 # Sequential Control of dual series-bridges

The Control dual series bridges with suitable examples and sketches are discuss here advanced. Control dual series bridges is needed to read.

Dual full-bridge converter uses two nos. of full-bridge converters in series or parallel.  These converters are called “12-pulse converters” because there are 12 thyristors, each requiring a separate trigger pulses.

Series-converter gives a higher output voltage and the parallel converter gives higher output current.  Per unit voltage-ripple at the output is lesser for both types.

Dual converter produces an input current having 12-steps, bringing it more towards the sinusoidal shape.  So, THD of input current is lesser.   Because of these reasons, most high power converters prefer dual-bridge arrangement.

In sequential control we use different α1 and α2, and control two bridges in sequence.  Here, only one bridge is controlled at a time. Accordingly, we get two sub-modes of control.

1. mode-1, where α2 = 0° and α1 is varied from 0° to 180°.

2. mode-2, where  α1 = 180° and α2 is varied from 0° to 180°

Sub-mode 1 gives variable positive voltage and sub-mode 2 gives variable negative voltage.  With this control, the displacement-factor at the input gets improved but THD tends to get deteriorated.

## Sub-mode 1 : Sequential Control of dual series-bridges

Let take  𝐼1′ and  𝐼2′ be components of input line-current at utility end, produced by bridge-1 and bridge-2, respectively.

According to operation of Dd0y1 transformer and full-bridge converters, 𝐼2′ is inphase with input voltage (because  𝛼2 = 0) and  𝐼1′ is 𝛼1lagging behind input voltage (because of  𝛼1).

Let

Thus input current  𝐼𝐴 contains harmonics of order  6𝑘 ± 1 , where k = 1, 2, 3, 4, ….⫽

## Sub-mode 2 : Sequential Control of dual series-bridges

𝐼2′ is 𝛼2lagging behind input voltage (because of  𝛼2) and  𝐼1′is out of phase with input voltage (because  𝛼1 = 180°).

Thus input current  𝐼𝐴 contains harmonics of order  6𝑘 ± 1 , where k = 1, 2, 3, 4, ….⫽

Variation of (IA)Fund and its Displacement angle for different delay angles in the concurrent and sequential control. (VA is phase-A voltage at Utility)

With respect to this diagram of fundamental current, too, we can identify the operational parameters for the dual series converter, as given below.

For sequential control Mode-1:

For sequential control Mode-2:

For concurrent control:

## Example

Three-phase, dual series-bridge thyristor converter is operating on 400 V, 50 Hz utility three-phase supply via 400V/350V/350V, Dd0y1, three-phase transformer.  The converter delivers 16 kW at 800 V to an inductive load.  Converter is operated with concurrent control.  Ignore supply internal inductance and transformer leakage inductances.  Determine, (i) Delay angles for the two bridges (ii) Displacement Factor at utility input (iii) RMS value of the fundamental current supplied by the utility (iv) RMS values of the two lowest order harmonics present in the utility input current. If the converter is operated in sequential control, how the values in (I) to (iii) above would be modified.

Ans:  𝑉𝐿 = 350 𝑉 𝑓 𝑠 = 50 𝐻𝑧  𝑉 𝑜 𝑚𝑒𝑎𝑛 = 800 𝑉 𝑃 𝑜 = 16 𝑘𝑊 𝑚 = 350 400 = 0.875  𝐷𝑑0𝑦1 normal connection

The Control dual series bridges with suitable examples and sketches are discuss here advanced. Control dual series bridges is needed to read.

## Reference Control dual series bridges

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