LECTURE 27 Controlled Rectifiers Dr. Rostamkolai ECE 452 Power Electronics

Single-Phase Series Converters For high voltage applications, two or more converters can be connected in series to share the voltage and also to improve the power factor The following figure shows two semiconverters that are connected in series Each transformer secondary has the same number of turns, and the turns ratio is 2

Twelve-Pulse Converter For high-power applications such as high-voltage dc transmission, a 12-pulse output is generally required to reduce the output ripples and to increase the ripple frequencies Two six-pulse bridges can be combined either in series or in parallel to produce a 12-pulse output Two configurations are shown in the following figure

A 30 degrees phase shift between secondary windings can be accomplished by connecting one secondary in wye and the other in delta

Design of Converter Circuits The design of converter circuits requires determining the ratings of switching devices and diodes The switches and diodes are specified by: Average current RMS current Peak current Peak inverse voltage

In case of the controlled rectifiers, the current rating of devices depend on the delay angle The ratings of power devices must be designed under the worst case condition This occurs when the converter delivers the maximum average output voltage

The output of converters contain harmonics that depend on the delay angle The worst-case condition is generally when the minimum output voltage occurs Input and output filters must be designed under the minimum output voltage conditions

Effect of Load and Source Inductances The load current harmonics depend on load inductances Also, the input power factor depends on the load power factor So far, in derivations of output voltages and the performance criteria of converters, we have assumed that the source has no inductances and resistances

The source resistance is generally very small The amount of voltage drop due to source inductance is independent of the delay angle, and: The voltage drop is not dependent on the delay angle α

However, the commutation (overlap) angle μ varies with the delay angle As the delay angle increases, the overlap angle decreases

If Vx is the average voltage drop per commutation due to overlap and Vy is the average voltage reduction due to phase angle control that is zero, then average output voltage is (when ignoring commutation overlap):

The average output voltage with overlap due to two commutations and phase angle control is:

Then: The overlap angle μ can be determined from the above equation for a single-phase full converter

Work on Example 10.10 α = 10o → µ = 4.66o α = 30o → µ = 1.94o