Evolution and operation of modern grids and networks necessitate reconsideration of several characteristics. For example transformers operating in power grid or in industrial environment are subjected nowadays also to transients or stimulus with different waveforms. Traditionally, in case of power transformers the stresses are related to lightning and switching phenomena. In industrial networks distribution transformers are exposed to many switching operations generating often overvoltages. Transformer windings subjected to such system disturbances are stressed due to the non-linear transient voltage distribution along the winding and potential resonance phenomena. Initial voltage distribution, reflects mainly the capacitive coupling between turns, windings, core and tank. The non-uniform distribution of voltage results in concentred stress in certain parts of the winding. Winding resonance might occur, if the frequency spectrum of incoming surge matches the winding natural frequencies. Therefore external transients occurring in power systems might trigger internal overvoltages with large maximum value in transformer windings. The frequency characteristic of the transformer winding may determine those regions in the frequency spectrum. The paper presents results of voltage distribution measurements performed on model windings with different constructions subjected to voltages with different rise time. The experimental setup allows for investigation of influence of the dielectric (air/oil) and temperature on the stress distribution along the winding. Also the impact of oil and temperature on frequency characteristics of internal voltages are shown. The presented results might be used both for design and optimization of transformers windings.

During operation transformers are typically subjected to overvoltages with different wave shapes. Even though overvoltage protection systems, such as metal oxide surge arresters, are often employed, the overvoltages generated in power systems can still have a significant impact on the insulation systems of the windings of a power transformer. There are numerous examples of transformer failures as a result of overvoltages occurring during operation, even in cases that standard impulse voltage tests show no indication of potential problems and the transformers are protected by surge arresters. Comparisons of the internal overvoltages generated in transformers during voltage tests as well as those generated during standard operation of electrical networks provides fundamental insights into these overvoltages; insights which in turn can be used to improve the construction of insulation systems and overvoltage protection approaches and ultimately increase the reliability of transformers. In this paper, internal overvoltages occurring in the windings of selected transformer during standard voltage tests are compared with those which occur during typical operation of a transformer in an electrical power system. Overvoltages measured in the windings of the transformer form the basis of this comparison. Measurements were conducted with the transformer being subjected to voltage impulse tests, as well as transient voltages with waveforms simulating overvoltages generated in electrical power systems. In order to investigate the response of the windings to voltage strokes, full and chopped lightning voltage impulses were applied to the transformer. The waveforms of overvoltages generated in the electrical power systems were modelled as a rectangular voltage stroke, sinusoidal voltage, voltage with shape of “sweep sinus” also often referred to as a chirp signal and voltage with shape of “sweep ramp”.

Vacuum circuit breakers are increasingly used as switching apparatus in electric power systems. The vacuum circuit breakers (VCBs) have very good operating properties. VCBs are characterized by specific physical phenomena that affect overvoltage exposure of the insulation systems of other devices. The most important phenomena are the ability to chop the current before the natural zero crossing, the ability to switch off high-frequency currents, and the rapid increase in dielectric strength recovery. One of the devices connected directly to vacuum circuit breakers is the distribution transformer. Overvoltages generated in electrical systems during switching off the transformers are a source of internal overvoltages in the windings. The analysis of the exposure of transformers operating in electrical networks equipped with vacuum circuit breakers is of great importance because of the impact on the insulation systems of switching overvoltages (SO). These types of overvoltages can be characterized by high maximum values and atypical waveforms, depending on the phenomena in the circuit breaker chambers, system configuration, parameters of electrical devices, and overvoltage protection. Overvoltages that stress the internal insulation systems are the result of the windings response to overvoltages at transformer terminals. This article presents an analysis of overvoltages that stress the transformer insulation systems, which occur while switching off transformers in systems with vacuum circuit breakers. The analysis was based on the results of laboratory measurements of switching overvoltages at transformer terminals and inside the winding, in a model medium-voltage electrical network with a vacuum circuit breaker.

Due to the increasing requirements for the reliability of electrical power supply and associated apparatus, it is necessary to provide a detailed analysis of the overvoltage risk of power transformer insulation systems and equipment connected to their terminals. Exposure of transformer windings to overvoltages is the result of the propagation condition of electromagnetic waves in electrical networks and transformer windings. An analysis of transformer winding responses to transients in power systems is of particular importance, especially when protection against surges by typical overvoltage protection systems is applied. The analysis of internal overvoltages in transformers during a typical transient related to switching operations and selected failures is of great importance, particularly to assess the overvoltage exposure of insulation systems in operating conditions. The random nature of overvoltage phenomena in electrical networks implies the usage of computer simulations for the analysis of overvoltage exposures of electrical devices in operation. This article presents the analysis of the impact of transient phenomena in a model of a medium-voltage electrical network during switching operations and ground faults on overvoltages in the internal insulation systems of transformer windings. The basis of the analysis is simulations of overvoltages in the windings, made in the Electromagnetic Transients Program/Alternative Transients Program (EMTP/ATP) using a model with lumped parameters of transformer windings. The analysis covers the impact of the cable line length and the ground fault resistance value on internal overvoltage distributions.

Power transformers are subjected to surge overvoltages which can lead to failure of the windings insulation system. The amplitude of the overvoltages in the windings is dependent on the parameters of the surge voltage (duration, waveform, steepness, etc.) and the design of the windings, thus various winding designs perform differently when surge voltage is applied to their terminals. The transformer winding vulnerability to surge phenomena can be estimated based on the initial voltage distribution. In practice, the initial voltage distribution can be improved by increasing the series or decreasing the ground capacitance of the winding, by design changes, or by application of electrostatic shields. This article presents the analysis of the initial voltage distributions in power transformer windings, comparing layer and disk designs. The analysis was done based on the results of measurements of surge voltage waveforms in real-scale winding models, while a lightning impulse is applied to their terminals. The experimental analysis was extended by simulations of the electrostatic potential distribution in the winding models, and the influence of selected factors (shielding, interleaving turns) on the initial voltage distribution was analyzed and discussed. The article is focused on the comparison of initial voltage distribution in transformer windings of disk and layer designs based on quantitative and qualitative analysis.