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 combines two fully controlled bridges in inverse-parallel across one DC load. Either bridge can act as a rectifier or an inverter, depending on its firing angle and power direction. The opposed bridges allow reversible DC voltage and current, which supports four-quadrant operation.

Modes of Operation of Dual Converter

A dual converter can operate without circulating current or with controlled circulating current between its bridges.

Non Circulating Current Mode

Only one bridge receives firing pulses at a time, so no intentional current circulates between the converters.
For positive rectification through converter 1, its firing angle α₁ satisfies 0<α₁< 90^o. In the sign convention shown, V_dc and I_dc are positive.
For negative rectification through the oppositely connected converter 2, its firing angle α₂ satisfies 0<α₂< 90^o. Under the same reference directions, V_dc and I_dc are negative.

Circulating Current Mode

Both bridges receive firing pulses. Small instantaneous voltage differences drive circulating current through the inter-converter reactor.
For ideal complementary control, the firing angles satisfy α₁ + α₂ = 180^o. This matches the bridges’ average DC voltage magnitudes.
For positive load current, converter 1 acts as a controlled rectifier with 0<α₁< 90^o, while converter 2 acts as an inverter with 90^o<α₂< 180^o. In the stated sign convention, V_dc and I_dc are positive.
For negative load current, converter 1 acts as an inverter with 90^o<α₁< 180^o, while converter 2 acts as a controlled rectifier with 0<α₂< 90^o. In the stated sign convention, V_dc and I_dc are negative.
The voltage-current plane below shows the four operating quadrants for motoring and regenerative power flow in either direction.

Ideal Dual Converter

Ideal Dual Converter: The ideal analysis uses ripple-free average output voltage. Diodes D₁ and D₂ in the shown equivalent circuit indicate the permitted bridge-current directions, while the combined converter permits load current of either polarity. Let the bridge average voltages be V₀₁ and V₀₂. The thyristors receive complementary firing commands so the opposed bridges have matched average voltage magnitudes.

Average output voltage of a single-phase converter =
Average output voltage of a three-phase converter =
For converter 1, the average output voltage is
For converter 2, the average output voltage is
The common load-voltage relation is
With each firing angle restricted to the controlled-converter range up to 180^o, the complementary relation is

Types of Dual Converters

The AC supply determines the two common forms: a Single-phase dual converter or a three-phase dual converter. Both forms connect opposed controlled bridges to one DC load.

Single Phase Dual Converter

A single-phase dual converter uses two single-phase fully controlled bridges connected in inverse parallel across the DC load. Converter 1 supports one current direction and converter 2 supports the opposite direction. The load remains on the shared DC terminals; the bridges do not form a cascaded AC-to-DC-to-AC converter.

Three Phase Dual Converter

A three-phase dual converter uses two three-phase fully controlled bridges connected in inverse parallel across the same DC load. Each bridge can rectify or regenerate according to its firing command and the load-current direction. This arrangement is used for higher-power reversible DC drives, but its rating depends on the selected valves, transformer and cooling system rather than a universal 2 MW limit.