Generation and measurement of mixed voltages, investigation on electrical discharge phenomena, and electric field analysis

Abstract

The insulation systems in power systems are frequently faced combinations of the operational voltage with over voltages. These types of voltage are called as "composite voltages" and "combined voltages" depending on the type of test object. They are superimposed two voltage signals with different properties (amplitude, frequency, time parameter, waveform). In order to generate them, it is necessary to connect different types of voltage generators together or types of two different voltage must be applied simultaneously to the device under test (DUT). In literature, tests of electrical insulation material are only applied for a single type of voltage wave. But, insulation of the system is forced with the electric field formed by the sum of the system voltage and overvoltage, when the internal and external overvoltage occurs in power systems. For example, insulation of the system is stressed with sum of the operation alternative voltage and lightning impulse voltage, when lightning strikes to power system line. During this event, the stress to which the insulation system is subjected differs according to the polarity of the lightning impulse and the polarity of the operating voltage at the time the lightning occurs. So, composite voltage conditions in the system must also be considered, when the insulation security and reliability of the system is defined. In this context, this thesis is based on three SCI articles on composite and combined voltage. In the first section of the thesis, 66 kV and 110 kV SiR insulators currently used in power transmission systems have been analyzed under combined AC–DC voltage using the finite element method (FEM). Insulators are the most crucial part of power systems. The insulation performance of insulators is vital for the sustainability of power systems. Recently, silicone rubber (SiR) insulators are used frequently in all sections of the power systems. In the analyzes made, positive and negative DC voltages in different amplitude ratios were superimposed over the phase-earth operating voltage of the insulator. In the study, the models were created based on time and analysis were applied in time-dependent. Alone DC voltage was applied to the insulator for the first 60 s, AC + DC voltage was applied between 60 to 120 s. Thus, the electric field behavior of the SiR insulator under combined AC–DC voltage has been obtained. The change of electric field based on positive and negative DC components was investigated. As a result of the study, the effect of the polarity of the DC component in the combined voltage was observed. The effect of the polarity of the DC component in the combined voltage on the maximum electric field intensity was observed. In the second section of the thesis, the effects of different electric fields, distance between electrodes and DC component of composite voltage on the breakdown voltage of air were investigated. The valve side of the converter valve in the HVDC transmission systems is subjected to mixed voltages such as composite AC & DC voltage. Normally, their structures have the geometry to create a uniform or less uniform electric field, but sharp points such as burrs on smooth surfaces can create non-uniform electric fields. In this study, four different electrode arrangements were used in the experiments to create different electric fields. The effects of the homogeneity of electric field on breakdown voltage were investigated for different ±DC component amplitudes of the composite voltage. The field efficiency factor was calculated using mean and maximum field strengths for all of them. Variation of breakdown voltage of air was examined under the composite AC & DC voltage for different ratios ±DC. As one result of the study, the breakdown occurs at the positive half-wave of the AC voltage despite −DC voltage being applied due to positive corona discharge pulses. This breakdown point is named as the polarity change point. The breakdown voltage increases with the decrease of DC voltage component up to polarity change point in non-uniform electric field. As a result of the experiments, it was seen that the polarity change point is closely related to the homogeneity of the electric field. As the homogeneity of the electric field increases, the polarity change point starts to be lower −DC voltage. In less uniform electric field, the AC breakdown voltage was measured slightly higher than the DC breakdown voltage. In less uniform electric field, as the ratio of the applied AC voltage to DC voltage increases, the breakdown voltage gradually approaches the AC breakdown voltage. This result is similar to the result obtained for the +DC component in non-uniform electric field experiments. In the last section of the thesis, firstly, experimental circuits were designed to generate and measure composite DC and LI high voltage using a simulation program. The voltage sources used in composite voltage generation must be isolated from each other with coupling elements so that they do not affect each other. In this context, it is critical to decide on the types and values of coupling elements. The coupling elements used were chosen according to simulation results. Afterward, experimental circuits were established in the laboratory according to the simulation results of the designed experimental circuit. Then, breakdown voltages under composite DC and LI voltage for less uniform and non-uniform electric fields were measured with four different electrode systems for positive and negative DC voltage pre-stresses with different amplitudes. The 50% breakdown voltage was calculated using the least-squares method. Finally, 3D models were created for the electrode systems used in the experiments using the finite element method. The efficiency factors of electrode systems calculated with the FEM results were correlated with the experimental breakdown voltage results. Thus, the breakdown behavior of air under bipolar and unipolar composite voltages (CV) was investigated. In conclusion, the experimental results showed that very fast polarity change in bipolar CV causes higher electrical stress compared to unipolar CV.