Hybrid Transformers - Power Electronics for Future Grids

Abstract

The awareness of the limited availability of fossil fuels and of their negative impact on the global environment is increasing. A transition towards a sustainable energy generation from renewable sources is of paramount importance but challenges the operation of the distribution grid. Among others, the distributed generation results in undesirable voltage fluctuations in distribution grids. As an alternative to conventional costly and tedious grid reinforcements, active grid voltage control has recently gained attention to ensure a high power quality in the presence of a high share of renewable generation. A significant enhancement of the grid controllability is possible with hybrid transformers, which combine a conventional low-frequency transformer with a fractionally rated power electronic converter. Since the major part of the power is transferred through the low-frequency transformer, the hybrid transformer concept utilizes the high efficiency, the high reliability and the low cost of conventional low-frequency transformers. By integrating a conventional transformer and a converter into one single system, the overall system complexity, cost and volume can be reduced. This thesis examines hybrid transformers from different perspectives to provide a comprehensive picture of their functionalities, protection requirements, performance and limitations. The focus is on hybrid transformers interconnecting the 20 kV and the 400V grids. Today's grid is a hostile environment for power electronic converters. Hence, a protection concept is of decisive importance for the reliable operation of hybrid transformers. In this thesis, protection requirements for hybrid transformers are derived from realistic fault scenarios and, where possible, from the standards for low-frequency transformers IEC 60076. A protection concept was developed, implemented and tested experimentally for a 100 kVA prototype. The converter protection was found to have a major impact on the design and the performance of the converter. However, the protection concept increases losses and volume of the complete hybrid transformer system only insignificantly due to the fractional converter rating. A converter optimization procedure is developed in this thesis to derive the theoretically achievable performance of the power electronic converter and of the total hybrid transformer. For the considered 100 kVA hybrid transformer with a control range of 10% of the voltage, active and reactive power, a full load efficiency of ηHT = 98.48% is possible. This is only 0.16 percent points lower than the assumed full load efficiency of the conventional transformer of ηLFT = 98.64%. The control system is an essential part of the hybrid transformer to allow the precise adjustment of grid voltages and currents while ensuring a safe converter operation. This thesis presents the overall hybrid transformer control structure and investigates a control concept which is generally applicable to three-phase inverters with parallel phase-legs. Besides the intuitive operation principle, the main benefit of the concept is the decoupling of the converter output voltage/current controller from the controller part equalizing the currents of the parallel phase legs. This allows an independent tuning of the two control parts. Simulation and experimental results from a 10 kVA converter prototype verify the effectiveness of the control concept. GaN switches are a promising technology for hard-switched, low-voltage, high-current applications like the converter of the hybrid transformer. The thesis presents Pareto optimizations which underscore the performance benefits of GaN over Si switches for such applications. However, currently available GaN devices necessitate a large number of parallel switches to achieve high output currents at low voltages. This thesis investigates challenges arising from this parallelization theoretically and experimentally. A half-bridge cell with four parallel GaN devices is presented and tested as the basic building block of general high-current GaN converters such as the converter of the hybrid transformer prototype. A three-phase 100 kVA hybrid transformer prototype was realized within the scope of this thesis. The 10 kVA GaN back-to-back converter is designed based on the mentioned converter optimization procedure. Measurement results successfully demonstrate the targeted voltage and reactive power control functionalities as well as the low impact of the converter on the total harmonic distortion of the grid voltage. A hybrid transformer efficiency of ηHT = 98.3% was determined from the measured converter efficiency of ηconv = 96.1% at a power level of 100 kW. A discussion of the loss contributions reveals potentials to further increase the converter efficiency. Finally, this thesis suggests additional functionalities of hybrid transformers to enhance the customer benefits with only a minor additional hardware effort. A method to eliminate the inrush currents occurring when the low-frequency transformer is connected to the grid was developed exemplarily.