A two-level voltage source converter (VSC) is a power electronic converter that produces a square-wave output voltage at the AC terminals. It has switches, which are arranged in a half-bridge configuration, which allows for the generation of both positive and negative output voltages. By turning the switches on and off in a specific sequence, a square-wave voltage of two-level is produced at the AC terminals. Pulse-width modulation (PWM) is often used to control the average output voltage and generate a sinusoidal AC waveform.
A three-level VSC is a power electronic converter that can produce a square-wave (step-wave) output voltage with three levels: positive, zero and negative. This provides more flexibility in controlling the output voltage compared to a two-level VSC. By connecting the top switch to the positive DC link and the bottom switch to ground, a positive output voltage is produced. By connecting the top switch to the ground and the bottom switch to the negative DC link, a negative output voltage is produced. Similarly, by connecting the top and bottom switches to ground, a zero-output voltage is produced.
Output waveform and harmonic content
A two-level VSC produces a square-wave output, which is rich in harmonics. But a three-level VSC generates a stepped waveform with three levels (positive, negative and zero), which is closer to a sinusoidal waveform, which has a lower harmonic content. The stepped waveform of a three-level VSC contains fewer higher-order harmonics compared to the square-wave of a two-level VSC. This is because the stepped waveform is a closer approximation of a sinusoidal waveform, which has a lower harmonic content by definition.
The lower harmonic content of a three-level VSC means that less filtering is required to meet harmonic standards or to reduce interference with other equipment. This can result in lower overall system costs and improved efficiency. Moreover, the additional level in a three-level VSC allows for lower voltage and current stress on the switching devices. This leads to longer device lifetimes and improved reliability.
Efficiency
Three-level VSCs generally outperform two-level VSCs in terms of efficiency due to lower switching losses. This is particularly advantageous in high-power applications where energy efficiency is a primary concern. For example, if a large-scale wind turbine power plant is being designed, the primary concern is to maximize energy efficiency and minimize energy losses during the conversion of DC power generated by the turbines to AC power for grid connection. A traditional choice for such applications is two-level VSC. It has a simpler structure and lower initial cost. However, the square-wave output results in higher harmonic content, leading to increased losses in the power grid. Additionally, the switching losses associated with the two-level topology can be significant, especially at high power levels.
In this case, three-level VSC offers a more efficient solution due to its stepped waveform and lower switching losses. The stepped waveform reduces harmonic distortion, minimizing losses in the grid. Moreover, the three-level topology allows for a more efficient distribution of current among the switches, reducing switching losses. This means more of the energy generated can be delivered to the grid, increasing the profitability of the wind turbine power plant. Moreover, the cleaner output waveform from the three-level VSC enhances power quality, benefiting other connected loads and reducing potential equipment damage.
Cost and complexity
Three-level VSCs are typically more complex and expensive to design and implement compared to two-level VSCs. However, the benefits of reduced harmonic content, lower switch stress, and higher efficiency often justify the increased cost and complexity in certain applications. Advances in power semiconductor technology and control techniques have made three-level VSCs more feasible and cost-effective.
Applications
Two-level VSCs
- Grid-connected inverters: Two-level VSCs are widely used in grid-connected inverters due to their simpler structure and lower cost. They are suitable for applications where lower power levels and moderate harmonic content are acceptable.
- Motor drives: In motor drives, two-level VSCs are often preferred because they offer a simpler control structure and are generally sufficient for most motor applications.
- Uninterruptible power supplies (UPS): Two-level VSCs are commonly used in UPS systems due to their reliability and ability to provide clean power during power outages.
Three-level VSCs
- High-power applications: Three-level VSCs are favored in high-power applications such as wind turbines, photovoltaic systems, and static VAR compensators (SVCs) due to their several advantages:
- Lower harmonic content: The stepped waveform of a three-level VSC produces fewer harmonics, which is crucial for grid-connected systems to minimize interference.
- Lower voltage and current stress on switches: This reduces the stress on the switching devices, extending their lifetime and improving reliability.
- Higher efficiency: The lower switching losses in a three-level VSC lead to higher overall system efficiency, especially in high-power applications.
- Reduced filtering requirements: The lower harmonic content means less filtering is needed, reducing system costs and complexity.
Conclusion
While two-level VSCs are simpler and less expensive, three-level VSCs offer advantages in terms of harmonic reduction, lower switch stress and higher efficiency, making them suitable for high-power and high-quality applications. The choice between the two depends on the specific requirements of the application such as power level, harmonic requirements, efficiency considerations, and cost constraints.
