Analysis and Verification of a Cascaded Advanced AC-Simulator with Non-Linear Loads

A cascaded advanced AC-simulator system for testing grid tied applications has been introduced recently. Such a system emulates a virtual impedance, which is implemented in the control loop to simulate a certain grid impedance. Therefore a small signal inverter is used in addition to a large signal inverter, which generates the fundamental voltage signal. Since the small signal inverter operates at lower DC-link voltages, fast switching devices can be used for the design. This enables high bandwidth and fast dynamics and on the other hand high accuracy of the voltage measurements. As both inverter systems are connected in series a galvanic isolation is mandatory and realized by a YY0 transformer. However, the output impedance of the large signal inverter and the stray inductance of the transformer have impact on the measurement results, these impedance characteristics have to be considered and compensated. This compensation can be done by the small signal inverter in addition to the impedance emulation. The behavior and accuracy of such a system with non-linear loads is going to be discussed in detail in this work. Based on the analysis of bridge rectifiers and the limitations of the proposed topology, an appropriate laboratory test setup is implemented. The measurement results are analyzed and compared to an ideal reference model. The analysis is done in the time domain and in the frequency domain with focus on Fast Fourier Transformation (FFT).