Short-circuit protection is a function of a device connected between a power source and a load, which effectively disconnects the short-circuit point from the power source when a short circuit occurs on the load side. For a device to have short-circuit protection, it must have a certain short-circuit breaking capacity; and a device with short-circuit breaking capacity indicates that it has short-circuit protection.
Taking a commonly used air circuit breaker as an example, when the short-circuit current exceeds the short-circuit breaking current of the air circuit breaker, the air circuit breaker will definitely have activated its protection. However, its breaking capacity is too small to completely break the short-circuit current, and it will eventually be damaged. Why is its breaking capacity less than the actual short-circuit current? There are two main reasons: first, the air circuit breaker selection; and second, the power grid capacity. The magnitude of the short-circuit current in a circuit can be calculated based on circuit parameters, which are related to the power supply capacity and line impedance—factors that should be considered during design and selection.
Just because the inverter is damaged after a short circuit doesn't mean it lacks short-circuit protection. The damage indicates a problem in some part of the system.
The inverter's output side doesn't have short-circuit protection because the power devices in inverters are currently IGBT power modules. When a short circuit occurs at the inverter output or load, the IGBT itself has short-circuit protection capabilities, locking itself out of the current output. This protects the power devices from damage by the short-circuit current. Conversely, adding short-circuit protection to the inverter output would cause problems. First, the short-circuit current is very fast; if the IGBT cannot latch itself, it won't have enough time to protect itself and will fail. Adding protection would be pointless because the speed is too high to stop. Second, and most importantly, the load must never be open when the IGBT is operating; otherwise, the high dv/dt will cause the IGBT to break down.
This is why IGBTs are self-short-circuit resistant. In other words, they are not afraid of load short circuits. If the output power components of the inverter fail, it's not due to the short circuit itself; there must be other reasons, such as a sudden open circuit in the load during operation, or an overload (IGBTs are susceptible to overloads but not short circuits).
First, let's discuss why the module explodes when the load suddenly disconnects (opens) while the inverter output is operating. This is because the dv/dt ratio is too high at this time (especially under heavy load), and the circuit is essentially operating like a fuse under load. Although the inverter's output power module has RC snubber protection, the excessively high dynamic voltage is still insufficient to prevent the IGBT from breaking down. Therefore, the module will inevitably explode at this point.
Furthermore, regarding thermal overload causing module failure, the output power module of the frequency converter is designed with heat dissipation and temperature rise control in mind, including the heat dissipation cross-section, airflow, and cooling medium flow rate. If it operates under overload conditions for an extended period, the heat accumulation in the tubes will increase, leading to an excessive temperature rise and inevitably resulting in module failure.
I remember back in the mid-to-late 1990s, the Xi'an Power Electronics Technology Research Institute held an academic exchange on IGBT power devices. At that time, IGBTs in China were still in the research and development stage. Foreign products were widely used, while domestic products were not yet widely available. At that time, domestic thyristor devices were already quite mature, with various power ratings available, but IGBTs were not yet advanced. I remember one of the engineers who gave a presentation saying that in their experiments, IGBTs were not afraid of short circuits; if a short circuit occurred, there was no output, and it quickly turned off. However, IGBTs are extremely sensitive to heat; if heat dissipation is inadequate, they can easily explode. Therefore, I have a particularly deep memory of the content of that exchange activity. I also learned what the weakness of IGBTs was.
As application engineers specializing in automation and drives, we should understand the characteristics and properties of the components we use. Only in this way can we scientifically, rationally, and correctly select and configure the systems we design, ensuring their safe and reliable operation. Adding unnecessary switches or protections to the inverter output, especially under load, would undoubtedly be a fatal mistake. Using substandard components and cutting corners to save money carries serious potential dangers.
Cutting corners in construction projects leads to collapses; cutting corners in automation-driven projects leads to explosions and fires. Both are major, high-profile incidents.
