Inverter short circuit protection precautions - Solutions - Huaqiang Electronic Network
The short circuit protection is a function of the device connected between the power source and the load capable of effectively breaking the short circuit point from the power source when a short circuit occurs on the load side thereof.
The short-circuit breaking capability refers to the ability of the device to reliably cut off the short-circuit current without causing damage to the device itself after a short circuit occurs on the load side, so as not to expand the fault range.
A device has a short-circuit protection function, so it must have a certain short-circuit breaking capability; and a device with a short-circuit breaking capability indicates that it has a short-circuit protection function. How can it be separated between the two?
Taking a common air switch as an example, when the short-circuit current is greater than the short-circuit breaking current of the air switch, the air switch must have been activated, but its ability is small and the short-circuit current cannot be broken, and eventually it will be damaged. Why is its breaking capacity less than the actual short-circuit current? The first is the problem of air switch selection, and the second is the problem of grid capacity. Because the short-circuit current in the circuit is calculated according to the circuit parameters, it is related to the power supply capacity and line impedance, which is the consideration of design selection.
It cannot be said that it has no short-circuit protection function because the inverter is damaged after a short circuit occurs. If it is damaged, it means that there is a problem in one of the links in the system.
The breaking capacity is usually used on high and low voltage circuit breakers and fuses, that is, the arc extinguishing capability when breaking short circuit current. We have used AEG's early frequency converters, the control board is fixed on a metal plate with a loose leaf, and the quick-melt on the DC bus can be seen by removing it. So far, we have used a large number of Siemens, basically in accordance with the output reactor, but the field line short circuit or motor burning did not cause the power components to burn out, the only exception is that there is a brake unit caused by a short circuit at the braking resistor. And the rectification part is burnt, and the incoming line side uses an ordinary circuit breaker. If the inverter is damaged due to the short circuit, as far as the inverter is concerned, I think that the response time from the detection of the protection is slow and there is no other protection measure. Now it is rare to see the DC circuit with the fast-melting inverter. It is not known whether the output reactor has a limit on the short-circuit current.
The knife-melt switch protects the incoming line side. The outlet is without any hardware protection (the output reactor can limit the short-circuit current), single-phase ground fault (full voltage) occurs during operation, and your inverter (with output reactor) is not discussed. Because the outlet is not short-circuit protected.
The output side of the inverter is not short-circuit protected because the output power device of the inverter is currently the power module of the IGBT. When the inverter output or the load is short-circuited, the IGBT itself has the function of resisting short-circuit, and it is locked by the function of not outputting current. The protection power device is not damaged by the short circuit current. On the contrary, if the inverter output is short-circuit protected, there is a problem. First, the short-circuit current is very fast. If the IGBT cannot self-lock, it is too late to protect itself. Adding protection is also a plus, because the speed is too fast to stop. Second, and most importantly, when the IGBT is in operation, the load is never allowed to open, otherwise the IGBT will break down due to high dv/dt.
Because of this, IGBTs are self-resisting to short circuits. In other words, it is not afraid of load short circuit If the output power device of the inverter is blown up, not because of the short circuit itself, there must be other reasons. For example, the load suddenly opens during operation, or it is overloaded (the IGBT is afraid of overload and is not afraid of short circuit).
The short circuit and the overload are of course different. The short circuit current is much larger than the overload current, and the IGBT has a fast self-shutdown function. The self-locking protection of the overload current IGBT is not turned off. At this time, when the overload time is long, the tube will heat up, causing the thermal overload to blow up the tube. Therefore, the fault protection of the frequency converter sets parameters for both overload and short circuit. Siemens inverters have these output protection features. The input of the inverter is diode or thyristor, there is no self-protection function, and external short-circuit protection should be used for short-circuit protection. If the rectifier unit is in the form of an IGBT (ALM), the input also has a self-locking protection function and is not afraid of a short circuit! The input does not speed up the melt.
The development process of IGBT is to study how to resist short circuit at the beginning, otherwise it can't be used. Because the external contact switch or fast-melting is not enough to safely protect the IGBT, the switch jumps, and the IGBT also follows. Therefore, a current cut-off protection function is added to the tube output, and if the short-circuit current comes, it is self-locking immediately. Make the pipe not finished. It is precisely because IGBT has the function of short-circuit resistance, so in the rectification or inverter circuit, he does not have "inverted subversion" like thyristor. This is also a highlight of IGBT.
When the inverter output or the load is short-circuited, the IGBT itself has the function of resisting short-circuit, and it is locked by the function of not outputting current. The protection power device is not damaged by the short circuit current.
When the inverter output is working, the load is suddenly disconnected (open circuit), and the module is blown up. This is because the dv/dt is too high at this time (especially when the load is large), and the state of the line is like a load-carrying switch. Although the output power module of the inverter has RC RC absorption function, it is too high dynamic voltage to stop the IGBT.
This kind of reverse surge shock is very similar to the river current in the natural world. For example, on the river with rapid currents, the gate of the dam suddenly falls down, which will inevitably cause the reverse surge to strongly impact the upstream. The upstream here is the IGBT, and the very high reverse surge dv/dt voltage makes the IGBT untenable. Therefore, the fried IGBT module is not discussed at this time.
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