Introduction to Diode Rectifiers | What Is 3-Phase Power?, Part 5 - MATLAB & Simulink
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    Introduction to Diode Rectifiers | What Is 3-Phase Power?, Part 5

    From the series: What Is 3-Phase Power?

    In 3-phase electrical power systems, the AC system is commonly connected to a DC system. The process of converting AC to DC is known as rectification.

    You will learn:

    1. The operating principle of a single diode acting as a half-wave rectifier
    2. How an H-bridge rectifier operates as a full-wave rectifier and allows both positive and negative cycles of an AC waveform to be converted to positive DC voltage
    3. How a 6-pulse diode rectifier converts 3-phase AC to DC
    4. How a 12-pulse diode rectifier is constructed using a three-winding transformer and requires appropriate phase shift on the secondary coils to operate correctly

    Published: 12 Jun 2022

    Hello, everyone. My name is Graham Dudgeon. Welcome to part 5 in a series of tutorials on 3-phase power. The aim of the video series is to build up our engineering knowledge on the design, analysis, and operation of 3-phase electrical power systems. Today we'll explore the conversion of AC power to DC power using diode rectifiers.

    The process of converting an AC waveform, which is both positive and negative polarity, into a DC waveform, which has only one polarity, is known as rectification. A device that conducts in only one polarity direction is known as a rectifier. The diode is a fundamental rectifier and is a passive component as it operates based on the voltage condition of the circuit it's a part of and is not require an external control signal to operate.

    Here we have a single diode that connects a single phase AC source to a resistive load. The diode will conduct when it's forward biased, meaning that the voltage across it is positive. It will stop conducting when it's negative biased, meaning the voltage across it is negative. In practice, a diode has what is known as a forward voltage, VF, which dictates at what positive voltage threshold the diode will conduct. At the top I've set forward voltage to zero and on the bottom I've set forward voltage to be 50% of the peak supply voltage.

    Note this is rather contrived and I've done it for visualization reasons. In practice forward voltage is around 0.6 volts or so. Choosing a forward voltage value of 0 is a reasonable approximation for this tutorial. In the animation the diode is colored red when it conducts and black when it's not conducting. A single diode is known as a half-wave rectifier as it does not conduct during the negative cycle. Note that when a diode switches its conduction state we refer to that as commutation.

    To rectify both the positive voltage and negative voltage on an AC waveform we can configure an architecture known as on H-bridge. The aim of an H-bridge is to allow a path for the negative half cycle of the AC waveform by connecting the negative AC terminal to the positive DC terminal and the positive AC terminal to the negative DC terminal. This means the negative half cycle will show up as a positive half cycle on the DC site. In the animation, you can see that during positive conduction the diodes one and four conduct and during the negative half cycle the diodes two and three conduct.

    Now we'll consider a three-phase diode rectifier. With a three-phase rectifier we have six diodes. You can see that the three phase configuration is a natural extension of the H-bridge with the addition of an additional arm with diodes 5 and 6. You can see the switching sequence in the animation, which I have color coded as a function of the line voltage. There are three observations.

    First, the diodes will conduct when a line voltage exceeds square root of 2 of the peak line voltage magnitude. So why is that? If you look at the plot on the lower left, I'm showing the absolute values of the line voltage magnitudes. The diodes will conduct when they are forward biased and so we need to look at the line voltage magnitudes relative to each other. Note that at square root over 2 threshold, a given line voltage is greater in magnitude than the other two line voltages. Hence, forward bias occurs and the related diodes will conduct.

    If we look at the commutation sequence, you can see that there are six commutations in a single cycle, so computation is occurring every 60 degrees. As there are six computations per cycle, we refer to this as a 6 pulse device. Our final observation on the six pulse rectifier is the effect it has on AC current. On the bottom left, you can see that line current is not a sinusoid. The commutation has caused the line current to be contaminated with harmonics. That is frequency components that are higher than the supply frequency. We'll cover harmonic analysis in a future tutorial.

    We'll now consider an architecture for a 12-pulse device. That is a rectifier that has two 6-pulse devices connected in parallel. A 12-pulse device is supplied by a three winding transformer that has two secondary windings. For additional information on three-phase transformers, please refer to part 3 in this tutorial series.

    In this case, the secondary windings are the same and are configured as delta D1. Let's take a look at what happens with this architecture. At the top left, we have the line voltages of the upper secondary and at the bottom left, we have the line voltages of the lower secondary. Note that because VAB1 is in phase with VAB2 and so on, that the upper and lower diodes commutate in sequence. This means that we have 6-pulse operation. The pulsing is happening every 60 degrees.

    As we have 6-pulse operation with 12 devices we're essentially wasting the potential of the architecture. If you can have the upper and lower rectifiers switching out of sequence we should be able to obtain lower ripple on the DC voltage. So 6-pulse operations switches at 60 degrees, 360 divided by 6. With a 12-pulse device we want to switch at 30 degrees, 360 divided by 12. To do this, we phase shift the lower secondary relative to the upper secondary.

    Here we have a 30 degree phase shift between the upper and lower secondaries. I've changed the lower secondary to a star configuration and left the upper secondary as a delta D1. Let's see what happens now. Now you can see that we are commutating at 30 degree intervals. As a consequence we've lowered the DC voltage ripple. We've now achieved true 12-pulse operation with our 12 devices. Note that we can extend these architectures to 18-pulse, 24- pulse, and so on, but we won't cover that in this tutorial.

    In summary, rectification is the process of converting an AC waveform, which is both positive and negative polarity, into a DC waveform, which has only one polarity. The diode is a fundamental rectifier and is a passive component, as it operates based on the voltage condition of the circuit it's a part of, and does not require an external control signal to operate. The minimum number of diodes we need to convert three-phase AC to DC is six. We refer to a rectifier with six diodes as a 6-pulse rectifier.

    In our 6-pulse rectifier commutation occurs when line voltage exceeds square root of 3 over 2 of peak line voltage and commutation occurs every 60 degrees. Two 6-pulse rectifiers in parallel is a 12-pulse device. By phase-shifting the line voltages of each rectifier by 30 degrees, the devices commutate at 30 degrees, which reduces DC voltage ripple relative to a 6-pulse device. Three-phase rectifiers introduce harmonics onto the AC supply. I hope you find this information useful. Thank you for listening.