Introduction:
A Voltage Source Inverter (VSI) is a power electronic device that converts a DC voltage into an AC voltage with variable frequency and magnitude. It is commonly used in applications such as motor drives, renewable energy systems, and UPS systems. In a VSI, the internal impedance of the DC source is considered negligible.

Explanation:

1. DC Source:
The DC source in a VSI is typically a rectifier that converts AC power from the grid into a DC voltage. The rectifier circuitry used in modern VSIs is designed to have a low internal impedance. This means that the DC source presents a very small resistance to the flow of current. As a result, the internal impedance of the DC source can be considered negligible.

2. Impedance:
Impedance is the opposition to the flow of alternating current (AC). It is a complex quantity that consists of resistance and reactance. In the case of a VSI, the internal impedance of the AC source (grid) is significant due to the presence of transmission lines and other components. However, the internal impedance of the DC source is intentionally made very low to minimize losses and ensure efficient power transfer.

3. IGBTs:
IGBTs (Insulated Gate Bipolar Transistors) are commonly used in VSIs as the switching devices. They are responsible for converting the DC input into an AC output by rapidly switching the voltage across the load. The firing of IGBTs is controlled by a pulse-width modulation (PWM) technique. The timing and duration of the switching signals determine the magnitude and frequency of the AC output voltage.

4. Firing Angle:
The firing angle is the phase difference between the reference voltage and the switching signals applied to the IGBTs. It determines the average output voltage and controls the power flow in the VSI. The firing angle is typically adjusted to control the speed of AC motors or the output voltage of the VSI.

Conclusion:
In a VSI, the internal impedance of the DC source is considered negligible. This is because the rectifier circuitry used in modern VSIs is designed to have a low internal impedance. On the other hand, the internal impedance of the AC source is significant due to the presence of transmission lines and other components. The firing of IGBTs is controlled by adjusting the firing angle, which determines the average output voltage and controls the power flow in the VSI.