Ideally, power system voltages are balanced, with equal magnitudes and phases 120° apart. However, in a real power system only generator voltages satisfy this criterion whereas upstream voltages may also be unbalanced. A major cause for unbalanced network voltage is the uneven distribution of single-phase loads (e.g. commercial facility loads, single-phase electric traction systems, rural electric systems). The effects of unbalanced voltages include added losses in power systems and performance deterioration of power-electronic converters and induction drives.
Fundamental balancing is usually provided by connecting single-phase loads across different phases or by static VAR compensators (capacitors and Thyristor Controlled Reactors). These techniques are only aimed at balancing the fundamental component, while the harmonics remain unbalanced.
A range of solutions can be applied for filtering unbalanced harmonics. Purely passive filtering that is nowadays commonly used is insensitive to unbalanced conditions. However, due to the resonant nature of these devices there may be unwanted resonant interactions with the supply system. With the development of power electronics, active filters are growing in importance, because they do not cause resonance with the system in both balanced and unbalanced conditions and they also enable fast dynamic responses to changing conditions. The most notable disadvantage of active filters is their high investment and operational costs. This paper concentrates on hybrid filter design, which offer lower ratings. It presents a hybrid active filter and current control strategy appropriate for filtering harmonics of an unbalanced nonlinear load in a three-phase three-wire power system. Paper demonstrates good filtering performance on a real-time simulator (RTDS) with hardware in-the-loop.
L. Herman, B. Blazic, A. Bozicek, I. Papic, Presented at the CIGRÉ Canada Conference on Power Systems, Winnipeg, Canada, Sep. 2015., Paper CIGRE 683
KEYWORDS: Harmonics, hybrid filters, proportional-resonant controller, unbalanced conditions.