Dual full-bridge Converters

The three phase Dual full bridge with suitable examples and sketches are discuss here advanced. three phase Dual full bridge 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.

3-phase dual series-bridge converter with inductive load

Dual series converter gives higher DC voltage output with still higher ripple frequency.  Two full-bridges are supplied with three-phase voltages of same RMS value but 30° phase-shift in between. High power applications, eg: HVDC, prefer this converter.

In general, dual series converter gives,  Higher DC voltage output  Higher ripple-frequency in output voltage  Lesser THD at the supply side AC current

Required 2-sets of three-phase voltages with 30° phase-shift in between is obtained using a single 3-winding transformer (or two separate transformers).  The same transformer does the current addition to result in a low THD in the net input current.

Turn-ratio of the Ddy transformer should be 𝑁1:𝑁2:𝑁3 ≡ 1:𝑚:𝑚 3  to give equal voltage to two bridges. Value of  𝑚 is decided according to the utility voltage and the required converter voltage.  Vector connection Dd0y1 gives necessary phase-shift, i.e. bridge-2 voltage 30° leading ahead of bridge-1 voltage.

To operate dual-bridge we can select one of two alternative controls. • Concurrent control • Sequential control

(i)  Concurrent Control of dual series-bridges

This is a popular mode of control.  Here, both bridges are operated at the same delay-angle, i.e. α1 = α2. Then,

𝑉𝑜2and 𝑉𝑜1of individual bridges each has 6 ripples per AC cycle. The net output 𝑉 𝑜 has 12 ripples per AC cycle.  This doubled ripple frequency is beneficial for reducing the size of filter components at output and input, both.

Adding up of bridge output voltages to give the net output voltage

Using Ampere’s Law, we can easily show that Input current at the utility side is given by,

Currents  𝐼𝑎 2,  𝐼𝑎 1 and  𝐼𝑏 1 are quasi-square currents having 120°wide single pulse in each half-cycle.    𝐼𝑎 2 is 30° leading ahead of  𝐼𝑎 1because bridge-2 voltage is 30°leading ahead of bridge-1.   𝐼𝑏 1 is 120° lagging behind  𝐼𝑎 1 as usual.

Utility side input current waveform has significantly improved in the form of a stepped sinusoidal waveform. Using Fourier expansion,

Orders of harmonics present at input current are  12𝑘 ±1 , where 𝑘 = 1,2,3,4,……

Contributions to   𝐼𝐴 𝐹𝑢𝑛𝑑 by bridge-2 and bridge-1 can be given separately, with respect to waveforms of  𝐼𝑎 2,  𝐼𝑎 1 and  𝐼𝑏 1. Contribution from bridge-2 is 𝑚 𝐼𝑎 2 and that from bridge-1 is  𝑚 3  𝐼𝑎 1 − 𝐼𝑏 1 .  Mathematically, both contributions are equal and inphase, and angle 𝛼 lagging behind utility phase-A voltage.

Taking input phase-voltage as 𝑉𝐴 = 𝑉𝑚 sin𝜔𝑡, and using Fourier expansion,

Order of harmonics in IA 𝑖𝑠 = 12𝑘 ±1 ,   𝑘 = 1,2,3,4,….⫽ Input Displacement Angle = 𝛼 ⫽ Input 𝐷𝑖𝑠𝐹 = cos𝛼 ⫽

The three phase Dual full bridge with suitable examples and sketches are discuss here advanced. three phase Dual full bridge is needed to read.

Reference

1. Page three phase Dual full bridge
2. Page three phase Dual full bridge