kW inverter uses two string inputs with one MPPT channel.

 IEC Standards for DC Side Protection in Photovoltaic Systems

  1. When is Overcurrent Protection Required?

    According to IEC 62548 6.5.3, the need for overcurrent protection devices in each string of a PV system is determined by the number of parallel strings. If the number of parallel strings exceeds two, each string requires overcurrent protection.

  2. Types of Overcurrent Protection Devices

    As per IEC 60364-7-712 712.533, DC side overcurrent protection devices in PV systems must be either PV-specific fuses compliant with IEC 60269-6 or circuit breakers compliant with IEC 60947-2.

Isolating Switches 2

Given these standards, can isolating switches effectively replace fuses and circuit breakers for overcurrent protection in fault conditions? To answer this, we need to understand the protection mechanisms of these devices:

  • Fuses: Fuses protect circuits by melting their internal element when the current exceeds a certain threshold, thereby interrupting the circuit.
  • Circuit Breakers: PV systems commonly use circuit breakers with magnetic and thermal tripping mechanisms. Magnetic tripping occurs when a large current generates a magnetic field strong enough to trigger the breaker. Thermal tripping relies on the deformation of a bimetal strip heated by excessive current to trip the breaker.
  • Intelligent Disconnect Isolating Switches: These switches use high-precision DC sensors to monitor each string’s current. Upon detecting a fault, the inverter automatically controls the DC disconnect switch to isolate the faulty circuit.

Analysis of Protection Mechanisms

  • Passive Protection: Fuses and circuit breakers provide passive protection, as they can operate independently of external circuits to protect against faults like short circuits.
  • Active Protection: Intelligent disconnect isolating switches rely on external circuits for detection and control. If the system loses power or the control circuit fails, these switches may not reliably isolate the fault.

Conclusion

While intelligent disconnect technology enhances system safety to some extent, it cannot entirely replace fuses and circuit breakers for DC protection. Specifically, when the number of parallel strings exceeds two, each string should still be equipped with fuses or circuit breakers in addition to the isolating switches. For systems with up to two parallel strings, adding isolating switches can improve safety. However, for systems with more than two parallel strings, the risk of sustained overcurrent and potential fire hazards remains if isolating switches fail to operate correctly. Thus, combining isolating switches with traditional overcurrent protection devices is crucial for comprehensive safety in PV systems.