In this paper, the research methodology and the results of the analysis carried out using the acoustic emission (AE) and thermal imaging for a selected oil power transformer are presented. The basis for the research, by means of the AE method, was the author’s patented research method. The AE descriptor maps on the side walls of the tested transformer along with the location of areas with increased AE activity and an analysis of the properties of AE signals recorded at the measurement points located in these areas have been performed. The results showed no partial discharges that could threaten further operation of the tested transformer as well as three areas where increased magnetoacoustic emission occurred. Thermal imaging studies were carried out in the 7.5 μm < λ < 13 μm band. Three areas were located on the calculated thermograms: the entire upper surface of the transformer tank and two areas on the side walls of the tested transformer in which increased IR radiation occurred. The results of the analysis of the research results for the two methods correspond with each other, having a common part, and complement each other giving a broader description of studied phenomena.

Transformerless inverters are widely used in different photovoltaic nonisolated ac module applications, mainly in grid-tied photovoltaic (PV) generation systems, due to the benefits of achieving high efficiency over a wide load range, and low cost. Various transformerless inverter topologies have been proposed to meet the safety requirement of low ground leakage currents, such as specified in the VDE-4105 standard and low-output ac-current distortion. Topology modifications of transformerless full bridge inverters are designed to balance and maintain a constant common mode output voltage, thereby eliminating or reducing leakage currents. This article reviews and compares the different methods for limiting leakage currents in known topologies of the full-bridge transformerless inverters, such as: H4, H5, H6, HERIC, and their improvements. The main topologies and strategies used to reduce the leakage current in transformerless schemes are summarized, highlighting advantages and disadvantages and establishing points of comparison with similar topologies. To compare the properties of different medium to high power inverters, PV inverter topologies were implemented using IGBTs and tested with the same components, same simulation parameters in PSPICE to evaluate their performance in terms of energy efficiency and leakage current characteristics. The detailed power stage operating principles, extended PWM modulator, and integrated universal gate driver with galvanic isolation in the transmission path of control signal for all IGBTs of the inverter, as well isolated and floating bias power supply for gate drivers are described.

The paper proposes an original single-phase transformerless three-level (S-PT) photovoltaic (PV) inverter in the cascade H bridge (CHB) configuration. The DC-link voltage of the inverter is created by two serial voltage sources with a voltage twice as low as the DC-link voltage. An appropriate V_CC DC-link voltage is generated by a two-phase DC-DC boost converter, fed from the string panel output at a level determined by the maximum power point tracking (MPPT) algorithm. Two symmetrical sources with V_CC/2 are formed by a divider of two series-connected capacitors of large and the same capacitance. The common mode (CM) voltage of the proposed inverter is constant, and the voltage stresses across all switches, diodes and gate drive circuits are half of the DC-link voltage. The principles of operation of the S-PT inverter, an implementation of a complete gate control system with galvanic isolation for all IGBTs, are also presented. The proposed inverter topologies have been implemented using high-speed IGBTs and simulated in PSPICE, as well as being experimentally validated.