Test: Current Source Inverters - Electrical Engineering (EE) MCQ

In VSIs the input voltage is maintained at a constant value and the amplitude of the output voltage does not depend on the load conditions. However, the waveform of the load current as well as its magnitude depends upon the nature of the load impedance.

In order that the current input to the CSI must remain ripple free an constant, L-filter is used before the CSI (in series with the energy source).

Considering the SCRs as ideal voltage sources, when T1, T2 are on the load current is = I. When T3, T4 conduct, the load current is negative with respect to the previous value and = -I.

The line voltage is the difference of two phase voltages, the 3rd harmonic gets cancelled out, though it is present in the phase voltages.

In VSIs the input voltage is maintained at a constant value and the amplitude of the output voltage does not depend on the load conditions. However, the waveform of the load current as well as its magnitude depends upon the nature of the load impedance.

In CSIs, the amplitude of the output current is independent on the load impedance, as the input current (to the CSI) is kept constant. However, the magnitude of output voltage and its waveform depends upon the nature of the load impedance.

CSI converts the input dc current to an ac current at its output terminals.

The load current waveform is a square wave with frequency = 1/T. As the load consists of a capacitor, dv/dt must be constant over every half cycle. This slope is positive from zero to T/2 and negative from T/2 to T.

All the CSIs need capacitors for their commutation if force commutation is required. Force commutation is essential for lagging power factors.

Current source inverter:

• In current source inverter, the input current remains constant, but this input current is adjustable.
• The current source inverter is also called the current fed inverter.
• The output voltage of the inverter is independent of the load.
• The magnitude and nature of the load current depend on the nature of load impedance.
• It is most suitable for supplying power to capacitive loads.
• In order that the current input to the CSI must remain ripple free and constant, L-filter is used before the CSI (in series with the energy source).

Advantages:

• The circuit for CSI is simple as it uses only converter grade thyristor which should have reverse blocking capability, and also able to withstand high voltage spikes during commutation.
• An output short circuit or simultaneous conduction in an inverter arm is controlled by the ‘controlled current source’ used here, i.e., a current limited voltage source in series with a large inductance.
• The commutation failure in the same leg does not occur due to the presence of a large inductance and has the capability to recover from occasional commutation failure.
• The converter-inverter combined configuration has inherent four-quadrant operation capability without any extra power component.

Disadvantages:

• A minimum load at the output is required, and the commutation capability is dependent upon load current. This limits the operating frequency, and also puts a limitation on its use for UPS systems.
• At light loads, and high frequency, these inverters have sluggish performance and stability problems.

Important Points

In VSIs the input voltage is maintained at a constant value and the amplitude of the output voltage does not depend on the load conditions. However, the waveform of the load current as well as its magnitude depends upon the nature of the load impedance.