Multipulse Converters II

Summary

This NPTEL course segment delves into advanced power electronics, focusing on multipulse converters. It discusses the 18-pulse converter's Zigzag transformer, which introduces a 20-degree phase shift, and its impact on harmonics and power quality. The lecture also covers the 24-pulse rectifier, which significantly reduces total harmonic distortion (THD) to 1.49%. The course explores the effects of source inductance on THD and power factor, emphasizing the trade-offs involved. Practical considerations like the overlapping angle due to source inductance in rectifiers are also examined, providing a comprehensive understanding of power electronics and control.

Takeaways

• 🔌 The 18-pulse converter uses a Zigzag transformer with a 20-degree phase difference to reduce complexity and improve performance.
• 🌐 The 18-pulse converter's individual rectifier currents, when superimposed, result in a primary current with a Total Harmonic Distortion (THD) of less than 3%, meeting power quality standards.
• 🚫 The 18-pulse converter eliminates lower order harmonics (6n±1, 12n±1) and starts from 18n±1 harmonics, improving the waveform quality.
• ⚙️ The 24-pulse diode rectifier further improves power quality with a THD as low as 1.49% due to its phase shift of 15 degrees and the use of a Zigzag transformer.
• 📈 The power factor for 18-pulse and 24-pulse converters remains relatively high (around 0.95 to 0.98), which is acceptable for utility standards.
• 🔗 The Separate-type Diode Rectifier, such as the 12-pulse version, offers a different topology with a star-delta configuration, suitable for sensitive loads.
• 🌠 Multilevel Cascade H-bridge Inverter Fed Drive is a high-voltage, precise drive application made possible by the use of separate-type rectifiers.
• 📉 The presence of source inductance reduces the Total Harmonic Distortion (THD) by ensuring continuous conduction and reducing the overlapping angle in the commutation process.
• 🔄 The overlapping angle (Mu) in rectifiers, caused by source inductance, affects the commutation process and output voltage, which is a critical factor in circuit design.
• ⏲️ The average output voltage in rectifiers is influenced by the overlapping angle, which needs to be calculated and accounted for in the design of power electronics systems.

Q & A

What is the significance of a Zigzag transformer in an 18-pulse converter?

A Zigzag transformer is crucial in an 18-pulse converter because it provides the necessary phase difference of 20 degrees between its secondary windings, which is essential for reducing harmonics and achieving a cleaner output waveform.

How does the phase difference in the Zigzag transformer affect the harmonic content in an 18-pulse converter?

The phase difference of 20 degrees in the Zigzag transformer of an 18-pulse converter helps to cancel out lower order harmonics, resulting in a waveform that starts with 18n±1 harmonics, significantly reducing the Total Harmonic Distortion (THD).

What is the typical THD percentage for the individual rectifiers in an 18-pulse converter before superimposition?

The individual rectifiers in an 18-pulse converter may have a Total Harmonic Distortion (THD) as high as 23% before the currents are superimposed.

How does the THD on the primary side of an 18-pulse converter compare to the individual rectifier's THD?

After superimposing the currents from the individual rectifiers in an 18-pulse converter, the THD on the primary side is typically less than 3%, which is significantly lower than the individual rectifier's THD.

What is the role of the iaaa9 standard in relation to the 18-pulse converter?

The iaaa9 standard requires that the Total Harmonic Distortion (THD) should be 5% or less for power quality. The 18-pulse converter can achieve this by reducing the THD to below 3%, thus meeting the standard.

What is the primary advantage of using a 24-pulse diode rectifier over an 18-pulse converter?

The primary advantage of a 24-pulse diode rectifier is its significantly lower Total Harmonic Distortion (THD), which can be as low as 1.49%, due to the additional phase shifts provided by the Zigzag transformer.

How does the phase shift in a 24-pulse converter differ from that in an 18-pulse converter?

In a 24-pulse converter, the phase shift provided by the Zigzag transformer is 15 degrees, as opposed to the 20 degrees in an 18-pulse converter, which allows for further reduction in harmonic content.

What is the impact of source inductance on the THD in a 24-pulse converter?

As the source inductance increases in a 24-pulse converter, the Total Harmonic Distortion (THD) generally decreases, with the THD starting from around 6% and potentially going below 2%.

How does the power factor vary with different firing angles in a 12-pulse Separate-type Diode Rectifier?

In a 12-pulse Separate-type Diode Rectifier, the power factor remains relatively constant and high for different firing angles, with values typically around 0.98, making it acceptable for utility standards.

What is the significance of the overlapping angle (Mu) in the operation of a rectifier?

The overlapping angle (Mu) is significant in rectifier operation because it accounts for the time during which two diodes conduct simultaneously due to the presence of source inductance, affecting the output voltage and average voltage calculations.