Mitigation of Remanence Flux in Power Transformers using Predetermined Method of De-Energization

Title from PDF of title page viewed October 30, 2017Thesis advisor: Preetham GoliVitaIncludes bibliographical references (pages 37-39)Thesis (M.S)--School of Computing and Engineering. University of Missouri--Kansas City, 2017Energization of large power transformers are subject to many transients that may complicate the successful completion of this process and ultimately reduce the expected life of these critical components. The first-time energization (commissioning), subsequent energizations (operational), methods of energization (abrupt or controlled energizations from the high voltage or low voltage winding) and the possibility/improbability of these transformers being preloaded all affect the transformer’s longevity. The consequences of such energizations during the conditions are inrush currents and voltage stresses on the affected components that may not be foremost on the designer’s mind. The designer may be more concerned with proper parameter application and not the effects of commissioning and operation on these massive components. These behemoths are a bit akin to elephants whose longevity is dependent on the sum of their life experiences and the scars they endure during this period. The reliability of electric system is directly affected by these series connected behemoths. The construction of power transformers has been optimized by the advent of computers (especially finite analyses) to the point that stray flux, eddy current, hysteresis loss and harmonic loss (embodied and represented within the non-linear Rₚ element and known as “core Watt losses”) have all attained significant improvements witnessed by their 99.8+ percent efficiency. The difficulties that remain are magnetizing inrush and remanence embodied within Xₚ which occur dependent on three parameters. The parameters are primary resistance Rₛ (dependent on the location of same for the equivalent circuit used), the time dependent voltage at the point on the voltage wave when the transformer is energized (referred to as “Point of Wave”) and the remanent (or residual flux) and its polarity all at the instant of energization. The magnetizing inrush problem has been thoroughly researched and commercial products exist to mitigate such difficulties by control system add-ons. This research recognizes that knowledge of Point on Wave has effectively mitigated the problems with transformer energization at zero voltage. The results obtained after hundreds of runs confirms a direct relationship between the point of the wave where current is extinguished for a fast acting air switch and minimal to zero remanence flux in a single-phase shell form transformer. This minimal to zero residual flux appears at the peak of the equivalent sinusoidal current wave (increasing or decreasing) without the effects of saturation. The conclusion of the experimental runs was that the use of multiple Hall-Effect transducers (multiple installations suggested for manufacturing errors or wiring failures) within the laminations of a transformer which would be used to confirm the near zero remanent flux once the current was extinguished as described above. These findings and recommendations are still subject to testing at nameplate loads of varying power factors upon three phase transformers of shell and core constructions.Introduction -- PCAD modelling of remanence in transformers -- Equipment -- Experiment and results -- Implementation of the conclusion

Smart switching in single-phase grid-connected photovoltaic power systems for inrush current elimination

Grid-connected photovoltaic (PV) power systems are one of the most promising tech- nologies to address growing energy demand and ecological challenges. This paper proposes smart switching to mitigate inrush currents during the connection of single-phase transformers used in PV systems. An effective inrush current mitigation contributes to the reliability of PV systems. The inrush current severity is influenced by the pseudorandom residual flux at the transformer core and the energization point-on-wave. The most common approach to avoid inrush currents is controlled connection, which requires prior knowledge of the residual flux. However, the residual flux can differ in each case, and its measurement or estimation can be impractical. The proposed smart switching is based on a comprehensive analysis of the residual flux and the de-energization trajectories, and only requires two pieces of data (fRM and f0, flux values of the static and dynamic loops when the respective currents are null), calculated from two simple no-load tests. It has a clear advantage over common approaches: no need to estimate or measure the residual flux before each connection, avoiding the need for expensive equipment or complex setups. Smart switching can be easily im- plemented in practical settings, as it considers different circuit breakers with distinctive aperture features, making it cost-effective for PV systems.Peer ReviewedPostprint (published version

Method to Eliminate Flux Linkage DC Component in Load Transformer for Static Transfer Switch

Many industrial and commercial sensitive loads are subject to the voltage sags and interruptions. The static transfer switch (STS) based on the thyristors is applied to improve the power quality and reliability. However, the transfer will result in severe inrush current in the load transformer, because of the DC component in the magnetic flux generated in the transfer process. The inrush current which is always 2~30 p.u. can cause the disoperation of relay protective devices and bring potential damage to the transformer. The way to eliminate the DC component is to transfer the related phases when the residual flux linkage of the load transformer and the prospective flux linkage of the alternate source are equal. This paper analyzes how the flux linkage of each winding in the load transformer changes in the transfer process. Based on the residual flux linkage when the preferred source is completely disconnected, the method to calculate the proper time point to close each phase of the alternate source is developed. Simulation and laboratory experiments results are presented to show the effectiveness of the transfer method

Hybrid cascaded modular multilevel converter with DC fault ride-through capability for HVDC transmission system

A new hybrid cascaded modular multilevel converter for high-voltage dc (HVDC) transmission system is presented. The half-bridge (HB) cells are used on the main power stage and the cascade full-bridge (FB) cells are connected to its ac terminals. The main power stage generates the fundamental voltages with quite low switching frequency, resulting relatively low losses. The cascaded FB cells only attenuate the harmonics generated by the main power stage, without contribution to the power transfer. Thus, the energy storage requirement of the cascaded FB cells is low and the capacitance of FB cells is reduced significantly. Due to the dc fault reverse blocking capability of the cascaded FB cells, the proposed topology can ride-through the pole-to-pole dc fault. In addition the soft restart is achieved after the fault eliminates, without exposing the system to significant inrush current. Besides, the average-value model of the proposed topology is derived, based on which the control strategy is presented. The results show the feasibility of the proposed converter

Virtually synchronous power plant control

During the last century, the electrical energy infrastructures have been governed by synchronous generators, producing electrical energy to the vast majority of the population worldwide. However, power systems are no longer what they used to be. During the last two decades of this new millennium the classical, centralized and hierarchical networks have experienced an intense integration of renewable energy sources, mainly wind and solar, thanks also to the evolution and development of power conversion and power electronics industry. Although the current electrical system was designed to have a core of generation power plants, responsible of producing the necessary energy to supply end users and a clear power flow, divided mainly into transmission and distribution networks, as well as scalable consumers connected at different levels, this scenario has dramatically changed with the addition of renewable generation units. The massive installation of wind and solar farms, connected at medium voltage networks, as well as the proliferation of small distributed generators interfaced by power converters in low voltage systems is changing the paradigm of energy generation, distribution and consumption. Despite the feasibility of this integration in the existing electrical network, the addition of these distributed generators made grid operators face new challenges, especially considering the stochastic profile of such energy producers. Furthermore, the replacement of traditional generation units for renewable energy sources has harmed the stability and the reliable response during grid contingencies. In order to cope with the difficult task of operating the electrical network, transmission system operators have increased the requirements and modified the grid codes for the newly integrated devices. In an effort to enable a more natural behavior of the renewable systems into the electrical grid, advanced control strategies were presented in the literature to emulate the behavior of traditional synchronous generators. These approaches focused mainly on the power converter relying on their local measurement points to resemble the operation of a traditional generating unit. However, the integration of those units into bigger systems, such as power plants, is still not clear as the effect of accumulating hundreds or thousands of units has not been properly addressed. In this regard, the work of this thesis deals with the study of the so-called virtual synchronous machine (VSM) in three control layers. Furthermore, an in-depth analysis of the general structure used for the different virtual synchronous machine approaches is presented, which constitutes the base implementation tree for all existent strategies of virtual synchronous generation. In a first stage, the most inner control loop is studied and analyzed regarding the current control on the power converter. This internal regulator is in charge of the current injection and the tracking of all external power reference. Afterward, the synchronous control is oriented to the device, where the generating unit relies on its local measurements to emulate a synchronous machine in the power converter. In this regard, a sensorless approach to the virtual synchronous machine is introduced, increasing the stability of the power converter and reducing the voltage measurements used. Finally, the model of the synchronous control is extrapolated into a power plant control layer to be able to regulate multiple units in a coordinated manner, thus emulating the behavior of a unique synchronous machine. In this regard, the local measurements are not used for the emulation of the virtual machine, but they are switched to PCC measurements, allowing to set the desired dynamic response at the power plant level.Postprint (published version