Frequently Asked Question

Modeling of Modular Multilevel Converter (MMC) can be demanding of HIL hardware resources but even more so for IO interface requirements. The MMC modeling approach and chosen control method modify these requirements. In our toolchain, we take the MMC switching function modeling approach and use the nearest level control (NLC) method which is only one control method of many that can be used. Finally, the user application decides if both hardware and IO requirements are to take into account. If you are primarily interested in real-time SIL (model-based controller implemented in Signal Processing) then there are no IO limitations. 

For C-HIL testing with an external controller, the IO number can become a more limiting factor than hardware requirements. Using a Bus Join component and Digital Inputs it is possible to create a Su vector for controlling the SM in MMC. While using the Bus Split component and Probes it is possible to provide feedback to the MMC controller.

The typical structure of a three-phase MMC is shown in Figure 1. Each phase leg of the converter has two arms, an upper and a lower. Each one is constituted by n number of sub-modules (SMs).

Our HIL sizing tests are not maximum load tests, rather they provide a ballpark figure based on realistic MMC applications. The number of levels can be expressed as: 

no. of SM per phase/2 + 1

As NLC assumes complementary-type switching, one digital input can control one MMC sub-module. Also, one analog output per sub-module is required for sending measurements back to the controller.

The table below provides HIL sizing for both RCP and C-HIL type applications.

*Real controller testing requires at least 72/78 of DI/AO, therefore one HIL604 device/HIL606 device with 64/64 of DI/AO is not enough. 

**Real controller testing requires 120/126 of DI/AO, therefore one HIL404 device with 32/32 DI/AO is not enough. 

***Real controller uses SFP Communication Protocol instead of IO (120/122 of DI/AO). Maximum payload size is 250 words (4 bytes each).

We highly recommend checking our application note for Modular Multi-level Converter (MMC) with Induction Machine. This example can be trialed in the free version of our software.