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Some fields of application demand absolutely everything of medium-voltage converters. They are expected to offer maximum availability while being motor or network-friendly. The most reliable option in these situations is the modular multilevel converter, or M2C for short. The technology patented by Siemens is based on a modular system that makes the converter extremely flexible, scalable, reliable to use and extremely easy to maintain.
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Modular multilevel converters and the theory behind them
An M2C medium-voltage converter consists of several low-voltage cells. Switched in series, they generate an output voltage at medium-voltage level. Within the converter, each cell can be controlled individually, and each has a local energy storage unit that contributes a proportion of the link voltage. This means the output voltage can be modulated so finely that it almost forms a sine curve. That’s what is innovative about the M2C technology, first patented by Siemens in 2002, which has gone on to capture more and more of the market ever since. A more detailed look will reveal even more advantages.
The M2C topology is DC bus-capable. This means that additional topologies can be combined with M2C at both the network and the motor end. This gives you huge freedom to configure the converter – whether for single or multi-axis drives. For a rectifier, for example, you can choose between different pulse counts, depending on how “clean” you need the input voltage to be.
At the same time, this freedom can be transferred to the system design as a whole. Technically, a converter based on M2C does not necessarily need a transformer, and if one is used (to adapt the voltage), it does not have to be located in the immediate vicinity of the converter. You can plan more flexibly as a result, and so, more than ever before, you can get the most out of the space available for installation.
A critical factor in day-to-day operations is what happens in an outage. This is where the cell-based principle kicks in. If a cell fails, a bypass is automatically activated within the converter – in other words, the critical point is short-circuited within an extremely short time (< 1 ms). This brief voltage failure is not perceptible from the outside, either during or after the outage. The module will cease to function only if several cells in a given branch should fail.
The advantage of the cell-based principle is also evident when it comes to maintenance. In a redundant configuration, cells do not have to be replaced immediately. Operators can wait until the next scheduled downtime, when the part can be swapped very quickly. As a rule it should take no more than 15 minutes. These conditions ensure that you can always enjoy maximum system availability.
The more scalable the converter, the better chance you have to achieve the ideal layout for the system as a whole. By itself, that’s enough to save costs and improve the overall quality of your system. The cell-based design used in M2C technology creates the best conditions to achieve this. The converter is easy to adapt – in terms of voltage, redundancy and the necessary power output. As a result, M2C can meet your requirements perfectly, even if what you need from the converter should change over time.
Just how scalable the topology is has already been demonstrated in other areas of application, like high-voltage DC transmission (HVDC applications). These involve switching up to 200 cells in series in a single M2C branch. Output voltages of up to 400 kV can be achieved as a result.
From theory to practice
The technological innovations contained in a modular multilevel converter provide many additional advantages in application, even compared to other medium-voltage converters.
Overview of M2C solutions
Two of our medium-voltage converters are currently available with M2C technology. Both have been proven in practice for a number of years now.