Dual Converter | Electrical4U

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Key learnings:

Dual Converter Definition: A dual converter is an electronic circuit with two converters—one as a rectifier and the other as an inverter—connected to the same DC load.
Modes of Operation: There are two modes—non-circulating current mode, where only one converter operates at a time, and circulating current mode, where both operate simultaneously.
Ideal Dual Converter: Ideal dual converters produce ripple-free output voltage, using diodes for unidirectional current while allowing flexibility in current direction.
Types of Dual Converters: They include single-phase dual converters and three-phase dual converters, differing in the type of AC supply they convert.
Applications: Dual converters are used in DC motor control, reversible DC needs, and industrial variable speed DC drives.

A dual converter is defined as an electronic circuit with two converters—one acts as a rectifier and the other as an inverter. These converters are arranged in an anti-parallel pattern and connected to the same DC load, allowing for four quadrant operations.

Modes of Operation of Dual Converter

There are two functional modes: Non-circulating current mode and circulating mode.

Non Circulating Current Mode

One converter will perform at a time. So there is no circulating current between the converters.
During the converter 1 operation, firing angle (α₁) will be 0<α₁< 90^o; V_dc and I_dc are positive.
During the converter 2 operation, firing angle (α₂) will be 0<α₂< 90^o; V_dc and I_dc are negative.

Circulating Current Mode

Two converters will be in the ON condition at the same time. So circulating current is present.
The firing angles are adjusted such that firing angle of converter 1 (α₁) + firing angle of converter 2 (α₂) = 180^o.
Converter 1 performs as a controlled rectifier when firing angle be 0<α₁< 90^o and Converter 2 performs as an inverter when the firing angle be 90^o<α₂< 180^o. In this condition, V_dc and I_dc are positive.
Converter 1 performs as an inverter when firing angle be 90^o<α₁< 180^o and Converter 2 performs as a controlled rectifier when the firing angle be 0<α₂< 90^o In this condition, V_dc and I_dc are negative.
The four quadrant operation is shown below.

Ideal Dual Converter

Ideal Dual Converter: An ideal dual converter provides ripple-free output voltage. Diodes (D₁ and D₂) ensure unidirectional DC current flow, though current can flow in any direction. The average output voltages of converters 1 and 2 are V₀₁ and V₀₂, respectively. The thyristors firing angles are controlled to ensure both converters have the same polarity and magnitude of output voltage.

Average output voltage of Single-phase converter =
Average output voltage of Three-phase converter =
For converter 1, the average output voltage,
For converter 2, the average output voltage,
Output voltage,
The firing angle can never be greater than 180^o. So,

Types of Dual Converters

They are of two types: Single-phase dual converter and three-phase dual converter.

Single Phase Dual Converter

This converter uses a single-phase supply. In non-circulating mode, converter 1 rectifies AC to DC, which is filtered and sent to the load. This DC is then input to converter 2, which inverts it back to AC, providing AC output.

Three Phase Dual Converter

Here, three-phase rectifier and three-phase inverter are used. The processes are similar to single-phase dual converter. The three-phase rectifier will do the conversion of the three-phase AC supply to the DC. This DC is filtered and given to the input of the second converter. It will do the DC to AC conversion and the output that we get is the three-phase AC. Applications where the output is up to 2 megawatts. The circuit is shown below.