A thyristor power converter is said to be in discontinuous when: 

Explanation:

Thyristor Power Converter in Discontinuous Mode

Definition: A thyristor power converter is said to operate in discontinuous mode when the load current becomes zero for a part of the output cycle, even though the load voltage may still be present. This typically occurs under light load conditions or in systems where the energy demand from the load is intermittent. The discontinuous mode is a critical operational condition to understand in power electronics as it affects the design, control, and performance of the converter.

Working Principle: In a thyristor-based converter, when the load current drops to zero during a portion of the cycle, the thyristors are not conducting, and the load is temporarily disconnected from the power source. This is referred to as the discontinuous conduction mode (DCM). The presence of load voltage even when the load current is zero is indicative of the stored energy in the reactive components (like inductors or capacitors) being released, maintaining a voltage across the load.

Correct Option Analysis:

The correct option is:

Option 1: The load current is zero even though the load voltage is present.

In the discontinuous mode, the load current ceases to flow for a part of the cycle, while the load voltage may still be present due to energy stored in inductive or capacitive components. This matches the behavior described in Option 1, making it the correct answer. Discontinuous conduction mode (DCM) is a common phenomenon in circuits with reactive elements, where the current waveform becomes discontinuous, but the voltage waveform remains continuous due to the energy stored in the reactive components.

Advantages of Discontinuous Mode:

• Reduced conduction losses as the thyristors are not conducting for the entire cycle.
• Better thermal management of thyristors due to reduced conduction duration.

Disadvantages of Discontinuous Mode:

• Higher voltage and current stresses on the thyristors and other components during transitions.
• Increased electromagnetic interference (EMI) due to rapid switching events.
• Complex control mechanisms required to manage the transitions between continuous and discontinuous modes.

Applications: Discontinuous mode operation is commonly observed in power converters used in scenarios where the load demand fluctuates significantly, such as in motor drives, power supplies, and renewable energy systems.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Both load voltage and load current are zero simultaneously.

This option is incorrect as it describes a state where neither voltage nor current is present, which typically indicates a complete disconnection or shutdown of the system. This is not characteristic of the discontinuous conduction mode, where the load voltage can still be present even when the load current is zero.

Option 3: The load current is present even though load voltage is zero.

This scenario is not possible in a properly functioning thyristor power converter. For the load current to flow, there must be a voltage across the load to drive the current. Hence, this option is incorrect.

Option 4: When load current is ripple free.

This option is incorrect as well. Ripple-free load current typically occurs in continuous conduction mode (CCM), where the load current does not drop to zero during the operation. Discontinuous conduction mode, by definition, involves a load current that becomes zero for a part of the cycle, which is the opposite of ripple-free behavior.

Conclusion:

Understanding the operational modes of thyristor power converters is essential for designing efficient and reliable power electronics systems. The discontinuous conduction mode occurs when the load current becomes zero for part of the cycle while the load voltage remains present. This mode has specific implications for the performance, control, and design of the converter, as highlighted in the explanation. Option 1 correctly describes this phenomenon, distinguishing it from the other options, which are either incorrect or describe different operational conditions.