Modular Multilevel Converters (MMCs) are a type of power converter topology that is used for medium and high-voltage applications. They are employed in various applications such as power transmission (especially in HVDC systems), renewable energy integration, and motor drives. MMCs have a modular design, making them scalable and flexible for different power levels and applications.

Structure and Operation of Modular Multilevel Converters

Conventional MMC topoogy for DC to AC Conversion

Each arm of an MMC consists of multiple cells or submodules connected in series. The cells are configured as half-bride or full-bridge topologies, containing switches (IGBT/MOSFET) and a capacitor. Half-bridge submodules (HBSMs) are simpler and more commonly used, while full-bridge submodules (FBSMs) provide DC fault-blocking capability but with a higher component.

The number of submodules per arm determines the number of voltage levels that the MMC can produce. More submodules result in a higher number of voltage levels and better output waveform quality. Also, the large number of voltage levels generated by the series-connected submodules leads to very low harmonic content in the output voltage and current waveforms, reducing filtering requirements. Inductors are placed in each arm to control the current ripple and balance energy flow between submodules.

In the half-bridge topology, A and O are the two ports of the cell. If switch S₁ is turned on, the output voltage V_OA is V_C.  If switch S₂ is turned on, the output voltage V_OA is zero. In the full-bridge topology, A and B are the two ports of the cell. If switches S₁ and S₄ are turned on, the output voltage, V_AB is V_C. On the other hand, if switches S₂ and S₃ are turned on, the output voltage, V_AB is -V_C. Turning ON S₁ and S₂ or S₃ and S₄ results in zero output voltage. Thus, the use of half-bridge topology results in two levels of voltage (0 and V_C), whereas the use of full-bridge topology results in three voltage levels (0, V_C,-V_C).

Thus, MMC operates by switching its submodules in and out of the circuit, controlling the number of active submodules at any given time to generate the desired AC or DC output voltage.

AC to DC Conversion (Rectification): When converting AC to DC, the AC voltage from the grid or another source is applied to the AC side of the converter. The switches in the submodules are controlled so that the converter outputs a stepped DC voltage across the DC terminals. Each submodule either inserts its capacitor voltage into the circuit or bypasses it. The voltage is controlled smoothly by adjusting the number of active submodules in the upper and lower arms.

DC to AC Conversion (Inversion): When converting DC to AC (inverter mode), a DC voltage is applied to the converter’s DC terminals. The MMC uses the submodules to generate a stepped AC voltage across the AC terminals. The switching pattern of the submodules in the upper and lower arms of each phase leg is controlled in such a way that the output voltage approximates a sine wave.

Stepped Waveform Generation: MMC generates multi-level voltages that are a staircase approximation of the desired AC waveform. The greater the number of submodules, the more voltage levels are produced, leading to a smoother output waveform and lower harmonic distortion.

Features of MMC

• Modular and Scalable Design: MMCs are constructed from many identical submodules connected in series, allowing easy scalability to different voltage levels and power ratings making them suitable for a wide range of applications.
• High Output Voltage Quality: The large number of voltage levels produced by the series-connected submodules results in a very low harmonic content in the output voltage and current waveforms.
• Reduced Voltage Stress on Power Semiconductors: The voltage across each submodule is much lower than the total converter voltage, allowing the use of lower-rated and more cost-effective switches.
• Improved Efficiency and Reliability: The modular design allows bypassing of faulty submodules and easy maintenance, while the lower switching frequencies reduce losses.

Advantages of MMC

• High Voltage Scalability: MMCs have a modular structure that allows easy scalability to higher voltages by adding more submodules in series. This enables direct connection to medium and high-voltage AC grids without the need for bulky transformers.
• Low Harmonic Distortion: The large number of voltage levels produced by the series-connected submodules results in very low harmonic content in the output voltage and current waveforms. This reduces filtering requirements and improves power quality.
• Reduced Voltage Stress on Switches: The voltage across each submodule is much lower than the total converter voltage, allowing the use of lower-rated and more cost-effective power semiconductor devices.
• Improved Efficiency and Reliability: The modular design enables bypassing of faulty submodules and easy maintenance. The lower switching frequencies reduce losses, improving efficiency.
• Inherent Redundancy: If a submodule fails, it can be bypassed, allowing the converter to continue operating and enhancing overall reliability.
• Flexible Control: MMCs offer independent control of active and reactive power, DC link voltage, and capacitor voltages of each submodule. This provides high flexibility in power flow control and system stability.

Disadvantages of MMC

While MMCs offer numerous advantages, they also have certain disadvantages that impact their performance and implementation in high-voltage direct current (HVDC) applications.

• Complexity of Control: The control strategies for MMCs are more complex than those for traditional converters. Managing the operation of multiple submodules, ensuring voltage balancing, and suppressing circulating currents require sophisticated algorithms and control schemes, which complicates system design and operation.
• Increased Component Count: MMCs consist of many submodules, leading to a higher number of components compared to traditional two-level converters. This increased component count can lead to higher initial costs, more extensive maintenance requirements, and potential reliability issues due to the greater number of failure points.
• Capacitor Voltage Ripple: The capacitor voltage ripple in each submodule can be significant, especially at lower carrier frequencies. High ripple may affect the performance and reliability of the converter, necessitating additional control measures to manage voltage levels effectively.
• Thermal Management: The modular design leads to uneven thermal distribution among submodules, which require additional thermal management solutions to ensure that all components operate within safe temperature limits. This further complicates the design and increase costs.
• Cost: The initial investment for MMCs is higher than other converter types due to the complexity, number of components, and advanced control systems required. This may pose a challenge for certain applications, especially when budget constraints are a factor.

Click here to learn more about Power Converters on everything PE