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Designing inductive voltage transformers for the highest voltage levels (550 kV and above) is a true challenge indeed. The main reason for this is their complex internal R-L-C structure, including an insulating system which needs to be resistant to all types of overvoltages and consequent dielectric stress it can encounter during its lifetime. This is why it is necessary to verify behavior of such transformers during the design process, and under four sets of standard test overvoltage types; the Power Frequency Withstand Voltage (PFWV), the Lightning Impulse Withstand Voltage (LIWV, i.e. BIL), Chopped Impulse Withstand Voltage (CIWV) and the Switching Impulse Withstand Voltage (SIWV). The main idea of the paper is to demonstrate that by understanding the influence of crucial parameters of the appropriate equivalent diagram on the voltage distribution across the active part of the transformer, it is possible to define the overall design of inductive voltage transformers so that they can satisfy even the most rigorous insulation requirements, thus pushing the boundaries of design even further.

Losses due to eddy currents in an open-type transformer core are significantly reduced by the lamination of the transformer core. In order to further reduce the eddy current losses, the open-type core often has a multi-part structure, i.e., it is composed of several more slender cores. The complete homogenization of such a core is not possible when an A→,V−A→ formulation is used, where A and V represent the magnetic vector potential and electric scalar potential, respectively. On the other hand, an A→,T→−A→ formulation, where T represents the electric vector potential, enables the complete homogenization of the general open-type core, but the simulation converges poorly due to the large number of degrees of freedom. By eliminating the redundant degrees of freedom, the convergence rate is significantly improved, and is at least twice as good as the convergence rate of the simulation based on the A→,V−A→ formulation. In this paper, a method for the calculation of the eddy current losses in an open-type core based on the A→,T→−A→ formulation with the elimination of redundant degrees of freedom is presented. The method is validated by comparison with a brute force simulation based on the A→,V−A→ formulation, and the efficiency of the method is determined by comparison with the standard homogenization method based on the A→,V−A→ formulation.

Modular Multilevel Converters (MMC) HVDC converter station topologies require the use of high-impedance grounding reactors on the converter side of power transformers. The main requirement for such units is the ability to withstand simultaneous AC and DC loading. The aim of this paper is to give insight on how to determine the DC capabilities of such units both through calculation and measurements. Additionally, the purpose of this paper is to show what the possibilities of different core designs in terms of DC capability are and what values can be expected and obtained.

This paper presents a fast and precise method for the calculation of internal voltage transients over the voltage instrument transformer windings. Lumped circuit parameters of the transformer winding are calculated using self-developed solvers based on the boundary element method and integral equations approach. A detailed equivalent circuit of the transformer winding is solved in time domain. Test model of the voltage instrument transformer is constructed with a number of measurement points along the windings. Results of the calculation are in a good agreement with the measured voltages.

Relevant international instrument transformer standards specify internal arc testing to prove the transformer behavior under internal fault conditions. However, the test is defined in a way that does not recognize that it is possible to limit and reduce the total fault energy. For such instances, currently defined testing is mostly inapplicable. The purpose of this paper is to provide a theoretical background on behavior of units with sectioned active part under fault conditions, aided by numerical analysis of different fault scenarios on a variety of inductive voltage transformers, spanning from 72.5–550 kV. Numerical calculations are verified with several tests performed on a 123 kV inductive voltage transformer. This paper is a part of a continuous broad research with an aim to develop and specify adequate routine, type and special testing sequence for qualifying the paper-oil insulation systems that limit internal arc energy. Furthermore, the idea is to prove performance of such systems by introducing criteria that exceed the practices of current standards and aim to improve the instrument transformer fault statistics.

Since the insulation defects, design miscalculations and material im-perfections in electric equipment lead to partial discharge, the detection of these discharges can be used not only to evaluate the insulation condition, but also to improve the production process of electric equipment. Therefore, these tests are often used in research and development departments. However, the procedure is usually time-consuming and can occupy significant human resources. Moreover, in repeated tests the human error can become apparent. As a solution to this prob-lem, automation of the tests presents itself as the solution. In this paper, an auto-mated system for detection of partial discharges in oil-paper insulation under al-ternating voltage is described. The automation test system consists of multiple control and acquisition subsystems. To keep the same test conditions in multiple tests, a heat insulated test tank with oil heaters is made for the regulation of oil and model temperature. Also, a voltage control system is implemented, which can raise and continuously regulate the test voltage, as well as rapidly switch off the voltage in case of partial discharge events to protect the test object. In high voltage tests the automation safety is a crucial factor, therefore measures are taken to prevent and detect equipment failure. Furthermore, communication of the automation system with the test engineer is enabled via status email messages, data logging and test recording. The above-mentioned system can follow the given test procedure and perform the experiments in place of the test engineer. The whole process is designed as a stand-alone automated test bed which can run continuously and independently for days.

Star-point reactors are grounding devices installed in HVDC stations between the converter transformer secondary side and AC side of converter arms to provide a reference to ground. Such reactors are used as high-impedance grounding on the converter side of power transformers, providing high impedance path for the fundamental harmonic (i.e. 50 or 60 Hz) and a low impedance path for DC current, eliminating DC current flowing through the transformer windings. Temperature rise and DC current capability tests of 420 kV star-point reactor are presented in this paper. The purpose of temperature rise test is to verify that temperatures that can damage the insulation of star-point reactor will not be reached with the specified service conditions. The temperature rise test was carried out according to the requirements of the IEC 61869-3 standard and client’s request which included simultaneous application of fundamental, 3rd harmonic voltage and DC excitation. The inclusion of 3rd harmonic excitation of an amplitude up to 15% simulates voltage harmonic distortion which may appear in the power system at the location of star-point reactor installation. Prior to temperature rise test, DC current capability test was performed. The goal of this test is to determine the value of DC current at which the saturation point is reached. DC current is injected through star-point reactor while AC voltage is applied. Two different cases are considered regarding AC voltage: test with fundamental voltage harmonic and test with fundamental voltage harmonic with superimposed third voltage harmonic. Test circuit is proposed which is suitable for generation of complex voltages composed of fundamental harmonic and superimposed third harmonic with amplitudes up to 15% of the applied fundamental harmonic. The proposed test circuit was also used during the temperature rise test and it is applicable for testing of HV equipment with rated voltage up to 420 kV.

The main drawback of wind energy is its intermittent nature. While wind power plants have shown great potential for larger scale system integration and electricity production, the reliability of their output is still an issue. This article outlines one possible way of decreasing the variations of wind energy. By grouping several geographically dispersed wind power plants into a virtual unit, the intermittency of wind power can be reduced and the daily load curve smoothed. The main goal of this artcle is to present the analysis of interconnecting several geographically dispersed locations into a virtual power plant, while placing the emphasis on the actual benefits of such a synergy in terms of increased availability and production of stable power with a constant output. The analyses presented in this article are based on actual measurements on several locations in Croatia.