A Modular Multilevel Converter (MMC) is a type of power electronic converter used for high-voltage and high-power applications in AC (alternating current) power systems. It's designed to efficiently and controllably convert power between different voltage levels, making it particularly useful for applications like high-voltage direct current (HVDC) transmission, flexible AC transmission systems (FACTS), and renewable energy integration.
The MMC consists of multiple submodules connected in series, forming a multilevel structure. Each submodule typically consists of several power semiconductor devices, such as insulated gate bipolar transistors (IGBTs) or gate turn-off thyristors (GTOs), and associated passive components like capacitors and resistors. These submodules are connected in such a way that they can generate multiple voltage levels between the positive and negative terminals of the converter.
The full-bridge submodule is a fundamental building block of the MMC. It's called "full-bridge" because it comprises four power semiconductor devices arranged in a bridge configuration, forming two arms. Each arm consists of two devices – one connected to the positive terminal and the other to the negative terminal. The load (AC system or DC system) is connected between the two midpoints of these arms.
The operation of the MMC involves controlling the switching of these power semiconductor devices to synthesize a staircase-like voltage waveform across the load. By adjusting the switching patterns and duty cycles of the submodules, the output voltage can be controlled and adjusted to match the desired AC waveform. The key advantages of the MMC with full-bridge submodules are:
Modularity: The MMC's modular structure allows for easy scalability and fault tolerance. If a submodule fails, it can be bypassed while the rest of the converter continues to operate, minimizing downtime.
High Voltage Capability: The multilevel structure enables the MMC to achieve high voltage levels without the need for excessively high device voltage ratings. This is particularly useful in HVDC applications.
Reduced Harmonics: The multiple voltage levels generated by the submodules result in a waveform that closely approximates a sine wave, leading to reduced harmonic content in the output voltage. This is beneficial for power quality and compatibility with the AC grid.
Low Switching Losses: The MMC's switching strategy typically involves using low switching frequencies, leading to lower switching losses and improved efficiency.
High Power Quality and Control Flexibility: The precise control over the voltage levels and waveform shapes enables effective power flow control, reactive power compensation, and voltage regulation, which are essential in modern power systems.
Less Electro-Magnetic Interference (EMI): The multilevel output waveform leads to lower EMI compared to conventional two-level converters.
In summary, the Modular Multilevel Converter (MMC) with full-bridge submodules is a versatile and efficient solution for AC power control applications, offering benefits like high voltage capability, modular architecture, reduced harmonics, and improved power quality. It has gained significant attention and adoption in high-power applications due to its ability to address the challenges of modern power systems.