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This paper proposes an inductively active filtering (IAF) method to comprehensively improve the power quality (PQ) for not only the distribution network (public grid) but also the power-supply system (nonlinear load) connected to the network. At first, a new main-circuit topology for implementing the IAF method is presented, which consists of an inductively filtered transformer and a fully tuned branch controlled by an inverter. Its operating principle and technical features are introduced by comparing with the traditional active filtering method. Based on this, the equivalent circuit is established and by means of mathematical modeling, the unique filtering mechanism of the IAF method is revealed in theory. Further, the control strategy of the FT branch and the impedance coordination for the inductively filtered transformer are designed based on theoretical analysis. A case study is investigated in detail to illustrate the operating characteristics of the IAF method. Both the theoretical and the case studies show that the IAF method can effectively prevent harmonic components from flowing into the primary (grid) winding of the transformer. Since the harmonic components are suppressed near the harmonic source, it is good for the power-supply system and especially good for the converter transformer. Besides, since the harmonic flow is limited to near the harmonic source, the PQ of the public network can be guaranteed completely. [ABSTRACT FROM AUTHOR]

Short circuit (SC) strength of a power transformer is influenced by its winding clamping pressure. The required level of clamping pressure is selected at the design stage based on the maximum allowable SC current level of the transformer. Progressive loss of clamping pressure is a common problem in power transformers. Shrinkage and fading of the stiffness of solid insulation due to thermal degradation are the most common causes of this problem. It has also been identified that changes in the moisture content and repetitive compression cycles on pressboard during high current faults tend to change the winding clamping pressure. This study aims to investigate the effects of moisture and compression cycles on winding clamping pressure through a set of laboratory experiments and finite element analyses. Thickness change and compressive stress-strain curves of pressboard were measured under different moisture conditions. The results were then used to calculate changes in winding geometry and clamping pressure of a 100 kVA three phase, disc type transformer winding. [ABSTRACT FROM PUBLISHER]

Broadband responses under random excitations can reflect the modal characteristics of a structure. This paper investigates the applicability of operational modal analysis (OMA) to extract modal parameters of transformer windings under random excitations. Firstly, random vibration signals are extracted from measured vibration signals by estimating an accurate power frequency and eliminating vibration harmonics. Then, modal characteristics are obtained by applying OMA to the extracted random vibration signals. The numeric study on a winding axial vibration model demonstrates the effectiveness of the proposed method in obtaining its modal characteristics. Laboratory experiments show that the natural frequencies calculated by OMA are a subset of the actual natural frequencies of windings and the tank. The field test results of a 110 kV power transformer show that the number of natural frequencies increase when winding deformation occurs. OMA of transformer windings may pave a way for online winding condition monitoring. [ABSTRACT FROM AUTHOR]