Matrix converter fed power electronic transformer with enhanced features.

Abstract

Power transformers are an integral part of power systems. In many applications, like wind energy conversion and electric ship, the large weight and volume of present 60 Hz power transformer is a limitation. The size of transformers can be reduced by replacing the power transformers with high frequency transformers. For use of high frequency transformer in power systems, first the low frequency voltages are converted to high frequency by a power electronic converter and then it is stepped up or down by the high frequency transformer and finally the high frequency voltage is converter to low frequency by a second power electronic converter. The whole system is termed as power electronic transformer (PET). Matrix converter(MC) based power electronic transformers are the focus of this doctoral research. Direct AC to AC conversion using matrix converters have the major advantage of absence of storage capacitors over AC-DC-AC based conversion systems. A matrix converter based PET with open ended primary is described in this thesis. It has the salient features of controllable output voltage frequency, controllable input power factor, bi-directional power flow, zero common mode voltage and voltage transfer ratio of 1.5. The described PET is analyzed and simulation results are presented. Commutation of current in the leakage inductance is one of the challenges in the matrix converter based PET. The effect on voltage regulation due to increase in leakage inductance is studied using extensive simulation. An alternative path is needed for change of direction of current during commutation time. One of the ways to provide alternative path is use of clamp circuits. But use of clamp circuits lead to energy loss unless efficient energy recovery systems are designed. A source based commutation method is proposed to eliminate the use of clamp circuits. Depending on the direction of current in the HF transformer, the matrix converters on primary side of the transformer are switched such that natural commutation takes place in the leakage inductances. The overall efficiency of the PET is significantly improved. The commutation time is dependent on the value of leakage inductance. So, higher the leakage inductance, larger the commutation time and therefore, lower is the switching frequency. The nano-crystalline materials used for making HF transformers have very low core losses at very high frequencies as compared to 60 Hz power transformers. Also, power devices made of SiC devices are available which can operate at very high switching frequencies with very low switching losses. Therefore, leakage inductance is the only factor that limits the switching frequency. To solve the above mentioned problem, three novel PET topologies are proposed in which the switching frequency is independent of the value of leakage inductance. The first one is sinusoidal input output three phase HF transformer. It has a three phase low pass filter and a matrix converter on both primary and secondary side of the HF transformer. Three square wave voltages at switching frequency, phase shifted by 120◦ are produced by the primary side MC. The square wave voltages are filtered by the primary side filter to give three sine waves at high frequency shifted by 120◦. The switched currents at the input of secondary side MC are filtered by the secondary side low pass filter. The HF transformer works like a three phase power transformer. The second topology proposed is sinusoidal input output three winding HF transformer. In this topology, the supply voltages are first converted to square wave voltage at high frequency by a three phase to single phase MC and then filtered to a sine wave by a low pass filter connected to primary of the HF transformer. Two opposite sine voltages at high frequency are produced by the three winding transformer, on the secondary side, which is further connected to two capacitors. The switched currents on the input of MC, connected to load, are filtered by these two capacitors. The transformer again sees sinusoidal voltages and currents as in the first topology. Zero common mode voltage is achieved in this topology by use of a modified pulse density modulation(PDM) strategy proposed. The third one proposed is sinusoidal current HF transformer. Unlike the first two, it does not has a filter on the primary side. A high frequency square wave voltage is produced by a three phase to single phase MC. A MC is connected between the secondary terminals and the load. PDM is used for secondary side MC. A low pass filter is formed with the transformer by connecting a capacitor at the secondary terminals. Both, the switched currents at input of secondary side MC and the square wave voltages across the primary windings are filtered by the low pass filter. Thus, only sinusoidal currents flow through the HF transformer. In all the above three mentioned topologies, the leakage inductance is used to form the low pass filter, minimizing the amount of reactive elements required. As these low pass filters are required to filter very high frequency voltages, the size of reactive elements are reduced. For the sinusoidal current HF transformer only one additional capacitor is needed. Also because of use of PDM, zero voltage switching is possible for the sinusoidal three winding and sinusoidal current transformers. All the proposed topologies are analyzed and simulated in MATLAB/ SIMULINK environment and the simulation results are presented. Mathematical model for filter design is provided. A laboratory prototype is built for sinusoidal current HF transformer and the experimental results are presented.