The development of the control unit for modern power converters is nowadays a very cost- and time-critical task. In contrast to this, quick answers are expected for the claims by the market, certainly without quality loss.
A general power converter consists of two main parts: a power level (main) circuit and a digital controller unit, which is usually realized by using some kind of DSP. Remarkably complex and multiple controls are required for the state-of-the-art power conversion tasks, and the field test (test on the real main circuit) of these control units can be dangerous not just for the system itself, but for the testers as well.
A low-power model of the main circuit can be built under laboratory conditions, but it will have parameters differing from the ones of the original system. Generally it is not possible to set the same time constants as in the real system, and the relative losses are also higher.
The Hardware-In-the-Loop (HIL) simulator is now the model of the power converter with its supply and its load implemented on an FPGA. The HIL simulation is a very useful and practical tool in power electronics. The simulator can be parameterized, monitored, it works on logic level, and thus the test of a newly designed control unit can be done quickly and without any danger. Extreme failure cases are also reproducible, which would happen very rarely in the real system.
The further benefit of this test method -in contrast to the computer-based simulation method- is that it is possible to use the same signal levels as in the interface (control and measured signals) between the control and the main circuit. Therefore the hardware- and the software test of the control electronics can be fulfilled.
In the development of the HIL simulator we have to try to get a proper ratio of the cost/quality, and for that the FPGA-implementable model of the main circuit can be defined on high level program language (MATLAB/Simulink), so that less work is required for the code optimization during the development.
The subject of my Master Thesis is the development of such a real-time HIL simulator using an FPGA. The system to be modeled is partly a three phase grid-connected, 36kW powered Sinamics S120 Active Line Module (ALM) with an Active Interface Module (AIM). The latter realizes line and EMC filtering, and the former is practically a three-phase IGBT active rectifier stage, which is able to work in inverter mode, so that is able to feed back to the grid.
The control part consists of the integrated control of the Sinamics S120 ALM, and of a central CU320 control unit. In the model the measured signals are state variables so that these come to the control after a conversion by the Σ/Δ converters implemented in the FPGA. When the simulation runs, it periodically re-calculates the state variables of the system. The goal is to reach a refreshing frequency of 10MHz, which means 100 ns time step resolution for the simulation.