This question asks to evaluate the truthfulness of three statements concerning three-phase voltage source inverters (VSIs). Let's analyze each statement:

Statement A Analysis: Voltage-Bidirectional Switches

Statement A claims that three-phase voltage source inverters require "voltage-bidirectional two-quadrant devices" for their switches. Standard VSIs typically employ semiconductor switches like Insulated Gate Bipolar Transistors (IGBTs) or Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). These devices, when used with an antiparallel diode (which is standard practice for handling inductive loads and providing a path for freewheeling current), are considered current-bidirectional but voltage-unidirectional. This means they can block voltage in only one direction (typically the reverse direction of the diode). A device that is "voltage-bidirectional" would imply it can block voltage in both forward and reverse directions, which is characteristic of four-quadrant devices or specific configurations not typically described as the fundamental requirement for basic VSI switches. Therefore, statement A is generally considered false for the primary switching elements in common VSI configurations.

Statement B Analysis: PWM and Harmonics

Statement B suggests that "PWM techniques are used to reduce the frequency of the harmonics in the output." Pulse Width Modulation (PWM) is a crucial control technique used in VSIs. By rapidly switching the semiconductor devices on and off according to a specific pattern determined by the PWM strategy, the fundamental component of the output voltage can be controlled. A primary benefit of PWM is the shaping of the output waveform's harmonic spectrum. Specifically, PWM techniques shift the energy of lower-order harmonics to higher frequencies and reduce the overall magnitude of harmonics. While PWM doesn't reduce the *number* of harmonics, it pushes them to higher frequencies, making them easier to filter out using smaller passive components (like inductors and capacitors). So, the statement correctly implies that PWM is employed for harmonic management, effectively dealing with harmonics at higher frequencies. Thus, statement B is true.

Statement C Analysis: 180° Conduction Mode

Statement C states that "In the 180° conduction mode, a new switch is gated in every 60° duration." The 180° conduction mode, often associated with six-step or fundamental switching frequency operation (though basic PWM can also achieve 180° conduction per device), involves each switching device in the inverter legs conducting for 180° of the fundamental output voltage cycle. In a typical three-phase VSI configuration using six switching devices (two per phase leg), the output waveform is synthesized by sequencing these switches. The fundamental cycle (360°) is divided into six intervals, each lasting 60°. During each 60° interval, a specific combination of switches is turned on to generate a particular output voltage waveform segment. A transition occurs at the beginning of each 60° interval, meaning a new switching state is activated. Therefore, a new switch (or a change in the active set of switches) is indeed gated or switched every 60° of the fundamental period. Statement C is true.

Conclusion

Based on the analysis, statements B and C are true, while statement A is false. Therefore, the correct option is the one that includes both B and C.

The correct option is B and C only.