Comparison between VSI and CSI

VSI vs CSI

·         When the DC voltage remains constant, then it is called Voltage Source Inverter(VSI) or Voltage Fed Inverter (VFI).

·         When input current is maintained constant, then it is called Current Source Inverter (CSI) or Current Fed Inverter (CFI).

·         Some times, the DC input voltage to the inverter is controlled to adjust the output. Such inverters are called Variable DC Link Inverters. The inverters can have single phase or three-phase output.

§  A voltage source inverter(VSI) is fed by a stiff DC voltage, whereas a current source inverter is fed by a stiff current source.

§  A voltage source can be converted to a current source by connecting a series inductance and then varying the voltage to obtain the desired current.

§  A VSI can also be operated in current-controlled mode, and similarly a CSI can also be operated in the voltage control mode.

§  The inverters are used in variable frequency ac motor drives, uninterrupted power supplies, induction heating, static VAR compensators, etc.

i)VSI requires constant DC-link voltage, and generates AC voltages in the form of voltage pulses; CSI requires constant DC-link current, and generates AC currents in the form of current pulses.

ii) VSI requires DC-link capacitor and AC filter inductors; CSI requires DC-link inductor and AC filter capacitors.

iii) VSI is susceptible to leg short circuit fault and requires dead time in switching upper and lower switches; CSI is susceptible to leg open-circuit fault and requires overlap time in switching upper and lower switches.

iv) VSI is a buck inverter (magnitude of ac voltage is always smaller or equal to the dc voltage) but a boost rectifier; CSI is a boost inverter but a buck rectifier.

for grid operation VSI is more common than CSI. VSI alone can work as either grid forming or grid following. For Grid forming, the VSI needs to work in Voltage Control Mode, where the output voltage of the VSI is controlled to a fixed value and frequency, to form a consistent grid. Since the voltage and frequency is fixed in this case, the current will not be directly controllable, but depends on the load connected (I = V/Z). The VSI will need to ensure that it is able to source the required current to maintain the voltage regulation, failing which, the "grid" will collapse.

For VSI operating in grid following mode, it will have to run in Current Control Mode. Instead of deciding the voltage output directly, the VSI will vary the voltage output such that the current injected to (or drawn from) the grid is controlled.