Mehdi Allahbakhshi

This article deals with the methods of finding partial discharge (PD) location in power transformers using ultra high frequency (UHF) measurements. The UHF technique utilises two methods to find the PD location, that is, the shortest path method and hyperbolic method. The shortest path method works based on the comparison of the measured data and the ones in the database. In the hyperbolic method, a hyperbolic equation is obtained between each two element subset of sensors. The coordinate that best fits all equations is known as the PD location, and can be obtained in three different ways, that is, iterative algorithms, the Fang method and Chan method. The convergence of iterative algorithms is limited by poor initial estimate, overshoot, mitigation of non‐convergence etc. The Fang and Chan methods are two closed‐form solutions that are used in the communication system to find the radiation source location. This article explains how to use these two methods to obtain the PD coordinate inside the power transformer. These two methods can find exactly the coordinate that best fits all hyperbolic equations. At the end of this article, several tests are carried out through CST software and the PD locations is estimated by all presented methods. The simulation results show how the Fang and Chan methods can overcome the limitations of the iterative method.

This article presents two novel high-voltage pulse generator (HVPG) structures. These modular topologies are based on the buck-boost converter (BBC) operated in discontinuous conduction mode (DCM). The proposed generators produce unipolar and bipolar bell-shaped pulses, which can be applied for applications with high resistive loads. In these structures, the capacitors can be charged up to several times the dc input voltage, simultaneously. Accordingly, they have a high voltage (HV) gain even with a low-voltage dc (LVDC) source. This allows for a compact structure with a low number of modules (switches, diodes, capacitors, etc.). By reducing the number of modules, the total cost and power losses are reduced and the reliability is increased and its maintenance and fault detection would become easier. The other significant advantage of the proposed topologies is the voltage rating reduction of switches and diodes. Also, the series connection of switches and diodes is not required in this structure, which decreases the complexity and costs. Furthermore, due to the simultaneous charging of the capacitors, high-frequency pulses can be generated. Moreover, pulsewidth and pulse rise time are designed arbitrary parameters in nanosecond to microsecond ranges and they are completely independent of the gate driver and switch speed. In the design section, the parameters of the structure are determined based on the pulse specifications. The structures are simulated in MATLAB/SIMULINK environment. The experimental results for 1 kV output voltage confirm the performance of the proposed structures.

Partial discharge (PD) measurement is an essential tool for evaluating the health condition of a transformer insulation system. The Conventional PD measurement systems have a major drawback since they cannot discriminate the internal PD pulses and external pulse-shaped interferences, especially in online and on-site conditions. Various de-noising techniques (e.g. wavelets) have been also used in the literature, which is effective for extracting the PD signal from the noisy signals or classifying the PD types. However, the PD pulses and some of the randomly external pulse-shaped interferences have similar time-frequency characteristics, which makes them difficult to separate. In this paper, a new cost-effective and simple logic method is proposed for the separation of these interferences. This uses the polarity comparison of the output voltages of the Rogowski Coil (RC) sensor, mounted on the bushing, and measurement impedance, connected to the test tap. The same polarity represents the internal pulse, while the opposite polarity indicates the external pulses. A distribution transformer ( $20/0.4\ {\bm{kV}},\ 500\ {\bm{kVA}})$ and a condenser bushing ( $72.5\ {\bm{kV}})$ were considered to evaluate the performance of the proposed system. The results confirm the effectiveness of the proposed PD measurement system for separating the pulses.