Introduction:
IGBTs (Insulated Gate Bipolar Transistors) are widely used in various applications such as power electronics, motor drives, and renewable energy systems. They provide high voltage and current handling capabilities with low conduction losses. In order to control the switching of IGBTs, it is important to understand the factors that can turn them off. This question specifically asks about the condition that turns off IGBTs.

Detailed Explanation:
IGBTs are turned off by removing the base signal. Let's understand this in more detail:

1. Load Commutation:
Load commutation refers to the process of transferring the load current from one device to another during the switching operation. It is not directly related to turning off IGBTs. Load commutation can occur during the turn-on process of IGBTs, but it does not affect the turn-off operation.

2. Line Commutation:
Line commutation refers to the process of transferring the load current to another device due to the change in the AC line voltage. Similar to load commutation, line commutation is not directly related to turning off IGBTs. It occurs during the turn-on process and does not affect the turn-off operation.

3. Applying a Negative Gate Pulse:
Applying a negative gate pulse to the gate terminal of an IGBT results in turning it on, not off. Negative gate voltage enhances the conductivity of the IGBT, allowing current to flow through it. Therefore, applying a negative gate pulse does not turn off IGBTs.

4. Removing the Base Signal:
The base signal is the control signal applied to the gate terminal of an IGBT. By removing the base signal, the IGBT is effectively turned off. When the base signal is removed, the IGBT enters the blocking state and does not conduct current. This is the correct condition that turns off IGBTs.

Conclusion:
In summary, IGBTs are turned off by removing the base signal. Load commutation, line commutation, and applying a negative gate pulse do not turn off IGBTs. Understanding the turn-off condition of IGBTs is crucial for designing reliable and efficient power electronic systems.