Determination of RMS Current Load on the DC-Link Capacitor of Voltage Source Converters Using Direct Current Control

RMS current load is one of the most important design criteria for the dc-link capacitors of voltage source converters. In the literature, determination of this rms current load is well documented for state-of-the-art modulation schemes, such as space vector modulation. But the methods presented cannot be applied for direct current control algorithms. Therefore, this article introduces a new approach especially for direct current controllers, which reflects and compares to the state-of-the-art calculation method for space vector modulation. As direct current control is a feedback control, no closed-form solution can be given. Instead, the dc-link capacitor rms current load is determined by an iterative calculation scheme as a function of the modulation level and the phase angle. Although the scheme presented is applied to a three-phase two-level voltage source converter, it can be adapted to any voltage source converter topology. An example is presented applying the calculation method to one specific direct current control algorithm. The calculated results are verified via a digital simulation model as well as test bench measurements showing good correlation. For correct comparison, different effects are considered and discussed between simulation, calculation, and measurement results. These contain transients, the commutation process between insulated gate bipolar transistor (IGBT) and diode, the current-dependent voltage drop of the semiconductors, and the stationary current error of the hysteresis control. The dc-link capacitor current load for space vector modulation is calculated analytically as a reference by using the state-of-the-art method. The comparison of results obtained in this article and the reference are very similar, which is discussed in the conclusion.