A new, universal Series Hybrid Cascaded H-Bridge Converter for Power-Hardware in the Loop Emulation

This paper presents a new Series-Hybrid Cascaded H-Bridge (SH-CHB) Converter for Power-Hardware in the Loop (PHIL) systems. The converter combines the benefits of CHB-Cells, high power density and high system efficiency, with the advantages of Linear Power Amplifiers (LPA), high bandwidth and high fidelity. The introduced system is used to emulate different AC-grids as well as DC-grids with simultaneous supply of sinusoidal test signals for impedance spectroscopy of converters. The system provides sinusoidal test signals up to 105 kHz with a three-phase output AC voltage of 400 V or a DC voltage of 1000 V. The maximum output power of the system is 60 kVA.

Introduction

In converter-fed grids, the converters connected to the point of common coupling (PCC) define the behavior of the PCC. Consequently, the simplified consideration of a grid behavior by means of passive grid impedances is obsolete. Additionally, the installed power transformed by converters in the European electrical grid is increasing, whereby the converters define more and more the PCC behavior. Hence, the behavior of the PCC depends among others on characteristics of those converters, such as the control parameters, the phase lock loop (PLL) design and the filter components. Due to the impact of the converter characteristics on the PCC behavior, parallel connected converters can influence each other. This could lead to unstable operating points of the grid. To predict the behavior of converters in converter-fed grids, models of the converters and the grid must be developed and parameterized. Hence, a Power-Hardware in the Loop (PHIL)-system is needed, which could determine the parameters of the grid and the converters. Therefore, the designed PHIL-system should be able to emulate different grid situations, so that the converter can be investigated under the same operating conditions under which it will be used later. Simultaneously, the PHIL-system must generate test signals to measure the behavior of the converter. One possible method to analyze the stability of a converter is the Nyquist stability criterion [1]. For the Nyquist stability criterion a small signal model or a harmonic impedance model of the analyzed converters and the grid are needed [1], [2]. These can be obtained by an impedance spectroscopy. To parameterize the models, sinusoidal test signals with frequencies up to the switching frequency of the analyzed converter must be added to the output voltage of the PHIL-system. For accurate measurements a high power, high fidelity and high dynamic AC-voltage source is necessary. Therefore, a new Series-Hybrid CHB (SH-CHB) concept was developed. The presented SH-CHB converter can provide a 400 V three-phase AC-Grid or 1000 V DC-Grid. The maximum output power of the system is 60 kVA with a maximum large signal bandwidth of 105 kHz.