Industrial Electronics

The significance of modular multilevel converters

29 September 2023
A three-phase MMC. Source: N. Mughees

As a relatively new addition to the multilevel converter family, the modular multilevel converter (MMC) was initially presented by Lesnicar and Marquardt in 2002. MMCs are a novel type of power electronic converter used in high-voltage direct current (HVDC) transmission and other medium-to-high voltage applications. Power conversion efficiency, decreased harmonic distortion and increased dependability are just a few of the reasons why this advanced voltage source converter has attracted so much interest.

How does the MMC work?

In order to produce the necessary output voltage waveform, a typical MMC combines numerous sub-modules in series. Capacitors and control circuits and power semiconductor devices like insulated-gate bipolar transistors (IGBTs) and metal-oxide semiconductor field-effect transistors (MOSFETs) make up these smaller modules. Depending on how the individual modules are set up, the converter can be categorized as either a half-bridge or a full-bridge MMC.

Typically, a three-phase MMC will include a DC terminal, an AC terminal and a converting kernel with three phase legs as shown in the diagram above. There are two arms — the top arm and the lower arm — that make up each leg/phase. Both the upper and lower arms have a series-connected set of similar submodules and a chock inductor to dampen the current's high-frequency components. Power may be converted in both directions using an MMC.

The basic operation of an MMC could be summarized as follows:

  • Sub-modules: The fundamental components are the sub-modules, also known as "arms" or "cells." Each sub-module contains power semiconductor devices (switches) and capacitors. The total number of sub-modules determines the number of voltage levels the converter can produce.
  • Series connection: These sub-modules are connected in series to achieve the desired output voltage level. Interestingly, the voltage rating of each sub-module is lower than the total output voltage, which enhances efficiency and voltage control.
  • Voltage synthesis: By controlling the switches in the sub-modules, the converter can create different voltage levels between the positive and negative terminals, thus generating the desired output voltage waveform.
  • Pulse width modulation (PWM): The control system employs PWM techniques to manage the switching of the power semiconductor devices. It adjusts the duty cycle of the PWM signals to control the amplitude of each voltage level and shape the output voltage waveform accordingly.
  • Capacitor voltage balancing: During operation, imbalances in capacitor voltages may arise due to varying loads or switching actions. To maintain stability and proper voltage distribution, the control system monitors and adjusts the capacitor voltages as necessary.
  • Control and synchronization: A sophisticated control system is crucial to effectively manage the MMC. It continuously monitors parameters like output voltage, current and capacitor voltages to ensure stable and efficient operation. In multi-module MMCs, synchronization between the sub-modules is essential to preserve the desired waveform integrity.

Advantages of MMCs

The benefits of MMCs are numerous. First, they can handle high voltage and power levels effectively, making them well-suited for applications demanding substantial power conversion, such as HVDC transmission systems. Second, they produce nearly sinusoidal output waveforms, which significantly reduces harmonic content and ensures the delivery of high-quality power.

Another advantage of MMCs is their fault tolerance and redundancy. Thanks to their modular design, these converters can continue operating even if a sub-module experiences a failure, ensuring system reliability. Additionally, MMCs offer scalability, easily adaptable to varying voltage and power requirements by adding or removing sub-modules as needed. These converters also exhibit higher efficiency due to their reduced switching losses and improved waveform quality. This makes them an eco-friendly and energy-efficient choice for power conversion needs.

Applications of MMCs

Apart from HVDC transmission and medium HV drivers, MMCs can be used in

1. Renewable energy integration: MMCs play a vital role in integrating renewable energy sources, such as wind and solar, into the power grid. Renewable energy systems often have variable output characteristics, and MMCs can efficiently convert the fluctuating AC power from these sources into stable DC power for grid integration, ensuring smooth and reliable power supply.

2. STATCOM (Static Synchronous Compensator): MMC-based STATCOM systems are utilized to stabilize and regulate voltage levels in power systems. They can respond rapidly to changes in load conditions and disturbances, thereby enhancing the stability and reliability of the grid.

3. Energy storage systems: MMCs are employed in energy storage applications, where they efficiently convert electrical energy into the appropriate voltage levels for charging and discharging batteries or other energy storage devices. This enables effective management of energy storage systems and facilitates their integration into the grid.

4. Traction systems: MMCs are used in traction systems for trains and cars that run on electricity. They are well-suited for today's electric transportation systems due to their high-power conversion efficiency, small size and enhanced controllability.

5. Hybrid AC/DC grids: In hybrid AC/DC grids, MMCs act as interface converters that enable smooth power exchange between AC and DC grids, facilitating the integration of various power sources and improving grid stability.

6. Power quality improvement: Industrial settings also make use of MMCs for power quality enhancement by reducing power fluctuations, voltage dips and other disturbances to provide a steady and consistent power supply to sensitive machinery.

Conclusion

MMCs will be crucial in creating tomorrow's power grids. The technology is of value because of the improvements in efficiency, harmonic distortion reduction, fault tolerance and scalability it provides. These qualities make it a preferred choice for various high-power applications, such as HVDC transmission systems, renewable energy integration and medium-to-high voltage power conversion.



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