This paper presents the first experimental validation of the stability analysis based on the online measurement of harmonic impedances exploiting the Linear Time Periodic (LTP) approach, applied to AC networks of power converters. Previous publications have provided the theoretical framework for the method, enabling the stability assessment of an unknown system adopting a blackbox approach, relying only on injected perturbations and local measurements. The experimental case study considered in this paper comprises two single-phase converters, one acting as source subsystem and the other as load subsystem. A third converter, the Stability Measurement Unit (SMU), is controlled to inject small current perturbations at the point of common coupling (PCC).From the measured small-signal perturbations of PCC voltage, source current and load current, the harmonic impedances of source and load subsystems are calculated. The LTP Nyquist Criterion is then applied to the ratio of the two harmonic impedances in order to assess the stability of the whole system. Theoretical and experimental results from a 5 kW laboratory prototype are provided and confirm the effectiveness of the method. In addition, the measurements do not require sophisticated equipment or control boards and can be easily performed from data sampled by commercial micro-controllers.

Abstract-Non-linear loads (NLLs) in three-phase systems are known to produce current harmonics at -5, 7, -11, 13… times the fundamental frequency; harmonics of the same frequencies are induced in microgrid voltage, reducing therefore the power quality. Dedicated equipment like active power filters can be used to compensate the microgrid harmonics; alternatively, each distributed generation (DG) unit present in the microgrid can be potentially used to compensate for those harmonics. The use of the virtual admittance concept combined with a PI-RES control structure has been recently proposed as a harmonic compensation sharing strategy when multiple DGs operate in parallel. The drawback of this methodology is that a large number of RES controllers might be required to compensate for all harmonic components induced by NLLs. This paper proposes the combined use of virtual admittance control loop and repetitive controller (RC) for harmonic compensation. The main advantage of the proposed method is that only one RC is required to compensate for all the harmonic components, significantly reducing the computational burden and the design complexity.

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Introduction of the Modular Multilevel Converter (MMC) has enabled the exploitation of Voltage Source Converters (VSCs) in an increasing number of High Voltage Direct Current (HVDC) applications. Subsequently, some new topologies and solutions have been presented to tailor the MMC concept to specific uses. Particular attention has been paid to reduction of the converter footprint for applications where plant size is a critical economic aspect, for example, in off-shore installations.This paper introduces a new series connected modular multilevel ac/dc converter, the Series Chain-link Converter (SCC), which gives a significant reduction in the required number of submodules (SMs) and a more compact distribution of the energy storage, compared to an MMC. In the paper, the operating principle of the converter and its design are discussed in detail; the sub-module count and energy storage requirement are also given. The basic control loops required for the practical operation of the converter are presented and designed. The SCC concept has been experimentally validated on a small-scale 450V dc, 415V ac, 4.5kVA laboratory prototype, confirming the practical viability of the topology.

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